Aspect-Ratio Controlled Synthesis and Tunable Luminescence of YF₃:Yb³⁺/Er³⁺ Upconversion Nanocrystals

We report a facile synthetic method for high-aspect-ratio YF₃:Yb³⁺/Er³⁺ nanocrystals by controlling the composition of the precursor and salts to manipulate the structures of YF₃:Yb³⁺/Er³⁺ nanocrystals. A controlled aspect ratio is attained by tuning the molar ratio of NH₄⁺/RE³⁺ (NH₄⁺ is the counterion released from the fluoride precursor (NH₄F) and RE³⁺ indicates the total amount of Y, Yb, and Er) and the presence of specific inorganic salts. A low molar ratio of NH₄⁺/RE³⁺ induced the production of high-aspect-ratio YF₃:Yb³⁺/Er³⁺ nanocrystals, while a high molar ratio of NH₄⁺/RE³⁺ induced the formation of irregular nanoplates. The selection of the inorganic salt as an additive is critical in controlling the aspect ratio and crystallinity of YF₃:Yb³⁺/Er³⁺ nanocrystals. The cation showed limited control over the aspect ratio, but the anions, Cl^- and Br^-, induced the production of the highest aspect ratio for YF₃:Yb³⁺/Er³⁺ nanocrystals. We found that the aspect ratio affected the luminescence properties of YF₃:Yb³⁺/Er³⁺ nanocrystals. The green emission from nanorod structures changed to orange when the morphology is transformed to nanoplates.