Unravelling Size-Dependent Upconversion Luminescence in Ytterbium and Erbium Codoped NaYF₄ Nanocrystals

The size of the lanthanide upconversion nanocrystals significantly impacts their luminescence properties, yet the underlying mechanisms remain unclear. In this work, we undertake a systematic examination of the size effects in the commonly studied hexagonal phase sodium yttrium fluoride (β-NaYF₄) nanocrystals codoped with ytterbium and erbium ions and their core-shell structure. We demonstrate the coexistence of surface quenching and finite-size-dependent energy transfer mechanisms, quantify the effects of size-dependent surface quenching and finite-size-dependent energy transfer, and determine an interaction energy transfer distance limit of ∼8.8 nm. A proposed theoretical model for the interplay between the two underlying mechanisms is shown to predict the experimental observations of size-dependent upconversion luminescence. Our findings provide a clear and fundamental understanding of the size effects on lanthanide upconversion luminescence at the nanoscale, thereby giving important implications for a variety of applications ranging from bioimaging and nanothermometry.