Fig. 2: Characterization of ~ 14 nm core-shell-shell UCNPs.

a Schematic illustration of high brightness capUCNPs via a sandwiched structure with NaYbF₄ as intermediate layer. b TEM images of capUCNPs of Yb:Er@Yb^{1.5 nm}@Lu^{0.5 nm}. c Upconversion emission spectra of the as-prepared capUCNPs, Yb:Er@Lu^{1.5 nm}@Lu^{0.5 nm} and Yb:Er@Yb:Er^{1.5 nm}@Lu^{0.5 nm} nanoparticle and their corresponding core-shell and core nanoparticles. All spectra were recorded under excitation of a 980 nm CW diode laser at a power density of 530 W cm^-2; d Upconversion luminescence decay curves of Er³⁺ emission at 654 nm for Yb: Er, Yb: Er@Yb and various sandwich structured nanoparticles with NaLuF₄ coating. The nanoparticles were stabilized with oleic acid (OA) ligands and dispersed in cyclohexane solution. “a. u.” represents “arbitrary units”.