Figure 2.

HRTEM studies of NPs: (a) HRTEM images of Fe₃O₄@NaYF₄(Yb, Tm); (b) fast Fourier transform of the selected area in part a, showing two sets of diffraction patterns. The diffraction pattern marked in blue belonged to cubic Fe₃O₄, and the one marked in red was assigned as cubic NaYF₄; (c) high angle annular dark field image of Fe₃O₄@NaYF₄(Yb, Tm), showing the Z contrast difference between the shell and core of particles induced by a slightly higher average atomic number in the shell after doping with heavy atoms Yb and Tm; (d) HRTEM image revealed the core–shell structure of NP Co_0.16Fe_2.84O₄@NaYF₄(Yb, Er). Atomic lattice fringes 2.97 and 4.14 Å corresponded to (022) and (200) planes of Fe₃O₄, respectively. The inset is a fast Fourier transform of the micrograph.