Influence of Cubic-to-Hexagonal-Phase Transformation on the Uniformity of NaLnF₄ (Ho, Tb, Eu, Sm) Nanoparticles

Although there have been many reports on the synthesis of NaLnF4 nanoparticles (NPs), in particular ones that show upconversion luminescence, there is still an insufficient understanding of the mechanism to design the set of reaction conditions that will result in NPs of a particular diameter with a narrow size distribution. Here, we describe experiments that lead to the synthesis of uniform NaLnF4 (Ln = Sm, Eu, Tb, Ho) nanoparticles (NPs) with diameters between 4 and 30 nm. NPs, in this size range, are particularly important as potential reagents for mass cytometry. The effects of varying the amount of ligand (oleic acid), precursor ratios, heating time, and temperature are discussed. In general, we found that identical reaction conditions cannot be used to synthesize NPs of identical sizes for different lanthanide elements. We found that to obtain larger uniform NaLnF4 NPs, conditions should be found such that the NPs initially nucleate in the cubic phase and undergo a subsequent phase change to the hexagonal phase. The nucleating phase was found to be affected by the Na/Ln and F/Ln precursor ratios, where larger ratios led to initial nucleation in the hexagonal phase and smaller ratios led to initial nucleation in the cubic phase and progressively larger NPs. The nucleating phase was also affected by the amount of the ligand; however, the trends we observed were not consistent between different lanthanides.