To further confirm the structure of RDs, we developed a new method to determine NP packing on a single-aggregate level directly from electron microscopy. The key steps of the procedure are summarized in Fig. . First, we acquired a series of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images over a wide range of tilt angles (-70º to +72º), focusing on a tip of a colloidal crystal (Fig. ). After aligning the tilt series, we reconstructed the volume of the imaged region (Fig. ) and calculated the corresponding 3D Fourier transform (Fig. ). Optimization of the reciprocal vectors (for details, see Supplementary Section 5) followed by inverse Fourier transformation led to a primitive unit cell representative of the entire imaged region. Then, the primitive unit cell was replicated in all directions and overlaid on the reconstructed volume (Fig. ). Finally, the second optimization process afforded the complete unit cell (Fig. ). We note that whereas the shape of the unit cell determined using the above procedure matched that elucidated by SAXS (α, β, γ = 88.8±2.1º and 90º, respectively; Fig. ), the cell's edge length decreased significantly (a, b, c = 7.93±0.03 nm vs. 9.40 nm for SAXS), which can be attributed to drying-induced collapse (40% volume loss) (see also Supplementary Fig. ).