Long-range propagation of protons in single-crystal VO involving structural transformation to HVO.

Vanadium dioxide (VO) is a strongly correlated electronic material with a metal-insulator transition (MIT) near room temperature. Ion-doping to VO dramatically alters its transport properties and the MIT temperature. Recently, insulating hydrogenated VO (HVO) accompanied by a crystal structure transformation from VO was experimentally observed. Despite the important steps taken towards realizing novel applications, essential physics such as the diffusion constant of intercalated protons and the crystal transformation energy between VO and HVO are still lacking. In this work, we investigated the physical parameters of proton diffusion constants accompanied by VO to HVO crystal transformation with temperature variation and their transformation energies. It was found that protons could propagate several micrometers with a crystal transformation between VO and HVO. The proton diffusion speed from HVO to VO was approximately two orders higher than that from VO to HVO The long-range propagation of protons leads to the possibility of realizing novel iontronic applications and energy devices.