Ferroelectric Epsilon-WO3 Nanoparticles and Its Bipolaron Driven Opto-electronic Properties at Room Temperature

Abstract: A unique polymorph of binary tungsten trioxide, the epsilon phase of WO3, has non-centrosymmetric ferroelectric structure, typically stable below -43 degree C in bulk. We have stabilized the epsilon-WO3 at room temperature (RT) and nanostructured powders via flame spray pyrolysis synthesis. These nanopowders are drop cast into uniform thin films to enable RT measurement of ferroelectric and optoelectronic properties. We report ferroelectric hysteresis, nanoscale domains, and dipole switching measured via Piezo-response force microscopy (PFM). The epsilon-WO3 films also display optical second harmonic generation (SHG) and anticlockwise ferroelectric butterfly capacitance versus voltage hysteresis, further demonstrating the ferroelectric nature of epsilon-WO3. Remarkably, epsilon-WO3 shows ferroelectric polarization responses to optical stimuli and form bipolaron at RT, a spin-zero quasiparticle previously found only in cryogenic temperatures. The bipolaron formation and its interaction with electro-optical stimuli results in a single layer solid-state blue coloration, a ferrochromic effect. A mechanism of the ferrochromic effect is discussed. In summary, epsilon-WO3 appears to be a ferroelectric with the simplest structure, forming bosonic spin-zero bipolaron at RT, and it's dipoles respond to opto-electrical signals; therefore, this material holds significant promise for transforming the field of optoelectronics.