Thermal-Sensitive Luminescence Dynamics of NaNdF4:Yb@CaF2 Nanostructures as Nanothermometers

Luminescence nanothermometry is arousing wide interestdue to itsnoninvasive, real-time, and nanometrically spatially precise potentials.The peculiar luminescence properties of rare-earth-doped nanomaterials,such as their superstability and long lifetime, demonstrate theirnecessity in high-accuracy thermal sensing. Among the rare-earth nanothermometers,the recently emerged energy-transfer-based nanothermometers (e.g.,NaNdF4:Yb@CaF2 nanocrystals) provide a crediblelifetime signal with high sensitivity. However, the rationale forthis property remains unexplored. The unclear rationale limits thesystematic and targeted optimization of energy-transfer-based nanothermometers.Here, we reveal the working principle of energy-transfer-based NaNdF4:Yb@CaF2 nanothermometers with the classical rateequation model and experimental verifications. Dominated by the proportionbetween the energy transfer and back transfer rates of Nd3+ and Yb3+, the F-2(5/2)(Yb3+) population decays mono-exponentially after 50 mu s of the withdrawalof excitation. This is the prerequisite for the F-2(5/2)(Yb3+) lifetime to be used as an accurate interference-freedetection signal. The rate equation model is also used to investigatethe concentration dependence of the thermal sensitivity of NaNdF4:Yb@CaF2 nanocrystals. The thermal sensitivitygets better with a declining Yb3+ concentration. Theseinsights into thermal-sensitive luminescence dynamics pave the wayfor further material optimization toward nanothermometers with betterperformance.