Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO₂ Thin Films

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 VO_{2- x} system via annealing under high vacuum (∼5 × 10^-7 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V⁴⁺ to V³⁺ and generates unpaired electron charges which result in the emergence of donor states near the 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 a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO_{2- x} thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO₂.