Ferroelectricity and Bipolaron-Driven Optoelectronic Behavior of ε-WO3 Nanoparticles at Room Temperature

A unique polymorph of tungsten trioxide, the epsilon phase (ε-WO3), has an acentric crystal structure, which suggests it may be ferroelectric in nature. Although typically stable below −43 °C in the bulk, it has been stabilized by our group at RT and above. This study explores the ferroelectric nature and bipolaronic behavior of ε-WO3 at room temperature (RT) for the first time. Nanostructured powders synthesized by flame spray pyrolysis (FSP) are shown to exhibit ferroelectric hysteresis and dipole switching, as confirmed via butterfly-type capacitance–voltage study at 1 MHz frequency, piezo-response force microscopy (PFM), and second harmonic generation (SHG) studies. The ferroelectric dipoles of ε-WO3 respond to electro-optical stimuli by forming a bipolaron. The latter interacts with light, producing a single-layer, all-solid-state blue coloration, namely, “the ferrochromic effect”. Our results lay the groundwork for exploring further the structurally simplest ferroelectric ε-WO3 for potentially transforming the field of optoelectronics.