The role of multiple scattering in the study of lattice images of graphitic carbons

The effect of multiple scattering upon the nature of the electron-microscope images of graphitic specimens has been investigated by theoretical calculations within the systematics multislice approach. For perfect, infinite specimens, imaged under axial illumination, the results agreed closely with those predicted by a simple three-beam dynamical treatment. In all cases the weak-phase-object approximation was found to be invalid, even qualitatively, whenever specimen thicknesses exceeded 50 Å. Calculations were also performed for finite crystal sizes and various plausible types of structurally imperfect graphites. The calculated images demonstrate the need for great care in interpreting experimental images of structurally defective graphitic solids. In particular, the positions of missing layers in a crystal of graphite can be 'filled' by spurious fringes under certain conditions of defocus, and fringes corresponding to layers of an intercalated species turn out to have ill defined positions. Characteristics such as these are equally prominent in all images, irrespective of the magnitude of multiple scattering, suggesting that these perturbations arise primarily from instrumental aberrations.