Structure and photoluminescence properties of ZnWO4 film prepared by depositing WO3/ZnO/WO3 heterolayer via magnetron sputtering technique

Novel ZnWO4 thin films were achieved by annealing sandwich WO3/ZnO/WO3 (WZW) heterolayer that were deposited on clean fused silica substrates by RF magnetron sputtering technique. The crystal structures, surface morphologies and photoluminescence (PL) properties of the annealed WZW heterolayers were characterized by x-ray diffraction (XRD), Raman, scanning electron microscopy (SEM) and PL measurements. The XRD and Raman patterns show that the ZnWO4 thin films formed from WZW heterolayers possess the monoclinic wolframite structure with a preferential orientation along the a-axis. The annealing temperature of WZW heterolayer plays a vital role in determining the crystallization, surface morphologies and PL properties of the formed ZnWO4 films. The SEM graphs and PL spectra indicate that the annealing treatment at about 750 °C for 30min is very effective for deposited WZW heterolayer to form the flat ZnWO4 thin film with the regular nanometer grains (200-400 nm) and the strongest PL emission near about 495 nm, while further increasing annealing temperature to 850 °C leads to a rough surface and weakened PL intensity. The PL spectra from all the annealed WZW films display a broad and asymmetrical shape and are depend on the excitation wavelengths in the range of 240-290 nm as well as annealing temperature. The related PL mechanisms were analyzed and discussed according to the excitation spectra monitored at 410 nm and 495 nm, respectively. It is suggested that the coexistence of the ZnWO4 and ZnO excitation bands in the film should be responsible for the evolving of PL shapes with the excitation wavelengths. Finally, the average PL decay time of the formed ZnWO4 film was determined to be about 22μs by time-resolving the three intrinsic emissions of the ZnWO4 crystallites. This work illustrates that magnetron sputtering is an effective technique to prepare the fluorescing ZnWO4 film.