Effect of Li co-doping with Er on up-conversion luminescence property and its temperature dependence of NaY(WO4)2

Abstract

NaY(WO4)2 phosphors with different Er3+ doping and Er3+/Li+ co-doping concentrations were prepared via high temperature solid state reaction. X-ray diffraction measurement demonstrated the presence of tetragonal-phase NaY(WO4)2, and no impurity phase was found in Er3+/Li+ co-doped samples. Well-crystallized NaY(WO4)2 phosphors showed a fine morphology with particle sizes of 1–6 μm determined by scanning electron microscope. Under excitation at 980 nm, the origins of three emission peaks located at 527 nm (green emission 1), 549 nm (green emission 2) and 665 nm (red emission), respectively, were identified. By introducing Li+, the up-converted (UC) emission intensity was enhanced by 0.5 times and 2 times in green and red emission region, respectively. The enhanced luminescence was attributed to the distortion of the local symmetry around Er3+ due to Li+ incorporation in the lattices. The 2H11/2 → 4I15/2 (green emission 1) and 4S3/2 → 4I15/2 (green emission 2) transitions of the Er3+ ion presented a temperature dependent behavior from 300 to 30 K and were proposed for temperature sensing (optical thermometry) by using the fluorescence intensity ratio (FIR) method. The FIR data obtained by experiment was well fitted with a theoretical function. A higher maximum value of sensitivity (0.0061 K-1) was obtained in 5% Er3+, 0% Li+ doped sample at 300 K. However, due to the distortion of local crystal field around Er3+ ions caused by the introduction of Li+, the sensitivity value of 5% Er3+ and 1% Li+ co-doped sample was higher than that of Li+ free sample below 270 K, which indicated that appropriate Li+ co-doping could optimize the temperature sensing behaviors of Er3+ doped NaY(WO4)2 phosphors.