ATOMIC SCALE STUDY OF TWO-DIMENSIONAL MATERIALS BY SCANNING TUNNELING MICROSCOPY AND FIRST-PRINCIPLES CALCULATIONS

Abstract

In this thesis, the atomic structure and electronic properties of 2D-materials were investigated by first-principles calculations in combination with LT-STM/S. We first present an investigation of the native point defects in SL-WSe2. DFT calculations show that Se adatoms and vacancies are the two most stable defects, but none of the native defects were observed as the dominant point defects in STM measurements. Combining STM/S and DFT calculations, we found that the observed point defects are oxygen contaminants formed by oxygen dissociative adsorption at the Se vacancies. Next, we studied perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on SL-WSe2, bare graphite and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO-LUMO gap on electronic screening effects from the substrate. The SL-WSe2 interlayer provides substantial – but not complete – screening at the organic/inorganic interface. Lastly, the PTCDA monolayer adsorbed on few layer h-BN were also measured by STM/S and the measured band gap (3.8 eV) is smaller than the theoretical predicated value for isolated PTCDA molecule (5.0 eV) and layer (4.1 eV). The results show that screening by few layer hBN can substantially reduce the HOMO-LUMO gap of adsorbed molecules.