Microstructural characterization of oxygen-defective VO_2−x films produced by precisely controlled oxidation of vanadium metal foil

Abstract

Vanadium oxides have attracted a great deal of attention because of their good electrical and magnetic properties, in addition to their excellent optical properties. VO₂ undergoes a phase transformation from a tetragonal to a monoclinic structure at 341 K, leading to changes in its electrical and optical properties. We successfully produced oxygen-defective VO_2−x films with thicknesses of the order of several hundreds of nanometers on the surface of V metal foils heat-treated at 773 K for 4 to 20 h under an oxygen partial pressure of 10.13 Pa. The thickness and the grain size of the oxides increased with increasing holding time. High-angle annular dark-field scanning transmission electron microscopy observations indicated that the transformation from a tetragonal to a monoclinic structure was accompanied by an atomic displacement along the c- and a-axes of the tetragonal VO₂ structure and that the monoclinic VO₂ structure was distributed in nanodomains within the tetragonal VO₂ grain. The existence of a stable tetragonal structure at room temperature probably originates from suppression of the atomic displacements accompanying the transformation through the introduction of oxygen deficiencies. The diffuse reflectance of oxygen-defective VO_2−x films was less than 30 %, indicating the absorption of visible and near-infrared lights.