Figure 5

(a–d) TEM images of NaYF₄ nanocrystals co-doped with 20% Yb³⁺ and 2% Er³⁺ synthesized by increasing the RE precurcors concentration in a constant solvent volume (120 mL): 5 mmol, 10 mmol, and 15 mmol LnCl₃; for TEM images with higher resolution: see SI, Fig. S1k,w–y; (e) UCL emission spectra of NaYF₄: x % Yb³⁺, 2% Er³⁺ UCNPs under 980 nm excitation; (f) Decay kinetics of the 540 nm Er³⁺ emission; (g) Decay kinetics of the 654 nm Er³⁺ emission.