Anion Additive‐Induced Size, Morphology, and Local Structure Tuning of Lanthanide‐Doped Upconversion Nanoparticles

Abstract Introducing additives during the preparation can effectively modulate characteristics of lanthanide‐doped upconversion nanoparticles (UCNPs) that are pivotal to applications of the nanoparticles, ranging from bioimaging to super‐resolution microscopy. Unfortunately, anion additive‐caused changes in the characteristics of lanthanide‐doped UCNPs are almost unexplored, even though anionic species are ubiquitous. Herein, Na(CCl₃COO), as a representative anion additive, is used to demonstrate that anion additives can tune characteristics of cubic NaYF₄:Yb/Er UCNPs, including size, morphology, and local structure. Experimental studies, together with theoretical calculations, reveal that Cl⁻ ions released from the anion additive can affect the growth kinetics and selectively cap on planes of crystals during the formation of the UCNPs. Consequently, tuning of characteristics can be achieved, which also enables the modulation of upconversion properties of resulting nanoparticles. Furthermore, the anion additive‐based modulation can facilitate the fabrication of ultrasmall UCNPs with bright luminescence and advance the upconversion‐based information encryption when other techniques are integrated. These results reveal that anionic species should be considered during the preparation of UCNPs, and these findings should shed light on new avenues to fabricate UCNPs with desired properties.