Hydrogen-induced band-filling-control of multivalent VO2 epitaxial thin films

Abstract

The ability to control a variety of functionalities with external stimuli is one of the main issues in correlated oxides. Due to the extreme sensitivity of those material systems to external stimuli, the control of the versatile functionalities can achieve unique phenomena. Among the various stimuli, intrinsic or extrinsic atomic defects have a strong influence on the d-band filling in correlated electronic systems. hydrogen also affects the change of the functionalities in correlated oxides as interstitial defect. In this regards, this thesis focuses on the correlation between VO2, a typical correlated oxide, and hydrogen, interstitial defect. The first topic is about reversible phase modulation in multivalent VO2 epitaxial thin films by hydrogenation. Hydrogen, the smallest and the lightest atomic element, is reversibly incorporated into interstitial sites in vanadium dioxide (VO2), a correlated oxide with a 3d1 electronic configuration, and induces electronic phase modulation. It is widely reported that low hydrogen concentrations stabilize the metallic phase, but the understanding of hydrogen in the high doping regime is limited. Here, it is demonstrated 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. This hydrogenation process allows to elucidate electronic phase modulation of vanadium oxyhydride, demonstrating two-step insulator (VO2)–metal (HxVO2)–insulator (HVO2) phase modulation during inter-integer d-band filling. This finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2. The second theme is about facet-dependent phase control by band filling and anisotropic electron-lattice coupling in HVO2 epitaxial films. Unlike the substitutional dopants, interstitial hydrogen effectively supplies significant amount of carriers in the empty narrow d band in correlated electronic systems by reversibly adding it into interstitial sites. Here, it is demonstrated that hydrogenated VO2, a heavily hydrogenated correlated insulating phase with 3d2 electronic configuration, can be thermodynamically stabilized by topotactically preserving its lattice framework regardless of the facet direction of VO2 epilayers. However, the kinetics of phase modulation and the response of out-of-plane lattice expansion are clearly affected by the orientation of the crystal facet, which is attributed to the anisotropy in the diffusion of hydrogen atoms and to anisotropic expansion induced by chemical stress. This result demonstrates universal electron-doping-induced phase transition near-integer numbers of d band filling in correlated electronic systems and also supports the idea that orientation of crystallographic facets can be exploited to control the rate of electronic phase transition and the degree of electron–lattice coupling. The third subject is about direct probing of oxygen loss from the surface lattice of VO2 epitaxial thin films during hydrogen spillover. Hydrogen spillover has been suggested as a strategy to reversibly provide a massive amount of hydrogen dopants while maintaining the crystal framework of correlated oxides under reducing atmosphere. Hydrogen spillover is a catalytic process that occurs by surface reaction and subsequent diffusion between gas and correlated oxides, but the mechanism and surface chemistry at the surface of correlated oxides with metal catalyst are not well understood. Here, it is shows that a significant amount of oxygen is released from the surface of correlated VO2 films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO2 films. By using ambient-pressure X-ray photoelectron spectroscopy combined with first-principles calculations, it is proved that the formation of surface oxygen vacancies during hydrogen spillover is a consequence of a favorable reaction for the generation of weakly-adsorbed H2O from surface O atoms that have low coordination and weak binding strength. This results reveal the importance of in-situ characterization to prove the dynamic change near the surface during redox reaction. The present studies will (i) opens up the potential application in proton-based Mottronics and novel hydrogen storage, (ii) offer a design rule for proton-based electronic devices and electrochemical actuators that exploit chemical stress, and (iii) give opportunity to control intrinsic defects at the surface that are different from those in the bulk.