Zhenlong Cheng,Mingzhou Meng,Jiaoyu Wang,Zhuoyue Li,Jiao He,Hao Liang,Xin Qiao,Yuanli Liu,Jun Ou
Non-contact optical temperature sensors are highly sought after by researchers due to their satisfactory temperature resolution (δ(T) < 0.1 °C), high relative thermal sensitivity (Sr > 1% °C−1), fast temporal response (t < 0.1 s), and long-term optical stability. In this study, NaYF4:Yb3+/Ho3+/Tm3+ upconversion nanoparticles were prepared by a solvothermal method, and their crystal structure, microscopic morphology, and luminescence mechanism, together with the temperature sensing properties of the specimens, were investigated. Under 980 nm laser excitation, the specimens exhibited strong upconversion luminescence, and the emission peaks corresponded to the characteristic energy level jumps of Ho3+ and Tm3+, respectively. The temperature-dependent luminescence spectra of the samples were investigated based on the fluorescence intensity ratio (FIR) technique over a temperature gradient of 295–495 K. The samples are based on thermally coupled energy levels (TCLs: 1G4(1,2) → 3H6(Tm3+)) and non-thermally coupled energy levels (NTCLs: 3F3 → 3H6(Tm3+) and 5F3 → 5I8(Ho3+), 3F3 → 3H6(Tm3+) and 1G4 → 3H6(Tm3+), 3F3 → 3H6(Tm3+) and 5F5 → 5I8(Ho3+), 3F3 → 3H6(Tm3+) and 5F4 → 5I8(Ho3+)) for temperature sensing performance. The maximum absolute sensitivity (Sa), relative sensitivity (Sr), and minimum temperature resolution δ(T) were found to be 0.0126 K−1 (495 K), 1.7966% K−1 (345 K), and 0.0167 K, respectively, which are better than those of most sensing materials, and the simultaneous action of multiple coupling energy levels can further improve the temperature precision. This study indicates that the sample has a good value for optical temperature measurement and also provides new ideas for the exploration of other high-quality optical temperature sensing materials.
Upconversion (UC) nanomaterials are of interest to researchers because of their excellent photostability, effective quantum efficiency and optical temperature dependence. However, most UC nanomaterials are thermally quenched, which causes optical nanothermometers to be susceptible to their own thermal effects, reducing the accuracy of temperature measurements. In this paper, NaYF 4 :Yb 3+ /Ho 3+ /Tm 3+ /Gd 3+ nanorods with UC luminescence and resistance to thermal quenching are prepared through a simple hydrothermal method. The UC luminescence intensity of NaYF 4 :Yb 3+ /Ho 3+ /Tm 3+ /Gd 3+ nanorods is adjusted by doping with Tm 3+ ions. The luminescence intensity at each location shows different temperature dependence as the temperature increases. When the temperature reaches 573 K, the integrated luminescence intensity is three times that at 293 K. At the same time, the intensity at all temperature points in the range of 293–573 K is greater than the intensity at 293 K, which shows that the nanorods have excellent resistance to thermal quenching. In addition, the relative sensitivity ( S r ) of LIR( I 696 / I 476 ), LIR( I 696 / I 541 ), LIR( I 696 /I 646 ), LIR( I 696 / I 802 ), and LIR( I 541 / I 476 ) is investigated based on the luminescence intensity ratio (LIR) technique. The S r of LIR( I 696 / I 802 ) is found to have larger values in the low-temperature range, while S r of LIR( I 696 / I 476 ) has larger values in the high-temperature range. The use of LIR( I 696 / I 802 ) and LIR( I 696 / I 476 ) in separate temperature bands allows S r to reach 0.93–1.67 %K −1 in the range of 293–573 K, confirming that the nanorods have high S r over a wide temperature interval. This study indicates that the NaYF 4 :Yb 3+ /Ho 3+ /Tm 3+ /Gd 3+ nanorods are promising candidates for optical thermometers with high sensitivity, wide temperature range, and resistance to thermal quenching.
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