title: Transparent conducting oxide induced by liquid electrolyte gating.
abstract: 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 WO3Here, 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 WO3is 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 W and 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.
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden 01187, Germany.
authors.forename: Carlos
authors.initials: C
authors.lastname: ViolBarbosa
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden 01187, Germany.
authors.forename: Julie
authors.initials: J
authors.lastname: Karel
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden 01187, Germany.
authors.forename: Janos
authors.initials: J
authors.lastname: Kiss
authors.affiliation: Technische Universität Chemnitz, Chemnitz 09107, Germany.
authors.forename: Ovidiu-Dorin
authors.initials: OD
authors.lastname: Gordan
authors.affiliation: IBM Research-Almaden, San Jose, CA 95120; Max Planck Institute of Microstructure Physics, Halle (Saale) 06120, Germany.
authors.forename: Simone G
authors.initials: SG
authors.lastname: Altendorf
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden 01187, Germany.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Utsumi
authors.affiliation: IBM Research-Almaden, San Jose, CA 95120.
authors.forename: Mahesh G
authors.initials: MG
authors.lastname: Samant
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan.
authors.forename: Yu-Han
authors.initials: YH
authors.lastname: Wu
authors.affiliation: National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan.
authors.forename: Ku-Ding
authors.initials: KD
authors.lastname: Tsuei
authors.affiliation: Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden 01187, Germany.
authors.forename: Claudia
authors.initials: C
authors.lastname: Felser
authors.affiliation: IBM Research-Almaden, San Jose, CA 95120; Max Planck Institute of Microstructure Physics, Halle (Saale) 06120, Germany; Martin Luther University Halle-Wittenberg, Halle (Saale) 06099, Germany stuart.parkin@mpi-halle.mpg.de.
authors.forename: Stuart S P
authors.initials: SS
authors.lastname: Parkin
keywords: TCO; electrolyte gating; metal–insulator transition; transparent conducting oxide