Figure 2
(a) Schematic illustration of the formation and growth of UCNPs by the thermal decomposition method according to Refs.52,53. The Ln precursor thermally decompose at a higher temperature to initially yield small cubic α-phase particles, which are then transformed into larger thermodynamically favored hexagonal β-phase particles via a thermodynamically driven dissolution–recrystallization process. (b–e) Reproducibility, robustness, and up-scalability of the basic synthetic procedure that yields 20 nm β-Yb,Er-co-doped NaYF4 UCNPs. TEM images of the UCNPs obtained from (b) a reaction performed with an increased temperature (335 °C instead of 325 °C) and a prolongated reaction time (60 min instead of 30 min); (c) different batch sizes (1–25 mmol RE precursors) to assess the robustness of the synthesis, and (d) ten independent reactions performed under the same reaction condition (STD 1–STD 10) to explore the reproducibility of the synthesis. (e) Size distribution of the obtained UCNPs as determined by TEM. TEM images of the UCNPs obtained with a higher resolution are given in the Supporting Information (SI) in Fig. S1a–o.
