Journal
JOURNAL OF ELECTRON MICROSCOPY
Volume 53, Issue 3, Pages 257-266Publisher
OXFORD UNIV PRESS
DOI: 10.1093/jmicro/53.3.257
Keywords
scanning transmission electron microscopy; coherent scattering; elastic scattering; incoherent imaging; Z-contrast; depth slicing
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A full coherent Bloch wave calculation is presented to investigate high-angle annular dark-field image formation for sub-angstrom probes in scanning transmission electron microscopy (STEM). With increasing illumination angle, the contribution of the Is bound state increases to a maximum at an optimum probe angle, after which we find increasing contributions from high-angle plane wave states around the periphery of the objective aperture. Examination of image contributions from different depths within a crystal shows an oscillatory behavior due to the beating between Is and non-Is states. The oscillation period reduces with decreasing probe size, while the relative contribution from a specific depth increases. This signifies a changeover from a projection mode of imaging to a depth-slicing mode of imaging. This new mode appears capable of resolving three-dimensional atomic structures in future generation aberration-corrected STEM.
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