Fig. 1: Structural characterization of optically active α-NaYF₄:Yb:Er core nanocrystals (NCs).

a Cubic crystal structure (top part) of the starting α-NaYF₄:Yb:Er core NCs along the [001] direction (perspective view) together with important crystallographic data (bottom part) such as atomic positions (x y z) and site occupation frequency (SOF). b Representative low magnification high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image. At least ten HAADF-STEM images were acquired at various magnifications and spatial locations on the grid. The synthesis of core particles (without Ce³⁺ doping) was performed twice with similar results. c Corresponding size distribution histogram. d Experimental (black open symbols) and calculated (red solid line) X-ray powder diffraction patterns together with their difference curve (blue solid line). The calculated powder X-ray diffraction (PXRD) pattern is the result of a full Rietveld refinement. The values for the expected R factor (R_exp) and weighted profile R factor (R_wp) are given to assess the quality of the Rietveld refinement when combined with the visual inspection of the difference curve. Note that the peak at 2θ ≈ 80° (*symbol) does not belong to the cubic crystal structure. Nevertheless, it is properly described by the calculated PXRD pattern because the background was modelled by scaling the signal of the sample holder. The latter is a low background (911)-oriented silicon wafer onto which NCs are drop casted to obtain, after solvent evaporation, a thin film of NCs. The PXRD pattern of the empty sample holder is given in the Supplementary Fig. 2. e High-angle annular dark-field (HAADF-STEM) image of an individual α-NaYF₄:Yb:Er core NC. f Integrated differential phase contrast (iDPC)-STEM image of a region of an individual α-NaYF₄:Yb:Er core NC. Atomic columns that contain rare-earth and fluorine atoms are in yellow and beige, respectively.