Figure 2.

(a) Photoluminescence spectra of NaYF₄:Yb³⁺/Er³⁺ samples under 980 nm laser illumination with a power of 60 mW; (b) peak intensities at 522, 541, and 654 nm as a function of the reaction time. (c) energy transfer mechanisms for the Yb³⁺–Er³⁺ couple under 980 nm laser illumination.