The Size Effects of Lanthanide Upconversion Nanoparticles

ABSTRACT Particle size plays a pivotal role in defining the optical performance and pharmacokinetic behaviors of lanthanide‐doped upconversion nanoparticles (UCNPs). Yet, the fundamental mechanisms underlying their size‐dependent upconversion luminescence remain elusive and controversial, with unresolved debates on the hierarchical contribution of phonon relaxation, surface quenching, and energy transfer across different size regimes. This review consolidates the evolution of mechanistic understanding—from early phonon relaxation models to the current framework that integrates size‐dependent surface quenching with lanthanide energy transfer processes, and deepens the analysis of conflicting experimental observations and scale‐dependent mechanistic transitions within this integrated model. This refined perspective achieves precise quantitative agreement between theoretical predictions and experimental data, while also identifying critical gaps in explaining extreme nanoscale behavior (