Materials Advances through Aberration-Corrected Electron Microscopy
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Materials Advances
through AberrationCorrected Electron Microscopy
S.J. Pennycook, M.Varela, C.J.D. Hetherington, and A.I. Kirkland Abstract Over the last few years, the performance of electron microscopes has undergone a dramatic improvement, with achievable resolution having more than doubled. It is now possible to probe individual atomic sites in many materials and to determine atomic and electronic structure with single-atom sensitivity. This revolution has been enabled by the successful correction of the dominant aberrations present in electron lenses. In this review, the authors present a brief overview of these instrumental advances, emphasizing the new insights they provide to several areas of materials research. Keywords: electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM).
Introduction Richard Feynman is widely associated with the nanoscience revolution as a result of his prophetic lecture “There’s Plenty of Room at the Bottom,” given in 1959.1 Less well known is that, in the same lecture, he challenged us to “improve the resolution of the electron microscope by 100 times,” his goal being to “just look at the thing.” At the time of the lecture, it was understood that the resolution of the electron microscope was fundamentally limited by the intrinsic spherical aberration (by which rays far from the optical axis are overfocused) in the primary, image-forming magnetic objective lens. This spherical aberration is unavoidable with rotationally symmetric magnetic fields, and Feynman’s response was simply “why must the field be symmetric?” Today, sophisticated electron-optical components are available which break this rotational symmetry, correcting spherical aberration and leading to higher resolution. The impact of these correctors on electron microscopy is evident from Figure 1, which shows the improvement in resolution from the era of optical microscopy to the present, although we
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are not yet close to Feynman’s challenge of 0.01 nm. The major reason that it has taken more than four decades to achieve this is the requirement for sophisticated computer control to measure the aberrations present and to iteratively adjust the necessary lens currents. As an example, optimization of the 12 independent thirdorder aberrations (including the spherical aberration) requires precise control of 40 or more optical elements. However, the current generation of correctors readily achieve this and are able to correct the electron wave front to a degree of perfection better than a quarter wavelength (0.5 pm) over 70 µm. There are presently two designs of aberration corrector available for electron microscopes produced by Nion Co.3 and by CEOS GmbH.4 The former has been exclusively used in scanning transmission electron microscopy (STEM), whereas the latter has been used in both STEM and in conventional transmission electron microscopy (CTEM). Although the optical design of these two systems differs, it is important to appreciate that both
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