Electronic transport in nano-devices based on graphene and topological insulators
In this thesis we experimentally focused on quantum transport in graphene and topological insulators and we worked on four main projects. 1) The extraordinary properties and potential applications of graphene have motivated the development of large-scale, synthetic graphene, such as chemical vapor deposition (CVD) on Cu. In this research, we study graphene grains (either isolated grains or a small number of merged grains) formed during the early stage of ambient CVD growth on Cu foils. We show that grain boundaries give a significant Raman 'D' peak, impede electrical transport, and induce prominent weak localization indicative of intervalley scattering in graphene. In order for graphene to realize its promise in \carbon-based" electronics, it is necessary to achieve a better control over the nucleation of individual graphene grains and to avoid the grain boundaries in fabricated graphene devices. 2) Topological Insulators are materials where the bandstructure has a bulk bandgap and conductive surface. The topological surface state is characterized by a linear dispersion, similar to graphene, but it is topologically protected, suppressing backscattering. One of the main sources of scattering in ultrathin nanowire devices is backscattering, and this could be suppressed in topological insulator nanowires (TINWs). In this study we have measured ambipolar field effect and metal-insulator transitions on topological insulator nanowire field effect devices. We also have measured the Aharonov-Bohn (A-B) and Shubnikov de Haas (SdH) oscillations showing electrons on the surface of TINWs behave like massless Dirac particles. 3) We measured the gate voltage dependent A-B oscillation phase and we infer the surface of TINWs is quantized forming subbands with a gap at the Dirac point when 0 or even multiples of half magnetic flux quantum (\Phi_0) is threaded through the TINWs (axial magnetic field). However, it undergoes through topological transitions when odd multiples of Φ0/2 is threaded through the TINWs and a spin-helical topological protected gapless surface mode forms. This spin-helical topological surface state has been predicted to host Majorana fermions (fundamental building blocks for topological quantum computers) when in proximity to s-wave superconductors. 4) In the last project we placed superconductor contacts on TINWs and we showed proximity effect with large induced superconducting gaps and critical currents in agreement with the clean limit. Because of the transparent superconductor contacts, we observed high-order sub-harmonic gaps.
Chen, Purdue University.
Electrical engineering|Condensed matter physics|Nanotechnology
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