Observation and Creation of Defects of Transition Metal and van der waal Surfaces through Scanning Tunneling Microscopy

The research described in this dissertation involves the study of two different kinds of surfaces: (i) Pt(111), which is a model surface for heterogeneous catalysis, and (ii) the surfaces of a range of 2D materials with technologically interesting electronic properties, which include TaS2, iron intercalated TaS2 and EuZn2As2.
Local real space observations were made of the dissociative chemisorption products of a branched alkane on a catalytic metal surface. The initial dissociative chemisorption of saturated alkanes on catalyst surfaces typically occurs through C-H bond cleavage, and initial dissociative chemisorption by C-C bond cleavage is rarely observed. Tetramethylbutane (TMB) is the only saturated alkane that has been conjectured to display an additional C-C bond cleavage pathway on Pt(111) at high temperatures based on dissociative sticking coefficient measurements. Scanning tunneling microscopy (STM) was used here to directly observe the fragment products from both C-H and C-C bond cleavage pathways for TMB on Pt(111) and thereby confirmed the mechanistic conjecture made earlier based on the macroscopic dissociative sticking coefficient experiments.
Interactions between the biased metallic STM tip and the surface of 2H-TaS2 bulk crystal were investigated. Tip-induced surface structure manipulation has displayed potential towards the application of scanning probes in surface nanolithography, and it is vital to understand the roles of surface and tip conditions to progress this application in a practical way. STM and scanning transmission electron microscopy (STEM) experiments provided a surface defect inventory of chemical vapor transport (CVT) grown 2H-TaS2 crystals which were consistent with a clustering and propagation of defect structures during the CVT growth. At high defect concentrations, the STM tip-induced growth of defect vacancy islands (VIs) in the monolayer was linear in time for the VI perimeter and parabolic in time for the VI area. New vacancies emerged from layers below the surface after the top layer was removed during the etching process. More importantly, mobile surface islands/flakes were revealed to play a crucial role in the tip-induced etching mechanism.
Anisotropic ferromagnetic phases can be introduced to transitional metal dichalcogenide (TMD) 2H-TaS2 through intercalating Fe in the van der Waals (vdW) gaps. By deviating from the commensurate values (x = ¼ or ⅓), the crystalline structure as well as the magnetotransport properties of the TMD system can be tuned. For instance, Fe1/4TaS2 has a centrosymmetric 2 × 2 structure while Fe1/3TaS2 has a non-centrosymmetric √3×√3 R30° supercell structure. The magnetic Curie temperature of FexTaS2 also exhibits a strong dependence on Fe concentration. We evaluate Fe0.28TaS2 and 2H-TaS2 samples using STM/Spectroscopy (STM/S) and density functional theory (DFT) to investigate the real-space intercalant electronic structure comparatively and the potential phase segregation between the two commensurate compounds. Fe0.28TaS2 shows a √3×√3 R30° supercell at 77 K, whereas 2H-TaS2 displays no apparent supercell at the same temperature. Fe vacancy defects and clusters are discovered in the intercalated surface, and their surrounding local density of states (LDOS) shows non-trivial differences at energies compared to the pristine Fe0.28TaS2 area, which is believed to be related to Fe orbitals contributions based on DFT calculations. These investigations are important to better understand the anomalously high peak in the magnetoresistance displayed by FexTaS2 materials with x = 0.28.
The surface electronic structure of EuZn2As2 was studied, and a deep-learning-based workflow and DFT were employed to determine the termination of the surface in a complex defect environment. EuM2As2 (M = Zn, Cd, In, Sn, etc.) are excellent material systems for studying electronic topological properties, which can be easily tuned by application of magnetic fields. Theoretical calculations predict gapped and flat bands in the LDOS for EuZn2As2 but a gapless structure in EuCd2As2. In this work, low-temperature (77 K) cleaved EuZn2As2 crystals were studied using STM/S and DFT calculations. Characteristic defects imaged as triangular shapes with modified LDOS helped identify the surface terminations: Eu versus As capped surfaces. While large bandgaps (~1.5 eV at 77 K) are observed on both pristine surfaces, the bandgap width is found to be very sensitive to local heterogeneities, such as defects and step edges, that tend to reduce the bandgap. Combining experimental data with DFT simulations, we conclude that the modified bandgap in the heterogeneous area arises from Zn sub-surface vacancies and/or substitution into those Zn vacancies by As atoms. Our investigation offers important information for evaluating the electronic structure topology of the EuM2As2 family of materials by establishing a methodology to correctly identify the exposed surface plane under study by STM/S.