Abstract
Aberration correction is an area of research with a long history in electron microscopy, dating back to the discovery by Scherzer in 1936 that the spherical and chromatic aberrations of conventional round lenses will always be positive [1]. The correction of spherical aberration (Cs) is achieved by incorporating a separate aberration corrector, containing non-round elements, into the electron optical system to eliminate the Cs of the round objective lens. We have recently fitted a quadrupole-octupole Cs-corrector [2], to the 100 kV VG501 STEM at ORNL, shown in Fig. 1. The STEM is particularly well suited to the correction of spherical aberration because the high-angle annular dark field (HA-ADF) image can be directly interpreted as an image of the atomic column positions [3], is relatively insensitive to chromatic aberration and allows the simultaneous acquisition of electron energy loss spectra (EELS).
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