Atomic deciphering of cation exchange mechanism in upconversion nanoparticles

Transition metal ion doping in upconversion nanoparticles (UCNPs) provides an effective way to enhance the luminescence for their wide array of applications. However, the doping sites of transition metal ions have not been comprehensively explored, and commonly assumed that transition metal ions replace the trivalent lanthanides within the lattice. Here we report that cation exchange of transition metal (Mn²⁺) in β-NaYF₄:Yb³⁺/Er³⁺ UCNPs occurs through alkaline metal (Na⁺) replacement, via 2Na⁺ ↔ Mn²⁺ + Vacancy reaction. This process distorts the LnF₉ polyhedrons and tailors the surrounding environment around the trivalent lanthanides, thereby improving the upconversion intensity from active lanthanides. Further confirmed by core-shell design and spectroscopic study, we prove that the transition–alkaline metal exchange enables both the emission enhancement and transition probability variation of activators.