Phase transformation studies within one-dimensional metal oxide nanowires

The synthesis methods for nanowires of elemental semiconductor and low-melting metal oxide materials have been developed fairly extensively. However, the extension of such methods for other types of materials and their compounds is not readily possible. Also, the synthesis methods for metal oxide nanowires for many materials systems are much easier than other compounds such as nitrides and sulfides. We proposed to produce binary oxide nanowires transform them into other compounds or compositions using gas-solid reactions. Specifically, in this dissertation, a large-scale synthesis method is investigated for producing nanowires of a set of high melting metal oxides and phase transformation of oxide to nitride and oxide to metal using reactions with gas phase. Primary objective of this dissertation is to understand the factors that govern the nucleation during phase transformation within one-dimensional systems and the resulting crystallinity.;Phase transformation studies were conducted within one-dimensional structures using tungsten oxide as the model system. Two types of gas-solid reactions were studied: reduction using hydrogen and nitridation using ammonia. In both cases, the transformation was found to be complete within minutes of reaction time scales for nanowires with diameters ranging from few rim to hundred nm. Depending upon the underlying nucleation mechanism presented, the size of critical nuclei can determine the extent of crystallinity within the resulting nanowires. A model for nitride nucleation is presented which suggested that the oxide nanowires with diameters less than 10 nanometers can be transformed to single crystalline nitride nanowires. A model for nucleation is presented in which lattice mismatch between the nitride phase and the oxide host determines the critical nuclei diameter.;In the first part of the dissertation, the synthesis of high melting metal oxide (tungsten oxide and molybdenum oxide) nanowire arrays is investigated using a hot-wire chemical vapor deposition (HWCVD) reactor. Experiments using variations in process conditions showed that the gas phase super-saturation of metal oxide vapor phase species affects the resulting morphology of nanowire films: interconnected thin films, branched nanowire structures, and nanoparticles. A mechanism for nucleation and growth of nanowires and their assembly into hollow tubular structures in the case of molybdenum oxide system is presented.