First discovered by Holm and Meissner in 1932, the superconducting proximity effect has remained a subject of experimental and theoretical interest. In recent years, it has been proposed that proximity effect in a semiconductor with large g-factor and spin-orbit coupling could lead to exotic phases of superconductivity. This thesis focuses on proximity effect in one of the prime semiconductor candidates -- InAs nanowires. The first set of experiments investigates the superconducting phase-dependent tunneling spectrum of a proximitized InAs quantum dot. We observe tunneling resonances of Andreev bound states in the Kondo regime, and induce quantum phase transitions of the quantum dot ground state with gate voltage and phase bias -- the latter being the first experimental observation of its kind. An additional zero-bias peak of unknown origin is observed to coexist with the Andreev bounds states. The second set of experiments extends upon the first with sharper tunneling resonances and an increase in the device critical field. By applying an external magnetic field, we observe spin-resolved Andreev bound states in proximitized InAs quantum dots. From the linear splitting of the tunneling resonances, we extract g-factors of 5 and 10 in two different devices. The third set of experiments utilizes a novel type of epitaxial core-shell InAs-Al nanowire. We compare the induced gaps of these nanowires with control devices proximitized with evaporated Al films. Our results show that the epitaxial core-shell nanowires possess a much harder induced gap -- up to two orders of magnitude in sub-gap conductance suppression as compared to a factor of five in evaporated control devices. This observation suggests that roughness in S-N interfaces plays a crucial role in the quality of the proximity effect. The fourth set of experiments investigates the gate-tunability of epitaxial half-shell nanowires. In a half-shell nanowire Josephson junction, we measure the normal state resistance, maximum supercurrent, and magnetic field-dependent supercurrent interference patterns. The gate dependences of these independent experimental parameters are consistent with one another and indicate that an InAs nanowire in good ohmic contact to a thin sliver of Al retains its proximity effect and is gate-tunable. / Physics

This dissertation involves the study of epitaxial behavior of one-dimensional nanomaterials like single-walled carbon nanotubes and Indium Arsenide nanowires grown on metallic catalyst surfaces. It has been previously observed in our novel microplasma based CVD growth of SWCNTs on Ni-Fe bimetallic nanoparticles that changes in the metal catalyst composition was accompanied by variations in the average metal-metal bond lengths of the nanoparticle and that in turn, affected nanotube chirality distributions. In this dissertation, we have developed a very simplistic model of the metal catalyst in order to explain the nanotube growth of specific nanotube chiralities on various Ni-Fe catalyst surfaces. The metal catalyst model is a two-dimensional flat surface with varying metal-metal bond lengths and comprising of constituent metal atoms. The effect of the composition change was modeled as a change in the bond length of the model catalyst surface and density functional theory based calculations were used to study specific nanotube caps. Our results indicated that nanotube caps like (8,4) and (6,5) show enhanced binding with increased metal-metal bond lengths in the nanoparticle in excellent agreement with the experimental observations. Later, we used this epitaxial nucleation model and combined with a previously proposed chirality-dependent growth rate model to explore better catalysts that will preferentially grow an enhanced chirality distribution of metallic nanotubes. From our DFT calculations and other geometrical considerations for nanotube growth, we demonstrated that the pure Ni0.5Cu0.5 metal nanoparticles and its lattice-strained surfaces can serve as a promising catalyst for enhanced growth of metallic nanotubes. Finally, we extended this model of epitaxial growth to study the growth of,andoriented nanowires on gold metal nanoparticles where a faster growth rate ofnanowires was previously observed in experiments on shaped nanoparticles than that on spherical nanoparticles. The DFT calculations indicated an enhanced growth selectivity of theoriented nanowires on the Au(111) surfaces. However, the DFT results also show that theandNWs will preferentially grow on the Au(100) surface than on the Au(100) surface. The epitaxial model based DFT calculations of nanotube and nanowire growth on metal catalyst surfaces presented in this dissertation, provide a deep insight into their epitaxial growth mechansims and, can be easily exploited to layout better design principles of synthesizing catalysts that helps in growing these one-dimensional nanomaterials with desired material properties.

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