2018

  • Thermodynamically consistent three-dimensional electrochemical model for polymeric membranes
    • M. Rossi, T. Wallmersperger
    • Electrochimica Acta 283, 1323-1338 (2018)
    • DOI   Abstract  

      In this paper, the behavior of an electrochemical thin polymeric film sandwiched between porous electrodes and under input voltage (potentiostatic) conditions is numerically investigated. Thin polymeric membranes, such as Nation, are widely used in micro-batteries and proton-exchange-membrane fuel cells. A three-dimensional continuum-based multi-field formulation for thin polymeric membranes is presented. The model is applied to configurations of the type electrode-membrane-electrode in order to represent the core of a characteristic electrochemical device. Various three-dimensional geometries are considered. The phenomena modeled within the polymeric membrane are (i) ion transport (chemical field) and (ii) electrical field. The field equations, i.e. the Poisson-Nernst-Planck equations, are determined by inserting the thermodynamically consistent constitutive equations within the balance equations of the electrochemical problem. Suitable initial and boundary conditions have to be imposed in order to solve the system of partial differential equations. In particular, the phenomena modeled at the membrane/electrode interface are (i) the electrochemical kinetics of the chemical reaction and (ii) the polarization effects. Time-dependent numerical simulations for potentiostatic conditions are performed within a finite element framework. It is shown that the presented fully coupled multi-field model reproduces, within a simulation framework, the behavior of electrochemical cells. In fact, the model is capable of well predicting the space and time evolution of the main electrochemical parameters as well as catching multi-dimensional effects. The presented model can be regarded as a starting point to develop further research concerning an electro-chemo-mechanical model in order to well understand the stress distribution which arises in polymeric membranes during operating conditions. (C) 2018 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Marco Rossi and Thomas Wallmersperger},
      title = {Thermodynamically consistent three-dimensional electrochemical model for polymeric membranes},
      journal = {Electrochimica Acta},
      volume = {283},
      pages = {1323-1338},
      abstract = {In this paper, the behavior of an electrochemical thin polymeric film sandwiched between porous electrodes and under input voltage (potentiostatic) conditions is numerically investigated. Thin polymeric membranes, such as Nation, are widely used in micro-batteries and proton-exchange-membrane fuel cells. A three-dimensional continuum-based multi-field formulation for thin polymeric membranes is presented. The model is applied to configurations of the type electrode-membrane-electrode in order to represent the core of a characteristic electrochemical device. Various three-dimensional geometries are considered. The phenomena modeled within the polymeric membrane are (i) ion transport (chemical field) and (ii) electrical field. The field equations, i.e. the Poisson-Nernst-Planck equations, are determined by inserting the thermodynamically consistent constitutive equations within the balance equations of the electrochemical problem. Suitable initial and boundary conditions have to be imposed in order to solve the system of partial differential equations. In particular, the phenomena modeled at the membrane/electrode interface are (i) the electrochemical kinetics of the chemical reaction and (ii) the polarization effects. Time-dependent numerical simulations for potentiostatic conditions are performed within a finite element framework. It is shown that the presented fully coupled multi-field model reproduces, within a simulation framework, the behavior of electrochemical cells. In fact, the model is capable of well predicting the space and time evolution of the main electrochemical parameters as well as catching multi-dimensional effects. The presented model can be regarded as a starting point to develop further research concerning an electro-chemo-mechanical model in order to well understand the stress distribution which arises in polymeric membranes during operating conditions. (C) 2018 Elsevier Ltd. All rights reserved.},
      year = {2018},
      url = http://dx.doi.org/{10.1016/j.electacta.2018.06.174},
      doi = {10.1016/j.electacta.2018.06.174},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Time-dependent framework for energy and charge currents in nanoscale systems
    • T. Lehmann, A. Croy, R. Gutiérrez, G. Cuniberti
    • Chemical Physics (2018)
    • DOI   Abstract  

      The calculation of time-dependent charge and energy currents in nanoscale systems is a challenging task. Nevertheless it is crucial for gaining a deep understanding of the relevant processes at the nanoscale. We extend the auxiliary-mode approach for time-dependent charge transport to allow for the calculation of energy currents for arbitrary time dependencies. We apply the approach to two illustrative examples, a single-level system and a benzene ring, demonstrating its usefulness for a wide range of problems beyond simple toy models, such as molecular devices.

      @article{,
      author = {Thomas Lehmann and Alexander Croy and Rafael Gutiérrez and Gianaurelio Cuniberti},
      title = {Time-dependent framework for energy and charge currents in nanoscale systems},
      journal = {Chemical Physics},
      abstract = {The calculation of time-dependent charge and energy currents in nanoscale systems is a challenging task. Nevertheless it is crucial for gaining a deep understanding of the relevant processes at the nanoscale. We extend the auxiliary-mode approach for time-dependent charge transport to allow for the calculation of energy currents for arbitrary time dependencies. We apply the approach to two illustrative examples, a single-level system and a benzene ring, demonstrating its usefulness for a wide range of problems beyond simple toy models, such as molecular devices.},
      year = {2018},
      url = http://dx.doi.org/{10.1016/j.chemphys.2018.01.011},
      doi = {10.1016/j.chemphys.2018.01.011},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Active crystals on a sphere
    • S. Praetorius, A. Voigt, R. Wittkowski, H. Loewen
    • Physical Review E 97(2018)
    • DOI   Abstract  

      Two-dimensional crystals on curved manifolds exhibit nontrivial defect structures. Here we consider "active crystals" on a sphere, which are composed of self-propelled colloidal particles. Our work is based on a phasefield-crystal-type model that involves a density and a polarization field on the sphere. Depending on the strength of the self-propulsion, three different types of crystals are found: a static crystal, a self-spinning "vortex-vortex" crystal containing two vortical poles of the local velocity, and a self-translating "source-sink" crystal with a source pole where crystallization occurs and a sink pole where the active crystal melts. These different crystalline states as well as their defects are studied theoretically here and can in principle be confirmed in experiments.

      @article{,
      author = {Simon Praetorius and Axel Voigt and Raphael Wittkowski and Hartmut Loewen},
      title = {Active crystals on a sphere},
      journal = {Physical Review E},
      volume = {97},
      number = {5},
      abstract = {Two-dimensional crystals on curved manifolds exhibit nontrivial defect structures. Here we consider "active crystals" on a sphere, which are composed of self-propelled colloidal particles. Our work is based on a phasefield-crystal-type model that involves a density and a polarization field on the sphere. Depending on the strength of the self-propulsion, three different types of crystals are found: a static crystal, a self-spinning "vortex-vortex" crystal containing two vortical poles of the local velocity, and a self-translating "source-sink" crystal with a source pole where crystallization occurs and a sink pole where the active crystal melts. These different crystalline states as well as their defects are studied theoretically here and can in principle be confirmed in experiments.},
      year = {2018},
      url = http://dx.doi.org/{10.1103/PhysRevE.97.052615},
      doi = {10.1103/PhysRevE.97.052615},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Analyzing the n-Doping Mechanism of an Air-Stable Small-Molecule Precursor
    • M. Schwarze, B. D. Naab, M. L. Tietze, R. Scholz, P. Pahner, F. Bussolotti, S. Kera, D. Kasemann, Z. Bao, K. Leo
    • Acs Applied Materials & Interfaces 10, 1340-1346 (2018)
    • DOI   Abstract  

      Efficient n-doping of organic semiconductors requires electron-donating molecules with small ionization energies, making such n-dopants usually sensitive to degradation under air exposure. A workaround consists in the usage of air-stable precursor molecules containing the actual n-doping species. Here, we systematically analyze the doping mechanism of the small-molecule precursor o-MeO-DMBI-Cl, which releases a highly reducing o-MeO-DMBI radical upon thermal evaporation. n-Doping of N,N-bis(fluoren-2-y1)-naphthalene tetracarboxylic diimide yields air-stable and highly conductive films suitable for application as electron transport layer in organic solar cells. By photoelectron spectroscopy, we determine a reduced doping efficiency at high doping concentrations. We attribute this reduction to a change of the precursor decomposition mechanism with rising crucible temperature, yielding an undesired demethylation at high evaporation rates. Our results do not only show the possibility of efficient and air-stable n-doping, but also support the design of novel air-stable precursor molecules of strong n-dopants.

      @article{,
      author = {Martin Schwarze and Benjamin D. Naab and Max L. Tietze and Reinhard Scholz and Paul Pahner and Fabio Bussolotti and Satoshi Kera and Daniel Kasemann and Zhenan Bao and Karl Leo},
      title = {Analyzing the n-Doping Mechanism of an Air-Stable Small-Molecule Precursor},
      journal = {Acs Applied Materials & Interfaces},
      volume = {10},
      number = {1},
      pages = {1340-1346},
      abstract = {Efficient n-doping of organic semiconductors requires electron-donating molecules with small ionization energies, making such n-dopants usually sensitive to degradation under air exposure. A workaround consists in the usage of air-stable precursor molecules containing the actual n-doping species. Here, we systematically analyze the doping mechanism of the small-molecule precursor o-MeO-DMBI-Cl, which releases a highly reducing o-MeO-DMBI radical upon thermal evaporation. n-Doping of N,N-bis(fluoren-2-y1)-naphthalene tetracarboxylic diimide yields air-stable and highly conductive films suitable for application as electron transport layer in organic solar cells. By photoelectron spectroscopy, we determine a reduced doping efficiency at high doping concentrations. We attribute this reduction to a change of the precursor decomposition mechanism with rising crucible temperature, yielding an undesired demethylation at high evaporation rates. Our results do not only show the possibility of efficient and air-stable n-doping, but also support the design of novel air-stable precursor molecules of strong n-dopants.},
      year = {2018},
      url = http://dx.doi.org/{10.1021/acsami.7b14034},
      doi = {10.1021/acsami.7b14034},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Bioinspired thermoresponsive nanoscaled coatings: Tailor-made polymer brushes with bioconjugated arginine-glycine-aspartic acid-peptides
    • U. Koenig, E. Psarra, O. Guskova, E. Bittrich, K. Eichhorn, M. Mueller, P. B. Welzel, M. Stamm, P. Uhlmann
    • Biointerphases 13(2018)
    • DOI   Abstract  

      The development of bioengineered surface coatings with stimuli-responsive properties is beneficial for a number of biomedical applications. Environmentally responsive and switchable polymer brush systems have a great potential to create such smart biointerfaces. This study focuses on the bioconjugation of cell-instructive peptides, containing the arginine-glycine-aspartic acid tripeptide sequence (RGD motif), onto well-defined polymer brush films. Herein, the highly tailored end-grafted homo polymer brushes are either composed of the polyelectrolyte poly(acrylic) acid (PAA), providing the reactive carboxyl functionalities, or of the temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm). Of particular interest is the preparation of grafted-to binary brushes using both polymers and their subsequent conversion to RGD-biofunctionalized PNIPAAm-PAA binary brushes by a carbodiimide conjugation method. The bioconjugation process of two linear RGD-peptides Gly-Arg-Gly-Asp-Ser and Gly-Arg-Gly-Asp-Ser-Pro-Lys and one cyclic RGD-peptide cyclo(Arg-Gly-Asp-D-Tyr-Lys) is comparatively investigated by complementary analysis methods. Both techniques, in situ attenuated total reflectance Fourier transform infrared spectroscopy measurements and the in situ spectroscopic ellipsometric analysis, describe changes of the brush surface properties due to biofunctionalization. Besides, the bound RGD-peptide amount is quantitatively evaluated by ellipsometry in comparison to high performance liquid chromatography analysis data. Additionally, molecular dynamic simulations of the RGD-peptides themselves allow a better understanding of the bioconjugation process depending on the peptide properties. The significant influence on the bioconjugation result can be derived, on the one hand, of the polymer brush composition, especially from the PNIPAAm content, and, on the other hand, of the peptide dimension and its reactivity. Published by the AVS.

      @article{,
      author = {Ulla Koenig and Evmorfia Psarra and Olga Guskova and Eva Bittrich and Klaus-Jochen Eichhorn and Martin Mueller and Petra B. Welzel and Manfred Stamm and Petra Uhlmann},
      title = {Bioinspired thermoresponsive nanoscaled coatings: Tailor-made polymer brushes with bioconjugated arginine-glycine-aspartic acid-peptides},
      journal = {Biointerphases},
      volume = {13},
      number = {2},
      abstract = {The development of bioengineered surface coatings with stimuli-responsive properties is beneficial for a number of biomedical applications. Environmentally responsive and switchable polymer brush systems have a great potential to create such smart biointerfaces. This study focuses on the bioconjugation of cell-instructive peptides, containing the arginine-glycine-aspartic acid tripeptide sequence (RGD motif), onto well-defined polymer brush films. Herein, the highly tailored end-grafted homo polymer brushes are either composed of the polyelectrolyte poly(acrylic) acid (PAA), providing the reactive carboxyl functionalities, or of the temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm). Of particular interest is the preparation of grafted-to binary brushes using both polymers and their subsequent conversion to RGD-biofunctionalized PNIPAAm-PAA binary brushes by a carbodiimide conjugation method. The bioconjugation process of two linear RGD-peptides Gly-Arg-Gly-Asp-Ser and Gly-Arg-Gly-Asp-Ser-Pro-Lys and one cyclic RGD-peptide cyclo(Arg-Gly-Asp-D-Tyr-Lys) is comparatively investigated by complementary analysis methods. Both techniques, in situ attenuated total reflectance Fourier transform infrared spectroscopy measurements and the in situ spectroscopic ellipsometric analysis, describe changes of the brush surface properties due to biofunctionalization. Besides, the bound RGD-peptide amount is quantitatively evaluated by ellipsometry in comparison to high performance liquid chromatography analysis data. Additionally, molecular dynamic simulations of the RGD-peptides themselves allow a better understanding of the bioconjugation process depending on the peptide properties. The significant influence on the bioconjugation result can be derived, on the one hand, of the polymer brush composition, especially from the PNIPAAm content, and, on the other hand, of the peptide dimension and its reactivity. Published by the AVS.},
      year = {2018},
      url = http://dx.doi.org/{10.1116/1.5020129},
      doi = {10.1116/1.5020129},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Chirality-Dependent Electron Spin Filtering by Molecular Monolayers of Helicenes
    • M. Kettner, V. V. Maslyuk, D. Nuerenberg, J. Seibel, R. Gutierrez, G. Cuniberti, K. Ernst, H. Zacharias
    • Journal of Physical Chemistry Letters 9, 2025-2030 (2018)
    • DOI   Abstract  

      The interaction of low-energy photoelectrons with well-ordered mono layers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about Pz = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin orbit coupling induced by the electrostatic molecular potential.

      @article{,
      author = {Matthias Kettner and Volodymyr V. Maslyuk and Daniel Nuerenberg and Johannes Seibel and Rafael Gutierrez and Gianaurelio Cuniberti and Karl-Heinz Ernst and Helmut Zacharias},
      title = {Chirality-Dependent Electron Spin Filtering by Molecular Monolayers of Helicenes},
      journal = {Journal of Physical Chemistry Letters},
      volume = {9},
      number = {8},
      pages = {2025-2030},
      abstract = {The interaction of low-energy photoelectrons with well-ordered mono layers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about Pz = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin orbit coupling induced by the electrostatic molecular potential.},
      year = {2018},
      url = http://dx.doi.org/{10.1021/acs.jpclett.8b00208},
      doi = {10.1021/acs.jpclett.8b00208},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Copper Electroplating with Polyethylene Glycol: Part II. Experimental Analysis and Determination of Model Parameters
    • H. Yang, R. Krause, C. Scheunert, R. Liske, B. Uhlig, A. Preusse, A. Dianat, M. Bobeth, G. Cuniberti
    • Journal of the Electrochemical Society 165, D13-D22 (2018)
    • DOI   Abstract  

      In the electrochemical deposition of copper on structured substrates, additives are commonly used as ingredients to refine the copper thin film properties. By means of cyclovoltametric (CV) measurements, we examine the process behavior of the additives PEG (polyethylene glycol) and chloride ions over a wide range of experimental parameters relevant for production-like conditions. In this plating process, additives practically are neither consumed in chemical reactions nor are they incorporated into the growing copper film. To understand the observed complex hysteresis behavior of the deposition current in CV scans, we have recently proposed a model which is able to qualitatively explain this behavior without supposing additive consumption. In the present study, we fit crucial parameters of this model from the experimental data to increase its predictive power. The quantitative agreement of performed simulations of CV scans with the measured scans demonstrates the validity of the proposed copper deposition model. Equipped with the determined parameter set, the model can help to optimize the copper plating process in industrial applications. (C) 2018 The Electrochemical Society.

      @article{,
      author = {H. Yang and R. Krause and C. Scheunert and R. Liske and B. Uhlig and A. Preusse and A. Dianat and M. Bobeth and Gianaurelio Cuniberti},
      title = {Copper Electroplating with Polyethylene Glycol: Part II. Experimental Analysis and Determination of Model Parameters},
      journal = {Journal of the Electrochemical Society},
      volume = {165},
      number = {2},
      pages = {D13-D22},
      abstract = {In the electrochemical deposition of copper on structured substrates, additives are commonly used as ingredients to refine the copper thin film properties. By means of cyclovoltametric (CV) measurements, we examine the process behavior of the additives PEG (polyethylene glycol) and chloride ions over a wide range of experimental parameters relevant for production-like conditions. In this plating process, additives practically are neither consumed in chemical reactions nor are they incorporated into the growing copper film. To understand the observed complex hysteresis behavior of the deposition current in CV scans, we have recently proposed a model which is able to qualitatively explain this behavior without supposing additive consumption. In the present study, we fit crucial parameters of this model from the experimental data to increase its predictive power. The quantitative agreement of performed simulations of CV scans with the measured scans demonstrates the validity of the proposed copper deposition model. Equipped with the determined parameter set, the model can help to optimize the copper plating process in industrial applications. (C) 2018 The Electrochemical Society.},
      year = {2018},
      url = http://dx.doi.org/{10.1149/2.0081802jes},
      doi = {10.1149/2.0081802jes},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Defects at grain boundaries: A coarse-grained, three-dimensional description by the amplitude expansion of the phase-field crystal model
    • M. Salvalaglio, R. Backofen, K. R. Elder, A. Voigt
    • Physical Review Materials 2(2018)
    • DOI   Abstract  

      We address a three-dimensional, coarse-grained description of dislocation networks at grain boundaries between rotated crystals. The so-called amplitude expansion of the phase-field crystal model is exploited with the aid of finite element method calculations. This approach allows for the description of microscopic features, such as dislocations, while simultaneously being able to describe length scales that are orders of magnitude larger than the lattice spacing. Moreover, it allows for the direct description of extended defects by means of a scalar order parameter. The versatility of this framework is shown by considering both fee and bee lattice symmetries and different rotation axes. First, the specific case of planar, twist gram boundaries is illustrated. The details of the method are reported and the consistency of the results with literature is discussed. Then, the dislocation networks forming at the interface between a spherical, rotated crystal embedded in an unrotated crystalline structure, are shown. Although explicitly accounting for dislocations which lead to an anisotropic shrinkage of the rotated gram, the extension of the spherical grain boundary is found to decrease linearly over time in agreement with the classical theory of grain growth and recent atomistic investigations. It is shown that the results obtained for a system with bee symmetry agree very well with existing results, validating the methodology. Furthermore, fully original results are shown for fee lattice symmetry, revealing the generality of the reported observations.

      @article{,
      author = {Marco Salvalaglio and Rainer Backofen and K. R. Elder and Axel Voigt},
      title = {Defects at grain boundaries: A coarse-grained, three-dimensional description by the amplitude expansion of the phase-field crystal model},
      journal = {Physical Review Materials},
      volume = {2},
      number = {5},
      abstract = {We address a three-dimensional, coarse-grained description of dislocation networks at grain boundaries between rotated crystals. The so-called amplitude expansion of the phase-field crystal model is exploited with the aid of finite element method calculations. This approach allows for the description of microscopic features, such as dislocations, while simultaneously being able to describe length scales that are orders of magnitude larger than the lattice spacing. Moreover, it allows for the direct description of extended defects by means of a scalar order parameter. The versatility of this framework is shown by considering both fee and bee lattice symmetries and different rotation axes. First, the specific case of planar, twist gram boundaries is illustrated. The details of the method are reported and the consistency of the results with literature is discussed. Then, the dislocation networks forming at the interface between a spherical, rotated crystal embedded in an unrotated crystalline structure, are shown. Although explicitly accounting for dislocations which lead to an anisotropic shrinkage of the rotated gram, the extension of the spherical grain boundary is found to decrease linearly over time in agreement with the classical theory of grain growth and recent atomistic investigations. It is shown that the results obtained for a system with bee symmetry agree very well with existing results, validating the methodology. Furthermore, fully original results are shown for fee lattice symmetry, revealing the generality of the reported observations.},
      year = {2018},
      url = http://dx.doi.org/{10.1103/PhysRevMaterials.2.053804},
      doi = {10.1103/PhysRevMaterials.2.053804},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • DFT study of interaction of additives with Cu(111) surface relevant to Cu electrodeposition
    • A. Dianat, H. Yang, M. Bobeth, G. Cuniberti
    • Journal of Applied Electrochemistry 48, 211-219 (2018)
    • DOI   Abstract  

      The interaction of additives and ions with the copper surface plays a crucial role in the copper electroplating process. In this work, the interaction of the additives polyethylene glycol (PEG) and bis(3-sulfopropyl)-disulfide (SPS) as well as of chloride with the Cu(111) surface is considered within the framework of density functional theory. In the presence of water, the adsorption energy of chloride diminishes by about 1 eV compared to the case in vacuum. The activation barrier for chloride desorption was found to be 0.8 eV. Simulations of the deposition of copper atoms on a Cl-covered copper surface revealed that Cl atoms are always displaced to the surface. Calculations of adsorption energies of additives in vacuum indicated that the accelerator molecule SPS is bound stronger to Cu(111) than the suppressor molecule PEG. A comparatively strong adsorption of additives was found on a copper surface covered with a Cl-Cu mixed layer. Investigation of the dynamics of additives on Cu(111) by means of first principles molecular dynamics revealed an occasional spontaneous decomposition of an SPS molecule into two MPS molecules. [GRAPHICS] .

      @article{,
      author = {Arezoo Dianat and Hongliu Yang and Manfred Bobeth and Gianaurelio Cuniberti},
      title = {DFT study of interaction of additives with Cu(111) surface relevant to Cu electrodeposition},
      journal = {Journal of Applied Electrochemistry},
      volume = {48},
      number = {2},
      pages = {211-219},
      abstract = {The interaction of additives and ions with the copper surface plays a crucial role in the copper electroplating process. In this work, the interaction of the additives polyethylene glycol (PEG) and bis(3-sulfopropyl)-disulfide (SPS) as well as of chloride with the Cu(111) surface is considered within the framework of density functional theory. In the presence of water, the adsorption energy of chloride diminishes by about 1 eV compared to the case in vacuum. The activation barrier for chloride desorption was found to be 0.8 eV. Simulations of the deposition of copper atoms on a Cl-covered copper surface revealed that Cl atoms are always displaced to the surface. Calculations of adsorption energies of additives in vacuum indicated that the accelerator molecule SPS is bound stronger to Cu(111) than the suppressor molecule PEG. A comparatively strong adsorption of additives was found on a copper surface covered with a Cl-Cu mixed layer. Investigation of the dynamics of additives on Cu(111) by means of first principles molecular dynamics revealed an occasional spontaneous decomposition of an SPS molecule into two MPS molecules. [GRAPHICS] .},
      year = {2018},
      url = http://dx.doi.org/{10.1007/s10800-018-1150-1},
      doi = {10.1007/s10800-018-1150-1},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • A Dual-Stimuli-Responsive Sodium-Bromine Battery with Ultrahigh Energy Density
    • F. Wang, H. Yang, J. Zhang, P. Zhang, G. Wang, X. Zhuang, G. Cuniberti, X. Feng
    • Advanced Materials 30(2018)
    • DOI   Abstract  

      Stimuli-responsive energy storage devices have emerged for the fast-growing popularity of intelligent electronics. However, all previously reported stimuli-responsive energy storage devices have rather low energy densities (<250 Wh kg(-1)) and single stimuli-response, which seriously limit their application scopes in intelligent electronics. Herein, a dual-stimuli-responsive sodium-bromine (Na//Br-2) battery featuring ultrahigh energy density, electrochromic effect, and fast thermal response is demonstrated. Remarkably, the fabricated Na//Br-2 battery exhibits a large operating voltage of 3.3 V and an energy density up to 760 Wh kg(-1), which outperforms those for the state-of-the-art stimuli-responsive electrochemical energy storage devices. This work offers a promising approach for designing multi-stimuli-responsive and high-energy rechargeable batteries without sacrificing the electrochemical performance.

      @article{,
      author = {Faxing Wang and Hongliu Yang and Jian Zhang and Panpan Zhang and Gang Wang and Xiaodong Zhuang and Gianaurelio Cuniberti and Xinliang Feng},
      title = {A Dual-Stimuli-Responsive Sodium-Bromine Battery with Ultrahigh Energy Density},
      journal = {Advanced Materials},
      volume = {30},
      number = {23},
      abstract = {Stimuli-responsive energy storage devices have emerged for the fast-growing popularity of intelligent electronics. However, all previously reported stimuli-responsive energy storage devices have rather low energy densities (<250 Wh kg(-1)) and single stimuli-response, which seriously limit their application scopes in intelligent electronics. Herein, a dual-stimuli-responsive sodium-bromine (Na//Br-2) battery featuring ultrahigh energy density, electrochromic effect, and fast thermal response is demonstrated. Remarkably, the fabricated Na//Br-2 battery exhibits a large operating voltage of 3.3 V and an energy density up to 760 Wh kg(-1), which outperforms those for the state-of-the-art stimuli-responsive electrochemical energy storage devices. This work offers a promising approach for designing multi-stimuli-responsive and high-energy rechargeable batteries without sacrificing the electrochemical performance.},
      year = {2018},
      url = http://dx.doi.org/{10.1002/adma.201800028},
      doi = {10.1002/adma.201800028},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Effect of Magnetic Zigzag Edges in Graphene-like Nanoribbons on the Thermoelectric Power Factor
    • S. Krompiewski, G. Cuniberti
    • Acta Physica Polonica A 133, 535-537 (2018)
    • DOI   Abstract  

      This study shows that magnetic edge states of graphene-like nanoribbons enhance effectively the thermoelectric performance. This is due to the antiparallel alignment of magnetic moments on opposite zigzag edges and the confinement effect, which jointly lead to the appearance of a gap in the electronic energy spectrum. Consequently, the Seebeck coefficient as well as the thermoelectric power factor get strongly enhanced (with respect to other alignment cases) at room temperature and energies not far away from the charge neutrality point. Moreover the corresponding figure of merit (ZT) is also improved as a result of the reduced electronic thermal conductance.

      @article{,
      author = {S. Krompiewski and G. Cuniberti},
      title = {Effect of Magnetic Zigzag Edges in Graphene-like Nanoribbons on the Thermoelectric Power Factor},
      journal = {Acta Physica Polonica A},
      volume = {133},
      number = {3},
      pages = {535-537},
      abstract = {This study shows that magnetic edge states of graphene-like nanoribbons enhance effectively the thermoelectric performance. This is due to the antiparallel alignment of magnetic moments on opposite zigzag edges and the confinement effect, which jointly lead to the appearance of a gap in the electronic energy spectrum. Consequently, the Seebeck coefficient as well as the thermoelectric power factor get strongly enhanced (with respect to other alignment cases) at room temperature and energies not far away from the charge neutrality point. Moreover the corresponding figure of merit (ZT) is also improved as a result of the reduced electronic thermal conductance.},
      year = {2018},
      url = http://dx.doi.org/{10.12693/APhysPolA.133.535},
      doi = {10.12693/APhysPolA.133.535},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Feasible Device Architectures for Ultrascaled CNTFETs
    • A. Pacheco-Sanchez, F. Fuchs, S. Mothes, A. Zienert, J. Schuster, S. Gemming, M. Claus
    • Ieee Transactions on Nanotechnology 17, 100-107 (2018)
    • DOI   Abstract  

      Feasible device architectures for ultrascaled carbon nanotubes field-effect transistors (CNTFETs) are studied down to 5.9 nm using a multiscale simulation approach covering electronic quantum transport simulations and numerical device simulations. Schottky-like and ohmiclike contacts are considered. The simplified approach employed in the numerical device simulator is critically evaluated and verified bymeans of comparing the results with electronic quantum simulation results of an identical device. Different performance indicators, such as the switching speed, switching energy, the subthreshold slope, I-on/I-off – ratio, among others, are extracted for different device architectures. These values guide the evaluation of the technology for different application scenarios. For high-performance logic applications, the buried gate CNTFET is claimed to be the most suitable structure.

      @article{,
      author = {Anibal Pacheco-Sanchez and Florian Fuchs and Sven Mothes and Andreas Zienert and Joerg Schuster and Sibylle Gemming and Martin Claus},
      title = {Feasible Device Architectures for Ultrascaled CNTFETs},
      journal = {Ieee Transactions on Nanotechnology},
      volume = {17},
      number = {1},
      pages = {100-107},
      abstract = {Feasible device architectures for ultrascaled carbon nanotubes field-effect transistors (CNTFETs) are studied down to 5.9 nm using a multiscale simulation approach covering electronic quantum transport simulations and numerical device simulations. Schottky-like and ohmiclike contacts are considered. The simplified approach employed in the numerical device simulator is critically evaluated and verified bymeans of comparing the results with electronic quantum simulation results of an identical device. Different performance indicators, such as the switching speed, switching energy, the subthreshold slope, I-on/I-off - ratio, among others, are extracted for different device architectures. These values guide the evaluation of the technology for different application scenarios. For high-performance logic applications, the buried gate CNTFET is claimed to be the most suitable structure.},
      year = {2018},
      url = http://dx.doi.org/{10.1109/tnano.2017.2774605},
      doi = {10.1109/tnano.2017.2774605},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • First-Principle-Based Phonon Transport Properties of Nanoscale Graphene Grain Boundaries
    • L. M. Sandonas, H. Sevincli, R. Gutierrez, G. Cuniberti
    • Advanced Science 5(2018)
    • DOI   Abstract  

      The integrity of phonon transport properties of large graphene (linear and curved) grain boundaries (GBs) is investigated under the influence of structural and dynamical disorder. To do this, density functional tight-binding (DFTB) method is combined with atomistic Green’s function technique. The results show that curved GBs have lower thermal conductance than linear GBs. Its magnitude depends on the length of the curvature and out-of-plane structural distortions at the boundary, having stronger influence the latter one. Moreover, it is found that by increasing the defects at the boundary, the transport properties can strongly be reduced in comparison to the effect produced by heating up the boundary region. This is due to the large reduction of the phonon transmission for in-plane and out-of-plane vibrational modes after increasing the structural disorder in the GBs.

      @article{,
      author = {Leonardo Medrano Sandonas and Haldun Sevincli and Rafael Gutierrez and Gianaurelio Cuniberti},
      title = {First-Principle-Based Phonon Transport Properties of Nanoscale Graphene Grain Boundaries},
      journal = {Advanced Science},
      volume = {5},
      number = {2},
      abstract = {The integrity of phonon transport properties of large graphene (linear and curved) grain boundaries (GBs) is investigated under the influence of structural and dynamical disorder. To do this, density functional tight-binding (DFTB) method is combined with atomistic Green's function technique. The results show that curved GBs have lower thermal conductance than linear GBs. Its magnitude depends on the length of the curvature and out-of-plane structural distortions at the boundary, having stronger influence the latter one. Moreover, it is found that by increasing the defects at the boundary, the transport properties can strongly be reduced in comparison to the effect produced by heating up the boundary region. This is due to the large reduction of the phonon transmission for in-plane and out-of-plane vibrational modes after increasing the structural disorder in the GBs.},
      year = {2018},
      url = http://dx.doi.org/{10.1002/advs.201700365},
      doi = {10.1002/advs.201700365},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Hexacene generated on passivated silicon
    • F. Eisenhut, J. Krueger, D. Skidin, S. Nikipar, J. M. Alonso, E. Guitian, D. Perez, D. A. Ryndyk, D. Pena, F. Moresco, G. Cuniberti
    • Nanoscale 10, 12582-12587 (2018)
    • DOI   Abstract  

      On-surface synthesis represents a successful strategy to obtain designed molecular structures on an ultra-clean metal substrate. While metal surfaces are known to favor adsorption, diffusion, and chemical bonding between molecular groups, on-surface synthesis on non-metallic substrates would allow the electrical decoupling of the resulting molecule from the surface, favoring application to electronics and spintronics. Here, we demonstrate the on-surface generation of hexacene by surface-assisted reduction on a H-passivated Si(001) surface. The reaction, observed by scanning tunneling microscopy and spectroscopy, is probably driven by the formation of Si-O complexes at dangling bond defects. Supported by density functional theory calculations, we investigate the interaction of hexacene with the passivated silicon surface, and with single silicon dangling bonds.

      @article{,
      author = {Frank Eisenhut and Justus Krueger and Dmitry Skidin and Seddigheh Nikipar and Jose M. Alonso and Enrique Guitian and Dolores Perez and Dmitry A. Ryndyk and Diego Pena and Francesca Moresco and Gianaurelio Cuniberti},
      title = {Hexacene generated on passivated silicon},
      journal = {Nanoscale},
      volume = {10},
      number = {26},
      pages = {12582-12587},
      abstract = {On-surface synthesis represents a successful strategy to obtain designed molecular structures on an ultra-clean metal substrate. While metal surfaces are known to favor adsorption, diffusion, and chemical bonding between molecular groups, on-surface synthesis on non-metallic substrates would allow the electrical decoupling of the resulting molecule from the surface, favoring application to electronics and spintronics. Here, we demonstrate the on-surface generation of hexacene by surface-assisted reduction on a H-passivated Si(001) surface. The reaction, observed by scanning tunneling microscopy and spectroscopy, is probably driven by the formation of Si-O complexes at dangling bond defects. Supported by density functional theory calculations, we investigate the interaction of hexacene with the passivated silicon surface, and with single silicon dangling bonds.},
      year = {2018},
      url = http://dx.doi.org/{10.1039/c8nr03422b},
      doi = {10.1039/c8nr03422b},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Influence of microstructure morphology on multi-scale modeling of low-alloyed TRIP-steels
    • S. Prueger, A. Gandhi, D. Balzani
    • Engineering Computations 35, 499-528 (2018)
    • DOI   Abstract  

      Purpose – The purpose of this study is to quantify the impact of the variation of microstructural features on macroscopic and microscopic fields. The application of multi-scale methods in the context of constitutive modeling of microheterogeneous materials requires the choice of a representative volume element (RVE) of the considered microstructure, which may be based on some idealized assumptions and/or on experimental observations. In any case, a realistic microstructure within the RVE is either computationally too expensive or not fully accessible by experimental measurement techniques, which introduces some uncertainty regarding the microstructural features. Design/methodology/approach – In this paper, a systematical variation of microstructural parameters controlling the morphology of an RVE with an idealized microstructure is conducted and the impact on macroscopic quantities of interest as well as microstructural fields and their statistics is investigated. The study is carried out under macroscopically homogeneous deformation states using the direct micro-macro scale transition approach. Findings – The variation of microstructural parameters, such as inclusion volume fraction, aspect ratio and orientation of the inclusion with respect to the overall loading, influences the macroscopic behavior, especially the micromechanical fields significantly. Originality/value – The systematic assessment of the impact of microstructural parameters on both macroscopic quantities and statistics of the micromechanical fields allows for a quantitative comparison of different microstructure morphologies and a reliable identification of microstructural parameters that promote failure initialization in microheterogeneous materials.

      @article{,
      author = {Stefan Prueger and Ashutosh Gandhi and Daniel Balzani},
      title = {Influence of microstructure morphology on multi-scale modeling of low-alloyed TRIP-steels},
      journal = {Engineering Computations},
      volume = {35},
      number = {2},
      pages = {499-528},
      abstract = {Purpose - The purpose of this study is to quantify the impact of the variation of microstructural features on macroscopic and microscopic fields. The application of multi-scale methods in the context of constitutive modeling of microheterogeneous materials requires the choice of a representative volume element (RVE) of the considered microstructure, which may be based on some idealized assumptions and/or on experimental observations. In any case, a realistic microstructure within the RVE is either computationally too expensive or not fully accessible by experimental measurement techniques, which introduces some uncertainty regarding the microstructural features. Design/methodology/approach - In this paper, a systematical variation of microstructural parameters controlling the morphology of an RVE with an idealized microstructure is conducted and the impact on macroscopic quantities of interest as well as microstructural fields and their statistics is investigated. The study is carried out under macroscopically homogeneous deformation states using the direct micro-macro scale transition approach. Findings - The variation of microstructural parameters, such as inclusion volume fraction, aspect ratio and orientation of the inclusion with respect to the overall loading, influences the macroscopic behavior, especially the micromechanical fields significantly. Originality/value - The systematic assessment of the impact of microstructural parameters on both macroscopic quantities and statistics of the micromechanical fields allows for a quantitative comparison of different microstructure morphologies and a reliable identification of microstructural parameters that promote failure initialization in microheterogeneous materials.},
      year = {2018},
      url = http://dx.doi.org/{10.1108/ec-01-2017-0009},
      doi = {10.1108/ec-01-2017-0009},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C-60 and ZnPc
    • C. Gaul, S. Hutsch, M. Schwarze, K. S. Schellhammer, F. Bussolotti, S. Kera, G. Cuniberti, K. Leo, F. Ortmann
    • Nature Materials 17, 439-+ (2018)
    • DOI   Abstract  

      Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C-60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Delta(1) between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Delta(1) may provide alternative strategies to optimize the electronic properties of organic semiconductors.

      @article{,
      author = {Christopher Gaul and Sebastian Hutsch and Martin Schwarze and Karl Sebastian Schellhammer and Fabio Bussolotti and Satoshi Kera and Gianaurelio Cuniberti and Karl Leo and Frank Ortmann},
      title = {Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C-60 and ZnPc},
      journal = {Nature Materials},
      volume = {17},
      number = {5},
      pages = {439-+},
      abstract = {Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C-60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Delta(1) between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Delta(1) may provide alternative strategies to optimize the electronic properties of organic semiconductors.},
      year = {2018},
      url = http://dx.doi.org/{10.1038/s41563-018-0030-8},
      doi = {10.1038/s41563-018-0030-8},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Lattice dynamics and metastability of fcc metals in the hcp structure and the crucial role of spin-orbit coupling in platinum
    • S. Schonecker, X. Li, M. Richter, L. Vitos
    • Physical Review B 97(2018)
    • DOI   Abstract  

      We investigate the lattice dynamical properties of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au in the nonequilibrium hcp structure by means of density-functional simulations, wherein spin-orbit coupling (SOC) was considered for Ir, Pt, and Au. The determined dynamical properties reveal that all eight elements possess a metastable hcp phase at zero temperature and pressure. The hcp Ni, Cu, Rh, Pd, and Au previously observed in nanostructures support this finding. We make evident that the inclusion of SOC is mandatory for an accurate description of the phonon dispersion relations and dynamical stability of hcp Pt. The underlying sensitivity of the interatomic force constants is ascribed to a SOC-induced splitting of degenerate band states accompanied by a pronounced reduction of electronic density of states at the Fermi level. To give further insight into the importance of SOC in Pt, we (i) focus on phase stability and examine a lattice transformation related to optical phonons in the hcp phase and (ii) focus on the generalized stacking fault energy (GSFE) of the fcc phase pertinent to crystal plasticity. We show that the intrinsic stable and unstable fault energies of the GSFE scale as in other common fcc metals, provided that the spin-orbit interaction is taken into account.

      @article{,
      author = {Stephan Schonecker and Xiaoqing Li and Manuel Richter and Levente Vitos},
      title = {Lattice dynamics and metastability of fcc metals in the hcp structure and the crucial role of spin-orbit coupling in platinum},
      journal = {Physical Review B},
      volume = {97},
      number = {22},
      abstract = {We investigate the lattice dynamical properties of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au in the nonequilibrium hcp structure by means of density-functional simulations, wherein spin-orbit coupling (SOC) was considered for Ir, Pt, and Au. The determined dynamical properties reveal that all eight elements possess a metastable hcp phase at zero temperature and pressure. The hcp Ni, Cu, Rh, Pd, and Au previously observed in nanostructures support this finding. We make evident that the inclusion of SOC is mandatory for an accurate description of the phonon dispersion relations and dynamical stability of hcp Pt. The underlying sensitivity of the interatomic force constants is ascribed to a SOC-induced splitting of degenerate band states accompanied by a pronounced reduction of electronic density of states at the Fermi level. To give further insight into the importance of SOC in Pt, we (i) focus on phase stability and examine a lattice transformation related to optical phonons in the hcp phase and (ii) focus on the generalized stacking fault energy (GSFE) of the fcc phase pertinent to crystal plasticity. We show that the intrinsic stable and unstable fault energies of the GSFE scale as in other common fcc metals, provided that the spin-orbit interaction is taken into account.},
      year = {2018},
      url = http://dx.doi.org/{10.1103/PhysRevB.97.224305},
      doi = {10.1103/PhysRevB.97.224305},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Microscale finite element model of brittle multifilament yarn failure behavior
    • O. Doebrich, T. Gereke, M. Hengstermann, C. Cherif
    • Journal of Industrial Textiles 47, 870-882 (2018)
    • DOI   Abstract  

      A microscale model of multifilament reinforcement yarns made of technical carbon fibers is established based on the finite element method. The model is used to perform simulations of tensile failure. The failure behavior of dry multifilament carbon yarns is modeled using a maximum stress criterion with statistical distribution of the strength. The maximum stress is assigned to every single element and varied according to a normal distribution found in experimental tests of single filaments. The Weibull distribution is used for calculating the local failure stress. The material parameters are calculated in function of the element size to account for the volume-specific statistical breaking effect. Representative simulations of the tensile failure behavior prove the concept of the introduced assumptions.

      @article{,
      author = {Oliver Doebrich and Thomas Gereke and Martin Hengstermann and Chokri Cherif},
      title = {Microscale finite element model of brittle multifilament yarn failure behavior},
      journal = {Journal of Industrial Textiles},
      volume = {47},
      number = {5},
      pages = {870-882},
      abstract = {A microscale model of multifilament reinforcement yarns made of technical carbon fibers is established based on the finite element method. The model is used to perform simulations of tensile failure. The failure behavior of dry multifilament carbon yarns is modeled using a maximum stress criterion with statistical distribution of the strength. The maximum stress is assigned to every single element and varied according to a normal distribution found in experimental tests of single filaments. The Weibull distribution is used for calculating the local failure stress. The material parameters are calculated in function of the element size to account for the volume-specific statistical breaking effect. Representative simulations of the tensile failure behavior prove the concept of the introduced assumptions.},
      year = {2018},
      url = http://dx.doi.org/{10.1177/1528083716674908},
      doi = {10.1177/1528083716674908},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Morphological evolution of Ge/Si nano-strips driven by Rayleigh-like instability
    • M. Salvalaglio, P. Zaumseil, Y. Yamamoto, O. Skibitzki, R. Bergamaschini, T. Schroeder, A. Voigt, G. Capellini
    • Applied Physics Letters 112(2018)
    • DOI   Abstract  

      We present the morphological evolution obtained during the annealing of Ge strips grown on Si ridges as a prototypical process for 3D device architectures and nanophotonic applications. In particular, the morphological transition occurring from Ge/Si nanostrips to nanoislands is illustrated. The combined effect of performing annealing at different temperatures and varying the lateral size of the Si ridge underlying the Ge strips is addressed by means of a synergistic experimental and theoretical analysis. Indeed, three-dimensional phase-field simulations of surface diffusion, including the contributions of both surface and elastic energy, are exploited to understand the outcomes of annealing experiments. The breakup of Ge/Si strips, due to the activation of surface diffusion at high temperature, is found to be mainly driven by surface-energy reduction, thus pointing to a Rayleigh-like instability. The residual strain is found to play a minor role, only inducing local effects at the borders of the islands and an enhancement of the instability. Published by AIP Publishing.

      @article{,
      author = {Marco Salvalaglio and Peter Zaumseil and Yuji Yamamoto and Oliver Skibitzki and Roberto Bergamaschini and Thomas Schroeder and Axel Voigt and Giovanni Capellini},
      title = {Morphological evolution of Ge/Si nano-strips driven by Rayleigh-like instability},
      journal = {Applied Physics Letters},
      volume = {112},
      number = {2},
      abstract = {We present the morphological evolution obtained during the annealing of Ge strips grown on Si ridges as a prototypical process for 3D device architectures and nanophotonic applications. In particular, the morphological transition occurring from Ge/Si nanostrips to nanoislands is illustrated. The combined effect of performing annealing at different temperatures and varying the lateral size of the Si ridge underlying the Ge strips is addressed by means of a synergistic experimental and theoretical analysis. Indeed, three-dimensional phase-field simulations of surface diffusion, including the contributions of both surface and elastic energy, are exploited to understand the outcomes of annealing experiments. The breakup of Ge/Si strips, due to the activation of surface diffusion at high temperature, is found to be mainly driven by surface-energy reduction, thus pointing to a Rayleigh-like instability. The residual strain is found to play a minor role, only inducing local effects at the borders of the islands and an enhancement of the instability. Published by AIP Publishing.},
      year = {2018},
      url = http://dx.doi.org/{10.1063/1.5007937},
      doi = {10.1063/1.5007937},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Nematic liquid crystals on curved surfaces: a thin film limit
    • I. Nitschke, M. Nestler, S. Praetorius, H. Lowen, A. Voigt
    • Proceedings of the Royal Society a-Mathematical Physical and Engineering Sciences 474(2018)
    • DOI   Abstract  

      We consider a thin film limit of a Landau-do Gennes Q-tensor model. In the limiting process, we observe a continuous transition where the normal and tangential parts of the Q-tensor decouple and various intrinsic and extrinsic contributions emerge. The main properties of the thin film model, like uniaxiality and parameter phase space, are preserved in the limiting process. For the derived surface Landau-dc Gennes model, we consider an L-2-gradient flow. The resulting tensor-valued surface partial differential equation is numerically solved to demonstrate realizations of the tight coupling of elastic and bulk free energy with geometric properties.

      @article{,
      author = {Ingo Nitschke and Michael Nestler and Simon Praetorius and Hartmut Lowen and Axel Voigt},
      title = {Nematic liquid crystals on curved surfaces: a thin film limit},
      journal = {Proceedings of the Royal Society a-Mathematical Physical and Engineering Sciences},
      volume = {474},
      number = {2214},
      abstract = {We consider a thin film limit of a Landau-do Gennes Q-tensor model. In the limiting process, we observe a continuous transition where the normal and tangential parts of the Q-tensor decouple and various intrinsic and extrinsic contributions emerge. The main properties of the thin film model, like uniaxiality and parameter phase space, are preserved in the limiting process. For the derived surface Landau-dc Gennes model, we consider an L-2-gradient flow. The resulting tensor-valued surface partial differential equation is numerically solved to demonstrate realizations of the tight coupling of elastic and bulk free energy with geometric properties.},
      year = {2018},
      url = http://dx.doi.org/{10.1098/rspa.2017.0686},
      doi = {10.1098/rspa.2017.0686},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Photosensitive Cationic Azobenzene Surfactants: Thermodynamics of Hydration and the Complex Formation with Poly(methacrylic acid)
    • M. Montagna, O. Guskova
    • Langmuir 34, 311-321 (2018)
    • DOI   Abstract  

      In this computational work, we investigate the photosensitive cationic surfactants with the trimethylammonium or polyamine hydrophilic head and the azobenzene-containing hydrophobic tail. The azobenzene-based molecules are known to undergo a reversible trans-cis-trans isomerization reaction when subjected to UV-visible light irradiation. Combining the density functional theory and the all-atom molecular dynamics simulations, the structural and the hydration properties of the trans- and the cis-isomers and their interaction with the oppositely charged poly(methacrylic acid) in aqueous solution are investigated. We establish and quantify the correlations of the molecular structure and the isomerization state of the surfactants and their hydrophilicity/hydrophobicity and the self-assembling altered by light. For this reason, we compare the hydration free energies of the trans- and the cis-isomers. Moreover, the investigations of the interaction strength between the azobenzene molecules and the polyanion provide additional elucidations of the recent experimental and theoretical studies on the light triggered reversible deformation behavior of the microgels and the polymer brushes loaded with azobenzene surfactants.

      @article{,
      author = {Maria Montagna and Olga Guskova},
      title = {Photosensitive Cationic Azobenzene Surfactants: Thermodynamics of Hydration and the Complex Formation with Poly(methacrylic acid)},
      journal = {Langmuir},
      volume = {34},
      number = {1},
      pages = {311-321},
      abstract = {In this computational work, we investigate the photosensitive cationic surfactants with the trimethylammonium or polyamine hydrophilic head and the azobenzene-containing hydrophobic tail. The azobenzene-based molecules are known to undergo a reversible trans-cis-trans isomerization reaction when subjected to UV-visible light irradiation. Combining the density functional theory and the all-atom molecular dynamics simulations, the structural and the hydration properties of the trans- and the cis-isomers and their interaction with the oppositely charged poly(methacrylic acid) in aqueous solution are investigated. We establish and quantify the correlations of the molecular structure and the isomerization state of the surfactants and their hydrophilicity/hydrophobicity and the self-assembling altered by light. For this reason, we compare the hydration free energies of the trans- and the cis-isomers. Moreover, the investigations of the interaction strength between the azobenzene molecules and the polyanion provide additional elucidations of the recent experimental and theoretical studies on the light triggered reversible deformation behavior of the microgels and the polymer brushes loaded with azobenzene surfactants.},
      year = {2018},
      url = http://dx.doi.org/{10.1021/acs.langmuir.7b03638},
      doi = {10.1021/acs.langmuir.7b03638},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3
    • G. Bastien, G. Garbarino, R. Yadav, F. J. Martinez-Casado, B. R. Rodriguez, Q. Stahl, M. Kusch, S. P. Limandri, R. Ray, P. Lampen-Kelley, D. G. Mandrus, S. E. Nagler, M. Roslova, A. Isaeva, T. Doert, L. Hozoi, A. U. B. Wolter, B. Buechner, J. Geck, J. van den Brink
    • Physical Review B 97(2018)
    • DOI   Abstract  

      Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p similar to 0.2GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in a-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.

