Abstract
Abstract As first pointed out by Lord Rayleigh a century ago, incoherent imaging offers a substantial resolution enhancement compared to coherent imaging, together with freedom from phase contrast interference effects and contrast oscillations. In the STEM configuration, with a high angle annular detector to provide the transverse incoherence, the image also shows strong Z-contrast, sufficient in the case of a 300 kV STEM to image single Pt and Rh atoms on a γ-alumina support. The annular detector provides incoherence by virtue of its large central hole, which is equivalent by Babinet's principle of complementarity to a bright field detector of the same size. For weakly scattering specimens, it shows greater contrast than the incoherent bright field image, and also facilitates EELS analysis at atomic resolution, using the Z-contrast image to locate the probe with sub-À precision. The inner radius of the annular detector can be chosen to reduce the transverse coherence length to well below the spacings needed to resolve the object, a significant advantage compared to light microscopy.
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