A Simple Strategy for the Controlled Synthesis of Ultrasmall Hexagonal‐Phase NaYF4:Yb,Er Upconversion Nanocrystals

Abstract

AbstractPrecise control over the size and crystallographic phase of nanocrystals allows the manipulation of their properties and applications as desired. Hexagonal‐phase NaYF4 (β‐NaYF4) upconversion nanocrystals with efficient luminescence are quite desirable for bio‐applications, but synthetic control over the nanocrystallite size remains challenging. Herein we verify experimentally that control of the hexagonal‐phase nucleation is crucial for controlling the final particle size and the number of hexagonal‐phase seeds strongly depends on the actual F(Na):Y ratio. A negligible number of hexagonal‐phase seeds were formed at low ratio whereas the high ratio yields a very large number of seeds. We also identify that the actual F(Na):Y ratio is more likely related to the different methods employed for the addition of NH4F and NaOH rather than its initial feeding ratio. By taking advantage of this dependence and varying the reaction conditions (temperature and time), β‐NaYF4:Yb,Er nanocrystals with a size smaller than 6 nm can be synthesized. Theoretical modeling shows a good agreement with the experimental results.