(524a) Nanocrystal-Mediated Crystallization of Silicon and Germanium Nanowires in Organic Solvents: the Role of Catalysis and Solid-Phase Seeding
Gold-seeded vapor-liquid-solid (VLS) growth is a common route to nanowire synthesis, which gives high yields of crystalline Si and Ge nanowires at low growth temperatures (~360C). Unfortunately, Au traps electrons and holes in both Si and Ge and poses a serious contamination problem for integration with Si CMOS. Here, we present results of Si and Ge nanowire synthesis using many different nanocrystals?Co, Ni, CuS, Mn, Ir, MnPt3, Fe2O3, and FePt?as seeds for Si and Ge nanowire synthesis to develop a more general understanding of the role of the seed particles in the nanowire growth process.
Si and Ge nanowires were grown by decomposing silanes or germanes in high temperature (450~500C), high pressure (10.3 MPa)?e.g. supercritical?toluene. Under these reaction conditions, Au nanocrystals seed nanowires via the ?supercritical fluid-liquid-solid? (SFLS) mechanism in which nanowires evolve from a liquid Au:Si (or Au:Ge) eutectic. Many of the seed materials studied here are interesting because their liquid eutectics form at temperatures well above 500C, but solid alloys form at temperatures below 500C. The nanocrystals used to seed Si and Ge nanowires had size distributions with standard deviations less than 20% about mean diameters ranging between 4.2 and 10.2 nm. All of the nanocrystals seeded Si and Ge nanowires from monophenylsilane (MPS) and diphenylgermane (DPG), but with varying success. Co nanocrystals gave the highest yield of straight, long (>10 µm) crystalline Si and Ge nanowires. Ni nanocrystals also produced crystalline Si and Ge nanowires with good yield. CuS nanocrystals produced straight crystalline Si nanowires with good yield but slightly shorter lengths (3-10 µm) and Fe2O3 nanocrystals produced high quality Ge nanowires with relatively high yield.
Of the nanocrystals studied, Co gave the highest yield and quality of both Si and Ge nanowires, rivaling Au-seeded reactions. The nanowire growth temperatures were well below the bulk eutectic temperature, indicating that the nanowires grow via a solid-phase seeding process. Solid-phase seeding can probably occur for any semiconductor with a high solubility in the seed metal; however, the growth temperature must be sufficiently high for fast saturation by solid-state diffusion. The very short diffusion lengths in nanometer-diameter seed particles help enable relatively low temperatures for solid-phase seeding. Co and Ni nanocrystals were also found to catalyze Si nanowire growth from octylsilane and trisilane--reactants that do not yield nanowires in the presence of Au nanocrystals. The use of catalytic transition metal seeds might be a general approach for lowering the nanowire growth temperature to limit sidewall deposition and provide better diameter control. Furthermore, the solid-phase growth mechanism may become as prevalent as liquid-eutectic seeding for nanowires as more seed metals are explored for nanowire growth.