On the origin of transverse incoherence in Z-contrast STEM

We use a Bloch wave approach to further investigate the origins of the incoherent nature of Z-contrast imaging using an ADF detector in a STEM. We discuss how, although at high angles the collected electrons will be mostly thermally scattered in addition to the elastic scattering, it is not the ther...

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Bibliographic Details
Published in:	Journal of electron microscopy 2001-01, Vol.50 (3), p.227-233
Main Authors:	Rafferty, B, Nellist, D, Pennycook, J
Format:	Article
Language:	eng
Subjects:	INCOHERENT SCATTERING INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY ORIGIN SCANNING ELECTRON MICROSCOPY TRANSMISSION ELECTRON MICROSCOPY
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Summary:	We use a Bloch wave approach to further investigate the origins of the incoherent nature of Z-contrast imaging using an ADF detector in a STEM. We discuss how, although at high angles the collected electrons will be mostly thermally scattered in addition to the elastic scattering, it is not the thermal scattering that destroys the coherence, rather the combination of the large detector with the high-angle elastic scattering. This incoherent nature of the elastic scattering arises through the filtering of the 1s-type Bloch states by the detector geometry. We show that it is this filtering that renders an atomic column an independent scatterer insensitive to the configuration of neighbouring columns. It also makes the image contrast insensitive to the effects of beam spreading onto neighbouring columns as the probe propagates through the crystal. We also discuss the implications of this for previous calculations of the intensity of Z-contrast images. Keywords:scanning transmission electron microscopy, Z-contrast, ADF, Bloch waves, thermal diffuse scattering, imaging theory
ISSN:	0022-0744 2050-5698 0022-2720 1477-9986 2050-5701 1365-2818