Hydrogen dopant is injected into interstitial sites rather than into substitutional sites (e.g., W, Nb in VO 2 lattice [5,6] ) without destroying the framework of the lattice. In this way, electrons (one per hydrogen) are effectively and reversibly supplied to the correlated oxide lattice. [12] Because hydrogen-induced carrier doping minimizes the dopant-induced disorder, the use of hydrogen is close to mimicking pure band-filling control of correlated systems. The charge carriers supplied by hydrogen are expected to cause large changes in electrical resistivity and optical transmission across the hydrogen-induced phase transition. These changes would make the correlated materials useful in "ionotronic" devices. [12,13,16,17] Recently, it was demonstrated that up to one electron can be introduced into each VO 2 chemical unit by hydrogen spillover method. [12] Hydrogen diffuses along the empty [001] R channel in the lattice and forms thermodynamically stable hydrogenated VO 2 [11,18,19] (HVO 2 ). The electron doping by hydrogen allows dynamic band filling of VO 2 and achieves a two-step phase transition from insulator (VO 2 , 3d 1 ) to metal (H x VO 2 ; 0 < x < 1) to insulator (HVO 2 , 3d 2 ) during interinteger d-band filling. These changes are accompanied by expansion of out-of-plane lattice parameters in an epitaxial (100) R -faceted VO 2 thin film. An important question is whether a new insulating phase universally emerges near-integer band filling of d regardless of facet orientation and how crystal facet orientation influences the stability and kinetics of the hydrogenated-insulating HVO 2 epilayer with 3d 2 configuration.Here, we demonstrate universal and distinct characteristics on the reversible injection of hydrogen dopants to, and release of hydrogen dopants from, (001) R -VO 2 epitaxial films, as well as (100) R -VO 2 epitaxial films. The doping caused remarkable expansion in the out-of-plane lattice parameter (a-axis in (100) R -faceted HVO 2 films; c-axis in (001) R -faceted HVO 2 films), and stabilized the hydrogenated-insulating phase at the highest hydrogen contents. These results suggest that metal-insulator transition induced by electron doping is universal in H x VO 2 regardless of the facet direction. Contrary to universal phase transition to HVO 2 , transition rates and the degree of lattice expansion were controlled by facet orientation: because of anisotropic diffusion of hydrogen in the rutile lattice, the transition to (001) R -HVO 2 that has a surface-exposed oxygen channel was faster than the transition to (100) R -HVO 2 . Moreover, as a result of the 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 VO 2 , a heavily hydrogenated correlated insulating phase with 3d 2 electronic configuration, can be thermodynamically stabilized by topotactically preserving its lattice framework r...
Transition-metal oxides (TMOs) with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the OVC orientation further leads to modifications in the hydrogenation of the BM-SrFeO thin films. The results demonstrate a strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.
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 (D*) and surface exchange coefficient (k*) 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.
Reversible phase transformation of correlated oxides by field‐driven ionic process present opportunity to efficiently transduce between ionic transfer and electrical currents in insertion‐based reconfigurable transistors. However, the switching rate of insertion transistors is fundamentally limited by the slow rate of ionic insertion into the lattices of correlated oxides. Here, it is demonstrated that preformed oxygen vacancies in VO2−δ lattices strongly accelerate proton insertion by low gate voltage in synaptic transistors. As the degree of oxygen deficiency δ increases in VO2−δ transistors, the steepness of phase transformation and transconductance increase during the voltage sweep at the expense of the channel current modulation. Theoretical and experimental analyses reveal that the accelerated of H+ kinetics in the VO2−δ lattice occurs because immobile oxygen vacancies reduce the energy barrier to H+ migration. In an electronic synapse, this facile H+ migration in VO2−δ lattices renders “inscribed” memory by positioning the H+ neurotransmitter far from the electrolyte/VO2−δ interface. This discovery suggests a strategy to improve the learning and memory processes of artificial synaptic devices by controlling the density of intrinsic defects in the lattice framework to achieve efficient ion exchange.
The control of field-driven ionic redistribution guided by crystal anisotropy increases the retention of H+s in VO2 lattices by locating H+ into the deep regions from the interfaces, and thus strengthens long-term memory in artificial synaptic devices.
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