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Crystallographic image processing
Crystallographic image processing (CIP) is a set of methods for determining the atomic structure of crystalline matter from high-resolution electron microscopy (HREM) images obtained in a transmission electron microscope (TEM). The term was created in the research group of (Sven Hovmöller ) at Stockholm University during the early 1980s and became rapidly a label for this approach.
== HREM image contrast and crystal potential reconstruction methods ==
Many beam HREM images are only directly interpretable in terms of a projected crystal structure if they have been recorded under special conditions, i.e. the so-called Scherzer defocus. In that case the positions of the atom columns appear as black blobs in the image. Difficulties for interpretation of HREM images arise for other defocus values because the transfer properties of the objective lens alter the image contrast as function of the defocus. Hence atom columns which appear at one defocus value as dark blobs can turn into white blobs at a different defocus and vice versa. In addition to the objective lens defocus (which can easily be changed by the TEM operator), the thickness of the crystal under investigation has also a significant influence on the image contrast. These two factors often mix and yield HREM images which cannot be straightforwardly interpreted as a projected structure. If the structure is unknown, so that image simulation techniques cannot be applied beforehand, image interpretation is even more complicated. Nowadays two approaches are available to overcome this problem: one method is the exit-wave function reconstruction method, which requires several HREM images from the same area at different defocus and the other method is crystallographic image processing (CIP) which processes only a single HREM image. Exit-wave function reconstruction 〔A. Thust, M.H.F. Overwijk, W.M.J. Coene, M. Lentzen (1996) "Numerical correction of lens aberrations in phase retrieval HRTEM" Ultramicroscopy 64, 249 - 264.〕〔L.J. Allen, W. McBride, N.L. O’Leary & M.P. Oxley (2004) "Exit wave reconstruction at atomic resolution" Ultramicroscopy vol. 100, 91-104.〕 provides an amplitude and phase image of the (effective) projected crystal potential over the whole field of view. The thereby reconstructed crystal potential is corrected for aberration and delocalisation and also not affected by possible transfer gaps since several images with different defocus are processed. CIP on the other side considers only one image and applies corrections on the averaged image amplitudes and phases. The result of the latter is a pseudo-potential map of one projected unit cell. The result can be further improved by crystal tilt compensation and search for the most likely projected symmetry. In conclusion one can say that the exit-wave function reconstruction method has most advantages for determining the (aperiodic) atomic structure of defects and small clusters and CIP is the method of choice if the periodic structure is in focus of the investigation or when defocus series of HREM images cannot be obtained, e.g. due to beam damage of the sample. However, a recent study on the catalyst related material Cs0.5() shows the advantages when both methods are linked in one study.〔J. Barthel, T.E. Weirich, G. Cox, H. Hibst, A. Thust (2010) "Structure of Cs0.5() analysed by focal-series reconstruction and crystallographic image processing" Acta Materialia vol. 58, 3764-3772. (article )〕
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