Upconversion luminescence and temperature sensing of NaLuF4:Yb/Er/Tm microcrystals at low temperatures

Abstract

In this study, Yb/Er/Tm-doped NaLuF₄ luminescent microcrystals were synthesised hydrothermally. Then, the upconversion spectra of these microcrystals at low temperatures (5–303 K) were investigated, and their potential for temperature sensing assessed. As the temperature decreased, the intensity of the emission peak correspondingly increased. However, the emission peak at 520 nm was not readily discernible at temperatures below 160 K. The kinetic process of luminescence decay typically increased with decreasing temperature. Furthermore, the emission peak at 539 nm exhibited a pronounced sensitivity to variations in temperature. The system's temperature-sensing capabilities, which rely on both thermally and non-thermally coupled energy states, were evaluated in low- (160–280 K) and high-temperature (333–493 K) ranges. At the lower end of the temperature scale (160 K), the maximum value of relative sensitivity was 4.29% K⁻¹, which was 4–5 times higher than that at high temperatures. These findings demonstrate the significant potential of the material for temperature-sensing applications and introduce a novel concept for such uses.