      @article{,
      author = {G. Bastien and G. Garbarino and R. Yadav and F. J. Martinez-Casado and R. Beltran Rodriguez and Q. Stahl and M. Kusch and S. P. Limandri and R. Ray and P. Lampen-Kelley and D. G. Mandrus and S. E. Nagler and M. Roslova and A. Isaeva and T. Doert and L. Hozoi and A. U. B. Wolter and B. Buechner and J. Geck and J. van den Brink},
      title = {Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet alpha-RuCl3},
      journal = {Physical Review B},
      volume = {97},
      number = {24},
      abstract = {Magnetization and high-resolution x-ray diffraction measurements of the Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced crystallographic and magnetic phase transition at a hydrostatic pressure of p similar to 0.2GPa. This structural transition into a triclinic phase is characterized by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru links, causing a sharp increase of the antiferromagnetic exchange interactions. These combined experimental and computational data show that the Kitaev spin-liquid phase in a-RuCl3 strongly competes with the crystallization of spin singlets into a valence bond solid.},
      year = {2018},
      url = http://dx.doi.org/{10.1103/PhysRevB.97.241108},
      doi = {10.1103/PhysRevB.97.241108},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Projection and transfer operators in adaptive isogeometric analysis with hierarchical B-splines
    • P. Hennig, M. Ambati, D. L. Lorenzis, M. Kaestner
    • Computer Methods in Applied Mechanics and Engineering 334, 313-336 (2018)
    • DOI   Abstract  

      We present projection methods and transfer operations required for adaptive mesh refinement/coarsening in problems with internal variables. We extend the results of Hennig et al. 2016 on Bezier extraction of truncated hierarchical B-splines and its application to adaptive isogeometric analysis. It is shown that isogeometric analysis improves the performance of transfer operations as already the coarsest mesh represents the exact geometry and the hierarchical structure allows for quadrature free projection methods. We propose two different local least squares projection methods for field variables and compare them to existing global and semi-local versions. We discuss the application of two different transfer operators for internal variables. An alternative new operator inspired by superconvergent patch recovery is also proposed. The presented projection methods and transfer operations are tested in benchmark problems and applied to phase-field modelling of spinodal decomposition and brittle and ductile fracture. (C) 2018 Elsevier B.V. All rights reserved.

      @article{,
      author = {P. Hennig and M. Ambati and L. De Lorenzis and M. Kaestner},
      title = {Projection and transfer operators in adaptive isogeometric analysis with hierarchical B-splines},
      journal = {Computer Methods in Applied Mechanics and Engineering},
      volume = {334},
      pages = {313-336},
      abstract = {We present projection methods and transfer operations required for adaptive mesh refinement/coarsening in problems with internal variables. We extend the results of Hennig et al. 2016 on Bezier extraction of truncated hierarchical B-splines and its application to adaptive isogeometric analysis. It is shown that isogeometric analysis improves the performance of transfer operations as already the coarsest mesh represents the exact geometry and the hierarchical structure allows for quadrature free projection methods. We propose two different local least squares projection methods for field variables and compare them to existing global and semi-local versions. We discuss the application of two different transfer operators for internal variables. An alternative new operator inspired by superconvergent patch recovery is also proposed. The presented projection methods and transfer operations are tested in benchmark problems and applied to phase-field modelling of spinodal decomposition and brittle and ductile fracture. (C) 2018 Elsevier B.V. All rights reserved.},
      year = {2018},
      url = http://dx.doi.org/{10.1016/j.cma.2018.01.017},
      doi = {10.1016/j.cma.2018.01.017},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Reusable Photocatalytic Optical Fibers for Underground, Deep-Sea, and Turbid Water Remediation
    • S. Teixeira, B. Magalhaes, P. M. Martins, K. Kuehn, L. Soler, S. Lanceros-Mendez, G. Cuniberti
    • Global Challenges 2(2018)
    • DOI   Abstract  

      An approach for underground, deep, and turbid water remediation is presented based on optical fibers with a photocatalytic coating. Thus, photocatalytic TiO2 P25 nanoparticles immobilized in a poly(vinylidene difluoride) (PVDF) matrix are coated on polymeric optical fibers (POFs) and the photocatalytic performance of the system is assessed under artificial sunlight. To the best of our knowledge, poly(methyl methacrylate)-POF coated with TiO2/PVDF and the reusability of any type of POF for photocatalytic applications are not previously reported. The photocatalytic efficiency of the hybrid material in the degradation of ciprofloxacin (CIP) and its reusability are evaluated here. It is shown that 50 w/w% of TiO2 P25 achieves a degradation of 95% after 72 h under artificial sunlight and a reusability of three times leads to a loss of activity inferior to 11%. The efficient removal of ciprofloxacin and the stability of the POF coated with TiO2 P25 successfully demonstrate its suitability in the degradation of pollutants with potential application in regions with low light illumination, as in underground and deep water.

      @article{,
      author = {Sara Teixeira and Bruno Magalhaes and Pedro M. Martins and Klaus Kuehn and Lluis Soler and Senentxu Lanceros-Mendez and Gianaurelio Cuniberti},
      title = {Reusable Photocatalytic Optical Fibers for Underground, Deep-Sea, and Turbid Water Remediation},
      journal = {Global Challenges},
      volume = {2},
      number = {3},
      abstract = {An approach for underground, deep, and turbid water remediation is presented based on optical fibers with a photocatalytic coating. Thus, photocatalytic TiO2 P25 nanoparticles immobilized in a poly(vinylidene difluoride) (PVDF) matrix are coated on polymeric optical fibers (POFs) and the photocatalytic performance of the system is assessed under artificial sunlight. To the best of our knowledge, poly(methyl methacrylate)-POF coated with TiO2/PVDF and the reusability of any type of POF for photocatalytic applications are not previously reported. The photocatalytic efficiency of the hybrid material in the degradation of ciprofloxacin (CIP) and its reusability are evaluated here. It is shown that 50 w/w% of TiO2 P25 achieves a degradation of 95% after 72 h under artificial sunlight and a reusability of three times leads to a loss of activity inferior to 11%. The efficient removal of ciprofloxacin and the stability of the POF coated with TiO2 P25 successfully demonstrate its suitability in the degradation of pollutants with potential application in regions with low light illumination, as in underground and deep water.},
      year = {2018},
      url = http://dx.doi.org/{10.1002/gch2.201700124},
      doi = {10.1002/gch2.201700124},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Self-Assembled Two-Dimensional Supramolecular Networks Characterized by Scanning Tunneling Microscopy and Spectroscopy in Air and under Vacuum
    • B. Naydenov, S. Torsney, A. S. Bonilla, M. E. Garah, A. Ciesielski, A. Gualandi, L. Mengozzi, P. G. Cozzi, R. Gutierrez, P. Samori, G. Cuniberti, J. J. Boland
    • Langmuir 34, 7698-7707 (2018)
    • DOI   Abstract  

      We combine ambient (air) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) investigations together with density functional theory (DFT) calculations to gain a subnanometer insight into the structure and dynamic of two-dimensional (2D) surface-supported molecular networks. The planar tetraferrocene-porphyrin molecules employed in this study undergo spontaneous self-assembly via the formation of hydrogen bonded networks at the gold substrate-solution interface. To mimic liquid phase ambient deposition conditions, film formation was accomplished in UHV by electro-spraying a solution of the molecule in chloroform onto an Au(111) substrate, thereby providing access to the full spectroscopic capabilities of STM that can be hardly attained under ambient conditions. We show that molecular assembly on Au (111) is identical in films prepared under the two different conditions, and in good agreement with the theoretical predictions. However, we observe the contrast found for a given STM bias condition to be different in ambient and UHV conditions despite the similarity of the structures, and we propose possible origins of the different imaging contrast. This approach could be valuable for the thorough characterization of surface systems that involve large molecules and are prepared mainly in ambient conditions.

      @article{,
      author = {Borislav Naydenov and Samuel Torsney and Alejandro Santana Bonilla and Mohamed El Garah and Artur Ciesielski and Andrea Gualandi and Luca Mengozzi and Pier Giorgio Cozzi and Rafael Gutierrez and Paolo Samori and Gianaurelio Cuniberti and John J. Boland},
      title = {Self-Assembled Two-Dimensional Supramolecular Networks Characterized by Scanning Tunneling Microscopy and Spectroscopy in Air and under Vacuum},
      journal = {Langmuir},
      volume = {34},
      number = {26},
      pages = {7698-7707},
      abstract = {We combine ambient (air) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) investigations together with density functional theory (DFT) calculations to gain a subnanometer insight into the structure and dynamic of two-dimensional (2D) surface-supported molecular networks. The planar tetraferrocene-porphyrin molecules employed in this study undergo spontaneous self-assembly via the formation of hydrogen bonded networks at the gold substrate-solution interface. To mimic liquid phase ambient deposition conditions, film formation was accomplished in UHV by electro-spraying a solution of the molecule in chloroform onto an Au(111) substrate, thereby providing access to the full spectroscopic capabilities of STM that can be hardly attained under ambient conditions. We show that molecular assembly on Au (111) is identical in films prepared under the two different conditions, and in good agreement with the theoretical predictions. However, we observe the contrast found for a given STM bias condition to be different in ambient and UHV conditions despite the similarity of the structures, and we propose possible origins of the different imaging contrast. This approach could be valuable for the thorough characterization of surface systems that involve large molecules and are prepared mainly in ambient conditions.},
      year = {2018},
      url = http://dx.doi.org/{10.1021/acs.langmuir.8b01374},
      doi = {10.1021/acs.langmuir.8b01374},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Solid-state dewetting of single-crystal silicon on insulator: effect of annealing temperature and patch size
    • M. Abbarchi, M. Naffouti, M. Lodari, M. Salvalaglio, R. Backofen, T. Bottein, A. Voigt, T. David, J. Claude, M. Bouabdellaoui, A. Benkouider, I. Fraj, L. Favre, A. Ronda, I. Berbezier, D. Grosso, M. Bollani
    • Microelectronic Engineering 190, 1-6 (2018)
    • DOI   Abstract  

      We address the solid state dewetting of ultra-thin and ultra-large patches of monocrystalline silicon on insulator. We show that the underlying instability of the thin Si film under annealing can be perfectly controlled to form monocrystalline, complex nanoarchitectures extending over several microns. These complex patterns are obtained guiding the dewetting fronts by etching ad-hoc patches prior to annealing. They can be reproduced over hundreds of repetitions extending over hundreds of microns. We discuss the effect of annealing temperature and patch size on the stability of the final result of dewetting showing that for simple patches (e.g. simple squares) the final outcome is stable and well reproducible at 720 degrees C and for similar to 1 mu m square size. Finally, we demonstrate that introducing additional features within squared patches (e.g. a hole within a square) stabilises the dewetting dynamic providing perfectly reproducible complex nanoarchitectures of 5 pm size. (C) 2018 Elsevier B.V. All rights reserved.

      @article{,
      author = {Marco Abbarchi and Meher Naffouti and Mario Lodari and Marco Salvalaglio and Rainer Backofen and Thomas Bottein and Axel Voigt and Thomas David and Jean-Benoit Claude and Mohammed Bouabdellaoui and Abdelmalek Benkouider and Ibtissem Fraj and Luc Favre and Antoine Ronda and Isabelle Berbezier and David Grosso and Monica Bollani},
      title = {Solid-state dewetting of single-crystal silicon on insulator: effect of annealing temperature and patch size},
      journal = {Microelectronic Engineering},
      volume = {190},
      pages = {1-6},
      abstract = {We address the solid state dewetting of ultra-thin and ultra-large patches of monocrystalline silicon on insulator. We show that the underlying instability of the thin Si film under annealing can be perfectly controlled to form monocrystalline, complex nanoarchitectures extending over several microns. These complex patterns are obtained guiding the dewetting fronts by etching ad-hoc patches prior to annealing. They can be reproduced over hundreds of repetitions extending over hundreds of microns. We discuss the effect of annealing temperature and patch size on the stability of the final result of dewetting showing that for simple patches (e.g. simple squares) the final outcome is stable and well reproducible at 720 degrees C and for similar to 1 mu m square size. Finally, we demonstrate that introducing additional features within squared patches (e.g. a hole within a square) stabilises the dewetting dynamic providing perfectly reproducible complex nanoarchitectures of 5 pm size. (C) 2018 Elsevier B.V. All rights reserved.},
      year = {2018},
      url = http://dx.doi.org/{10.1016/j.mee.2018.01.002},
      doi = {10.1016/j.mee.2018.01.002},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Solving the incompressible surface Navier-Stokes equation by surface finite elements
    • S. Reuther, A. Voigt
    • Physics of Fluids 30(2018)
    • DOI   Abstract  

      We consider a numerical approach for the incompressible surface Navier-Stokes equation on surfaces with arbitrary genus g(S). The approach is based on a reformulation of the equation in Cartesian coordinates of the embedding R-3, penalization of the normal component, a Chorin projection method, and discretization in space by surface finite elements for each component. The approach thus requires only standard ingredients which most finite element implementations can offer. We compare computational results with discrete exterior calculus simulations on a torus and demonstrate the interplay of the flow field with the topology by showing realizations of the Poincare-Hopf theorem on n-tori. Published by AIP Publishing.

      @article{,
      author = {Sebastian Reuther and Axel Voigt},
      title = {Solving the incompressible surface Navier-Stokes equation by surface finite elements},
      journal = {Physics of Fluids},
      volume = {30},
      number = {1},
      abstract = {We consider a numerical approach for the incompressible surface Navier-Stokes equation on surfaces with arbitrary genus g(S). The approach is based on a reformulation of the equation in Cartesian coordinates of the embedding R-3, penalization of the normal component, a Chorin projection method, and discretization in space by surface finite elements for each component. The approach thus requires only standard ingredients which most finite element implementations can offer. We compare computational results with discrete exterior calculus simulations on a torus and demonstrate the interplay of the flow field with the topology by showing realizations of the Poincare-Hopf theorem on n-tori. Published by AIP Publishing.},
      year = {2018},
      url = http://dx.doi.org/{10.1063/1.5005142},
      doi = {10.1063/1.5005142},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Unimolecular Logic Gate with Classical Input by Single Gold Atoms
    • D. Skidin, O. Faizy, J. Krueger, F. Eisenhut, A. Jancarik, N. Khanh-Hung, G. Cuniberti, A. Gourdon, F. Moresco, C. Joachim
    • Acs Nano 12, 1139-1145 (2018)
    • DOI   Abstract  

      By a combination of solution and on-surface chemistry, we synthesized an asymmetric starphene molecule with two long anthracenyl input branches and a short naphthyl output branch on the Au(111) surface. Starting from this molecule, we could demonstrate the working principle of a single molecule NAND logic gate by selectively contacting single gold atoms by atomic manipulation to the longer branches of the molecule. The logical input "1" ("0") is defined by the interaction (noninteraction) of a gold atom with one of the input branches. The output is measured by scanning tunneling spectroscopy following the shift in energy of the electronic tunneling resonances at the end of the short branch of the molecule.

      @article{,
      author = {Dmitry Skidin and Omid Faizy and Justus Krueger and Frank Eisenhut and Andrej Jancarik and Nguyen Khanh-Hung and Gianaurelio Cuniberti and Andre Gourdon and Francesca Moresco and Christian Joachim},
      title = {Unimolecular Logic Gate with Classical Input by Single Gold Atoms},
      journal = {Acs Nano},
      volume = {12},
      number = {2},
      pages = {1139-1145},
      abstract = {By a combination of solution and on-surface chemistry, we synthesized an asymmetric starphene molecule with two long anthracenyl input branches and a short naphthyl output branch on the Au(111) surface. Starting from this molecule, we could demonstrate the working principle of a single molecule NAND logic gate by selectively contacting single gold atoms by atomic manipulation to the longer branches of the molecule. The logical input "1" ("0") is defined by the interaction (noninteraction) of a gold atom with one of the input branches. The output is measured by scanning tunneling spectroscopy following the shift in energy of the electronic tunneling resonances at the end of the short branch of the molecule.},
      year = {2018},
      url = http://dx.doi.org/{10.1021/acsnano.7b06650},
      doi = {10.1021/acsnano.7b06650},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

2017

  • Ab initio study of electron-phonon coupling in rubrene
    • P. Ordejon, D. Boskovic, M. Panhans, F. Ortmann
    • Physical Review B 96(2017)
    • DOI   Abstract  

      The use of ab initio methods for accurate simulations of electronic, phononic, and electron-phonon properties of molecular materials such as organic crystals is a challenge that is often tackled stepwise based on molecular properties calculated in gas phase and perturbatively treated parameters relevant for solid phases. In contrast, in this work we report a full first-principles description of such properties for the prototypical rubrene crystals. More specifically, we determine a Holstein-Peierls-type Hamiltonian for rubrene, including local and nonlocal electron-phonon couplings. Thereby, a recipe for circumventing the issue of numerical inaccuracies with low-frequency phonons is presented. In addition, we study the phenyl group motion with a molecular dynamics approach.

      @article{,
      author = {P. Ordejon and D. Boskovic and M. Panhans and F. Ortmann},
      title = {Ab initio study of electron-phonon coupling in rubrene},
      journal = {Physical Review B},
      volume = {96},
      number = {3},
      abstract = {The use of ab initio methods for accurate simulations of electronic, phononic, and electron-phonon properties of molecular materials such as organic crystals is a challenge that is often tackled stepwise based on molecular properties calculated in gas phase and perturbatively treated parameters relevant for solid phases. In contrast, in this work we report a full first-principles description of such properties for the prototypical rubrene crystals. More specifically, we determine a Holstein-Peierls-type Hamiltonian for rubrene, including local and nonlocal electron-phonon couplings. Thereby, a recipe for circumventing the issue of numerical inaccuracies with low-frequency phonons is presented. In addition, we study the phenyl group motion with a molecular dynamics approach.},
      year = {2017},
      url = http://dx.doi.org/{10.1103/PhysRevB.96.035202},
      doi = {10.1103/PhysRevB.96.035202},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Absorption Tails of Donor:C-60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation
    • K. Vandewal, J. Benduhn, K. S. Schellhammer, T. Vangerven, J. E. Rueckert, F. Piersimoni, R. Scholz, O. Zeika, Y. Fan, S. Barlow, D. Neher, S. R. Marder, J. Manca, D. Spoltore, G. Cuniberti, F. Ortmann
    • Journal of the American Chemical Society 139, 1699-1704 (2017)
    • DOI   Abstract  

      In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D A complexes occur at photon energies below the optical gaps of both the donors and the C-60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C-60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.

      @article{,
      author = {Koen Vandewal and Johannes Benduhn and Karl Sebastian Schellhammer and Tim Vangerven and Janna E. Rueckert and Fortunato Piersimoni and Reinhard Scholz and Olaf Zeika and Yeli Fan and Stephen Barlow and Dieter Neher and Seth R. Marder and Jean Manca and Donato Spoltore and Gianaurelio Cuniberti and Frank Ortmann},
      title = {Absorption Tails of Donor:C-60 Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation},
      journal = {Journal of the American Chemical Society},
      volume = {139},
      number = {4},
      pages = {1699-1704},
      abstract = {In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)-acceptor (A) interfaces. Weak CT absorption bands for D A complexes occur at photon energies below the optical gaps of both the donors and the C-60 acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C-60 CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D-A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/jacs.6b12857},
      doi = {10.1021/jacs.6b12857},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Active forming manipulation of composite reinforcements for the suppression of forming defects
    • F. N. Nezami, T. Gereke, C. Cherif
    • Composites Part a-Applied Science and Manufacturing 99, 94-101 (2017)
    • DOI   Abstract  

      For composite applications in automotive serial production, reinforcement textiles are brought into the desired shape by forming processes. The preform quality depends on the interacting factors of tool geometry, textile material, and forming process. The primary cause of defects in multilayer draping is relative movements of the plies, and the occurrence of wrinkles. Thus, the reduction of the interactions between plies is crucial to enhance preform quality. This was achieved with active metal sheets between the fabric layers. Those intermediate layers are additionally stimulated with piezo actors to reduce friction. Additional local and ply-specific clamping of layers was achieved with tension rods and segmented actuators. Defects and wrinkles in the preform could be eliminated or reduced significantly and fiber orientation could be controlled. Thus, a forming process providing high-quality preforms from multiple fabric layers was developed. Furthermore, automation complexity could be reduced significantly by utilizing rigid interlayers. (C) 2017 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Farbod Nosrat Nezami and Thomas Gereke and Chokri Cherif},
      title = {Active forming manipulation of composite reinforcements for the suppression of forming defects},
      journal = {Composites Part a-Applied Science and Manufacturing},
      volume = {99},
      pages = {94-101},
      abstract = {For composite applications in automotive serial production, reinforcement textiles are brought into the desired shape by forming processes. The preform quality depends on the interacting factors of tool geometry, textile material, and forming process. The primary cause of defects in multilayer draping is relative movements of the plies, and the occurrence of wrinkles. Thus, the reduction of the interactions between plies is crucial to enhance preform quality. This was achieved with active metal sheets between the fabric layers. Those intermediate layers are additionally stimulated with piezo actors to reduce friction. Additional local and ply-specific clamping of layers was achieved with tension rods and segmented actuators. Defects and wrinkles in the preform could be eliminated or reduced significantly and fiber orientation could be controlled. Thus, a forming process providing high-quality preforms from multiple fabric layers was developed. Furthermore, automation complexity could be reduced significantly by utilizing rigid interlayers. (C) 2017 Elsevier Ltd. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.compositesa.2017.04.011},
      doi = {10.1016/j.compositesa.2017.04.011},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Approaches for process and structural finite element simulations of braided ligament replacements
    • T. Gereke, O. Doebrich, D. Aibibu, J. Nowotny, C. Cherif
    • Journal of Industrial Textiles 47, 408-425 (2017)
    • DOI   Abstract  

      To prevent the renewed rupture of ligaments and tendons prior to the completed healing process, which frequently occurs in treated ruptured tendons, a temporary support structure is envisaged. The limitations of current grafts have motivated the investigation of tissue-engineered ligament replacements based on the braiding technology. This technology offers a wide range of flexibility and adjustable geometrical and structural parameters. The presented work demonstrates the possible range for tailoring the mechanical properties of polyester braids and a variation of the braiding process parameters. A finite element simulation model of the braiding process was developed, which allows the optimization of production parameters without the performance of further experimental trials. In a second modelling and simulation step, mechanical properties of the braided structures were virtually determined and compared with actual tests. The digital element approach was used for the yarns in the numerical model. The results show very good agreement for the process model in terms of braiding angles and good agreement for the structural model in terms of force-strain behaviour. With a few adaptions, the models can, thus, be applied to actual ligament replacements made of resorbable polymers.

      @article{,
      author = {Thomas Gereke and Oliver Doebrich and Dilbar Aibibu and Jorg Nowotny and Chokri Cherif},
      title = {Approaches for process and structural finite element simulations of braided ligament replacements},
      journal = {Journal of Industrial Textiles},
      volume = {47},
      number = {3},
      pages = {408-425},
      abstract = {To prevent the renewed rupture of ligaments and tendons prior to the completed healing process, which frequently occurs in treated ruptured tendons, a temporary support structure is envisaged. The limitations of current grafts have motivated the investigation of tissue-engineered ligament replacements based on the braiding technology. This technology offers a wide range of flexibility and adjustable geometrical and structural parameters. The presented work demonstrates the possible range for tailoring the mechanical properties of polyester braids and a variation of the braiding process parameters. A finite element simulation model of the braiding process was developed, which allows the optimization of production parameters without the performance of further experimental trials. In a second modelling and simulation step, mechanical properties of the braided structures were virtually determined and compared with actual tests. The digital element approach was used for the yarns in the numerical model. The results show very good agreement for the process model in terms of braiding angles and good agreement for the structural model in terms of force-strain behaviour. With a few adaptions, the models can, thus, be applied to actual ligament replacements made of resorbable polymers.},
      year = {2017},
      url = http://dx.doi.org/{10.1177/1528083716648765},
      doi = {10.1177/1528083716648765},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Charge carrier mobility in one-dimensional aligned pi-stacks of conjugated small molecules with a benzothiadiazole central unit
    • D. Raychev, O. Guskova
    • Physical Chemistry Chemical Physics 19, 8330-8339 (2017)
    • DOI   Abstract  

      A theoretical study is applied to gain insight into the microscopic electron and hole transport in benzothiadiazole-cored molecular semiconductors either with furan or thiophene flanks arranged in p-stacks. For the characterization of the energetics of the reduction and oxidation processes and their impact on the molecular geometry, the internal reorganization energy is defined for isolated molecules in the gas phase. The outer-shell reorganization energy is evaluated within the frequency-resolved cavity model and as an electrostatic contribution within the polarizable continuum model. The intermolecular electronic coupling interaction for the Marcus charge hopping is calculated using the energy splitting in dimer method, the generalized Mulliken-Hush approach and the fragment charge difference scheme. In order to probe the relation between the charge hopping rate/charge carrier mobility and the molecular organization within the p-stacks, different stacking modes are investigated: (i) dimers with a perfect registry, i. e. segregated stacking motif, when molecules are placed face-to-face, and (ii) dimers forming slipped cofacial orientations with longitudinal and transverse shifts, i. e. mixed stacking motif. Besides, the effects of molecular planarity and rigidity, influencing internal molecular relaxation upon charging, the effects of non-covalent interactions within stacks and the heteroatom replacement on the charge carrier mobility are studied. The results obtained in the simulations of one-dimensional aligned p-stacks of molecular semiconductors are compared with available experimental data for small conjugated benzothiadiazolecored molecules with thiophene flanks and benzothiadiazole-quaterthiophene-based copolymers.

      @article{,
      author = {Deyan Raychev and Olga Guskova},
      title = {Charge carrier mobility in one-dimensional aligned pi-stacks of conjugated small molecules with a benzothiadiazole central unit},
      journal = {Physical Chemistry Chemical Physics},
      volume = {19},
      number = {12},
      pages = {8330-8339},
      abstract = {A theoretical study is applied to gain insight into the microscopic electron and hole transport in benzothiadiazole-cored molecular semiconductors either with furan or thiophene flanks arranged in p-stacks. For the characterization of the energetics of the reduction and oxidation processes and their impact on the molecular geometry, the internal reorganization energy is defined for isolated molecules in the gas phase. The outer-shell reorganization energy is evaluated within the frequency-resolved cavity model and as an electrostatic contribution within the polarizable continuum model. The intermolecular electronic coupling interaction for the Marcus charge hopping is calculated using the energy splitting in dimer method, the generalized Mulliken-Hush approach and the fragment charge difference scheme. In order to probe the relation between the charge hopping rate/charge carrier mobility and the molecular organization within the p-stacks, different stacking modes are investigated: (i) dimers with a perfect registry, i. e. segregated stacking motif, when molecules are placed face-to-face, and (ii) dimers forming slipped cofacial orientations with longitudinal and transverse shifts, i. e. mixed stacking motif. Besides, the effects of molecular planarity and rigidity, influencing internal molecular relaxation upon charging, the effects of non-covalent interactions within stacks and the heteroatom replacement on the charge carrier mobility are studied. The results obtained in the simulations of one-dimensional aligned p-stacks of molecular semiconductors are compared with available experimental data for small conjugated benzothiadiazolecored molecules with thiophene flanks and benzothiadiazole-quaterthiophene-based copolymers.},
      year = {2017},
      url = http://dx.doi.org/{10.1039/c7cp00798a},
      doi = {10.1039/c7cp00798a},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Chemical Gating of a Weak Topological Insulator: Bi14Rh3I9
    • M. P. Ghimire, M. Richter
    • Nano Letters 17, 6303-6308 (2017)
    • DOI   Abstract  

      The compound Bi14Rh3I9 has recently been suggested as a weak three-dimensional topological insulator on the basis of angle resolved photoemission and scanning-tunneling experiments in combination with density functional (DF) electronic structure calculations. These methods unanimously support the topological character of the headline compound, but a compelling confirmation could only be obtained by dedicated transport experiments. The latter, however, are biased by an intrinsic n-doping of the material’s surface due to its polarity. Electronic reconstruction of the polar surface shifts the topological gap below the Fermi energy, which would also prevent any future device application. Here, we report the results of DF slab calculations for chemically gated and counter-doped surfaces of Bi14Rh3I9. We demonstrate that both methods can be used to compensate the surface polarity without closing the electronic gap.

      @article{,
      author = {Madhav Prasad Ghimire and Manuel Richter},
      title = {Chemical Gating of a Weak Topological Insulator: Bi14Rh3I9},
      journal = {Nano Letters},
      volume = {17},
      number = {10},
      pages = {6303-6308},
      abstract = {The compound Bi14Rh3I9 has recently been suggested as a weak three-dimensional topological insulator on the basis of angle resolved photoemission and scanning-tunneling experiments in combination with density functional (DF) electronic structure calculations. These methods unanimously support the topological character of the headline compound, but a compelling confirmation could only be obtained by dedicated transport experiments. The latter, however, are biased by an intrinsic n-doping of the material's surface due to its polarity. Electronic reconstruction of the polar surface shifts the topological gap below the Fermi energy, which would also prevent any future device application. Here, we report the results of DF slab calculations for chemically gated and counter-doped surfaces of Bi14Rh3I9. We demonstrate that both methods can be used to compensate the surface polarity without closing the electronic gap.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acs.nanolett.7b03001},
      doi = {10.1021/acs.nanolett.7b03001},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Coherent spin dynamics in a helical arrangement of molecular dipoles
    • E. Diaz, R. Gutierrez, C. Gaul, G. Cuniberti, F. Dominguez-Adame
    • Aims Materials Science 4, 1052-1061 (2017)
    • DOI   Abstract  

      Experiments on electron transport through helical molecules have demonstrated the appearance of high spin selectivity, in spite of the rather weak spin-orbit coupling in organic compounds. Theoretical models usually rely on different mechanisms to explain these experiments, such as large spin-orbit coupling, quantum dephasing, the role of metallic contacts, or the interplay between a helicity-induced spin-orbit coupling and a strong dipole electric field. In this work we consider the coherent electron dynamics in the electric field created by the helical arrangement of dipoles of the molecule backbone, giving rise to an effective spin-orbit coupling. WC calculate the spin projection onto the helical axis as a figure of merit for the assessment of the spin dynamics in a very long helical molecule. We prove that the spin projection reaches a steady state regime after a short transient. We compare its asymptotic value for different initial conditions, aiming to better understand the origin of the spin selectivity found in experiments.

      @article{,
      author = {Elena Diaz and Rafael Gutierrez and Christopher Gaul and Gianaurelio Cuniberti and Francisco Dominguez-Adame},
      title = {Coherent spin dynamics in a helical arrangement of molecular dipoles},
      journal = {Aims Materials Science},
      volume = {4},
      number = {5},
      pages = {1052-1061},
      abstract = {Experiments on electron transport through helical molecules have demonstrated the appearance of high spin selectivity, in spite of the rather weak spin-orbit coupling in organic compounds. Theoretical models usually rely on different mechanisms to explain these experiments, such as large spin-orbit coupling, quantum dephasing, the role of metallic contacts, or the interplay between a helicity-induced spin-orbit coupling and a strong dipole electric field. In this work we consider the coherent electron dynamics in the electric field created by the helical arrangement of dipoles of the molecule backbone, giving rise to an effective spin-orbit coupling. WC calculate the spin projection onto the helical axis as a figure of merit for the assessment of the spin dynamics in a very long helical molecule. We prove that the spin projection reaches a steady state regime after a short transient. We compare its asymptotic value for different initial conditions, aiming to better understand the origin of the spin selectivity found in experiments.},
      year = {2017},
      url = http://dx.doi.org/{10.3934/matersci.2017.5.1052},
      doi = {10.3934/matersci.2017.5.1052},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Complex dewetting scenarios of ultrathin silicon films for large-scale nanoarchitectures
    • M. Naffouti, R. Backofen, M. Salvalaglio, T. Bottein, M. Lodari, A. Voigt, T. David, A. Benkouider, I. Fraj, L. Favre, A. Ronda, I. Berbezier, D. Grosso, M. Abbarchi, M. Bollani
    • Science Advances 3(2017)
    • DOI   Abstract  

      Dewetting is a ubiquitous phenomenon in nature; many different thin films of organic and inorganic substances (such as liquids, polymers, metals, and semiconductors) share this shape instability driven by surface tension and mass transport. Via templated solid-state dewetting, we frame complex nanoarchitectures of monocrystalline silicon on insulator with unprecedented precision and reproducibility over large scales. Phase-field simulations reveal the dominant role of surface diffusion as a driving force for dewetting and provide a predictive tool to further engineer this hybrid top-down/bottom-up self-assembly method. Our results demonstrate that patches of thin monocrystalline films of metals and semiconductors share the same dewetting dynamics. We also prove the potential of our method by fabricating nanotransfer molding of metal oxide xerogels on silicon and glass substrates. This method allows the novel possibility of transferring these Si-based patterns on different materials, which do not usually undergo dewetting, offering great potential also for microfluidic or sensing applications.

      @article{,
      author = {Meher Naffouti and Rainer Backofen and Marco Salvalaglio and Thomas Bottein and Mario Lodari and Axel Voigt and Thomas David and Abdelmalek Benkouider and Ibtissem Fraj and Luc Favre and Antoine Ronda and Isabelle Berbezier and David Grosso and Marco Abbarchi and Monica Bollani},
      title = {Complex dewetting scenarios of ultrathin silicon films for large-scale nanoarchitectures},
      journal = {Science Advances},
      volume = {3},
      number = {11},
      abstract = {Dewetting is a ubiquitous phenomenon in nature; many different thin films of organic and inorganic substances (such as liquids, polymers, metals, and semiconductors) share this shape instability driven by surface tension and mass transport. Via templated solid-state dewetting, we frame complex nanoarchitectures of monocrystalline silicon on insulator with unprecedented precision and reproducibility over large scales. Phase-field simulations reveal the dominant role of surface diffusion as a driving force for dewetting and provide a predictive tool to further engineer this hybrid top-down/bottom-up self-assembly method. Our results demonstrate that patches of thin monocrystalline films of metals and semiconductors share the same dewetting dynamics. We also prove the potential of our method by fabricating nanotransfer molding of metal oxide xerogels on silicon and glass substrates. This method allows the novel possibility of transferring these Si-based patterns on different materials, which do not usually undergo dewetting, offering great potential also for microfluidic or sensing applications.},
      year = {2017},
      url = http://dx.doi.org/{10.1126/sciadv.aao1472},
      doi = {10.1126/sciadv.aao1472},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Conformational and electronic properties of small benzothiadiazolecored oligomers with aryl flanking units: Thiophene versus Furan
    • D. Raychev, O. Guskova, G. Seifert, J. Sommer
    • Computational Materials Science 126, 287-298 (2017)
    • DOI   Abstract  

      Symmetrical benzothiadiazole-cored (BTZ) oligomers with aromatic flanks are widely used in experiments as structural blocks for organic electronic materials. Along with chemical composition, the molecular conformation plays a crucial role in crystal packing/self-assembly of these building blocks in thin films. In this study, we perform an extensive theoretical comparison of conformational preferences, electronic and optical properties of small pi-conjugated molecules having BTZ central unit symmetrically decorated with thiophene (Th) or furan (Fu) rings using DFT calculations. In addition to the conformational screening of small molecules, the torsion potentials of the internal rotation and the energetics of weak intramolecular S…N, O…N, O…H and N…H nonbonded interactions stabilizing certain conformations are evaluated. The conformational properties of -[BTZ-Fu](n)- and -[BTZ-Th](n)- chains are predicted applying the hindered rotation model. Our calculation shows that substitution of one atom, here sulphur by oxygen, leads to huge stiffening of the resulting polymer as estimated by the Kuhn length based on the rotational isomeric model. On the other hand, the calculation of the band gaps and the UV-vis spectra shows that the electronic and optical properties of both compounds are almost identical. This offers the possibility to decouple the intramolecular electronic properties from the intermolecular arrangement and the morphology of the materials since the latter properties are sensitive to the local stiffness of oligomers and polymers. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {Deyan Raychev and Olga Guskova and Gotthard Seifert and Jens-Uwe Sommer},
      title = {Conformational and electronic properties of small benzothiadiazolecored oligomers with aryl flanking units: Thiophene versus Furan},
      journal = {Computational Materials Science},
      volume = {126},
      pages = {287-298},
      abstract = {Symmetrical benzothiadiazole-cored (BTZ) oligomers with aromatic flanks are widely used in experiments as structural blocks for organic electronic materials. Along with chemical composition, the molecular conformation plays a crucial role in crystal packing/self-assembly of these building blocks in thin films. In this study, we perform an extensive theoretical comparison of conformational preferences, electronic and optical properties of small pi-conjugated molecules having BTZ central unit symmetrically decorated with thiophene (Th) or furan (Fu) rings using DFT calculations. In addition to the conformational screening of small molecules, the torsion potentials of the internal rotation and the energetics of weak intramolecular S...N, O...N, O...H and N...H nonbonded interactions stabilizing certain conformations are evaluated. The conformational properties of -[BTZ-Fu](n)- and -[BTZ-Th](n)- chains are predicted applying the hindered rotation model. Our calculation shows that substitution of one atom, here sulphur by oxygen, leads to huge stiffening of the resulting polymer as estimated by the Kuhn length based on the rotational isomeric model. On the other hand, the calculation of the band gaps and the UV-vis spectra shows that the electronic and optical properties of both compounds are almost identical. This offers the possibility to decouple the intramolecular electronic properties from the intermolecular arrangement and the morphology of the materials since the latter properties are sensitive to the local stiffness of oligomers and polymers. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.commatsci.2016.09.044},
      doi = {10.1016/j.commatsci.2016.09.044},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Controlling the energy of defects and interfaces in the amplitude expansion of the phase-field crystal model
    • M. Salvalaglio, R. Backofen, A. Voigt, K. R. Elder
    • Physical Review E 96(2017)
    • DOI   Abstract  

      One of the major difficulties in employing phase-field crystal (PFC) modeling and the associated amplitude (APFC) formulation is the ability to tune model parameters to match experimental quantities. In this work, we address the problem of tuning the defect core and interface energies in the APFC formulation. We show that the addition of a single term to the free-energy functional can be used to increase the solid-liquid interface and defect energies in a well-controlled fashion, without any major change to other features. The influence of the newly added term is explored in two-dimensional triangular and honeycomb structures as well as bcc and fcc lattices in three dimensions. In addition, a finite-element method (FEM) is developed for the model that incorporates a mesh refinement scheme. The combination of the FEM and mesh refinement to simulate amplitude expansion with a new energy term provides a method of controlling microscopic features such as defect and interface energies while simultaneously delivering a coarse-grained examination of the system.

      @article{,
      author = {Marco Salvalaglio and Rainer Backofen and Axel Voigt and Ken R. Elder},
      title = {Controlling the energy of defects and interfaces in the amplitude expansion of the phase-field crystal model},
      journal = {Physical Review E},
      volume = {96},
      number = {2},
      abstract = {One of the major difficulties in employing phase-field crystal (PFC) modeling and the associated amplitude (APFC) formulation is the ability to tune model parameters to match experimental quantities. In this work, we address the problem of tuning the defect core and interface energies in the APFC formulation. We show that the addition of a single term to the free-energy functional can be used to increase the solid-liquid interface and defect energies in a well-controlled fashion, without any major change to other features. The influence of the newly added term is explored in two-dimensional triangular and honeycomb structures as well as bcc and fcc lattices in three dimensions. In addition, a finite-element method (FEM) is developed for the model that incorporates a mesh refinement scheme. The combination of the FEM and mesh refinement to simulate amplitude expansion with a new energy term provides a method of controlling microscopic features such as defect and interface energies while simultaneously delivering a coarse-grained examination of the system.},
      year = {2017},
      url = http://dx.doi.org/{10.1103/PhysRevE.96.023301},
      doi = {10.1103/PhysRevE.96.023301},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • A convergence study of phase-field models for brittle fracture
    • T. Linse, P. Hennig, M. Kaestner, R. de Borst
    • Engineering Fracture Mechanics 184, 307-318 (2017)
    • DOI   Abstract  

      A crucial issue in phase-field models for brittle fracture is whether the functional that describes the distributed crack converges to the functional of the discrete crack when the internal length scale introduced in the distribution function goes to zero. Theoretical proofs exist for the original theory. However, for continuous media as well as for discretised media, significant errors have been reported in numerical solutions regarding the approximated crack surface, and hence for the dissipated energy. We show that for a practical setting, where the internal length scale and the spacing of the discretisation are small but finite, the observed discrepancy partially stems from the fact that numerical studies consider specimens of a finite length, and partially relates to the irreversibility introduced when casting the variational theory for brittle fracture in a damage-like format. While some form of irreversibility may be required in numerical implementations, the precise form significantly influences the accuracy and convergence towards the discrete crack. (C) 2017 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Thomas Linse and Paul Hennig and Markus Kaestner and Rene de Borst},
      title = {A convergence study of phase-field models for brittle fracture},
      journal = {Engineering Fracture Mechanics},
      volume = {184},
      pages = {307-318},
      abstract = {A crucial issue in phase-field models for brittle fracture is whether the functional that describes the distributed crack converges to the functional of the discrete crack when the internal length scale introduced in the distribution function goes to zero. Theoretical proofs exist for the original theory. However, for continuous media as well as for discretised media, significant errors have been reported in numerical solutions regarding the approximated crack surface, and hence for the dissipated energy. We show that for a practical setting, where the internal length scale and the spacing of the discretisation are small but finite, the observed discrepancy partially stems from the fact that numerical studies consider specimens of a finite length, and partially relates to the irreversibility introduced when casting the variational theory for brittle fracture in a damage-like format. While some form of irreversibility may be required in numerical implementations, the precise form significantly influences the accuracy and convergence towards the discrete crack. (C) 2017 Elsevier Ltd. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.engfracmech.2017.09.013},
      doi = {10.1016/j.engfracmech.2017.09.013},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Coordination Polymer Framework Based On-Chip Micro-Supercapacitors with AC Line-Filtering Performance
    • C. Yang, K. S. Schellhammer, F. Ortmann, S. Sun, R. Dong, M. Karakus, Z. Mics, M. Loeffler, F. Zhang, X. Zhuang, E. Canovas, G. Cuniberti, M. Bonn, X. Feng
    • Angewandte Chemie-International Edition 56, 3920-3924 (2017)
    • DOI   Abstract  

      On-chip micro-supercapacitors (MSCs) are important Si-compatible power-source backups for miniaturized electronics. Despite their tremendous advantages, current on-chip MSCs require harsh processing conditions and typically perform like resistors when filtering ripples from alternating current (AC). Herein, we demonstrated a facile layer-by-layer method towards on-chip MSCs based on an azulene-bridged coordination polymer framework (PiCBA). Owing to the good carrier mobility (5 x 10(-3) cm(2)V(-1) s(-1)) of PiCBA, the permanent dipole moment of azulene skeleton, and ultralow band gap of PiCBA, the fabricated MSCs delivered high specific capacitances of up to 34.1 Fcm(-3) at 50 mV s(-1) and a high volumetric power density of 1323 W cm(-3). Most importantly, such MCSs exhibited AC line-filtering performance (-73 degrees at 120 Hz) with a short resistance-capacitance constant of circa 0.83 ms.

      @article{,
      author = {Chongqing Yang and Karl Sebastian Schellhammer and Frank Ortmann and Sai Sun and Renhao Dong and Melike Karakus and Zoltan Mics and Markus Loeffler and Fan Zhang and Xiaodong Zhuang and Enrique Canovas and Gianaurelio Cuniberti and Mischa Bonn and Xinliang Feng},
      title = {Coordination Polymer Framework Based On-Chip Micro-Supercapacitors with AC Line-Filtering Performance},
      journal = {Angewandte Chemie-International Edition},
      volume = {56},
      number = {14},
      pages = {3920-3924},
      abstract = {On-chip micro-supercapacitors (MSCs) are important Si-compatible power-source backups for miniaturized electronics. Despite their tremendous advantages, current on-chip MSCs require harsh processing conditions and typically perform like resistors when filtering ripples from alternating current (AC). Herein, we demonstrated a facile layer-by-layer method towards on-chip MSCs based on an azulene-bridged coordination polymer framework (PiCBA). Owing to the good carrier mobility (5 x 10(-3) cm(2)V(-1) s(-1)) of PiCBA, the permanent dipole moment of azulene skeleton, and ultralow band gap of PiCBA, the fabricated MSCs delivered high specific capacitances of up to 34.1 Fcm(-3) at 50 mV s(-1) and a high volumetric power density of 1323 W cm(-3). Most importantly, such MCSs exhibited AC line-filtering performance (-73 degrees at 120 Hz) with a short resistance-capacitance constant of circa 0.83 ms.},
      year = {2017},
      url = http://dx.doi.org/{10.1002/anie.201700679},
      doi = {10.1002/anie.201700679},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Copper Electroplating with Polyethylene Glycol I. An Alternative Hysteresis Model without Additive Consumption
    • H. Yang, A. Dianat, M. Bobeth, G. Cuniberti
    • Journal of the Electrochemical Society 164, D196-D203 (2017)
    • DOI   Abstract  

      Additives play an important role in electrochemical deposition and understanding their working mechanism is a great challenge. In cyclic voltammetry measurements of copper deposition, hysteresis is ubiquitously observed. Correct prediction of hysteresis behavior is an important test for deposition models. In previous models, including poly(ethylene glycol) (PEG) and chloride ions as additives, a common assumption to explain the hysteresis is the consumption of additives during copper deposition. However, second-ion mass spectrometry measurements often detected comparatively low levels of impurities in deposits. Therefore, we propose here an alternative mechanism for explaining hysteresis curves without invoking additive consumption. Essential ingredients of our models are: (i) a strongly nonlinear dependence of the maximal possible PEG coverage on the chloride coverage on the copper surface, (ii) a nonlinear dependence of the deposition current on the PEG surface coverage, and (iii) an additional activation of the desorption of additives with increasing copper deposition current. We demonstrate that our model reproduces characteristic features of cyclic voltammograms measured under vastly different conditions and exhibiting pronounced hysteresis. Furthermore, simulations are compared well to PEG adsorption/desorption experiments with varying additive concentrations. The proposed model may serve to describe deposition situations with negligible additive consumption. (C) 2017 The Electrochemical Society. All rights reserved.

      @article{,
      author = {Hongliu Yang and Arezoo Dianat and Manfred Bobeth and Gianaurelio Cuniberti},
      title = {Copper Electroplating with Polyethylene Glycol I. An Alternative Hysteresis Model without Additive Consumption},
      journal = {Journal of the Electrochemical Society},
      volume = {164},
      number = {4},
      pages = {D196-D203},
      abstract = {Additives play an important role in electrochemical deposition and understanding their working mechanism is a great challenge. In cyclic voltammetry measurements of copper deposition, hysteresis is ubiquitously observed. Correct prediction of hysteresis behavior is an important test for deposition models. In previous models, including poly(ethylene glycol) (PEG) and chloride ions as additives, a common assumption to explain the hysteresis is the consumption of additives during copper deposition. However, second-ion mass spectrometry measurements often detected comparatively low levels of impurities in deposits. Therefore, we propose here an alternative mechanism for explaining hysteresis curves without invoking additive consumption. Essential ingredients of our models are: (i) a strongly nonlinear dependence of the maximal possible PEG coverage on the chloride coverage on the copper surface, (ii) a nonlinear dependence of the deposition current on the PEG surface coverage, and (iii) an additional activation of the desorption of additives with increasing copper deposition current. We demonstrate that our model reproduces characteristic features of cyclic voltammograms measured under vastly different conditions and exhibiting pronounced hysteresis. Furthermore, simulations are compared well to PEG adsorption/desorption experiments with varying additive concentrations. The proposed model may serve to describe deposition situations with negligible additive consumption. (C) 2017 The Electrochemical Society. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1149/2.1051704jes},
      doi = {10.1149/2.1051704jes},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Copper Induced Conformational Changes of Tripeptide Monolayer Based Impedimetric Biosensor
    • E. Mervinetsky, I. Alshanski, Y. Hamo, L. M. Sandonas, A. Dianat, J. Buchwald, R. Gutierrez, G. Cuniberti, M. Hurevich, S. Yitzchaik
    • Scientific Reports 7(2017)
    • DOI   Abstract  

      Copper ions play a major role in biological processes. Abnormal Cu2+ ions concentrations are associated with various diseases, hence, can be used as diagnostic target. Monitoring copper ion is currently performed by non-portable, expensive and complicated to use equipment. We present a label free and a highly sensitive electrochemical ion-detecting biosensor based on a Gly-Gly-His tripeptide layer that chelate with Cu2+ ions. The proposed sensing mechanism is that the chelation results in conformational changes in the peptide that forms a denser insulating layer that prevents RedOx species transfer to the surface. This chelation event was monitored using various electrochemical methods and surface chemistry analysis and supported by theoretical calculations. We propose a highly sensitive ion-detection biosensor that can detect Cu2+ ions in the pM range with high SNR parameter.

