07. May. 2015, 16:15 Uhr, Gebäude NW1, Raum H3
Measuring structure parameters from electron microscopy images: what are the present possibilities and how to improve the limits?
Prof. Dr. Sandra Van Aert, EMAT Antwerpen, Belgien
New developments in the field of nanoscience and nanotechnology drive the need for advanced quantitative materials characterisation techniques that can be applied to complex nanostructures. The physical properties of these nanostructures are obviously controlled by composition and chemical bonding, but also by the positions of the atoms. Indeed, changing the interatomic distances by picometers can turn an insulator into a conductor. Transmission electron microscopy (TEM) is an excellent technique to study nanostructures because of the strong interaction of electrons with small volumes of matter. Over the past few years, remarkable high-technology developments in the lens design greatly improved the image resolution. Nowadays, a resolution of the order of 50 pm can be achieved. For most atom types, this exceeds the point where the electrostatic potential of the atoms is the limiting factor. Furthermore, new data collection geometries are emerging that allow one to optimise the experimental settings. In addition, detectors behave more and more as ideal quantum detectors. In this manner, the microscope itself becomes less restricting and the quality of the experimental images is mainly set by the unavoidable presence of electron counting noise and environmental disturbances. In order to measure the atom positions and atom types as accurately and precisely as possible from atomic resolution TEM image, quantitative methods are required. To reach this goal, the use of statistical parameter estimation theory is of great help. This methodology allows one to measure 2D atomic column positions with subpicometer precision, to measure compositional changes at interfaces, to count atoms with single atom sensitivity, and to reconstruct 3D atomic structures. In this talk, the basic principles of the method will be explained and illustrated using current state-of-the-art experimental examples