Tailoring the Emission Behavior of WO₃ Thin Films by Eu³⁺ Ions for Light-Emitting Applications

The article reports the successful fabrication of Eu³⁺-doped WO₃ thin films via the radio-frequency magnetron sputtering (RFMS) technique. To our knowledge, this is the first study showing the tunable visible emission (blue to bluish red) from a WO₃:Eu³⁺ thin film system using RFMS. X-ray diffractograms revealed that the crystalline nature of these thin films increased upto 3 wt% of the Eu³⁺ concentration. The diffraction peaks in the crystalline films are matched well with the monoclinic crystalline phase of WO₃, but for all the films', micro-Raman spectra detected bands related to WO₃ monoclinic phase. Vibrational and surface studies reveal the amorphous/semi-crystalline behavior of the 10 wt% Eu³⁺-doped sample. Valence state determination shows the trivalent state of Eu ions in doped films. In the 400-900 nm regions, the fabricated thin films show an average optical transparency of ~51-85%. Moreover, the band gap energy gradually reduces from 2.95 to 2.49 eV, with an enhancement of the Eu³⁺-doping content. The doped films, except the one at a higher doping concentration (10 wt%), show unique emissions of Eu³⁺ ions, besides the band edge emission of WO₃. With an enhancement of the Eu³⁺ content, the concentration quenching process of the Eu³⁺ ions' emission intensities is visible. The variation in CIE chromaticity coordinates suggest that the overall emission color can be altered from blue to bluish red by changing the Eu³⁺ ion concentration.