Semiconductor nanowires : from a nanoscale system to a macroscopic material
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Semiconductor nanowires are one-dimensional nanoscale systems that exhibit many unique properties. Their nanoscale size can lead to defect densities and impurity populations different than bulk materials, resulting in altered diffusion behavior and mechanical properties. Synthetic methods now support the large-scale production of semiconductor nanowires, enabling a new class of materials and devices that use macroscopic quantities of nanowires. These advances have created an opportunity to fabricate bulk structures which exhibit the unique physical properties of semiconductor nanowires, bridging the properties of a nanoscale system with macroscopic materials. High aspect ratio germanium nanowires were synthesized in supercritical organic solvents using colloidal gold nanocrystal seeds. The nanowires were chemically passivated inside the reactor system using in situ thermal hydrogermylation and thiolation. The chemical stability of the passivated nanowires was studied by exposure to highly corrosive and oxidative environments. Chemical surface functionalization of germanium nanowires was investigated by covalently tethering carboxylic acid groups to the surface, as a general platform for the further functionalization of nanowire surfaces with molecules such as polyethylene glycol. Surface functionalization with dopant-containing molecules was also explored as a potential route for doping nanowires. In addition, static charging was exploited in the development of an electrostatic deposition method for semiconductor nanowires. In situ transmission electron microscopy experiments were conducted on gold-seeded germanium nanowires encapsulated within a volume-restricting carbon shell. A depressed eutectic melting temperature was observed, along with strong capillary effects, and the solid-state diffusion of gold into the crystalline stem of the germanium nanowire, occurring at rates orders of magnitude slower than in the bulk. Copper, nickel, and gold diffusion in silicon nanowires were also investigated. The rate of gold diffusion was found to be a strong function of the amount of gold available to the system. Finally, germanium nanowires were found to exhibit exceptional mechanical properties, with bending strengths approaching that of an ideal, defect-free, perfect crystal, and strength-to-weight ratios greater than either Kevlar or carbon fiber. Macroscopic quantities of nanowires were used to fabricate large sheets of free-standing semiconductor nanowire fabric, and the physical, morphological, and optical properties of the material were investigated.