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TEM images of (a) NaGdF4 core, (b) NaGdF4@NaYF4:Yb,Er, core–inner shell structure, (c) NaGdF4@NaYF4:Yb,Er@NaYF4 sandwich structure. (d) High-resolution TEM image and (e) SAED pattern of SWUCNPs. (f) XRD patterns of the obtained nanoparticles. (g) Emission spectra of NaGdF4@NaYF4:Yb,Er@NaYF4 and NaGdF4@NaYF4:Yb,Er UCNPs.
