| Summary: | Accurate and reliable non-contact temperature sensors are imperative for industrial production and scientific research. Here, Er<sup>3+</sup>/Tm<sup>3+</sup>/Yb<sup>3+</sup> co-doped NaYF<sub>4</sub> phosphors were studied as an optical thermometry material. The typical hydrothermal method was used to synthesize hexagonal Er<sup>3+</sup>/Tm<sup>3+</sup>/Yb<sup>3+</sup> co-doped NaYF<sub>4</sub> phosphors and the morphology was approximately rod-like. The up-conversion emissions of the samples were located at 475, 520, 550, 650, 692 and 800 nm. Thermo-responsive emissions from the samples were monitored to evaluate the relative sensing sensitivity. The thermal coupled energy level- and non-thermal coupled energy level-based luminescence intensity ratio thermometry of the sample demonstrated that these two methods can be used to test temperature. Two green emissions (520 and 550 nm), radiated from <sup>2</sup>H<sub>11/2</sub>/<sup>4</sup>S<sub>3/2</sub> levels, were monitored, and the maximum relative sensing sensitivities reached to 0.013 K<sup>−1</sup> at 297 K. The emissions located in the first biological window (650, 692 and 800 nm) were monitored and the maximum relative sensing sensitivities reached to 0.027 (<i>R</i><sub>692/650</sub>) and 0.028 K<sup>−1</sup> (<i>R</i><sub>692/800</sub>) at 297 K, respectively. These results indicate that Er<sup>3+</sup>/Tm<sup>3+</sup>/Yb<sup>3+</sup> co-doped NaYF<sub>4</sub> phosphors have potential applications for temperature determination in the visible and the first biological window ranges.
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