      @article{,
      author = {Evgeniy Mervinetsky and Israel Alshanski and Yonatan Hamo and Leonardo Medrano Sandonas and Arezoo Dianat and Joerg Buchwald and Rafael Gutierrez and Gianaurelio Cuniberti and Mattan Hurevich and Shlomo Yitzchaik},
      title = {Copper Induced Conformational Changes of Tripeptide Monolayer Based Impedimetric Biosensor},
      journal = {Scientific Reports},
      volume = {7},
      abstract = {Copper ions play a major role in biological processes. Abnormal Cu2+ ions concentrations are associated with various diseases, hence, can be used as diagnostic target. Monitoring copper ion is currently performed by non-portable, expensive and complicated to use equipment. We present a label free and a highly sensitive electrochemical ion-detecting biosensor based on a Gly-Gly-His tripeptide layer that chelate with Cu2+ ions. The proposed sensing mechanism is that the chelation results in conformational changes in the peptide that forms a denser insulating layer that prevents RedOx species transfer to the surface. This chelation event was monitored using various electrochemical methods and surface chemistry analysis and supported by theoretical calculations. We propose a highly sensitive ion-detection biosensor that can detect Cu2+ ions in the pM range with high SNR parameter.},
      year = {2017},
      url = http://dx.doi.org/{10.1038/s41598-017-10288-z},
      doi = {10.1038/s41598-017-10288-z},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Curvature controlled defect dynamics in topological active nematics
    • F. Alaimo, C. Koehler, A. Voigt
    • Scientific Reports 7(2017)
    • DOI   Abstract  

      We study the spatiotemporal patterns that emerge when an active nematic film is topologically constraint. These topological constraints allow to control the non-equilibrium dynamics of the active system. We consider ellipsoidal shapes for which the resulting defects are 1/2 disclinations and analyze the relation between their location and dynamics and local geometric properties of the ellipsoid. We highlight two dynamic modes: a tunable periodic state that oscillates between two defect configurations on a spherical shape and a tunable rotating state for oblate spheroids. We further demonstrate the relation between defects and high Gaussian curvature and umbilical points and point out limits for a coarse-grained description of defects as self-propelled particles.

      @article{,
      author = {Francesco Alaimo and Christian Koehler and Axel Voigt},
      title = {Curvature controlled defect dynamics in topological active nematics},
      journal = {Scientific Reports},
      volume = {7},
      abstract = {We study the spatiotemporal patterns that emerge when an active nematic film is topologically constraint. These topological constraints allow to control the non-equilibrium dynamics of the active system. We consider ellipsoidal shapes for which the resulting defects are 1/2 disclinations and analyze the relation between their location and dynamics and local geometric properties of the ellipsoid. We highlight two dynamic modes: a tunable periodic state that oscillates between two defect configurations on a spherical shape and a tunable rotating state for oblate spheroids. We further demonstrate the relation between defects and high Gaussian curvature and umbilical points and point out limits for a coarse-grained description of defects as self-propelled particles.},
      year = {2017},
      url = http://dx.doi.org/{10.1038/s41598-017-05612-6},
      doi = {10.1038/s41598-017-05612-6},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Developing a Customized Perfusion Bioreactor Prototype with Controlled Positional Variability in Oxygen Partial Pressure for Bone and Cartilage Tissue Engineering
    • P. S. Lee, H. Eckert, R. Hess, M. Gelinsky, D. Rancourt, R. Krawetz, G. Cuniberti, D. Scharnweber
    • Tissue Engineering Part C-Methods 23, 286-297 (2017)
    • DOI   Abstract  

      Skeletal development is a multistep process that involves the complex interplay of multiple cell types at different stages of development. Besides biochemical and physical cues, oxygen tension also plays a pivotal role in influencing cell fate during skeletal development. At physiological conditions, bone cells generally reside in a relatively oxygenated environment whereas chondrocytes reside in a hypoxic environment. However, it is technically challenging to achieve such defined, yet diverse oxygen distribution on traditional in vitro cultivation platforms. Instead, engineered osteochondral constructs are commonly cultivated in a homogeneous, stable environment. In this study, we describe a customized perfusion bioreactor having stable positional variability in oxygen tension at defined regions. Further, engineered collagen constructs were coaxed into adopting the shape and dimensions of defined cultivation platforms that were precasted in 1.5% agarose bedding. After cultivating murine embryonic stem cells that were embedded in collagen constructs for 50 days, mineralized constructs of specific dimensions and a stable structural integrity were achieved. The end-products, specifically constructs cultivated without chondroitin sulfate A (CSA), showed a significant increase in mechanical stiffness compared with their initial gel-like constructs. More importantly, the localization of osteochondral cell types was specific and corresponded to the oxygen tension gradient generated in the bioreactor. In addition, CSA in complementary with low oxygen tension was also found to be a potent inducer of chondrogenesis in this system. In summary, we have demonstrated a customized perfusion bioreactor prototype that is capable of generating a more dynamic, yet specific cultivation environment that could support propagation of multiple osteochondral lineages within a single engineered construct in vitro. Our system opens up new possibilities for in vitro research on human skeletal development.

      @article{,
      author = {Poh Soo Lee and Hagen Eckert and Ricarda Hess and Michael Gelinsky and Derrick Rancourt and Roman Krawetz and Gianaurelio Cuniberti and Dieter Scharnweber},
      title = {Developing a Customized Perfusion Bioreactor Prototype with Controlled Positional Variability in Oxygen Partial Pressure for Bone and Cartilage Tissue Engineering},
      journal = {Tissue Engineering Part C-Methods},
      volume = {23},
      number = {5},
      pages = {286-297},
      abstract = {Skeletal development is a multistep process that involves the complex interplay of multiple cell types at different stages of development. Besides biochemical and physical cues, oxygen tension also plays a pivotal role in influencing cell fate during skeletal development. At physiological conditions, bone cells generally reside in a relatively oxygenated environment whereas chondrocytes reside in a hypoxic environment. However, it is technically challenging to achieve such defined, yet diverse oxygen distribution on traditional in vitro cultivation platforms. Instead, engineered osteochondral constructs are commonly cultivated in a homogeneous, stable environment. In this study, we describe a customized perfusion bioreactor having stable positional variability in oxygen tension at defined regions. Further, engineered collagen constructs were coaxed into adopting the shape and dimensions of defined cultivation platforms that were precasted in 1.5% agarose bedding. After cultivating murine embryonic stem cells that were embedded in collagen constructs for 50 days, mineralized constructs of specific dimensions and a stable structural integrity were achieved. The end-products, specifically constructs cultivated without chondroitin sulfate A (CSA), showed a significant increase in mechanical stiffness compared with their initial gel-like constructs. More importantly, the localization of osteochondral cell types was specific and corresponded to the oxygen tension gradient generated in the bioreactor. In addition, CSA in complementary with low oxygen tension was also found to be a potent inducer of chondrogenesis in this system. In summary, we have demonstrated a customized perfusion bioreactor prototype that is capable of generating a more dynamic, yet specific cultivation environment that could support propagation of multiple osteochondral lineages within a single engineered construct in vitro. Our system opens up new possibilities for in vitro research on human skeletal development.},
      year = {2017},
      url = http://dx.doi.org/{10.1089/ten.tec.2016.0244},
      doi = {10.1089/ten.tec.2016.0244},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Doping of graphene induced by boron/silicon substrate
    • A. Dianat, Z. Liao, M. Gall, T. Zhang, R. Gutierrez, E. Zschech, G. Cuniberti
    • Nanotechnology 28(2017)
    • DOI   Abstract  

      In this work, we show the doping of graphene most likely from heteroatoms induced by the substrate using Raman spectra, x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy and ab initio molecular dynamics (MD) simulations. The doping of graphene on a highly boron-doped silicon substrate was achieved by an annealing at 400 K for about 3 h in an oven with air flow. With the same annealing, only the Raman features similar to that from the pristine graphene were observed in the freestanding graphene and the graphene on a typical Si/SiO2 wafer. Ab initio MD simulations were performed for defected graphene on boron-doped silicon substrate at several temperatures. All vacancy sites in the graphene are occupied either with B atoms or Si atoms resulting in the mixed boron-silicon doping of the graphene. The MD simulations validated the experimetal finding of graphene doped behavior observed by Raman spectrum. The electronic structure analysis indicated the p-type nature of doped graphene. The observed doping by the possible incorporation of heteroatoms into the graphene, simply only using 400 K annealing the boron-doped Si substrate, could provide a new approach to synthesize doped graphene in a more economic way.

      @article{,
      author = {Arezoo Dianat and Zhongquan Liao and Martin Gall and Tao Zhang and Rafael Gutierrez and Ehrenfried Zschech and Gianaurelio Cuniberti},
      title = {Doping of graphene induced by boron/silicon substrate},
      journal = {Nanotechnology},
      volume = {28},
      number = {21},
      abstract = {In this work, we show the doping of graphene most likely from heteroatoms induced by the substrate using Raman spectra, x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy and ab initio molecular dynamics (MD) simulations. The doping of graphene on a highly boron-doped silicon substrate was achieved by an annealing at 400 K for about 3 h in an oven with air flow. With the same annealing, only the Raman features similar to that from the pristine graphene were observed in the freestanding graphene and the graphene on a typical Si/SiO2 wafer. Ab initio MD simulations were performed for defected graphene on boron-doped silicon substrate at several temperatures. All vacancy sites in the graphene are occupied either with B atoms or Si atoms resulting in the mixed boron-silicon doping of the graphene. The MD simulations validated the experimetal finding of graphene doped behavior observed by Raman spectrum. The electronic structure analysis indicated the p-type nature of doped graphene. The observed doping by the possible incorporation of heteroatoms into the graphene, simply only using 400 K annealing the boron-doped Si substrate, could provide a new approach to synthesize doped graphene in a more economic way.},
      year = {2017},
      url = http://dx.doi.org/{10.1088/1361-6528/aa6ce9},
      doi = {10.1088/1361-6528/aa6ce9},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Edge magnetism impact on electrical conductance and thermoelectric properties of graphenelike nanoribbons
    • S. Krompiewski, G. Cuniberti
    • Physical Review B 96(2017)
    • DOI   Abstract  

      Edge states in narrow quasi-two-dimensional nanostructures determine, to a large extent, their electric, thermoelectric, and magnetic properties. Nonmagnetic edge states may quite often lead to topological-insulator-type behavior. However, another scenario develops when the zigzag edges are magnetic and the time reversal symmetry is broken. In this work we report on the electronic band structure modifications, electrical conductance, and thermoelectric properties of narrow zigzag nanoribbons with spontaneously magnetized edges. Theoretical studies based on the Kane-Mele-Hubbard tight-binding model show that for silicene, germanene, and stanene both the Seebeck coefficient and the thermoelectric power factor are strongly enhanced for energies close to the charge neutrality point. A perpendicular gate voltage lifts the spin degeneracy of energy bands in the ground state with antiparallel magnetized zigzag edges and makes the electrical conductance significantly spin polarized. Simultaneously the gate voltage worsens the thermoelectric performance. Estimated room-temperature figures of merit for the aforementioned nanoribbons can exceed a value of 3 if phonon thermal conductances are adequately reduced.

      @article{,
      author = {Stefan Krompiewski and Gianaurelio Cuniberti},
      title = {Edge magnetism impact on electrical conductance and thermoelectric properties of graphenelike nanoribbons},
      journal = {Physical Review B},
      volume = {96},
      number = {15},
      abstract = {Edge states in narrow quasi-two-dimensional nanostructures determine, to a large extent, their electric, thermoelectric, and magnetic properties. Nonmagnetic edge states may quite often lead to topological-insulator-type behavior. However, another scenario develops when the zigzag edges are magnetic and the time reversal symmetry is broken. In this work we report on the electronic band structure modifications, electrical conductance, and thermoelectric properties of narrow zigzag nanoribbons with spontaneously magnetized edges. Theoretical studies based on the Kane-Mele-Hubbard tight-binding model show that for silicene, germanene, and stanene both the Seebeck coefficient and the thermoelectric power factor are strongly enhanced for energies close to the charge neutrality point. A perpendicular gate voltage lifts the spin degeneracy of energy bands in the ground state with antiparallel magnetized zigzag edges and makes the electrical conductance significantly spin polarized. Simultaneously the gate voltage worsens the thermoelectric performance. Estimated room-temperature figures of merit for the aforementioned nanoribbons can exceed a value of 3 if phonon thermal conductances are adequately reduced.},
      year = {2017},
      url = http://dx.doi.org/{10.1103/PhysRevB.96.155447},
      doi = {10.1103/PhysRevB.96.155447},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Effects of octahedral tilting on the electronic structure and optical properties of d(0) double perovskites A(2)ScSbO(6) (A = Sr, Ca)
    • R. Ray, A. K. Himanshu, P. Sen, U. Kumar, M. Richter, T. P. Sinha
    • Journal of Alloys and Compounds 705, 497-506 (2017)
    • DOI   Abstract  

      With increasing temperature, Sr2ScSbO6 undergoes three structural phase transitions at approximately 400K, 560K and 650K, leading to the following sequence of phases: P2(1)/n -> I2= m -> I4/m -> Fm (3) over barm, making it an ideal candidate to study the effects of octahedral tilting while keeping other parameters fixed. To ascertain the isolated effects of octahedral distortions, the electronic and optical properties of the monoclinic P2(1)/ n (at room temperature), monoclinic I2/m (at 430K), tetragonal I4= m (at 613K) and the cubic Fm (3) over barm (at 660K) phases have been studied in terms of the electronic structure, dielectric constant, optical conductivity and electron energy loss spectrum using density functional theory. Ca2ScSbO6, on the other hand, shows only a P2(1)/n phase at room temperature and its properties have been compared with the corresponding Sr compound. UV-Vis spectroscopic studies of the optical properties of the room-temperature phase of these d(0) double perovskites have been performed and presence of large direct bandgap for both the compounds have been reported. The electronic bandgaps for the room temperature phases are found to be in good agreement with the corresponding experimental values obtained using the Kubelka-Munk function. Interestingly, in contrast to other Sc-based d(0) double perovskites, with increasing octahedral distortions, the effective t(2g) bandwidth remains unaffected while the states forming the band change due to changes in unit cell orientation, leading to small effects on the electronic and optical properties. (C) 2017 Elsevier B.V. All rights reserved.

      @article{,
      author = {Rajyavardhan Ray and A. K. Himanshu and Pintu Sen and Uday Kumar and Manuel Richter and T. P. Sinha},
      title = {Effects of octahedral tilting on the electronic structure and optical properties of d(0) double perovskites A(2)ScSbO(6) (A = Sr, Ca)},
      journal = {Journal of Alloys and Compounds},
      volume = {705},
      pages = {497-506},
      abstract = {With increasing temperature, Sr2ScSbO6 undergoes three structural phase transitions at approximately 400K, 560K and 650K, leading to the following sequence of phases: P2(1)/n -> I2= m -> I4/m -> Fm (3) over barm, making it an ideal candidate to study the effects of octahedral tilting while keeping other parameters fixed. To ascertain the isolated effects of octahedral distortions, the electronic and optical properties of the monoclinic P2(1)/ n (at room temperature), monoclinic I2/m (at 430K), tetragonal I4= m (at 613K) and the cubic Fm (3) over barm (at 660K) phases have been studied in terms of the electronic structure, dielectric constant, optical conductivity and electron energy loss spectrum using density functional theory. Ca2ScSbO6, on the other hand, shows only a P2(1)/n phase at room temperature and its properties have been compared with the corresponding Sr compound. UV-Vis spectroscopic studies of the optical properties of the room-temperature phase of these d(0) double perovskites have been performed and presence of large direct bandgap for both the compounds have been reported. The electronic bandgaps for the room temperature phases are found to be in good agreement with the corresponding experimental values obtained using the Kubelka-Munk function. Interestingly, in contrast to other Sc-based d(0) double perovskites, with increasing octahedral distortions, the effective t(2g) bandwidth remains unaffected while the states forming the band change due to changes in unit cell orientation, leading to small effects on the electronic and optical properties. (C) 2017 Elsevier B.V. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.jallcom.2017.02.080},
      doi = {10.1016/j.jallcom.2017.02.080},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Elastic and piezoresistive properties of nickel carbides from first principles
    • J. Kelling, P. Zahn, J. Schuster, S. Gemming
    • Physical Review B 95(2017)
    • DOI   Abstract  

      The nickel-carbon system has received increased attention over the past years due to the relevance of nickel as a catalyst for carbon nanotube and graphene growth, where nickel carbide intermediates may be involved or carbide interface layers form in the end. Nickel-carbon composite thin films comprising Ni3C are especially interesting in mechanical sensing applications. Due to the metastability of nickel carbides, formation conditions and the coupling between mechanical and electrical properties are not yet well understood. Using first-principles electronic structure methods, we calculated the elastic properties of Ni3C, Ni2C, and NiC, as well as changes in electronic properties under mechanical strain. We observe that the electronic density of states around the Fermi level does not change under the considered strains of up to 1%, which correspond to stresses up to 3 GPa. Relative changes in conductivity of Ni3C range up to maximum values of about 10%.

      @article{,
      author = {Jeffrey Kelling and Peter Zahn and Joerg Schuster and Sibylle Gemming},
      title = {Elastic and piezoresistive properties of nickel carbides from first principles},
      journal = {Physical Review B},
      volume = {95},
      number = {2},
      abstract = {The nickel-carbon system has received increased attention over the past years due to the relevance of nickel as a catalyst for carbon nanotube and graphene growth, where nickel carbide intermediates may be involved or carbide interface layers form in the end. Nickel-carbon composite thin films comprising Ni3C are especially interesting in mechanical sensing applications. Due to the metastability of nickel carbides, formation conditions and the coupling between mechanical and electrical properties are not yet well understood. Using first-principles electronic structure methods, we calculated the elastic properties of Ni3C, Ni2C, and NiC, as well as changes in electronic properties under mechanical strain. We observe that the electronic density of states around the Fermi level does not change under the considered strains of up to 1%, which correspond to stresses up to 3 GPa. Relative changes in conductivity of Ni3C range up to maximum values of about 10%.},
      year = {2017},
      url = http://dx.doi.org/{10.1103/PhysRevB.95.024113},
      doi = {10.1103/PhysRevB.95.024113},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Electronic transport in metallic carbon nanotubes with mixed defects within the strong localization regime
    • F. Teichert, A. Zienert, J. Schuster, M. Schreiber
    • Computational Materials Science 138, 49-57 (2017)
    • DOI   Abstract  

      We study the electron transport in metallic carbon nanotubes (CNTs) with realistic defects of different types. We focus on large CNTs with many defects in the mesoscopic range. In a recent paper we demonstrated that the electronic transport in those defective CNTs is in the regime of strong localization. We verify by quantum transport simulations that the localization length of CNTs with defects of mixed types can be related to the localization lengths of CNTs with identical defects by taking the weighted harmonic average. Secondly, we show how to use this result to estimate the conductance of arbitrary defective CNTs, avoiding time consuming transport calculations. (C) 2017 Elsevier B.V. All rights reserved.

      @article{,
      author = {Fabian Teichert and Andreas Zienert and Joerg Schuster and Michael Schreiber},
      title = {Electronic transport in metallic carbon nanotubes with mixed defects within the strong localization regime},
      journal = {Computational Materials Science},
      volume = {138},
      pages = {49-57},
      abstract = {We study the electron transport in metallic carbon nanotubes (CNTs) with realistic defects of different types. We focus on large CNTs with many defects in the mesoscopic range. In a recent paper we demonstrated that the electronic transport in those defective CNTs is in the regime of strong localization. We verify by quantum transport simulations that the localization length of CNTs with defects of mixed types can be related to the localization lengths of CNTs with identical defects by taking the weighted harmonic average. Secondly, we show how to use this result to estimate the conductance of arbitrary defective CNTs, avoiding time consuming transport calculations. (C) 2017 Elsevier B.V. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.commatsci.2017.06.001},
      doi = {10.1016/j.commatsci.2017.06.001},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Enhancement of thermal transport properties of asymmetric Graphene/hBN nanoribbon heterojunctions by substrate engineering
    • L. M. Sandonas, G. Cuba-Supanta, R. Gutierrez, A. Dianat, C. V. Landauro, G. Cuniberti
    • Carbon 124, 642-650 (2017)
    • DOI   Abstract  

      Two-dimensional heterostructures offer a new route to manipulate phonons at the nanoscale. By performing non-equilibrium molecular dynamics simulations we address the thermal transport properties of structurally asymmetric graphene/hBN nanoribbon heterojunctions deposited on several substrates: graphite, Si(100), SiC(0001), and SiO2. Our results show a reduction of the interface thermal resistance in coplanar G/hBN heterojunctions upon substrate deposition which is mainly related to the increment on the power spectrum overlap. This effect is more pronounced for deposition on Si(100) and SiO2 substrates, independently of the planar stacking order of the materials. Moreover, it has been found that the thermal rectification factor increases as a function of the degree of structural asymmetry for hBN-G nanoribbons, reaching values up to similar to 24%, while it displays a minimum (is an element of[0.7, 2.4]) for G-hBN nanoribbons. More importantly, these properties can also be tuned by varying the substrate temperature, e.g., thermal rectification of symmetric hBN-G nanoribbon is enhanced from 8.8% to 79% by reducing the temperature of Si(100) substrate. Our investigation yields new insights into the physical mechanisms governing heat transport in G/hBN heterojunctions, and thus opens potential new routes to the design of phononic devices. (C) 2017 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Leonardo Medrano Sandonas and G. Cuba-Supanta and Rafael Gutierrez and Arezoo Dianat and Carlos V. Landauro and Gianaurelio Cuniberti},
      title = {Enhancement of thermal transport properties of asymmetric Graphene/hBN nanoribbon heterojunctions by substrate engineering},
      journal = {Carbon},
      volume = {124},
      pages = {642-650},
      abstract = {Two-dimensional heterostructures offer a new route to manipulate phonons at the nanoscale. By performing non-equilibrium molecular dynamics simulations we address the thermal transport properties of structurally asymmetric graphene/hBN nanoribbon heterojunctions deposited on several substrates: graphite, Si(100), SiC(0001), and SiO2. Our results show a reduction of the interface thermal resistance in coplanar G/hBN heterojunctions upon substrate deposition which is mainly related to the increment on the power spectrum overlap. This effect is more pronounced for deposition on Si(100) and SiO2 substrates, independently of the planar stacking order of the materials. Moreover, it has been found that the thermal rectification factor increases as a function of the degree of structural asymmetry for hBN-G nanoribbons, reaching values up to similar to 24%, while it displays a minimum (is an element of[0.7, 2.4]) for G-hBN nanoribbons. More importantly, these properties can also be tuned by varying the substrate temperature, e.g., thermal rectification of symmetric hBN-G nanoribbon is enhanced from 8.8% to 79% by reducing the temperature of Si(100) substrate. Our investigation yields new insights into the physical mechanisms governing heat transport in G/hBN heterojunctions, and thus opens potential new routes to the design of phononic devices. (C) 2017 Elsevier Ltd. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.carbon.2017.09.025},
      doi = {10.1016/j.carbon.2017.09.025},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Exciton Binding Energy in Molecular Triads
    • S. Kraner, G. Prampolini, G. Cuniberti
    • Journal of Physical Chemistry C 121, 17088-17095 (2017)
    • DOI   Abstract  

      The power conversion efficiency of state of the art organic photovoltaics is predicted to be limited at about 15%. This limit can be increased by an improved charge carrier mobility and by a lower exciton binding energy. In order to achieve this, we suggest a concept based on organic triads, comprising a donor, spacer, and acceptor submolecule. On the basis of time dependent density functional theory (TD-DFT) simulations we investigate the lowest excited state of the well-known Carotenoid-Porphyrin-C60 triad and obtain a calculated exciton binding energy of 25 meV, justifying the experimentally observed and reported separation of photo generated charges. Further, we introduce a new triad with the ability to not only improve and control the separation process, but also to improve the charge carrier transport properties. We used molecular dynamics (MD) simulations to optimize the geometry of a cluster of 25 triads. From this organic cluster, we picked one triad with its four neighbors, again calculated the exciton binding energy of this structure and obtained 39 meV. We conclude that the exciton binding energy is stable for a variety of different basis set and functionals, and does not significantly change if one or a cluster of five triads is simulated. This stable behavior occurs, since changes in the wave functions do not significantly influence the exciton binding energy, as long the distance between the positive and negative charge remains the same. For photovoltaic applications and based on organic materials with a dielectric constant of about four, we suggest the use of spacer molecules larger than two nanometers.

      @article{,
      author = {Stefan Kraner and Giacomo Prampolini and Gianaurelio Cuniberti},
      title = {Exciton Binding Energy in Molecular Triads},
      journal = {Journal of Physical Chemistry C},
      volume = {121},
      number = {32},
      pages = {17088-17095},
      abstract = {The power conversion efficiency of state of the art organic photovoltaics is predicted to be limited at about 15%. This limit can be increased by an improved charge carrier mobility and by a lower exciton binding energy. In order to achieve this, we suggest a concept based on organic triads, comprising a donor, spacer, and acceptor submolecule. On the basis of time dependent density functional theory (TD-DFT) simulations we investigate the lowest excited state of the well-known Carotenoid-Porphyrin-C60 triad and obtain a calculated exciton binding energy of 25 meV, justifying the experimentally observed and reported separation of photo generated charges. Further, we introduce a new triad with the ability to not only improve and control the separation process, but also to improve the charge carrier transport properties. We used molecular dynamics (MD) simulations to optimize the geometry of a cluster of 25 triads. From this organic cluster, we picked one triad with its four neighbors, again calculated the exciton binding energy of this structure and obtained 39 meV. We conclude that the exciton binding energy is stable for a variety of different basis set and functionals, and does not significantly change if one or a cluster of five triads is simulated. This stable behavior occurs, since changes in the wave functions do not significantly influence the exciton binding energy, as long the distance between the positive and negative charge remains the same. For photovoltaic applications and based on organic materials with a dielectric constant of about four, we suggest the use of spacer molecules larger than two nanometers.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acs.jpcc.7b03923},
      doi = {10.1021/acs.jpcc.7b03923},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Graphene nanoribbons on gold: understanding superlubricity and edge effects
    • L. Gigli, N. Manini, A. Benassi, E. Tosatti, A. Vanossi, R. Guerra
    • 2d Materials 4(2017)
    • DOI   Abstract  

      We address the atomistic nature of the longitudinal static friction against sliding of graphene nanoribbons (GNRs) deposited on gold, a system whose structural and mechanical properties have been recently the subject of intense experimental investigation. By means of numerical simulations and modeling we show that the GNR interior is structurally lubric (‘superlubric’) so that the static friction is dominated by the front/tail regions of the GNR, where the residual uncompensated lateral forces arising from the interaction with the underneath gold surface opposes the free sliding. As a result of this edge pinning the static friction does not grow with the GNR length, but oscillates around a fairly constant mean value. These friction oscillations are explained in terms of the GNR-Au(111) lattice mismatch: at certain GNR lengths close to an integer number of the beat (or moire) length there is good force compensation and superlubric sliding; whereas close to half odd-integer periods there is significant pinning of the edge with larger friction. These results make qualitative contact with recent state-of-the-art atomic force microscopy experiment, as well as with the sliding of other different incommensurate systems.

      @article{,
      author = {L. Gigli and N. Manini and A. Benassi and E. Tosatti and A. Vanossi and R. Guerra},
      title = {Graphene nanoribbons on gold: understanding superlubricity and edge effects},
      journal = {2d Materials},
      volume = {4},
      number = {4},
      abstract = {We address the atomistic nature of the longitudinal static friction against sliding of graphene nanoribbons (GNRs) deposited on gold, a system whose structural and mechanical properties have been recently the subject of intense experimental investigation. By means of numerical simulations and modeling we show that the GNR interior is structurally lubric ('superlubric') so that the static friction is dominated by the front/tail regions of the GNR, where the residual uncompensated lateral forces arising from the interaction with the underneath gold surface opposes the free sliding. As a result of this edge pinning the static friction does not grow with the GNR length, but oscillates around a fairly constant mean value. These friction oscillations are explained in terms of the GNR-Au(111) lattice mismatch: at certain GNR lengths close to an integer number of the beat (or moire) length there is good force compensation and superlubric sliding; whereas close to half odd-integer periods there is significant pinning of the edge with larger friction. These results make qualitative contact with recent state-of-the-art atomic force microscopy experiment, as well as with the sliding of other different incommensurate systems.},
      year = {2017},
      url = http://dx.doi.org/{10.1088/2053-1583/aa7fdf},
      doi = {10.1088/2053-1583/aa7fdf},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Improved recursive Green’s function formalism for quasi one-dimensional systems with realistic defects
    • F. Teichert, A. Zienert, J. Schuster, M. Schreiber
    • Journal of Computational Physics 334, 607-619 (2017)
    • DOI   Abstract  

      We derive an improved version of the recursive Green’s function formalism (RGF), which is a standard tool in the quantum transport theory. We consider the case of disordered quasi one-dimensional materials where the disorder is applied in form of randomly distributed realistic defects, leading to partly periodic Hamiltonian matrices. The algorithm accelerates the common RGF in the recursive decimation scheme, using the iteration steps of the renormalization decimation algorithm. This leads to a smaller effective system, which is treated using the common forward iteration scheme. The computational complexity scales linearly with the number of defects, instead of linearly with the total system length for the conventional approach. We show that the scaling of the calculation time of the Green’s function depends on the defect density of a random test system. Furthermore, we discuss the calculation time and the memory requirement of the whole transport formalism applied to defective carbon nanotubes. (C) 2017 Elsevier Inc. All rights reserved.

      @article{,
      author = {Fabian Teichert and Andreas Zienert and Jorg Schuster and Michael Schreiber},
      title = {Improved recursive Green's function formalism for quasi one-dimensional systems with realistic defects},
      journal = {Journal of Computational Physics},
      volume = {334},
      pages = {607-619},
      abstract = {We derive an improved version of the recursive Green's function formalism (RGF), which is a standard tool in the quantum transport theory. We consider the case of disordered quasi one-dimensional materials where the disorder is applied in form of randomly distributed realistic defects, leading to partly periodic Hamiltonian matrices. The algorithm accelerates the common RGF in the recursive decimation scheme, using the iteration steps of the renormalization decimation algorithm. This leads to a smaller effective system, which is treated using the common forward iteration scheme. The computational complexity scales linearly with the number of defects, instead of linearly with the total system length for the conventional approach. We show that the scaling of the calculation time of the Green's function depends on the defect density of a random test system. Furthermore, we discuss the calculation time and the memory requirement of the whole transport formalism applied to defective carbon nanotubes. (C) 2017 Elsevier Inc. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.jcp.2017.01.024},
      doi = {10.1016/j.jcp.2017.01.024},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • In Situ Electron Driven Carbon Nanopillar-Fullerene Transformation through Cr Atom Mediation
    • L. Zhao, H. Q. Ta, A. Dianat, A. Soni, A. Fediai, W. Yin, T. Gemming, B. Trzebicka, G. Cuniberti, Z. Liu, A. Bachmatiuk, M. H. Rummeli
    • Nano Letters 17, 4725-4732 (2017)
    • DOI   Abstract  

      The promise of sp(2) nanomaterials remains immense, and ways to strategically combine and manipulate these nanostructures will further enhance their potential as well as advance nanotechnology as a whole. The scale of these structures requires precision at the atomic scale. In this sense electron microscopes are attractive as they offer both atomic imaging and a means to structurally modify structures. Here we show how Cr atoms can be used as physical linkers to connect carbon nanotubes and fullerenes to graphene. Crucially, while under electron irradiation, the Cr atoms can drive transformations such as catalytic healing of a hole in graphene with simultaneous transformation of a single wall carbon nanotube into a fullerene. The atomic resolution of the electron microscopy along with density functional theory based total energy calculations provide insight into the dynamic transformations of Cr atom linkers. The work augments the potential of transmission electron microscopes as nanolaboratories.

      @article{,
      author = {Liang Zhao and Huy Q. Ta and Arezoo Dianat and Akash Soni and Artem Fediai and Wajian Yin and Thomas Gemming and Barbara Trzebicka and Gianaurelio Cuniberti and Zhongfan Liu and Alicja Bachmatiuk and Mark H. Rummeli},
      title = {In Situ Electron Driven Carbon Nanopillar-Fullerene Transformation through Cr Atom Mediation},
      journal = {Nano Letters},
      volume = {17},
      number = {8},
      pages = {4725-4732},
      abstract = {The promise of sp(2) nanomaterials remains immense, and ways to strategically combine and manipulate these nanostructures will further enhance their potential as well as advance nanotechnology as a whole. The scale of these structures requires precision at the atomic scale. In this sense electron microscopes are attractive as they offer both atomic imaging and a means to structurally modify structures. Here we show how Cr atoms can be used as physical linkers to connect carbon nanotubes and fullerenes to graphene. Crucially, while under electron irradiation, the Cr atoms can drive transformations such as catalytic healing of a hole in graphene with simultaneous transformation of a single wall carbon nanotube into a fullerene. The atomic resolution of the electron microscopy along with density functional theory based total energy calculations provide insight into the dynamic transformations of Cr atom linkers. The work augments the potential of transmission electron microscopes as nanolaboratories.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acs.nanolett.7b01406},
      doi = {10.1021/acs.nanolett.7b01406},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • In-Plane Edge Magnetism in Graphene-Like Nanoribbons
    • S. Krompiewski, G. Cuniberti
    • Acta Physica Polonica A 131, 828-829 (2017)
    • DOI   Abstract  

      This paper is devoted to identification of the most important factors responsible for formation of magnetic moments at edges of graphene-like nanoribbons. The main role is attributed to the Hubbard correlations (within unrestricted Hartree-Pock approximation) and intrinsic spin orbit interactions, but additionally a perpendicular electric field is also taken into account. Of particular interest is the interplay of the in-plane edge magnetism and the energy band gap. It is shown that, with the increasing electric field, typically the following phases develop: magnetic insulator (with in-plane spins). nomnagnetic narrow-band semiconductor, and nonmagnetic band insulator.

      @article{,
      author = {S. Krompiewski and G. Cuniberti},
      title = {In-Plane Edge Magnetism in Graphene-Like Nanoribbons},
      journal = {Acta Physica Polonica A},
      volume = {131},
      number = {4},
      pages = {828-829},
      abstract = {This paper is devoted to identification of the most important factors responsible for formation of magnetic moments at edges of graphene-like nanoribbons. The main role is attributed to the Hubbard correlations (within unrestricted Hartree-Pock approximation) and intrinsic spin orbit interactions, but additionally a perpendicular electric field is also taken into account. Of particular interest is the interplay of the in-plane edge magnetism and the energy band gap. It is shown that, with the increasing electric field, typically the following phases develop: magnetic insulator (with in-plane spins). nomnagnetic narrow-band semiconductor, and nonmagnetic band insulator.},
      year = {2017},
      url = http://dx.doi.org/{10.12693/APhysPolA.131.828},
      doi = {10.12693/APhysPolA.131.828},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • In-Situ Stretching Patterned Graphene Nanoribbons in the Transmission Electron Microscope
    • Z. Liao, L. M. Sandonas, T. Zhang, M. Gall, A. Dianat, R. Gutierrez, U. Muehle, J. Gluch, R. Jordan, G. Cuniberti, E. Zschech
    • Scientific Reports 7(2017)
    • DOI   Abstract  

      The mechanical response of patterned graphene nanoribbons (GNRs) with a width less than 100 nm was studied in-situ using quantitative tensile testing in a transmission electron microscope (TEM). A high degree of crystallinity was confirmed for patterned nanoribbons before and after the in-situ experiment by selected area electron diffraction (SAED) patterns. However, the maximum local true strain of the nanoribbons was determined to be only about 3%. The simultaneously recorded low-loss electron energy loss spectrum (EELS) on the stretched nanoribbons did not reveal any bandgap opening. Density Functional Based Tight Binding (DFTB) simulation was conducted to predict a feasible bandgap opening as a function of width in GNRs at low strain. The bandgap of unstrained armchair graphene nanoribbons (AGNRs) vanished for a width of about 14.75 nm, and this critical width was reduced to 11.21 nm for a strain level of 2.2%. The measured low tensile failure strain may limit the practical capability of tuning the bandgap of patterned graphene nanostructures by strain engineering, and therefore, it should be considered in bandgap design for graphene-based electronic devices by strain engineering.

      @article{,
      author = {Zhongquan Liao and Leonardo Medrano Sandonas and Tao Zhang and Martin Gall and Arezoo Dianat and Rafael Gutierrez and Uwe Muehle and Juergen Gluch and Rainer Jordan and Gianaurelio Cuniberti and Ehrenfried Zschech},
      title = {In-Situ Stretching Patterned Graphene Nanoribbons in the Transmission Electron Microscope},
      journal = {Scientific Reports},
      volume = {7},
      abstract = {The mechanical response of patterned graphene nanoribbons (GNRs) with a width less than 100 nm was studied in-situ using quantitative tensile testing in a transmission electron microscope (TEM). A high degree of crystallinity was confirmed for patterned nanoribbons before and after the in-situ experiment by selected area electron diffraction (SAED) patterns. However, the maximum local true strain of the nanoribbons was determined to be only about 3%. The simultaneously recorded low-loss electron energy loss spectrum (EELS) on the stretched nanoribbons did not reveal any bandgap opening. Density Functional Based Tight Binding (DFTB) simulation was conducted to predict a feasible bandgap opening as a function of width in GNRs at low strain. The bandgap of unstrained armchair graphene nanoribbons (AGNRs) vanished for a width of about 14.75 nm, and this critical width was reduced to 11.21 nm for a strain level of 2.2%. The measured low tensile failure strain may limit the practical capability of tuning the bandgap of patterned graphene nanostructures by strain engineering, and therefore, it should be considered in bandgap design for graphene-based electronic devices by strain engineering.},
      year = {2017},
      url = http://dx.doi.org/{10.1038/s41598-017-00227-3},
      doi = {10.1038/s41598-017-00227-3},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid-Liquid Interface
    • M. E. Garah, E. Borre, A. Ciesielski, A. Dianat, R. Gutierrez, G. Cuniberti, S. Bellemin-Laponnaz, M. Mauro, P. Samori
    • Small 13(2017)
    • DOI   Abstract  

      The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive "smart" surfaces organized with an atomic precision.

      @article{,
      author = {Mohamed El Garah and Etienne Borre and Artur Ciesielski and Arezoo Dianat and Rafael Gutierrez and Gianaurelio Cuniberti and Stephane Bellemin-Laponnaz and Matteo Mauro and Paolo Samori},
      title = {Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid-Liquid Interface},
      journal = {Small},
      volume = {13},
      number = {40},
      abstract = {The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive "smart" surfaces organized with an atomic precision.},
      year = {2017},
      url = http://dx.doi.org/{10.1002/smll.201701790},
      doi = {10.1002/smll.201701790},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Method for the quantification of rupture probability in soft collagenous tissues
    • D. Balzani, T. Schmidt, M. Ortiz
    • International Journal for Numerical Methods in Biomedical Engineering 33(2017)
    • DOI   Abstract  

      A computational method is presented for the assessment of rupture probabilities in soft collagenous tissues. This may in particular be important for the quantitative analysis of medical diseases such as atherosclerotic arteries or abdominal aortic aneurysms, where an unidentified rupture has in most cases fatal consequences. The method is based on the numerical minimization and maximization of probabilities of failure, which arise from random input quantities, for example, tissue properties. Instead of assuming probability distributions for these quantities, which are typically unknown especially for soft collagenous tissues, only restricted knowledge of these distributions is taken into account. Given this limited statistical input data, the minimized/maximized probabilities represent optimal bounds on the rupture probability, which enable a quantitative estimation of potential risks of performing or not performing medical treatment. Although easily extendable to all kinds of mechanical rupture criteria, the approach presented here incorporates stretch-based and damage-based criteria. These are evaluated based on numerical simulations of loaded tissues, where continuum mechanical material formulations are considered, which capture the supra-physiological behavior of soft collagenous tissues. Numerical examples are provided demonstrating the applicability of the method in an overstretched atherosclerotic artery. Copyright (C) 2016 John Wiley & Sons, Ltd.

      @article{,
      author = {D. Balzani and T. Schmidt and M. Ortiz},
      title = {Method for the quantification of rupture probability in soft collagenous tissues},
      journal = {International Journal for Numerical Methods in Biomedical Engineering},
      volume = {33},
      number = {1},
      abstract = {A computational method is presented for the assessment of rupture probabilities in soft collagenous tissues. This may in particular be important for the quantitative analysis of medical diseases such as atherosclerotic arteries or abdominal aortic aneurysms, where an unidentified rupture has in most cases fatal consequences. The method is based on the numerical minimization and maximization of probabilities of failure, which arise from random input quantities, for example, tissue properties. Instead of assuming probability distributions for these quantities, which are typically unknown especially for soft collagenous tissues, only restricted knowledge of these distributions is taken into account. Given this limited statistical input data, the minimized/maximized probabilities represent optimal bounds on the rupture probability, which enable a quantitative estimation of potential risks of performing or not performing medical treatment. Although easily extendable to all kinds of mechanical rupture criteria, the approach presented here incorporates stretch-based and damage-based criteria. These are evaluated based on numerical simulations of loaded tissues, where continuum mechanical material formulations are considered, which capture the supra-physiological behavior of soft collagenous tissues. Numerical examples are provided demonstrating the applicability of the method in an overstretched atherosclerotic artery. Copyright (C) 2016 John Wiley & Sons, Ltd.},
      year = {2017},
      url = http://dx.doi.org/{10.1002/cnm.2781},
      doi = {10.1002/cnm.2781},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Method for the unique identification of hyperelastic material properties using full-field measures. Application to the passive myocardium material response
    • L. E. Perotti, A. V. S. Ponnaluri, S. Krishnamoorthi, D. Balzani, D. B. Ennis, W. S. Klug
    • International Journal for Numerical Methods in Biomedical Engineering 33(2017)
    • DOI   Abstract  

      Quantitative measurement of the material properties (eg, stiffness) of biological tissues is poised to become a powerful diagnostic tool. There are currently several methods in the literature to estimating material stiffness, and we extend this work by formulating a framework that leads to uniquely identified material properties. We design an approach to work with full-field displacement dataie, we assume the displacement field due to the applied forces is known both on the boundaries and also within the interior of the body of interestand seek stiffness parameters that lead to balanced internal and external forces in a model. For in vivo applications, the displacement data can be acquired clinically using magnetic resonance imaging while the forces may be computed from pressure measurements, eg, through catheterization. We outline a set of conditions under which the least-square force error objective function is convex, yielding uniquely identified material properties. An important component of our framework is a new numerical strategy to formulate polyconvex material energy laws that are linear in the material properties and provide one optimal description of the available experimental data. An outcome of our approach is the analysis of the reliability of the identified material properties, even for material laws that do not admit unique property identification. Lastly, we evaluate our approach using passive myocardium experimental data at the material point and show its application to identifying myocardial stiffness with an in silico experiment modeling the passive filling of the left ventricle.

      @article{,
      author = {Luigi E. Perotti and Aditya V. S. Ponnaluri and Shankarjee Krishnamoorthi and Daniel Balzani and Daniel B. Ennis and William S. Klug},
      title = {Method for the unique identification of hyperelastic material properties using full-field measures. Application to the passive myocardium material response},
      journal = {International Journal for Numerical Methods in Biomedical Engineering},
      volume = {33},
      number = {11},
      abstract = {Quantitative measurement of the material properties (eg, stiffness) of biological tissues is poised to become a powerful diagnostic tool. There are currently several methods in the literature to estimating material stiffness, and we extend this work by formulating a framework that leads to uniquely identified material properties. We design an approach to work with full-field displacement dataie, we assume the displacement field due to the applied forces is known both on the boundaries and also within the interior of the body of interestand seek stiffness parameters that lead to balanced internal and external forces in a model. For in vivo applications, the displacement data can be acquired clinically using magnetic resonance imaging while the forces may be computed from pressure measurements, eg, through catheterization. We outline a set of conditions under which the least-square force error objective function is convex, yielding uniquely identified material properties. An important component of our framework is a new numerical strategy to formulate polyconvex material energy laws that are linear in the material properties and provide one optimal description of the available experimental data. An outcome of our approach is the analysis of the reliability of the identified material properties, even for material laws that do not admit unique property identification. Lastly, we evaluate our approach using passive myocardium experimental data at the material point and show its application to identifying myocardial stiffness with an in silico experiment modeling the passive filling of the left ventricle.},
      year = {2017},
      url = http://dx.doi.org/{10.1002/cnm.2866},
      doi = {10.1002/cnm.2866},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Microscale simulation of adhesive and cohesive failure in rough interfaces
    • F. Hirsch, M. Kaestner
    • Engineering Fracture Mechanics 178, 416-432 (2017)
    • DOI   Abstract  

      Multi-material lightweight designs, e.g. the combination of aluminum with fiber reinforced composites, are a key feature for the development of innovative and resource efficient products. The connection properties of such bi-material interfaces are influenced by the geometric structure on different length scales. In this article a modeling strategy is presented to study the failure behavior of rough interfaces within a computational homogenization scheme. We study different local phenomena and their effects on the overall interface characteristics, e.g. the surface roughness and different local failure types as cohesive failure of the bulk material and adhesive failure of the local interface. Since there is a large separation in the length scales of the surface roughness, which is in the micrometer range, and conventional structural components, we employ a numerical homogenization approach to extract effective traction-separation laws to derive effective interface parameters. Adhesive interface failure is modeled by cohesive elements based on a traction separation law and cohesive failure of the bulk material is described by an elastic-plastic model with progressive damage evolution. (C) 2017 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Franz Hirsch and Markus Kaestner},
      title = {Microscale simulation of adhesive and cohesive failure in rough interfaces},
      journal = {Engineering Fracture Mechanics},
      volume = {178},
      pages = {416-432},
      abstract = {Multi-material lightweight designs, e.g. the combination of aluminum with fiber reinforced composites, are a key feature for the development of innovative and resource efficient products. The connection properties of such bi-material interfaces are influenced by the geometric structure on different length scales. In this article a modeling strategy is presented to study the failure behavior of rough interfaces within a computational homogenization scheme. We study different local phenomena and their effects on the overall interface characteristics, e.g. the surface roughness and different local failure types as cohesive failure of the bulk material and adhesive failure of the local interface. Since there is a large separation in the length scales of the surface roughness, which is in the micrometer range, and conventional structural components, we employ a numerical homogenization approach to extract effective traction-separation laws to derive effective interface parameters. Adhesive interface failure is modeled by cohesive elements based on a traction separation law and cohesive failure of the bulk material is described by an elastic-plastic model with progressive damage evolution. (C) 2017 Elsevier Ltd. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.engfracmech.2017.02.026},
      doi = {10.1016/j.engfracmech.2017.02.026},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Modeling and Simulation of Electrochemical Cells under Applied Voltage
    • M. Rossi, T. Wallmersperger, S. Neukamm, K. Padberg-Gehle
    • Electrochimica Acta 258, 241-254 (2017)
    • DOI   Abstract  

      The behavior of an electrochemical thin film under input voltage (potentiostatic) conditions is numerically investigated. Thin films are used in micro-batteries and proton-exchange-membrane fuel cells: these devices are expected to play a significant role in the next generation energy systems for use in vehicles as a replacement to combustion engines. The electrochemical investigation of thin films is a relevant topic for a wide range of applications such as hydrogels, ionic polymer metal composites, biological membranes, and treatment of tumors. In this work, a continuum-based model is presented in order to describe the behavior of thin membranes. The electrochemical behavior of thin membranes is usually hard to investigate with experiments. Therefore, numerical simulations are carried out in order to enable a better understanding of the chemical reactions occurring within microscopic regions at the electrode/electrolyte interfaces. Diffusive-migrative ionic fluxes and electric field distribution are considered. A one-dimensional domain is employed. The fully-coupled electrochemical field is given by the Poisson-Nernst-Planck equations. The model involves initial and interface/boundary conditions appropriate for an electrolytic/galvanic cell. The latter are the Stern layer conditions for polarization (or diffuse charge) effects and the Frumkin-Butler- Volmer equations for electrochemical kinetics of chemical reactions. Time-dependent numerical simulations within a finite element framework are performed using the commercial tools MATLAB and COMSOL Multiphysics. The results are consistent with the physical behavior of electrolytic cells under potentiostatic conditions. The time evolution of the main electrochemical parameters is in accordance with the imposed boundary/interface conditions. Interestingly, the ion flux and the electric field show slight asymmetries at the boundaries. Moreover, the model well predicts the behavior of systems, such as redox flow cells or rechargeable batteries, that can either run under applied voltage or applied current conditions. In fact, the field equations and the boundary conditions, presented here for electrolytic cells under applied voltage, can be applied also for galvanic cells under applied current. Equations and boundary conditions for applied voltage and applied current working conditions are presented in a compact form in order to emphasize differences and similarities. (c) 2017 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Marco Rossi and Thomas Wallmersperger and Stefan Neukamm and Kathrin Padberg-Gehle},
      title = {Modeling and Simulation of Electrochemical Cells under Applied Voltage},
      journal = {Electrochimica Acta},
      volume = {258},
      pages = {241-254},
      abstract = {The behavior of an electrochemical thin film under input voltage (potentiostatic) conditions is numerically investigated. Thin films are used in micro-batteries and proton-exchange-membrane fuel cells: these devices are expected to play a significant role in the next generation energy systems for use in vehicles as a replacement to combustion engines. The electrochemical investigation of thin films is a relevant topic for a wide range of applications such as hydrogels, ionic polymer metal composites, biological membranes, and treatment of tumors. In this work, a continuum-based model is presented in order to describe the behavior of thin membranes. The electrochemical behavior of thin membranes is usually hard to investigate with experiments. Therefore, numerical simulations are carried out in order to enable a better understanding of the chemical reactions occurring within microscopic regions at the electrode/electrolyte interfaces. Diffusive-migrative ionic fluxes and electric field distribution are considered. A one-dimensional domain is employed. The fully-coupled electrochemical field is given by the Poisson-Nernst-Planck equations. The model involves initial and interface/boundary conditions appropriate for an electrolytic/galvanic cell. The latter are the Stern layer conditions for polarization (or diffuse charge) effects and the Frumkin-Butler- Volmer equations for electrochemical kinetics of chemical reactions. Time-dependent numerical simulations within a finite element framework are performed using the commercial tools MATLAB and COMSOL Multiphysics. The results are consistent with the physical behavior of electrolytic cells under potentiostatic conditions. The time evolution of the main electrochemical parameters is in accordance with the imposed boundary/interface conditions. Interestingly, the ion flux and the electric field show slight asymmetries at the boundaries. Moreover, the model well predicts the behavior of systems, such as redox flow cells or rechargeable batteries, that can either run under applied voltage or applied current conditions. In fact, the field equations and the boundary conditions, presented here for electrolytic cells under applied voltage, can be applied also for galvanic cells under applied current. Equations and boundary conditions for applied voltage and applied current working conditions are presented in a compact form in order to emphasize differences and similarities. (c) 2017 Elsevier Ltd. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.electacta.2017.10.047},
      doi = {10.1016/j.electacta.2017.10.047},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Modeling of magnetic hystereses in soft MREs filled with NdFeB particles
    • K. A. Kalina, J. Brummund, P. Metsch, M. Kaestner, Y. D. Borin, J. M. Linke, S. Odenbach
    • Smart Materials and Structures 26(2017)
    • DOI   Abstract  

      Herein, we investigate the structure-property relationships of soft magnetorheological elastomers (MREs) filled with remanently magnetizable particles. The study is motivated from experimental results which indicate a large difference between the magnetization loops of soft MREs filled with NdFeB particles and the loops of such particles embedded in a comparatively stiff matrix, e.g. an epoxy resin. We present a microscale model for MREs based on a general continuum formulation of the magnetomechanical boundary value problem which is valid for finite strains. In particular, we develop an energetically consistent constitutive model for the hysteretic magnetization behavior of the magnetically hard particles. The microstructure is discretized and the problem is solved numerically in terms of a coupled nonlinear finite element approach. Since the local magnetic and mechanical fields are resolved explicitly inside the heterogeneous microstructure of the MRE, our model also accounts for interactions of particles close to each other. In order to connect the microscopic fields to effective macroscopic quantities of the MRE, a suitable computational homogenization scheme is used. Based on this modeling approach, it is demonstrated that the observable macroscopic behavior of the considered MREs results from the rotation of the embedded particles. Furthermore, the performed numerical simulations indicate that the reversion of the sample’s magnetization occurs due to a combination of particle rotations and internal domain conversion processes. All of our simulation results obtained for such materials are in a good qualitative agreement with the experiments.

