Electron tomography for functional nanomaterials
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ectron tomography born Tomography, from the Greek words tomos (to slice) and graphe (to draw), describes a technique where materials are sectioned to reveal hidden internal structure. However, electron tomography does not measure specimen slices directly, but instead reconstructs the volumetric structure of nanomaterials from a set of high-resolution projection images across many viewing angles. Electron tomography is advantageous for measuring volumes in the range of (1000 nm)3 to (10 nm)3 at resolutions around 30 to 3 Å.1 Precise and accurate threedimensional (3D) reconstruction of nanoscale materials has had decades of advancements in electron microscopy, and data processing and visualization. Understanding material structure in three dimensions is significant for both biological and inorganic systems—natural or synthetic. Despite the recent surge of interest, electron tomography remarkably predates the personal computer. In 1968, De Rosier and Klug famously reconstructed the helical structure of the T4 bacteriophage tail from a single transmission electron microscope (TEM) projection using prior knowledge about its helical symmetry.2 In 1970, spherical symmetry was exploited to reconstruct a negatively stained human wart virus and tomato bushy virus from multiple projections.3 Electron
tomography, as we know it, was achieved with 3D reconstruction of the low-symmetry fatty acid synthetase molecule in 1974.4,5 In the last two years (2018–2019), more biological structures have been reported by electron tomography than all of the previous years combined in the European Protein Data Bank.6 The demand for electron tomography of nanomaterials has now blossomed across many fields—such as semiconductors and clean energy nanocatalysts. Initially, biological TEM techniques were applied to reconstruct polymer morphologies where similar contrast mechanisms could be employed.7 In 2000, TEM tomography of silver particles on zeolite opened up 3D characterization of inorganic matter.8 While biological specimens are radiation sensitive (
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