We discuss interesting haloing effects observed in experimental images of gold nanoparticles obtained using aberration-corrected scanning transmission electron microscopy (STEM) employing the low-angle annular dark-field (LAADF) imaging mode. The LAADF images contained bright rings of intensity (halos) with a diameter equal to or smaller than the diameter of the nanoparticle, the diameter varying as a function of the defocus of the STEM probe. Numerical simulations reveal that the halos are only present if the nanoparticles are imaged down a zone axis. Since the halos were observed in nearly all experimental images, this suggests that the nanoparticles become oriented along crystal zone axes during imaging.
This paper was originally published in Ultramicroscopy (2012) 120, 10-15.
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