A concept is presented for synthesizing metal nanowires directly from the vapor phase using chemical vapor transport to temperatures higher than the corresponding metal oxide decomposition temperature. Specifically, this concept is demonstrated with the synthesis of tungsten metal nanowires with sizes ranging from 70 to 40 nm by increasing the condensation temperature. The simultaneous condensation and decomposition of the tungsten oxide species during nucleation and growth is suggested for 1-D growth of metallic tungsten nanowires. This synthesis concept could potentially be extended to the vapor phase synthesis of metal nanowires of several other nonvolatile and refractory metals. The tungsten nanowires could find potential applications in gas sensors and as electron sources in electron microscopes.

In this paper, we provide a theoretical basis using thermodynamic stability analysis for explaining the spontaneous nucleation and growth of a high density of 1-D structures of a variety of materials from lowmelting metals such as Ga, In, or Sn. The thermodynamic stability analysis provides a theoretical estimate of the extent of supersaturation of solute species in molten metal solvent. Using the extent of maximum supersaturation, the size and density of critical nucleus were estimated and compared with experimental results using nucleation and growth of Ge nanowires using Ga droplets. The consistency of the proposed model is validated with the size and density of the resulting nanowires as a function of the synthesis temperature and droplet size. Both the experimental evidence and the theoretical model predictions point that the diameters of the resulting nanowires decrease with the lowering of synthesis temperatures and that the nucleation density decreases with the size of metal droplet diameter and increasing synthesis temperature.

Textured gallium nitride (GaN) films were grown on polished, polycrystalline and amorphous substrates in sub-atmospheric pressures, by direct nitradation of a thin molten gallium films using electron cyclotron resonance (ECR) microwave nitrogen plasma. C-plane texturing was achieved, independent of the substrate crystallinity. Single crystal quality GaN nanowires with diameters ranging from 40-50 nm were also synthesized using direct nitridation of thin gallium films with nitrogen plasma. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS) and Cross-sectional transmission electron microscopy (CS-TEM), high-resolution TEM (HRTEM) and Micro-Raman spectroscopy were used to characterize the synthesized gallium nitride films and GaN nanowires.

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We report the bulk synthesis of hydrogenated, amorphous Si x N y and Si x O y nanowires using pools of molten gallium as the solvent medium and microwave plasma consisting of silane in nitrogen and silane in oxygen respectively. High densities of multiple nanowires nucleated and grew from molten gallium pools. The resulting nanowires were tens of nanometers in diameter and tens of microns long. Electron energy loss spectroscopy (EELS) and Fourier transform infra-red (FTIR) spectroscopy showed that the silicon nitride nanowires are hydrogenated amorphous silicon nitride (a-Si x N y H). The results of energy dispersive X-ray spectroscopy (EDS) yielded N : Si and O : Si ratios less than the stoichiometric composition of silicon nitride (Si 3 N 4) and silica (SiO 2). Studies on the chemical stability and refractive index (RI) measurements demonstrate a-Si x N y H nanowires are potential candidates for use as etching masks in nanoscale lithography, and as high index materials in optical coatings.