Accelerated Hydrogen Diffusion and Surface Exchange by Domain Boundaries in Epitaxial VO₂ Thin Films

Abstract

Electronic phase modulation based on hydrogen insertion/extraction is kinetically limited by the bulk hydrogen diffusion or surface exchange reaction, so slow hydrogen kinetics has been a fundamental challenge to be solved for realizing faster solid-state electrochemical switching devices. Here we accelerate electronic phase modulation that occurs by hydrogen insertion in VO₂ through vertically aligned 2D defects induced by symmetry mismatch between epitaxial films and substrates. By using domain-matching epitaxial growth of monoclinic VO₂ films with lattice rotation and twinning on hexagonal Al₂O₃ substrates, the domain boundaries naturally align vertically; they provide a "highway" for hydrogen diffusion and surface exchange in VO₂ films and overcome the limited rates of bulk diffusion and surface reaction. From the quantitative analysis of the deuterium (²H) isotope tracer exchange, it is confirmed that the tracer diffusion coefficient (<i>D*</i>) and surface exchange coefficient (<i>k*</i>) were increased by several orders of magnitude in VO₂ films that had domain boundaries. These results yield fundamental insights into the mechanism by which mobile ions are inserted along extended defects and provide a strategy to overcome a limitation to switching speed in electrochemical devices that exploit ion insertion.