Thermally coupled energy levels of rare-earth Er-based nanomaterials for high-precision and reliable temperature sensing

Traditional optical temperature sensors rely on strong fluorescence emission materials, high-homogeneity nanoparticles, and individual temperature calibration to ensure measurement accuracy and reliability. However, these requirements inevitably introduce disadvantages such as significant time costs, extensive labor inputs, and challenges in material synthesis, which hinder large-scale applications. Consequently, this paper investigates the effects of size, homogeneity, metal doping, and core-shell structures of rare-earth nanoparticles on the thermally coupled energy levels ( 2 H 11/2 and 4 S 3/2 ) of Er-based upconversion nanoparticles (UCNPs). The nanoparticles NaLuF 4 :Yb,Er, NaLuF 4 :Yb,Er,Li, NaLuF 4 :Yb,Er,Mg and NaLuF 4 :Yb,Er@NaLuF 4 :Yb,Tm@NaLuF 4 were prepared using a thermal decomposition method. The morphology and luminescence properties of the four UCNPs were characterized through transmission electron microscopy and fluorescence spectrophotometry. Additionally, variable-temperature luminescence intensity measurements of the four UCNPs were conducted under 980   nm laser excitation. The experimental results demonstrate a strong linear relationship between temperature and the luminescence intensity ratios (LIRs) of the thermally coupled energy levels of Er-based UCNPs with varying shapes, sizes, core-shell structures, and luminescence intensities. Notably, the fitted straight lines relating LIR for different Er-based UCNPs to temperature exhibit approximately equal slopes (sensitivity). This characteristic allows for the calibration of thermometers using only a single temperature point, significantly simplifying the calibration process. Furthermore, the synthesis of the materials is streamlined, enabling mass production without the need to consider the homogeneity and symmetry of the material. Therefore, thermometers based on rare-earth Er-based UCNPs hold significant potential for large-scale production and application.