Controlling the diameter, length, and crystalline structure of semiconductor nanowires (s-NWs) during synthesis is essential for their envisioned technological applications. Among the various growth parameters, the seed particle plays a critical role in determining the final structure of s-NWs. However, metal seed particles are typically less than 5 nm in size and obtaining well-defined nanoparticles (NPs) in terms of their size, composition, and morphology remains a significant synthetic challenge.
In this dissertation, we present several different approaches in the gas and liquid phase for the preparation of metal NPs that can ultimately be used for the growth of high quality s-NWs. In one approach, an atmospheric-pressure microplasma is used to dissociate organometallic precursors and produce clean, surfactant-free metal NPs in the gas phase. Combining vapor precursors at different ratios in the microplasma allows multimetallic NPs of tunable composition to be generated. The availability of a wide range of metal and alloyed NPs addresses an important problem in Si nanowire synthesis where metals such as Au create a deep level impurity.
A particularly novel aspect of this thesis work is the first studies of shaped seed particles for nanowire nucleation and growth. Shaped Au particles are synthesized in the liquid phase by an established technique that relies on surfactant directed growth. These particles are then used to grow InAs NWs. We find that shaped Au particles enhance the nucleation/growth by increasing the apparent growth
rate of InAs NWs by a factor of two as compared to the more typical spherical Au particles. In addition, we find that the shape of the seed particles is retained after growth. These studies suggest that the generally accepted vapor-liquid-solid (VLS) growth mechanism is not widely applicable, particularly in the case of shaped particles where the initial morphology is found to influence the nanowire nucleation/growth kinetics. Overall, our experiments emphasize the importance of the seed particle and show that controlling its structure in terms of composition and shape can lead to the synthesis of well-defined s-NWs.