Phase transition induced formation of hollow structures in colloidal lanthanide-doped NaYF4 nanocrystals

Formation of colloidal hollow structures in NaYF4 nanocrystals (NCs) with and without lanthanide ion doping has been observed and investigated via the co-thermolysis of a mixture of trifluoroacetate precursors in trioctylphosphine oxide. The Kirkendall effect in this one-step reaction is driven by the monomer diffusion and crystal phase transition. It is found that three kinetic stages which include rapid precipitation of cubic phase NCs, cubic to hexagonal phase transition concurrent with an inward transport of NaF species, and vacancy diffusion are attributed to the hollow structure formation. X-ray energy dispersive spectroscopy (XEDS) point analysis is applied to examine the ions distribution and crystalline components in the lanthanides (Yb and Er)-doped NaYF4 up-conversion nanophosphors (UCNPs). The hollow structures increase the surface-to-volume of a single NC and thus have a significant effect on the photoluminescence of the lanthanide-doped NPs and provide synthetic applications for achieving novel NaYF4-based NCs.