Figure 3

(a) TEM image of β-NaYF₄:5%Er³⁺ nanocrystals calcined at 700 °C for 2 h. (b,c) HRTEM images of the little particle in red circle and the micro blocks in (a), respectively. (d) TEM image of α-NaYF₄:5%Er³⁺ nanocrystals calcined at 700 °C for 2 h. (e) HRTEM image of samples in (d). The inserts in (a,d) are corresponding SAED pattern.