Practical Electron Microscopy and Database

In aberration-corrected EMs (electron microscopes) a combination of hexapole or octupole lenses are used in a aberration corrector which lacks rotational (circular) symmetry and thus doesn’t have to have a positive spherical aberration like conventional, round magnetic lenses. The overall spherical aberration of the objective lens and this corrector can be minimized by the operator, which can significantly improve the point to point resolution by choosing a smaller spherical aberration. Furthermore, the corrector has also made it possible to use negative spherical aberrations giving bright atoms on a dark background, which actually enhances the contrast in the images compared with the images taken with a large, positive spherical aberration.

For instance, improvement of spatial resolution based on aberration correction both in scanning and stationary modes in TEM (transmission electron microscopes) is ~20 – 40% [1-6]. The attainable STEM resolution has improved by about 2.5 ×.

Corrections of spherical aberration and off-axial coma have been recently used for improving the spatial resolution of TEM imaging. However, these corrections cannot further improve the resolution when reaching 0.5 Å at voltages ≤ 200 kV due to chromatic aberration. Fortunately, the chromatic aberration can decrease significantly by employing a monochromator or be corrected by crossed electric and magnetic quadrupole elements.

[1] Batson, P. E., Dellby, N. & Krivanek, O. L. Sub-angstrom resolution using aberration corrected electron optic. Nature 418, 617-620 (2002).
[2] Jia, C. L., Lentzen, M. & Urban, K. Atomic-resolution imaging of oxygen in perovskite ceramics. Science 299, 870-873 (2003).
[3] Nellist, P. D. et al. Direct sub-angstrom imaging of a crystal lattice. Science 305, 1741 (2004).
[4] Muller, D. A. et al. Atomic-scale chemical imaging of composition and bonding by aberration-corrected microscopy. Science 319, 1073-1076 (2008).
[5] Kimoto, K. et al. Element-selective imaging of atomic columns in a crystal using STEM and EELS. Nature 450, 702-704 (2007).
[6] Zhu, Y. & Wall, J. in Aberration-corrected Electron Microscopy, A thematic volume of Advances in Imaging & Electron Physics (ed. Hawkes, P. W.) 481-523 (Elsevier/Academic, 2008).