Enhancing upconversion via constructing local energy clusters in lanthanide-doped fluoride nanoparticles

Upconversion (UC) nanoparticles (NPs) have gained significant attention in many fields, such as super resolution nanoscopy and background-free bioimaging. UC efficiency is one of the most important parameters in these practical applications. However, enhancing the UC efficiency of core-only NPs synthesized in a single step, without the need for additional post-processing, has remained a great challenge. In this study, we propose an effective strategy for significantly enhancing the UC intensity by constructing local energy clusters within the lanthanide-doped fluoride NPs. By introducing local energy clusters within the NaGdF₄ crystal lattice, the optimal Er³⁺, Ho³⁺ and Tm³⁺ doping concentrations were increased to 8 mol%, 8 mol%, and 2 mol%, respectively. The UC intensity of the 20Yb/8Er/20Ca:NaGdF₄ NPs is approximately 54.3 times higher than that of typical 20Yb/2Er:NaGdF₄ NPs, which is attributed to the increased energy transfer efficiency from Yb³⁺ to Er³⁺ and the reduced energy loss caused by surface vibrations. Moreover, with the formation of energy clusters within the crystal lattice, the temperature-dependent UC intensity behavior shifted from a negative thermal quenching effect to a positive thermal quenching effect, which results in enhanced temperature sensing performance. The violet UC from Tm³⁺ activators was significantly enhanced through the energy clusters, which facilitates the noticeable inter-particle energy migration UC of Tb³⁺ ions. Overall, our study presents a novel approach to significantly enhance the optimal doping concentrations of lanthanide activators and the UC intensities of NPs. These findings open up exciting opportunities for the development of high-performance UCNPs with improved applications in various fields.