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

) 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.