Enhance the Er³⁺ Upconversion Luminescence by Constructing NaGdF₄:Er³⁺@NaGdF₄:Er³⁺ Active-Core/Active-Shell...

NaGdF₄:12%Er³⁺@NaGdF₄:x%Er³⁺ (x = 0, 6, 8, 10, and 12) active-core/active-shell nanoparticles (NPs) were peculiarly synthesized via a delayed nucleation pathway with procedures. The phase, shape, and size of the resulting core-shell NPs are confirmed by transmission electron microscopy and X-ray diffraction. Coated with a NaGdF₄:10%Er³⁺ active shell around the NaGdF₄:12%Er³⁺ core NPs, a maximum luminescent enhancement of about 336 times higher than the NaGdF₄:12%Er³⁺ core-only NPs was observed under the 1540 nm excitation. The intensity ratio of green to red was adjusted through the construction of the core-shell structure and the change of Er³⁺ concentration in the shell. By analyzing the lifetimes of emission bands and exploring the energy transition mechanism, the giant luminescence enhancement is mainly attributed to the significant increase in the near-infrared absorption at 1540 nm and efficient energy migration from the shell to core.