Growth of SiOx nanowires by laser ablation

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Amorphous SiOx nanowires (NWs) were synthesized using laser ablation of silicon-containing targets.The influence of various parameters such as target composition, substrate type,substrate temperature and carrier gas on the growth process was studied. TheNWs were characterized using high resolution scanning and transmission electronmicroscopes (HRSEM and HRTEM) with their attachments: electron dispersivespectroscopy (EDS) and energy electron loss spectroscopy (EELS). A metal catalystwas found essential for the NW growth. A growth temperature higher than1000 °C was necessary for the NW formation using an Ar-based carrier gas at 500 Torr. The use ofAr–5%H2 instead of pure Ar resulted in a higher yield and longer NWs. Application of a diffusion barrieron top of the Si substrate guaranteed the availability of metal catalyst droplets on the surface,essential for the NW growth. Ni was found to be a better catalyst than Au in terms of theNW yield and length. Two alternative sequences for the evolution of the amorphousSiOx NWs were considered: (a) the formation of Si NWs first and their completeoxidation afterwards, which seems to be doubtful, (b) the direct formation ofSiOx NWs, which is more likely to occur. The direct formation mechanismwas proposed to advance in three stages: preferential adsorption ofSiOx clusters on the catalyst surface first, a successive surface diffusion to the catalyst dropletlower hemisphere, and finally the formation and growth of the NW between the catalystand the substrate.