Synthesis, Assembly, and Integration of Semiconductor Nanowires

Thesis (Ph.D.)--University of Washington, 2020Semiconductor nanowires are a class of highly anisotropic crystalline materials with nanoscale diameters and lengths that range from micrometers to millimeters. The electronic, optical, and mechanical properties of semiconductor nanowires can be considerably different than their bulk counterparts, making them attractive for a range of applications including sensors, energy storage, and quantum information systems. Solution-based synthesis is a promising strategy to produce semiconductor nanowires in a scalable, cost-effective matter. However, many solution-based methods are limited in their ability to produce nanowires with increasingly complex compositions—including doped, alloyed, and heterostructured architectures—as well as to rapidly screen synthetic parameters for combinatorial discovery and optimization. In addition, chemistries and growth dynamics can be difficult to track with nanowire syntheses that require high temperature and extreme pressure equipment. Moreover, the widespread integration of semiconductor nanowires into devices will also require new methods of assembly as well as careful consideration of surface chemistry. After an introduction to current methods of semiconductor nanowire synthesis, existing tactics for nanowire assembly, and strategies to improve the energy density of lithium ion batteries with group IV nanomaterials, this dissertation will cover three main topics related to (i) new synthetic methods for nanowire growth, (ii) a novel light-based nanowire assembly process, and (iii) the integration of nanowires into high-energy-density composite electrodes for lithium ion batteries. Herein, we demonstrate a new continuous-flow, laser-driven, nanowire growth process that exploits the light absorption of colloidal metal nanocrystals to drive semiconductor nanowire growth in an optically accessible reactor on the benchtop, potentially opening the door for both rapid screening of synthetic parameters as well as in situ studies of nanowire growth dynamics. Investigations of solution-based nanowire growth using this system establish that laser-driven syntheses can achieve rapid, on-demand growth of semiconductor nanowires. Importantly, the integration of nanowires into future device architectures will require a wide range of assembly strategies. While current solution-based nanowire assembly processes struggle to create deterministic heterojunctions, here, we demonstrate a novel example of nanowire assembly in a high-Prandtl-number organic solvent system, using an optical trap to orient, align, and “solder” metal-seeded semiconductor nanowires into periodic axial heterostructures. Finally, we investigate the role of surface functionalization on the integration of group-IV nanowires into high-capacity alloying electrodes for lithium ion batteries. We demonstrate that interfacial chemistry affects electrochemical access to different phases of lithiated germanium, and by carefully controlling the nanowire surface chemistry, we eliminate the need for the fluorinated electrolyte additives typically required for the stable cycling of group-IV-based, lithium-ion battery electrodes. In addition, we demonstrate that by balancing precursor decomposition kinetics, alloyed silicon-germanium (SiGe) nanowires can be synthesized through supercritical-fluid-based processes, potentially improving the rate capability of high-capacity silicon-based electrode materials produced via scalable processes. We anticipate that the information gained from these solution-based synthetic methods, assembly techniques, and surface chemistry studies will inform synthetic compositional control, elucidate relationships between solution-based reaction parameters and emergent properties, and advance the integration of solution-grown semiconductor nanowires into next-generation devices