Electron energy-loss spectroscopy study of the metal-insulator transition in VO2

Electron energy-loss spectra of VO2 have been measured from perfect single crystalline areas of 100-180 nm diameter in the metallic and insulating phases. A sharp peak has been observed at 1.2eV in the valence electron excitation spectra of the metallic phase but not in those of the insulating phase. We assign the peak to an interband transition or d-d transition by inspecting the dielectric function derived from the present loss function with the help of an energy band diagram already given, although an excitation at about 1eV already observed by optical measurement was assigned to the excitation of a free-carrier plasmon. The peak due to the O 1s --> V 3d(t(2g)) transition in the O 1s excitation spectra decreased in intensity but increased in the full width at half-maximum (FWHM) at the transition from the metallic phase to the insulating phase. These changes are considered to have been due to the fact that the d(//) band splits into the upper unoccupied and lower occupied d(//) bands and the upper d(//) band rises above the pi* band in the insulating phase, but that the d(//) band is unsplit and located within the pi* band in the metallic phase.