A facile methodology for regulating the size of hexagonal NaYF₄:Yb³⁺,Er³⁺ upconversion nanocrystals

Hexagonal phase (β-) NaYF₄:Yb³⁺,Er³⁺ upconversion nanocrystals have attracted great attention due to their potential applications in a variety of fields, such as biomedical imaging, labeling, and photodynamic therapy. However, the precise adjustment of the particle size of pure β-NaYF₄:Yb³⁺,Er³⁺ nanocrystals (without doping), especially below 20 nm, is still a great challenge, which is critical for their biomedical utilization. In this paper, a facile methodology for regulating the size of β-NaYF₄:Yb³⁺,Er³⁺ upconversion nanoparticles (UCNPs) is demonstrated by varying the relative ratio of oleic acid to rare-earth acetates. The roles of oleic acid and 1-octadecene in the synthesis of UCNPs are studied in detail and their internal mechanism for the nucleation and growth of the hexagonal UCNPs is also proposed.