Insulator to metal transition in WO3 induced by electrolyte gating

Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an alkali metal) are well known for their interesting optical and electrical characteristics. We have modified the transport properties of thin WO3 films by electrolyte gating using both ionic liquids and polymer electrolytes. We are able to tune the resistivity of the gated film by more than five orders of magnitude, and a clear insulator-to-metal transition is observed. To clarify the doping mechanism, we have performed a series of incisive operando experiments, ruling out both a purely electronic effect (charge accumulation near the interface) and oxygen-related mechanisms. We propose instead that hydrogen intercalation is responsible for doping WO3 into a highly conductive ground state and provide evidence that it can be described as a dense polaronic gas.Electrolyte gating: Hydrogenation mechanism in WO 3The mechanism leading to large carrier density changes and even concomitant electronic phase transitions with electrolyte gating is under debate. An international team led by Ivan Božović at USA’s Brookhaven National Laboratory and Yale University report a series of experiments based on WO3 films, which is found to exhibit an insultator-to-metal transition under gating, with both ionic liquids and polymer electrolytes. The experimental results allow to rule out some mechanisms—such as charge accumulation near the interface or oxygen vacancy formation—previously suggested in other material systems. Instead, the authors propose that the primary effect of electrolyte gating in WO3 is hydrogen intercalation. Hydrogenation leads to the formation of a dense polaronic gas that explains the conductive ground state. The doping mechanism behind electrolyte gating seems to be material dependent.