For the last decade, semiconducting nanowires synthesized by bottom-up methods have opened up new opportunities, stimulated innovative scientific research, and led to applications in materials science, electronics, optics, and biology at the nanoscale. Notably, nanowire building blocks with precise control of size, structure, morphology, and even composition in one, two, and three dimensions can successfully demonstrate high-performance electrical characteristics of field-effect transistors (FETs) and highly sensitive, selective, label-free, real-time biosensors in the fields of nanoelectronics and nano-biosensing, respectively. This thesis has focused on the design, synthesis, assembly, fabrication and electrical characterization of nanowire heterostructures for a proof-of-concept nanoprocessor and morphology-modulated kinked nanowire molecular nanosensor. / Physics

Central to the implementation of colloidal nanomaterials in commercial applications is the development of high throughput synthesis strategies for technologically relevant materials. Solution based synthesis approaches provide the controllability, high throughput, and scalability needed to meet commercial demand. A flow through supercritical fluid reactor was used to synthesize silicon nanowires in high yield with production rates of ~45 mg/hr. The high temperature and high pressure of the supercritical medium facilitated the decomposition of monophenylsilane and seeded growth of silicon nanowires by gold seeds. Crystalline nanowires with diameters of ~25 nm and lengths greater than 20 [micrometers] were routinely synthesized. Accumulation of nanowires in the reactor resulted in deposition of a conformal amorphous shell on the crystalline surface of the wire. X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy were used to determine the shell composition. The shell was identified as polyphenylsilane formed by polymerization of the silicon precursor monophenylsilane. A post synthesis etch was developed to remove the shell while still maintaining the integrity of the crystalline silicon nanowire core. Subsequent surface passivation was achieved through thermal hydrosilylation with a terminal alkene. The development colloidal nanomaterials into commercial applications also requires simple and robust bottom-up assembly strategies to facilitate device fabrication. A Langmuir-Blodgett trough was used to assemble continuous monolayers of hexagonally ordered spherical nanocrystals over areas greater than 1 cm². Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates using pre-patterned polydimethylsiloxane (PDMS) stamps and a modified Langmuir Schaefer transfer technique. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room temperature MFM could detect a remanent (permanent) magnetization from multilayers (>3 nanocrystals thick) films of chemically-ordered L1₀ FePt nanocrystals. Grazing incidence small angle X-ray scattering was used to quantitatively characterize the grain size, crystal structure, lattice disorder, and edge-to-edge spacing of the nanocrystal films prepared on the Langmuir-Blodgett trough both on the air-water interface and after transfer. / text

Transparent electrodes (TEs) have become important components of displays, touch screens, and solar photovoltaic (PV) energy conversion devices. As electrodes, they must be electrically conductive while being transparent. Transparent materials are normally poor conductors and materials with high electrical conductivity, such as metals, are typically not transparent. From the few candidate materials, indium tin oxide (ITO) is currently the best available, but indium is an expensive material and ITO cost has risen with increasing demand. Therefore, alternative materials or methods are sought to encourage production needs of applications and help in reducing their price. This thesis presents and discusses results of experimental work for a method, field-directed chaining, to produce a TE device which is nanowire-based, with a figure of merit FoM= 2.39 x10E-4 Ohm E-1, comparable to ITO but with potential for far lower cost. Using electric field-directed chaining, multiple parallel long chains of metal nanowires are assembled on inexpensive transparent materials such as glass by field directed nanowire chaining, using methods first demonstrated in our laboratory. In this work, we have improved the fraction of functional chains, by tuning the field/voltage, a key step in increasing the FoM and lowering the cost. The effect of operating parameters on TE optical and electrical properties has been studied and identified as well. From experiments with twenty seven substrates, each with a range of electric field and nanowire concentration, the highest light transmission achieved is 78% and the lowest sheet resistance achieved is 100 Ohm/sq. Among all the operating parameters, the electric field has the most significant influence on the fraction of nanowire chains that are functional. In the operating range of electric field strength available to us, we observed a monotonic increase in the fraction of functional nanowire chains. We found a counter-intuitive change in TE properties in a sub-range of nanowire concentration, associated with a change in the structure of chained patterns. / Graduate