Silicon-Catalyzed Growth of Amorphous SiOx Nanowires by Laser Vaporization of Si and Si/SiO2

Products from the laser vaporization of targets of four types Si, (1:1) Si/SiO2, (1:9) Si/SiO2, and SiO2 without the addition of any metal catalysts in the presence of high-pressure (0.9 MPa) Ar gas were characterized by transmission electron microscopy (TEM), high-resolution TEM, energy-filtered TEM, transmission electron diffractometry, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and powder X-ray diffractometry. Amorphous SiOx (0.8 <= x <= 2.0) nanowires (NWs) (10-40 nm in diameter and up to 1 mu m long), having crystalline Si nanoparticles (NPs) covered with thin amorphous SiOx layers at the NW tips, were observed for the products from the Si and Si/SiO2 composite targets, while only amorphous SiOx NPs were produced from the SiO2 target. The 0 content relative to that of the Si in SiOx NWs was also increased with increasing the content of SiO2 in the targets. We propose a novel mechanism for the growth of amorphous SiOx NWs, in which nuclei of the NWs are formed on the Si-rich molten SiOx NPs due to precipitation of SiOx via its supersaturation. Si acts as both a catalyst to precipitate SiOx and a source material of the NWs. A successive supply of Si, SiO, O, and others to the molten NPs, their diffusion and precipitation, and the oxidation of the precipitated NWs result in the further growth of the NWs.