Effect of annealing temperature on structural and optical properties of WO3:Yb3+, Er3+ upconversion phosphors by high-energy wet ball milling

Yb³⁺ and Er³⁺-co-doped WO₃ upconversion phosphors (WO₃:Yb³⁺, Er³⁺ UCPs) were successfully synthesized using a high-energy wet ball milling method. The structural and optical properties of the phosphors were systematically examined at various annealing temperatures. X-ray diffraction (XRD) patterns confirmed that all samples corresponded to the monoclinic WO₃ phase without any secondary phases. Both the XRD and Raman peak intensities increased as the annealing temperature increased, indicating enhanced crystallinity. Similarly, photoluminescence (PL) emission intensity was positively correlated with annealing temperature. Under 980 nm excitation, PL spectra exhibited strong green emissions at 522 and 546 nm, and a weaker red emission at 654 nm, corresponding to the Er³⁺ transitions of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2, respectively. Notably, at 522 nm, the intensity of PL spectra for the sample annealed at 1000 ℃ was approximately five times that of the sample annealed at 400 ℃.