Contrast reversal in atomic-scale phonon spectroscopic imaging

It was recently shown that phonon spectral mapping of atomic structure is possible in scanning transmission electron microscopy. These results were obtained by deflecting the transmitted beam to detect probe electrons that had been scattered through large momentum transfer into a detector centered at an angle more than twice the radius of the bright-field disk. Atomic-scale spectral maps are still obtained for electrons scattered to angles smaller than the probe convergence angle, albeit at lower contrast and despite the smaller momentum transfers associated with these electrons. It is reported here that apparent reversals of contrast can be observed for these smaller momentum transfers, which is not the case when imaging using probe electrons scattered through larger momentum transfers. This phenomenon can be explained in terms of the channeling of the incident probe. We discuss how such contrast reversals may be avoided and atomic resolution maps reliably obtained.