HAADF-STEM imaging of CuAg core-shell nanoparticles

Scanning transmission electron microscopy (STEM) is used in this study to determine the location of silver in bimetallic CuAg nanoparticles. The Z-contrast effect in STEM is particularly useful in our case because of the large difference in Z number between the two materials. However, the channeling effect that occurs when electrons are transmitted through a crystalline nanoparticle near a zone axis orientation prevents a direct interpretation of the STEM image contrast in terms of silver location. Two simple procedures are presented in this paper to estimate the contribution due to channeling, by varying the detection angle and by tilting the specimen a few degrees from its original orientation.