A quantitative assessment of the capabilities of 2 1/2D microscopy for analysing crystalline solids

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Abstract
The technique of 2 1/2D electron microscopy is shown to have a much greater potential than previously thought for determining the orientation changes or difference in lattice plane spacings in crystalline solids. A theoretical model and experimental evidence are presented to determine the limit to which diffraction spots may be closely spaced and yet still be correlated with their microstructural features. Because 2 1/2D imaging depends on an image shift produced by objective lens defocus, it is inherently limited in its usefulness by the loss in image resolution with defocus. However, the present treatment indicates that the depth of field in an electron microscope, under conditions of diffraction contrast and a very low beam divergence, is much greater than commonly believed. The minimum distance between diffraction spots (δ[gbar]) which may be resolved this way is found to be inversely related to the required image resolution. The results indicate that δ[gbar] separations of 0·5 nm−1 can be measured and unambiguously correlated with features in the dark-field stereo pair while maintaining an image resolution of 1 nm. If the required resolution is reduced to 10 nm, separations of less than 0·05 nm−1 can feasibly be studied.

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