Iterative structure retrieval techniques in HREM: a comparative study and a modular program package

A retrieval technique for crystal structures using high-resolution electron microscopy is presented. The inversion of the complex structure-to-image relation is performed by numerical optimization of the configuration space of object models. Unlike structure refinement in X-ray crystallography, the method operates on unknown defect structures on a nanometre scale. The diversity of crystal defects examined and the differences in microscope types and alignment conditions makes it necessary to adapt the basic algorithm to a broad variety of needs resulting in a modular program package for general use. New developments, such as a real space slice function calculation, problem adapted optimization strategies, and finally an examination of iterative matching of image series and exit wavefunctions are presented.     

Quantitative HREM observation of the Σ11(113)/[1¯10] grain-boundary structure in aluminium and comparison with atomistic simulation

Quantitative high-resolution electron microscopy (QHREM) involves the detailed comparison of experimental high-resolution images with image simulation based on a model and weighted by the estimated uncertainty in the experimental results. For simple metals, such as Al, models have been systematically improved using nonlinear least-squares methods to obtain simulated images that are indistinguishable from experimental images within the experimental error. QHREM has been used to study the atomic structure of the Σ11(113)/[1¯10] in Al. In this paper, we focus on the method of refining electron-optical imaging parameters and atomic structure to bring the simulated HREM image into agreement with the experimental result to within the experimental error and thus yield a result more useful to the materials scientist. Uncertainties in fitted parameters are studied using the conditional probability distribution function. We discuss experimental results for atomic column locations compared with atomistic simulations of the structure of the grain boundary.     

Towards 1.0 Å resolution: taking advantage of dynamical scattering, and the benefits for structure retrieval

This paper is an exploration of the behaviour of high-resolution transmission electron microscope (HRTEM) images at up to 1 Å resolution. The ultimate limits to HRTEM (structure) resolution and the manner in which strong scattering may lead to weak diffraction in heavy fcc metals are discussed. A resolution of 1.0 Å is somewhat better than the ultimate resolution presently achievable in a 400-kV electron microscope. In heavy metals, such as platinum, it is found that the lattice fringe contrast is very low in the [011] projection, but that fringe contrast may be improved by imaging in the [111] projection. For atomic resolution imaging of the heavy metals in the [111] projection a resolution of 1.2 Å is required. For the study of oxygen position in high-temperature superconducting (HTS) oxides a resolution of between 1.2 and 1.4 Å is required. At better than 1.2 Å resolution the thick crystal images in HTS oxides remain simple and are easily interpreted. At such resolution all atomic columns are separated for the HTS [010] projection and the dynamical diffraction effects improve the contrast of oxygen atoms relative to the metal atoms.