HAADF-STEM plan view images of FeзAuo(a) and Fe₃Au₂(b) samples showing the drastic morphological transformation of the Fe thin film when covered by a thin Au layer. A high-resolution HAADF-STEM image of a typical Fe NP is displayed in inset at the top of Figure (a). Note that, owing to the sensitivity of HAADF-STEM images to the atomic number of the compounds, the Fe NPs exhibit a bright contrast in (a) but a grey contrast in (b), the bright contrast being displayed by Au. The darker regions correspond to the silica matrix in both images. Bottom insets show the size histograms of the Fe NPs (Fig. a) and the Fe cores (Fig. b).