      @article{,
      author = {K. A. Kalina and J. Brummund and P. Metsch and M. Kaestner and D. Yu Borin and J. M. Linke and S. Odenbach},
      title = {Modeling of magnetic hystereses in soft MREs filled with NdFeB particles},
      journal = {Smart Materials and Structures},
      volume = {26},
      number = {10},
      abstract = {Herein, we investigate the structure-property relationships of soft magnetorheological elastomers (MREs) filled with remanently magnetizable particles. The study is motivated from experimental results which indicate a large difference between the magnetization loops of soft MREs filled with NdFeB particles and the loops of such particles embedded in a comparatively stiff matrix, e.g. an epoxy resin. We present a microscale model for MREs based on a general continuum formulation of the magnetomechanical boundary value problem which is valid for finite strains. In particular, we develop an energetically consistent constitutive model for the hysteretic magnetization behavior of the magnetically hard particles. The microstructure is discretized and the problem is solved numerically in terms of a coupled nonlinear finite element approach. Since the local magnetic and mechanical fields are resolved explicitly inside the heterogeneous microstructure of the MRE, our model also accounts for interactions of particles close to each other. In order to connect the microscopic fields to effective macroscopic quantities of the MRE, a suitable computational homogenization scheme is used. Based on this modeling approach, it is demonstrated that the observable macroscopic behavior of the considered MREs results from the rotation of the embedded particles. Furthermore, the performed numerical simulations indicate that the reversion of the sample's magnetization occurs due to a combination of particle rotations and internal domain conversion processes. All of our simulation results obtained for such materials are in a good qualitative agreement with the experiments.},
      year = {2017},
      url = http://dx.doi.org/{10.1088/1361-665X/aa7f81},
      doi = {10.1088/1361-665X/aa7f81},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Modeling the effects of lanthanum, nitrogen, and fluorine treatments of Si-SiON-HfO2-TiN gate stacks in 28 nm high-k-metal gate technology
    • R. Leitsmann, F. Lazarevic, M. Drescher, E. Erben
    • Journal of Applied Physics 121(2017)
    • DOI   Abstract  

      We have carried out a combined experimental and theoretical study on the influence of lanthanum, nitrogen, and fluorine treatments on the electric properties of high-k metal gate (HKMG) devices. In particular, we have developed a theoretical gate stack model which is able to predict qualitatively and quantitatively the influence of nitrogen, fluorine, and lanthanum treatments on the characteristic electric properties of Si-SiON-HfO2 gate stacks. The combination of this theoretical model with experimental investigations of several differently treated HKMG devices allows the estimation of the amount of incorporated impurity atoms in different material layers. Furthermore, we propose an atomistic mechanism for the incorporation of lanthanum and fluorine impurity atoms and we can explain the results of recent leakage current measurements by a passivation of oxygen vacancies within the HfO2 layer. Published by AIP Publishing.

      @article{,
      author = {Roman Leitsmann and Florian Lazarevic and Maximilian Drescher and Elke Erben},
      title = {Modeling the effects of lanthanum, nitrogen, and fluorine treatments of Si-SiON-HfO2-TiN gate stacks in 28 nm high-k-metal gate technology},
      journal = {Journal of Applied Physics},
      volume = {121},
      number = {23},
      abstract = {We have carried out a combined experimental and theoretical study on the influence of lanthanum, nitrogen, and fluorine treatments on the electric properties of high-k metal gate (HKMG) devices. In particular, we have developed a theoretical gate stack model which is able to predict qualitatively and quantitatively the influence of nitrogen, fluorine, and lanthanum treatments on the characteristic electric properties of Si-SiON-HfO2 gate stacks. The combination of this theoretical model with experimental investigations of several differently treated HKMG devices allows the estimation of the amount of incorporated impurity atoms in different material layers. Furthermore, we propose an atomistic mechanism for the incorporation of lanthanum and fluorine impurity atoms and we can explain the results of recent leakage current measurements by a passivation of oxygen vacancies within the HfO2 layer. Published by AIP Publishing.},
      year = {2017},
      url = http://dx.doi.org/{10.1063/1.4986494},
      doi = {10.1063/1.4986494},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Molecular and Ionic Dipole Effects on the Electronic Properties of Si-/SiO2-Grafted Alkylamine Monolayers
    • A. Gankin, R. Sfez, E. Mervinetsky, J. Buchwald, A. Dianat, L. M. Sandonas, R. Gutierrez, G. Cuniberti, S. Yitzchaik
    • Acs Applied Materials & Interfaces 9, 44873-44879 (2017)
    • DOI   Abstract  

      In this work, we demonstrate the tunability of electronic properties of Si/SiO2 substrates by molecular and ionic surface modifications. The changes in the electronic properties such as the work function (WF) and electron affinity were experimentally measured by the contact potential difference technique and theoretically supported by density functional theory calculations. We attribute these molecular electronic effects mainly to the variations of molecular and surface dipoles of the ionic and neutral species. We have previously shown that for the alkylhalide monolayers, changing the tail group from Cl to I decreased the WF of the substrate. Here, we report on the opposite trend of WF changes, that is, the increase of the WF, obtained by using the anions of these halides from Cl- to I-. This trend was observed on self-assembled alkylammonium halide (-NH3+ X-, where X- = Cl-, Br-, or I-) monolayer-modified substrates. The monolayer’s formation was supported by ellipsometry measurements, X-ray photoelectron spectroscopy, and atomic force microscopy. Comparison of the theoretical and experimental data suggests that the ionic surface dipole depends mainly on the polarizability and the position of the counter halide anion along with the organization and packaging of the layer. The described ionic modification can be easily used for facile tailoring and design of the electronic properties Si/SiO2 substrates for various device applications.

      @article{,
      author = {Alma Gankin and Ruthy Sfez and Evgeniy Mervinetsky and Jorg Buchwald and Arezoo Dianat and Leonardo Medrano Sandonas and Rafael Gutierrez and Gianaurelio Cuniberti and Shlomo Yitzchaik},
      title = {Molecular and Ionic Dipole Effects on the Electronic Properties of Si-/SiO2-Grafted Alkylamine Monolayers},
      journal = {Acs Applied Materials & Interfaces},
      volume = {9},
      number = {51},
      pages = {44873-44879},
      abstract = {In this work, we demonstrate the tunability of electronic properties of Si/SiO2 substrates by molecular and ionic surface modifications. The changes in the electronic properties such as the work function (WF) and electron affinity were experimentally measured by the contact potential difference technique and theoretically supported by density functional theory calculations. We attribute these molecular electronic effects mainly to the variations of molecular and surface dipoles of the ionic and neutral species. We have previously shown that for the alkylhalide monolayers, changing the tail group from Cl to I decreased the WF of the substrate. Here, we report on the opposite trend of WF changes, that is, the increase of the WF, obtained by using the anions of these halides from Cl- to I-. This trend was observed on self-assembled alkylammonium halide (-NH3+ X-, where X- = Cl-, Br-, or I-) monolayer-modified substrates. The monolayer's formation was supported by ellipsometry measurements, X-ray photoelectron spectroscopy, and atomic force microscopy. Comparison of the theoretical and experimental data suggests that the ionic surface dipole depends mainly on the polarizability and the position of the counter halide anion along with the organization and packaging of the layer. The described ionic modification can be easily used for facile tailoring and design of the electronic properties Si/SiO2 substrates for various device applications.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acsami.7b12218},
      doi = {10.1021/acsami.7b12218},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Molecular Self-Assembly Driven by On-Surface Reduction: Anthracene and Tetracene on Au(111)
    • J. Krueger, F. Eisenhut, T. Lehmann, J. M. Alonso, J. Meyer, D. Skidin, R. Ohmann, D. A. Ryndyk, D. Perez, E. Guitian, D. Pena, F. Moresco, G. Cuniberti
    • Journal of Physical Chemistry C 121, 20353-20358 (2017)
    • DOI   Abstract  

      Epoxyacenes adsorbed on metal surfaces form acenes during thermally induced reduction in ultrahigh vacuum conditions. The incorporation of oxygen bridges into a hydrocarbon backbone leads to an enhanced stability of these molecular precursors under ambient condition; however, it has also a distinct influence on their adsorption and self assembly on metal surfaces. Here, a low-temperature scanning tunneling microscopy (LT-STM) study of two different epoxyacenes on the Au(111) surface at submonolayer coverage is presented. Both molecules show self-assembly based on hydrogen bonding. While for the molecules with a single epoxy moiety nanostructures of three molecules are formed, extended molecular networks are achieved with two epoxy moieties and a slightly higher surface coverage. Upon annealing at 390 K, the molecules are reduced to the respective acene; however, both systems keep a similar assembled structure. The experimental STM images supported by theoretical calculations show that the self-assembly of the on-surface fabricated acenes is greatly influenced by the on-surface reaction and strongly differs from the adsorption pattern of directly deposited acenes, highlighting the importance of the cleaved oxygen in the self-assembly.

      @article{,
      author = {Justus Krueger and Frank Eisenhut and Thomas Lehmann and Jose M. Alonso and Joerg Meyer and Dmitry Skidin and Robin Ohmann and Dmitry A. Ryndyk and Dolores Perez and Enrique Guitian and Diego Pena and Francesca Moresco and Gianaurelio Cuniberti},
      title = {Molecular Self-Assembly Driven by On-Surface Reduction: Anthracene and Tetracene on Au(111)},
      journal = {Journal of Physical Chemistry C},
      volume = {121},
      number = {37},
      pages = {20353-20358},
      abstract = {Epoxyacenes adsorbed on metal surfaces form acenes during thermally induced reduction in ultrahigh vacuum conditions. The incorporation of oxygen bridges into a hydrocarbon backbone leads to an enhanced stability of these molecular precursors under ambient condition; however, it has also a distinct influence on their adsorption and self assembly on metal surfaces. Here, a low-temperature scanning tunneling microscopy (LT-STM) study of two different epoxyacenes on the Au(111) surface at submonolayer coverage is presented. Both molecules show self-assembly based on hydrogen bonding. While for the molecules with a single epoxy moiety nanostructures of three molecules are formed, extended molecular networks are achieved with two epoxy moieties and a slightly higher surface coverage. Upon annealing at 390 K, the molecules are reduced to the respective acene; however, both systems keep a similar assembled structure. The experimental STM images supported by theoretical calculations show that the self-assembly of the on-surface fabricated acenes is greatly influenced by the on-surface reaction and strongly differs from the adsorption pattern of directly deposited acenes, highlighting the importance of the cleaved oxygen in the self-assembly.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acs.jpcc.7b06131},
      doi = {10.1021/acs.jpcc.7b06131},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Morphological Evolution of Pit-Patterned Si(001) Substrates Driven by Surface-Energy Reduction
    • M. Salvalaglio, R. Backofen, A. Voigt, F. Montalenti
    • Nanoscale Research Letters 12(2017)
    • DOI   Abstract  

      Lateral ordering of heteroepitaxial islands can be conveniently achieved by suitable pit-patterning of the substrate prior to deposition. Controlling shape, orientation, and size of the pits is not trivial as, being metastable, they can significantly evolve during deposition/annealing. In this paper, we exploit a continuum model to explore the typical metastable pit morphologies that can be expected on Si(001), depending on the initial depth/shape. Evolution is predicted using a surface-diffusion model, formulated in a phase-field framework, and tackling surface-energy anisotropy. Results are shown to nicely reproduce typical metastable shapes reported in the literature. Moreover, long time scale evolutions of pit profiles with different depths are found to follow a similar kinetic pathway. The model is also exploited to treat the case of heteroepitaxial growth involving two materials characterized by different facets in their equilibrium Wulff’s shape. This can lead to significant changes in morphologies, such as a rotation of the pit during deposition as evidenced in Ge/Si experiments.

      @article{,
      author = {Marco Salvalaglio and Rainer Backofen and Axel Voigt and Francesco Montalenti},
      title = {Morphological Evolution of Pit-Patterned Si(001) Substrates Driven by Surface-Energy Reduction},
      journal = {Nanoscale Research Letters},
      volume = {12},
      abstract = {Lateral ordering of heteroepitaxial islands can be conveniently achieved by suitable pit-patterning of the substrate prior to deposition. Controlling shape, orientation, and size of the pits is not trivial as, being metastable, they can significantly evolve during deposition/annealing. In this paper, we exploit a continuum model to explore the typical metastable pit morphologies that can be expected on Si(001), depending on the initial depth/shape. Evolution is predicted using a surface-diffusion model, formulated in a phase-field framework, and tackling surface-energy anisotropy. Results are shown to nicely reproduce typical metastable shapes reported in the literature. Moreover, long time scale evolutions of pit profiles with different depths are found to follow a similar kinetic pathway. The model is also exploited to treat the case of heteroepitaxial growth involving two materials characterized by different facets in their equilibrium Wulff's shape. This can lead to significant changes in morphologies, such as a rotation of the pit during deposition as evidenced in Ge/Si experiments.},
      year = {2017},
      url = http://dx.doi.org/{10.1186/s11671-017-2320-5},
      doi = {10.1186/s11671-017-2320-5},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Nearest-neighbor Kitaev exchange blocked by charge order in electron-doped alpha-RuCl3
    • A. Koitzsch, C. Habenicht, E. Mueller, M. Knupfer, B. Buechner, S. Kretschmer, M. Richter, J. van den Brink, F. Boerrnert, D. Nowak, A. Isaeva, T. Doert
    • Physical Review Materials 1(2017)
    • DOI   Abstract  

      A quantum spin liquid might be realized in alpha-RuCl3, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, alpha-RuCl3 is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at K0.5RuCl3. This gives rise to a peculiar charge disproportionation into formally Ru2+ (4d(6)) and Ru3+ (4d(5)). Every Ru 4d(5) site with one hole in the t(2g) shell is surrounded by nearest neighbors of 4d(6) character, where the t(2g) level is full and magnetically inert. Thus, each type of Ru site forms a triangular lattice, and nearest-neighbor interactions of the original honeycomb are blocked.

      @article{,
      author = {A. Koitzsch and C. Habenicht and E. Mueller and M. Knupfer and B. Buechner and S. Kretschmer and M. Richter and J. van den Brink and F. Boerrnert and D. Nowak and A. Isaeva and Th Doert},
      title = {Nearest-neighbor Kitaev exchange blocked by charge order in electron-doped alpha-RuCl3},
      journal = {Physical Review Materials},
      volume = {1},
      number = {5},
      abstract = {A quantum spin liquid might be realized in alpha-RuCl3, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, alpha-RuCl3 is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at K0.5RuCl3. This gives rise to a peculiar charge disproportionation into formally Ru2+ (4d(6)) and Ru3+ (4d(5)). Every Ru 4d(5) site with one hole in the t(2g) shell is surrounded by nearest neighbors of 4d(6) character, where the t(2g) level is full and magnetically inert. Thus, each type of Ru site forms a triangular lattice, and nearest-neighbor interactions of the original honeycomb are blocked.},
      year = {2017},
      url = http://dx.doi.org/{10.1103/PhysRevMaterials.1.052001},
      doi = {10.1103/PhysRevMaterials.1.052001},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Nitrogen Engineering in the Ultrathin SiO2 Interface Layer of High-k CMOS Devices: A First-Principles Investigation of Fluorine, Oxygen, and Boron Defect Migration
    • F. Lazarevic, R. Leitsmann, M. Drescher, E. Erben, P. Plaenitz, M. Schreiber
    • Ieee Transactions on Electron Devices 64, 5073-5080 (2017)
    • DOI   Abstract  

      The further development of future semiconductor devices necessitates methods for characterization on an atomic scale. This ab initio investigation reveals consequences of nitrogen treatment of the state-of-the-art high-k gate-stacks. The model allows a profound characterization of the SiO2 interface layer for different impurity concentrations. The presented results explain recent experimental observations qualitatively as well as quantitatively. Beyond that, a fundamental understanding is given, which can be used as an essential instrument for future reliability engineering.

      @article{,
      author = {Florian Lazarevic and Roman Leitsmann and Maximilian Drescher and Elke Erben and Philipp Plaenitz and Michael Schreiber},
      title = {Nitrogen Engineering in the Ultrathin SiO2 Interface Layer of High-k CMOS Devices: A First-Principles Investigation of Fluorine, Oxygen, and Boron Defect Migration},
      journal = {Ieee Transactions on Electron Devices},
      volume = {64},
      number = {12},
      pages = {5073-5080},
      abstract = {The further development of future semiconductor devices necessitates methods for characterization on an atomic scale. This ab initio investigation reveals consequences of nitrogen treatment of the state-of-the-art high-k gate-stacks. The model allows a profound characterization of the SiO2 interface layer for different impurity concentrations. The presented results explain recent experimental observations qualitatively as well as quantitatively. Beyond that, a fundamental understanding is given, which can be used as an essential instrument for future reliability engineering.},
      year = {2017},
      url = http://dx.doi.org/{10.1109/ted.2017.2766083},
      doi = {10.1109/ted.2017.2766083},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • On-Surface Annulation Reaction Cascade for the Selective Synthesis of Diindenopyrene
    • F. Eisenhut, T. Lehmann, A. Viertel, D. Skidin, J. Kruger, S. Nikipar, D. A. Ryndyk, C. Joachim, S. Hecht, F. Moresco, G. Cuniberti
    • Acs Nano 11, 12419-12425 (2017)
    • DOI   Abstract  

      We investigated the thermally induced on surface cyclization of 4,10-bis(2′-bromo-4′-methylpheny1)1,3-dimethylpyrene to form the previously unknown, nonalternant polyaromatic hydrocarbon diindeno[1,2,3-cd:1′,2′,3′-mn]pyrene on Au(111) using scanning tunneling microscopy and spectroscopy. The observed unimolecular reaction involves thermally induced debromination followed by selective ring closure to fuse the neighboring benzene moieties via a five-membered ring. The structure of the product has been verified experimentally as well as theoretically. Our results demonstrate that on-surface reactions give rise to unusual chemical reactivities and selectivities and provide access to nonalternant polyaromatic molecules.

      @article{,
      author = {Frank Eisenhut and Thomas Lehmann and Andreas Viertel and Dmitry Skidin and Justus Kruger and Seddigheh Nikipar and Dmitry A. Ryndyk and Christian Joachim and Stefan Hecht and Francesca Moresco and Gianaurelio Cuniberti},
      title = {On-Surface Annulation Reaction Cascade for the Selective Synthesis of Diindenopyrene},
      journal = {Acs Nano},
      volume = {11},
      number = {12},
      pages = {12419-12425},
      abstract = {We investigated the thermally induced on surface cyclization of 4,10-bis(2'-bromo-4'-methylpheny1)1,3-dimethylpyrene to form the previously unknown, nonalternant polyaromatic hydrocarbon diindeno[1,2,3-cd:1',2',3'-mn]pyrene on Au(111) using scanning tunneling microscopy and spectroscopy. The observed unimolecular reaction involves thermally induced debromination followed by selective ring closure to fuse the neighboring benzene moieties via a five-membered ring. The structure of the product has been verified experimentally as well as theoretically. Our results demonstrate that on-surface reactions give rise to unusual chemical reactivities and selectivities and provide access to nonalternant polyaromatic molecules.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acsnano.7b06459},
      doi = {10.1021/acsnano.7b06459},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Photocatalytic degradation of recalcitrant micropollutants by reusable Fe3O4/SiO2/TiO2 particles
    • S. Teixeira, H. Mora, L. Blasse, P. M. Martins, S. A. C. Carabineiro, S. Lanceros-Mendez, K. Kuehn, G. Cuniberti
    • Journal of Photochemistry and Photobiology a-Chemistry 345, 27-35 (2017)
    • DOI   Abstract  

      Photocatalysis promotes the degradation of contaminants in water, transforming them into by-products with lower or no toxicity. The photocatalysts can be applied in suspension or immobilized onto a support. The aim of using the immobilized form against the suspension form is that the costly extra final filtration process can be avoided, which is particularly important in water decontamination. This work reports on reusable Fe3O4/SiO2/TiO2 particles and the assessment of their photocatalytic activity on the degradation of methylene blue (MB), ciprofloxacin (CIP), norfloxacin (NOR) and ibuprofen (IBP). To achieve the most efficient photocatalyst it is necessary to determine the optimal thermal treatment parameters therefore, the Fe3O4/SiO2/TiO2 particles were calcined at different temperatures (500 degrees C and 600 degrees C) and times (30 min and 60 min). Higher degradation rates were achieved with calcination at 600 degrees C, reaching a total degradation of CIP, NOR, and MB, and half of IBP. The reuse of the magnetic particles is an economic and eco-friendly way to treat polluted water. The produced Fe3O4/SiO2/TiO2 particles showed a remarkable photocatalytic degradation of recalcitrant micropollutants, without significant efficiency loss after five uses. In addition, no other works reporting on the degradation of recalcitrant micropollutants using similar magnetic particles were found in the literature. (C) 2017 Elsevier B.V. All rights reserved.

      @article{,
      author = {Sara Teixeira and H. Mora and Lisa-Marie Blasse and P. M. Martins and S. A. C. Carabineiro and S. Lanceros-Mendez and Klaus Kuehn and Gianaurelio Cuniberti},
      title = {Photocatalytic degradation of recalcitrant micropollutants by reusable Fe3O4/SiO2/TiO2 particles},
      journal = {Journal of Photochemistry and Photobiology a-Chemistry},
      volume = {345},
      pages = {27-35},
      abstract = {Photocatalysis promotes the degradation of contaminants in water, transforming them into by-products with lower or no toxicity. The photocatalysts can be applied in suspension or immobilized onto a support. The aim of using the immobilized form against the suspension form is that the costly extra final filtration process can be avoided, which is particularly important in water decontamination. This work reports on reusable Fe3O4/SiO2/TiO2 particles and the assessment of their photocatalytic activity on the degradation of methylene blue (MB), ciprofloxacin (CIP), norfloxacin (NOR) and ibuprofen (IBP). To achieve the most efficient photocatalyst it is necessary to determine the optimal thermal treatment parameters therefore, the Fe3O4/SiO2/TiO2 particles were calcined at different temperatures (500 degrees C and 600 degrees C) and times (30 min and 60 min). Higher degradation rates were achieved with calcination at 600 degrees C, reaching a total degradation of CIP, NOR, and MB, and half of IBP. The reuse of the magnetic particles is an economic and eco-friendly way to treat polluted water. The produced Fe3O4/SiO2/TiO2 particles showed a remarkable photocatalytic degradation of recalcitrant micropollutants, without significant efficiency loss after five uses. In addition, no other works reporting on the degradation of recalcitrant micropollutants using similar magnetic particles were found in the literature. (C) 2017 Elsevier B.V. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.jphotochem.2017.05.024},
      doi = {10.1016/j.jphotochem.2017.05.024},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Photoisomers of Azobenzene Star with a Flat Core: Theoretical Insights into Multiple States from DFT and MD Perspective
    • M. Koch, M. Saphiannikova, S. Santer, O. Guskova
    • Journal of Physical Chemistry B 121, 8854-8867 (2017)
    • DOI   Abstract  

      This study focuses on comparing physical properties of photoisomers of an azobenzene star with benzene-1,3,5-tricarboxamide core. Three azobenzene arms of the molecule undergo a reversible trans-cis isomerization upon UV-vis light illumination giving rise to multiple states from the planar all-trans one, via two mixed states to the kinked all-cis isomer. Employing density functional theory, we characterize the structural and photophysical properties of each state indicating a role the planar core plays in the coupling between azobenzene chromophores. To characterize the light-triggered switching of solvophilicity/solvophobicity of the star, the difference in solvation free energy is calculated for the transfer of an azobenzene star from its gas phase to implicit or explicit solvents. For the latter case, classical all-atom molecular dynamics simulations of aqueous solutions of azobenzene star are performed employing the polymer consistent force field to shed light on the thermodynamics of explicit hydration as a function of the isomerization state and on the structuring of water around the star. From the analysis of two contributions to the free energy of hydration, the nonpolar van der Waals and the electrostatic terms, it is concluded that isomerization specificity largely determines the polarity of the molecule and the solute-solvent electrostatic interactions. This convertible hydrophilicity/hydrophobicity together with readjustable occupied volume and the surface area accessible to water, affects the self-assembly/disassembly of the azobenzene star with a flat core triggered by light.

      @article{,
      author = {Markus Koch and Marina Saphiannikova and Svetlana Santer and Olga Guskova},
      title = {Photoisomers of Azobenzene Star with a Flat Core: Theoretical Insights into Multiple States from DFT and MD Perspective},
      journal = {Journal of Physical Chemistry B},
      volume = {121},
      number = {37},
      pages = {8854-8867},
      abstract = {This study focuses on comparing physical properties of photoisomers of an azobenzene star with benzene-1,3,5-tricarboxamide core. Three azobenzene arms of the molecule undergo a reversible trans-cis isomerization upon UV-vis light illumination giving rise to multiple states from the planar all-trans one, via two mixed states to the kinked all-cis isomer. Employing density functional theory, we characterize the structural and photophysical properties of each state indicating a role the planar core plays in the coupling between azobenzene chromophores. To characterize the light-triggered switching of solvophilicity/solvophobicity of the star, the difference in solvation free energy is calculated for the transfer of an azobenzene star from its gas phase to implicit or explicit solvents. For the latter case, classical all-atom molecular dynamics simulations of aqueous solutions of azobenzene star are performed employing the polymer consistent force field to shed light on the thermodynamics of explicit hydration as a function of the isomerization state and on the structuring of water around the star. From the analysis of two contributions to the free energy of hydration, the nonpolar van der Waals and the electrostatic terms, it is concluded that isomerization specificity largely determines the polarity of the molecule and the solute-solvent electrostatic interactions. This convertible hydrophilicity/hydrophobicity together with readjustable occupied volume and the surface area accessible to water, affects the self-assembly/disassembly of the azobenzene star with a flat core triggered by light.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acs.jpcb.7b07350},
      doi = {10.1021/acs.jpcb.7b07350},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Polycyclic heteroaromatic hydrocarbons containing a benzoisoindole core
    • M. Richter, K. S. Schellhammer, P. Machata, G. Cuniberti, A. Popov, F. Ortmann, R. Berger, K. Muellen, X. Feng
    • Organic Chemistry Frontiers 4, 847-852 (2017)
    • DOI   Abstract  

      By the combination of 9a-azaphenalene and a perpendicularly oriented acene, we have synthesized three derivatives of a series of novel, fully-conjugated nitrogen-containing polycyclic aromatic hydrocarbons (PAHs), namely [7,8] naphtho[2′, 3′: 1,2] indolizino[6,5,4,3-def] phenanthridine, with an acetylene triisopropylsilyl (TIPS), phenyl or benzothiophenyl substituent. Their optoelectronic properties were studied via UV-Vis-NIR absorption, fluorescence spectroscopy and cyclic voltammetry. In addition, in situ spectroelectrochemistry was performed to investigate the optical and magnetic properties of the mono-radical cation and anion by quasi-reversible oxidation and reduction of 11-(tert-butyl)-5,17-bis((triisopropylsilyl) ethynyl)[7,8] naphtho[2′, 3′: 1,2] indolizino[6,5,4,3-def] phenanthridine (1a). Theoretical modelling confirmed the predominately closed-shell electronic ground state with a weak diradical character depending on the geometry.

      @article{,
      author = {Marcus Richter and Karl Sebastian Schellhammer and Peter Machata and Gianaurelio Cuniberti and Alexey Popov and Frank Ortmann and Reinhard Berger and Klaus Muellen and Xinliang Feng},
      title = {Polycyclic heteroaromatic hydrocarbons containing a benzoisoindole core},
      journal = {Organic Chemistry Frontiers},
      volume = {4},
      number = {5},
      pages = {847-852},
      abstract = {By the combination of 9a-azaphenalene and a perpendicularly oriented acene, we have synthesized three derivatives of a series of novel, fully-conjugated nitrogen-containing polycyclic aromatic hydrocarbons (PAHs), namely [7,8] naphtho[2', 3': 1,2] indolizino[6,5,4,3-def] phenanthridine, with an acetylene triisopropylsilyl (TIPS), phenyl or benzothiophenyl substituent. Their optoelectronic properties were studied via UV-Vis-NIR absorption, fluorescence spectroscopy and cyclic voltammetry. In addition, in situ spectroelectrochemistry was performed to investigate the optical and magnetic properties of the mono-radical cation and anion by quasi-reversible oxidation and reduction of 11-(tert-butyl)-5,17-bis((triisopropylsilyl) ethynyl)[7,8] naphtho[2', 3': 1,2] indolizino[6,5,4,3-def] phenanthridine (1a). Theoretical modelling confirmed the predominately closed-shell electronic ground state with a weak diradical character depending on the geometry.},
      year = {2017},
      url = http://dx.doi.org/{10.1039/c7qo00180k},
      doi = {10.1039/c7qo00180k},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • The relaxed-polar mechanism of locally optimal Cosserat rotations for an idealized nanoindentation and comparison with 3D-EBSD experiments
    • A. Fischle, P. Neff, D. Raabe
    • Zeitschrift Fur Angewandte Mathematik Und Physik 68(2017)
    • DOI   Abstract  

      The rotation polar(F) is an element of SO(3) arises as the unique orthogonal factor of the right polar decomposition F = polar(F) U of a given invertible matrix F is an element of GL(+)(3). In the context of nonlinear elasticity Grioli (Boll Un Math Ital 2:252-255, 1940) discovered a geometric variational characterization of polar(F) as a unique energy-minimizing rotation. In preceding works, we have analyzed a generalization of Grioli’s variational approach with weights (material parameters) mu > 0 and mu(c) >= 0 (Grioli: mu = mu(c)). The energy subject to minimization coincides with the Cosserat shear-stretch contribution arising in any geometrically nonlinear, isotropic and quadratic Cosserat continuum model formulated in the deformation gradient field F := del phi : Omega -> GL(+)(3) and the microrotation field R : Omega -> SO(3). The corresponding set of non-classical energy-minimizing rotations rpolar(mu,mu c)(+/-) (F) := arg min (R is an element of SO(3)) {W-mu,W-mu c (R; F) := mu parallel to sym(R-T F – 1)parallel to(2) + mu(c)parallel to skew(R-T F – 1)parallel to(2)} represents a new relaxed-polar mechanism. Our goal is to motivate this mechanism by presenting it in a relevant setting. To this end, we explicitly construct a deformation mapping phi(nano) which models an idealized nanoindentation and compare the corresponding optimal rotation patterns rpolar(1,0)(+/-)(F-nano) with experimentally obtained 3D-EBSD measurements of the disorientation angle of lattice rotations due to a nanoindentation in solid copper. We observe that the non-classical relaxed-polar mechanism can produce interesting counter-rotations. A possible link between Cosserat theory and finite multiplicative plasticity theory on small scales is also explored.

      @article{,
      author = {Andreas Fischle and Patrizio Neff and Dierk Raabe},
      title = {The relaxed-polar mechanism of locally optimal Cosserat rotations for an idealized nanoindentation and comparison with 3D-EBSD experiments},
      journal = {Zeitschrift Fur Angewandte Mathematik Und Physik},
      volume = {68},
      number = {4},
      abstract = {The rotation polar(F) is an element of SO(3) arises as the unique orthogonal factor of the right polar decomposition F = polar(F) U of a given invertible matrix F is an element of GL(+)(3). In the context of nonlinear elasticity Grioli (Boll Un Math Ital 2:252-255, 1940) discovered a geometric variational characterization of polar(F) as a unique energy-minimizing rotation. In preceding works, we have analyzed a generalization of Grioli's variational approach with weights (material parameters) mu > 0 and mu(c) >= 0 (Grioli: mu = mu(c)). The energy subject to minimization coincides with the Cosserat shear-stretch contribution arising in any geometrically nonlinear, isotropic and quadratic Cosserat continuum model formulated in the deformation gradient field F := del phi : Omega -> GL(+)(3) and the microrotation field R : Omega -> SO(3). The corresponding set of non-classical energy-minimizing rotations rpolar(mu,mu c)(+/-) (F) := arg min (R is an element of SO(3)) {W-mu,W-mu c (R; F) := mu parallel to sym(R-T F - 1)parallel to(2) + mu(c)parallel to skew(R-T F - 1)parallel to(2)} represents a new relaxed-polar mechanism. Our goal is to motivate this mechanism by presenting it in a relevant setting. To this end, we explicitly construct a deformation mapping phi(nano) which models an idealized nanoindentation and compare the corresponding optimal rotation patterns rpolar(1,0)(+/-)(F-nano) with experimentally obtained 3D-EBSD measurements of the disorientation angle of lattice rotations due to a nanoindentation in solid copper. We observe that the non-classical relaxed-polar mechanism can produce interesting counter-rotations. A possible link between Cosserat theory and finite multiplicative plasticity theory on small scales is also explored.},
      year = {2017},
      url = http://dx.doi.org/{10.1007/s00033-017-0834-4},
      doi = {10.1007/s00033-017-0834-4},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Simulation of self-piercing rivetting processes in fibre reinforced polymers: Material modelling and parameter identification
    • F. Hirsch, S. Mueller, M. Machens, R. Staschko, N. Fuchs, M. Kaestner
    • Journal of Materials Processing Technology 241, 164-177 (2017)
    • DOI   Abstract  

      This paper addresses the numerical simulation of self-piercing rivetting processes to join fibre reinforced polymers and sheet metals. Special emphasis is placed on the modelling of the deformation and failure behaviour of the composite material. Different from the simulation of rivetting processes in metals, which requires the modelling of large plastic deformations, the mechanical response of composites is typically governed by intra- and interlaminar damage phenomena. Depending on the polymeric matrix, viscoelastic effects can interfere particularly with the long-term behaviour of the joint. We propose a systematic approach to the modelling of composite laminates, discuss limitations of the used model, and present details of parameter identification. Homogenisation techniques are applied to predict the mechanical behaviour of the composite in terms of effective anisotropic elastic and viscoelastic material properties. In combination with a continuum damage approach this model represents the deformation and failure behaviour of individual laminae. Cohesive zones enable the modelling of delamination processes. The parameters of the latter models are identified from experiments. The defined material model for the composite is eventually utilised in the simulation of an exemplary self-piercing rivetting process. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {Franz Hirsch and Sebastian Mueller and Michael Machens and Robert Staschko and Normen Fuchs and Markus Kaestner},
      title = {Simulation of self-piercing rivetting processes in fibre reinforced polymers: Material modelling and parameter identification},
      journal = {Journal of Materials Processing Technology},
      volume = {241},
      pages = {164-177},
      abstract = {This paper addresses the numerical simulation of self-piercing rivetting processes to join fibre reinforced polymers and sheet metals. Special emphasis is placed on the modelling of the deformation and failure behaviour of the composite material. Different from the simulation of rivetting processes in metals, which requires the modelling of large plastic deformations, the mechanical response of composites is typically governed by intra- and interlaminar damage phenomena. Depending on the polymeric matrix, viscoelastic effects can interfere particularly with the long-term behaviour of the joint. We propose a systematic approach to the modelling of composite laminates, discuss limitations of the used model, and present details of parameter identification. Homogenisation techniques are applied to predict the mechanical behaviour of the composite in terms of effective anisotropic elastic and viscoelastic material properties. In combination with a continuum damage approach this model represents the deformation and failure behaviour of individual laminae. Cohesive zones enable the modelling of delamination processes. The parameters of the latter models are identified from experiments. The defined material model for the composite is eventually utilised in the simulation of an exemplary self-piercing rivetting process. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2017},
      url = http://dx.doi.org/{10.1016/j.jmatprotec.2016.10.010},
      doi = {10.1016/j.jmatprotec.2016.10.010},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Spin-orbit coupling in nearly metallic chiral carbon nanotubes: a density-functional based study
    • V. V. Maslyuk, R. Gutierrez, G. Cuniberti
    • Physical Chemistry Chemical Physics 19, 8848-8853 (2017)
    • DOI   Abstract  

      Spin-orbit interaction in carbon nanotubes has been under debate for several years and a variety of theoretical calculations and experimental results have been published. Here, we present an accurate implementation of spin-orbit interactions in a density-functional theory framework including both core and valence orbital contributions, thus using the full potential of the system. We find that the spin-splitting of the frontier bands of armchair nanotubes is of the order of several mu eV and does not strongly depend on the diameter of the nanotube. We also provide a systematic analysis of the band splitting in chiral nanotubes as a function of the diameter and the chiral angle. Very good agreement with previous theoretical studies and experimental results is overall found. In particular, our approach can be of great relevance in view of the recently discovered chirality-induced spin selectivity, since it allows us to include not only atomic contributions to the spin-orbit interaction, but more importantly, global contributions to the potential arising from the geometric structure (topology) of the system. Our methodology can thus encode effects such as helical symmetry in a straightforward way.

      @article{,
      author = {Volodymyr V. Maslyuk and Rafael Gutierrez and Gianaurelio Cuniberti},
      title = {Spin-orbit coupling in nearly metallic chiral carbon nanotubes: a density-functional based study},
      journal = {Physical Chemistry Chemical Physics},
      volume = {19},
      number = {13},
      pages = {8848-8853},
      abstract = {Spin-orbit interaction in carbon nanotubes has been under debate for several years and a variety of theoretical calculations and experimental results have been published. Here, we present an accurate implementation of spin-orbit interactions in a density-functional theory framework including both core and valence orbital contributions, thus using the full potential of the system. We find that the spin-splitting of the frontier bands of armchair nanotubes is of the order of several mu eV and does not strongly depend on the diameter of the nanotube. We also provide a systematic analysis of the band splitting in chiral nanotubes as a function of the diameter and the chiral angle. Very good agreement with previous theoretical studies and experimental results is overall found. In particular, our approach can be of great relevance in view of the recently discovered chirality-induced spin selectivity, since it allows us to include not only atomic contributions to the spin-orbit interaction, but more importantly, global contributions to the potential arising from the geometric structure (topology) of the system. Our methodology can thus encode effects such as helical symmetry in a straightforward way.},
      year = {2017},
      url = http://dx.doi.org/{10.1039/c7cp00059f},
      doi = {10.1039/c7cp00059f},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • A Stable Saddle-Shaped Polycyclic Hydrocarbon with an Open-Shell Singlet Ground State
    • J. Ma, J. Liu, M. Baumgarten, Y. Fu, Y. Tan, K. S. Schellhammer, F. Ortmann, G. Cuniberti, H. Komber, R. Berger, K. Muellen, X. Feng
    • Angewandte Chemie-International Edition 56, 3280-3284 (2017)
    • DOI   Abstract  

      Diindeno-fused bischrysene, a new diindeno-based polycyclic hydrocarbon (PH), was synthesized and characterized. It was elucidated in detailed experimental and theoretical studies that this cyclopenta-fused PH possesses an open-shell singlet biradical structure in the ground state and exhibits high stability under ambient conditions (t(1/2) = 39 days). The crystal structure unambiguously shows a novel saddle-shaped pi-conjugated carbon skeleton due to the steric hindrance of the central cove-edged bischrysene unit. UV/Vis spectral measurements revealed that the title molecule has a very narrow optical energy gap of 0.92 eV, which is consistent with the electrochemical analysis and further supported by density functional theory (DFT) calculations.

      @article{,
      author = {Ji Ma and Junzhi Liu and Martin Baumgarten and Yubin Fu and Yuan-Zhi Tan and Karl Sebastian Schellhammer and Frank Ortmann and Gianaurelio Cuniberti and Hartmut Komber and Reinhard Berger and Klaus Muellen and Xinliang Feng},
      title = {A Stable Saddle-Shaped Polycyclic Hydrocarbon with an Open-Shell Singlet Ground State},
      journal = {Angewandte Chemie-International Edition},
      volume = {56},
      number = {12},
      pages = {3280-3284},
      abstract = {Diindeno-fused bischrysene, a new diindeno-based polycyclic hydrocarbon (PH), was synthesized and characterized. It was elucidated in detailed experimental and theoretical studies that this cyclopenta-fused PH possesses an open-shell singlet biradical structure in the ground state and exhibits high stability under ambient conditions (t(1/2) = 39 days). The crystal structure unambiguously shows a novel saddle-shaped pi-conjugated carbon skeleton due to the steric hindrance of the central cove-edged bischrysene unit. UV/Vis spectral measurements revealed that the title molecule has a very narrow optical energy gap of 0.92 eV, which is consistent with the electrochemical analysis and further supported by density functional theory (DFT) calculations.},
      year = {2017},
      url = http://dx.doi.org/{10.1002/anie.201611689},
      doi = {10.1002/anie.201611689},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Switchable Multiple Spin States in the Kondo description of Doped Molecular Magnets
    • R. Ray, S. Kumar
    • Scientific Reports 7(2017)
    • DOI   Abstract  

      We show that introducing electrons in magnetic clusters and molecular magnets lead to rich phase diagrams with a variety of low-spin and high-spin states allowing for multiple switchability. The analysis is carried out for a quantum spin-fermion model using the exact diagonalization, and the cluster mean-field approach. The model is relevant for a number of molecular magnets with triangular motifs consisting of transition metal ions such as Cr, Cu and V. Re-entrant spin-state behavior and chirality on-off transitions exist over a wide parameter regime. A subtle competition among geometrical frustration effects, electron itinerancy, and Kondo coupling at the molecular level is highlighted. Our results demonstrate that electron doping provides a viable mean to tame the magnetic properties of molecular magnets towards potential technological applications.

      @article{,
      author = {Rajyavardhan Ray and Sanjeev Kumar},
      title = {Switchable Multiple Spin States in the Kondo description of Doped Molecular Magnets},
      journal = {Scientific Reports},
      volume = {7},
      abstract = {We show that introducing electrons in magnetic clusters and molecular magnets lead to rich phase diagrams with a variety of low-spin and high-spin states allowing for multiple switchability. The analysis is carried out for a quantum spin-fermion model using the exact diagonalization, and the cluster mean-field approach. The model is relevant for a number of molecular magnets with triangular motifs consisting of transition metal ions such as Cr, Cu and V. Re-entrant spin-state behavior and chirality on-off transitions exist over a wide parameter regime. A subtle competition among geometrical frustration effects, electron itinerancy, and Kondo coupling at the molecular level is highlighted. Our results demonstrate that electron doping provides a viable mean to tame the magnetic properties of molecular magnets towards potential technological applications.},
      year = {2017},
      url = http://dx.doi.org/{10.1038/srep42255},
      doi = {10.1038/srep42255},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Theoretical models for magneto-sensitive elastomers: A comparison between continuum and dipole approaches
    • D. Romeis, P. Metsch, M. Kaestner, M. Saphiannikova
    • Physical Review E 95(2017)
    • DOI   Abstract  

      In the literature, different theoretical models have been proposed to describe the properties of systems which consist of magnetizable particles that are embedded into an elastomer matrix. It is well known that such magneto-sensitive elastomers display a strong magneto-mechanical coupling when subjected to an external magnetic field. Nevertheless, the predictions of available models often vary significantly since they are based on different assumptions and approximations. Up to now the actual accuracy and the limits of applicability are widely unknown. In the present work, we compare the results of a microscale continuum and a dipolar mean field approach with regard to their predictions for the magnetostrictive response of magneto-sensitive elastomers and reveal some fundamental relations between the relevant quantities in both theories. It turns out that there is a very good agreement between both modeling strategies, especially for entirely random microstructures. In contrast, a comparison of the finite-element results with a modified approach, which-similar to the continuum model-is based on calculations with discrete particle distributions, reveals clear deviations. Our systematic analysis of the differences shows to what extent the dipolar mean field approach is superior to other dipole models.

      @article{,
      author = {Dirk Romeis and Philipp Metsch and Markus Kaestner and Marina Saphiannikova},
      title = {Theoretical models for magneto-sensitive elastomers: A comparison between continuum and dipole approaches},
      journal = {Physical Review E},
      volume = {95},
      number = {4},
      abstract = {In the literature, different theoretical models have been proposed to describe the properties of systems which consist of magnetizable particles that are embedded into an elastomer matrix. It is well known that such magneto-sensitive elastomers display a strong magneto-mechanical coupling when subjected to an external magnetic field. Nevertheless, the predictions of available models often vary significantly since they are based on different assumptions and approximations. Up to now the actual accuracy and the limits of applicability are widely unknown. In the present work, we compare the results of a microscale continuum and a dipolar mean field approach with regard to their predictions for the magnetostrictive response of magneto-sensitive elastomers and reveal some fundamental relations between the relevant quantities in both theories. It turns out that there is a very good agreement between both modeling strategies, especially for entirely random microstructures. In contrast, a comparison of the finite-element results with a modified approach, which-similar to the continuum model-is based on calculations with discrete particle distributions, reveals clear deviations. Our systematic analysis of the differences shows to what extent the dipolar mean field approach is superior to other dipole models.},
      year = {2017},
      url = http://dx.doi.org/{10.1103/PhysRevE.95.042501},
      doi = {10.1103/PhysRevE.95.042501},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Tuning Near-Infrared Absorbing Donor Materials: A Study of Electronic, Optical, and Charge-Transport Properties of aza-BODIPYs
    • K. S. Schellhammer, T. Li, O. Zeika, C. Koerner, K. Leo, F. Ortmann, G. Cuniberti
    • Chemistry of Materials 29, 5525-5536 (2017)
    • DOI   Abstract  

      The class of 4,4′-difluoro-4-bora-3a,4a,8-triaza-s-indacenes (aza-BODIPYs) are promising near-infrared absorber materials which are successfully used in organic solar cells to extend their absorption to the near-infrared regime. We computationally studied electronic properties, internal reorganization energies, and the optical properties of more than 100 promising candidates and derived design principles, including novel functionalization routes, to improve their performance as donor materials. We synthesized and characterized several of the promising molecules, confirming the predicted trends. The best charge transport properties and absorption characteristics are obtained for naphthalene-annulated molecular cores due to optimally delocalized frontier molecular orbitals. Further optimization can be achieved by alpha-functionalization with fluorinated groups, beta-functionalization with accepting substituents, and modification of the borondifluoride group. For such molecules, we predict a bathochromic shift in the absorption, which should not significantly reduce the open-circuit voltage. Torsional restriction of alpha-substituents by carbon bridges can further improve both charge transport and absorption. The theoretically and experimentally observed independence of most of the functionalization BODIPYs an ideal material class for tailor-made absorber materials that can cover a broad range of absorption, charge transport, and energetic regimes, calling for further exploration in organic solar cell applications, fluorescence microscopy, and photodynamic therapy.

      @article{,
      author = {Karl Sebastian Schellhammer and Tian-Yi Li and Olaf Zeika and Christian Koerner and Karl Leo and Frank Ortmann and Gianaurelio Cuniberti},
      title = {Tuning Near-Infrared Absorbing Donor Materials: A Study of Electronic, Optical, and Charge-Transport Properties of aza-BODIPYs},
      journal = {Chemistry of Materials},
      volume = {29},
      number = {13},
      pages = {5525-5536},
      abstract = {The class of 4,4'-difluoro-4-bora-3a,4a,8-triaza-s-indacenes (aza-BODIPYs) are promising near-infrared absorber materials which are successfully used in organic solar cells to extend their absorption to the near-infrared regime. We computationally studied electronic properties, internal reorganization energies, and the optical properties of more than 100 promising candidates and derived design principles, including novel functionalization routes, to improve their performance as donor materials. We synthesized and characterized several of the promising molecules, confirming the predicted trends. The best charge transport properties and absorption characteristics are obtained for naphthalene-annulated molecular cores due to optimally delocalized frontier molecular orbitals. Further optimization can be achieved by alpha-functionalization with fluorinated groups, beta-functionalization with accepting substituents, and modification of the borondifluoride group. For such molecules, we predict a bathochromic shift in the absorption, which should not significantly reduce the open-circuit voltage. Torsional restriction of alpha-substituents by carbon bridges can further improve both charge transport and absorption. The theoretically and experimentally observed independence of most of the functionalization BODIPYs an ideal material class for tailor-made absorber materials that can cover a broad range of absorption, charge transport, and energetic regimes, calling for further exploration in organic solar cell applications, fluorescence microscopy, and photodynamic therapy.},
      year = {2017},
      url = http://dx.doi.org/{10.1021/acs.chemmater.7b00653},
      doi = {10.1021/acs.chemmater.7b00653},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Tuning quantum electron and phonon transport in two-dimensional materials by strain engineering: a Green’s function based study
    • L. M. Sandonas, R. Gutierrez, A. Pecchia, G. Seifert, G. Cuniberti
    • Physical Chemistry Chemical Physics 19, 1487-1495 (2017)
    • DOI   Abstract  

      Novel two-dimensional (2D) materials show unusual physical properties which combined with strain engineering open up the possibility of new potential device applications in nanoelectronics. In particular, transport properties have been found to be very sensitive to applied strain. In the present work, using a density-functional based tight-binding (DFTB) method in combination with Green’s function (GF) approaches, we address the effect of strain engineering of the transport setup (contact-device(scattering)contact regions) on the electron and phonon transport properties of two-dimensional materials, focusing on hexagonal boron-nitride (hBN), phosphorene, and MoS2 monolayers. Considering unstretched contact regions, we show that the electronic bandgap displays an anomalous behavior and the thermal conductance continuously decreases after increasing the strain level in the scattering region. However, when the whole system (contact and device regions) is homogeneously strained, the bandgap for hBN and MoS2 monolayers decreases, while for phosphorene it first increases and then tends to zero with larger strain levels. Additionally, the thermal conductance shows specific strain dependence for each of the studied 2D materials. These effects can be tuned by modifying the strain level in the stretched contact regions.

