fig1

Figure 1. TEM images of cubic (A-C, α-phase) and hexagonal (D-F, β-phase) NaYF₄:10%Er core, NaYF₄:10%Er @NaYbF₄ core-shell; NaYF₄:10%Er@NaYbF₄@NaYF₄ core-shell-shell nanoparticles. The size distribution is shown in the insets.