Crystal Growth of a <i>β</i>-Phase Molybdenum Trioxide Thin Film on a Lattice-Matched Substrate and its Electrochromic Properties

We have successfully grown monoclinic (β) phase single-crystalline molybdenum trioxide (MoO3) films on a lattice-matched (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) substrate with a 5-nm-thick WO3 buffer layer by molecular beam epitaxy (MBE). We investigated the structural changes and electrochromic properties of a 40-nm-thick β-MoO3 film prepared by proton injection for smart window applications. Proton injection into the film up to 12 mC/cm2 resulted in the formation of cubic H0.99MoO3 with an associated significant reduction in transmittance in the visible to near-infrared range. The coloration efficiencies at λ = 650 and 1000 nm are ∼32 and ∼65 cm2/C, respectively. Such high values are due to the unique properties of the high-quality β-phase MoO3 epitaxial thin films. X-ray photoelectron spectroscopy measurements showed that Mo5+ and Mo4+ related peaks appeared simultaneously owing to protonation, which may have introduced oxygen vacancies into the molybdenum bronze. Our results provide valuable information for the application of MoO3-based electrochromic devices.