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. 2019 Sep 23;14:7759–7780. doi: 10.2147/IJN.S221433

Figure 4.

Figure 4

(A) Schematic illustrations of the typical structures of epitaxial and non-epitaxial shells for core-shell tuned UCNPs. Reproduced with permission from Chen X, Peng D, Ju Q, Wang F. Photon upconversion in core-shell nanoparticles. Chem Soc Rev. 2015;44(6):1318–1330.88 Copyright the Royal Society of Chemistry 2015. (B) Schematic illustration of synthesizing and coating process for multilayer NaYF4:Yb,Tm nanoparticles and TEM images from each step. Reproduced with permission from Su Q, Han S, Xie X, et al. The effect of surface coating on energy migration-mediated upconversion. J Am Chem Soc. 2012;134 (51):20849–20857.96 Copyright American Chemistry Society 2012. (C) The left panel shows high-angle annular dark-field micrograph of a NaYF4:Yb,Er/NaGdF4 nanoparticle with the chemical maps from the presence of yttrium (Y) in the core and gadolinium (Gd) in the shell of nanocrystal. The right panel shows the electron energy loss spectroscopy of Y and Gd edges from the nanoparticle with 2.4 nm shell. Reproduced with permission from Zhang F, Che R, Li X, et al. Direct imaging the upconversion nanocrystal core/shell structure at the subnanometer level: shell thickness dependence in upconverting optical properties. Nano Lett. 2012;12(6):2852–2858.98 Copyright American Chemistry Society 2012.