      @article{,
      author = {Leonardo Medrano Sandonas and Rafael Gutierrez and Alessandro Pecchia and Gotthard Seifert and Gianaurelio Cuniberti},
      title = {Tuning quantum electron and phonon transport in two-dimensional materials by strain engineering: a Green's function based study},
      journal = {Physical Chemistry Chemical Physics},
      volume = {19},
      number = {2},
      pages = {1487-1495},
      abstract = {Novel two-dimensional (2D) materials show unusual physical properties which combined with strain engineering open up the possibility of new potential device applications in nanoelectronics. In particular, transport properties have been found to be very sensitive to applied strain. In the present work, using a density-functional based tight-binding (DFTB) method in combination with Green's function (GF) approaches, we address the effect of strain engineering of the transport setup (contact-device(scattering)contact regions) on the electron and phonon transport properties of two-dimensional materials, focusing on hexagonal boron-nitride (hBN), phosphorene, and MoS2 monolayers. Considering unstretched contact regions, we show that the electronic bandgap displays an anomalous behavior and the thermal conductance continuously decreases after increasing the strain level in the scattering region. However, when the whole system (contact and device regions) is homogeneously strained, the bandgap for hBN and MoS2 monolayers decreases, while for phosphorene it first increases and then tends to zero with larger strain levels. Additionally, the thermal conductance shows specific strain dependence for each of the studied 2D materials. These effects can be tuned by modifying the strain level in the stretched contact regions.},
      year = {2017},
      url = http://dx.doi.org/{10.1039/c6cp06621f},
      doi = {10.1039/c6cp06621f},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

2016

  • An algorithmic scheme for the automated calculation of fiber orientations in arterial walls
    • S. Fausten, D. Balzani, J. Schroder
    • Computational Mechanics 58, 861-878 (2016)
    • DOI   Abstract  

      We propose an algorithmic scheme for the numerical calculation of fiber orientations in arterial walls. The basic assumption behind the procedure is that the fiber orientations are mainly governed by the principal tensile stress directions resulting in an improved load transfer within the artery as a consequence of the redistribution of stresses. This reflects the biological motivation that soft tissues continuously adapt to their mechanical environment in order to optimize their load-bearing capacities. The algorithmic scheme proposed here enhances efficiency of the general procedure given in Hariton et al. (Biomech Model Mechanobiol 6(3):163-175, 2007), which consists of repeatedly identifying a favored fiber orientation based on the principal tensile stresses under a certain loading scenario, and then re-calculating the stresses for that loading scenario with the modified favored fiber orientation. Since the method still depends on a highly accurate stress approximation of the finite element formulation, which is not straightforward to obtain in particular for incompressible and highly anisotropic materials, furthermore, a modified model is introduced. This model defines the favored fiber orientation not only in terms of the local principal stresses, but in terms of the volume averages of the principal stresses computed over individual finite elements. Thereby, the influence of imperfect stress approximations can be weakened leading to a stabilized convergence of the reorientation procedure and a more reasonable fiber orientation with less numerical noise. The performance of the proposed fiber reorientation scheme is investigated with respect to different finite element formulations and different favored fiber orientation models, Hariton et al. (Biomech Model Mechanobiol 6(3):163-175, 2007) and Cyron and Humphrey (Math Mech Solids 1-17, 2014). In addition, it is applied to calculate the fiber orientation in a patient-specific arterial geometry.

      @article{,
      author = {Simon Fausten and Daniel Balzani and Joerg Schroder},
      title = {An algorithmic scheme for the automated calculation of fiber orientations in arterial walls},
      journal = {Computational Mechanics},
      volume = {58},
      number = {5},
      pages = {861-878},
      abstract = {We propose an algorithmic scheme for the numerical calculation of fiber orientations in arterial walls. The basic assumption behind the procedure is that the fiber orientations are mainly governed by the principal tensile stress directions resulting in an improved load transfer within the artery as a consequence of the redistribution of stresses. This reflects the biological motivation that soft tissues continuously adapt to their mechanical environment in order to optimize their load-bearing capacities. The algorithmic scheme proposed here enhances efficiency of the general procedure given in Hariton et al. (Biomech Model Mechanobiol 6(3):163-175, 2007), which consists of repeatedly identifying a favored fiber orientation based on the principal tensile stresses under a certain loading scenario, and then re-calculating the stresses for that loading scenario with the modified favored fiber orientation. Since the method still depends on a highly accurate stress approximation of the finite element formulation, which is not straightforward to obtain in particular for incompressible and highly anisotropic materials, furthermore, a modified model is introduced. This model defines the favored fiber orientation not only in terms of the local principal stresses, but in terms of the volume averages of the principal stresses computed over individual finite elements. Thereby, the influence of imperfect stress approximations can be weakened leading to a stabilized convergence of the reorientation procedure and a more reasonable fiber orientation with less numerical noise. The performance of the proposed fiber reorientation scheme is investigated with respect to different finite element formulations and different favored fiber orientation models, Hariton et al. (Biomech Model Mechanobiol 6(3):163-175, 2007) and Cyron and Humphrey (Math Mech Solids 1-17, 2014). In addition, it is applied to calculate the fiber orientation in a patient-specific arterial geometry.},
      year = {2016},
      url = http://dx.doi.org/{10.1007/s00466-016-1321-z},
      doi = {10.1007/s00466-016-1321-z},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Analyses of interaction mechanisms during forming of multilayer carbon woven fabrics for composite applications
    • F. N. Nezami, T. Gereke, C. Cherif
    • Composites Part a-Applied Science and Manufacturing 84, 406-416 (2016)
    • DOI   Abstract  

      The use of textile reinforcing structures provides enormous possibilities in the design of lightweight composites. However, the physical mechanisms during fabric forming are complex and far from being fully understood especially in multilayer draping. The aims of this study are the analyses of interaction mechanisms of individual textile layers during the forming operation and of defects arising from interactions. In basic experiments of the carbon woven fabric, friction properties and the fabric integrity were investigated. In single and multilayer draping experiments the findings were transferred to the composite preforming process. Interaction defects are characterised as interdependency between the acting inter-ply shear forces and the structural integrity of the fabric. The defects resulting from the interactions depend on the configuration of the fabric (e.g. shearing) and the acting normal forces. The reduction of friction is crucial for the preform quality but is opposite to an actively force-controlled material manipulation. (C) 2016 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Farbod Nosrat Nezami and Thomas Gereke and Chokri Cherif},
      title = {Analyses of interaction mechanisms during forming of multilayer carbon woven fabrics for composite applications},
      journal = {Composites Part a-Applied Science and Manufacturing},
      volume = {84},
      pages = {406-416},
      abstract = {The use of textile reinforcing structures provides enormous possibilities in the design of lightweight composites. However, the physical mechanisms during fabric forming are complex and far from being fully understood especially in multilayer draping. The aims of this study are the analyses of interaction mechanisms of individual textile layers during the forming operation and of defects arising from interactions. In basic experiments of the carbon woven fabric, friction properties and the fabric integrity were investigated. In single and multilayer draping experiments the findings were transferred to the composite preforming process. Interaction defects are characterised as interdependency between the acting inter-ply shear forces and the structural integrity of the fabric. The defects resulting from the interactions depend on the configuration of the fabric (e.g. shearing) and the acting normal forces. The reduction of friction is crucial for the preform quality but is opposite to an actively force-controlled material manipulation. (C) 2016 Elsevier Ltd. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.compositesa.2016.02.023},
      doi = {10.1016/j.compositesa.2016.02.023},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Anisotropic Thermoelectric Response in Two-Dimensional Puckered Structures
    • L. M. Sandonas, D. Teich, R. Gutierrez, T. Lorenz, A. Pecchia, G. Seifert, G. Cuniberti
    • Journal of Physical Chemistry C 120, 18841-18849 (2016)
    • DOI   Abstract  

      Two-dimensional semiconductor materials with puckered structure offer a novel playground to implement nanoscale thermoelectric, electronic, and optoelectronic devices with improved functionality. Using a combination of approaches to compute the electronic and phonon band structures with Green’s function based transport techniques, we address the thermoelectric performance of phosphorene, arsenene, and SnS monolayers. In particular, we study the influence of anisotropy in the electronic and phononic transport properties and its impact on the thermoelectric figure of merit ZT. Our results show no strong electronic anisotropy, but a strong thermal one, the effect being most pronounced in the.case of SnS monolayers. This material also displays the largest figure of merit at room temperature for both transport directions, zigzag (ZT similar to 0.95) and armchair (ZT similar to 1.6), thus hinting at the high potential of these new material’s in thermoelectric applications.

      @article{,
      author = {Leonardo Medrano Sandonas and David Teich and Rafael Gutierrez and Tommy Lorenz and Alessandro Pecchia and Gotthard Seifert and Gianaurelio Cuniberti},
      title = {Anisotropic Thermoelectric Response in Two-Dimensional Puckered Structures},
      journal = {Journal of Physical Chemistry C},
      volume = {120},
      number = {33},
      pages = {18841-18849},
      abstract = {Two-dimensional semiconductor materials with puckered structure offer a novel playground to implement nanoscale thermoelectric, electronic, and optoelectronic devices with improved functionality. Using a combination of approaches to compute the electronic and phonon band structures with Green's function based transport techniques, we address the thermoelectric performance of phosphorene, arsenene, and SnS monolayers. In particular, we study the influence of anisotropy in the electronic and phononic transport properties and its impact on the thermoelectric figure of merit ZT. Our results show no strong electronic anisotropy, but a strong thermal one, the effect being most pronounced in the.case of SnS monolayers. This material also displays the largest figure of merit at room temperature for both transport directions, zigzag (ZT similar to 0.95) and armchair (ZT similar to 1.6), thus hinting at the high potential of these new material's in thermoelectric applications.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acs.jpcc.6b04969},
      doi = {10.1021/acs.jpcc.6b04969},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Atomically Precise Prediction of 2D Self-Assembly of Weakly Bonded Nanostructures: STM Insight into Concentration-Dependent Architectures
    • M. E. Garah, A. Dianat, A. Cadeddu, R. Gutierrez, M. Cecchini, T. R. Cook, A. Ciesielski, P. J. Stang, G. Cuniberti, P. Samori
    • Small 12, 343-350 (2016)
    • DOI   Abstract  

      A joint experimental and computational study is reported on the concentration-dependant self-assembly of a flat C-3-symmetric molecule on a graphite surface. As a model system a tripodal molecule, 1,3,5-tris(pyridin-3-ylethynyl) benzene, has been chosen, which can adopt either C-3h or C-s symmetry when planar, as a result of pyridyl rotation along the alkynyl spacers. Density functional theory (DFT) simulations of 2D nanopatterns with different surface coverage reveal that the molecule can generate different types of self-assembled motifs. The stability of fourteen 2D patterns and the influence of concentration are analyzed. It is found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy (STM) investigation of this molecular self-assembly system at a solution/graphite interface reveals four supramolecular motifs, which are in perfect agreement with those predicted by simulations. Therefore, this DFT method represents a key step forward toward the atomically precise prediction of molecular self-assembly on surfaces and at interfaces.

      @article{,
      author = {Mohamed El Garah and Arezoo Dianat and Andrea Cadeddu and Rafael Gutierrez and Marco Cecchini and Timothy R. Cook and Artur Ciesielski and Peter J. Stang and Gianaurelio Cuniberti and Paolo Samori},
      title = {Atomically Precise Prediction of 2D Self-Assembly of Weakly Bonded Nanostructures: STM Insight into Concentration-Dependent Architectures},
      journal = {Small},
      volume = {12},
      number = {3},
      pages = {343-350},
      abstract = {A joint experimental and computational study is reported on the concentration-dependant self-assembly of a flat C-3-symmetric molecule on a graphite surface. As a model system a tripodal molecule, 1,3,5-tris(pyridin-3-ylethynyl) benzene, has been chosen, which can adopt either C-3h or C-s symmetry when planar, as a result of pyridyl rotation along the alkynyl spacers. Density functional theory (DFT) simulations of 2D nanopatterns with different surface coverage reveal that the molecule can generate different types of self-assembled motifs. The stability of fourteen 2D patterns and the influence of concentration are analyzed. It is found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy (STM) investigation of this molecular self-assembly system at a solution/graphite interface reveals four supramolecular motifs, which are in perfect agreement with those predicted by simulations. Therefore, this DFT method represents a key step forward toward the atomically precise prediction of molecular self-assembly on surfaces and at interfaces.},
      year = {2016},
      url = http://dx.doi.org/{10.1002/smll.201502957},
      doi = {10.1002/smll.201502957},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Ba2NiOsO6: A Dirac-Mott insulator with ferromagnetism near 100 K
    • H. L. Feng, S. Calder, M. P. Ghimire, Y. Yuan, Y. Shirako, Y. Tsujimoto, Y. Matsushita, Z. Hu, C. Kuo, L. H. Tjeng, T. Pi, Y. Soo, J. He, M. Tanaka, Y. Katsuya, M. Richter, K. Yamaura
    • Physical Review B 94(2016)
    • DOI   Abstract  

      The ferromagnetic semiconductor Ba2NiOsO6 (T-mag similar to 100 K) was synthesized at 6 GPa and 1500 degrees C. It crystallizes into a double perovskite structure [Fm-3m; a = 8.0428(1) angstrom], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga, Mn) As and (Cd, Mn) Te (T-mag < 180 K), the spin-gapless semiconductor Mn2CoAl (T-mag similar to 720 K), and the ferromagnetic insulators EuO (T-mag similar to 70 K) and Bi3Cr3O11 (T-mag similar to 220 K). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.

      @article{,
      author = {Hai L. Feng and Stuart Calder and Madhav Prasad Ghimire and Ya-Hua Yuan and Yuichi Shirako and Yoshihiro Tsujimoto and Yoshitaka Matsushita and Zhiwei Hu and Chang-Yang Kuo and Liu Hao Tjeng and Tun-Wen Pi and Yun-Liang Soo and Jianfeng He and Masahiko Tanaka and Yoshio Katsuya and Manuel Richter and Kazunari Yamaura},
      title = {Ba2NiOsO6: A Dirac-Mott insulator with ferromagnetism near 100 K},
      journal = {Physical Review B},
      volume = {94},
      number = {23},
      abstract = {The ferromagnetic semiconductor Ba2NiOsO6 (T-mag similar to 100 K) was synthesized at 6 GPa and 1500 degrees C. It crystallizes into a double perovskite structure [Fm-3m; a = 8.0428(1) angstrom], where the Ni2+ and Os6+ ions are perfectly ordered at the perovskite B site. We show that the spin-orbit coupling of Os6+ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a > 21 kOe magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga, Mn) As and (Cd, Mn) Te (T-mag < 180 K), the spin-gapless semiconductor Mn2CoAl (T-mag similar to 720 K), and the ferromagnetic insulators EuO (T-mag similar to 70 K) and Bi3Cr3O11 (T-mag similar to 220 K). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of Ba2NiOsO6 should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.},
      year = {2016},
      url = http://dx.doi.org/{10.1103/PhysRevB.94.235158},
      doi = {10.1103/PhysRevB.94.235158},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Band structure engineering in organic semiconductors
    • M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking, K. Ortstein, A. A. Guenther, D. Kasemann, D. Andrienko, K. Leo
    • Science 352, 1446-1449 (2016)
    • DOI   Abstract  

      A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

      @article{,
      author = {Martin Schwarze and Wolfgang Tress and Beatrice Beyer and Feng Gao and Reinhard Scholz and Carl Poelking and Katrin Ortstein and Alrun A. Guenther and Daniel Kasemann and Denis Andrienko and Karl Leo},
      title = {Band structure engineering in organic semiconductors},
      journal = {Science},
      volume = {352},
      number = {6292},
      pages = {1446-1449},
      abstract = {A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.},
      year = {2016},
      url = http://dx.doi.org/{10.1126/science.aaf0590},
      doi = {10.1126/science.aaf0590},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Bezier extraction and adaptive refinement of truncated hierarchical NURBS
    • P. Hennig, S. Mueller, M. Kaestner
    • Computer Methods in Applied Mechanics and Engineering 305, 316-339 (2016)
    • DOI   Abstract  

      This contribution presents Bezier extraction of truncated hierarchical B-splines and the application of the approach to adaptive isogeometric analysis. The developed procedures allow for the implementation of hierarchical B-splines and NURBS without the need for an explicit truncation of the basis. Moreover, standard procedures of adaptive finite element analysis for error estimation and marking of elements are directly applicable due to the strict use of an element viewpoint. Starting from a multi-level nested mesh that results from uniform h-refinement, standard Bezier extraction is applied to active elements that contribute to the hierarchical approximation. This results in a multi-level system of equations without communication between individual hierarchy levels. A hierarchical subdivision operator is developed to recover this communication by transforming the multi-level system of equations into a hierarchical system of equations. It is demonstrated that this approach implicitly defines the truncated hierarchical basis in terms of a simple matrix multiplication. In this way, the implementation effort is reduced to a minimum as shape function routines and Bezier extraction procedures remain unchanged compared to standard isogeometric analysis. The convergence and the computational efficiency of the approach are examined in three different demonstration problems of heat conduction, linear elasticity, and the phase-field modelling of brittle fracture. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {P. Hennig and S. Mueller and M. Kaestner},
      title = {Bezier extraction and adaptive refinement of truncated hierarchical NURBS},
      journal = {Computer Methods in Applied Mechanics and Engineering},
      volume = {305},
      pages = {316-339},
      abstract = {This contribution presents Bezier extraction of truncated hierarchical B-splines and the application of the approach to adaptive isogeometric analysis. The developed procedures allow for the implementation of hierarchical B-splines and NURBS without the need for an explicit truncation of the basis. Moreover, standard procedures of adaptive finite element analysis for error estimation and marking of elements are directly applicable due to the strict use of an element viewpoint. Starting from a multi-level nested mesh that results from uniform h-refinement, standard Bezier extraction is applied to active elements that contribute to the hierarchical approximation. This results in a multi-level system of equations without communication between individual hierarchy levels. A hierarchical subdivision operator is developed to recover this communication by transforming the multi-level system of equations into a hierarchical system of equations. It is demonstrated that this approach implicitly defines the truncated hierarchical basis in terms of a simple matrix multiplication. In this way, the implementation effort is reduced to a minimum as shape function routines and Bezier extraction procedures remain unchanged compared to standard isogeometric analysis. The convergence and the computational efficiency of the approach are examined in three different demonstration problems of heat conduction, linear elasticity, and the phase-field modelling of brittle fracture. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.cma.2016.03.009},
      doi = {10.1016/j.cma.2016.03.009},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Borophene as an anode material for Ca, Mg, Na or Li ion storage: A first-principle study
    • B. Mortazavi, A. Dianat, O. Rahaman, G. Cuniberti, T. Rabczuk
    • Journal of Power Sources 329, 456-461 (2016)
    • DOI   Abstract  

      Borophene, the boron atom analogue to graphene, being atomic thick have been just recently experimentally fabricated. In this work, we employ first-principles density functional theory calculations to investigate the interaction of Ca, Mg, Na or Li atoms with single-layer and free-standing borophene. We first identified the most stable binding sites and their corresponding binding energies as well and then we gradually increased the ions concentration. Our calculations predict strong binding energies of around 4.03 eV, 2.09 eV, 2.92 eV and 3.28 eV between the borophene substrate and Ca, Mg, Na or Li ions, respectively. We found that the binding energy generally decreases by increasing the ions content. Using the Bader charge analysis, we evaluate the charge transfer between the adatoms and the borophene sheet. Our investigation proposes the borophene as a 2D material with a remarkably high capacity of around 800 mA h/g, 1960 mA h/g, 1380 mA h/g and 1720 mA h/g for Ca, Mg, Na or Li ions storage, respectively. This study can be useful for the possible application of borophene for the rechargeable ion batteries. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {Bohayra Mortazavi and Arezoo Dianat and Obaidur Rahaman and Gianaurelio Cuniberti and Timon Rabczuk},
      title = {Borophene as an anode material for Ca, Mg, Na or Li ion storage: A first-principle study},
      journal = {Journal of Power Sources},
      volume = {329},
      pages = {456-461},
      abstract = {Borophene, the boron atom analogue to graphene, being atomic thick have been just recently experimentally fabricated. In this work, we employ first-principles density functional theory calculations to investigate the interaction of Ca, Mg, Na or Li atoms with single-layer and free-standing borophene. We first identified the most stable binding sites and their corresponding binding energies as well and then we gradually increased the ions concentration. Our calculations predict strong binding energies of around 4.03 eV, 2.09 eV, 2.92 eV and 3.28 eV between the borophene substrate and Ca, Mg, Na or Li ions, respectively. We found that the binding energy generally decreases by increasing the ions content. Using the Bader charge analysis, we evaluate the charge transfer between the adatoms and the borophene sheet. Our investigation proposes the borophene as a 2D material with a remarkably high capacity of around 800 mA h/g, 1960 mA h/g, 1380 mA h/g and 1720 mA h/g for Ca, Mg, Na or Li ions storage, respectively. This study can be useful for the possible application of borophene for the rechargeable ion batteries. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jpowsour.2016.08.109},
      doi = {10.1016/j.jpowsour.2016.08.109},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Ciprofloxacin wastewater treated by UVA photocatalysis: contribution of irradiated TiO2 and ZnO nanoparticles on the final toxicity as assessed by Vibrio fischeri
    • A. R. Silva, P. M. Martins, S. Teixeira, S. A. C. Carabineiro, K. Kuehn, G. Cuniberti, M. M. Alves, S. Lanceros-Mendez, L. Pereira
    • Rsc Advances 6, 95494-95503 (2016)
    • DOI   Abstract  

      Photocatalysis has become an attractive process to treat wastewater since it allows a rapid and efficient degradation of micropollutants in water. A solution of ciprofloxacin (CIP) was photocatalytically treated by ultraviolet A light (UVA) and titanium dioxide (TiO2) or zinc oxide (ZnO) nanoparticles. Toxicity of CIP and of the treated CIP solutions, as well as the toxicity of TiO2 and ZnO irradiated nanoparticles, was evaluated towards Vibrio fischeri. The lowest concentration of CIP tested, 10 mu g L-1, leads to 50% of luminescence inhibition. Regarding irradiated nanoparticles, ZnO presented higher bacteria luminescence inhibition than TiO2, 97 and 38%, respectively. Due to high toxicity of ZnO, it was only possible to evaluate the CIP solution treated by UVA/TiO2. Initially, the toxicity decreased with the time of the process, but after 15 min the toxicity increased significantly (55%) and after 45 min of treatment, was 70%. High-performance liquid chromatography (HPLC) and Fourier transform infrared spectroscopy (FTIR) analysis proved that the initial decrease of toxicity was caused by CIP adsorption on catalyst surface, which latter increased due to the generation of by-products and toxicity contribution of soluble nanoparticles. Ten by-products were identified by liquid chromatography-mass spectrometry (LC-MS) and the mechanism of CIP photocatalytic degradation was proposed.

      @article{,
      author = {A. R. Silva and P. M. Martins and S. Teixeira and S. A. C. Carabineiro and K. Kuehn and G. Cuniberti and M. M. Alves and S. Lanceros-Mendez and L. Pereira},
      title = {Ciprofloxacin wastewater treated by UVA photocatalysis: contribution of irradiated TiO2 and ZnO nanoparticles on the final toxicity as assessed by Vibrio fischeri},
      journal = {Rsc Advances},
      volume = {6},
      number = {98},
      pages = {95494-95503},
      abstract = {Photocatalysis has become an attractive process to treat wastewater since it allows a rapid and efficient degradation of micropollutants in water. A solution of ciprofloxacin (CIP) was photocatalytically treated by ultraviolet A light (UVA) and titanium dioxide (TiO2) or zinc oxide (ZnO) nanoparticles. Toxicity of CIP and of the treated CIP solutions, as well as the toxicity of TiO2 and ZnO irradiated nanoparticles, was evaluated towards Vibrio fischeri. The lowest concentration of CIP tested, 10 mu g L-1, leads to 50% of luminescence inhibition. Regarding irradiated nanoparticles, ZnO presented higher bacteria luminescence inhibition than TiO2, 97 and 38%, respectively. Due to high toxicity of ZnO, it was only possible to evaluate the CIP solution treated by UVA/TiO2. Initially, the toxicity decreased with the time of the process, but after 15 min the toxicity increased significantly (55%) and after 45 min of treatment, was 70%. High-performance liquid chromatography (HPLC) and Fourier transform infrared spectroscopy (FTIR) analysis proved that the initial decrease of toxicity was caused by CIP adsorption on catalyst surface, which latter increased due to the generation of by-products and toxicity contribution of soluble nanoparticles. Ten by-products were identified by liquid chromatography-mass spectrometry (LC-MS) and the mechanism of CIP photocatalytic degradation was proposed.},
      year = {2016},
      url = http://dx.doi.org/{10.1039/c6ra19202e},
      doi = {10.1039/c6ra19202e},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Collective migration under hydrodynamic interactions: a computational approach
    • W. Marth, A. Voigt
    • Interface Focus 6(2016)
    • DOI   Abstract  

      We consider a generic model for cell motility. Even if a comprehensive understanding of cell motility remains elusive, progress has been achieved in its modelling using a whole-cell physical model. The model takes into account the main mechanisms of cell motility, actin polymerization, actin-myosin dynamics and substrate mediated adhesion (if applicable), and combines them with steric cell-cell and hydrodynamic interactions. The model predicts the onset of collective cell migration, which emerges spontaneously as a result of inelastic collisions of neighbouring cells. Each cell here modelled as an active polar gel is accomplished with two vortices if it moves. Upon collision of two cells, the two vortices which come close to each other annihilate. This leads to a rotation of the cells and together with the deformation and the reorientation of the actin filaments in each cell induces alignment of these cells and leads to persistent translational collective migration. The effect for low Reynolds numbers is as strong as in the non- hydrodynamic model, but it decreases with increasing Reynolds number.

      @article{,
      author = {W. Marth and A. Voigt},
      title = {Collective migration under hydrodynamic interactions: a computational approach},
      journal = {Interface Focus},
      volume = {6},
      number = {5},
      abstract = {We consider a generic model for cell motility. Even if a comprehensive understanding of cell motility remains elusive, progress has been achieved in its modelling using a whole-cell physical model. The model takes into account the main mechanisms of cell motility, actin polymerization, actin-myosin dynamics and substrate mediated adhesion (if applicable), and combines them with steric cell-cell and hydrodynamic interactions. The model predicts the onset of collective cell migration, which emerges spontaneously as a result of inelastic collisions of neighbouring cells. Each cell here modelled as an active polar gel is accomplished with two vortices if it moves. Upon collision of two cells, the two vortices which come close to each other annihilate. This leads to a rotation of the cells and together with the deformation and the reorientation of the actin filaments in each cell induces alignment of these cells and leads to persistent translational collective migration. The effect for low Reynolds numbers is as strong as in the non- hydrodynamic model, but it decreases with increasing Reynolds number.},
      year = {2016},
      url = http://dx.doi.org/{10.1098/rsfs.2016.0037},
      doi = {10.1098/rsfs.2016.0037},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Comment on "Degenerate mobilities in phase field models are insufficient to capture surface diffusion" Appl. Phys. Lett. 107, 081603 (2015)
    • A. Voigt
    • Applied Physics Letters 108(2016)
    • DOI  
      @article{,
      author = {Axel Voigt},
      title = {Comment on "Degenerate mobilities in phase field models are insufficient to capture surface diffusion" Appl. Phys. Lett. 107, 081603 (2015)},
      journal = {Applied Physics Letters},
      volume = {108},
      number = {3},
      year = {2016},
      url = http://dx.doi.org/{10.1063/1.4939930},
      doi = {10.1063/1.4939930},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Computational study of structure, electronic, and microscopic charge transport properties of small conjugated diketopyrrolopyrrole-thiophene molecules
    • M. V. Makarova, S. G. Semenov, O. A. Guskova
    • International Journal of Quantum Chemistry 116, 1459-1466 (2016)
    • DOI   Abstract  

      -Conjugated small molecules containing diketopyrrolopyrrole (DPP) and thiophene moieties represent a modern class of functional materials that exhibit promising charge transport properties and therefore have great potential as building blocks of active elements of electronic devices. As a starting point of this computational study, the molecular structure, electronic characteristics, and reorganization energies associated with electron or hole transfer are considered. Prediction of molecular crystal packing is followed by the calculation of couplings between adjacent molecules and detection of the effective charge transfer pathways. Finally, the rates of charge transfer process are evaluated. The obtained results shed light not only on the properties of materials containing low-molecular species but also serve as a benchmark for further classical force-field simulations of DPP-based polymers.

      @article{,
      author = {Maria V. Makarova and Sergey G. Semenov and Olga A. Guskova},
      title = {Computational study of structure, electronic, and microscopic charge transport properties of small conjugated diketopyrrolopyrrole-thiophene molecules},
      journal = {International Journal of Quantum Chemistry},
      volume = {116},
      number = {20},
      pages = {1459-1466},
      abstract = {-Conjugated small molecules containing diketopyrrolopyrrole (DPP) and thiophene moieties represent a modern class of functional materials that exhibit promising charge transport properties and therefore have great potential as building blocks of active elements of electronic devices. As a starting point of this computational study, the molecular structure, electronic characteristics, and reorganization energies associated with electron or hole transfer are considered. Prediction of molecular crystal packing is followed by the calculation of couplings between adjacent molecules and detection of the effective charge transfer pathways. Finally, the rates of charge transfer process are evaluated. The obtained results shed light not only on the properties of materials containing low-molecular species but also serve as a benchmark for further classical force-field simulations of DPP-based polymers.},
      year = {2016},
      url = http://dx.doi.org/{10.1002/qua.25205},
      doi = {10.1002/qua.25205},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Contact-dependent mechanical properties of graphene nanoribbons: an ab initio study
    • A. Dianat, D. A. Ryndyk, G. Cuniberti
    • Nanotechnology 27(2016)
    • DOI   Abstract  

      The mechanical properties of graphene nanoribbons on Ni(111) surfaces with different contact sizes are investigated by means of density functional theory. For finite contact sizes, the stress behavior of graphene nanoribbons on metal electrodes is likely to be similar to that of suspended graphene, however the critical strain is not reached due to the sliding friction at the interface. The competition between frictional and external forces is responsible for the nonmonotonic stress behavior. It is indicated that the stick-slip motions of graphene on Ni(111) are as a result of applied external forces on the GNR/metal contact. Moreover, the effect of vacancies and chemical doping on the sliding friction are addressed. Graphene starts to slide on the surface under a much lower external force in the case of defected graphene, due to the weaker binding to the surface. For infinite contact sizes, a linear relationship between stress and strain are found until structural failure occurs by 11% applied strain. The corresponding critical strain for the suspended GNR (without electrodes) has been found to be 13%.

      @article{,
      author = {Arezoo Dianat and Dmitry A. Ryndyk and Gianaurelio Cuniberti},
      title = {Contact-dependent mechanical properties of graphene nanoribbons: an ab initio study},
      journal = {Nanotechnology},
      volume = {27},
      number = {2},
      abstract = {The mechanical properties of graphene nanoribbons on Ni(111) surfaces with different contact sizes are investigated by means of density functional theory. For finite contact sizes, the stress behavior of graphene nanoribbons on metal electrodes is likely to be similar to that of suspended graphene, however the critical strain is not reached due to the sliding friction at the interface. The competition between frictional and external forces is responsible for the nonmonotonic stress behavior. It is indicated that the stick-slip motions of graphene on Ni(111) are as a result of applied external forces on the GNR/metal contact. Moreover, the effect of vacancies and chemical doping on the sliding friction are addressed. Graphene starts to slide on the surface under a much lower external force in the case of defected graphene, due to the weaker binding to the surface. For infinite contact sizes, a linear relationship between stress and strain are found until structural failure occurs by 11% applied strain. The corresponding critical strain for the suspended GNR (without electrodes) has been found to be 13%.},
      year = {2016},
      url = http://dx.doi.org/{10.1088/0957-4484/27/2/025702},
      doi = {10.1088/0957-4484/27/2/025702},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Continuum modelling of semiconductor heteroepitaxy: an applied perspective
    • R. Bergamaschini, M. Salvalaglio, R. Backofen, A. Voigt, F. Montalenti
    • Advances in Physics-X 1, 331-367 (2016)
    • DOI   Abstract  

      Semiconductor heteroepitaxy involves a wealth of qualitatively different, competing phenomena. Examples include three-dimensional island formation, injection of dislocations, mixing between film and substrate atoms. Their relative importance depends on the specific growth conditions, giving rise to a very complex scenario. The need for an optimal control over heteroepitaxial films and/or nanostructures is widespread: semiconductor epitaxy by molecular beam epitaxy or chemical vapour deposition is nowadays exploited also in industrial environments. Simulation models can be precious in limiting the parameter space to be sampled while aiming at films/nanostructures with the desired properties. In order to be appealing (and useful) to an applied audience, such models must yield predictions directly comparable with experimental data. This implies matching typical time scales and sizes, while offering a satisfactory description of the main physical driving forces. It is the aim of the present review to show that continuum models of semiconductor heteroepitaxy evolved significantly, providing a promising tool (even a working tool, in some cases) to comply with the above requirements. Several examples, spanning from the nanometre to the micron scale, are illustrated. Current limitations and future research directions are also discussed. [GRAPHICS] .

      @article{,
      author = {R. Bergamaschini and M. Salvalaglio and R. Backofen and A. Voigt and F. Montalenti},
      title = {Continuum modelling of semiconductor heteroepitaxy: an applied perspective},
      journal = {Advances in Physics-X},
      volume = {1},
      number = {3},
      pages = {331-367},
      abstract = {Semiconductor heteroepitaxy involves a wealth of qualitatively different, competing phenomena. Examples include three-dimensional island formation, injection of dislocations, mixing between film and substrate atoms. Their relative importance depends on the specific growth conditions, giving rise to a very complex scenario. The need for an optimal control over heteroepitaxial films and/or nanostructures is widespread: semiconductor epitaxy by molecular beam epitaxy or chemical vapour deposition is nowadays exploited also in industrial environments. Simulation models can be precious in limiting the parameter space to be sampled while aiming at films/nanostructures with the desired properties. In order to be appealing (and useful) to an applied audience, such models must yield predictions directly comparable with experimental data. This implies matching typical time scales and sizes, while offering a satisfactory description of the main physical driving forces. It is the aim of the present review to show that continuum models of semiconductor heteroepitaxy evolved significantly, providing a promising tool (even a working tool, in some cases) to comply with the above requirements. Several examples, spanning from the nanometre to the micron scale, are illustrated. Current limitations and future research directions are also discussed. [GRAPHICS] .},
      year = {2016},
      url = http://dx.doi.org/{10.1080/23746149.2016.1181986},
      doi = {10.1080/23746149.2016.1181986},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Correlation between topological band character and chemical bonding in a Bi14Rh3I9-based family of insulators
    • B. Rasche, A. Isaeva, M. Ruck, K. Koepernik, M. Richter, J. van den Brink
    • Scientific Reports 6(2016)
    • DOI   Abstract  

      Recently the presence of topologically protected edge-states in Bi14Rh3I9 was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi14Rh3I9 parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi14Rh3I9 and graphene in terms of p(z) -molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt-and Pd-based systems, whereas the Os-and Ru-systems remain trivial. Furthermore, the energy position of the metal d-band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The d-band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.

      @article{,
      author = {Bertold Rasche and Anna Isaeva and Michael Ruck and Klaus Koepernik and Manuel Richter and Jeroen van den Brink},
      title = {Correlation between topological band character and chemical bonding in a Bi14Rh3I9-based family of insulators},
      journal = {Scientific Reports},
      volume = {6},
      abstract = {Recently the presence of topologically protected edge-states in Bi14Rh3I9 was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi14Rh3I9 parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi14Rh3I9 and graphene in terms of p(z) -molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt-and Pd-based systems, whereas the Os-and Ru-systems remain trivial. Furthermore, the energy position of the metal d-band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The d-band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.},
      year = {2016},
      url = http://dx.doi.org/{10.1038/srep20645},
      doi = {10.1038/srep20645},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Discrete polygonal supramolecular architectures of isocytosine-based Pt(II) complexes at the solution/graphite interface
    • M. E. Garah, S. Sinn, A. Dianat, A. Santana-Bonilla, R. Gutierrez, L. D. Cola, G. Cuniberti, A. Ciesielski, P. Samori
    • Chemical Communications 52, 11163-11166 (2016)
    • DOI   Abstract  

      Polygonal supramolecular architectures of a Pt(II) complex including trimers, tetramers, pentamers and hexamers were self-assembled via hydrogen bonding between isocytosine moieties; their structure at the solid/liquid interface was unravelled by in situ scanning tunneling microscopy imaging. Density functional theory calculations provided in-depth insight into the thermodynamics of their formation by exploring the different energy contributions attributed to the molecular self-assembly and adsorption processes.

      @article{,
      author = {Mohamed El Garah and Stephan Sinn and Arezoo Dianat and Alejandro Santana-Bonilla and Rafael Gutierrez and Luisa De Cola and Gianaurelio Cuniberti and Artur Ciesielski and Paolo Samori},
      title = {Discrete polygonal supramolecular architectures of isocytosine-based Pt(II) complexes at the solution/graphite interface},
      journal = {Chemical Communications},
      volume = {52},
      number = {74},
      pages = {11163-11166},
      abstract = {Polygonal supramolecular architectures of a Pt(II) complex including trimers, tetramers, pentamers and hexamers were self-assembled via hydrogen bonding between isocytosine moieties; their structure at the solid/liquid interface was unravelled by in situ scanning tunneling microscopy imaging. Density functional theory calculations provided in-depth insight into the thermodynamics of their formation by exploring the different energy contributions attributed to the molecular self-assembly and adsorption processes.},
      year = {2016},
      url = http://dx.doi.org/{10.1039/c6cc05087e},
      doi = {10.1039/c6cc05087e},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • An efficient coarse-grained approach for the electron transport through large molecular systems under dephasing environment
    • D. Nozaki, R. Bustos-Marun, C. J. Cattena, G. Cuniberti, H. M. Pastawski
    • European Physical Journal B 89(2016)
    • DOI   Abstract  

      Dephasing effects in electron transport in molecular systems connected between contacts average out the quantum characteristics of the system, forming a bridge to the classical behavior as the size of the system increases. For the evaluation of the conductance of the molecular systems which have sizes within this boundary domain, it is necessary to include these dephasing effects. These effects can be calculated by using the D’Amato-Pastawski model. However, this method is computationally demanding for large molecular systems since transmission functions for all pairs of atomic orbitals need to be calculated. To overcome this difficulty, we develop an efficient coarse-grained model for the calculation of conductance of molecular junctions including decoherence. By analyzing the relationship between chemical potential and inter-molecular coupling, we find that the chemical potential drops stepwise in the systems with weaker inter-unit coupling. Using this property, an efficient coarse-grained algorithm which can reduce computational costs considerably without losing the accuracy is derived and applied to one-dimensional organic systems as a demonstration. This model can be used for the study of the orientation dependence of conductivity in various phases (amorphous, crystals, and polymers) of large molecular systems such as organic semiconducting materials.

      @article{,
      author = {Daijiro Nozaki and Raul Bustos-Marun and Carlos J. Cattena and Gianaurelio Cuniberti and Horacio M. Pastawski},
      title = {An efficient coarse-grained approach for the electron transport through large molecular systems under dephasing environment},
      journal = {European Physical Journal B},
      volume = {89},
      number = {4},
      abstract = {Dephasing effects in electron transport in molecular systems connected between contacts average out the quantum characteristics of the system, forming a bridge to the classical behavior as the size of the system increases. For the evaluation of the conductance of the molecular systems which have sizes within this boundary domain, it is necessary to include these dephasing effects. These effects can be calculated by using the D'Amato-Pastawski model. However, this method is computationally demanding for large molecular systems since transmission functions for all pairs of atomic orbitals need to be calculated. To overcome this difficulty, we develop an efficient coarse-grained model for the calculation of conductance of molecular junctions including decoherence. By analyzing the relationship between chemical potential and inter-molecular coupling, we find that the chemical potential drops stepwise in the systems with weaker inter-unit coupling. Using this property, an efficient coarse-grained algorithm which can reduce computational costs considerably without losing the accuracy is derived and applied to one-dimensional organic systems as a demonstration. This model can be used for the study of the orientation dependence of conductivity in various phases (amorphous, crystals, and polymers) of large molecular systems such as organic semiconducting materials.},
      year = {2016},
      url = http://dx.doi.org/{10.1140/epjb/e2016-70013-y},
      doi = {10.1140/epjb/e2016-70013-y},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9
    • C. Pauly, B. Rasche, K. Koepernik, M. Richter, S. Borisenko, M. Liebmann, M. Ruck, J. van den Brink, M. Morgenstern
    • Acs Nano 10, 3995-4003 (2016)
    • DOI   Abstract  

      Compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)(3)I](2+) and insulating [Bi2I8](2-) layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level, which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well-screened toward the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both on the surface and in the bulk; that is, it is not surface-doped due to iodine desorption. The well-screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically nontrivial surface layer, which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.

      @article{,
      author = {Christian Pauly and Bertold Rasche and Klaus Koepernik and Manuel Richter and Sergey Borisenko and Marcus Liebmann and Michael Ruck and Jeroen van den Brink and Markus Morgenstern},
      title = {Electronic Structure of the Dark Surface of the Weak Topological Insulator Bi14Rh3I9},
      journal = {Acs Nano},
      volume = {10},
      number = {4},
      pages = {3995-4003},
      abstract = {Compound Bi14Rh3I9 consists of ionic stacks of intermetallic [(Bi4Rh)(3)I](2+) and insulating [Bi2I8](2-) layers and has been identified to be a weak topological insulator. Scanning tunneling microscopy revealed the robust edge states at all step edges of the cationic layer as a topological fingerprint. However, these edge states are found 0.25 eV below the Fermi level, which is an obstacle for transport experiments. Here, we address this obstacle by comparing results of density functional slab calculations with scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. We show that the n-type doping of the intermetallic layer is intrinsically caused by the polar surface and is well-screened toward the bulk. In contrast, the anionic "spacer" layer shows a gap at the Fermi level, both on the surface and in the bulk; that is, it is not surface-doped due to iodine desorption. The well-screened surface dipole implies that a buried edge state, probably already below a single spacer layer, is located at the Fermi level. Consequently, a multilayer step covered by a spacer layer could provide access to the transport properties of the topological edge states. In addition, we find a lateral electronic modulation of the topologically nontrivial surface layer, which is traced back to the coupling with the underlying zigzag chain structure of the spacer layer.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acsnano.6b00841},
      doi = {10.1021/acsnano.6b00841},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Electronically Driven Single-Molecule Switch on Silicon Dangling Bonds
    • A. Nickel, T. Lehmann, J. Meyer, F. Eisenhut, R. Ohmann, D. A. Ryndyk, C. Joachim, F. Moresco, G. Cuniberti
    • Journal of Physical Chemistry C 120, 27027-27032 (2016)
    • DOI   Abstract  

      We demonstrate that a single 4-acetylbiphenyl molecule adsorbed along the dimer row of a Si(100)-(2 X 1) surface can be reversibly switched between two stable conformations using the tunneling current of a scanning tunneling microscope. The experiment supported by density functional theory calculations demonstrates that the molecule by switching selectively passivates and depassivates a dangling-bond pair on the silicon surface, opening new routes for the logical input in dangling-bond-based atomic-scale circuits.

      @article{,
      author = {Anja Nickel and Thomas Lehmann and Jorg Meyer and Frank Eisenhut and Robin Ohmann and Dmitry A. Ryndyk and Christian Joachim and Francesca Moresco and Gianaurelio Cuniberti},
      title = {Electronically Driven Single-Molecule Switch on Silicon Dangling Bonds},
      journal = {Journal of Physical Chemistry C},
      volume = {120},
      number = {47},
      pages = {27027-27032},
      abstract = {We demonstrate that a single 4-acetylbiphenyl molecule adsorbed along the dimer row of a Si(100)-(2 X 1) surface can be reversibly switched between two stable conformations using the tunneling current of a scanning tunneling microscope. The experiment supported by density functional theory calculations demonstrates that the molecule by switching selectively passivates and depassivates a dangling-bond pair on the silicon surface, opening new routes for the logical input in dangling-bond-based atomic-scale circuits.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acs.jpcc.6b05680},
      doi = {10.1021/acs.jpcc.6b05680},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Empirical transport model of strained CNT transistors used for sensor applications
    • C. Wagner, J. Schuster, T. Gessner
    • Journal of Computational Electronics 15, 881-890 (2016)
    • DOI   Abstract  

      We present an empirical model for the near-ballistic transport in carbon nanotube (CNT) transistors used as strain sensors. This model describes the intrinsic effect of strain on the transport in CNTs by taking into account phonon scattering and thermally activated charge carriers. As this model relies on a semiempirical description of the electronic bands, different levels of electronic structure calculations can be used as input. The results show that the electronic structure of strained single-walled CNTs with a radius larger than 0.7 nm can be described by a fully analytical model in the sensing regime. For CNTs with smaller diameter, parameterized data from electronic structure calculations can be used for the model. Depending on the type of CNTs, the conductance can vary by several orders of magnitude when strain is applied, which is consistent with the current literature. Further, we demonstrate the tuning of the sensor by an external gate which allows shifting the signal amplitude. These parameters have to be balanced to get good sensing properties. The impact of (semi-)metallic CNTs on the sensor performance is evaluated, too. Metallic CNTs have to be avoided in order to construct working sensing devices. Due to its basically analytical nature, the transport model can be evolved towards a compact model for circuit simulations.

      @article{,
      author = {Christian Wagner and Joerg Schuster and Thomas Gessner},
      title = {Empirical transport model of strained CNT transistors used for sensor applications},
      journal = {Journal of Computational Electronics},
      volume = {15},
      number = {3},
      pages = {881-890},
      abstract = {We present an empirical model for the near-ballistic transport in carbon nanotube (CNT) transistors used as strain sensors. This model describes the intrinsic effect of strain on the transport in CNTs by taking into account phonon scattering and thermally activated charge carriers. As this model relies on a semiempirical description of the electronic bands, different levels of electronic structure calculations can be used as input. The results show that the electronic structure of strained single-walled CNTs with a radius larger than 0.7 nm can be described by a fully analytical model in the sensing regime. For CNTs with smaller diameter, parameterized data from electronic structure calculations can be used for the model. Depending on the type of CNTs, the conductance can vary by several orders of magnitude when strain is applied, which is consistent with the current literature. Further, we demonstrate the tuning of the sensor by an external gate which allows shifting the signal amplitude. These parameters have to be balanced to get good sensing properties. The impact of (semi-)metallic CNTs on the sensor performance is evaluated, too. Metallic CNTs have to be avoided in order to construct working sensing devices. Due to its basically analytical nature, the transport model can be evolved towards a compact model for circuit simulations.},
      year = {2016},
      url = http://dx.doi.org/{10.1007/s10825-016-0823-4},
      doi = {10.1007/s10825-016-0823-4},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Experimental characterisation and numerical modelling of cutting processes in viscoelastic solids
    • M. Boisly, S. Schuldt, M. Kaestner, Y. Schneider, H. Rohm
    • Journal of Food Engineering 191, 1-9 (2016)
    • DOI   Abstract  

      Rate dependency is an important phenomenon that can be observed during cutting of viscoelastic materials. This paper reports on results of both experimental and theoretical investigations that were obtained by using a polymeric food model system. Viscoelasticity was experimentally observed in relaxation experiments at small strains, and tensile tests were performed for large deformations at different tensile speeds. Finite viscoelasticity with MOONEY-RIVLIN elasticity valid for large displacements was applied for modelling the viscoelastic properties of the food model, and degradation and crack propagation were considered by cohesive fracture mechanisms. Model predictions were prepared by applying the finite element method using Abaqus and compared with experimental results. The good agreement between simulation and measurement especially for the maximum cutting force and the characteristic plateau validate the modelling strategy. (C) 2016 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Martin Boisly and Stefan Schuldt and Markus Kaestner and Yvonne Schneider and Harald Rohm},
      title = {Experimental characterisation and numerical modelling of cutting processes in viscoelastic solids},
      journal = {Journal of Food Engineering},
      volume = {191},
      pages = {1-9},
      abstract = {Rate dependency is an important phenomenon that can be observed during cutting of viscoelastic materials. This paper reports on results of both experimental and theoretical investigations that were obtained by using a polymeric food model system. Viscoelasticity was experimentally observed in relaxation experiments at small strains, and tensile tests were performed for large deformations at different tensile speeds. Finite viscoelasticity with MOONEY-RIVLIN elasticity valid for large displacements was applied for modelling the viscoelastic properties of the food model, and degradation and crack propagation were considered by cohesive fracture mechanisms. Model predictions were prepared by applying the finite element method using Abaqus and compared with experimental results. The good agreement between simulation and measurement especially for the maximum cutting force and the characteristic plateau validate the modelling strategy. (C) 2016 Elsevier Ltd. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jfoodeng.2016.06.019},
      doi = {10.1016/j.jfoodeng.2016.06.019},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Friction and adhesion mediated by supramolecular host-guest complexes
    • R. Guerra, A. Benassi, A. Vanossi, M. Ma, M. Urbakh
    • Physical Chemistry Chemical Physics 18, 9248-9254 (2016)
    • DOI   Abstract  

      The adhesive and frictional response of an AFM tip connected to a substrate through supramolecular host-guest complexes is investigated by dynamic Monte Carlo simulations. Here, the variation of the pull-off force with the unloading rate recently observed in experiments is unraveled by evidencing simultaneous (progressive) breaking of the bonds at fast (slow) rates. The model reveals the origin of the observed plateaus in the retraction force as a function of the tip-surface distance, showing that they result from the tip geometrical features. In lateral sliding, the model exhibits a wide range of dynamic behaviors ranging from smooth sliding to stick-slip at different velocities, with the average friction force determined by the characteristic formation/rupture rates of the complexes. In particular, it is shown that for some molecular complexes friction can become almost constant over a wide range of velocities. Also, we show the possibility of exploiting the ageing effect through slide-hold-slide experiments, in order to infer the characteristic formation rate. Finally, our model predicts a novel "antiageing” effect which is characterized by a decrease of the static friction force with the hold time. Such an effect is explained in terms of enhancement of adhesion during sliding, especially observed at high driving velocities.

