Epitaxy of Monoclinic VO2 on Large-Misfit 3m Template Enabled by a Metastable Interfacial Layer

We report the epitaxial growth of a monoclinic VO₂ thin film on the CoFe₂O₄(111) template, assisted by an interfacial layer of the metastable orthorhombic phase. The interface between orthorhombic VO₂ and CoFe₂O₄ is atomically sharp without noticeable interfacial diffusion. The (010)-faceted orthorhombic VO₂ layer is lattice-matched to both the CoFe₂O₄(111) template and the monoclinic phase, although they have different surface symmetries. The occurrence of an orthorhombic VO₂ thin layer significantly lowers the in-plane misfit strains of the monoclinic VO₂ epilayer, along both the [100] and [001] axes. Our first-principles calculations confirm that the low-misfit orthorhombic VO₂ is preferred on CoFe₂O₄(111) over the large-misfit monoclinic phase, at the initial growth stage. Additionally, upon increasing the film thickness to ∼8 nm, the orthorhombic phase is no longer favored, and the bulk stable monoclinic VO₂ appears to minimize the free energy of the system. Moreover, we show that the metal-to-insulator transition of our VO₂ epilayer can be efficiently triggered by both the temperature and Joule self-heating effect.