Oxygen vacancy induced band gap narrowing of the low-temperature vanadium dioxide phase

Abstract

Switching of vanadium dioxide (VO2) from low-temperature insulating phase to high-temperature rutile phase can be induced by photons with a certain energy. Photoinduced insulator-metal transition is found experimentally in VO2 polycrystalline film by photos with energy even below 0.67 eV. However, insulator-metal transition in single crystal can only be induced when photo energyis above 0.67 eV. In order to understand these experimental phenomena, we make a first-principle study on low-temperature non-magnetic M1 phase of VO2 with oxygen vacancy by density functional theory calculations based on the Heyd-Scuseria-Ernzerhof screened hybrid functional. According to symmetry, M1 phase has two kinds of different oxygen vacancies, O1 and O2 vacancies. Calculations are made on structures and electronic properties of nonmagnetic M1 phases with O1 and O2 vacancies, respectively. The present theoretical results show that neither the short vanadium-vanadium (VV) bond length near O1 or O2 vacancy nor the lattice parameters almost change but the long VV bond length near O1 or O2 vacancy decreases due to the oxygen vacancy. The long VV bond lengths near O1 and O2 vacancies are about 2.80 and 2.95 , respectively, but the long VV bond length is 3.17 in pure M1. The insulating band gap is opened between V 3d bands, and hybridization happens between V 3d and O 2p orbitals. Furthermore, the present theoretical results demonstrate that the band gap of pure nonmagnetic M1 is 0.68 eV while M1 with O1 vacancy, O2 vacancy, and two oxygen vacancies including O1 and O2, have band gaps of 0.23 eV, 0.20 eV, and 0.15 eV, respectively. The band gap decreases probably because oxygen vacancy results in the decease of the long VV bond length near it. The present results can explain the experimental results well.