Identification of surface defects and in situ lattice reconstruction of upconversion nanoparticles

Lanthanide doped upconversion nanoparticles have attracted widespread attention due to their unique and efficient anti-Stokes emission. However, the large specific surface area and high surface quenching rates pose significant challenges in achieving small upconversion nanoparticles with strong emission intensity. Herein, we identify the surface defects that disrupt the crystal lattice periodicity as lanthanide cation vacancies and propose an effective localized lattice reconstruction strategy to block undesired energy transfer from excited states to surface quenching sites in LiYF₄:Yb,Tm upconversion nanosystems. The improvement in upconversion performance is verified at the single nanoparticle level, eliminating the macroscopic statistical averaging inherent in ensemble measurements using solution- or powder-based systems. Notably, the emission intensity enhancement becomes more pronounced as nanoparticle size decreases. An ∼60-fold emission enhancement of the ¹G₄ → ³H₆ transition is achieved on 13.5 nm nanoparticles without increasing the particle size, which demonstrates the significance of suppressing surface quenching for small nanoparticles. This lanthanide ion-assisted post-annealing strategy for surface lattice reconstruction could promote the development of small but bright upconversion nanoparticles for advanced applications.