Structural stability and phase transitions in WO₃ thin films

Tungsten oxide (WO₃) thin films have been produced by KrF excimer laser (λ = 248 nm) ablation of bulk ceramic WO₃ targets. The crystal structure, surface morphology, chemical composition, and structural stability of the WO₃ thin films have been studied in detail. Characterization of freshly grown WO₃ thin films has been performed using X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy (RS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) measurements. The results indicate that the freshly grown WO₃ thin films are nearly stoichiometric and well crystallized as monoclinic WO ₃. The surface morphology of the resulting WO₃ thin film has grains of ∼60 nm in size with a root-mean-square Xrms) surface roughness of 10 nm. The phase transformations in the WO₃ thin films were investigated by annealing in the TEM column at 30-500 °C. The phase transitions in the WO₃ thin films occur in sequence as the temperature is increased: monoclinic → orthorhombic → hexagonal. Distortion and tilting of the WO₆ octahedra occurs with the phase transitions and significantly affects the electronic properties and, hence, the electrochemical device applications of WO₃.