Fig. 2.

Comparative characterization of the NaYF₄:Er and NaYF₄:Er@NaYF₄ nanocrystals. a TEM images of the NaYF₄:Er (2−100%) core and the NaYF₄:Er (2−100%)@NaYF₄ core−shell nanocrystals. Scale bars are 50 nm. b HAADF scanning TEM image of the NaErF₄@NaYF₄ nanoparticle highlighting the core−shell structure. c Proposed energy diagram showing upconversion processes in Er³⁺ under 1532 nm excitation. The full, wavy, and colored arrows represent excitation, multiphonon relaxation, and emission processes, respectively. d, e Lifetimes of various excited states of Er³⁺ as a function of the dopant concentration in the NaYF₄:Er (2−100%) core and the NaYF₄:Er (2−100%)@NaYF₄ core−shell nanoparticles, respectively. The excitation pulse energy density was set at 1 mJ mm⁻² for decay measurements