Plasma assisted pulsed laser deposition of WO₃ films for thermochromism

Tungsten trioxide (WO₃) is one of the extensively investigated transition metal oxides. Its excellent chromogenic properties make WO₃ promising for a variety of scientific and technological applications. We report a new method for reactively depositing WO₃ films by plasma assisted pulsed laser deposition that combines electron cyclotron resonance microwave discharge and pulsed laser ablation. The plasma formed during film deposition was spectroscopically characterized by optical emission measurement, revealing that the plasma contains high density of reactive gaseous oxygen and tungsten species which are essential for efficiently synthesizing WO_x precursors and depositing WO₃ films. The structure of the deposited films and the effect of post-deposition thermal annealing on the film structure were characterized by X-ray diffraction and Raman spectroscopy. The as-deposited WO₃ film appears amorphous in nature. Annealing resulted in the growth of crystalline grains and the improvement in the crystallinity of monoclinic WO₃. Optical properties of the prepared WO₃ films were studied by measuring ultraviolet–visible–near infrared transmission spectra. With the increase of annealing temperature, the WO₃ films show a continuous change in color, decline in transmittance, red shift in absorption edge, and narrowing in band gap, clearly manifesting the thermochromic features of the deposited WO₃ films.