Figure 1

(a,b) SEM images of the as-prepared β-NaYF₄ and β-NaYF₄:5%Er³⁺ nanocrystals. (c) HRTEM image and (d) EDS spectrum of as-prepared β-NaYF₄:5%Er³⁺ nanocrystals. (e) The MIR emission spectra of as-prepared β-NaYF₄ and β-NaYF₄:5%Er³⁺ nanocrystals. The inset in (b) is corresponding SAED pattern.