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

Luminescence nanothermometry is arousing wide interest due to its noninvasive, real-time, and nanometrically spatially precise potentials. The peculiar luminescence properties of rare-earth-doped nanomaterials, such as their superstability and long lifetime, demonstrate their necessity 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 credible lifetime signal with high sensitivity. However, the rationale for this property remains unexplored. The unclear rationale limits the systematic 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 rate equation model and experimental verifications. Dominated by the proportion between the energy transfer and back transfer rates of Nd3+ and Yb3+, the 2F5/2(Yb3+) population decays mono-exponentially after 50 μs of the withdrawal of excitation. This is the prerequisite for the 2F5/2(Yb3+) lifetime to be used as an accurate interference-free detection signal. The rate equation model is also used to investigate the concentration dependence of the thermal sensitivity of NaNdF4:Yb@CaF2 nanocrystals. The thermal sensitivity gets better with a declining Yb3+ concentration. These insights into thermal-sensitive luminescence dynamics pave the way for further material optimization toward nanothermometers with better performance.