Fig. 4
A TEM and HRTEM images of the as-prepared cubic core crystals and heterogeneous core/shell crystals. TEM images of (a) cubic NaGdF4 core nanocrystals, (b)–(e) heterogeneous NaGdF4/ NaYF4 core/shell crystals with the shell growth time of 2, 6, 12, and 24 h, respectively, (f) a single particle of the 24-h-reaction NaGdF4/ NaYF4 core/shell crystals which stand on the TEM grids on their bottom faces. (g) SEM image of the 24-h-reaction NaGdF4/NaYF4 core/shell crystals, which exhibit a hexagonal nanoprism look. HRTEM images of (h) cubic NaGdF4 core nanocrystals. (i)–(l) heterogeneous NaGdF4/NaYF4 core/shell crystals with the shell growth time of 2, 6, 12, and 24 h, respectively. B Histograms of size distribution of (m) cubic NaGdF4 core nanocrystals, (n)–(p) heterogeneous NaGdF4/NaYF4 core/shell crystals with the shell growth time of 2, 6, and 12 h, respectively, (q) and (r) diameter of the hexagonal cross section and the nanoprism height of the 24-h-reaction NaGdF4/ NaYF4 core/shell crystals