Thermoelectric probe of defect state induced by ionic liquid gating in vanadium dioxide

Thermoelectric measurements detect the asymmetry between the density of states above and below the chemical potential in a material. They provide insights into small variations in the density of states near the chemical potential, complementing electron transport measurements. Here, we report combined resistance and thermoelectric power measurements of vanadium dioxide (VO2), a prototypical correlated electron material, under ionic-liquid (IL) gating. We show that under our gating conditions, the charge transport below the metal-to-insulator-transition (MIT) temperature remains in the thermally activated regime, while the Seebeck coefficient exhibits an apparent transition from semiconducting to metallic behavior. The contrasting behavior indicates changes in the electronic structure upon IL gating, due to the formation of oxygen defect states. The experimental results are corroborated by numerical simulations based on a model density of states incorporating a gating-induced defect band. Our study reveals thermoelectric measurements to be a convenient and sensitive probe for the role of defect states induced by IL gating in suppressing the MIT in VO2, which remains benign in charge transport measurements, and possibly for studying defect states in other materials.