Enhanced thermal stability and charge transfer band redshift induced ratiometric thermometry of Sr2Al2SiO7:Sm3+/K+ phosphors

Abstract

In this study, Sr₂Al₂SiO₇: Sm³⁺ and Sr₂Al₂SiO₇: Sm³⁺/K⁺ phosphors were synthesized via a high-temperature processing for remote temperature sensing applications. X-ray diffraction (XRD) analysis confirmed the tetragonal gehlenite structure of Sr₂Al₂SiO₇ with space group P$\bar{4}$2 ₁m and the presence of impurity phases, which also confirmed the effective replacement of Sr²⁺ by Sm³⁺ in the crystal structure. Scanning electron microscopy (SEM) observations showed irregular grain morphology and severe particle agglomeration. The particle size distribution of the sample ranges from 5 to 95 μm, with the particles mainly concentrated around 22.54 μm. EDS mappings verified the presence of Sm³⁺ and K⁺ ions within the crystal structure. UV-vis diffuse reflection spectra and bandgap calculations indicated a reduced bandgap due to doping. PL and PLE spectra demonstrated concentration-dependent characteristics, with optimal concentrations of 0.03 for Sm³⁺ and 0.04 for K⁺. Temperature-dependent PL and PLE measurements revealed enhanced thermal stability and temperature sensitivity upon K⁺ doping, with thermal stability improving from 86.015% to 90.86% at 423 K. The observed redshift in PLE spectra with increasing temperature was explained. Furthermore, the temperature-dependent redshift of CTB and the ⁶H_5/2→⁴F_7/2 transition were then effectively utilized for thermometry. The relative sensitivity reached 0.472% K^-1 at 473 K, showing competitive performance compared to other phosphors despite a higher working temperature. A single band ratio (SBR) thermometry based on LIR of the peak at 601 nm was constructed using two excitation wavelengths (250 nm and 404 nm). The maximum absolute sensitivity was 1.86×10^-3 K^-1 at 573 K, and the relative sensitivity was 0.558% K^-1 at 462 K. These results highlight the potential of Sr₂Al₂SiO₇: Sm³⁺/K⁺ for high-temperature optical thermometry applications.