Chemical analysis by high-angle hollow cone illumination

The high-angle hollow cone illumination realized in a conventional transmission microscope is known to suppress coherent contrast mechanisms. It is shown that under suitable experimental conditions, orientation and strain contrasts are sufficiently suppressed to obtain a Z-contrast image. Various factors, influencing the image intensity, are determined by a quantitative image analysis of pure metals. Based on these measurements an incoherent multiscattering model is generalized to binary alloys in order to describe the chemical image contrast and to determine the local composition of the specimen from an intensity measurement. To test the concept, the early stages of interdiffusion between thin films of Cu and Au are investigated. The results demonstrate that a useful chemical analysis is possible, even in the case of nanocrystalline specimens. Complete composition profiles are determined on a length scale of 10 nm. The spatial and chemical resolution of the method are discussed.