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Chromatic aberration affects the performance of the electron microscopes such as the contract of the bright field images and the quality of the dark field images in TEMs. According to Scherzer theorem, spherical (Cs) and chromatic (Cc) aberrations can in principle be corrected if at least one of the four conditions in Scherzer theorem has been broken, while in the most successful practices the corrections have been obtained by breaking rotational symmetry. However, so far no applicable corrector can compensate for both the chromatic and spherical aberration without introducing obvious off-axial aberrations.
Up to 1990s the correction of Cc had experimentally been studied only by Hardy [1], Koops [2] and Hely [3].
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.
In summary, the correction of the chromatic aberrations offers a range of benefits, especially for TEM and STEM:
i) High spatial resolutions in STEM and TEM imaging;
ii) High energy resolution benefiting to fine structure analysis in EELS measurements;
iii) High signal-to-noise ratio in EFTEM by using large energy windows without degradation of spatial resolution;
iv) High contrast transfer;
v) High beam currents available for different analytical analyses;
vi) Additional space, at the sample location, for in situ EMs;
vii) Lower voltages applicable to imaging unstained biological objects with sufficient contrast;
viii) Easy data interpretation.
[1] D.F. Hardy, Dissertation, University of Cambridge 1967.
[2] H. Koops, G. Kuck, O. Scherzer, Optik 48 (1977) 225.
[3] H. Hely, Optik 60 (1982) 353.
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