Structure of tungsten oxide nanoclusters

Tungsten oxide clusters were prepared by oxidizing tungsten clusters deposited on sapphire. The oxidation was carried out by thermal treatment under various conditions. The morphology and the structure of the oxide clusters were investigated by AFM and RHEED. It was found that the tungsten deposit on the Al2O3 substrate was partially oxidized to WO2 by annealing in vacuum. The Al2O3 substrate exhibited a modified surface indicating that the tungsten clusters were oxidized by the substrate oxygen. Oxidation under low oxygen pressure (10 Pa) gave rise to a WO2 phase while oxidation in air led to WO3. In all experiments, the epitaxial orientations of tungsten or tungsten oxide clusters were deduced from the RHEED patterns.