Evolution of Insulator–Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating

Nishihaya, Shinichi; Uchida, Masaki; Kozuka, Y.; Iwasa, Yoshihiro; Kawasaki, Masashi

doi:10.1021/acsami.6b06593

Cited by 34 publications

(34 citation statements)

References 28 publications

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“…This differs with previous XRD data taken while electrolyte gating WO3 that showed the lattice constants changing by at least 2%. 21,22 However, we point out that there is really no agreement on this particular point as the c-axis lattice constants have been observed to both decrease 21 and increase 22 while charging in the range of gate voltages used here (up to +3 V). Several differences between our work and that of Refs.…”

Section: Resultsmentioning

confidence: 64%

“…None, however, have observed any superconducting phases. Several explanations for how the excess conductivity is created in WO3 by electrolyte gating have been put forth from electrostatic, 19,22 to a mix of electrostatic and electrochemical processes, 20 to a structural phase transition driven by oxygen removed from WO3 by the applied electric field, 21 with no apparent consensus.…”

Section: Introductionmentioning

confidence: 99%

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Insulator to metal transition in WO3 induced by electrolyte gating

et al. 2017

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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.

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Section: Resultsmentioning

confidence: 64%

Section: Introductionmentioning

confidence: 99%

Insulator to metal transition in WO3 induced by electrolyte gating

et al. 2017

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“…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.…”

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confidence: 99%

“…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:

κ_{total} = κ_{normale} + κ_{normalL}

where κ e is electronic thermal conductivity, which can be estimated by the Wiedemann–Franz law

κ_{normale} = L σ T

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.…”

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confidence: 99%

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO₃ Thin Films

et al. 2019

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According to the Boltzmann transport theory of phonons, the lattice thermal conductivity (κ L ) depends mainly on structure-related parameters including the specific heat, the velocity, and the scattering of phonons. One common strategy for the manipulation of phonon transport has been to introduce lattice imperfections in materials which alters the mean free path of phonons, particularly for applications in thermoelectric energy conversion. [1] Based on the well-established Abeles theory of interactions between phonons and defects in alloys or semiconductors, [2] point defects, including substitutionals, [3] interstitials, [4] and vacancies, [5] 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. [6][7][8][9][10] 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.In this work, we focus on epitaxial tungsten trioxide (WO 3 ), which exhibits an ABO 3 perovskite structure with the A-sites vacant and can accommodate a range of lattice distortions via octahedral tilts and rotations. [11][12][13] We find that the roomtemperature (RT) thermal conductivity of the WO 3 thin films evolves significantly upon electrolyte gating, accompanied by reversible structure changes caused by the intercalation or deintercalation of hydrogen (H i ), enabling a reversible control of thermal conductivity. The modulation in thermal conductivity is comparable with that reported in LiCoO 2 [14] and MoS 2 [15] by electrochemically tuning the degree of lithiation, but works in a much simpler configuration. What is more, the variation of thermal conductivity is highly dependent on the substrate on which the epitaxial WO 3 thin films are grown. An unusual increase of κ L is observed after gating with the intercalation of hydrogen, which contradicts the expectation that point defects cause additional thermal resistance and therefore reduce κ L . [16] We further investigate how another common type of point defect, the oxygen vacancy (V O ), influences the lattice structure and κ L in WO 3 thin films. The results collectively indicate that the dominant factor determining κ L is the lattice dimension, i.e., the unit-cell volume. κ L increases as the lattice contracts and Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO 3 thin films. Depending on the substrate, the lattice of epitaxial WO 3 expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, ins...

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“…We chose WO 3 as a model system for demonstrating how the oxygen octahedral rotation can be manipulated by an electric field. WO 3 maintains the perovskite structure with no A‐site cation, thus consisting entirely of oxygen octahedra; furthermore, ionic liquid gating was recently shown to produce significant changes in the electronic properties of WO 3 films . We synthesized epitaxial WO 3 (001) heterostructures by deposition onto SrTiO 3 (STO) (001) single‐crystal substrates (see the Experimental Section).…”

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confidence: 99%

Dynamic Field Modulation of the Octahedral Framework in Metal Oxide Heterostructures

Liu

Dong

et al. 2018

Advanced Materials

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Control over the oxygen octahedral framework is widely recognized as key to the design of functional properties in perovskite oxide heterostructures. Although the oxygen octahedral framework can be manipulated during synthesis, the as‐grown oxygen octahedra generally remain fixed, preventing the development of adaptive behavior in electronic and ionotronic systems. Here, it is demonstrated that the oxygen octahedral framework can be dynamically and reversibly manipulated by an electric field through the coupling with oxygen vacancies. Studying model WO3 heterostructures during ionic liquid gating with a combination of in situ X‐ray scattering and spectroscopy, it is shown that large changes in electronic properties can arise due to the increased flexibility of the octahedral network at high vacancy concentrations. The results describe a generic framework for the construction of dynamic systems and devices with an array of field‐tunable properties.

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Evolution of Insulator–Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating

Cited by 34 publications

References 28 publications

Insulator to metal transition in WO3 induced by electrolyte gating

Insulator to metal transition in WO3 induced by electrolyte gating

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO₃ Thin Films

Dynamic Field Modulation of the Octahedral Framework in Metal Oxide Heterostructures

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Evolution of Insulator–Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating

Cited by 34 publications

References 28 publications

Insulator to metal transition in WO3 induced by electrolyte gating

Insulator to metal transition in WO3 induced by electrolyte gating

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

Dynamic Field Modulation of the Octahedral Framework in Metal Oxide Heterostructures

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Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO₃ Thin Films