New Direct Methods for Phase and Structure Retrieval in HREM

Abstract

Reliable interpretation of high resolution electron micrographs is only possible by comparison with computer simulations. However, simulation is a tedious trial and error technique which can only be successful if the experimental parameters are known and if the number of plausible structure models is small. For instance the interpretation of images of amorphous objects by simulation is hopeless. This makes the power of HREM very much dependent on the amount of prior information available from other techniques such as X-ray diffraction. HREM would be much more powerful and independent if a direct method could be established to retrieve the structural information of the object directly from the electron micrographs.We present a new “focus variation method” in which the phase is retrieved in the image plane in a deterministic way from a combination of images at closely spaced focus values, as inspired on the method proposed in [1] [2]. In a sense the whole information is used in the 3D image area of the electron microscope. The method allows to correct for chromatic aberration, spherical aberration and focus and is robust against noise. In a second stage we retrieve the projected structure of the object directly from the knowledge of the wavefunction at the exit face, using the channelling theory proposed in [3]. The first results are very promising.