Cubic-NaYF4/ transition interface layers/ hexagonal-NaLuF4 slow-phase-transition nanoparticles: Growth mechanism and luminescence enhancement

Lanthanide-doped sodium rare-earth tetrafluoride (NaREF 4 :Ln) nanocrystals with small sizes (below 20 nm) have shown great potential in biomedical applications, but their low luminescence efficiency has limited practical use. Here, we report the synthesis and characterization of small-sized α-NaYF 4 /transition interface layers (TIL)/β-NaLuF 4 :Yb,Er slow-phase-transition nanoparticles (SPTNPs) using a combined cation exchange and epitaxial growth strategy. These SPTNPs feature a continuous lattice evolution from α-phase core to β-phase shell, enhancing upconversion luminescence (UCL) intensity by 23.57 times compared to traditional UCNPs, with violet emission enhanced by up to 70.7 times. The absolute quantum yields reached 2.36 % for SPTNPs and 3.44 % for SPTNPs with an inert shell. We verified that the phase transition was driven by strain energy accumulation within the TIL, and demonstrated that the crystallinity of the α-core significantly impacted the phase transition of the shell. Eu³ ⁺ ions were used as structural probes to investigate the lattice environment evolution during SPTNPs growth, and the intermixing between the core and shell of the SPTNPs was further quantified. This work provides new insights into designing rare-earth-doped nanomaterials for biomedical and sensing applications.