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

Epitaxial europium oxide on Ni(100) with single-crystal quality

International audienceHigh quality epitaxy of EuO on Ni(100) is developed in an in situ scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) study. A careful selection of the initial growth parameters is decisive to obtain a surface oxide suitable for the subsequent epitaxy of single phase EuO(100). After the creation of a three layer thick coalesced oxide film for the subsequent growth a distillation technique is applied. Appropriate annealing of films with up to 100 nm thickness generates sufficient conductivity for STM and electron spectroscopies. Oxygen vacancies are directly imaged by STM. They are of decisive importance for the metal-to-insulator transition around the temperature of the ferromagnetic-to-paramagnetic transition. A fast relaxation of the initial biaxial strain observed by LEED leaves little hope for an increase of the Curie temperature through epitaxial compression. Ex situ x-ray adsorption spectroscopy and magneto-optical Kerr effect microscopy measurements of thicker films are consistent with the stoichiometric single phase EuO with bulk properties