Electron microscope observations of lattice imperfections and their movement on the atomic scale

The effect of partial coherence of illuminating electron waves on the image contrast is discussed. It is shown that aberration-free focus (AFF) imaging in axial and symmetrical illumination, which is generally believed to be the best imaging condition for coherent illumination, produces an inherent contrast which does not correspond to the atomic structure of the specimen. However, tilted-illumination AFF imaging produces the correct image. Atomic structure images of gold crystals containing edge, screw and mixed dislocations have been photographed, with tilted-illumination AFF imaging and by adjusting the specimen thickness to the optimum value, and the atomic structure of the core of these dislocations is discussed. The movement of these dislocations has been recorded on an atomic scale using a TV system, and the movement of atom columns accompanying the movement of dislocations is analyzed. (orig.)