International summer school MATERIALS 4.0
Rational Computational Materials Design using Quantum Confinement or Beyond Structural Materials Engineering
Prof. Dr. Thomas Heine, University of Leipzig
Friday, Sept. 15th, 9:30 – 10:30
Görgesbau (Helmholtzstr. 9), room 229
Abstract: Quantum confinement is one of the design principles in nanotechnology. Well-known examples are quantum dots, which are nanocrystals whose electronic band gaps depend crucially on their spatial extension. Thus, it is possible to create light emitting devices with the colour defined by the diameter of the quantum dot. Latest applications include large-scale displays and lasers.
In my presentation I will show that quantum confinement is not restricted to quantum dots and optoelectronic applications. I will show how quantum confinement can be exploited as strategy for the rational design of functional nanomaterials.
The first examples are taken from the field of layered materials, where quantum confinement can be used to tailor the band gap, but also the character of the band gap. For example, transition metal dichalcogenides MX2 (M=Mo, W, X=S, Se) are indirect band gap semiconductors as bulk and multilayer phases, but direct band gap semiconductors with appreciable photoluminescence signal as single-layer material. These ultrathing materials are also called two-dimensional crystals. The exploitation of quantum confinement gets even more interesting if the symmetry of the material is changed by changing the layer number. For example, by the absence (monolayer) or presence (bilayer) of inversion symmetry in two-dimensional crystals strong spin polarization effects are observed. The strongest quantum confinement effect so far we have predicted for PdS2, a two dimensional crystal that is semiconducting as monolayer, but metallic as bilayer. Similarly, GeP3 is a semiconductor in mono- and bilayer form, but metallic for trilayers and thicker stacks.
Another way of exploiting quantum confinement is the application of external fields, most notably electric fields, which are conveniently applied using a gate voltage, and strain fields. I will present various examples where the electronic band gap and/or the density of states are strongly affected by an external gate voltage and by strain fields. For the latter, I will also show some topological phase transitions.
In the final part of my presentation I will show how quantum confinement can be exploited to separate isotopes of light-weight elements, e.g. hydrogen or helium. I will present two independent mechanisms that have been realized in metal-organic frameworks (MOFs): MOFs with well-defined apartures that are about the same size as the effective diameter as the dihydrogen isotopologue show a so-called kinetic quantum sieving effect, where the diffusion of heavier isotopologues is favoured. On the contrary, chemical affinity sieving is observed for materials with strong adsorption sites, such as undercoordinated metal ions. Here, the isotopologues have different zero-point energy contributions to the adsorption energy. Using this effect, separation coefficients of 10 and higher can be achieved even at temperatures above 100 K.