Highly Sensitive Optical Temperature Sensing Based on FIR of CaWO₄: Sm³⁺ Nanophosphor

Abstract In this study, the hydrothermal method is used to prepare CaWO₄: 0.5 mol% Sm³⁺ phosphors with different particle sizes using Na₃Cit (Na₃C₆H₅O₇) as a chelating agent. The experimental results show that when the sodium citrate content increases, particle size decreases. The temperature‐dependent emission spectra of different samples are measured and found that the WO₄²⁻ luminescence exhibits the conventional thermal quenching trend, whereas the luminescence intensity of Sm³⁺ gradually enhances as the temperature increases. The anomalous thermal enhancement phenomenon of Sm³⁺ luminescence arises from the increased energy transfer effect from WO₄²⁻ to Sm³⁺ when the temperature increases. Since the luminescence thermal quenching trends of WO₄²⁻ and Sm³⁺ are significantly different, the temperature can be characterized by the fluorescence intensity ratio (FIR) between WO₄²⁻ and Sm³⁺. Through the comparison of different samples, it is found that the relative sensitivity (S_r) value increases as the particle size decreases. The maximum S_r values for phosphors with particle sizes of 4.4 µm, 1.3 µm, and 63 nm all occur at 303 K, with 5.86% K⁻¹, 8.05% K⁻¹, and 9.26% K⁻¹, respectively. In summary, the CaWO₄: 0.5 mol% Sm³⁺ nanophosphor have a high relative sensitivity, suggesting their promising application in temperature sensing.