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

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 VO through vertically aligned 2D defects induced by symmetry mismatch between epitaxial films and substrates. By using domain-matching epitaxial growth of monoclinic VO films with lattice rotation and twinning on hexagonal AlO substrates, the domain boundaries naturally align vertically; they provide a "highway" for hydrogen diffusion and surface exchange in VO films and overcome the limited rates of bulk diffusion and surface reaction. From the quantitative analysis of the deuterium (H) isotope tracer exchange, it is confirmed that the tracer diffusion coefficient () and surface exchange coefficient () were increased by several orders of magnitude in VO 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.