Aberration-Corrected Electron Microscopy
BF images of exceptional quality can now be recorded [19].
Challenges remain. The increased sensitivity of TEM
Meanwhile, it is not intuitively obvious that TEM imaging image appearance to slight variations in orientation, thickness,
with the spherical aberration coefficient set at exactly zero or lens defocus underscores the need for image simulations
is necessarily beneficial because amplitude contrast rather in support of image interpretation. Sample preparation also
than phase contrast will dominate the image characteristics. becomes more demanding as resolution limits improve. On
Moreover, slight changes in sample thickness can have a the one hand, clean sample surfaces are needed to ensure that
remarkable impact on TEM image appearance when using oxide or contamination layers do not degrade image quality.
very small Cs values, so that image simulations must still On the other hand, the absence of any amorphous material
be considered as highly desirable in order to avoid possibly makes accurate focusing and astigmatism correction much
erroneous conclusions about very fine image features. Further more difficult to achieve. Also, specimen regions suitable for
investigation of microscope and specimen parameter space is assessment and correction of aberrations using Ronchigrams
urgently required to determine the imaging conditions most or diffractogram tableaus are still required. Thinner crystals are
appropriate for studying specific types of materials. needed for higher-resolution imaging, but local crystal bending
is then more likely to occur. These somewhat conflicting
Prospects and Challenges
and demanding specimen requirements will need concerted
The field of aberration correction is expanding rapidly. All
attention if the potential of the ACEM for solving real materials
of the major TEM manufacturers are aggressively developing
problems is to be fully realized.
and marketing ACEMs, and novel applications of these
Finally, despite the considerable recent technical advances,
instruments are being reported with increasing frequency,
many other problems remain to be addressed. These include
as will be readily evident by reference to the proceedings of
improved instrumental stabilities, better detectors, image
recent electron microscopy meetings. Even more sophisticated
and signal quantification, better sample preparation, and
ACEMs that incorporate correction of lens aberrations of up to
capabilities for in situ chemical studies. Perhaps the ultimate
fifth order have been designed and are being tested [17, 20]. The
goal is three-dimensional tomography at atomic resolution to
first promising results from a prototype system for chromatic
locate and identify all the atoms and to determine how they
aberration correction developed under the DoE-supported
are bonded.
TEAM project have just been reported [21, 22]. The prospects
seem excellent for exciting times ahead for both the microscopy
References
and materials communities. Otto Scherzer would surely be
[1] O Scherzer, Z Physik 101 (1936) 593-603.
delighted to see the fruits of aberration correction!
[2] CL Jia, M Lentzen, and K Urban, Science 299 (2003)
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[3] O Scherzer, Optik 2 (1947) 114-132.
[4] H Rose, Ultramicroscopy 85 (1990) 11-25.
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[5] AV Crewe, Microsc Microanal 10 (2004) 414-419.
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[8] OL Krivanek and PE Mooney, Ultramicroscopy 49 (1993)
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[9] M Haider, et al., Nature 392 (1998) 768-769.
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[10] OL Krivanek, N Dellby, and AR Lupini, Ultramicroscopy
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[11] DJ Smith, Microsc Microanal 14 (2008) 2-14.
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[12] A Orchowski, WD Rau, and H Lichte, Phys Rev Lett 74
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[13] W Coene, et al., Phys Rev Lett 74 (1992) 3743-3746.
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[14] D Gabor, Proc Roy Soc A 197 (1949) 454-487.
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[15] A Tonomura, et al., J Electron Microsc 28 (1979) 1-11.
hard and soft matter is an anticipated growth area where the
[16] D Geiger, et al., Microsc Microanal 14 (2008) 68-81.
new developments in electron microscopy instrumentation
[17] C Kisielowski, et al., Microsc Microanal 14 (2008) 469-477.
and technique can potentially have a great impact, even though
[18] A Borisevich, et al., Proc Nat Acad Sci 103 (2006)
sensitivity to electron-beam irradiation will always remain a
3044-3048.
problematic issue. Likewise, real-time observations of such [19] SJ Pennycook, et al., Adv Imaging Electron Phys 153 (2008)
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diffusion, and phase transformations are among the more [20] OL Krivanek, et al., Ultramicroscopy 96 (2003) 229-237.
important scientific needs for aberration-corrected microscopy. [21] See http://ncem.lbl.gov/team/TEAMpage/TEAMpage.html.
For advances in all of these areas, faster, more sensitive [22] B Kabius, et al., J Electron Microsc 58 (2009) 147-155.
detectors will make a substantial contribution.
2009 September • www.microscopy-today.com 13
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