      @article{,
      author = {Roberto Guerra and Andrea Benassi and Andrea Vanossi and Ming Ma and Michael Urbakh},
      title = {Friction and adhesion mediated by supramolecular host-guest complexes},
      journal = {Physical Chemistry Chemical Physics},
      volume = {18},
      number = {13},
      pages = {9248-9254},
      abstract = {The adhesive and frictional response of an AFM tip connected to a substrate through supramolecular host-guest complexes is investigated by dynamic Monte Carlo simulations. Here, the variation of the pull-off force with the unloading rate recently observed in experiments is unraveled by evidencing simultaneous (progressive) breaking of the bonds at fast (slow) rates. The model reveals the origin of the observed plateaus in the retraction force as a function of the tip-surface distance, showing that they result from the tip geometrical features. In lateral sliding, the model exhibits a wide range of dynamic behaviors ranging from smooth sliding to stick-slip at different velocities, with the average friction force determined by the characteristic formation/rupture rates of the complexes. In particular, it is shown that for some molecular complexes friction can become almost constant over a wide range of velocities. Also, we show the possibility of exploiting the ageing effect through slide-hold-slide experiments, in order to infer the characteristic formation rate. Finally, our model predicts a novel "antiageing'' effect which is characterized by a decrease of the static friction force with the hold time. Such an effect is explained in terms of enhancement of adhesion during sliding, especially observed at high driving velocities.},
      year = {2016},
      url = http://dx.doi.org/{10.1039/c6cp00661b},
      doi = {10.1039/c6cp00661b},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • From Fluorine to FluoreneA Route to Thermally Stable aza-BODIPYs for Organic Solar Cell Application
    • M. Lorenz-Rothe, K. S. Schellhammer, T. Jaegeler-Hoheisel, R. Meerheim, S. Kraner, M. P. Hein, C. Schuenemann, M. L. Tietze, M. Hummert, F. Ortmann, G. Cuniberti, C. Koerner, K. Leo
    • Advanced Electronic Materials 2(2016)
    • DOI   Abstract  

      Despite favorable absorption characteristics, borondipyrromethenes (BODIPYs) often lack thermal stability preventing their application in vacuum-processed organic solar cells. In this paper, the replacement of the BF2 unit by borafluorene as a new functionalization strategy for this molecule class is explored. This approach is applied to a set of prototype molecules and demonstrates improved thermal stability, strong absorption in the red and near-infrared region of the sun spectrum, as well as excellent solar cell performance. Synthesis is realized from free ligands via complexation with 9-chloro-9-borafluorene giving high yields up to 81%. Planar heterojunction cells of these complexes exhibit high fill factors of more than 70%. Bulk heterojunction solar cells with C-60 are optimized yielding power conversion efficiencies up to 4.5%, rendering the investigated prototype compounds highly competitive among other NIR-absorbing small-molecule donor materials. Comprehensive experimental material characterization and solar cell analysis are carried out, and the results are discussed together with simulations of molecular properties. Based on this analysis, additional performance improvements are proposed by engineering the intramolecular steric interactions towards further red-shifted absorption.

      @article{,
      author = {Melanie Lorenz-Rothe and Karl Sebastian Schellhammer and Till Jaegeler-Hoheisel and Rico Meerheim and Stefan Kraner and Moritz P. Hein and Christoph Schuenemann and Max L. Tietze and Markus Hummert and Frank Ortmann and Gianaurelio Cuniberti and Christian Koerner and Karl Leo},
      title = {From Fluorine to FluoreneA Route to Thermally Stable aza-BODIPYs for Organic Solar Cell Application},
      journal = {Advanced Electronic Materials},
      volume = {2},
      number = {10},
      abstract = {Despite favorable absorption characteristics, borondipyrromethenes (BODIPYs) often lack thermal stability preventing their application in vacuum-processed organic solar cells. In this paper, the replacement of the BF2 unit by borafluorene as a new functionalization strategy for this molecule class is explored. This approach is applied to a set of prototype molecules and demonstrates improved thermal stability, strong absorption in the red and near-infrared region of the sun spectrum, as well as excellent solar cell performance. Synthesis is realized from free ligands via complexation with 9-chloro-9-borafluorene giving high yields up to 81%. Planar heterojunction cells of these complexes exhibit high fill factors of more than 70%. Bulk heterojunction solar cells with C-60 are optimized yielding power conversion efficiencies up to 4.5%, rendering the investigated prototype compounds highly competitive among other NIR-absorbing small-molecule donor materials. Comprehensive experimental material characterization and solar cell analysis are carried out, and the results are discussed together with simulations of molecular properties. Based on this analysis, additional performance improvements are proposed by engineering the intramolecular steric interactions towards further red-shifted absorption.},
      year = {2016},
      url = http://dx.doi.org/{10.1002/aelm.201600152},
      doi = {10.1002/aelm.201600152},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Impact of incomplete metal coverage on the electrical properties of metal-CNT contacts: A large-scale ab initio study
    • A. Fediai, D. A. Ryndyk, G. Seifert, S. Mothes, M. Schroter, M. Claus, G. Cuniberti
    • Applied Physics Letters 109, 159-162 (2016)
    • DOI   Abstract  

      Using a dedicated combination of the non-equilibrium Green function formalism and large-scale density functional theory calculations, we investigated how incomplete metal coverage influences two of the most important electrical properties of carbon nanotube (CNT)-based transistors: contact resistance and its scaling with contact length, and maximum current. These quantities have been derived from parameter-free simulations of atomic systems that are as close as possible to experimental geometries. Physical mechanisms that govern these dependences have been identified for various metals, representing different CNT-metal interaction strengths from chemisorption to physisorption. Our results pave the way for an application-oriented design of CNT-metal contacts. Published by AIP Publishing.

      @article{,
      author = {Artem Fediai and Dmitry A. Ryndyk and Gotthard Seifert and Sven Mothes and Michael Schroter and Martin Claus and Gianaurelio Cuniberti},
      title = {Impact of incomplete metal coverage on the electrical properties of metal-CNT contacts: A large-scale ab initio study},
      journal = {Applied Physics Letters},
      volume = {109},
      number = {10},
      pages = {159-162},
      abstract = {Using a dedicated combination of the non-equilibrium Green function formalism and large-scale density functional theory calculations, we investigated how incomplete metal coverage influences two of the most important electrical properties of carbon nanotube (CNT)-based transistors: contact resistance and its scaling with contact length, and maximum current. These quantities have been derived from parameter-free simulations of atomic systems that are as close as possible to experimental geometries. Physical mechanisms that govern these dependences have been identified for various metals, representing different CNT-metal interaction strengths from chemisorption to physisorption. Our results pave the way for an application-oriented design of CNT-metal contacts. Published by AIP Publishing.},
      year = {2016},
      url = http://dx.doi.org/{10.1063/1.4962439},
      doi = {10.1063/1.4962439},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Implementation of incremental variational formulations based on the numerical calculation of derivatives using hyper dual numbers
    • M. Tanaka, D. Balzani, J. Schroeder
    • Computer Methods in Applied Mechanics and Engineering 301, 216-241 (2016)
    • DOI   Abstract  

      In this paper, novel implementation schemes for the automatic calculation of internal variables, stresses and consistent tangent moduli for incremental variational formulations (IVFs) describing inelastic material behavior are proposed. IVFs recast inelasticity theory as an equivalent optimization problem where the incremental stress potential within a discrete time interval is minimized in order to obtain the values of internal variables. In the so-called Multilevel Newton-Raphson method for the inelasticity theory, this minimization problem is typically solved by using second derivatives with respect to the internal variables. In addition to that, to calculate the stresses and moduli further second derivatives with respect to deformation tensors are required. Compared with classical formulations such as the return mapping method, the IVFs are relatively new and their implementation is much less documented. Furthermore, higher order derivatives are required in the algorithms demanding increased implementation efforts. Therefore, even though IVFs are mathematically and physically elegant, their application is not standard. Here, novel approaches for the implementation of IVFs using HDNs of second and higher order are presented to arrive at a fully automatic and robust scheme with computer accuracy. The proposed formulations are quite general and can be applied to a broad range of different constitutive models, which means that once the proposed schemes are implemented as a framework, any other dissipative material model can be implemented in a straightforward way by solely modifying the constitutive functions. These include the Helmholtz free energy function, the dissipation potential function and additional side constraints such as e.g. the yield function in the case of plasticity. Its uncomplicated implementation for associative finite strain elasto-plasticity and performance is illustrated by some representative numerical examples. (C) 2015 Elsevier B.V. All rights reserved.

      @article{,
      author = {Masato Tanaka and Daniel Balzani and Joerg Schroeder},
      title = {Implementation of incremental variational formulations based on the numerical calculation of derivatives using hyper dual numbers},
      journal = {Computer Methods in Applied Mechanics and Engineering},
      volume = {301},
      pages = {216-241},
      abstract = {In this paper, novel implementation schemes for the automatic calculation of internal variables, stresses and consistent tangent moduli for incremental variational formulations (IVFs) describing inelastic material behavior are proposed. IVFs recast inelasticity theory as an equivalent optimization problem where the incremental stress potential within a discrete time interval is minimized in order to obtain the values of internal variables. In the so-called Multilevel Newton-Raphson method for the inelasticity theory, this minimization problem is typically solved by using second derivatives with respect to the internal variables. In addition to that, to calculate the stresses and moduli further second derivatives with respect to deformation tensors are required. Compared with classical formulations such as the return mapping method, the IVFs are relatively new and their implementation is much less documented. Furthermore, higher order derivatives are required in the algorithms demanding increased implementation efforts. Therefore, even though IVFs are mathematically and physically elegant, their application is not standard. Here, novel approaches for the implementation of IVFs using HDNs of second and higher order are presented to arrive at a fully automatic and robust scheme with computer accuracy. The proposed formulations are quite general and can be applied to a broad range of different constitutive models, which means that once the proposed schemes are implemented as a framework, any other dissipative material model can be implemented in a straightforward way by solely modifying the constitutive functions. These include the Helmholtz free energy function, the dissipation potential function and additional side constraints such as e.g. the yield function in the case of plasticity. Its uncomplicated implementation for associative finite strain elasto-plasticity and performance is illustrated by some representative numerical examples. (C) 2015 Elsevier B.V. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.cma.2015.12.010},
      doi = {10.1016/j.cma.2015.12.010},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Incompressible two-phase flows with an inextensible Newtonian fluid interface
    • S. Reuther, A. Voigt
    • Journal of Computational Physics 322, 850-858 (2016)
    • DOI   Abstract  

      We introduce a diffuse interface approximation for an incompressible two-phase flow problem with an inextensible Newtonian fluid interface. This approach allows to model lipid membranes as viscous fluids. In the present setting the membranes are assumed to be stationary. We validate the model and the numerical approach, which is based on a stream function formulation for the surface flow problem, an operator splitting approach and a semi-implicit adaptive finite element discretization, against observed flow patterns in vesicles, which are adhered to a solid surface and are subjected to shear flow. The influence of the Gaussian curvature on the surface flow pattern is discussed. (C) 2016 Elsevier Inc. All rights reserved.

      @article{,
      author = {Sebastian Reuther and Axel Voigt},
      title = {Incompressible two-phase flows with an inextensible Newtonian fluid interface},
      journal = {Journal of Computational Physics},
      volume = {322},
      pages = {850-858},
      abstract = {We introduce a diffuse interface approximation for an incompressible two-phase flow problem with an inextensible Newtonian fluid interface. This approach allows to model lipid membranes as viscous fluids. In the present setting the membranes are assumed to be stationary. We validate the model and the numerical approach, which is based on a stream function formulation for the surface flow problem, an operator splitting approach and a semi-implicit adaptive finite element discretization, against observed flow patterns in vesicles, which are adhered to a solid surface and are subjected to shear flow. The influence of the Gaussian curvature on the surface flow pattern is discussed. (C) 2016 Elsevier Inc. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jcp.2016.07.023},
      doi = {10.1016/j.jcp.2016.07.023},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Influence of organic ligands on the line shape of the Kondo resonance
    • J. Meyer, R. Ohmann, A. Nickel, C. Toher, R. Gresser, K. Leo, D. A. Ryndyk, F. Moresco, G. Cuniberti
    • Physical Review B 93(2016)
    • DOI   Abstract  

      The Kondo resonance of an organic molecule containing a Co atom is investigated by scanning tunneling spectroscopy and ab initio calculations on a Ag(100) surface. High resolution mapping of the line shape shows evidence of local nonradially symmetric variations of the Fano factor and the Kondo amplitude, revealing a strong influence of the molecular ligand. We show that the decay of the amplitude of the Kondo resonance is determined by the spatial distribution of the ligand’s orbital being hybridized with the singly occupied Co d(z2) orbital, forming together the singly occupied Kondo-active orbital.

      @article{,
      author = {Joerg Meyer and Robin Ohmann and Anja Nickel and Cormac Toher and Roland Gresser and Karl Leo and Dmitry A. Ryndyk and Francesca Moresco and Gianaurelio Cuniberti},
      title = {Influence of organic ligands on the line shape of the Kondo resonance},
      journal = {Physical Review B},
      volume = {93},
      number = {15},
      abstract = {The Kondo resonance of an organic molecule containing a Co atom is investigated by scanning tunneling spectroscopy and ab initio calculations on a Ag(100) surface. High resolution mapping of the line shape shows evidence of local nonradially symmetric variations of the Fano factor and the Kondo amplitude, revealing a strong influence of the molecular ligand. We show that the decay of the amplitude of the Kondo resonance is determined by the spatial distribution of the ligand's orbital being hybridized with the singly occupied Co d(z2) orbital, forming together the singly occupied Kondo-active orbital.},
      year = {2016},
      url = http://dx.doi.org/{10.1103/PhysRevB.93.155118},
      doi = {10.1103/PhysRevB.93.155118},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Influence of the synthesis conditions of gold nanoparticles on the structure and architectonics of dipeptide composites
    • A. I. Loskutov, O. A. Guskova, S. N. Grigoriev, V. B. Oshurko, A. V. Tarasiuk, O. Y. Uryupina
    • Journal of Nanoparticle Research 18(2016)
    • DOI   Abstract  

      A wide variety of peptides and their natural ability to self-assemble makes them very promising candidates for the fabrication of solid-state devices based on nano- and mesocrystals. In this work, we demonstrate an approach to form peptide composite layers with gold nanoparticles through in situ reduction of chloroauric acid trihydrate by dipeptide and/or dipeptide/formaldehyde mixture in the presence of potassium carbonate at different ratios of components. Appropriate composition of components for the synthesis of highly stable gold colloidal dispersion with particle size of 34-36 nm in dipeptide/formaldehyde solution is formulated. Infrared spectroscopy results indicate that dipeptide participates in the reduction process, conjugation with gold nanoparticles and the self-assembly in 2D, which accompanied by changing peptide chain conformations. The structure and morphology of the peptide composite solid layers with gold nanoparticles on gold, mica and silica surfaces are characterized by atomic force microscopy. In these experiments, the flat particles, dendrites, chains, mesocrystals and Janus particles are observed depending on the solution composition and the substrate/interface used. The latter aspect is studied on the molecular level using computer simulations of individual peptide chains on gold, mica and silica surfaces.

      @article{,
      author = {Alexander I. Loskutov and Olga A. Guskova and Sergey N. Grigoriev and Vadim B. Oshurko and Aleksei V. Tarasiuk and Olga Ya Uryupina},
      title = {Influence of the synthesis conditions of gold nanoparticles on the structure and architectonics of dipeptide composites},
      journal = {Journal of Nanoparticle Research},
      volume = {18},
      number = {8},
      abstract = {A wide variety of peptides and their natural ability to self-assemble makes them very promising candidates for the fabrication of solid-state devices based on nano- and mesocrystals. In this work, we demonstrate an approach to form peptide composite layers with gold nanoparticles through in situ reduction of chloroauric acid trihydrate by dipeptide and/or dipeptide/formaldehyde mixture in the presence of potassium carbonate at different ratios of components. Appropriate composition of components for the synthesis of highly stable gold colloidal dispersion with particle size of 34-36 nm in dipeptide/formaldehyde solution is formulated. Infrared spectroscopy results indicate that dipeptide participates in the reduction process, conjugation with gold nanoparticles and the self-assembly in 2D, which accompanied by changing peptide chain conformations. The structure and morphology of the peptide composite solid layers with gold nanoparticles on gold, mica and silica surfaces are characterized by atomic force microscopy. In these experiments, the flat particles, dendrites, chains, mesocrystals and Janus particles are observed depending on the solution composition and the substrate/interface used. The latter aspect is studied on the molecular level using computer simulations of individual peptide chains on gold, mica and silica surfaces.},
      year = {2016},
      url = http://dx.doi.org/{10.1007/s11051-016-3548-1},
      doi = {10.1007/s11051-016-3548-1},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Isogeometric analysis of the Cahn-Hilliard equation – a convergence study
    • M. Kaestner, P. Metsch, R. de Borst
    • Journal of Computational Physics 305, 360-371 (2016)
    • DOI   Abstract  

      Herein, we present a numerical convergence study of the Cahn-Hilliard phase-field model within an isogeometric finite element analysis framework. Using a manufactured solution, a mixed formulation of the Cahn-Hilliard equation and the direct discretisation of the weak form, which requires a C-1-continuous approximation, are compared in terms of convergence rates. For approximations that are higher than second-order in space, the direct discretisation is found to be superior. Suboptimal convergence rates occur when splines of order p = 2 are used. This is validated with a priori error estimates for linear problems. The convergence analysis is completed with an investigation of the temporal discretisation. Second-order accuracy is found for the generalised-alpha method. This ensures the functionality of an adaptive time stepping scheme which is required for the efficient numerical solution of the Cahn-Hilliard equation. The isogeometric finite element framework is eventually validated by two numerical examples of spinodal decomposition. (C) 2015 Elsevier Inc. All rights reserved.

      @article{,
      author = {Markus Kaestner and Philipp Metsch and Rene de Borst},
      title = {Isogeometric analysis of the Cahn-Hilliard equation - a convergence study},
      journal = {Journal of Computational Physics},
      volume = {305},
      pages = {360-371},
      abstract = {Herein, we present a numerical convergence study of the Cahn-Hilliard phase-field model within an isogeometric finite element analysis framework. Using a manufactured solution, a mixed formulation of the Cahn-Hilliard equation and the direct discretisation of the weak form, which requires a C-1-continuous approximation, are compared in terms of convergence rates. For approximations that are higher than second-order in space, the direct discretisation is found to be superior. Suboptimal convergence rates occur when splines of order p = 2 are used. This is validated with a priori error estimates for linear problems. The convergence analysis is completed with an investigation of the temporal discretisation. Second-order accuracy is found for the generalised-alpha method. This ensures the functionality of an adaptive time stepping scheme which is required for the efficient numerical solution of the Cahn-Hilliard equation. The isogeometric finite element framework is eventually validated by two numerical examples of spinodal decomposition. (C) 2015 Elsevier Inc. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jcp.2015.10.047},
      doi = {10.1016/j.jcp.2015.10.047},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Microscale modeling and simulation of magnetorheological elastomers at finite strains: A study on the influence of mechanical preloads
    • K. A. Kalina, P. Metsch, M. Kaestner
    • International Journal of Solids and Structures 102, 286-296 (2016)
    • DOI   Abstract  

      Herein, we present a numerical study on the deformation dependent behavior of magnetorheological elastomers with structured and unstructured particle distributions. To this end, finite element simulations are performed in order to calculate the effective magnetization and macroscopic actuation stresses for different specimens with realistic microstructures and varying mechanical preloads. Since the proposed microscale model is based on a continuum formulation of the magnetomechanical boundary value problem, the local magnetic and mechanical fields are resolved explicitly within the microstructures. The consideration of finite strains results in a finite element implementation of the coupled field problem for which a consistent linearization scheme is presented. In order to provide a better understanding of the deformation dependent behavior in real specimens, a study on chain-like structures is performed. It reveals that the interaction of the constituents in chain-like structures yields different material responses depending on their position. These findings are used to explain the influence of mechanical preloads on the behavior of samples with structured and unstructured arrangements of the particles. All of our results are in good agreement with experimental investigations which have been carried out for magnetorheological elastomers comprising a structured particle distribution. (C) 2016 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Karl A. Kalina and Philipp Metsch and Markus Kaestner},
      title = {Microscale modeling and simulation of magnetorheological elastomers at finite strains: A study on the influence of mechanical preloads},
      journal = {International Journal of Solids and Structures},
      volume = {102},
      pages = {286-296},
      abstract = {Herein, we present a numerical study on the deformation dependent behavior of magnetorheological elastomers with structured and unstructured particle distributions. To this end, finite element simulations are performed in order to calculate the effective magnetization and macroscopic actuation stresses for different specimens with realistic microstructures and varying mechanical preloads. Since the proposed microscale model is based on a continuum formulation of the magnetomechanical boundary value problem, the local magnetic and mechanical fields are resolved explicitly within the microstructures. The consideration of finite strains results in a finite element implementation of the coupled field problem for which a consistent linearization scheme is presented. In order to provide a better understanding of the deformation dependent behavior in real specimens, a study on chain-like structures is performed. It reveals that the interaction of the constituents in chain-like structures yields different material responses depending on their position. These findings are used to explain the influence of mechanical preloads on the behavior of samples with structured and unstructured arrangements of the particles. All of our results are in good agreement with experimental investigations which have been carried out for magnetorheological elastomers comprising a structured particle distribution. (C) 2016 Elsevier Ltd. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.ijsolstr.2016.10.019},
      doi = {10.1016/j.ijsolstr.2016.10.019},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • A microscopic field theoretical approach for active systems
    • F. Alaimo, S. Praetorius, A. Voigt
    • New Journal of Physics 18(2016)
    • DOI   Abstract  

      We consider a microscopic modeling approach for active systems. The approach extends the phase field crystal (PFC) model and allows us to describe generic properties of active systems within a continuum model. The approach is validated by reproducing results obtained with corresponding agent-based and microscopic phase field models. We consider binary collisions, collective motion and vortex formation. For larger numbers of particles we analyze the coarsening process in active crystals and identify giant number fluctuation in a cluster formation process.

      @article{,
      author = {F. Alaimo and S. Praetorius and A. Voigt},
      title = {A microscopic field theoretical approach for active systems},
      journal = {New Journal of Physics},
      volume = {18},
      abstract = {We consider a microscopic modeling approach for active systems. The approach extends the phase field crystal (PFC) model and allows us to describe generic properties of active systems within a continuum model. The approach is validated by reproducing results obtained with corresponding agent-based and microscopic phase field models. We consider binary collisions, collective motion and vortex formation. For larger numbers of particles we analyze the coarsening process in active crystals and identify giant number fluctuation in a cluster formation process.},
      year = {2016},
      url = http://dx.doi.org/{10.1088/1367-2630/18/8/083008},
      doi = {10.1088/1367-2630/18/8/083008},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • The modular approach enables a fully ab initio simulation of the contacts between 3D and 2D materials
    • A. Fediai, D. A. Ryndyk, G. Cuniberti
    • Journal of Physics-Condensed Matter 28(2016)
    • DOI   Abstract  

      Up to now, the electrical properties of the contacts between 3D metals and 2D materials have never been computed at a fully ab initio level due to the huge number of atomic orbitals involved in a current path from an electrode to a pristine 2D material. As a result, there are still numerous open questions and controversial theories on the electrical properties of systems with 3D/2D interfaces-for example, the current path and the contact length scalability. Our work provides a first-principles solution to this long-standing problem with the use of the modular approach, a method which rigorously combines a Green function formalism with the density functional theory (DFT) for this particular contact type. The modular approach is a general approach valid for any 3D/2D contact. As an example, we apply it to the most investigated among 3D/2D contacts-metal/graphene contacts-and show its abilities and consistency by comparison with existing experimental data. As it is applicable to any 3D/2D interface, the modular approach allows the engineering of 3D/2D contacts with the pre-defined electrical properties.

      @article{,
      author = {Artem Fediai and Dmitry A. Ryndyk and Gianaurelio Cuniberti},
      title = {The modular approach enables a fully ab initio simulation of the contacts between 3D and 2D materials},
      journal = {Journal of Physics-Condensed Matter},
      volume = {28},
      number = {39},
      abstract = {Up to now, the electrical properties of the contacts between 3D metals and 2D materials have never been computed at a fully ab initio level due to the huge number of atomic orbitals involved in a current path from an electrode to a pristine 2D material. As a result, there are still numerous open questions and controversial theories on the electrical properties of systems with 3D/2D interfaces-for example, the current path and the contact length scalability. Our work provides a first-principles solution to this long-standing problem with the use of the modular approach, a method which rigorously combines a Green function formalism with the density functional theory (DFT) for this particular contact type. The modular approach is a general approach valid for any 3D/2D contact. As an example, we apply it to the most investigated among 3D/2D contacts-metal/graphene contacts-and show its abilities and consistency by comparison with existing experimental data. As it is applicable to any 3D/2D interface, the modular approach allows the engineering of 3D/2D contacts with the pre-defined electrical properties.},
      year = {2016},
      url = http://dx.doi.org/{10.1088/0953-8984/28/39/395303},
      doi = {10.1088/0953-8984/28/39/395303},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • A novel mixed finite element for finite anisotropic elasticity; the SKA-element Simplified Kinematics for Anisotropy
    • J. Schroeder, N. Viebahn, D. Balzani, P. Wriggers
    • Computer Methods in Applied Mechanics and Engineering 310, 475-494 (2016)
    • DOI   Abstract  

      A variety of numerical approximation schemes for boundary value problems suffer from so-called locking-phenomena. It is well known that in such cases several finite element formulations exhibit poor convergence rates in the basic variables. A serious locking phenomenon can be observed in the case of anisotropic elasticity, due to high stiffness in preferred directions. The main goal of this paper is to overcome this locking problem in anisotropic hyperelasticity by introducing a novel mixed variational framework. Therefore we split the strain energy into two main parts, an isotropic and an anisotropic part. For the isotropic part we can apply different well-established approximation schemes and for the anisotropic part we apply a constant approximation of the deformation gradient or the right Cauchy-Green tensor. This additional constraint is attached to the strain energy function by a second-order tensorial Lagrange-multiplier, governed by a Simplified Kinematic for the Anisotropic part. As a matter of fact, for the tested boundary value problems the SKA-element based on quadratic ansatz functions for the displacements, performs excellent and behaves more robust than competitive formulations. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {Joerg Schroeder and Nils Viebahn and Daniel Balzani and Peter Wriggers},
      title = {A novel mixed finite element for finite anisotropic elasticity; the SKA-element Simplified Kinematics for Anisotropy},
      journal = {Computer Methods in Applied Mechanics and Engineering},
      volume = {310},
      pages = {475-494},
      abstract = {A variety of numerical approximation schemes for boundary value problems suffer from so-called locking-phenomena. It is well known that in such cases several finite element formulations exhibit poor convergence rates in the basic variables. A serious locking phenomenon can be observed in the case of anisotropic elasticity, due to high stiffness in preferred directions. The main goal of this paper is to overcome this locking problem in anisotropic hyperelasticity by introducing a novel mixed variational framework. Therefore we split the strain energy into two main parts, an isotropic and an anisotropic part. For the isotropic part we can apply different well-established approximation schemes and for the anisotropic part we apply a constant approximation of the deformation gradient or the right Cauchy-Green tensor. This additional constraint is attached to the strain energy function by a second-order tensorial Lagrange-multiplier, governed by a Simplified Kinematic for the Anisotropic part. As a matter of fact, for the tested boundary value problems the SKA-element based on quadratic ansatz functions for the displacements, performs excellent and behaves more robust than competitive formulations. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.cma.2016.06.029},
      doi = {10.1016/j.cma.2016.06.029},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Numerical modeling of fluid-structure interaction in arteries with anisotropic polyconvex hyperelastic and anisotropic viscoelastic material models at finite strains
    • D. Balzani, S. Deparis, S. Fausten, D. Forti, A. Heinlein, A. Klawonn, A. Quarteroni, O. Rheinbach, J. Schroder
    • International journal for numerical methods in biomedical engineering 32(2016)
    • DOI   Abstract  

      The accurate prediction of transmural stresses in arterial walls requires on the one hand robust and efficient numerical schemes for the solution of boundary value problems including fluid-structure interactions and on the other hand the use of a material model for the vessel wall that is able to capture the relevant features of the material behavior. One of the main contributions of this paper is the application of a highly nonlinear, polyconvex anisotropic structural model for the solid in the context of fluid-structure interaction, together with a suitable discretization. Additionally, the influence of viscoelasticity is investigated. The fluid-structure interaction problem is solved using a monolithic approach; that is, the nonlinear system is solved (after time and space discretizations) as a whole without splitting among its components. The linearized block systems are solved iteratively using parallel domain decomposition preconditioners. A simple – but nonsymmetric – curved geometry is proposed that is demonstrated to be suitable as a benchmark testbed for fluid-structure interaction simulations in biomechanics where nonlinear structural models are used. Based on the curved benchmark geometry, the influence of different material models, spatial discretizations, and meshes of varying refinement is investigated. It turns out that often-used standard displacement elements with linear shape functions are not sufficient to provide good approximations of the arterial wall stresses, whereas for standard displacement elements or F-bar formulations with quadratic shape functions, suitable results are obtained. For the time discretization, a second-order backward differentiation formula scheme is used. It is shown that the curved geometry enables the analysis of non-rotationally symmetric distributions of the mechanical fields. For instance, the maximal shear stresses in the fluid-structure interface are found to be higher in the inner curve that corresponds to clinical observations indicating a high plaque nucleation probability at such locations. Copyright 2015 John Wiley & Sons, Ltd.

      @article{,
      author = {Daniel Balzani and Simone Deparis and Simon Fausten and Davide Forti and Alexander Heinlein and Axel Klawonn and Alfio Quarteroni and Oliver Rheinbach and Joerg Schroder},
      title = {Numerical modeling of fluid-structure interaction in arteries with anisotropic polyconvex hyperelastic and anisotropic viscoelastic material models at finite strains},
      journal = {International journal for numerical methods in biomedical engineering},
      volume = {32},
      number = {10},
      abstract = {The accurate prediction of transmural stresses in arterial walls requires on the one hand robust and efficient numerical schemes for the solution of boundary value problems including fluid-structure interactions and on the other hand the use of a material model for the vessel wall that is able to capture the relevant features of the material behavior. One of the main contributions of this paper is the application of a highly nonlinear, polyconvex anisotropic structural model for the solid in the context of fluid-structure interaction, together with a suitable discretization. Additionally, the influence of viscoelasticity is investigated. The fluid-structure interaction problem is solved using a monolithic approach; that is, the nonlinear system is solved (after time and space discretizations) as a whole without splitting among its components. The linearized block systems are solved iteratively using parallel domain decomposition preconditioners. A simple - but nonsymmetric - curved geometry is proposed that is demonstrated to be suitable as a benchmark testbed for fluid-structure interaction simulations in biomechanics where nonlinear structural models are used. Based on the curved benchmark geometry, the influence of different material models, spatial discretizations, and meshes of varying refinement is investigated. It turns out that often-used standard displacement elements with linear shape functions are not sufficient to provide good approximations of the arterial wall stresses, whereas for standard displacement elements or F-bar formulations with quadratic shape functions, suitable results are obtained. For the time discretization, a second-order backward differentiation formula scheme is used. It is shown that the curved geometry enables the analysis of non-rotationally symmetric distributions of the mechanical fields. For instance, the maximal shear stresses in the fluid-structure interface are found to be higher in the inner curve that corresponds to clinical observations indicating a high plaque nucleation probability at such locations. Copyright 2015 John Wiley & Sons, Ltd.},
      year = {2016},
      url = http://dx.doi.org/{10.1002/cnm.2756},
      doi = {10.1002/cnm.2756},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Numerical modeling of fluid-structure interaction in arteries with anisotropic polyconvex hyperelastic and anisotropic viscoelastic material models at finite strains
    • D. Balzani, S. Deparis, S. Fausten, D. Forti, A. Heinlein, A. Klawonn, A. Quarteroni, O. Rheinbach, J. Schroeder
    • International Journal for Numerical Methods in Biomedical Engineering 32(2016)
    • DOI   Abstract  

      The accurate prediction of transmural stresses in arterial walls requires on the one hand robust and efficient numerical schemes for the solution of boundary value problems including fluid-structure interactions and on the other hand the use of a material model for the vessel wall that is able to capture the relevant features of the material behavior. One of the main contributions of this paper is the application of a highly nonlinear, polyconvex anisotropic structural model for the solid in the context of fluid-structure interaction, together with a suitable discretization. Additionally, the influence of viscoelasticity is investigated. The fluid-structure interaction problem is solved using a monolithic approach; that is, the nonlinear system is solved (after time and space discretizations) as a whole without splitting among its components. The linearized block systems are solved iteratively using parallel domain decomposition preconditioners. A simple – but nonsymmetric – curved geometry is proposed that is demonstrated to be suitable as a benchmark testbed for fluid-structure interaction simulations in biomechanics where nonlinear structural models are used. Based on the curved benchmark geometry, the influence of different material models, spatial discretizations, and meshes of varying refinement is investigated. It turns out that often-used standard displacement elements with linear shape functions are not sufficient to provide good approximations of the arterial wall stresses, whereas for standard displacement elements or F-bar formulations with quadratic shape functions, suitable results are obtained. For the time discretization, a second-order backward differentiation formula scheme is used. It is shown that the curved geometry enables the analysis of non-rotationally symmetric distributions of the mechanical fields. For instance, the maximal shear stresses in the fluid-structure interface are found to be higher in the inner curve that corresponds to clinical observations indicating a high plaque nucleation probability at such locations. Copyright (C) 2015 John Wiley & Sons, Ltd.

      @article{,
      author = {Daniel Balzani and Simone Deparis and Simon Fausten and Davide Forti and Alexander Heinlein and Axel Klawonn and Alfio Quarteroni and Oliver Rheinbach and Joerg Schroeder},
      title = {Numerical modeling of fluid-structure interaction in arteries with anisotropic polyconvex hyperelastic and anisotropic viscoelastic material models at finite strains},
      journal = {International Journal for Numerical Methods in Biomedical Engineering},
      volume = {32},
      number = {10},
      abstract = {The accurate prediction of transmural stresses in arterial walls requires on the one hand robust and efficient numerical schemes for the solution of boundary value problems including fluid-structure interactions and on the other hand the use of a material model for the vessel wall that is able to capture the relevant features of the material behavior. One of the main contributions of this paper is the application of a highly nonlinear, polyconvex anisotropic structural model for the solid in the context of fluid-structure interaction, together with a suitable discretization. Additionally, the influence of viscoelasticity is investigated. The fluid-structure interaction problem is solved using a monolithic approach; that is, the nonlinear system is solved (after time and space discretizations) as a whole without splitting among its components. The linearized block systems are solved iteratively using parallel domain decomposition preconditioners. A simple - but nonsymmetric - curved geometry is proposed that is demonstrated to be suitable as a benchmark testbed for fluid-structure interaction simulations in biomechanics where nonlinear structural models are used. Based on the curved benchmark geometry, the influence of different material models, spatial discretizations, and meshes of varying refinement is investigated. It turns out that often-used standard displacement elements with linear shape functions are not sufficient to provide good approximations of the arterial wall stresses, whereas for standard displacement elements or F-bar formulations with quadratic shape functions, suitable results are obtained. For the time discretization, a second-order backward differentiation formula scheme is used. It is shown that the curved geometry enables the analysis of non-rotationally symmetric distributions of the mechanical fields. For instance, the maximal shear stresses in the fluid-structure interface are found to be higher in the inner curve that corresponds to clinical observations indicating a high plaque nucleation probability at such locations. Copyright (C) 2015 John Wiley & Sons, Ltd.},
      year = {2016},
      url = http://dx.doi.org/{10.1002/cnm.2756},
      doi = {10.1002/cnm.2756},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • A numerical study on magnetostrictive phenomena in magnetorheological elastomers
    • P. Metsch, K. A. Kalina, C. Spieler, M. Kaestner
    • Computational Materials Science 124, 364-374 (2016)
    • DOI   Abstract  

      Herein, we present an investigation on magnetostrictive phenomena in magnetorheological elastomers. By using a continuum approach, constitutive as well as geometric properties on the microscale are taken into account in order to predict the effective behavior of these composites by means of a computational homogenization. Thus, the magnetic and mechanical fields are resolved explicitly without the simplifying assumption of dipoles. In the present work, a modeling strategy which accounts for elastic constituents and a nonlinear magnetization behavior of the particles is pursued. In order to provide a better understanding of fundamental deformation mechanisms, idealized lattices as well as compact and wavy chains are considered within a first study. Our results confirm assumptions stated in the literature according to which macroscopic magnetostriction can be ascribed to microscopic particle movements that result in an improved microstructure. The simulations that are performed for the subsequent investigations on random microstructures with different particle-volume fractions are evaluated statistically to ensure validity of our findings. They reveal anisotropic as well as isotropic macroscopic behavior for structured and unstructured particle distributions, respectively. In view of the macroscopic magnetostriction, all the results presented in this contribution are in good agreement with current experimental and theoretical findings. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {Philipp Metsch and Karl A. Kalina and Christian Spieler and Markus Kaestner},
      title = {A numerical study on magnetostrictive phenomena in magnetorheological elastomers},
      journal = {Computational Materials Science},
      volume = {124},
      pages = {364-374},
      abstract = {Herein, we present an investigation on magnetostrictive phenomena in magnetorheological elastomers. By using a continuum approach, constitutive as well as geometric properties on the microscale are taken into account in order to predict the effective behavior of these composites by means of a computational homogenization. Thus, the magnetic and mechanical fields are resolved explicitly without the simplifying assumption of dipoles. In the present work, a modeling strategy which accounts for elastic constituents and a nonlinear magnetization behavior of the particles is pursued. In order to provide a better understanding of fundamental deformation mechanisms, idealized lattices as well as compact and wavy chains are considered within a first study. Our results confirm assumptions stated in the literature according to which macroscopic magnetostriction can be ascribed to microscopic particle movements that result in an improved microstructure. The simulations that are performed for the subsequent investigations on random microstructures with different particle-volume fractions are evaluated statistically to ensure validity of our findings. They reveal anisotropic as well as isotropic macroscopic behavior for structured and unstructured particle distributions, respectively. In view of the macroscopic magnetostriction, all the results presented in this contribution are in good agreement with current experimental and theoretical findings. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.commatsci.2016.08.012},
      doi = {10.1016/j.commatsci.2016.08.012},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Orthogonal experimental design of titanium dioxide-Poly(methyl methacrylate) electrospun nanocomposite membranes for photocatalytic applications
    • A. Vild, S. Teixeira, K. Kuehn, G. Cuniberti, V. Sencadas
    • Journal of Environmental Chemical Engineering 4, 3151-3158 (2016)
    • DOI   Abstract  

      An orthogonal experimental method was designed to assess the influence of the electrospinning processing parameters on average diameter and distribution of poly(methyl methacrylate) (PMMA) fibers. Based on the orthogonal experimental design analysis, electrospun TiO2-PMMA nanocomposites were processed with the optimal polymer processing conditions to obtain thin fibers with a high overall surface area. Further it was found that the average fiber diameter decreases from 2.0 +/- 0.5 down to 1.2 +/- 0.2 mu m with increasing photocatalyst content. Moreover, the wettability of samples was independent of the filler amount, and showed strong hydrophobic behavior. Thermogravimetric analysis showed that for polymer solutions with concentrations higher than 10 wt%, there was a loss of the photocatalytic particles during processing, being more evident for the sample with 40 wt% particles present in the solution, with a loss of 8 wt% of ceramic particles. The immobilization of the TiO2 nanoparticles in the polymer fibers led to an increase of the thermal stability. The photocatalytic performance was assessed by using methylene blue (MB). The nanocomposite electrospun fiber membranes had a remarkable photocatalytic activity, especially the one with higher amount of TiO2, with all the MB dye being removed from the solution after 100 min, under UV. The orthogonal experimental design is an efficient way to save time and materials in the production of photocatalytic membranes. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.

      @article{,
      author = {Andrew Vild and Sara Teixeira and Klaus Kuehn and Gianaurelio Cuniberti and Vitor Sencadas},
      title = {Orthogonal experimental design of titanium dioxide-Poly(methyl methacrylate) electrospun nanocomposite membranes for photocatalytic applications},
      journal = {Journal of Environmental Chemical Engineering},
      volume = {4},
      number = {3},
      pages = {3151-3158},
      abstract = {An orthogonal experimental method was designed to assess the influence of the electrospinning processing parameters on average diameter and distribution of poly(methyl methacrylate) (PMMA) fibers. Based on the orthogonal experimental design analysis, electrospun TiO2-PMMA nanocomposites were processed with the optimal polymer processing conditions to obtain thin fibers with a high overall surface area. Further it was found that the average fiber diameter decreases from 2.0 +/- 0.5 down to 1.2 +/- 0.2 mu m with increasing photocatalyst content. Moreover, the wettability of samples was independent of the filler amount, and showed strong hydrophobic behavior. Thermogravimetric analysis showed that for polymer solutions with concentrations higher than 10 wt%, there was a loss of the photocatalytic particles during processing, being more evident for the sample with 40 wt% particles present in the solution, with a loss of 8 wt% of ceramic particles. The immobilization of the TiO2 nanoparticles in the polymer fibers led to an increase of the thermal stability. The photocatalytic performance was assessed by using methylene blue (MB). The nanocomposite electrospun fiber membranes had a remarkable photocatalytic activity, especially the one with higher amount of TiO2, with all the MB dye being removed from the solution after 100 min, under UV. The orthogonal experimental design is an efficient way to save time and materials in the production of photocatalytic membranes. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jece.2016.06.029},
      doi = {10.1016/j.jece.2016.06.029},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Photocatalytic degradation of pharmaceuticals present in conventional treated wastewater by nanoparticle suspensions
    • S. Teixeira, R. Gurke, H. Eckert, K. Kuehn, J. Fauler, G. Cuniberti
    • Journal of Environmental Chemical Engineering 4, 287-292 (2016)
    • DOI   Abstract  

      Pharmaceuticals have become an important public health issue as environmental pollutants over the last years. After ingestion, pharmaceuticals are partly excreted unchanged. They can reach the wastewater treatment plant (WWTP) via the sewer network. Because the conventional treatments are ineffective in their removal, new methods should be approached, for example semiconductor photocatalysis. Several of the hitherto published studies analyzed the degradation of model pollutants but for the degradation of pharmaceuticals in unspiked real wastewater further investigations are required. Therefore, we want to focus on the removal of pharmaceuticals in an actual effluent from a WWTP and investigate the effluent background effect. This study shows the heterogeneous photocatalytic degradation of 14 pharmaceuticals with initial concentrations C-i > 0.3 mu g L (1) present in a WWTP effluent. We found that UVA (1.5 mW cm (2), intensity peak at 365 nm) irradiation of TiO2 P25 (A(s) = 56 m(2) g (1)) or ZnO (A(s) = 5.23 m(2) g (1)) nanoparticles leads to considerable degradation of the analyzed pharmaceuticals. With ZnO nanoparticles, 40 min UVA irradiation was sufficient to degrade over 95% of these pharmaceuticals (k(app) = 8.6 x 10 (2) s (1)). Using TiO2 P25 on the other hand, it would take more than six times longer to reach the same level (k(app) = 1.4 x 10 (2) s (1)). Carbamazepine dissolved in millipore water served as a comparison model. Also in this system ZnO presents faster degradation. (C) 2015 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Sara Teixeira and Robert Gurke and Hagen Eckert and Klaus Kuehn and Joachim Fauler and Gianaurelio Cuniberti},
      title = {Photocatalytic degradation of pharmaceuticals present in conventional treated wastewater by nanoparticle suspensions},
      journal = {Journal of Environmental Chemical Engineering},
      volume = {4},
      number = {1},
      pages = {287-292},
      abstract = {Pharmaceuticals have become an important public health issue as environmental pollutants over the last years. After ingestion, pharmaceuticals are partly excreted unchanged. They can reach the wastewater treatment plant (WWTP) via the sewer network. Because the conventional treatments are ineffective in their removal, new methods should be approached, for example semiconductor photocatalysis. Several of the hitherto published studies analyzed the degradation of model pollutants but for the degradation of pharmaceuticals in unspiked real wastewater further investigations are required. Therefore, we want to focus on the removal of pharmaceuticals in an actual effluent from a WWTP and investigate the effluent background effect. This study shows the heterogeneous photocatalytic degradation of 14 pharmaceuticals with initial concentrations C-i > 0.3 mu g L (1) present in a WWTP effluent. We found that UVA (1.5 mW cm (2), intensity peak at 365 nm) irradiation of TiO2 P25 (A(s) = 56 m(2) g (1)) or ZnO (A(s) = 5.23 m(2) g (1)) nanoparticles leads to considerable degradation of the analyzed pharmaceuticals. With ZnO nanoparticles, 40 min UVA irradiation was sufficient to degrade over 95% of these pharmaceuticals (k(app) = 8.6 x 10 (2) s (1)). Using TiO2 P25 on the other hand, it would take more than six times longer to reach the same level (k(app) = 1.4 x 10 (2) s (1)). Carbamazepine dissolved in millipore water served as a comparison model. Also in this system ZnO presents faster degradation. (C) 2015 Elsevier Ltd. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jece.2015.10.045},
      doi = {10.1016/j.jece.2015.10.045},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Probing Silica-Biomolecule Interactions by Solid-State NMR and Molecular Dynamics Simulations
    • S. I. Brueckner, S. Donets, A. Dianat, M. Bobeth, R. Gutierrez, G. Cuniberti, E. Brunner
    • Langmuir 32, 11698-11705 (2016)
    • DOI   Abstract  

      Understanding the molecular interactions between inorganic phases such as silica and organic material is fundamental for chromatographic applications, for tailoring silica enzyme interactions, and for elucidating the mechanisms of biomineralization. The formation, structure, and properties of the organic/inorganic interface is crucial in this context. Here, we investigate the interaction of selectively C-13-labeled choline with Si-29-labeled monosilicic acid/silica at the molecular level. Silica/choline nanocomposites were analyzed by solid-state NMR spectroscopy in combination with extended molecular dynamics (MD) simulations to understand the silica/organic interface. Cross polarization magic angle spinning (CP MAS)-based NMR experiments like H-1-C-13 CP-REDOR (rotational-echo double resonance), H-1-C-13 HETCOR (heteronuclear correlation), and H-1-Si-29-H-1 double CP are employed to determine spatial parameters. The measurement of Si-29-C-13 internuclear distances for selectively C-13-labeled choline provides an experimental parameter that allows the direct verification of MD simulations. Atomistic modeling using classical MD methodologies is performed using the INTERFACE force field. The modeling results are in excellent agreement with the experimental data and reveal the relevant molecular conformations as well as the nature and interplay of the interactions between the choline cation and the silica surface. Electrostatic interactions and hydrogen bonding are both important and depend strongly on the hydration level as well as the charge state of the silica surface.

      @article{,
      author = {Stephan Ingmar Brueckner and Sergii Donets and Arezoo Dianat and Manfred Bobeth and Rafael Gutierrez and Gianaurelio Cuniberti and Eike Brunner},
      title = {Probing Silica-Biomolecule Interactions by Solid-State NMR and Molecular Dynamics Simulations},
      journal = {Langmuir},
      volume = {32},
      number = {44},
      pages = {11698-11705},
      abstract = {Understanding the molecular interactions between inorganic phases such as silica and organic material is fundamental for chromatographic applications, for tailoring silica enzyme interactions, and for elucidating the mechanisms of biomineralization. The formation, structure, and properties of the organic/inorganic interface is crucial in this context. Here, we investigate the interaction of selectively C-13-labeled choline with Si-29-labeled monosilicic acid/silica at the molecular level. Silica/choline nanocomposites were analyzed by solid-state NMR spectroscopy in combination with extended molecular dynamics (MD) simulations to understand the silica/organic interface. Cross polarization magic angle spinning (CP MAS)-based NMR experiments like H-1-C-13 CP-REDOR (rotational-echo double resonance), H-1-C-13 HETCOR (heteronuclear correlation), and H-1-Si-29-H-1 double CP are employed to determine spatial parameters. The measurement of Si-29-C-13 internuclear distances for selectively C-13-labeled choline provides an experimental parameter that allows the direct verification of MD simulations. Atomistic modeling using classical MD methodologies is performed using the INTERFACE force field. The modeling results are in excellent agreement with the experimental data and reveal the relevant molecular conformations as well as the nature and interplay of the interactions between the choline cation and the silica surface. Electrostatic interactions and hydrogen bonding are both important and depend strongly on the hydration level as well as the charge state of the silica surface.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acs.langmuir.6b03311},
      doi = {10.1021/acs.langmuir.6b03311},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • ReaxFF(+)-A New Reactive Force Field Method for the Accurate Description of Ionic Systems and Its Application to the Hydrolyzation of Aluminosilicates
    • O. Boehm, S. Pfadenhauer, R. Leitsmann, P. Planitz, E. Schreiner, M. Schreiber
    • Journal of Physical Chemistry C 120, 10849-10856 (2016)
    • DOI   Abstract  

      In this paper we present a powerful extension of the reactive force field method ReaxFF, which we call ReaxFF(+). It combines the charge equilibrium scheme with the bond order principle. The main advantage of this procedure is the correct distinction and description of covalent and ionic bonds. It allows reactive molecular dynamic simulations in ionic gases and liquids. To demonstrate the accuracy of this new method, we study the hydrolyzation of aluminosilicates. Comparing the results with experimental and ab initio data, we can prove the high accuracy of our method. This shows that ReaxFF(+) is a powerful force field simulation tool for reactions in acidic or alkaline environments.

