Strain engineering on the metal-insulator transition of VO₂/TiO₂ epitaxial films dependent on the strain state of vanadium dimers

In this letter, a series of vanadium dioxide (VO₂) epitaxial films were deliberately deposited on TiO₂ substrates with different orientations [(001), (110), and (101)], in an attempt to gain insights into the strained VO₂ epitaxial film. We found in-plane [100] and [1-10] directions, obviously anisotropic metal-insulator transition (MIT) in (110)-oriented VO₂ films. In combination with synchrotron radiation high-resolution x-ray diffraction characterizations, electronic transport data reveal that the critical temperature of MIT depends on the strain state of the dimeric vanadium atomic chain along the c axis of the rutile phase. The anisotropy of MIT is closely related to the orientation of the VO₂ films, which is caused by the varied orientation configuration of V-V atomic chain dimerization in the films. Soft x-ray absorption spectroscopy results further indicate that this anisotropy may be driven by the directional hybridization of O 2p and V 3d orbitals with respect to the orientation of VO₂ thin films. The polarization-dependent V L-edge and O K-edge XAS data suggest that the elongation of the apical V-O bond length increases the p-d orbital overlap; thus, the energy level of the d_// orbital is raised relative to that of the π^* orbital. These anisotropic MIT behaviors will help us to understand how the strain engineering depends on the strain state of vanadium dimers in VO₂ films.