High resolution imaging of amorphous materials

Abstract

The important role of electron microscopy in the study of amorphous materials is in the examination of the local variation in structure, whether inherent or caused by deformation, irradiation or the nucleation of crystallisation. This requires that the image contain information about local spacings and their variability at the atomic level even if only in relation to a projection. Until recently commercial instruments could not do this for spacings less than about 0.3 nm, and material scientists have either concentrated on the interpretation of coarser structural irregularities in amorphous alloys or used indirect methods to get round the lack of resolving power of the objective lens of their microscopes (see ref. 1 for review). The interpretative dangers of the most obvious approach of using tilted illumination conditions are now well understood^2–5. While the prospects for dark field interference techniques are a little more promising⁶ the essential problem of a nonlinear transfer function remains⁷. More interesting, though with other limitations, is the use of hollow cone illumination⁸ with axial transfer^8–11. Here we present high resolution, axial illumination electron micrographs of such materials with detail at spacings less than 0.2 nm and discuss the difficulties involved in the interpretation of such images.