Effects of optically inert ions on up-conversion luminescence and temperature-sensing properties of Y2O2S: Er3+/Yb3+/Tm3+ phosphors

The quenching of luminescence in Er3+/Yb3+ co-doped with Tm3+ presents a dilemma as it substantially diminishes thermometric performance. In this study, Y2O2S: Er3+/Yb3+/Tm3+ phosphors were synthesized using a homogeneous precipitation method combined with a sulfurization process. In addition, three sets of fluorescence intensity ratio (FIR) models were designed, which have self-calibration function. The effects of incorporation of Li+ or Li+/Ba2+ on the up-conversion luminescence (UCL) intensity and thermometric performance of the phosphors were systematically studied. The results show that the introduction of Li+ or Li+/Ba2+ co-doping leads to lattice distortion and the formation of oxygen ion vacancies in the lattice, which further accelerates the energy transfer process and mixes the charge transfer states, and ultimately dramatically improves the UCL and thermometric performance. The introduction of Li+ and Li+/Ba2+ co-doping enhanced the UCL of the materials by about 3.7 times and 5.6 times compared to Y2O2S: Er3+/Yb3+/Tm3+. When based on nonthermally coupled levels (NTCLs), the Sr-max of Li+/Ba2+ co-doped sample is 0.080 K−1 at 573 K, which is 87 % higher than that based on TCLs. These findings not only provide a new avenue for improving the UCL and thermometric performance but also provide an accurate self-calibration FIR model over a wide temperature range.