      @article{,
      author = {Oliver Boehm and Stephan Pfadenhauer and Roman Leitsmann and Philipp Planitz and Eduard Schreiner and Michael Schreiber},
      title = {ReaxFF(+)-A New Reactive Force Field Method for the Accurate Description of Ionic Systems and Its Application to the Hydrolyzation of Aluminosilicates},
      journal = {Journal of Physical Chemistry C},
      volume = {120},
      number = {20},
      pages = {10849-10856},
      abstract = {In this paper we present a powerful extension of the reactive force field method ReaxFF, which we call ReaxFF(+). It combines the charge equilibrium scheme with the bond order principle. The main advantage of this procedure is the correct distinction and description of covalent and ionic bonds. It allows reactive molecular dynamic simulations in ionic gases and liquids. To demonstrate the accuracy of this new method, we study the hydrolyzation of aluminosilicates. Comparing the results with experimental and ab initio data, we can prove the high accuracy of our method. This shows that ReaxFF(+) is a powerful force field simulation tool for reactions in acidic or alkaline environments.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acs.jpcc.6b00720},
      doi = {10.1021/acs.jpcc.6b00720},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Relaxed incremental variational approach for the modeling of damage-induced stress hysteresis in arterial walls
    • T. Schmidt, D. Balzani
    • Journal of the Mechanical Behavior of Biomedical Materials 58, 149-162 (2016)
    • DOI   Abstract  

      In this paper, a three-dimensional relaxed incremental variational damage model is proposed, which enables the description of complex softening hysteresis as observed in supra-physiologically loaded arterial tissues, and which thereby avoids a loss of convexity of the underlying formulation. The proposed model extends the relaxed formulation of Balzani and Ortiz [2012. Relaxed incremental variational formulation for damage at large strains with application to fiber-reinforced materials and materials with truss-like microstructures. Int. J. Numer. Methods Eng. 92, 551-570], such that the typical stress-hysteresis observed in arterial tissues under cyclic loading can be described. This is mainly achieved by constructing a modified one-dimensional model accounting for cyclic loading in the individual fiber direction and numerically homogenizing the response taking into account a fiber orientation distribution function. A new solution strategy for the identification of the convexified stress potential is proposed based on an evolutionary algorithm which leads to an improved robustness compared to solely Newton-based optimization schemes. In order to enable an efficient adjustment of the new model to experimentally observed softening hysteresis, an adjustment scheme using a surrogate model is proposed. Therewith, the relaxed formulation is adjusted to experimental data in the supra-physiological domain of the media and adventitia of a human carotid artery. The performance of the model is then demonstrated in a finite element example of an overstretched artery. Although here three-dimensional thick-walled atherosclerotic arteries are considered, it is emphasized that the formulation can also directly be applied to thin-walled simulations of arteries using shell elements or other fiber-reinforced biomembranes. (C) 2015 Elsevier Ltd. All rights reserved.

      @article{,
      author = {Thomas Schmidt and Daniel Balzani},
      title = {Relaxed incremental variational approach for the modeling of damage-induced stress hysteresis in arterial walls},
      journal = {Journal of the Mechanical Behavior of Biomedical Materials},
      volume = {58},
      pages = {149-162},
      abstract = {In this paper, a three-dimensional relaxed incremental variational damage model is proposed, which enables the description of complex softening hysteresis as observed in supra-physiologically loaded arterial tissues, and which thereby avoids a loss of convexity of the underlying formulation. The proposed model extends the relaxed formulation of Balzani and Ortiz [2012. Relaxed incremental variational formulation for damage at large strains with application to fiber-reinforced materials and materials with truss-like microstructures. Int. J. Numer. Methods Eng. 92, 551-570], such that the typical stress-hysteresis observed in arterial tissues under cyclic loading can be described. This is mainly achieved by constructing a modified one-dimensional model accounting for cyclic loading in the individual fiber direction and numerically homogenizing the response taking into account a fiber orientation distribution function. A new solution strategy for the identification of the convexified stress potential is proposed based on an evolutionary algorithm which leads to an improved robustness compared to solely Newton-based optimization schemes. In order to enable an efficient adjustment of the new model to experimentally observed softening hysteresis, an adjustment scheme using a surrogate model is proposed. Therewith, the relaxed formulation is adjusted to experimental data in the supra-physiological domain of the media and adventitia of a human carotid artery. The performance of the model is then demonstrated in a finite element example of an overstretched artery. Although here three-dimensional thick-walled atherosclerotic arteries are considered, it is emphasized that the formulation can also directly be applied to thin-walled simulations of arteries using shell elements or other fiber-reinforced biomembranes. (C) 2015 Elsevier Ltd. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.jmbbm.2015.08.005},
      doi = {10.1016/j.jmbbm.2015.08.005},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Reproducibility in density functional theory calculations of solids
    • K. Lejaeghere, G. Bihlmayer, T. Bjoerkman, P. Blaha, S. Bluegel, V. Blum, D. Caliste, I. E. Castelli, S. J. Clark, A. D. Corso, S. de Gironcoli, T. Deutsch, J. K. Dewhurst, I. D. Marco, C. Draxl, M. Dulak, O. Eriksson, J. A. Flores-Livas, K. F. Garrity, L. Genovese, P. Giannozzi, M. Giantomassi, S. Goedecker, X. Gonze, O. Granaes, E. K. U. Gross, A. Gulans, F. Gygi, D. R. Hamann, P. J. Hasnip, N. A. W. Holzwarth, D. Iusan, D. B. Jochym, F. Jollet, D. Jones, G. Kresse, K. Koepernik, E. Kuecuekbenli, Y. O. Kvashnin, I. L. M. Locht, S. Lubeck, M. Marsman, N. Marzari, U. Nitzsche, L. Nordstrom, T. Ozaki, L. Paulatto, C. J. Pickard, W. Poelmans, M. I. J. Probert, K. Refson, M. Richter, G. Rignanese, S. Saha, M. Scheffler, M. Schlipf, K. Schwarz, S. Sharma, F. Tavazza, P. Thunstroem, A. Tkatchenko, M. Torrent, D. Vanderbilt, M. J. van Setten, V. V. Speybroeck, J. M. Wills, J. R. Yates, G. Zhang, S. Cottenier
    • Science 351(2016)
    • DOI   Abstract  

      The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.

      @article{,
      author = {Kurt Lejaeghere and Gustav Bihlmayer and Torbjoern Bjoerkman and Peter Blaha and Stefan Bluegel and Volker Blum and Damien Caliste and Ivano E. Castelli and Stewart J. Clark and Andrea Dal Corso and Stefano de Gironcoli and Thierry Deutsch and John Kay Dewhurst and Igor Di Marco and Claudia Draxl and Marcin Dulak and Olle Eriksson and Jose A. Flores-Livas and Kevin F. Garrity and Luigi Genovese and Paolo Giannozzi and Matteo Giantomassi and Stefan Goedecker and Xavier Gonze and Oscar Granaes and E. K. U. Gross and Andris Gulans and Francois Gygi and D. R. Hamann and Phil J. Hasnip and N. A. W. Holzwarth and Diana Iusan and Dominik B. Jochym and Francois Jollet and Daniel Jones and Georg Kresse and Klaus Koepernik and Emine Kuecuekbenli and Yaroslav O. Kvashnin and Inka L. M. Locht and Sven Lubeck and Martijn Marsman and Nicola Marzari and Ulrike Nitzsche and Lars Nordstrom and Taisuke Ozaki and Lorenzo Paulatto and Chris J. Pickard and Ward Poelmans and Matt I. J. Probert and Keith Refson and Manuel Richter and Gian-Marco Rignanese and Santanu Saha and Matthias Scheffler and Martin Schlipf and Karlheinz Schwarz and Sangeeta Sharma and Francesca Tavazza and Patrik Thunstroem and Alexandre Tkatchenko and Marc Torrent and David Vanderbilt and Michiel J. van Setten and Veronique Van Speybroeck and John M. Wills and Jonathan R. Yates and Guo-Xu Zhang and Stefaan Cottenier},
      title = {Reproducibility in density functional theory calculations of solids},
      journal = {Science},
      volume = {351},
      number = {6280},
      abstract = {The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.},
      year = {2016},
      url = http://dx.doi.org/{10.1126/science.aad3000},
      doi = {10.1126/science.aad3000},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Reusability of photocatalytic TiO2 and ZnO nanoparticles immobilized in poly(vinylidene difluoride)-co-trifluoroethylene
    • S. Teixeira, P. M. Martins, S. Lanceros-Mendez, K. Kuehn, G. Cuniberti
    • Applied Surface Science 384, 497-504 (2016)
    • DOI   Abstract  

      Pollutants present in water are increasingly becoming an important public health issue. After their transportation across the sewer network they can pass through the wastewater treatment plants (WWTPs) mostly unchanged because WWTPs are not designed to remove pollutants present at trace levels. Conventional treatments are therefore ineffective. Immobilized photocatalytic systems are thus an advantage for the treatment of contaminated water, because they are ecofriendly, cost-effective and allow reusability. This work reports on TiO2 and ZnO commercial nanoparticles immobilized in poly(vinylidene difluoride)co-trifluoroethylene (P(VDF-TrFE)). Nanocomposites of P(VDF-TrFE) with different concentrations of TiO2 nanoparticles (5, 10, and 15 wt.%) and ZnO nanoparticles (15 wt.%) were produced by solvent casting and tested on the degradation of methylene blue, a model organic dye. Each nanocomposite was tested three times to assess its reusability. It is shown that increasing the photocatalyst concentration results in higher photocatalytic efficiencies; the degradation rates of 15% of TiO2 and ZnO are similar; and the photoactivity decreases 6%, 16%, 13%, and 11% after three utilizations, for TiO2 5%, TiO2 10%, TiO2 15%, and ZnO 15%, respectively. Thus, the low decrease in the photocatalytic activity after three uses makes the nanocomposites suitable for applications in which reusability is an important key factor. (C) 2016 Elsevier B.V. All rights reserved.

      @article{,
      author = {Sara Teixeira and P. M. Martins and S. Lanceros-Mendez and Klaus Kuehn and Gianaurelio Cuniberti},
      title = {Reusability of photocatalytic TiO2 and ZnO nanoparticles immobilized in poly(vinylidene difluoride)-co-trifluoroethylene},
      journal = {Applied Surface Science},
      volume = {384},
      pages = {497-504},
      abstract = {Pollutants present in water are increasingly becoming an important public health issue. After their transportation across the sewer network they can pass through the wastewater treatment plants (WWTPs) mostly unchanged because WWTPs are not designed to remove pollutants present at trace levels. Conventional treatments are therefore ineffective. Immobilized photocatalytic systems are thus an advantage for the treatment of contaminated water, because they are ecofriendly, cost-effective and allow reusability. This work reports on TiO2 and ZnO commercial nanoparticles immobilized in poly(vinylidene difluoride)co-trifluoroethylene (P(VDF-TrFE)). Nanocomposites of P(VDF-TrFE) with different concentrations of TiO2 nanoparticles (5, 10, and 15 wt.%) and ZnO nanoparticles (15 wt.%) were produced by solvent casting and tested on the degradation of methylene blue, a model organic dye. Each nanocomposite was tested three times to assess its reusability. It is shown that increasing the photocatalyst concentration results in higher photocatalytic efficiencies; the degradation rates of 15% of TiO2 and ZnO are similar; and the photoactivity decreases 6%, 16%, 13%, and 11% after three utilizations, for TiO2 5%, TiO2 10%, TiO2 15%, and ZnO 15%, respectively. Thus, the low decrease in the photocatalytic activity after three uses makes the nanocomposites suitable for applications in which reusability is an important key factor. (C) 2016 Elsevier B.V. All rights reserved.},
      year = {2016},
      url = http://dx.doi.org/{10.1016/j.apsusc.2016.05.073},
      doi = {10.1016/j.apsusc.2016.05.073},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Spin dynamics and relaxation in graphene dictated by electron-hole puddles
    • T. D. Van, F. Ortmann, A. W. Cummings, D. Soriano, S. Roche
    • Scientific Reports 6(2016)
    • DOI   Abstract  

      The understanding of spin dynamics and relaxation mechanisms in clean graphene, and the upper time and length scales on which spin devices can operate, are prerequisites to realizing graphenebased spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with Rashba spin-orbit fields as low as a few tens of mu eV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from a Dyakonov-Perel type for SiO2 substrates to a broadening-induced dephasing for hBN substrates is described. The energy dependence of spin lifetimes, their ratio for spins pointing out-of-plane and in-plane, and the scaling with disorder provide a global picture about spin dynamics and relaxation in ultraclean graphene in the presence of electron-hole puddles.

      @article{,
      author = {Tuan Dinh Van and Frank Ortmann and Aron W. Cummings and David Soriano and Stephan Roche},
      title = {Spin dynamics and relaxation in graphene dictated by electron-hole puddles},
      journal = {Scientific Reports},
      volume = {6},
      abstract = {The understanding of spin dynamics and relaxation mechanisms in clean graphene, and the upper time and length scales on which spin devices can operate, are prerequisites to realizing graphenebased spintronic technologies. Here we theoretically reveal the nature of fundamental spin relaxation mechanisms in clean graphene on different substrates with Rashba spin-orbit fields as low as a few tens of mu eV. Spin lifetimes ranging from 50 picoseconds up to several nanoseconds are found to be dictated by substrate-induced electron-hole characteristics. A crossover in the spin relaxation mechanism from a Dyakonov-Perel type for SiO2 substrates to a broadening-induced dephasing for hBN substrates is described. The energy dependence of spin lifetimes, their ratio for spins pointing out-of-plane and in-plane, and the scaling with disorder provide a global picture about spin dynamics and relaxation in ultraclean graphene in the presence of electron-hole puddles.},
      year = {2016},
      url = http://dx.doi.org/{10.1038/srep21046},
      doi = {10.1038/srep21046},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Stress Induced Branching of Growing Crystals on Curved Surfaces
    • C. Koehler, R. Backofen, A. Voigt
    • Physical Review Letters 116(2016)
    • DOI   Abstract  

      If two-dimensional crystals grow on a curved surface, the Gaussian curvature of the surface induces elastic stress and affects the growth pathway. The elastic stress can be alleviated by incorporating defects or, if this is energetically unfavorable, via an elastic instability which leads to anisotropic growth with branched ribbonlike structures. This instability provides a generic route to grow defect-free crystals on curved surfaces. Depending on the elastic properties of the crystal and the geometric properties of the surface, different growth morphologies with two-, four-, and sixfold symmetry develop. Using a phase field crystal type modeling approach, we provide a microscopic understanding of the morphology selection.

      @article{,
      author = {Christian Koehler and Rainer Backofen and Axel Voigt},
      title = {Stress Induced Branching of Growing Crystals on Curved Surfaces},
      journal = {Physical Review Letters},
      volume = {116},
      number = {13},
      abstract = {If two-dimensional crystals grow on a curved surface, the Gaussian curvature of the surface induces elastic stress and affects the growth pathway. The elastic stress can be alleviated by incorporating defects or, if this is energetically unfavorable, via an elastic instability which leads to anisotropic growth with branched ribbonlike structures. This instability provides a generic route to grow defect-free crystals on curved surfaces. Depending on the elastic properties of the crystal and the geometric properties of the surface, different growth morphologies with two-, four-, and sixfold symmetry develop. Using a phase field crystal type modeling approach, we provide a microscopic understanding of the morphology selection.},
      year = {2016},
      url = http://dx.doi.org/{10.1103/PhysRevLett.116.135502},
      doi = {10.1103/PhysRevLett.116.135502},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Synergistically Enhanced Polysulfide Chemisorption Using a Flexible Hybrid Separator with N and S Dual-Doped Mesoporous Carbon Coating for Advanced Lithium-Sulfur Batteries
    • J. Balach, H. K. Singh, S. Gomoll, T. Jaumann, M. Klose, S. Oswald, M. Richter, J. Eckert, L. Giebeler
    • Acs Applied Materials & Interfaces 8, 14586-14595 (2016)
    • DOI   Abstract  

      Because of the outstanding high theoretical specific energy density of 2600 Wh kg(-1), the lithium-sulfur (Li-S) battery is regarded as a promising candidate for post lithium-ion battery systems eligible to meet the forthcoming market requirements. However, its commercialization on large scale is thwarted by fast capacity fading caused by the Achilles’ heel of Li-S systems: the polysulfide shuttle. Here, we merge the physical features of carbon-coated separators and the unique chemical properties of N and S codoped mesoporous carbon to create a functional hybrid separator with superior polysulfide affinity and electrochemical benefits. DFT calculations revealed that carbon materials with N and S codoping possess a strong binding energy to high-order polysulfide species, which is essential to keep the active material in the cathode side. As a result of the synergistic effect of N, S dual-doping, an advanced Li-S cell with high specific capacity and ultralow capacity degradation of 0.041% per cycle is achieved. Pushing our simple-designed and scalable cathode to a highly increased sulfur loading of 5.4 mg cm(-2), the Li-S cell with the functional hybrid separator can deliver a remarkable areal capacity of 5.9 mAh cm(-2), which is highly favorable for practical applications.

      @article{,
      author = {Juan Balach and Harish K. Singh and Selina Gomoll and Tony Jaumann and Markus Klose and Steffen Oswald and Manuel Richter and Juergen Eckert and Lars Giebeler},
      title = {Synergistically Enhanced Polysulfide Chemisorption Using a Flexible Hybrid Separator with N and S Dual-Doped Mesoporous Carbon Coating for Advanced Lithium-Sulfur Batteries},
      journal = {Acs Applied Materials & Interfaces},
      volume = {8},
      number = {23},
      pages = {14586-14595},
      abstract = {Because of the outstanding high theoretical specific energy density of 2600 Wh kg(-1), the lithium-sulfur (Li-S) battery is regarded as a promising candidate for post lithium-ion battery systems eligible to meet the forthcoming market requirements. However, its commercialization on large scale is thwarted by fast capacity fading caused by the Achilles' heel of Li-S systems: the polysulfide shuttle. Here, we merge the physical features of carbon-coated separators and the unique chemical properties of N and S codoped mesoporous carbon to create a functional hybrid separator with superior polysulfide affinity and electrochemical benefits. DFT calculations revealed that carbon materials with N and S codoping possess a strong binding energy to high-order polysulfide species, which is essential to keep the active material in the cathode side. As a result of the synergistic effect of N, S dual-doping, an advanced Li-S cell with high specific capacity and ultralow capacity degradation of 0.041% per cycle is achieved. Pushing our simple-designed and scalable cathode to a highly increased sulfur loading of 5.4 mg cm(-2), the Li-S cell with the functional hybrid separator can deliver a remarkable areal capacity of 5.9 mAh cm(-2), which is highly favorable for practical applications.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acsami.6b03642},
      doi = {10.1021/acsami.6b03642},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif
    • X. Wang, F. Zhang, K. S. Schellharnmer, P. Machata, F. Ortmann, G. Cuniberti, Y. Fu, J. Hunger, R. Tang, A. A. Popov, R. Berger, K. Muellen, X. Feng
    • Journal of the American Chemical Society 138, 11606-11615 (2016)
    • DOI   Abstract  

      A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen boron nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized: Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilie bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP 5a was nearly quantitatively converted to sigma-dimer 5a,-2 through an open-shell intermediate, as indicated by UV-vis-NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as Well as 2D NMR spectra analyses. In situ spectroelectrochemistiy was used to confirm the formation process of the dimer radical cation 5a-2(center dot+). Finally, the developed new synthetic strategy could also be applied to obtain pi-extended NBN-clibentoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons.

      @article{,
      author = {Xinyang Wang and Fan Zhang and Karl Sebastian Schellharnmer and Peter Machata and Frank Ortmann and Gianaurelio Cuniberti and Yubin Fu and Jens Hunger and Ruizhi Tang and Alexey A. Popov and Reinhard Berger and Klaus Muellen and Xinliang Feng},
      title = {Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif},
      journal = {Journal of the American Chemical Society},
      volume = {138},
      number = {36},
      pages = {11606-11615},
      abstract = {A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen boron nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized: Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilie bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP 5a was nearly quantitatively converted to sigma-dimer 5a,-2 through an open-shell intermediate, as indicated by UV-vis-NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as Well as 2D NMR spectra analyses. In situ spectroelectrochemistiy was used to confirm the formation process of the dimer radical cation 5a-2(center dot+). Finally, the developed new synthetic strategy could also be applied to obtain pi-extended NBN-clibentoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/jacs.6b04445},
      doi = {10.1021/jacs.6b04445},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Temperature-Dependent Hole Mobility and Its Limit in Crystal-Phase P3HT Calculated from First Principles
    • A. Luecke, F. Ortmann, M. Panhans, S. Sanna, E. Rauls, U. Gerstmann, W. G. Schmidt
    • Journal of Physical Chemistry B 120, 5572-5580 (2016)
    • DOI   Abstract  

      We study temperature-dependent hole transport in ideal crystal-phase poly(3-hexylthiophene) (P3HT) with ab initio calculations, with the aim of estimating the maximum mobility in the limit of perfect order. To this end, the molecular transfer integrals, phonon frequencies, and electron phonon coupling constants are obtained from density functional theory (DFT). This allows the determination of transport properties without fit parameters. The strong coupling between charge carriers and vibrations leads to strong scattering and polaronic effects that impact carrier transport. By providing an intrinsic mobility limit to ideal P3HT crystals, this work allows identification of the impact of disorder on the temperature-dependent transport in real samples. A detailed analysis of the transport-relevant phonon modes is provided that gives microscopic insight into the polaron effects and hints toward mobility optimization strategies.

      @article{,
      author = {Andreas Luecke and Frank Ortmann and Michel Panhans and Simone Sanna and Eva Rauls and Uwe Gerstmann and Wolf Gero Schmidt},
      title = {Temperature-Dependent Hole Mobility and Its Limit in Crystal-Phase P3HT Calculated from First Principles},
      journal = {Journal of Physical Chemistry B},
      volume = {120},
      number = {24},
      pages = {5572-5580},
      abstract = {We study temperature-dependent hole transport in ideal crystal-phase poly(3-hexylthiophene) (P3HT) with ab initio calculations, with the aim of estimating the maximum mobility in the limit of perfect order. To this end, the molecular transfer integrals, phonon frequencies, and electron phonon coupling constants are obtained from density functional theory (DFT). This allows the determination of transport properties without fit parameters. The strong coupling between charge carriers and vibrations leads to strong scattering and polaronic effects that impact carrier transport. By providing an intrinsic mobility limit to ideal P3HT crystals, this work allows identification of the impact of disorder on the temperature-dependent transport in real samples. A detailed analysis of the transport-relevant phonon modes is provided that gives microscopic insight into the polaron effects and hints toward mobility optimization strategies.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acs.jpcb.6b03598},
      doi = {10.1021/acs.jpcb.6b03598},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Tetracene Formation by On-Surface Reduction
    • J. Krueger, N. Pavlicek, J. M. Alonso, D. Perez, E. Guitian, T. Lehmann, G. Cuniberti, A. Gourdon, G. Meyer, L. Gross, F. Moresco, D. Pena
    • Acs Nano 10, 4538-4542 (2016)
    • DOI   Abstract  

      We present the on-surface reduction of diepoxytetracenes to form genuine tetracene on Cu(111). The conversion is achieved by scanning tunneling microscopy (STM) tip-induced manipulation as well as thermal activation and is conclusively demonstrated by means of atomic force microscopy (AFM) with atomic resolution. We observe that the metallic surface plays an important role in the deoxygenation and for the planarization after bond cleavage.

      @article{,
      author = {Justus Krueger and Niko Pavlicek and Jose M. Alonso and Dolores Perez and Enrique Guitian and Thomas Lehmann and Gianaurelio Cuniberti and Andre Gourdon and Gerhard Meyer and Leo Gross and Francesca Moresco and Diego Pena},
      title = {Tetracene Formation by On-Surface Reduction},
      journal = {Acs Nano},
      volume = {10},
      number = {4},
      pages = {4538-4542},
      abstract = {We present the on-surface reduction of diepoxytetracenes to form genuine tetracene on Cu(111). The conversion is achieved by scanning tunneling microscopy (STM) tip-induced manipulation as well as thermal activation and is conclusively demonstrated by means of atomic force microscopy (AFM) with atomic resolution. We observe that the metallic surface plays an important role in the deoxygenation and for the planarization after bond cleavage.},
      year = {2016},
      url = http://dx.doi.org/{10.1021/acsnano.6b00505},
      doi = {10.1021/acsnano.6b00505},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Thermoelectric properties of nanocarbons: Atomistic modeling
    • T. Lehmann, D. A. Ryndyk, G. Cuniberti
    • Physica Status Solidi a-Applications and Materials Science 213, 591-602 (2016)
    • DOI   Abstract  

      We present a general atomistic ab initio-based modeling approach and numerical implementation for the calculation of thermoelectric properties of carbon nanomaterials. The approach is based on density functional theory calculations of electronic and vibrational properties in combination with quantum transport theory in the Green function formalism. It allows to calculate charge and heat transport, and therefore electrical conductance, thermopower (Seebeck coefficient), electron thermal conductance, phonon thermal conductance, and thermoelectric efficiency, i.e., figure of merit. We systematically investigated temperature, doping, and disorder dependence of the thermoelectric properties of the fundamental types of nanocarbons, such as graphene, metallic and semiconducting nanoribbons, as well as metallic and semiconducting nanotubes. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

      @article{,
      author = {Thomas Lehmann and Dmitry A. Ryndyk and Gianaurelio Cuniberti},
      title = {Thermoelectric properties of nanocarbons: Atomistic modeling},
      journal = {Physica Status Solidi a-Applications and Materials Science},
      volume = {213},
      number = {3},
      pages = {591-602},
      abstract = {We present a general atomistic ab initio-based modeling approach and numerical implementation for the calculation of thermoelectric properties of carbon nanomaterials. The approach is based on density functional theory calculations of electronic and vibrational properties in combination with quantum transport theory in the Green function formalism. It allows to calculate charge and heat transport, and therefore electrical conductance, thermopower (Seebeck coefficient), electron thermal conductance, phonon thermal conductance, and thermoelectric efficiency, i.e., figure of merit. We systematically investigated temperature, doping, and disorder dependence of the thermoelectric properties of the fundamental types of nanocarbons, such as graphene, metallic and semiconducting nanoribbons, as well as metallic and semiconducting nanotubes. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim},
      year = {2016},
      url = http://dx.doi.org/{10.1002/pssa.201532610},
      doi = {10.1002/pssa.201532610},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Thin-film growth dynamics with shadowing effects by a phase-field approach
    • M. Salvalaglio, R. Backofen, A. Voigt
    • Physical Review B 94(2016)
    • DOI   Abstract  

      Shadowing effects during the growth of nano-and microstructures are crucial for the realization of several technological applications. They are given by the shielding of the incoming material flux provided by the growing structures themselves. Their features have been deeply investigated by theoretical approaches, revealing important information to support experimental activities. However, comprehensive investigations able to follow every stage of the growth processes as a whole, particularly useful to design and understand targeted experiments, are still challenging. In this work, we study the thin-film growth dynamics by means of a diffuse interface approach accounting for both deposition with shadowing effects and surface diffusion driven by the minimization of the surface energy. In particular, we introduce the coupling between a phase-field model and the detailed calculation of the incoming material flux at the surface deposited from vacuum or vapor phase in the ballistic regime. This allows us to finely reproduce the realistic morphological evolution during the growth on nonflat substrates, also accounting for different flux distributions. A general assessment of the method, focusing on two-dimensional profiles, is provided thanks to the comparison with a sharp-interface approach for the evolution of the early stages. Then, the long-time-scale dynamics is shown in two and three dimensions, providing a general overview of the features observed during deposition on corrugated surfaces involving flattening, increasing of surface roughness with the growth of columnar structures, and voids formation.

      @article{,
      author = {Marco Salvalaglio and Rainer Backofen and Axel Voigt},
      title = {Thin-film growth dynamics with shadowing effects by a phase-field approach},
      journal = {Physical Review B},
      volume = {94},
      number = {23},
      abstract = {Shadowing effects during the growth of nano-and microstructures are crucial for the realization of several technological applications. They are given by the shielding of the incoming material flux provided by the growing structures themselves. Their features have been deeply investigated by theoretical approaches, revealing important information to support experimental activities. However, comprehensive investigations able to follow every stage of the growth processes as a whole, particularly useful to design and understand targeted experiments, are still challenging. In this work, we study the thin-film growth dynamics by means of a diffuse interface approach accounting for both deposition with shadowing effects and surface diffusion driven by the minimization of the surface energy. In particular, we introduce the coupling between a phase-field model and the detailed calculation of the incoming material flux at the surface deposited from vacuum or vapor phase in the ballistic regime. This allows us to finely reproduce the realistic morphological evolution during the growth on nonflat substrates, also accounting for different flux distributions. A general assessment of the method, focusing on two-dimensional profiles, is provided thanks to the comparison with a sharp-interface approach for the evolution of the early stages. Then, the long-time-scale dynamics is shown in two and three dimensions, providing a general overview of the features observed during deposition on corrugated surfaces involving flattening, increasing of surface roughness with the growth of columnar structures, and voids formation.},
      year = {2016},
      url = http://dx.doi.org/{10.1103/PhysRevB.94.235432},
      doi = {10.1103/PhysRevB.94.235432},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • TiO2/graphene oxide immobilized in P(VDF-TrFE) electrospun membranes with enhanced visible-light-induced photocatalytic performance
    • N. A. Almeida, P. M. Martins, S. Teixeira, J. L. A. da Silva, V. Sencadas, K. Kuehn, G. Cuniberti, S. Lanceros-Mendez, P. A. A. P. Marques
    • Journal of Materials Science 51, 6974-6986 (2016)
    • DOI   Abstract  

      Here, we report on the electrospinning of poly(vinylidene difluoride-co-trifluoroethylene) (P(VDF-TrFE)) copolymer fibrous membranes decorated with titanium dioxide/graphene oxide (TiO2/GO). The presence of the TiO2/GO increases the photocatalytic efficiency of the nanocomposite membrane towards the degradation of methylene blue (MB) when compared with the membranes prepared with naked TiO2, in UV and particularly in the visible range. Even a low content (3 %, w/w) of TiO2/GO in the fibers yields excellent photocatalytic performance by degrading similar to 100 % of a MB solution after 90 min of visible light exposure. This may be attributed to a rapid electron transport and the delayed recombination of electron-hole pairs due to improved ionic interaction between titanium and carbon combined with the advantageous electric properties of the polymer, such as high polarization and dielectric constant combined with low dielectric loss. Thus, a promising system to degrade organic pollutants in aqueous or gaseous systems under visible light irradiation has been developed.

      @article{,
      author = {Nuno A. Almeida and Pedro M. Martins and Sara Teixeira and Jose A. Lopes da Silva and Vitor Sencadas and K. Kuehn and G. Cuniberti and S. Lanceros-Mendez and Paula A. A. P. Marques},
      title = {TiO2/graphene oxide immobilized in P(VDF-TrFE) electrospun membranes with enhanced visible-light-induced photocatalytic performance},
      journal = {Journal of Materials Science},
      volume = {51},
      number = {14},
      pages = {6974-6986},
      abstract = {Here, we report on the electrospinning of poly(vinylidene difluoride-co-trifluoroethylene) (P(VDF-TrFE)) copolymer fibrous membranes decorated with titanium dioxide/graphene oxide (TiO2/GO). The presence of the TiO2/GO increases the photocatalytic efficiency of the nanocomposite membrane towards the degradation of methylene blue (MB) when compared with the membranes prepared with naked TiO2, in UV and particularly in the visible range. Even a low content (3 %, w/w) of TiO2/GO in the fibers yields excellent photocatalytic performance by degrading similar to 100 % of a MB solution after 90 min of visible light exposure. This may be attributed to a rapid electron transport and the delayed recombination of electron-hole pairs due to improved ionic interaction between titanium and carbon combined with the advantageous electric properties of the polymer, such as high polarization and dielectric constant combined with low dielectric loss. Thus, a promising system to degrade organic pollutants in aqueous or gaseous systems under visible light irradiation has been developed.},
      year = {2016},
      url = http://dx.doi.org/{10.1007/s10853-016-9986-4},
      doi = {10.1007/s10853-016-9986-4},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Towards an optimal contact metal for CNTFETs
    • A. Fediai, D. A. Ryndyk, G. Seifert, S. Mothes, M. Claus, M. Schroeter, G. Cuniberti
    • Nanoscale 8, 10240-10251 (2016)
    • DOI   Abstract  

      Downscaling of the contact length L-c of a side-contacted carbon nanotube field-effect transistor (CNTFET) is challenging because of the rapidly increasing contact resistance as L-c falls below 20-50 nm. If in agreement with existing experimental results, theoretical work might answer the question, which metals yield the lowest CNT-metal contact resistance and what physical mechanisms govern the geometry dependence of the contact resistance. However, at the scale of 10 nm, parameter-free models of electron transport become computationally prohibitively expensive. In our work we used a dedicated combination of the Green function formalism and density functional theory to perform an overall ab initio simulation of extended CNT-metal contacts of an arbitrary length (including infinite), a previously not achievable level of simulations. We provide a systematic and comprehensive discussion of metal-CNT contact properties as a function of the metal type and the contact length. We have found and been able to explain very uncommon relations between chemical, physical and electrical properties observed in CNT-metal contacts. The calculated electrical characteristics are in reasonable quantitative agreement and exhibit similar trends as the latest experimental data in terms of: (i) contact resistance for L-c = infinity, (ii) scaling of contact resistance R-c(L-c); (iii) metal-defined polarity of a CNTFET. Our results can guide technology development and contact material selection for downscaling the length of side-contacts below 10 nm.

      @article{,
      author = {Artem Fediai and Dmitry A. Ryndyk and Gotthard Seifert and Sven Mothes and Martin Claus and Michael Schroeter and Gianaurelio Cuniberti},
      title = {Towards an optimal contact metal for CNTFETs},
      journal = {Nanoscale},
      volume = {8},
      number = {19},
      pages = {10240-10251},
      abstract = {Downscaling of the contact length L-c of a side-contacted carbon nanotube field-effect transistor (CNTFET) is challenging because of the rapidly increasing contact resistance as L-c falls below 20-50 nm. If in agreement with existing experimental results, theoretical work might answer the question, which metals yield the lowest CNT-metal contact resistance and what physical mechanisms govern the geometry dependence of the contact resistance. However, at the scale of 10 nm, parameter-free models of electron transport become computationally prohibitively expensive. In our work we used a dedicated combination of the Green function formalism and density functional theory to perform an overall ab initio simulation of extended CNT-metal contacts of an arbitrary length (including infinite), a previously not achievable level of simulations. We provide a systematic and comprehensive discussion of metal-CNT contact properties as a function of the metal type and the contact length. We have found and been able to explain very uncommon relations between chemical, physical and electrical properties observed in CNT-metal contacts. The calculated electrical characteristics are in reasonable quantitative agreement and exhibit similar trends as the latest experimental data in terms of: (i) contact resistance for L-c = infinity, (ii) scaling of contact resistance R-c(L-c); (iii) metal-defined polarity of a CNTFET. Our results can guide technology development and contact material selection for downscaling the length of side-contacts below 10 nm.},
      year = {2016},
      url = http://dx.doi.org/{10.1039/c6nr01012a},
      doi = {10.1039/c6nr01012a},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Unconventional features in the quantum Hall regime of disordered graphene: Percolating impurity states and Hall conductance quantization
    • N. Leconte, F. Ortmann, A. Cresti, S. Roche
    • Physical Review B 93(2016)
    • DOI   Abstract  

      We report on the formation of critical states in disordered graphene, at the origin of variable and unconventional transport properties in the quantum Hall regime, such as a zero-energy Hall conductance plateau in the absence of an energy band gap and Landau-level degeneracy breaking. By using efficient real-space transport methodologies, we compute both the dissipative and Hall conductivities of large-size graphene sheets with random distribution of model single and double vacancies. By analyzing the scaling of transport coefficients with defect density, system size, and magnetic length, we elucidate the origin of anomalous quantum Hall features as magnetic-field-dependent impurity states, which percolate at some critical energies. These findings shed light on unidentified states and quantum-transport anomalies reported experimentally.

      @article{,
      author = {Nicolas Leconte and Frank Ortmann and Alessandro Cresti and Stephan Roche},
      title = {Unconventional features in the quantum Hall regime of disordered graphene: Percolating impurity states and Hall conductance quantization},
      journal = {Physical Review B},
      volume = {93},
      number = {11},
      abstract = {We report on the formation of critical states in disordered graphene, at the origin of variable and unconventional transport properties in the quantum Hall regime, such as a zero-energy Hall conductance plateau in the absence of an energy band gap and Landau-level degeneracy breaking. By using efficient real-space transport methodologies, we compute both the dissipative and Hall conductivities of large-size graphene sheets with random distribution of model single and double vacancies. By analyzing the scaling of transport coefficients with defect density, system size, and magnetic length, we elucidate the origin of anomalous quantum Hall features as magnetic-field-dependent impurity states, which percolate at some critical energies. These findings shed light on unidentified states and quantum-transport anomalies reported experimentally.},
      year = {2016},
      url = http://dx.doi.org/{10.1103/PhysRevB.93.115404},
      doi = {10.1103/PhysRevB.93.115404},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

2015

  • The breakdown of superlubricity by driving-induced commensurate dislocations
    • A. Benassi, M. Ma, M. Urbakh, A. Vanossi
    • Scientific Reports 5(2015)
    • DOI   Abstract  

      In the framework of a Frenkel-Kontorova-like model, we address the robustness of the superlubricity phenomenon in an edge-driven system at large scales, highlighting the dynamical mechanisms leading to its failure due to the slider elasticity. The results of the numerical simulations perfectly match the length critical size derived from a parameter-free analytical model. By considering different driving and commensurability interface configurations, we explore the distinctive nature of the transition from superlubric to high-friction sliding states which occurs above the critical size, discovering the occurrence of previously undetected multiple dissipative jumps in the friction force as a function of the slider length. These driving-induced commensurate dislocations in the slider are then characterized in relation to their spatial localization and width, depending on the system parameters. Setting the ground to scale superlubricity up, this investigation provides a novel perspective on friction and nanomanipulation experiments and can serve as a theoretical basis for designing high-tech devices with specific superlow frictional features.

      @article{,
      author = {A. Benassi and Ming Ma and M. Urbakh and A. Vanossi},
      title = {The breakdown of superlubricity by driving-induced commensurate dislocations},
      journal = {Scientific Reports},
      volume = {5},
      abstract = {In the framework of a Frenkel-Kontorova-like model, we address the robustness of the superlubricity phenomenon in an edge-driven system at large scales, highlighting the dynamical mechanisms leading to its failure due to the slider elasticity. The results of the numerical simulations perfectly match the length critical size derived from a parameter-free analytical model. By considering different driving and commensurability interface configurations, we explore the distinctive nature of the transition from superlubric to high-friction sliding states which occurs above the critical size, discovering the occurrence of previously undetected multiple dissipative jumps in the friction force as a function of the slider length. These driving-induced commensurate dislocations in the slider are then characterized in relation to their spatial localization and width, depending on the system parameters. Setting the ground to scale superlubricity up, this investigation provides a novel perspective on friction and nanomanipulation experiments and can serve as a theoretical basis for designing high-tech devices with specific superlow frictional features.},
      year = {2015},
      url = http://dx.doi.org/{10.1038/srep16134},
      doi = {10.1038/srep16134},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Consequences of a Single Double Bond within a Side Group on the Ordering of Supramolecular Polymers
    • R. Shokri, O. Guskova, A. Jamal, K. Jahanshahi, B. Isare, L. Bouteiller, L. Simon, J. Sommer, G. Reiter
    • Journal of Physical Chemistry C 119, 22596-22603 (2015)
    • DOI   Abstract  

      By combining atomic force microscopy experiments and full-atomistic computer simulations, we compared the two-dimensional ordering dynamics of two variants of supramolecular polymers of bis-urea molecules which differed only by a single cis-double bond in their side groups. At early stages of ordering, the double bonds favored kinks at the level of individual molecules, which induced transient steric constraints hindering the spontaneous formation of long supramolecular polymers. In addition, due to these kinks, molecule substrate interactions were disturbed. At later stages, however, due to a progressively increasing number of established directional hydrogen bonds between molecules, the self-assembly process improved and thereby increased the length of the supramolecular polymers. Large domains of micrometer-long and aligned supramolecular polymers were formed, epitaxially guided by the graphite substrate and having a constant width consistent with the length of the molecule. Thus, introducing flexible (kinked) side chains can reduce the nucleation probability and slow the growth of supramolecular polymers due to incommensurablility with the crystalline substrate. Such an elementary control of nucleation and growth via the introduction of a single double bond represents a powerful pathway for the formation of large ordered domains of aligned one-dimensional supramolecular polymers.

      @article{,
      author = {Roozbeh Shokri and Olga Guskova and Asad Jamal and Kaiwan Jahanshahi and Benjamin Isare and Laurent Bouteiller and Laurent Simon and Jens-Uwe Sommer and Guenter Reiter},
      title = {Consequences of a Single Double Bond within a Side Group on the Ordering of Supramolecular Polymers},
      journal = {Journal of Physical Chemistry C},
      volume = {119},
      number = {39},
      pages = {22596-22603},
      abstract = {By combining atomic force microscopy experiments and full-atomistic computer simulations, we compared the two-dimensional ordering dynamics of two variants of supramolecular polymers of bis-urea molecules which differed only by a single cis-double bond in their side groups. At early stages of ordering, the double bonds favored kinks at the level of individual molecules, which induced transient steric constraints hindering the spontaneous formation of long supramolecular polymers. In addition, due to these kinks, molecule substrate interactions were disturbed. At later stages, however, due to a progressively increasing number of established directional hydrogen bonds between molecules, the self-assembly process improved and thereby increased the length of the supramolecular polymers. Large domains of micrometer-long and aligned supramolecular polymers were formed, epitaxially guided by the graphite substrate and having a constant width consistent with the length of the molecule. Thus, introducing flexible (kinked) side chains can reduce the nucleation probability and slow the growth of supramolecular polymers due to incommensurablility with the crystalline substrate. Such an elementary control of nucleation and growth via the introduction of a single double bond represents a powerful pathway for the formation of large ordered domains of aligned one-dimensional supramolecular polymers.},
      year = {2015},
      url = http://dx.doi.org/{10.1021/acs.jpcc.5b07644},
      doi = {10.1021/acs.jpcc.5b07644},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Contact effects and quantum interference in engineered dangling bond loops on silicon surfaces
    • A. Kleshchonok, R. Gutierrez, G. Cuniberti
    • Nanoscale 7, 13967-13973 (2015)
    • DOI   Abstract  

      Dangling bond structures created on H-passivated silicon surfaces offer a novel platform for engineering planar nanoscale circuits, compatible with conventional semiconductor technologies. In this investigation we focus on the electronic structure and quantum transport signatures of dangling bond loops built on H-passivated Si(100) surfaces contacted by carbon nanoribbons, thus leading to a two-terminal planar, nanoscale setup. The computational studies were carried out to rationalize the influence of the local atomic-scale contacts of the dangling bond system to the mesoscopic electrodes as well as the possibility of revealing quantum interference effects in the dangling bond loops. Our results reveal a strong sensitivity of the low-energy quantum transmission to the loop topology and to the atomistic details of the electrode-loop contact. Varying the length of the loop or the spatial position of at least one of the electrodes has a drastic impact on the quantum interference pattern; depending on whether constructive or destructive interference within the loop takes place, the conductance of the system can be tuned over several orders of magnitude, thus suggesting the possibility of exploiting such quantum mechanical effects in the design of two-dimensional, atomic-scale electronic devices such as logic gates.

      @article{,
      author = {Andrii Kleshchonok and Rafael Gutierrez and Gianaurelio Cuniberti},
      title = {Contact effects and quantum interference in engineered dangling bond loops on silicon surfaces},
      journal = {Nanoscale},
      volume = {7},
      number = {33},
      pages = {13967-13973},
      abstract = {Dangling bond structures created on H-passivated silicon surfaces offer a novel platform for engineering planar nanoscale circuits, compatible with conventional semiconductor technologies. In this investigation we focus on the electronic structure and quantum transport signatures of dangling bond loops built on H-passivated Si(100) surfaces contacted by carbon nanoribbons, thus leading to a two-terminal planar, nanoscale setup. The computational studies were carried out to rationalize the influence of the local atomic-scale contacts of the dangling bond system to the mesoscopic electrodes as well as the possibility of revealing quantum interference effects in the dangling bond loops. Our results reveal a strong sensitivity of the low-energy quantum transmission to the loop topology and to the atomistic details of the electrode-loop contact. Varying the length of the loop or the spatial position of at least one of the electrodes has a drastic impact on the quantum interference pattern; depending on whether constructive or destructive interference within the loop takes place, the conductance of the system can be tuned over several orders of magnitude, thus suggesting the possibility of exploiting such quantum mechanical effects in the design of two-dimensional, atomic-scale electronic devices such as logic gates.},
      year = {2015},
      url = http://dx.doi.org/{10.1039/c5nr01251a},
      doi = {10.1039/c5nr01251a},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Design Proposals for Organic Materials Exhibiting a Low Exciton Binding Energy
    • S. Kraner, R. Scholz, C. Koerner, K. Leo
    • Journal of Physical Chemistry C 119, 22820-22825 (2015)
    • DOI   Abstract  

      Organic photovoltaics (OPV) have the potential for a low cost energy conversion from solar to electrical power. However, minimizing voltage losses, inherent to the currently used intermolecular donor acceptor system, represents the main challenge to make this technology suitable for commercial large area applications. One key parameter to overcome is the high exciton binding energy in organic materials. Based on density functional theory (DFT) simulations, it is shown how polar side chains may significantly increase the dielectric constant, which lowers the exciton binding energy and enables charge generation in organic materials without a donor acceptor system. Furthermore, based on time dependent (TD)-DFT calculations, we show another path for low exciton binding energy organic materials, by inducing a charge transfer state between two hydrogen bonded oligomers along the backbone. The charge transfer is achieved by fluorination of the acceptor oligomer investigated, leading to a wide-range intermolecular exciton, where the frontier orbitals are spatially separated from each other. Furthermore, we show that intramolecular charge transfer (CT) states and CT states between two pi-stacked molecules do not significantly influence the Coulomb interactions.

      @article{,
      author = {Stefan Kraner and Reinhard Scholz and Christian Koerner and Karl Leo},
      title = {Design Proposals for Organic Materials Exhibiting a Low Exciton Binding Energy},
      journal = {Journal of Physical Chemistry C},
      volume = {119},
      number = {40},
      pages = {22820-22825},
      abstract = {Organic photovoltaics (OPV) have the potential for a low cost energy conversion from solar to electrical power. However, minimizing voltage losses, inherent to the currently used intermolecular donor acceptor system, represents the main challenge to make this technology suitable for commercial large area applications. One key parameter to overcome is the high exciton binding energy in organic materials. Based on density functional theory (DFT) simulations, it is shown how polar side chains may significantly increase the dielectric constant, which lowers the exciton binding energy and enables charge generation in organic materials without a donor acceptor system. Furthermore, based on time dependent (TD)-DFT calculations, we show another path for low exciton binding energy organic materials, by inducing a charge transfer state between two hydrogen bonded oligomers along the backbone. The charge transfer is achieved by fluorination of the acceptor oligomer investigated, leading to a wide-range intermolecular exciton, where the frontier orbitals are spatially separated from each other. Furthermore, we show that intramolecular charge transfer (CT) states and CT states between two pi-stacked molecules do not significantly influence the Coulomb interactions.},
      year = {2015},
      url = http://dx.doi.org/{10.1021/acs.jpcc.5b07097},
      doi = {10.1021/acs.jpcc.5b07097},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Diameter-Selective Dispersion of Carbon Nanotubes via Polymers: A Competition between Adsorption and Bundling
    • H. Yang, V. Bezugly, J. Kunstmann, A. Filoramo, G. Cuniberti
    • Acs Nano 9, 9012-9019 (2015)
    • DOI   Abstract  

      The mechanism of the selective dispersion of single-walled carbon nanotubes (CNTs) by polyfluorene polymers is studied in this paper. Using extensive molecular dynamics simulations, it is demonstrated that diameter selectivity is the result of a competition between bundling of CNTs and adsorption of polymers on CNT surfaces. The preference for certain diameters corresponds to local minima of the binding energy difference between these two processes. Such minima in the diameter dependence occur due to abrupt changes in the CNT’s coverage with polymers, and their calculated positions are in quantitative agreement with preferred diameters reported experimentally. The presented approach defines a theoretical framework for the further understanding and improvement of dispersion/extraction processes.

      @article{,
      author = {Hongliu Yang and Viktor Bezugly and Jens Kunstmann and Arianna Filoramo and Gianaurelio Cuniberti},
      title = {Diameter-Selective Dispersion of Carbon Nanotubes via Polymers: A Competition between Adsorption and Bundling},
      journal = {Acs Nano},
      volume = {9},
      number = {9},
      pages = {9012-9019},
      abstract = {The mechanism of the selective dispersion of single-walled carbon nanotubes (CNTs) by polyfluorene polymers is studied in this paper. Using extensive molecular dynamics simulations, it is demonstrated that diameter selectivity is the result of a competition between bundling of CNTs and adsorption of polymers on CNT surfaces. The preference for certain diameters corresponds to local minima of the binding energy difference between these two processes. Such minima in the diameter dependence occur due to abrupt changes in the CNT's coverage with polymers, and their calculated positions are in quantitative agreement with preferred diameters reported experimentally. The presented approach defines a theoretical framework for the further understanding and improvement of dispersion/extraction processes.},
      year = {2015},
      url = http://dx.doi.org/{10.1021/acsnano.5b03051},
      doi = {10.1021/acsnano.5b03051},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Efficient linear scaling approach for computing the Kubo Hall conductivity
    • F. Ortmann, N. Leconte, S. Roche
    • Physical Review B 91(2015)
    • DOI   Abstract  

      We report an order-N approach to compute the Kubo Hall conductivity for disorderd two-dimensional systems reaching tens of millions of orbitals, and realistic values of the applied external magnetic fields (as low as a few Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity sigma(xy) using a wave-packet propagation method and a continued fraction expansion for the computation of diagonal and off-diagonal matrix elements of the Green functions. The validity of the method is demonstrated by comparison of results with brute-force diagonalization of the Kubo formula, using (disordered) graphene as the system of study. This approach to mesoscopic system sizes is opening an unprecedented perspective for so-called reverse engineering in which the available experimental transport data are used to get a deeper understanding of the microscopic structure of the samples. Besides, this will not only allow addressing subtle issues in terms of resistance standardization of large-scale materials (such as wafer scale polycrystalline graphene), but will also enable the discovery of new quantum transport phenomena in complex two-dimensional materials, out of reach with classical methods.

