Heterogeneous interfaces exhibit the unique phenomena by the redistribution of charged species to equilibrate the chemical potentials. Despite recent studies on the electronic charge accumulation across chemically inert interfaces, the systematic research to investigate massive reconfiguration of charged ions has been limited in heterostructures with chemically reacting interfaces so far. Here, we demonstrate that a chemical potential mismatch controls oxygen ionic transport across TiO 2 /VO 2 interfaces, and that this directional transport unprecedentedly stabilizes high-quality rutile TiO 2 epitaxial films at the lowest temperature (≤ 150°C) ever reported, at which rutile phase is difficult to be crystallized. Comprehensive characterizations reveal that this unconventional low-temperature epitaxy of rutile TiO 2 phase is achieved by lowering the activation barrier by increasing the "effective" oxygen pressure through a facile ionic pathway from VO 2-δ sacrificial templates. This discovery shows a robust control of defect-induced properties at oxide interfaces by the mismatch of thermodynamic driving force, and also suggests a strategy to overcome a kinetic barrier to phase stabilization at exceptionally low temperature.

The use of gate bias to control electronic phases in VO 2 , an archetypical correlated oxide, offers a powerful method to probe their underlying physics, as well as for the potential to develop novel electronic devices. Up to date, purely electrostatic gating in 3-terminal devices with correlated channel shows the limited electrostatic gating efficiency due to insufficiently induced carrier density and short electrostatic screening length. Here massive and reversible conductance modulation is shown in a VO 2 channel by applying gate bias V G at low voltage by a solid-state proton (H + ) conductor. By using porous silica to modulate H + concentration in VO 2 , gate-induced reversible insulator-tometal (I-to-M) phase transition at low voltage, and unprecedented two-step insulator-to-metal-to-insulator (I-to-M-to-I) phase transition at high voltage are shown. V G strongly and efficiently injects H + into the VO 2 channel without creating oxygen deficiencies; this H + -induced electronic phase transition occurs by giant modulation (≈7%) of out-of-plane lattice parameters as a result of H + -induced chemical expansion. The results clarify the role of H + on the electronic state of the correlated phases, and demonstrate the potentials for electronic devices that use ionic/electronic coupling.

Designing energy-efficient artificial synapses with adaptive and programmable electronic signals is essential to effectively mimic synaptic functions for brain-inspired computing systems. Here, we report all-solid-state three-terminal artificial synapses that exploit proton-doped metal–insulator transition in a correlated oxide NdNiO3 (NNO) channel by proton (H+) injection/extraction in response to gate voltage. Gate voltage reversibly controls the H+ concentration in the NNO channel with facile H+ transport from a H+-containing porous silica electrolyte. Gate-induced H+ intercalation in the NNO gives rise to nonvolatile multilevel analogue states due to H+-induced conductance modulation, accompanied by significant modulation of the out-of-plane lattice parameters. This correlated transistor operated by a proton pump shows synaptic characteristics such as long-term potentiation and depression, with nonvolatile and distinct multilevel conductance switching by a low voltage pulse (≥ 50 mV), with high energy efficiency (∼1 pJ) and tolerance to heat (≤150 °C). These results will guide the development of scalable, thermally-stable solid-state electronic synapses that operate at low voltage.

In article number https://doi.org/10.1002/adfm.201802003, Ji Young Jo, Junwoo Son, and co‐workers apply a gate bias to solid‐state proton conductors to control the electronic phases in the VO2 channel, yielding a reversible phase transition between metal and insulator. The proton‐induced electronic phase transition occurs by giant modulation of out‐of‐plane lattice parameters. The results demonstrate the potential for electronic devices that use ionic/electronic coupling.

Mott threshold switching, which is observed in quantum materials featuring an electrically fired insulator-to-metal transition, calls for delicate control of the percolative dynamics of electrically switchable domains on a nanoscale. Here, we demonstrate that embedded metallic nanoparticles (NP) dramatically promote metastability of switchable metallic domains in single-crystal-like VO2 Mott switches. Using a model system of Pt-NP-VO2 single-crystal-like films, interestingly, the embedded Pt NPs provide 33.3 times longer ‘memory’ of previous threshold metallic conduction by serving as pre-formed ‘stepping-stones’ in the switchable VO2 matrix by consecutive electical pulse measurement; persistent memory of previous firing during the application of sub-threshold pulses was achieved on a six orders of magnitude longer timescale than the single-pulse recovery time of the insulating resistance in Pt-NP-VO2 Mott switches. This discovery offers a fundamental strategy to exploit the geometric evolution of switchable domains in electrically fired transition and potential applications for non-Boolean computing using quantum materials.