The metal-non-metal transition in VO2: X-ray photoemission and resistivity measurements

Abstract

The semiconductor-metal transition in VO₂ has been investigated by means of XPS measurements. The band gap in the semiconducting region is found to be temperature-dependent with a room temperature value of E_g=0.3 eV, in agreement with resistivity measurements. The core lines, which are asymmetric below T_nu broaden and become symmetric above T_t. A model is discussed to explain the broadening in the metallic region, which requires that the core hole-valence electron interaction is comparable to the bandwidth. The relative shift of the O 1s and V 2p peak positions below and above T_t indicates that the charges on the ions are highest in the metallic phase, suggesting a more ionic bond in this region.