Influence of Er³⁺ doping concentration and temperature on upconversion photoluminescence property of NaY(WO₄)₂ phosphor

NaY(WO₄)₂: Er³⁺ phosphors with various doping concentrations of [Er³⁺]/([Er³⁺]+[Y³⁺]) (0, 5, 7, 10, 12, 15%) were successfully synthesized by high-temperature solid-state reaction. X-ray diffraction (XRD) measurement demonstrated the presence of tetragonal-phase NaY(WO₄)₂ phosphors and Fourier transform infrared (FT-IR) spectra showed a strong υ₃ stretching vibration mode of WO₄^2-, indicating the good crystallization of the phosphors. Well-crystallized phosphors showed a fine morphology with particle sizes of 0.8-10 μm by scanning electron microscope (SEM). Under excitation of 980 nm laser, Er³⁺-doped phosphors exhibited strong green emissions centered at 527 and 549 nm in addition to a 665 nm emission band in the red region, which were assigned to the ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. When the doping concentration was 10%, the intensities of green and red emissions reached their maximum values, stronger than that in 5% Er³⁺ doped one by 13 times. The double-logarithm plot of the upconversion (UC) emission intensity to the excitation power for different Er³⁺ doping concentrations depicted that at least two infrared photons were involved for every emitted green and red photon. UC emission intensity changing with temperature displayed different trends for the phosphors with different Er³⁺ concentrations. The enhancement for green emission band centered on 549 nm could reach 50 times at 30 K in sample with 5% Er³⁺ concentration. Under excitation by 355 nm laser, all the phosphors showed a strong PL emission band at 430 nm, while green luminescence band locating from 540 to 570 nm was only observed in 7, 10, and 12% Er³⁺-doped phosphors, which was ascribed to ⁴S_3/2 → ⁴I_15/2 transition. (orig.)