Transparent conducting oxide induced by liquid electrolyte gating

Carlos ViolBarbosa, Julie Karel, Janos Kiss, Ovidiu-dorin Gordan, Simone G. Altendorf, Yukumi Utsumi, Mahesh G. Samant, Yu-Han Wu, Ku-Ding Tsuei, Claudia Felser, Stuart S.P. Parkin

Research output: Contribution to journalArticleOtherpeer-review

9 Citations (Scopus)


Optically transparent conducting materials are essential in modern technology. These materials are used as electrodes in displays, photovoltaic cells, and touchscreens; they are also used in energy conserving windows to reflect the infrared spectrum. The most ubiquitous transparent conducting material is tin-doped indium oxide (ITO), a wide-gap oxide whose conductivity is ascribed to n-type chemical doping. Recently, it has been shown that ionic liquid gating can induce a reversible, nonvolatile metallic phase in initially insulating films of WO3. Here, we use hard X-ray photoelectron spectroscopy and spectroscopic ellipsometry to show that the metallic phase produced by the electrolyte gating does not result from a significant change in the bandgap but rather originates from new in-gap states. These states produce strong absorption below ∼1 eV, outside the visible spectrum, consistent with the formation of a narrow electronic conduction band. Thus WO3 is metallic but remains colorless, unlike other methods to realize tunable electrical conductivity in this material. Core-level photoemission spectra show that the gating reversibly modifies the atomic coordination of Wand O atoms without a substantial change of the stoichiometry; we propose a simple model relating these structural changes to the modifications in the electronic structure. Thus we show that ionic liquid gating can tune the conductivity over orders of magnitude while maintaining transparency in the visible range, suggesting the use of ionic liquid gating for many applications.
Original languageEnglish
Pages (from-to)11148-11151
Number of pages4
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number40
Publication statusPublished - 4 Oct 2016
Externally publishedYes


  • Electrolyte gating
  • Metal-insulator transition
  • Transparent conducting
  • Oxide
  • TCO

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