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High Resolution Imaging

The central objective of the research group in high resolution electron microscopy at the Chemical Center in Lund has been the study of the local crystal structure and the nature of structural defects by direct imaging of atomic positions.

For those crystals which are sufficiently resistant to irradiation damage, excluding most organic materials, it has become common-place to obtain images using electron beams with an energy of 100 keV to 1 MeV directly parallel to a crystal axis (preferably shorter than 10 ) so that a two-dimensional image of the structure can be observed showing details of the arrangements of atoms.

The resolution of electron microscopes has improved gradually, and it is now possible to achieve a resolution better than 1.7 . This of course results in images with clearer details of crystal structures.

It is no trivial matter to make full use of the resolution in the instruments now available. The precision needed for controlling the experimental variables increases rapidly as the resolution limit is improved. For this reason the use of the microscopes must be done together with a well trained scientist of the microscopy group.

Interpretation of Images

Many published images suggest direct representation of the crystal structure of the sample, i.e. black dots indicating the projected atomic positions. It must be emphasized that these are special images recorded under carefully chosen
conditions.

In general, high resolution images do not provide direct representation of the crystal structure. Even though black and white dots may appear they are often not at the atomic positions. The image appearance varies rapidly with the defocus of the objective lens and with the crystal thickness. Interpretation of images is strongly dependent upon accurate alignment of the microscope, correction of lens astigmatism and other factors.

It is not straightforward to ensure that the details seen in the image can be interpreted directly in terms of projections of the structure.

For reliable high resolution electron microscopy, even for thin crystals, it is essential that comparison of observed and calculated intensities is made. Computer programs for such calculations are available at the facility both for PCs and microcomputers.