Metallization of Epitaxial VO₂ Films by Ionic Liquid Gating through Initially Insulating TiO₂ Layers

Ionic liquid gating has been shown to metallize initially insulating layers formed from several different oxide materials. Of these vanadium dioxide (VO₂) is of especial interest because it itself is metallic at temperatures above its metal–insulator transition. Recent studies have shown that the mechanism of ionic liquid gated induced metallization is entirely distinct from that of the thermally driven metal–insulator transition and is derived from oxygen migration through volume channels along the (001) direction of the rutile structure of VO₂. Here we show that it is possible to metallize the entire volume of 10 nm thick layers of VO₂ buried under layers of rutile titanium dioxide (TiO₂) up to 10 nm thick. Key to this process is the alignment of volume channels in the respective oxide layers, which have the same rutile structure with clamped in-plane lattice constants. The metallization of the VO₂ layers is accompanied by large structural expansions of up to ∼6.5% in the out-of-plane direction, but the structure of the TiO₂ layer is hardly affected by gating. The TiO₂ layers become weakly conducting during the gating process, but in contrast to the VO₂ layers, the conductivity disappears on exposure to air. Indeed, even after air exposure, X-ray photoelectron spectroscopy studies show that the VO₂ films have a reduced oxygen content after metallization. Ionic liquid gating of the VO₂ films through initially insulating TiO₂ layers is not consistent with conventional models that have assumed the gate induced carriers are of electrostatic origin

Distinct Electronic Structure of the Electrolyte Gate-Induced Conducting Phase in Vanadium Dioxide Revealed by High-Energy Photoelectron Spectroscopy

The development of new phases of matter at oxide interfaces and surfaces by extrinsic electric fields is of considerable significance both scientifically and technologically. Vanadium dioxide (VO₂), a strongly correlated material, exhibits a temperature-driven metal-to-insulator transition, which is accompanied by a structural transformation from rutile (high-temperature metallic phase) to monoclinic (low-temperature insulator phase). Recently, it was discovered that a low-temperature conducting state emerges in VO₂ thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO₂ thin films, we show that electronic features in the gate-induced conducting phase are distinct from those of the temperature-induced rutile metallic phase. Moreover, polarization-dependent measurements reveal that the V 3d orbital ordering, which is characteristic of the monoclinic insulating phase, is partially preserved in the gate-induced metallic phase, whereas the thermally induced metallic phase displays no such orbital ordering. Angle-dependent measurements show that the electronic structure of the gate-induced metallic phase persists to a depth of at least ∼40 Å, the escape depth of the high-energy photoexcited electrons used here. The distinct electronic structures of the gate-induced and thermally induced metallic phases in VO₂ thin films reflect the distinct mechanisms by which these states originate. The electronic characteristics of the gate-induced metallic state are consistent with the formation of oxygen vacancies from electrolyte gating