Achieving Single-Orientation VO₂ Films by Substrate-Tuned Topotactic Oxidation

Topotactic reactions offer a promising approach for fabricating highly oriented crystalline films of various functional materials. However, conventional topotactic processes often produce polycrystalline films with multiple orientation domains, which largely depend on the crystallographic symmetry of the reactants and products. In this study, we identified an additional key factor that influences the orientation of the product crystals and proposed a strategy for producing single-orientation crystalline VO₂ films via topotactic reactions. To elucidate the mechanism governing the orientation of the VO₂ crystals formed during oxidation, we investigated the oxidation of epitaxial V₂O₃ films grown on various crystallographic planes (C, M, R, A, and N planes) of sapphire substrates. Our results demonstrate that the orientation of the precursor V₂O₃ film with respect to the substrate surface significantly affects the number of orientation domains in the resulting VO₂ film. Specifically, VO₂ films with three orientation domains are formed on C-plane substrates, those with two domains are formed on the M- and R-plane substrates, and those with a single domain are formed on the A- and N-plane substrates. This variation in the number of orientation domains is attributed to the anisotropic lattice contraction associated with topotactic transformation. The transformation proceeds preferentially in the direction in which the lattice mismatch at the transformation interface is minimized. These findings enhance our understanding of orientation control in thin-film fabrication via topotactic reactions and open new avenues for developing high-performance thin-film materials.