Electronic and transport properties of 2D Dirac materials: graphene and topological insulators.
Kretz, Bernhard Gunter
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This thesis presents a set of contributions to the field of 2D Dirac materials, which have emerged as promising candidates for future nanoelectronics. The electronic and transport properties of various graphene-based nanostructures and surfaces of topological insulators reported in this work, provide further insight into the functionalities of this class of materials. For the characterization of the materials reported in this thesis, we applied DFT and a NEGF approach.Most graphene-based electronic devices, require the formation of contacts between the 2D material and metal electrodes. Within this context, it is crucial to design graphene-metal interfaces with low contact resistances. In this work, we analyzed the structural evolution and transport properties of various metal-graphene contacts with and without functionalization or contamination of the graphene edge, with special focus on their influence on the contact resistance. For metal-graphene edge contacts, we found a strong metal dependence of the stability on graphene edge contaminants. In general, we found the contact resistance to increase upon graphene edge contamination, although the strength of the relative increment dependent significantly on the metal and edge contamination. Nevertheless, our study provides valuable insight into the mechanisms responsible for device-to-device variations of metal-graphene contacts in experiments.Further, we studied the electronic and transport properties of a novel type of graphene nanoribbon (GNR) and the nanoporous graphene (NPG) derived from it. Our characterization of the electronic properties of the NPG revealed that, like the ribbon, it is a semiconductor. Furthermore, it inherits uniquely localized states from ribbon, which form a 1D band dispersing in the direction perpendicular to the ribbon's backbone. Moreover, we found states localized in the vacuum region of the ribbon. Within the NPG, these states interact with each other, forming bonding and anti-bonding pore states. STM experiments have confirmed the existence of these pore states and demonstrated the uniform growth of the NPG over a larger area. Due to its semiconducting behavior, this NPG offers high potential for many electronic applications (e.g., FETs), as well as for molecular sensing and sieving applications.Due to their extraordinary properties, topological insulators (TIs) have been proposed for a wide range of applications, especially for spintronics. With the aim of spintronic applications, the challenge in this field is to find a TI with a surface state protected against magnetic perturbations. Within this context, we investigate the influence of chemical disorder on the structural and magnetic properties of a Co adsorbed on the surface of a given ternary TI surface. In combination with experiments, we could demonstrate that Co tends to adsorbed away from the high-symmetry position whenever it is surrounded by different species of the TI's surface atoms. This adsorption behavior leads to a reduced hybridization between Co and the TI's surface state and, consequently, the surface state is still protected by time-reversal symmetry.Similar results can be expected for other ternary TI's with chemical disorder in the surface layer.