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Optimization of exit-plane waves restored from HRTEM through-focal series
Authors:Rolf Erni  Marta D. Rossell  Philip N.H. Nakashima
Affiliation:1. Electron Microscopy Center, Swiss Federal Laboratories for Materials Testing and Research (Empa), Überlandstr. 129, CH-8600 Dübendorf, Switzerland;2. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, One Cyclotron Rd., Berkeley CA 94704, USA;3. Laboratory for Multifunctional Materials, Department of Materials, ETH Zürich, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich, Switzerland;4. ARC Centre of Excellence for Design in Light Metals, Monash University, Victoria 3800, Australia
Abstract:Atomic-resolution transmission electron microscopy has largely benefited from the implementation of aberration correctors in the imaging part of the microscope. Though the dominant geometrical axial aberrations can in principle be corrected or suitably adjusted, the impact of higher-order aberrations, which are mainly due to the implementation of non-round electron optical elements, on the imaging process remains unclear. Based on a semi-empirical criterion, we analyze the impact of residual aperture aberrations on the quality of exit-plane waves that are retrieved from through-focal series recorded using an aberration-corrected and monochromated instrument which was operated at 300 kV and enabled for an information transfer of ∼0.05 nm. We show that the impact of some of the higher-order aberrations in retrieved exit-plane waves can be balanced by a suitable adjustment of symmetry equivalent lower-order aberrations. We find that proper compensation and correction of 1st and 2nd order aberrations is critical, and that the required accuracy is difficult to achieve. This results in an apparent insensitivity towards residual higher-order aberrations. We also investigate the influence of the detector characteristics on the image contrast. We find that correction for the modulation transfer function results in a contrast gain of up to 40%.
Keywords:HRTEM   Through-focal series reconstruction   Exit-plane wave   Aberration correction   Modulation transfer function
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