Reversible Redox Control of Optoelectronic Properties of Hexagonal Tungsten Oxide Epitaxial Films Grown on YSZ Solid Electrolyte

Controlling the oxygen concentration in metal oxides is one of the most effective ways to modulate their optoelectronic properties. However, such redox control is difficult for metal oxide epitaxial films due to serious damages induced in the lattice, especially around the film/substrate interface during the large volume change upon the redox treatment. For overcoming this problem, the use of metal oxides with a stress-resistant crystal structure can be effective. Here, we show a reversible redox control in the optoelectronic properties of hexagonal tungsten oxide (h-WOx) epitaxial films with a honeycomb structure. We fabricated highly c-axis-oriented h-WOx epitaxial films on the (111) yttria-stabilized zirconia (YSZ) single-crystal substrate. Upon electrochemical redox treatment at 300 °C with the application of ±3 V to the YSZ solid electrolyte, the oxygen content x of h-WOx was reversibly controlled in the range of 2.93 ≤ x ≤ 2.99 without inducing damage to the crystal lattice. Simultaneously, the electrical conductivity was controlled from ∼400 S cm–1 to an insulator, and the optical transmission at 1.5 μm in the wavelength was controlled in the range of 35–70%. The present results would be useful for developing electrochemical optoelectronic devices based on metal oxide epitaxial films.