Insulator to metal transition in WO3 induced by electrolyte gating

Abstract

Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an alkali metal) are well known for their interesting optical and electrical characteristics. We have modified the transport properties of thin WO3 films by electrolyte gating using both ionic liquids and polymer electrolytes. We are able to tune the resistivity of the gated film by more than five orders of magnitude, and a clear insulator-to-metal transition is observed. To clarify the doping mechanism, we have performed a series of incisive operando experiments, ruling out both a purely electronic effect (charge accumulation near the interface) and oxygen-related mechanisms. We propose instead that hydrogen intercalation is responsible for doping WO3 into a highly conductive ground state and provide evidence that it can be described as a dense polaronic gas.

Anomalous screening of an electrostatic field at the surface of niobium nitride

“…In the past few years, this technique has led to the discovery and control of new phases (including the superconducting one) in various materials. In their native state, most of these featured a low to moderate carrier density ( 10 14 cm −2 ), and ranged from insulating oxides [2][3][4][5], to various layered materials [6][7][8][9][10][11][12][13][14], to cuprate superconductors [15][16][17][18]. A more limited attention has been paid instead to materials with a large intrinsic carrier density, where the effect of the field was generally thought to be undetectable because of the strong electrostatic screening.…”

Manipulate the Electronic and Magnetic States in NiCo2O4 Films through Electric‐Field‐Induced Protonation at Elevated Temperature

“…In the conventional ILG, the tunability is mainly dominated by an electrostatic effect occurred at the liquid/solid interface, while any electrochemical reaction during the gating is carefully avoided . However, it has been demonstrated recently that the residual water would exist ubiquitously within the ionic liquid (IL) when experiments performed in the air . The water molecular would facilitate the electrochemical reaction through electrolysis into H + and O 2‐ ions, which would then be inserted into materials depending on the polarity of the gating bias (as shown in Figure a).…”

“…However, it has been demonstrated recently that the residual water would exist ubiquitously within the ionic liquid (IL) when experiments performed in the air . The water molecular would facilitate the electrochemical reaction through electrolysis into H + and O 2‐ ions, which would then be inserted into materials depending on the polarity of the gating bias (as shown in Figure a). Clearly, the electrochemical reaction related modulation is a bulk effect, and therefore its associated charge modulation should not be confined by the 2D limit as the conventional ILG, which is about 10 15 cm −2 at the sample surface .…”

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

“…In addition, due to the defect‐driven volume insulator‐metal‐transition, we are also able to tune the RT electrical conductivity (σ) of epitaxial WO 3 thin films by more than 5 orders of magnitude, along with their electrochromic behavior, using oxygen pressure during growth and electrolyte gating. The independent sensitivities of thermal and electrical conductivity to lattice dimensions and defect concentrations enable us to selectively control both conductivities in WO 3 thin films and obtain specific combinations of thermal and electrical properties by appropriate modification of the lattice dimension and stoichiometry, making WO 3 a good candidate for various applications including smart windows, thermal barriers and thermoelectrics.…”

“…To separate the contributions to κ L , we should consider contributions of both charge carriers and the lattice, as the electrolyte gating leads to an insulator‐metal‐transition in WO 3 and a change in carrier concentration . The total thermal conductivity (κ total ) can be written as: where κ e is electronic thermal conductivity, which can be estimated by the Wiedemann–Franz law in which L , the Lorenz constant, is about 2.4 × 10 −8 W Ω K −2 (= L 0 ) at RT for most metals and varies little from metals to semiconductors.…”