Energy-Transfer Pathways in Nd-Sensitized Er-Rich Nanostructures: The Competition between Cross-Relaxation and Phonon-Assisted Sensitization

Abstract Rare earth-doped upconversion nanomaterials, which convert near-infrared light into visible or ultraviolet light, are widely recognized as having a huge potential application market, which unfortunately is restricted by their obstinate insufficient luminescence efficiency. Very recently, combining the strategies of phonon regulation and rare-earth high-level doping, nearly two orders of magnitude upconversion luminescence enhancement have been reported from the Er-based cryogenic materials. However, the inherent weak absorption of Er3+ at 808 nm remains a barrier. On the other hand, Nd sensitization has been widely used to extend excitation to around 808 nm, but its sensitization effect at cryogenic temperatures remains largely unexplored. In this work, a well-defined Nd-sensitized strategy (i.e., employing NaErF4@NaYbF4@NaYF4:30%Nd) that allows 808 nm absorption in both core and outer layer was developed to break through this limitation. By combining Nd3+ sensitization with cryogenic regulation, the optimized Er@Yb@Nd nanostructure achieves a 28.5-fold enhancement at room temperature and an overall 342-fold enhancement at 40 K. The observed temperature response is governed by the competition among the suppression of Er−Er cross-relaxation, the restriction of Nd → Yb transfer, and Er−Yb energy exchange. Importantly, temperature-dependent spectroscopy further reveals that the Nd-to-Yb energy transfer is strongly phonon assisted, making the sensitization pathway increasingly restricted at low temperatures. This finding uncovers the competition between cross-relaxation and phonon-assisted sensitization in Nd-sensitized Er-rich nanostructures and provides a mechanistic framework for the development of high-performance, highly doped luminescent materials.