First-Principles Study On The Effect Of Oxygen Vacancies On The Phase Transition Of Vanadium Dioxide

Vanadium dioxide（VO₂）is a phase change material that has wide application.With the reduction of temperature,VO₂ undergoes the transformation from metallic phase to insulatoring phase with the change of the lattice symmetry from tetragonal rutile to monoclinic.At the same time,the optical properties has also changed greatly.The phase transition temperature is about 340 K.The phase change is reversible and it can be triggered by a variety of incentives,such as heat,light,electricity,stress and so on.So VO₂ materials have been widely used in communications,information storage,sensing and other fields.In terms of basic research,metal-insulator transition is a very important phenomenon in condensed matter physics and it is a focus in the study of electronic properties in strong correlation system.We calculated the electronic structures and optical properties of VO₂ supercells containing different amount of oxygen vacancies before and after the phase transition based on density functional theory（DFT）.The electronic structures and optical properties of M1-phase VO₂ bulk supercells were calculated by GGA+U and that of R-phase VO₂ were calculated by GGA.We created the oxygen vacancies by remove the O atoms out of the supercells and performed the geometry optimization and calculated the properties to study the effect of oxygen vacancy defects on VO₂ materials of both crystals and nanowires.The results show that the band gap of VO₂（M1）crystal supercell is 0.772 eV and that with one oxygen vacancy and two oxygen vacancies are 0.796 eV and 0.687 eV,respectively.The calculations indicate that oxygen vacancies can reduce the energy barrier for phase transition.While in the R-phase VO₂ crystals,the impacts of different oxygen-vacancy concentrations on properties vary with the specific location of oxygen vacancies.The band gap of VO₂ nanowire supercells contains no oxygen vacancy and one oxygen vacancy are 0.692 eV and 0.471 eV,showing the similar tendency as M1-phase VO₂ bulk materials.In the R-phase VO₂ nanowires with and without oxygen vacancies,the Fermi level goes through the conduction band and they are both metallic.Therefore,the introduction of oxygen vacancies in the nanowire structure VO₂ can also promote the phase transition process.