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HREM and STEM of defects in multiply‐twinned particles

The structure of defects in multiply‐twinned particles has been studied in detail using high‐resolution lattice imaging, dark field and microdiffraction techniques. Icosahedral particles with sizes greater than about 15 nm were found to contain defects, in the form of stacking fault loops parallel with the external surface, which were extremely difficult to detect by conventional amplitude contrast techniques. Microdiffraction mappings correlated with these results, showing large rotations of the face‐centred cubic segments. Results for decahedral particles indicated the presence of stacking faults running adjacent to, and parallel with, the twin boundaries. Microdiffraction maps confirmed that the particle structure was face‐centred cubic, and also verified that the apparent epitaxy of these particles was highly variable. Models for the defects are proposed and discussed, and the relative merits of HREM and STEM for elucidating structural details in small particles is briefly considered. Finally, the potential for direct imaging at surfaces, as demonstrated by some recent images, is discussed.