Analytical electron tomography
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uction Over the past 15 years or so, electron tomography using the (scanning) transmission electron microscope ((S)TEM) has become a technique used routinely to determine the threedimensional (3D) nanoscale morphology of a variety of materials.1–4 As described elsewhere in this issue of MRS Bulletin, a tilt series of images are acquired that, if considered as a series of projections, can be “back-projected” to reconstruct the 3D object of interest. STEM high-angle annular dark-field (HAADF) imaging, which has become the de facto standard for electron tomography of strongly scattering crystalline specimens in the physical sciences, provides some compositional contrast through the atomic-number dependence of the high-angle scattered electrons, but often a more direct determination of composition is needed through electron energy-loss spectroscopy (EELS) or energy-dispersive x-ray spectroscopy (EDXS). Combining spectroscopy and tomography has proven to be a valuable method to determine not just 3D composition, but also chemistry (e.g., local valency), electronic properties, and optical properties. More recently, the ability to undertake 3D nanocrystallography has added to the ensemble of 3D nanoscale imaging techniques now available that, together, called analytical electron tomography (AET) (see Figure 1). Other advanced techniques available in electron microscopy can be coupled to tomography, electron holography in
particular.5 The number of different imaging modes used in electron tomography has seen fivefold growth in the last 15 years. While some of these could also be considered under the umbrella of AET, this review focuses on the aforementioned due to (1) EELS, EDXS, and diffraction being the most widespread techniques in analytical electron microscopy in general; (2) EELS and EDXS being the most widely implemented forms of AET to date; and (3) crystallographic tomography being an area in which we anticipate notable future development.
EFTEM and STEM-EELS tomography An electron spectrometer, placed either in-column or postcolumn, enables the electron beam, having traversed through the sample, to be dispersed in energy, forming an energyloss spectrum. This spectrum can be collected pixel by pixel over a region of interest, a technique known as STEM-EELS spectrum-imaging (described in detail later), or by using energyfiltered TEM (EFTEM), where an energy window may be placed over a characteristic energy-loss feature, and an image is formed using only those electrons that lie within that energy window. For EFTEM using core-loss ionization edges (whose onset is determined by the characteristic energy required to promote an inner-shell electron of a particular type of atom), the background signal under the edge can be estimated from additional EFTEM images (normally two, recorded before the loss) and
Rowan K. Leary, Department of Materials Science and Metallurgy, University of Cambridge, UK; [email protected] Paul A. Midgley, Department of Materials Science and Metallurgy, University of Cambridge, UK; [email protected] d
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