Porous nanocrystalline WO₃ thin films: fabrication, electrical and optical properties

A well-designed procedure, including ion beam sputtering of aluminium (Al)–tungsten trioxide (WO₃) composite films, annealing at an elevated temperature and dealloying of aluminium component, was carried out in sequence to fabricate porous nanocrystalline tungsten trioxide thin films. The texture, morphology and electrical and optical properties of the tungsten trioxide thin films were each characterised. It was found that when the annealing temperature of the aluminium–tungsten trioxide composite film reached 550°C, the obtained tungsten trioxide thin film after dealloying was nanocrystalline with a more pronounced porous structure. Electrical property tests showed that in contrast to that of amorphous tungsten trioxide films, the resistance of the porous nanocrystalline tungsten trioxide film was much greater, with a positive temperature coefficient of resistance (TCR) of +4.1 × 10⁻²/°C at temperatures of −10 to 5°C and a negative TCR of −2.4 × 10⁻²/°C at temperatures of 5 to 30°C. It was further demonstrated by ultraviolet–visible spectroscopy that compared with amorphous tungsten trioxide films, the porous crystalline tungsten trioxide film had much higher transmittance at the wavelength of visible light and a larger forbidden band width of 2.75 eV. Therefore, the porous nanocrystalline tungsten trioxide film exhibits unique or excellent electrical and optical properties, which indicate new potential applications in the fields of photoelectric devices and thermistors.