Synchrotron x-ray study of hydrogen-induced phase transition in VO2 epitaxial thin films

Phase transition by band filling control is one of the core concepts in correlated electronic systems. Unlike the substitutional dopants, hydrogen, the smallest and the lightest atom, plays a key role in effectively filling significant amount of carriers in the empty narrow d band by reversibly adding it into interstitial sites and supplying carriers. Vanadium dioxide (VO2), typical correlated oxide with 3d1 electronic configuration, can also reversibly incorporate hydrogen atoms into its interstitial sites and simultaneously occurs phase transition by its 3d band filling. Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO2 unit cell, and that hydrogen is reversibly absorbed into, and released from, VO2 without destroying its lattice framework due to the low temperature annealing process. This hydrogenation process demonstrates twostep insulator (VO2) – metal (HxVO2) – insulator (HVO2) phase modulation during inter-integer d-band filling. Moreover, HVO2 can be thermodynamically stabilized regardless of facet direction of VO2 epi-layer, but remarkable discrepancy in kinetics of phase modulation was clearly visualized depending on the crystal facet. Based on in situ XRD, XPS and NEXAFS in synchrotron, the unprecedented insulating HVO2 with 3d2 configuration is attributed to highly doped electrons via hydrogenation process in conjunction with huge lattice expansion. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2 and opens up the potential application in proton-based Mottronics and novel hydrogen storage.2