Remote Epitaxy and Phase Diagram of Single-Crystalline Ultrathin VO₂ Films on the TiO₂(001) Substrate

Ultrathin vanadium dioxide (UT-VO₂) films on TiO₂(001) provide a model platform to investigate how epitaxial strain, interfacial clamping, and thickness govern structure and the metal-insulator transition (MIT). A quantitative phase-strain-thickness framework is established for single-crystalline VO₂ grown by magnetron sputtering, and remote epitaxy─implemented via a nanometer-thin amorphous Al₂O₃ interlayer deposited by atomic layer deposition─alleviates interfacial clamping to stabilize the monoclinic M1 phase at room temperature while preserving epitaxial registry. High-resolution characterizations yield a thickness-temperature phase diagram featuring a pinned-rutile regime, a rutile-monoclinic coexistence regime, and a fully relaxed monoclinic regime. Aberration-corrected transmission electron microscopy quantifies the progressive relaxation of the out-of-plane strain across the amorphous interlayer. Combined with first-principles calculations, the results reveal the mechanism of strain-driven phase dynamics and coexistence in UT-VO₂, showing that direct epitaxy pins the rutile phase at room temperature and suppresses the resistivity contrast in the ultrathin limit, whereas remote epitaxy reduces residual strain to the subpercent regime, preserves a lowered transition temperature, and substantially amplifies the MIT amplitude without degrading crystallinity. These findings position remote epitaxy as a scalable strain-engineering route for deterministic control of phase and transport in ultrathin correlated-oxide heterostructures.