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

Abstract

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.