Revisiting the optimized doping ratio in core/shell nanostructured upconversion particles

Abstract

The development of rare-earth doped upconversion nanoparticles (RE-UCNPs) in various applications is fuelling the demand for nanoparticles with highly enhanced upconversion luminescence (UCL). Although the core/shell structure is proved to enhance the UCL effectively, there is still plenty of room to further improve the UCL by optimizing the doping ratio of the materials. In this article, a general strategy is demonstrated to achieve highly-enhanced visible UCL in core/shell nanostructured NaREF₄ by increasing the doping ratio of Yb³⁺ in the core region. The energy transfer from RE-UCNPs to surface quenching sites through Yb³⁺–Yb³⁺ energy migration is demonstrated to be the main reason for restricting the doping ratio of Yb³⁺. Notable UCL enhancement (ca. 15 times) of core/shell structured α-NaYF₄:Yb,Er@CaF₂ nanoparticles is observed by increasing the concentration of Yb³⁺ to 98 mol%. The highly-enhanced visible UCL signal is used to guide the lymphatic vessel resection with the naked eye.