Electric‐Field‐Controlled Phase Transformation in WO₃ Thin Films through Hydrogen Evolution

Abstract

Field-effect transistors with ionic-liquid gating (ILG) have been widely employed and have led to numerous intriguing phenomena in the last decade, due to the associated excellent carrier-density tunability. However, the role of the electrochemical effect during ILG has become a heavily debated topic recently. Herein, using ILG, a field-induced insulator-to-metal transition is achieved in WO thin films with the emergence of structural transformations of the whole films. The subsequent secondary-ion mass spectrometry study provides solid evidence that electrochemically driven hydrogen evolution dominates the discovered electrical and structural transformation through surface absorption and bulk intercalation.

Emergent electric field control of phase transformation in oxide superlattices

“…The voltage between the bottom electrode and the top electrode was initially set to 0 V and then ramped 13 to desired values. Meanwhile, the XRD spectra were collected continuously with a scanning rate of 3° per minute.…”

“…To fully exploit this mechanism, materials that can be electrically switched from one crystalline phase to another, with distinct physical properties, are highly desirable. Although ion transfer has been reported in many singlephase TMOs [8][9][10][11][12][13][14][15][16][17][18] , reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature (RT) has been limited to a few systems, including binary oxides VO2 8 and WO3 12,13 , and perovskite oxides SrCoO3-δ 14 and SrFeO3-δ 17 that exhibit topotactic transformations. Among these systems, coupled electronic, magnetic and optical phase transitions have only been demonstrated in SrCoO3-δ thus far 14 .…”

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

“…Ex situ Raman analysis was conducted to examine the Raman peaks S 1 and S 2 corresponding to the stretching modes of WO and OWO bonds, respectively. As Figure d shows, mode S 1 completely disappears after gating, which is a characteristic of hydrogenated tungsten oxide (H x WO 3 ), meaning that the defect involved during electrolyte gating with a positive bias is H i . After relaxation, the re‐appearance of mode S 1 suggests that H i has escaped from the WO 3 lattice.…”

“…Based on the well‐established Abeles theory of interactions between phonons and defects in alloys or semiconductors, point defects, including substitutionals, interstitials, and vacancies, are considered to cause additional scattering and therefore lower κ L due to the mass differences and local strain that they introduce. However, in addition to localized mass and strain effects, point defects can also yield significant structural changes, particularly in complex oxides . In this case, the role that defects play in the phonon transport could be more subtle, which merits re‐evaluation of the typical expectation that defects lower the thermal conductivity.…”

Electric‐Field Control of Dirac Two‐Dimensional Electron Gas in PbTe/CdTe Heterostructures

“…All the curves show completely metallic behaviors down to 2 K, suggesting that charged carriers are less scattered in this system. The sheet resistance decreases monotonously from 520 to 200 Ω/□ as the V g is increased from −2 to 1 V. Note that the residual resistance keeps the same trend, unlike the upturn resistance resulting from the Kondo effect or weak localization in strongly correlated oxides. To fully demonstrate the modulation of mobility and carrier density by IL gating, hall effect measurements were carried out by sweeping the magnetic field between +1 T and −1 T at various V g .…”