Y₂Mo₄O₁₅:Er³⁺/Tm³⁺/Yb³⁺ Nanophosphors for High-Sensitivity Optical Temperature Sensing

Noncontact optical nanothermometers are increasingly recognized for their high temperature resolution (δT), excellent relative thermal sensitivity (S_r > 1% K^-1), rapid response times (t < 0.1 s), and robust long-term optical stability. In this study, upconversion nanoparticles (UCNPs) based on Y₂Mo₄O₁₅ nanophosphors, codoped with 2% Er³⁺, 1% Tm³⁺, and x% Yb³⁺ (x = 5, 10, 15 and 20%), were synthesized using the sol-gel method. The crystal structure, morphology, luminescence mechanisms, and temperature-sensing capabilities of these nanoparticles were systematically characterized. Under 975 nm laser excitation, the UCNPs exhibited intense upconversion luminescence, with emission peaks corresponding to well-defined energy-level transitions of Er³⁺ and Tm³⁺ ions. Temperature-dependent luminescence spectra were measured over the 300-520 K range using the fluorescence intensity ratio technique. The material exhibits both thermally coupled levels (TCLs) and nonthermally coupled levels, resulting from intraionic and interionic transitions involving Er³⁺-Er³⁺, Tm³⁺-Tm³⁺, Er³⁺-Tm³⁺, and Tm³⁺-Er³⁺ interactions. This complex energy-transfer network significantly enhances the temperature-sensing performance. Among the investigated transitions, the intensity ratio I₇₀₀/I₈₀₆ derived from the TCL approach showed the highest relative sensitivity, reaching S_r = 2.18% K^-1 at 300 K. Additionally, the system achieved a minimum temperature uncertainty of δT = 0.26 K. These findings highlight the superior thermometric performance of the synthesized nanophosphors and underscore their potential for optimization through the synergistic interplay of multiple luminescent centers. This work validates the applicability of these nanomaterials for advanced optical nanothermometry and provides a foundation for developing next-generation temperature nanosensors.