Formation and Luminescence of Sodium Rare Earth Fluoride Nanocrystals in the Presence of Chelators

The single crystal structure of sodium yttrium fluoride (NaYF₄) nanoparticles formed mainly by aggregation in aqueous phase was first confirmed by high resolution transmission electron microscope (HRTEM). Then, effects of reactant concentrations and pH on the formation of NaYF₄ nanocrystals were investigated and described in the presence of sodium citrate (Na-citrate). The results showed that the non-aggregation formation of NaYF₄ was dominant under a low concentration range of reactants, and the aggregation formation was dominant under a high concentration range of reactants. For comparison, NaYF₄ nanocrystals were also synthesized by co-precipitation in the presence of various other chelators, not only amino-carboxyl (H₂N-C) types involving (Na-EDTA) and sodium nitrilotriacetate (Na-NTA) but also hydroxyl-carboxyl (HO-C) types including sodium citrate (Na-citrate), sodium malate (Na-malate), and potassium sodium tartrate (K, Na-tartate). All the chelators showed a similar influence on the particle formation. However, the size of the final product was different corresponding to the different chelators. An upconversion fluorescent material, cubic NaYF₄ nanocrystal co-doped with ytterbium and erbium with diameter about 10 nm was successfully achieved at room temperature. Under 980 nm laser excitation, the red emissions at 652 and 679 nm were much stronger than green emissions at 520 and 541 nm. Additionally, effects of concentrations of the different chelators on the fluorescent properties of the formed NaYF₄:Yb, Er nanocrystals were also investigated. Different chelators affected the fluorescent properties differently, according to their respective physical and chemical properties.