Structure and chemistry of interfaces

It is most desirable to understand the structure and chemistry of the internal interfaces for all classes of materials since the materials' properties often depend on the properties of the interfaces which, in turn, are controlled by their structure and chemistry. In contrast to surface science, there exist only a few techniques for studying the structure and chemistry of internal interfaces. One of the most powerful techniques seems to be transmission electron microscopy (TEM) by which short segments of interfaces can be analyzed. In high-resolution electron microscopy (HREM) a direct image is formed of the projection of the interfaces. A simple analysis of HREM micrographs is not possible owing to the complex image forming processes within HREM. In addition to experimental investigations, calculations of the structures must be performed using material specific interatomic potentials. From the calculated structure, HREM images must be simulated for the specific imaging conditions. The experimental micrographs must be compared to simulated images. An agreement between experimental micrographs and the simulated images results in the best possible atomistic configuration. A quantitative measure for this agreement is the difference image, D, between the experimental micrograph and the simulated image. Best agreement is reached if only the noise is visible in the difference image D. Analytical electron microscopy with high-spatial resolution (typical probe size <0.05 nm) allows the identification of impurities segregated at the interface. However the limit of detectability depends sensitively on the combination between the different elements. Recently developed techniques on spatially resolved electron energy loss spectra give information on bonding and coordination. As an example, the different TEM techniques have been applied to the investigation of grain boundaries in -Al₂O₃. It should be emphasized, however, that the TEM techniques could also be applied to internal interfaces in different boundaries.