  Thermoelectric measurements detect the asymmetry between the density of
states above and below the chemical potential in a material. It provides
insights into small variations in the density of states near the chemical
potential, complementing electron transport measurements. Here, combined
resistance and thermoelectric power measurements are performed on vanadium
dioxide (VO2), a prototypical correlated electron material, under ionic-liquid
(IL) gating. With IL gating, 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 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. This 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 sates in other materials.
