Auto-correlation analysis of high resolution electron micrographs of near-amorphous thin films

Abstract

In an attempt to characterize the degree of local ordering of amorphous, near-amorphous and microcrystalline materials from information contained in high resolution electron micrographs, a new method is considered in which auto-correlation functions are obtained from small image regions and then correlated in rotation. By finding the rotation which gives the best correlation with a particular local correlation function showing features of interest and then adding the local auto-correlation functions, an assessment of the local order for the whole image is achieved as a basis for consideration of the form and extent of the order in the specimen. The method has been applied to high resolution transmission electron microscope images recorded in digital form by use of a TV-based image intensifier and digital frame store and later analysed by use of an image processing system. The samples used were thin carbon films having different degrees of graphitization. Even the least-ordered carbon films showed significant local correlations.