2 research outputs found
Metallization of Epitaxial VO<sub>2</sub> Films by Ionic Liquid Gating through Initially Insulating TiO<sub>2</sub> Layers
Ionic
liquid gating has been shown to metallize initially insulating layers
formed from several different oxide materials. Of these vanadium dioxide
(VO<sub>2</sub>) 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 VO<sub>2</sub>. Here we show that it is possible to metallize the
entire volume of 10 nm thick layers of VO<sub>2</sub> buried under
layers of rutile titanium dioxide (TiO<sub>2</sub>) 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 VO<sub>2</sub> layers
is accompanied by large structural expansions of up to ∼6.5%
in the out-of-plane direction, but the structure of the TiO<sub>2</sub> layer is hardly affected by gating. The TiO<sub>2</sub> layers become
weakly conducting during the gating process, but in contrast to the
VO<sub>2</sub> layers, the conductivity disappears on exposure to
air. Indeed, even after air exposure, X-ray photoelectron spectroscopy
studies show that the VO<sub>2</sub> films have a reduced oxygen content
after metallization. Ionic liquid gating of the VO<sub>2</sub> films
through initially insulating TiO<sub>2</sub> 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 (VO<sub>2</sub>), 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 VO<sub>2</sub> thin films upon gating with a liquid electrolyte. Using photoemission spectroscopy measurements of the core and valence band states of electrolyte-gated VO<sub>2</sub> 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 VO<sub>2</sub> 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