STEM images and size distributions (deduced from STEM image analysis) of Ag _x Al_1-x NPs, for x = 0.25, 0.50 and 0.75 (from left to right). The largest images show BNPs sandwiched in silica. The indicated diameters correspond to the ones measured by mass spectrometry. Because of the deposition geometry, there is a shadowing effect around the particles that leads to a darker area on one side of the BNPs without SiO₂, and a brighter area on the other side where SiO₂ accumulates. The images in insert show carbon-protected Ag_0.75Al_0.25 particles and unprotected Ag_0.50Al_0.50 and Ag_0.25Al_0.75 BNPs. A core@shell structure is clearly visible for the last two compositions. Here, only the cores were considered for the size distributions, explaining the difference between the average diameters deduced from the size distributions (noted <Ø>) and those measured by mass spectrometry during the deposition (indicated in the largest images).