Extended Data Fig. 2: STEM-EDX analysis of iodide-masked octahedra.

a, The workflow of STEM-EDX analysis for iodide-masked octahedra. The particle shown in Fig. 2d is used here to illustrate the workflow. b–e, Low-magnification bright-field image (b), HAADF-STEM image (c), Au EDX map (d) and I EDX map (e) with corresponding stacked line profiles, from which atomic ratios of Au and I are obtained. The reduced shape uniformity of gold NPs in this sample is presumably because of the extensive surfactant removal (Methods) required for high-sensitivity STEM-EDX detection of submonolayer iodide, which may destabilize unprotected gold surfaces. For EDX line profile analysis, we focus on intact octahedra oriented in the [110] projection (examples highlighted with dotted circles in d and e). Stacked line profiles are obtained along the contour of the projected octahedra as illustrated in the line profile step in a. A total of 400 line profiles are stacked in d and e. Scale bars, 50 nm (a); 200 nm (b–e).