摘要： Vanadium dioxide (VO2) is a strongly-correlated material with 3d-electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO2 thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron-correlations. These ultrathin (~10 nm) epitaxial VO2 films were grown on NiO (111)/Al2O3 (0001) pseudomorphically, where the large misfit between NiO and Al2O3 were fully relaxed by domain matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies (x~0.20±0.02) into the VO2-x system via annealing under high vacuum (~5x10-7 Torr) and elevated temperature (450 oC). Systematic introduction of oxygen vacancies partially converts V4+ to V3+ and generates unpaired electron charges which result in the emergence of donor states near Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V-V vibrational modes and stabilization of V-V dimers are observed in vacuum-annealed VO2 films providing conclusive evidence for stabilization of monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO2-x thin films resulting in the metallic character at room temperature. With these results, we propose a defect engineering driven pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO2.