Figure 4

(a–e) TEM images of the NaYF₄ UCNPs co-doped with 2% Er³⁺ and various concentrations of Yb³⁺: 20% Yb³⁺, 40% Yb³⁺, 60% Yb³⁺, 70% Yb³⁺, and NaYbF₄: 2% Er³⁺. For TEM images with higher resolution: see SI, Fig. S1a,s–v. The photographs in the insets show the visible emission of each sample under 980 nm excitation; (f) UCL emission spectra of NaYF₄: x % Yb³⁺, 2% Er³⁺ UCNPs under 980 nm excitation; (g) Decay kinetics of the 540 nm Er³⁺ emission; (h) Decay kinetics of the 654 nm Er³⁺ emission.