Preparation of core–shell structured NaYF 4 :Yb 3+ /Tm 3+ @NaYF 4 :Yb 3+ /Er 3+ nanoparticles with high sensitivity, low resolution and good reliability and application of their fluorescence temperature properties

A series of NaYF 4 :Yb 3+ /Tm 3+ @NaYF 4 :Yb 3+ /Er 3+ nanoparticles doped with Tm 3+ and Er 3+ were successfully prepared by the solvothermal method. Under 980 nm laser excitation, intense upconversion emission peaks of Tm 3+ and Er 3+ were observed for all samples. By doping Tm 3+ and Er 3+ with core–shell partitioning, not only a significant increase in fluorescence intensity could be achieved, but also simultaneous temperature measurements on multiple thermocouple energy levels could be realised. In addition, the temperature sensing performance of different thermocouple energy levels was also investigated, and it was found that the 3 F 3 → 3 H 6 and 1 G 4 → 3 F 4 thermocouple energy level pairs of Tm 3+ were the best, with maximum absolute sensitivity and maximum relative sensitivity of up to 0.0250 K −1 and 2.155% K −1 respectively, higher than the sensitivity of other thermocouple energy levels. It has a temperature resolution of less than 0.0139 K, which is lower than that of most materials available today. By using this material as a probe to build a fiber optic temperature sensor platform, it was found to have reliable temperature measurement performance.