Non-Boltzmann Luminescence in NaYF4:Eu3+: Implications for Luminescence Thermometry

Luminescence (nano) thermometry is an important technique for remote temperature sensing. The recent development of lanthanide-doped nanoparticles with temperature-dependent emission has expanded the field of applications, especially for ratiometric methods relying on the temperature variation of relative emission intensities from thermally coupled energy levels. Analysis and calibration of the temperature dependence is based on a Boltzmann equilibrium for the coupled levels. To investigate the validity of this assumption, we analyze and model thermal equilibration for Eu3+ D-5(1) and D-5(0) emission in NaYF4. The results show that for low Eu3+ concentrations, temperature-dependent multiphonon relaxation can accurately explain both the intensity ratio and emission decay dynamics. The analysis also reveals that a Boltzmann equilibrium is not realized in the temperature regime investigated (300-900 K). By increasing the Eu3+ concentration, cross relaxation between neighboring Eu3+ ions enhances D-5(1)-D-5(0) relaxation rates and extends the temperature range in which emission intensity ratios can be used for temperature sensing (500-900+ K). The results obtained are important for recognizing, understanding, and controlling deviations from Boltzmann behavior in luminescence (nano) thermometry. By varying the dopant concentration, the range for accurate temperature sensing can be adjusted. These insights are crucial in the development and understanding of reliable temperature sensors.