First-principles study of the effect of oxygen vacancy and strain on the phase transition temperature of VO2

Vanadium dioxide (VO2) has been extensively studied as a thermochromic material due to its metal-insulator transition at a critical temperature (Tc) of similar to 340 K. Our first-principles calculations show that V-V chains in rutile VO2 with oxygen vacancy (VO2-x(R)) exhibit dimerization, and the band gap of monoclinic VO2 with oxygen vacancy (VO2-x(M)) can be narrowed to 0.51 eV compared to the 0.69 eV of pure monoclinic VO2 (VO2(M)), resulting in increased near-IR absorption. Furthermore, the smaller energy barrier of oxygen vacancy in VO2-x(M) (0.51 eV) with respect to VO2-x(R) (0.55 eV) indicates that VO2-x(M) could easily capture the oxygen from air and transition back to the normal VO2(M). However, precisely the opposite case is found for VO2-x(R), such that an oxygen vacancy in VO2-x(R) can stabilize the rutile phase at a low temperature. In addition, VO2-x is more sensitive to strain than pure VO2, implying that combining the effect of the oxygen vacancy and compressive strain could effectively tune the phase transition behavior and further reduce its phase transition temperatures. These results provide a theoretical guidance for the improvement of smart devices.