Influence of Coulomb interaction and spatial inhomogeneities on two-dimensional materials

Abstract

Nowadays a large variety of two-dimensional (2d) materials ranging from (functionalized) graphene, graphene analogues like hexagonal boron nitride to metallic, semiconducting or superconducting transition metal dichalcogenides are studied theoretically and experimentally. Their remarkable material features, resulting from the unique two-dimensional physics, as well as flexibility in tuning of their properties made them interesting for various applications. For example regarding electronic devices novel kinds of heterostructures were created with the possibility for on demand tailoring through the stacking of different layered materials. Coulomb interaction effects play a major role in characterizing 2d materials. Due to the low dimensionality of the systems, the interaction effects are enhanced and highly sensitive to external screening. In this thesis we make use of these peculiar interaction effects to create lateral heterojunctions within otherwise homogeneous monolayers through the external manipulation of the Coulomb interaction. Therefore we study the band gap modulation in semiconducting transition metal dichalcogenides placed on laterally structured substrates and show spatially sharp band gap transitions on the order of a few unit cells. Contrary to other kinds of heterostructures, the proposed mechanism is non-invasive leaving the active material of the heterojunction untouched. With respect to optical properties we study the response of the exciton to tuning of the Coulomb interaction and find that the lowest energy excited state is nearly unaffected by dielectric environments. However, higher energy excitations can be strongly manipulated. For the construction of optimal tailor-made devices a comprehensive understanding of the underlying Coulomb interaction effects is necessary. However, interaction effects are often not well understood and can be difficult to describe. To this end we utilize models based on ab-initio calculations which include the main features of the investigated materials and suitable descriptions of the screening effects. To find optimal candidates for these kind of heterostructures we compare the effect of external dielectric environments on different semiconducting transition metal dichalcogenides. All materials under investigation show the same relative changes in the band gap for increasing dielectric screening rendering this class of materials equally suitable for further applications. Not only the dielectric but also the chemical environment, e.g. different gaseous atmospheres, can alter the material properties of a 2d material. Finally we investigate the influence of O2 adsorption on (doped) monolayer MoS2 as a promising candidate for sensing applications.