Giant reversible, facet-dependent, structural changes in a correlated-electron insulator induced by ionic liquid gating

Abstract

The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO 2 ) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO 2 . Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO 2 crystal facet, which we relate to the ease of electric-field-induced motion of oxygen ions along chains of edge-sharing VO 6 octahedra that exist along the (rutile) c axis.T he use of electric fields to influence the transport properties of various materials by electrostatic injection of charge at an interface is the foundation of much of modern day electronics (1). Using a three-terminal field-effect transistor geometry, the magnitude of the electric fields provided by conventional gate dielectrics is limited by their dielectric properties. Much higher electric fields are possible by replacing the conventional gate material with an ionic liquid. Consequently, much higher electrostatically induced charge densities are possible, leading to the control or creation of novel metallic (2-3) and superconducting phases (4-7). Materials that are insulating by virtue of strong electron-electron correlations, namely Mott-Hubbard and charge-transfer insulators (8), are anticipated to be particularly sensitive to the injection of small numbers of carriers that could result in their metallization (9-11). Often these materials exhibit a thermally driven insulator to metal transition: one of these, VO 2 , exhibits such a transition near room temperature (12, 13) and, for this reason, has been extensively studied (14-16). In VO 2 the metal to insulator transition (MIT) is accompanied by a structural phase transition (SPT) in which the monoclinic insulating phase transforms to a rutile metallic phase (17). Recently, both Nakano et al. (18) and Jeong et al. (19) showed that ionic liquid (IL) gating of thin films of VO 2 results in the suppression of the MIT to temperatures below ∼10 K and, moreover, that the entire film becomes metallic even though gating takes place at the top surface of the film in contact with the IL. However, whereas Nakano et al. (18) claimed the metallization phenomenon was a direct ...

Metallic monoclinic phase in VO 2 induced by electrochemical gating: In situ Raman study

“…Our study shows that the VO 2 insulating monoclinic phase becomes metallic at a gate voltage of 2.6 V retaining the monoclinic symmetry. Further, our data suggest that the mechanism of metalization is likely due to oxygen vacancy formation as shown by Jeong et al 18,19 .…”

“…This type of switching process requires high electric field, which cannot be achieved by present day dielectric gates and hence calls for electrolytic top gating 14-16 as has been done using ionic liquid (IL) by two groups of Iwasa et al 8,17 and Jeong et al 18,19 . Both the groups showed that above a certain positive gate voltage, the MIT temperature is suppressed.…”

“…On the other hand, Jeong et al 18,19 proposed that the metalization of VO 2 is due to the electric field induced oxygen vacancy formation, arising from the migration of oxygen ions from the channels in to the ionic liquid. Further Iwasa et al 8,17 claimed that the MIT was accompanied by an increase of the unit cell along the c-axis by 1% on gating.…”

Facet‐Independent Electric‐Field‐Induced Volume Metallization of Tungsten Trioxide Films

“…[9] Recently, we have shown that not only is the gateinduced metallization dependent on the crystal orientation of epitaxially grown single-crystalline VO 2 films, but that the metallization is accompanied by a significant expansion of the VO 2 lattice. [13] Moreover, both these effects are non-volatile. [13] Adv.…”

Transparent conducting oxide induced by liquid electrolyte gating

“…Here we investigate an alternative method for controlling the electronic properties via ionic liquid gating. Previous work on VO 2 thin films has shown that liquid electrolyte gating produces structural modifications and leads to the suppression of the metal-insulator transition (11,12). Recent gating experiments on epitaxial WO 3 thin films indicate changes in the out-of-plane lattice parameter, concomitant with the metallization throughout the film volume (13).…”