NIC Excellence Project 2022/2
Spectra of 2D layered materials
John von Neumann Exzellenzprojekt 2022/2
Prof. Michael Rohlfing (Universität Münster)
Structural and opto-electronic properties of condensed matter are determined by electronic quantum mechanics on the atomic length scale, ranging from molecules to extended crystals. This local quantum mechanics competes with the geometry structure on the nanometer scale, asking for theoretical description by ab-initio (or ''first-principles'') techniques, in which the atomic orbitals constitute the smallest active unit. This is especially true for low-dimensional materials like monolayers of transition-metal dichalcogenides (e.g., molybdenum disulfide).
The intrinsic properties of a material can be manipulated by external influences, like deposition on a substrate, geometric strain, external electric or magnetic fields, gating from applying a voltage, and dielectric environment, opening up the possibility of functional design. In this context the theoretical understanding of the response to such external stimuli appears particularly important. Our main topic is the investigations of (opto)electronic spectra, i.e. single-particle spectra (band structures) and two-particle excitations (optical spectra) from first principles.
Such first-principles or ab-initio methods constitute a hierarchy of several steps, each of which follows naturally from the physical properties it shall describe. The basis is given by density-functional theory, which is the standard tool to describe the physical and chemical structure of condensed matter, including thermodynamical properties, elasticity, etc. As the next step we address electronic spectra (i.e. molecular levels or band structures of crystals) within the GW approximation of many-body perturbation theory. The challenge in here is to incorporate the many-particle nature of electrons in terms of exchange and correlation. In principle, electronic spectra also include electronic transport. Finally, optical excitations (excitons) are obtained from the Bethe-Salpeter equation of correlated electron-hole pairs, based on the electronic levels of the GW step. This gives access to all (linear) optical properties and light-matter interaction, like light absorption, transmission, reflectivity etc.
We are in particular interested in the precise determination of relative effects as resulting from differences between related systems, e.g., system-specific states of nanostructures (like monolayer states as compared to bulk states), and modifications due to changes in the environment. Among other effects, we investigate the red-shift of excitons due to environmental polarizability, the emergence of exciplex states due to the admixture of charge-transfer configurations if a monolayer is brought in close contact to further layers (like in a bilayer or bulk system). Further studies concern the spectral shift of excitons when a monolayer is exposed to magnetic fields, and the coupling of excitons of WSe2 when contacted with a two-dimensional ferromagnet, CrI3. Last but not least, atomic defects in low-dimensional systems yield local electronic state and local excitations much different from the surrounding extended material.
