Aberration Assessment and Correction using High-Resolution Holographic Methods in Electron Microscopy

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Compared to other techniques used to obtain structural information of materials, high resolution electron microscopy (HREM) has the great advantage that it yields local information about the atomic arrangements, projected along the direction of electron incidence at a resolution comparable to the interatomic distances. This enables the study of complicated structures, crystal defects, precipitates, etc. down to the atomic level. With the increasing need of the accurate determination of the atomic arrangements at non-periodic structures in materials design and control of microstructures and nanostructures, the accessibility to a high resolution electron microscope yielding quantitative structural information at the sub-angstrom level will be indispensable. The interpretation of HREM images, however, is not simple due to the strong interaction of electrons with matter and due the aberrations of the microscope. The latter result in complicated images, which do not reveal the real structure. Unfortunately, aberrations cannot be corrected from conventional electron micrographs since the phase information is lost during recording. Only holographic methods like through focus series and off-axis electron holography retrieve - within the reconstruction process - amplitude and phase hence make possible a subsequent correction of aberrations. Thus the correction for the microscope aberration is crucial for the further development of HREM, in particular when one wants to apply HREM in a quantitative manner. The Dresden and Delft groups have strong research programs focussed of off-axis electron holography (Dresden) and through focus series (Delft) which both includes the careful correction for the microscope aberrations. By means of a DFG-NWO grant they can join their forces in a better structured cooperation with frequent mutual visits for discussions und experiments. The cooperation will be focussed on the optimisation (method and software development) of aberration assessment and correction.