Figure 3: Evolution of morphology and composition in migration growth.

(a,b) NaGdF₄ growth along the longitudinal direction onto the ends of the NaYF₄ core; (c) transmission electron microscope image of the sample stopped 5 min after reacting with NaGdF₄/NaYF₄ nanocrystals in the presence of Na⁺, K⁺, Nd³⁺, OA⁻ and in the absence of F⁻ at 310 °C, dissolution occurs first; (d–g) real-time monitoring of the epitaxial growth of NaNdF₄ along the longitudinal direction onto NaYF₄–NaGdF₄ nanocrystals, involving the dissolution of NaYF₄ and NaGdF₄ from the transversal surfaces of the crystal and their subsequent re-growth onto the NaNdF₄ nanocrystals in the presence of Na⁺, K⁺, Nd³⁺, OA⁻ and absence of F⁻ ions at 310 °C. (h) HAADF-STEM image with elemental mapping results of the samples stopped after 60 min of reaction to confirm the distributions of Y³⁺, Gd³⁺, Nd³⁺ ions within a single NaYF₄/NaGdF₄/NaNdF₄ nanocrystal. (i) schematic processes of dissolution of NaYF₄/NaGdF₄ and the sequent epitaxial growth of NaNdF₄ in the longitudinal direction and the migration growth of F⁻, Y³⁺ and Gd³⁺ ions (scale bar, 100 nm).