Invited: Nanowire Growth Mechanism upon Treatment of Metals with Non-Equilibrium Oxygen Plasma

Recent results on the growth of metal oxide nanowires upon exposure of metallic samples to non-equilibrium oxygen plasma are presented. The first steps in nucleation and growing of the nanowires were studied using pure niobium foils. A strong gradient of sample temperature was applied by heating the sample on one side only and the formation of nanoparticles along the gradient was studied by scanning electron microscopy. Nanowires were found only on the surface already covered with a rather uniform oxide film. First nanowires were observed growing at the cracks between oxide crystallites allowing for a conclusion that the necessary condition for nanowire growth was preferential migration of metallic atoms along the grain boundaries. A higher temperature allowed for higher density of nanowires and formation of bundles was observed at optimal temperature. The hypothesis was confirmed using iron sample with thick oxide films on the surface formed by thermal oxidation. The thickness of the oxide film was roughly 10 micrometers. Bundles of nanowires on the surface of such materials grew only at elevated temperature higher than about 1100 K what was explained by poor migration of iron atoms through the oxide film at lower temperature. The density of nanowire bundles increased with increasing temperature and at high temperature the one-dimensional growth was lost what has been explained by migration of metal atoms throughout the oxide film rather independent from the local structure.