Aberration-corrected scanning transmission electron microscopy (STEM) has become a powerful technique for materials characterization of complex nanostructures. Recent progress in the development of quantitative methods allows us to extract structural and chemical information from experimental images in 2D as well as in 3D. Most studies concern nanostructures which are relatively stable under the incoming electron beam and therefore the atomic structure under investigation can be assumed to remain unchanged during illumination. However, radiation damage becomes increasingly relevant not only in biological studies but also in the study of nanostructures. In a post aberration-correction era, an important challenge is therefore to push the development of quantitative methods toward its fundamental limits. Therefore, the allowable electron dose needs to be used in the most optimal way. For that purpose, new strategies to optimize the microscope and detector settings and to analyze the experimental images will be demonstrated.
In quantitative STEM, images are treated as datasets from which structure parameters are determined by comparison with image simulations or by using parameter estimation-based methods. In order to retrieve the 3D atomic structure, the use of scattering cross-sections to count atoms along the viewing direction has become a successful technique and is extended to unscramble mixed elements. Cross-sections define the total scattered intensity for each atomic column. Their high sensitivity in combination with a statistical analysis enables us to count atoms with single-atom sensitivity. However, sufficient statistics is needed, limiting its usefulness to study beam-sensitive nanoparticles. Therefore, a hybrid method combining the benefits of the statistics-based and image simulations-based method has been developed and applied. Finally, the effect of electron dose, scan noise, and the choice of detector settings on the atom-counting performance will be demonstrated. It will be shown how to balance atom-counting reliability and structural damage as a function of electron dose.