Insulator to Metal Transition in WO$_{3}$ Induced by Electrolyte Gating*

We have modified the transport properties of thin WO$_{3}$ films by the electric field effect using ionic liquids and solid electrolytes. Atomically flat films were prepared on different substrates by RF sputtering. The huge electric field that is generated in the double-layer induces an extraordinarily large change of the mobile charge carrier density in the sample. The sheet resistance of the gated film drops by more than 10 orders of magnitude at the lowest temperature, and a clear insulator-to-metal transition is observed. The thickness dependence has been studied and the mechanism of doping by electrolyte gating will be discussed.