Tese de Doutorado #368 – Thaís Chagas Peixoto Silva – 05/05/2020

In this work we have used scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES) to study the interplay between atomic and electronic structure of two-dimensional materials. In particular, this thesis is set of three complementary projects, related to three different materials, namely, triangular graphene nanostructures, topological insulators and transition metal dichalcogenides.

First, a dual electronic response of triangular graphene nanostructures grown on SiC was investigated. We report on the observation of triangular nanostructures resulted from extended stacking faults in the SiC substrate and their effects on graphene layers that are formed on top of them. Spectroscopic measurements revealed distinct electronic responses as a consequence of the incomplete hydrogen intercalation process in a fraction of the observed nanostructures, in which the bottom layer of the bilayer graphene is still bonded to the substrate. Such configuration generates regions where the bilayer graphene presents an electronic signature that resembles those of single layer graphene systems.

In addition, the topological insulator Bi4Te3, synthetized using the Bridgman method, was also investigated here. X-ray diffraction measurements confirmed the formation of Bi4Te3 as a major phase in a system also composed of minority phases of Bi2 and Bi2Te3. Density functional theory of Bi4Te3 and its constituents allowed, through the calculation of their band structures and density of states, for a systematic study of their electronic properties. The combined use of STM and STS, analyzing height-profiles and conductance spectra, permitted us to identify different terminations of Bi4Te3 with different electronic responses. ARPES measurements highly resemble quintuple-layer terminated Bi4Te3 signatures.

Finally, in the last part of this thesis, a study of TaS2 films grown on Au(111) is presented. This material was synthetized through the evaporation of Ta on Au(111) in a background of H2S; and it was investigated using low energy electron diffraction and STM/STS. It was found that under specific growth conditions the quantity of TaS2 islands on the substrate is significantly reduced giving rise to a number of trimers. The emergence of these trimers with triangular morphology distort the Au(111) herringbone reconstruction. Atomically-resolved STM images revealed distinct orientations of the trimers depending on the regions of the Au(111) surface where they sit.