Abstract
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+ D15 and D05 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 D15-D05 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.
Highlights
Luminescence thermometry [1,2,3,4] is a powerful tool for remote temperature sensing
Emission from the thermally coupled 5D0 and 5D1 states is measured and analyzed for a low-dopant concentration (0.4% Eu3+) by using models for the temperature dependence of multiphonon relaxation (MPR) processes
The present study provides fundamental insight into thermal equilibration processes in the promising class of Lndoped temperature sensors
Summary
Luminescence thermometry [1,2,3,4] is a powerful tool for remote temperature sensing. Emission from the thermally coupled 5D0 and 5D1 states is measured and analyzed for a low-dopant concentration (0.4% Eu3+) by using models for the temperature dependence of multiphonon relaxation (MPR) processes. The time dependence of the luminescence intensity for emission from both the 5D0 and 5D1 states can be modeled as a function of temperature and provide information on the temperature dependence of the relaxation rates between the 5D0 and 5D1 levels. We analyze both the temperaturedependent excited-state dynamics and intensity ratio based on Eqs. Results for the low Eu3+ concentration (0.4%) are considered to obtain information on the parameters for isolated Eu3+ ions and the role of additional relaxation processes by cross-relaxation processes at higher Eu3+ concentrations is discussed
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