Population Control of Thermally Coupled Energy Levels via Energy Transfer in CaWO4:Dy/Er Phosphors for High-Sensitivity Ratiometric Thermometer and Multicolor Anticounterfeiting

The population of thermally coupled energy levels (TCLs) of lanthanide ions (Ln3+) has a significant impact on their photoluminescence (PL) properties, which are influenced by multiple physical mechanisms, including temperature, crystal field environment, ion concentration, and external excitation conditions, among others. The switchable multicolor emissions of Dy3+ and Er3+ codoped CaWO4 (CaWO4:Dy/Er) phosphors were obtained by varying the doping concentration and excitation wavelength. The excitation wavelength-dependent PL intensities from TCL transitions of Dy3+ and Er3+, combined with the corresponding fluorescence dynamics of TCLs, confirmed the significant energy transfer (ET) from Dy3+ to Er3+. Temperature-dependent PL studies demonstrated a substantial discrepancy between the theoretical and calculated energy differences (ΔE) of two TCLs, further verifying the influence of the ET process on the Boltzmann population distribution of TCLs. By leveraging the significantly enhanced calculated ΔE of TCLs in Dy3+ and Er3+, the phosphors exhibited an excellent TCL-based ratiometric thermometric performance with high absolute and relative temperature sensitivities. Additionally, the tunable PL color under different excitation wavelengths highlighted their applicability in anticounterfeiting and information encoding applications. This work advances the understanding of the factors that affect the TCL population for optimizing material design and offers novel insights for designing multifunctional luminescent materials for thermometry and secure information encoding.