Photoemission from VO2

Abstract

Photoemission quantum yield and energy distribution curves have been measured for V${\mathrm{O}}_{2}$ in the tetragonal (100\ifmmode^\circ\else\textdegree\fi{}C) and monoclinic (25\ifmmode^\circ\else\textdegree\fi{}C) phases over the photon energy range from 7 to 11.5 eV. The V${\mathrm{O}}_{2}$ studied was a polycrystalline film of several microns thickness formed by oxidizing vanadium. The experimental data, although somewhat broadened, show most of the major features expected on the basis of a simple model of V${\mathrm{O}}_{2}$, and support the interpretation of optical data given by Verleur et al. In both the semiconductor and metallic phases, there is evidence of a high-density-of-states band starting approximately 2.5 eV below the Fermi level. This band is believed to be due primarily to the oxygen $2p$ orbitals in V${\mathrm{O}}_{2}$. The photoelectric yield is considerably higher at 100\ifmmode^\circ\else\textdegree\fi{}C than at temperatures below 68\ifmmode^\circ\else\textdegree\fi{}C. This effect, and changes in the energy distribution curves, indicate that a band gap in excess of 0.6 eV has collapsed on heating V${\mathrm{O}}_{2}$ above 68\ifmmode^\circ\else\textdegree\fi{}C.