| The possible adverse effects of contamination of semiconducting nanowires, with the high-melting transition metals (Au and Fe) used for their synthesis, necessitates the need for developing schemes based on more benign low-melting metals (Ga, In and Sn) or schemes that do not use any foreign metal catalysts. In the first case involving low-melting metals, it is important to determine how and what conditions promote the tip-led growth of nanowires. In the second case it is important to investigate the possibility of using vapor-solid schemes directly for obtaining metal and metal oxide nanowires, without the use of any catalysts.; In this work, it is shown that the chemical vapor transport of metal oxide species onto substrates maintained above the decomposition temperature of the respective metal oxide leads to the formation metal nanowires. Similarly, the vapor transport of metal oxides onto substrates maintained at temperatures lower than the decomposition temperatures, is shown to lead to the formation of metal oxide nanowires. The nucleation and selective growth through sub-oxide clusters is proposed and determined to be the operating mechanism in this scheme. Such rationalization allowed for the extension of this scheme towards the synthesis of nanowires of several materials systems (W, Fe, WO3, Ta2O5, NiO etc.).; Secondly, the reactive vapor transport of indium in the presence of dissociated ammonia for InN nanowire synthesis is investigated in detail to understand the nucleation and growth steps involved in the observed tip-led growth mode. Here, it is shown that the selective wetting and formation of indium droplets on top of the InN crystal nuclei, followed by liquid phase epitaxy through the droplets, leads to the formation of InN nanowires. The self-catalysis schemes involving tip-led growth using Group III metals such as In, Ga and Al metal droplets has been shown to produce vertically oriented nanowire arrays and 3-dimensional "tree-like" structures of the respective Group III-nitrides. In addition, the above direct synthesis schemes are also investigated for synthesizing Group III-antimonide nanowires. The lasing and waveguiding in the near-Infrared (NIR) wavelength regime using thicker GaSb nanowires is also studied.; Finally, a new post-synthesis strategy involving the controlled decomposition of nanowires for reducing their radial sizes (to sizes below 5 nm) is also investigated, specifically for Group III-nitrides. It is shown that the process of decomposition of nanowires is dependent on their diameter. In the case of thicker nanowires (with diameters of 200 nm), the process of decomposition is observed to be similar to that of bulk crystals. However, the decomposition of thinner (diameters of 20 nm) nanowires resulted in uniform thinning of the nanowires. A mechanism involving the ballistic diffusion or evaporation of smaller molten metal droplets on thinner nanowires is proposed to explain the observed uniform shrinkage in the diameters of the nanowires. |
