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

Abstract

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.

Dominant role of processing temperature in electric field induced superconductivity in layered ZrNBr

“…Although this kind of average estimation always has a non-negligible error, the Br content obviously decreases in the gating process. A similar vacancy forming process was reported in VO 2 film, in which suppression of metal-insulator transition was caused by electric field induced oxygen vacancy formation [10,28,29]. As recently reported on YBa 2 Cu 3 O 7−x [30], the EDLT induced deoxygenation of the CuO chains causes the modification of Cu coordination, leading to a superconductor-to-insulator transition.…”

Role of Oxygen in Ionic Liquid Gating on Two-Dimensional Cr2Ge2Te6: A Non-oxide Material

“…Despite the large enhancement of the gate leakage current, the gating response of the channel resistances is not affected. These experimental results further support previous studies that the ionic liquid gate induced metallization in various oxide materials (TiO2, VO2, WO3, SrTiO3, etc) arises from the oxygen vacancies 18,[21][22][23]35.…”

Ionic Gating of Ultrathin and Leaky Ferroelectrics

“…Ionic liquid–solid interfaces have garnered broad interest for their wide range of properties . Due to their wide electrochemical stability window, ionic liquid (IL) gating allows large electric fields to be achieved at the interface—providing an ability to control, manipulate, and elevate unique phenomena that can arise in the interfaced solid materials . IL gating has advantage over conventional solid gating, as the large electric fields can be achieved in an initially insulating solid layer without significant current flow .…”