Electron Crystallography

Image Processing in High Resolution Electron Microscopy

Background
        For the structure analysis of crystalline materials, electron crystallographic methods are in some cases superior to that of X-rays. Firstly, many crystalline materials important in science and technology, such as high Tc superconductors, are too small in grain size and too imperfect in periodicity for an X-ray single crystal analysis to be carried out, but they are suitable for electron microscopic observation. Secondly, the atomic scattering factors for electrons differ greatly from those for X-rays and, it is easier for electron diffraction to observe light atoms in the presence of much heavier atoms. Finally the electron microscope is the only instrument that can simultaneously produce, for a crystalline sample, a microscopic image and a diffraction pattern at atomic resolution. In principle either the electron micrograph (EM) or the electron diffraction (ED) pattern could lead to a structure image. However the combination of the two will make the procedure much more efficient and powerful.
        There are two difficulties in using electron microscopic methods in solving crystal structures. Firstly, an electron microscope image is not a true structure image of the object but just a convolution of the true image with the Fourier transform of the contrast transfer function. Secondly, the point to point resolution of an electron microscope image is not enough to reveal individual atoms in most cases. Hence it is essential to have some technique for both image deconvolution and resolution enhancement. A two-stage image processing procedure making use of direct methods has been developed, which combines information from an electron microscope image and the corresponding electron diffraction pattern.