      @article{,
      author = {Frank Ortmann and Nicolas Leconte and Stephan Roche},
      title = {Efficient linear scaling approach for computing the Kubo Hall conductivity},
      journal = {Physical Review B},
      volume = {91},
      number = {16},
      abstract = {We report an order-N approach to compute the Kubo Hall conductivity for disorderd two-dimensional systems reaching tens of millions of orbitals, and realistic values of the applied external magnetic fields (as low as a few Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity sigma(xy) using a wave-packet propagation method and a continued fraction expansion for the computation of diagonal and off-diagonal matrix elements of the Green functions. The validity of the method is demonstrated by comparison of results with brute-force diagonalization of the Kubo formula, using (disordered) graphene as the system of study. This approach to mesoscopic system sizes is opening an unprecedented perspective for so-called reverse engineering in which the available experimental transport data are used to get a deeper understanding of the microscopic structure of the samples. Besides, this will not only allow addressing subtle issues in terms of resistance standardization of large-scale materials (such as wafer scale polycrystalline graphene), but will also enable the discovery of new quantum transport phenomena in complex two-dimensional materials, out of reach with classical methods.},
      year = {2015},
      url = http://dx.doi.org/{10.1103/PhysRevB.91.165117},
      doi = {10.1103/PhysRevB.91.165117},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Electron transport in extended carbon-nanotube/metal contacts: Ab initio based Green function method
    • A. Fediai, D. A. Ryndyk, G. Cuniberti
    • Physical Review B 91(2015)
    • DOI   Abstract  

      We have developed a newmethod that is able to predict the electrical properties of the source and drain contacts in realistic carbon nanotube field effect transistors (CNTFETs). It is based on large-scale ab initio calculations combined with a Green function approach. For the first time, both internal and external parts of a realistic CNT-metal contact are taken into account at the ab initio level. We have developed the procedure allowing direct calculation of the self-energy for an extended contact. Within the method, it is possible to calculate the transmission coefficient through a contact of both finite and infinite length; the local density of states can be determined in both free and embedded CNT segments. We found perfect agreement with the experimental data for Pd and Al contacts. We have explained why CNTFETs with Pd electrodes are p-type FETs with ohmic contacts, which can carry current close to the ballistic limit (provided contact length is large enough), whereas in CNT-Al contacts transmission is suppressed to a significant extent, especially for holes.

      @article{,
      author = {Artem Fediai and Dmitry A. Ryndyk and Gianaurelio Cuniberti},
      title = {Electron transport in extended carbon-nanotube/metal contacts: Ab initio based Green function method},
      journal = {Physical Review B},
      volume = {91},
      number = {16},
      abstract = {We have developed a newmethod that is able to predict the electrical properties of the source and drain contacts in realistic carbon nanotube field effect transistors (CNTFETs). It is based on large-scale ab initio calculations combined with a Green function approach. For the first time, both internal and external parts of a realistic CNT-metal contact are taken into account at the ab initio level. We have developed the procedure allowing direct calculation of the self-energy for an extended contact. Within the method, it is possible to calculate the transmission coefficient through a contact of both finite and infinite length; the local density of states can be determined in both free and embedded CNT segments. We found perfect agreement with the experimental data for Pd and Al contacts. We have explained why CNTFETs with Pd electrodes are p-type FETs with ohmic contacts, which can carry current close to the ballistic limit (provided contact length is large enough), whereas in CNT-Al contacts transmission is suppressed to a significant extent, especially for holes.},
      year = {2015},
      url = http://dx.doi.org/{10.1103/PhysRevB.91.165404},
      doi = {10.1103/PhysRevB.91.165404},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Engineering thermal rectification in MoS2 nanoribbons: a non-equilibrium molecular dynamics study
    • L. M. Sandonas, R. Gutierrez, A. Dianat, G. Cuniberti
    • Rsc Advances 5, 54345-54351 (2015)
    • DOI   Abstract  

      Phononics in two-dimensional (2D) materials is an emergent field with a high potential impact from the basic as well as applied research points of view. Thus it is crucial to provide strategies to control heat flow via atomic-scale engineering of the materials. In this study, thermal diodes made of single layer MoS2 nanoribbons are investigated using non-equilibrium classical molecular dynamics. Specifically, we focus on the influence of shape asymmetries of the nanoribbons on the thermal current, and obtain thermal rectification ratios up to 30% for the T-shaped nanoribbons. This behavior is then rationalized through a detailed analysis of the vibrational spectrum of the ribbons. In particular, it turns out that thermal rectification is mostly related to (i) the transversal finite size of the ribbon and (ii) to the different localization behavior of high-frequency modes for forward and backward heat flow directions. We expect our results to shed light on the potential of 2D materials for the engineering of highly efficient nanoscale thermal devices.

      @article{,
      author = {Leonardo Medrano Sandonas and Rafael Gutierrez and Arezoo Dianat and Giovanni Cuniberti},
      title = {Engineering thermal rectification in MoS2 nanoribbons: a non-equilibrium molecular dynamics study},
      journal = {Rsc Advances},
      volume = {5},
      number = {67},
      pages = {54345-54351},
      abstract = {Phononics in two-dimensional (2D) materials is an emergent field with a high potential impact from the basic as well as applied research points of view. Thus it is crucial to provide strategies to control heat flow via atomic-scale engineering of the materials. In this study, thermal diodes made of single layer MoS2 nanoribbons are investigated using non-equilibrium classical molecular dynamics. Specifically, we focus on the influence of shape asymmetries of the nanoribbons on the thermal current, and obtain thermal rectification ratios up to 30% for the T-shaped nanoribbons. This behavior is then rationalized through a detailed analysis of the vibrational spectrum of the ribbons. In particular, it turns out that thermal rectification is mostly related to (i) the transversal finite size of the ribbon and (ii) to the different localization behavior of high-frequency modes for forward and backward heat flow directions. We expect our results to shed light on the potential of 2D materials for the engineering of highly efficient nanoscale thermal devices.},
      year = {2015},
      url = http://dx.doi.org/{10.1039/c5ra05733g},
      doi = {10.1039/c5ra05733g},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Enhanced thermoelectric figure of merit in polycrystalline carbon nanostructures
    • T. Lehmann, D. A. Ryndyk, G. Cuniberti
    • Physical Review B 92(2015)
    • DOI   Abstract  

      Grain boundaries are commonly observed in carbon nanostructures, but their influence on thermal and electric properties is still not completely understood. Using a combined approach of density functional tight-binding theory and nonequilibrium Green functions we investigate electron and phonon transport in carbon-based systems. In this work, quantum transport and thermoelectric properties are summarized for graphene sheets, graphene nanoribbons, and carbon nanotubes with a variety of grain boundary types in a wide temperature range. Motivated by previous findings that disorder scatters phonons more effectively than electrons, a significant improvement in the thermoelectric performance for polycrystalline systems is expected. As the effect is marginally sensitive to the grain boundary type, we demonstrate that grain boundaries are a viable tool to greatly enhance the figure of merit, paving the way for the design of new thermoelectric materials.

      @article{,
      author = {Thomas Lehmann and Dmitry A. Ryndyk and Gianaurelio Cuniberti},
      title = {Enhanced thermoelectric figure of merit in polycrystalline carbon nanostructures},
      journal = {Physical Review B},
      volume = {92},
      number = {3},
      abstract = {Grain boundaries are commonly observed in carbon nanostructures, but their influence on thermal and electric properties is still not completely understood. Using a combined approach of density functional tight-binding theory and nonequilibrium Green functions we investigate electron and phonon transport in carbon-based systems. In this work, quantum transport and thermoelectric properties are summarized for graphene sheets, graphene nanoribbons, and carbon nanotubes with a variety of grain boundary types in a wide temperature range. Motivated by previous findings that disorder scatters phonons more effectively than electrons, a significant improvement in the thermoelectric performance for polycrystalline systems is expected. As the effect is marginally sensitive to the grain boundary type, we demonstrate that grain boundaries are a viable tool to greatly enhance the figure of merit, paving the way for the design of new thermoelectric materials.},
      year = {2015},
      url = http://dx.doi.org/{10.1103/PhysRevB.92.035418},
      doi = {10.1103/PhysRevB.92.035418},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Exciton size and binding energy limitations in one-dimensional organic materials
    • S. Kraner, R. Scholz, F. Plasser, C. Koerner, K. Leo
    • Journal of Chemical Physics 143(2015)
    • DOI   Abstract  

      In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy. (C) 2015 AIP Publishing LLC.

      @article{,
      author = {S. Kraner and R. Scholz and F. Plasser and C. Koerner and K. Leo},
      title = {Exciton size and binding energy limitations in one-dimensional organic materials},
      journal = {Journal of Chemical Physics},
      volume = {143},
      number = {24},
      abstract = {In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy. (C) 2015 AIP Publishing LLC.},
      year = {2015},
      url = http://dx.doi.org/{10.1063/1.4938527},
      doi = {10.1063/1.4938527},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Guanosine-based hydrogen-bonded 2D scaffolds: metal-free formation of G-quartet and G-ribbon architectures at the solid/liquid interface
    • M. E. Garah, R. C. Perone, A. S. Bonilla, S. Haar, M. Campitiello, R. Gutierrez, G. Cuniberti, S. Masiero, A. Ciesielski, P. Samori
    • Chemical Communications 51, 11677-11680 (2015)
    • DOI   Abstract  

      We report on the synthesis and self-assembly of three novel lipophilic guanosine derivatives exposing a ferrocene moiety in the C(5′) position of the sugar unit. Their self-association in solution, and at the solid/liquid interface, can be tuned by varying the size and nature of the C(8)-substituent, leading to the generation of either G-ribbons, lamellar G-dimer based arrays or the G(4) cation-free architectures.

      @article{,
      author = {Mohamed El Garah and Rosaria C. Perone and Alejandro Santana Bonilla and Sebastien Haar and Marilena Campitiello and Rafael Gutierrez and Gianaurelio Cuniberti and Stefano Masiero and Artur Ciesielski and Paolo Samori},
      title = {Guanosine-based hydrogen-bonded 2D scaffolds: metal-free formation of G-quartet and G-ribbon architectures at the solid/liquid interface},
      journal = {Chemical Communications},
      volume = {51},
      number = {58},
      pages = {11677-11680},
      abstract = {We report on the synthesis and self-assembly of three novel lipophilic guanosine derivatives exposing a ferrocene moiety in the C(5') position of the sugar unit. Their self-association in solution, and at the solid/liquid interface, can be tuned by varying the size and nature of the C(8)-substituent, leading to the generation of either G-ribbons, lamellar G-dimer based arrays or the G(4) cation-free architectures.},
      year = {2015},
      url = http://dx.doi.org/{10.1039/c5cc03197d},
      doi = {10.1039/c5cc03197d},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Impact of ultrasonic dispersion on the photocatalytic activity of titania aggregates
    • L. H. Nga, F. Babick, K. Kuehn, N. M. Tan, M. Stintz, G. Cuniberti
    • Beilstein Journal of Nanotechnology 6, 2423-2430 (2015)
    • DOI   Abstract  

      The effectiveness of photocatalytic materials increases with the specific surface area, thus nanoscale photocatalyst particles are preferred. However, such nanomaterials are frequently found in an aggregated state, which may reduce the photocatalytic activity due to internal obscuration and the extended diffusion path of the molecules to be treated. This paper investigates the effect of aggregate size on the photocatalytic activity of pyrogenic titania (Aeroxide (R) P25, Evonik), which is widely used in fundamental photocatalysis research. Well-defined and reproducible aggregate sizes were achieved by ultrasonic dispersion. The photocatalytic activity was examined by the color removal of methylene blue (MB) with a laboratory-scale setup based on a plug flow reactor (PFR) and planar UV illumination. The process parameters such as flow regime, optical path length and UV intensity are well-defined and can be varied. Our results firstly show that a complete dispersion of the P25 aggregates is not practical. Secondly, the photocatalytic activity is not further increased beyond a certain degree of dispersion, which probably corresponds to a critical size for which UV irradiation can penetrate the aggregate without significant obscuration.

      @article{,
      author = {Le Hoai Nga and Frank Babick and Klaus Kuehn and Nguyen Minh Tan and Michael Stintz and Gianaurelio Cuniberti},
      title = {Impact of ultrasonic dispersion on the photocatalytic activity of titania aggregates},
      journal = {Beilstein Journal of Nanotechnology},
      volume = {6},
      pages = {2423-2430},
      abstract = {The effectiveness of photocatalytic materials increases with the specific surface area, thus nanoscale photocatalyst particles are preferred. However, such nanomaterials are frequently found in an aggregated state, which may reduce the photocatalytic activity due to internal obscuration and the extended diffusion path of the molecules to be treated. This paper investigates the effect of aggregate size on the photocatalytic activity of pyrogenic titania (Aeroxide (R) P25, Evonik), which is widely used in fundamental photocatalysis research. Well-defined and reproducible aggregate sizes were achieved by ultrasonic dispersion. The photocatalytic activity was examined by the color removal of methylene blue (MB) with a laboratory-scale setup based on a plug flow reactor (PFR) and planar UV illumination. The process parameters such as flow regime, optical path length and UV intensity are well-defined and can be varied. Our results firstly show that a complete dispersion of the P25 aggregates is not practical. Secondly, the photocatalytic activity is not further increased beyond a certain degree of dispersion, which probably corresponds to a critical size for which UV irradiation can penetrate the aggregate without significant obscuration.},
      year = {2015},
      url = http://dx.doi.org/{10.3762/bjnano.6.250},
      doi = {10.3762/bjnano.6.250},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • In Situ Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes
    • H. T. Quang, A. Bachmatiuk, A. Dianat, F. Ortmann, J. Zhao, J. H. Warner, J. Eckert, G. Cunniberti, M. H. Ruemmeli
    • Acs Nano 9, 11408-11413 (2015)
    • DOI   Abstract  

      ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the in situ formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures.

      @article{,
      author = {Huy T. Quang and Alicja Bachmatiuk and Arezoo Dianat and Frank Ortmann and Jiong Zhao and Jamie H. Warner and Juergen Eckert and Gianaurelio Cunniberti and Mark H. Ruemmeli},
      title = {In Situ Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes},
      journal = {Acs Nano},
      volume = {9},
      number = {11},
      pages = {11408-11413},
      abstract = {ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the in situ formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures.},
      year = {2015},
      url = http://dx.doi.org/{10.1021/acsnano.5b05481},
      doi = {10.1021/acsnano.5b05481},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Influence of side groups on the performance of infrared absorbing aza-BODIPY organic solar cells
    • S. Kraner, J. Widmer, J. Benduhn, E. Hieckmann, T. Jaegeler-Hoheisel, S. Ullbrich, D. Schuetze, S. K. Radke, G. Cuniberti, F. Ortmann, M. Lorenz-Rothe, R. Meerheim, D. Spoltore, K. Vandewal, C. Koerner, K. Leo
    • Physica Status Solidi a-Applications and Materials Science 212, 2747-2753 (2015)
    • DOI   Abstract  

      Organic solar cells are a promising technology for a large area conversion of sunlight into electricity. In particular for solar cells based on oligomers (small molecules), efficient donor materials absorbing wavelengths larger than 780 nm are still rare. Here, we investigate three aza-BODIPY dyes absorbing in the infrared. The addition of side groups leads to a red shift of the optical gap from 802 to 818 nm. In optimized devices using these donors in a bulk heterojunction with C-60, we observe a higher charge carrier mobility and a higher power conversion efficiency for the molecules without a methyl or methoxy side group lowering the molecular reorganization energy. Surprisingly, the donor-acceptor blend with the lowest energy loss during the electron transfer to the C-60 yields the highest short circuit current. With increasing size of the attached side chain, the devices exhibit a larger trap density, measured by impedance spectroscopy. Based on the investigation of different blend ratios, we conclude that these traps are mainly present in the donor phase. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

      @article{,
      author = {Stefan Kraner and Johannes Widmer and Johannes Benduhn and Ellen Hieckmann and Till Jaegeler-Hoheisel and Sascha Ullbrich and Daniel Schuetze and K. Sebastian Radke and Gianaurelio Cuniberti and Frank Ortmann and Melanie Lorenz-Rothe and Rico Meerheim and Donato Spoltore and Koen Vandewal and Christian Koerner and Karl Leo},
      title = {Influence of side groups on the performance of infrared absorbing aza-BODIPY organic solar cells},
      journal = {Physica Status Solidi a-Applications and Materials Science},
      volume = {212},
      number = {12},
      pages = {2747-2753},
      abstract = {Organic solar cells are a promising technology for a large area conversion of sunlight into electricity. In particular for solar cells based on oligomers (small molecules), efficient donor materials absorbing wavelengths larger than 780 nm are still rare. Here, we investigate three aza-BODIPY dyes absorbing in the infrared. The addition of side groups leads to a red shift of the optical gap from 802 to 818 nm. In optimized devices using these donors in a bulk heterojunction with C-60, we observe a higher charge carrier mobility and a higher power conversion efficiency for the molecules without a methyl or methoxy side group lowering the molecular reorganization energy. Surprisingly, the donor-acceptor blend with the lowest energy loss during the electron transfer to the C-60 yields the highest short circuit current. With increasing size of the attached side chain, the devices exhibit a larger trap density, measured by impedance spectroscopy. Based on the investigation of different blend ratios, we conclude that these traps are mainly present in the donor phase. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim},
      year = {2015},
      url = http://dx.doi.org/{10.1002/pssa.201532385},
      doi = {10.1002/pssa.201532385},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Influence of Structural Defects and Oxidation onto Hole Conductivity in P3HT
    • A. Luecke, W. G. Schmidt, E. Rauls, F. Ortmann, U. Gerstmann
    • Journal of Physical Chemistry B 119, 6481-6491 (2015)
    • DOI   Abstract  

      The effect of structural imperfections as well as oxygen impurities on the quantum conductance of poly(3-hexylthiophene) is calculated from first principles by solving the scattering problem for molecular structures obtained within density functional theory It is shown that the conductivity of molecular crystals perpendicular to the polymer chains depends strongly on the stacking geometry and is roughly described within the Wentzel-Kramers-Brillouin approximation Furthermore it is found that local relaxation for twisted or bent 6320 polymer chains efficiently restores the conductance that drops 13 to substantially for sharp kinks with curvature radii smaller than 17 angstrom and rotations in excess of similar to 60 degrees In contrast isomer defects in the coupling along the chain direction are of minor importance for the intrachain transmission Also oxidation of the side chains as well as molecular sulfur barely changes the coherent transport properties, whereas oxidation of thiophene group carbon atoms drastically reduces the conductance

      @article{,
      author = {A. Luecke and W. G. Schmidt and E. Rauls and F. Ortmann and U. Gerstmann},
      title = {Influence of Structural Defects and Oxidation onto Hole Conductivity in P3HT},
      journal = {Journal of Physical Chemistry B},
      volume = {119},
      number = {21},
      pages = {6481-6491},
      abstract = {The effect of structural imperfections as well as oxygen impurities on the quantum conductance of poly(3-hexylthiophene) is calculated from first principles by solving the scattering problem for molecular structures obtained within density functional theory It is shown that the conductivity of molecular crystals perpendicular to the polymer chains depends strongly on the stacking geometry and is roughly described within the Wentzel-Kramers-Brillouin approximation Furthermore it is found that local relaxation for twisted or bent 6320 polymer chains efficiently restores the conductance that drops 13 to substantially for sharp kinks with curvature radii smaller than 17 angstrom and rotations in excess of similar to 60 degrees In contrast isomer defects in the coupling along the chain direction are of minor importance for the intrachain transmission Also oxidation of the side chains as well as molecular sulfur barely changes the coherent transport properties, whereas oxidation of thiophene group carbon atoms drastically reduces the conductance},
      year = {2015},
      url = http://dx.doi.org/{10.1021/acs.jpcb.5b03615},
      doi = {10.1021/acs.jpcb.5b03615},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Influence of surface charge on the transport characteristics of nanowire-field effect transistors in liquid environments
    • D. Nozaki, J. Kunstmann, F. Zoergiebel, G. Cuniberti
    • Applied Physics Letters 106(2015)
    • DOI   Abstract  

      One dimensional nanowire field effect transistors (NW-FETs) are a promising platform for sensor applications. The transport characteristics of NW-FETs are strongly modified in liquid environment due to the charging of surface functional groups accompanied with protonation or deprotonation. In order to investigate the influence of surface charges and ionic concentrations on the transport characteristics of Schottky-barrier NW-FETs, we have combined the modified Poisson-Boltzmann theory with the Landauer-Buttiker transport formalism. For a typical device, the model is able to capture the reduction of the sensitivity of NW-FETs in ionic solutions due to the screening from counter ions as well as a local gating from surface functional groups. Our approach allows to model, to investigate, and to optimize realistic Schottky-barrier NW-FET devices in liquid environment. (C) 2015 AIP Publishing LLC.

      @article{,
      author = {Daijiro Nozaki and Jens Kunstmann and Felix Zoergiebel and Gianaurelio Cuniberti},
      title = {Influence of surface charge on the transport characteristics of nanowire-field effect transistors in liquid environments},
      journal = {Applied Physics Letters},
      volume = {106},
      number = {20},
      abstract = {One dimensional nanowire field effect transistors (NW-FETs) are a promising platform for sensor applications. The transport characteristics of NW-FETs are strongly modified in liquid environment due to the charging of surface functional groups accompanied with protonation or deprotonation. In order to investigate the influence of surface charges and ionic concentrations on the transport characteristics of Schottky-barrier NW-FETs, we have combined the modified Poisson-Boltzmann theory with the Landauer-Buttiker transport formalism. For a typical device, the model is able to capture the reduction of the sensitivity of NW-FETs in ionic solutions due to the screening from counter ions as well as a local gating from surface functional groups. Our approach allows to model, to investigate, and to optimize realistic Schottky-barrier NW-FET devices in liquid environment. (C) 2015 AIP Publishing LLC.},
      year = {2015},
      url = http://dx.doi.org/{10.1063/1.4921401},
      doi = {10.1063/1.4921401},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Interplay between Mechanical and Electronic Degrees of Freedom in pi-Stacked Molecular Junctions: From Single Molecules to Mesoscopic Nanoparticle Networks
    • T. Ghane, D. Nozaki, A. Dianat, A. Vladyka, R. Gutierrez, J. P. Chinta, S. Yitzchaik, M. Calame, G. Cuniberti
    • Journal of Physical Chemistry C 119, 6344-6355 (2015)
    • DOI   Abstract  

      Functionalized nanoparticle networks offer a model system for the study of charge transport in low-dimensional systems as well as a potential platform to implement and test electronic functionalities. The electrical response of a nanoparticle network is expected to sensitively depend on the molecular interconnects, i.e., on the linker chemistry. If these linkers have complex charge transport properties, then phenomenological models addressing the large-scale properties of the network need to be complemented with microscopic calculations of the network building blocks. In this study we focus on the interplay between conformational fluctuations and electronic p-stacking in single-molecule junctions and use the obtained microscopic information on their electrical transport properties to parametrize transition rates describing charge diffusion in mesoscopic nanoparticle networks. Our results point out the strong influence of mechanical degrees of freedom on the electronic transport signatures of the studied molecules. This is then reflected in the varying charge diffusion at the network level. The modeling studies are complemented with first charge transport measurements at the single-molecule level of p-stacked molecular dimers using state-of-the-art mechanically controllable break junction techniques in a liquid environment.

      @article{,
      author = {Tahereh Ghane and Daijiro Nozaki and Arezoo Dianat and Anton Vladyka and Rafael Gutierrez and Jugun Prakash Chinta and Shlomo Yitzchaik and Michel Calame and Gianaurelio Cuniberti},
      title = {Interplay between Mechanical and Electronic Degrees of Freedom in pi-Stacked Molecular Junctions: From Single Molecules to Mesoscopic Nanoparticle Networks},
      journal = {Journal of Physical Chemistry C},
      volume = {119},
      number = {11},
      pages = {6344-6355},
      abstract = {Functionalized nanoparticle networks offer a model system for the study of charge transport in low-dimensional systems as well as a potential platform to implement and test electronic functionalities. The electrical response of a nanoparticle network is expected to sensitively depend on the molecular interconnects, i.e., on the linker chemistry. If these linkers have complex charge transport properties, then phenomenological models addressing the large-scale properties of the network need to be complemented with microscopic calculations of the network building blocks. In this study we focus on the interplay between conformational fluctuations and electronic p-stacking in single-molecule junctions and use the obtained microscopic information on their electrical transport properties to parametrize transition rates describing charge diffusion in mesoscopic nanoparticle networks. Our results point out the strong influence of mechanical degrees of freedom on the electronic transport signatures of the studied molecules. This is then reflected in the varying charge diffusion at the network level. The modeling studies are complemented with first charge transport measurements at the single-molecule level of p-stacked molecular dimers using state-of-the-art mechanically controllable break junction techniques in a liquid environment.},
      year = {2015},
      url = http://dx.doi.org/{10.1021/jp512524z},
      doi = {10.1021/jp512524z},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Lateral damage in graphene carved by high energy focused gallium ion beams
    • Z. Liao, T. Zhang, M. Gall, A. Dianat, R. Rosenkranz, R. Jordan, G. Cuniberti, E. Zschech
    • Applied Physics Letters 107(2015)
    • DOI   Abstract  

      Raman mapping is performed to study the lateral damage in supported monolayer graphene carved by 30 keV focused Ga+ beams. The evolution of the lateral damage is tracked based on the profiles of the intensity ratio between the D (1341 cm(-1)) and G (1582 cm(-1)) peaks (I-D/I-G) of the Raman spectra. The I-D/I-G profile clearly reveals the transition from stage 2 disorder into stage 1 disorder in graphene along the direction away from the carved area. The critical lateral damage distance spans from <1 mu m up to more than 30 mu m in the experiment, depending on the parameters used for carving the graphene. The wide damage in the lateral direction is attributed to the deleterious tail of unfocused ions in the ion beam probe. The study raises the attention on potential sample damage during direct patterning of graphene nanostructures using the focused ion beam technique. Minimizing the total carving time is recommended to mitigate the lateral damage. (C) 2015 AIP Publishing LLC.

      @article{,
      author = {Zhongquan Liao and Tao Zhang and Martin Gall and Arezoo Dianat and Ruediger Rosenkranz and Rainer Jordan and Gianaurelio Cuniberti and Ehrenfried Zschech},
      title = {Lateral damage in graphene carved by high energy focused gallium ion beams},
      journal = {Applied Physics Letters},
      volume = {107},
      number = {1},
      abstract = {Raman mapping is performed to study the lateral damage in supported monolayer graphene carved by 30 keV focused Ga+ beams. The evolution of the lateral damage is tracked based on the profiles of the intensity ratio between the D (1341 cm(-1)) and G (1582 cm(-1)) peaks (I-D/I-G) of the Raman spectra. The I-D/I-G profile clearly reveals the transition from stage 2 disorder into stage 1 disorder in graphene along the direction away from the carved area. The critical lateral damage distance spans from <1 mu m up to more than 30 mu m in the experiment, depending on the parameters used for carving the graphene. The wide damage in the lateral direction is attributed to the deleterious tail of unfocused ions in the ion beam probe. The study raises the attention on potential sample damage during direct patterning of graphene nanostructures using the focused ion beam technique. Minimizing the total carving time is recommended to mitigate the lateral damage. (C) 2015 AIP Publishing LLC.},
      year = {2015},
      url = http://dx.doi.org/{10.1063/1.4926647},
      doi = {10.1063/1.4926647},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Light Weight and Flexible High-Performance Diagnostic Platform
    • D. Karnaushenko, B. Ibarlucea, S. Lee, G. Lin, L. Baraban, S. Pregl, M. Melzer, D. Makarov, W. M. Weber, T. Mikolajick, O. G. Schmidt, G. Cuniberti
    • Advanced Healthcare Materials 4, 1517-1525 (2015)
    • DOI   Abstract  

      A flexible diagnostic platform is realized and its performance is demonstrated for early detection of avian influenza virus (AIV) subtype H1N1 DNA sequences. The key component of the platform is high-performance biosensors based on high output currents and low power dissipation Si nanowire field effect transistors (SiNW-FETs) fabricated on flexible 100 mu m thick polyimide foils. The devices on a polymeric support are about ten times lighter compared to their rigid counterparts on Si wafers and can be prepared on large areas. While the latter potentially allows reducing the fabrication costs per device, the former makes them cost efficient for high-volume delivery to medical institutions in, e.g., developing countries. The flexible devices withstand bending down to a 7.5 mm radius and do not degrade in performance even after 1000 consecutive bending cycles. In addition to these remarkable mechanical properties, on the analytic side, the diagnostic platform allows fast detection of specific DNA sequences of AIV subtype H1N1 with a limit of detection of 40 x 10(-12) M within 30 min suggesting its suitability for early stage disease diagnosis.

      @article{,
      author = {Daniil Karnaushenko and Bergoi Ibarlucea and Sanghun Lee and Gungun Lin and Larysa Baraban and Sebastian Pregl and Michael Melzer and Denys Makarov and Walter M. Weber and Thomas Mikolajick and Oliver G. Schmidt and Gianaurelio Cuniberti},
      title = {Light Weight and Flexible High-Performance Diagnostic Platform},
      journal = {Advanced Healthcare Materials},
      volume = {4},
      number = {10},
      pages = {1517-1525},
      abstract = {A flexible diagnostic platform is realized and its performance is demonstrated for early detection of avian influenza virus (AIV) subtype H1N1 DNA sequences. The key component of the platform is high-performance biosensors based on high output currents and low power dissipation Si nanowire field effect transistors (SiNW-FETs) fabricated on flexible 100 mu m thick polyimide foils. The devices on a polymeric support are about ten times lighter compared to their rigid counterparts on Si wafers and can be prepared on large areas. While the latter potentially allows reducing the fabrication costs per device, the former makes them cost efficient for high-volume delivery to medical institutions in, e.g., developing countries. The flexible devices withstand bending down to a 7.5 mm radius and do not degrade in performance even after 1000 consecutive bending cycles. In addition to these remarkable mechanical properties, on the analytic side, the diagnostic platform allows fast detection of specific DNA sequences of AIV subtype H1N1 with a limit of detection of 40 x 10(-12) M within 30 min suggesting its suitability for early stage disease diagnosis.},
      year = {2015},
      url = http://dx.doi.org/{10.1002/adhm.201500128},
      doi = {10.1002/adhm.201500128},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Materials Meets Concepts in Molecule-Based Electronics
    • F. Ortmann, S. K. Radke, A. Guenther, D. Kasemann, K. Leo, G. Cuniberti
    • Advanced Functional Materials 25, 1933-1954 (2015)
    • DOI   Abstract  

      In this contribution, molecular materials are highlighted as an important topic in the diverse field of condensed matter physics, with focus on their particular electronic and transport properties. A better understanding of their performance in various applications and devices demands for an extension of basic theoretical approaches to describe charge transport in molecular materials, including the accurate description of electron-phonon coupling. Starting with the simplest case of a molecular junction and moving on to larger aggregates of bulk organic semiconductors, charge-transport regimes from ballistic motion to incoherent hopping, which are frequently encountered in molecular systems under respective conditions, are discussed. Transport features of specific materials are described through ab initio material parameters whose determination is addressed.

      @article{,
      author = {Frank Ortmann and K. Sebastian Radke and Alrun Guenther and Daniel Kasemann and Karl Leo and Gianaurelio Cuniberti},
      title = {Materials Meets Concepts in Molecule-Based Electronics},
      journal = {Advanced Functional Materials},
      volume = {25},
      number = {13},
      pages = {1933-1954},
      abstract = {In this contribution, molecular materials are highlighted as an important topic in the diverse field of condensed matter physics, with focus on their particular electronic and transport properties. A better understanding of their performance in various applications and devices demands for an extension of basic theoretical approaches to describe charge transport in molecular materials, including the accurate description of electron-phonon coupling. Starting with the simplest case of a molecular junction and moving on to larger aggregates of bulk organic semiconductors, charge-transport regimes from ballistic motion to incoherent hopping, which are frequently encountered in molecular systems under respective conditions, are discussed. Transport features of specific materials are described through ab initio material parameters whose determination is addressed.},
      year = {2015},
      url = http://dx.doi.org/{10.1002/adfm.201402334},
      doi = {10.1002/adfm.201402334},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Microfluidic alignment and trapping of 1D nanostructures – a simple fabrication route for single-nanowire field effect transistors
    • A. Gang, N. Haustein, L. Baraban, W. Weber, T. Mikolajick, J. Thiele, G. Cuniberti
    • Rsc Advances 5, 94702-94706 (2015)
    • DOI   Abstract  

      We present a simple method to microfluidically align and trap 1D nanostructures from suspension at well-defined positions on a receiver substrate for the fabrication of single-nanowire field effect transistors (NWFETs). Our approach allows for subsequent contacting of deposited NWs via standard UV-lithography. We demonstrate that silicon as well as copper(II)oxide NWs can be processed, and that up to 13 out of 32 designated trapping sites are occupied with single-NW FETs.

      @article{,
      author = {A. Gang and N. Haustein and L. Baraban and W. Weber and T. Mikolajick and J. Thiele and G. Cuniberti},
      title = {Microfluidic alignment and trapping of 1D nanostructures - a simple fabrication route for single-nanowire field effect transistors},
      journal = {Rsc Advances},
      volume = {5},
      number = {115},
      pages = {94702-94706},
      abstract = {We present a simple method to microfluidically align and trap 1D nanostructures from suspension at well-defined positions on a receiver substrate for the fabrication of single-nanowire field effect transistors (NWFETs). Our approach allows for subsequent contacting of deposited NWs via standard UV-lithography. We demonstrate that silicon as well as copper(II)oxide NWs can be processed, and that up to 13 out of 32 designated trapping sites are occupied with single-NW FETs.},
      year = {2015},
      url = http://dx.doi.org/{10.1039/c5ra20414c},
      doi = {10.1039/c5ra20414c},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Modeling of photocatalytic degradation of organic components in water by nanoparticle suspension
    • H. Eckert, M. Bobeth, S. Teixeira, K. Kuehn, G. Cuniberti
    • Chemical Engineering Journal 261, 67-75 (2015)
    • DOI   Abstract  

      Photocatalytic degradation of organic components in water by means of TiO2 nanosuspensions under ultraviolet (UV) irradiation represents an efficient method for water purification. In the present paper, a modeling approach is proposed to simulate the involved kinetic processes based on the Langmuir-Hinshelwood mechanism. The extended model also includes the formation of intermediate organic components either by an incremental degradation mechanism or by a fragmentation mechanism. Model parameters were estimated from comparison with experimental findings. To demonstrate these models, adsorption and degradation experiments were performed using the antibiotic ciprofloxacin and the dye methylene blue as organic compounds and TiO2 and ZnO as photocatalytic materials. By comparing our simulations with concentration measurements, we found that the adsorption of organic molecules on the surface of the photocatalyst was rate determining at an irradiation intensity of about 20 W m(-2). The derived adsorption rates for ZnO were considerably higher than those for TiO2. The calculated concentration evolution of intermediates as well as the TOC evolution are discussed for different model assumptions with respect to their desorption rates from the photocatalyst surface. (C) 2014 Elsevier B.V. All rights reserved.

      @article{,
      author = {Hagen Eckert and Manfred Bobeth and Sara Teixeira and Klaus Kuehn and Gianaurelio Cuniberti},
      title = {Modeling of photocatalytic degradation of organic components in water by nanoparticle suspension},
      journal = {Chemical Engineering Journal},
      volume = {261},
      pages = {67-75},
      abstract = {Photocatalytic degradation of organic components in water by means of TiO2 nanosuspensions under ultraviolet (UV) irradiation represents an efficient method for water purification. In the present paper, a modeling approach is proposed to simulate the involved kinetic processes based on the Langmuir-Hinshelwood mechanism. The extended model also includes the formation of intermediate organic components either by an incremental degradation mechanism or by a fragmentation mechanism. Model parameters were estimated from comparison with experimental findings. To demonstrate these models, adsorption and degradation experiments were performed using the antibiotic ciprofloxacin and the dye methylene blue as organic compounds and TiO2 and ZnO as photocatalytic materials. By comparing our simulations with concentration measurements, we found that the adsorption of organic molecules on the surface of the photocatalyst was rate determining at an irradiation intensity of about 20 W m(-2). The derived adsorption rates for ZnO were considerably higher than those for TiO2. The calculated concentration evolution of intermediates as well as the TOC evolution are discussed for different model assumptions with respect to their desorption rates from the photocatalyst surface. (C) 2014 Elsevier B.V. All rights reserved.},
      year = {2015},
      url = http://dx.doi.org/{10.1016/j.cej.2014.05.147},
      doi = {10.1016/j.cej.2014.05.147},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Modeling of Solvent Effects in the Electrical Response of pi-Stacked Molecular Junctions
    • T. Ghane, A. Kleshchonok, R. Gutierrez, G. Cuniberti
    • Journal of Physical Chemistry C 119, 20201-20209 (2015)
    • DOI   Abstract  

      We present theoretical modeling of the influence of THF solvents on the mechanical stability and the electrical response of two different pi-stacked molecular junctions based on cysteamine conjugates of naphthalic anhydride and of pyrene. Combining molecular dynamics simulations and quantum transport calculations, we show that for junctions with a weaker pi-pi stacking as measured by the stacking energy dynamical breaking of the stacking induced by the solvent can take place. However, contrary to what may be expected, the conductance of the system is not suppressed due to the emergence of an additional transport channel which bypasses the broken pi overlap of the perylene cores. However, an additional gating-like effect in such a situation does reduce the low bias current when comparing with situations, where pi-stacking is preserved.

      @article{,
      author = {Tahereh Ghane and Andrii Kleshchonok and Rafael Gutierrez and Gianaurelio Cuniberti},
      title = {Modeling of Solvent Effects in the Electrical Response of pi-Stacked Molecular Junctions},
      journal = {Journal of Physical Chemistry C},
      volume = {119},
      number = {34},
      pages = {20201-20209},
      abstract = {We present theoretical modeling of the influence of THF solvents on the mechanical stability and the electrical response of two different pi-stacked molecular junctions based on cysteamine conjugates of naphthalic anhydride and of pyrene. Combining molecular dynamics simulations and quantum transport calculations, we show that for junctions with a weaker pi-pi stacking as measured by the stacking energy dynamical breaking of the stacking induced by the solvent can take place. However, contrary to what may be expected, the conductance of the system is not suppressed due to the emergence of an additional transport channel which bypasses the broken pi overlap of the perylene cores. However, an additional gating-like effect in such a situation does reduce the low bias current when comparing with situations, where pi-stacking is preserved.},
      year = {2015},
      url = http://dx.doi.org/{10.1021/acs.jpcc.5b06867},
      doi = {10.1021/acs.jpcc.5b06867},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Optoelectronic switching of nanowire-based hybrid organic/oxide/semiconductor field-effect transistors
    • E. Baek, S. Pregl, M. Shaygan, L. Roemhildt, W. M. Weber, T. Mikolajick, D. A. Ryndyk, L. Baraban, G. Cuniberti
    • Nano Research 8, 1229-1240 (2015)
    • DOI   Abstract  

      A novel photosensitive hybrid field-effect transistor (FET) which consists of a multiple-shell of organic porphyrin film/oxide/silicon nanowires is presented. Due to the oxide shell around the nanowires, photoswitching of the current in the hybrid nanodevices is guided by the electric field effect, induced by charge redistribution within the organic film. This principle is an alternative to a photoinduced electron injection, valid for devices relying on direct junctions between organic molecules and metals or semiconductors. The switching dynamics of the hybrid nanodevices upon violet light illumination is investigated and a strong dependence on the thickness of the porphyrin film wrapping the nanowires is found. Furthermore, the thickness of the organic films is found to be a crucial parameter also for the switching efficiency of the nanowire FET, represented by the ratio of currents under light illumination (ON) and in dark conditions (OFF). We suggest a simple model of porphyrin film charging to explain the optoelectronic behavior of nanowire FETs mediated by organic film/oxide/semiconductor junctions.

      @article{,
      author = {Eunhye Baek and Sebastian Pregl and Mehrdad Shaygan and Lotta Roemhildt and Walter M. Weber and Thomas Mikolajick and Dmitry A. Ryndyk and Larysa Baraban and Gianaurelio Cuniberti},
      title = {Optoelectronic switching of nanowire-based hybrid organic/oxide/semiconductor field-effect transistors},
      journal = {Nano Research},
      volume = {8},
      number = {4},
      pages = {1229-1240},
      abstract = {A novel photosensitive hybrid field-effect transistor (FET) which consists of a multiple-shell of organic porphyrin film/oxide/silicon nanowires is presented. Due to the oxide shell around the nanowires, photoswitching of the current in the hybrid nanodevices is guided by the electric field effect, induced by charge redistribution within the organic film. This principle is an alternative to a photoinduced electron injection, valid for devices relying on direct junctions between organic molecules and metals or semiconductors. The switching dynamics of the hybrid nanodevices upon violet light illumination is investigated and a strong dependence on the thickness of the porphyrin film wrapping the nanowires is found. Furthermore, the thickness of the organic films is found to be a crucial parameter also for the switching efficiency of the nanowire FET, represented by the ratio of currents under light illumination (ON) and in dark conditions (OFF). We suggest a simple model of porphyrin film charging to explain the optoelectronic behavior of nanowire FETs mediated by organic film/oxide/semiconductor junctions.},
      year = {2015},
      url = http://dx.doi.org/{10.1007/s12274-014-0608-7},
      doi = {10.1007/s12274-014-0608-7},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Photoassisted transport in silicon dangling bond wires
    • A. Kleshchonok, R. Gutierrez, C. Joachim, G. Cuniberti
    • Applied Physics Letters 107(2015)
    • DOI   Abstract  

      We theoretically investigate charge transport through dangling bond (DB) nanostructures built on a passivated silicon (100) surface by selectively removing hydrogen atoms. We focus on dangling bond wires and on T-junctions. In the latter case, destructive quantum interference effects lead to a strong suppression of charge transport mediated by the DB electronic states. We demonstrate, however, that by applying a time periodic voltage, mimicking irradiation with monochromatic light, a dramatic enhancement of the current up to the mu A range can be achieved. This result is however limited by the restriction on the AC field strength and frequency that bulk states should minimally contribute to charge transport; otherwise current leakage will set in. Despite this constraint, transconductance values of the order of 10(-6) A/V can be achieved, illustrating the potential of the discussed systems to find applications in nanoscale electronics. (c) 2015 AIP Publishing LLC.

      @article{,
      author = {Andrii Kleshchonok and Rafael Gutierrez and Christian Joachim and Gianaurelio Cuniberti},
      title = {Photoassisted transport in silicon dangling bond wires},
      journal = {Applied Physics Letters},
      volume = {107},
      number = {20},
      abstract = {We theoretically investigate charge transport through dangling bond (DB) nanostructures built on a passivated silicon (100) surface by selectively removing hydrogen atoms. We focus on dangling bond wires and on T-junctions. In the latter case, destructive quantum interference effects lead to a strong suppression of charge transport mediated by the DB electronic states. We demonstrate, however, that by applying a time periodic voltage, mimicking irradiation with monochromatic light, a dramatic enhancement of the current up to the mu A range can be achieved. This result is however limited by the restriction on the AC field strength and frequency that bulk states should minimally contribute to charge transport; otherwise current leakage will set in. Despite this constraint, transconductance values of the order of 10(-6) A/V can be achieved, illustrating the potential of the discussed systems to find applications in nanoscale electronics. (c) 2015 AIP Publishing LLC.},
      year = {2015},
      url = http://dx.doi.org/{10.1063/1.4936182},
      doi = {10.1063/1.4936182},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Quantum interference based Boolean gates in dangling bond loops on Si(100):H surfaces
    • A. Kleshchonok, R. Gutierrez, C. Joachim, G. Cuniberti
    • Scientific Reports 5(2015)
    • DOI   Abstract  

      Implementing atomic and molecular scale electronic functionalities represents one of the major challenges in current nano-electronic developments. Engineered dangling bond nanostructures on Silicon or Germanium surfaces posses the potential to provide novel routes towards the development of non-conventional electronic circuits. These structures are built by selectively removing hydrogen atoms from an otherwise fully passivated Si(100) or Ge(100) substrate. In this theoretical study, we demonstrate how dangling bond loops can be used to implement different Boolean logic gates. Our approach exploits quantum interference effects in such ring-like structures combined with an appropriate design of the interfacing of the dangling bond system with mesoscopic electrodes. We show how OR, AND, and NOR gates can be realized by tuning either the global symmetry of the system in a multi-terminal setup-by arranging the position of the input and output electrodes-or, alternatively, by selectively applying electrostatic gates in a two-terminal configuration.

      @article{,
      author = {Andrii Kleshchonok and Rafael Gutierrez and Christian Joachim and Gianaurelio Cuniberti},
      title = {Quantum interference based Boolean gates in dangling bond loops on Si(100):H surfaces},
      journal = {Scientific Reports},
      volume = {5},
      abstract = {Implementing atomic and molecular scale electronic functionalities represents one of the major challenges in current nano-electronic developments. Engineered dangling bond nanostructures on Silicon or Germanium surfaces posses the potential to provide novel routes towards the development of non-conventional electronic circuits. These structures are built by selectively removing hydrogen atoms from an otherwise fully passivated Si(100) or Ge(100) substrate. In this theoretical study, we demonstrate how dangling bond loops can be used to implement different Boolean logic gates. Our approach exploits quantum interference effects in such ring-like structures combined with an appropriate design of the interfacing of the dangling bond system with mesoscopic electrodes. We show how OR, AND, and NOR gates can be realized by tuning either the global symmetry of the system in a multi-terminal setup-by arranging the position of the input and output electrodes-or, alternatively, by selectively applying electrostatic gates in a two-terminal configuration.},
      year = {2015},
      url = http://dx.doi.org/{10.1038/srep14136},
      doi = {10.1038/srep14136},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Spin transport in hydrogenated graphene
    • D. Soriano, D. V. Tuan, S. M. M. Dubois, M. Gmitra, A. W. Cummings, D. Kochan, F. Ortmann, J. Charlier, J. Fabian, S. Roche
    • 2d Materials 2(2015)
    • DOI   Abstract  

      In this review we discuss the multifaceted problem of spin transport in hydrogenated graphene from a theoretical perspective. The current experimental findings suggest that hydrogenation can either increase or decrease spin lifetimes, which calls for clarification. We first discuss the spin-orbit coupling induced by locals sigma – pi re-hybridization and sp(3) C-H defect formation together with the formation of a local magnetic moment. First-principles calculations of hydrogenated graphene unravel the strong interplay of spin-orbit and exchange couplings. The concept of magnetic scattering resonances, recently introduced by Kochan et al (2014 Phys. Rev. Lett. 112 116602) is revisited by describing the local magnetism through the self-consistent Hubbard model in the mean field approximation in the dilute limit, while spin relaxation lengths and transport times are computed using an efficient real space order N wavepacket propagation method. Typical spin lifetimes on the order of 1 ns are obtained for 1 ppm of hydrogen impurities (corresponding to a transport time of about 50 ps), and the scaling of spin lifetimes with impurity density is described by the Elliott-Yafet mechanism. This reinforces the statement that local defect-induced magnetism can be at the origin of the substantial spin polarization loss in the clean graphene limit.

      @article{,
      author = {David Soriano and Dinh Van Tuan and Simon M. M. Dubois and Martin Gmitra and Aron W. Cummings and Denis Kochan and Frank Ortmann and Jean-Christophe Charlier and Jaroslav Fabian and Stephan Roche},
      title = {Spin transport in hydrogenated graphene},
      journal = {2d Materials},
      volume = {2},
      number = {2},
      abstract = {In this review we discuss the multifaceted problem of spin transport in hydrogenated graphene from a theoretical perspective. The current experimental findings suggest that hydrogenation can either increase or decrease spin lifetimes, which calls for clarification. We first discuss the spin-orbit coupling induced by locals sigma - pi re-hybridization and sp(3) C-H defect formation together with the formation of a local magnetic moment. First-principles calculations of hydrogenated graphene unravel the strong interplay of spin-orbit and exchange couplings. The concept of magnetic scattering resonances, recently introduced by Kochan et al (2014 Phys. Rev. Lett. 112 116602) is revisited by describing the local magnetism through the self-consistent Hubbard model in the mean field approximation in the dilute limit, while spin relaxation lengths and transport times are computed using an efficient real space order N wavepacket propagation method. Typical spin lifetimes on the order of 1 ns are obtained for 1 ppm of hydrogen impurities (corresponding to a transport time of about 50 ps), and the scaling of spin lifetimes with impurity density is described by the Elliott-Yafet mechanism. This reinforces the statement that local defect-induced magnetism can be at the origin of the substantial spin polarization loss in the clean graphene limit.},
      year = {2015},
      url = http://dx.doi.org/{10.1088/2053-1583/2/2/022002},
      doi = {10.1088/2053-1583/2/2/022002},
      openaccess = yes,
      peerreview = yes,
      keywords = {nanotechnology}
      }

  • Spin-Dependent Effects in Helical Molecular Systems with Rashba-Like Spin-Orbit Interaction
    • R. Gutierrez, G. Cuniberti
    • Acta Physica Polonica A 127, 185-191 (2015)