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Specimen Charging on Thin Films with One Conducting Layer: Discussion of Physical Principles

Published online by Cambridge University Press:  01 December 2004

Robert M. Glaeser
Affiliation:
Department of Molecular and Cell Biology, Stanley/Donner ASU, University of California, Berkeley, CA 94720-3206, USA Life Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA
Kenneth H. Downing
Affiliation:
Life Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA

Abstract

Although the most familiar consequences of specimen charging in transmission electron microscopy can be eliminated by evaporating a thin conducting film (such as a carbon film) onto an insulating specimen or by preparing samples directly on such a conducting film to begin with, a more subtle charging effect still remains. We argue here that specimen charging is in this case likely to produce a dipole sheet rather than a layer of positive charge at the surface of the specimen. A simple model of the factors that control the kinetics of specimen charging, and its neutralization, is discussed as a guide for experiments that attempt to minimize the amount of specimen charging. Believable estimates of the electrostatic forces and the electron optical disturbances that are likely to occur suggest that specimen bending and warping may have the biggest impact on degrading the image quality at high resolution. Electron optical effects are likely to be negligible except in the case of a specimen that is tilted to high angle. A model is proposed to explain how both the mechanical and electron-optical effects of forming a dipole layer would have much greater impact on the image resolution in a direction perpendicular to the tilt axis, a well-known effect in electron microscopy of two-dimensional crystals.

Type
Research Article
Copyright
© 2004 Microscopy Society of America

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References

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