Thomas Vogt
Aberration-Corrected Scanning Transmission Electron Microscopy Imaging and Its Use in Materials Science
Technologically important properties of materials are inherently connected to their atomic structure. Aberration-corrected scanning transmission electron microscopy (STEM) is currently transforming imaging at the nanoscale and will become increasingly important in the near future for materials and life sciences. In STEM imaging incoherent scattering is collected, which from a specimen with appropriate thicknesses is a monotonic function without contrast reversal; this greatly simplifies the interpretation of the resulting images.
This talk will present examples of both dark and bright field STEM imaging. In dark field imaging the detector collects electrons which have interacted with the atomic nuclei and can be approximated by ~Z2 Rutherford scattering (Z = atomic number of the element imaged). In bright field imaging detection of lighter elements such as oxygen next to heavier ones is possible, however, the Z2 scattering approximation no longer holds. This requires extensive simulations of the contrast variations in the images using multi-slice or Bloch wave simulations. Such simulations of image projections with unprecedented structural and compositional complexity guide our experiments and analysis.
However, we need to be mindful of our tendency to “image and imagine structure” which is well documented in the cognitive sciences. Recognizing structure, shape and form is one of the earliest tasks our brain excels at. Solid state chemistry, condensed matter physics and materials science are areas where “pictures” play an important, sometimes heuristic role. It is therefore vital to develop an iconoclastic culture as “the human mind gives way reluctantly when empirical observations curtail perceptual preference” (Anonymous).