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High-Resolution Electron Microscopy$

John C. H. Spence

Print publication date: 2013

Print ISBN-13: 9780199668632

Published to Oxford Scholarship Online: January 2014

DOI: 10.1093/acprof:oso/9780199668632.001.0001

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(p.402) Appendix 5 The challenge of HREM

(p.402) Appendix 5 The challenge of HREM

Source:
High-Resolution Electron Microscopy
Publisher:
Oxford University Press

This book is intended to teach students and research workers how to record and interpret atomic-resolution transmission electron microscope images. A well-defined aim is the greatest stimulus to progress. The following project therefore provides a thorough test of all the experimental skills taught in this book, together with a severe test of the underlying theory. Any student who completes it may reasonably claim to be an authority on high-resolution electron microscopy!

Magnesium oxide crystals of sub-micrometre dimensions are easily made by burning magnesium ribbon in air (see Section 10.1). A ‘holey carbon’ grid passed through the smoke will collect particles for examination in the electron microscope. The particles form in perfect cubes, and can be found in the (110) orientation with the electron beam passing along the cube face diagonal. The thickness is therefore known exactly at every point in a lattice image. It is possible to record a structure image of this material on the newest electron microscopes, and to determine the focus setting from diffractogram analysis (see Section 10.6). The image can then be matched as a function of thickness against computed images by the methods described in Sections 5.6 or 5.7. All the thickness-dependent contrast reversals in the lattice fringes due to multiple scattering should be correctly reproduced in the computed images.

Challenging aspects of this project include: (1) the variation of focus along the lower face of the crystal; (2) the possible need for ‘absorption’ corrections in the calculations for the thick regions (see Goodman and Lehmpfuhl 1967); (3) difficulties in matching the Fresnel fringes along the crystal edge of varying thickness, where a ‘profile image’ of the MgO surface will be seen; (4) refinement of the atomic scattering factors for ionicity effects; and (5) the need for very accurate alignment of the crystal. To the author’s knowledge, a thorough analysis of this problem has yet to be published. Ultimately, the credibility of the dynamical imaging theory given in this book depends on its ability to reproduce these experimental results.

This project represents a summation of practically all the useful knowledge in this book.

Reference

Goodman, P. and Lehmpfuhl, G. (1967). Electron diffraction study of MgO h00-systematic interactions. Acta Crystallogr. 22, 14.