Figure 3

(a–d) SEM characterization of NaYF₄:Yb³⁺, Er³⁺ (20, 2 mol%) nanoparticles doped with various concentrations of Gd³⁺ ions. (a) shows the SEM image of the NaYF₄:Yb³⁺, Er³⁺ (20, 2 mol%) UC nanoparticles. (b) shows the SEM image of the NaYF₄:Yb³⁺, Er³⁺ (20, 2 mol%) @Gd³⁺ doping (30 mol%) UC nanoparticles. (c) shows the SEM image of the NaYF₄:Yb³⁺, Er³⁺ (20, 2 mol%) @Gd³⁺ doping (50 mol%). (d) shows the SEM image of the NaYF₄:Yb³⁺, Er³⁺ (20, 2 mol%) @Gd³⁺ doping (70 mol%) UC nanoparticles. The scale bars in figure (a–d) are 500 nm.