Atomic resolution electron tomography
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troduction Solid matter is a three-dimensional (3D) agglomeration of atoms. The properties of materials are determined by the positions of the atoms, their chemical nature, and the bonding between them. If one is able to determine these parameters in 3D, it becomes possible to provide the necessary input into calculations for predicting the physicochemical properties of these atomic agglomerates. Moreover, this will guide the synthesis and development of new nanomaterials. The development of aberration-corrected transmission electron microscopes has enabled structural investigation of nanostructures with resolution on the order of 50 pm.1–4 However, transmission electron microscopy (TEM) images are only twodimensional (2D) projections of 3D (nano) objects. Electron tomography has therefore been developed as a powerful tool to investigate the morphology, 3D structure, and composition of a broad range of materials, many examples of which are presented elsewhere in this issue.5,6 Most tomography results have been achieved at the nanometer level, but open questions in materials science demanded further development of the technique and have pushed the resolution to the atomic scale. For example, it is known that the surface morphology of Au nanocrystals mainly determines their catalytic and optical properties.7,8 Although
the morphology can be characterized using conventional electron tomography with a resolution around a few nanometers or below, it is impossible to precisely determine the exact type of surface planes from such reconstructions. In addition to the morphology, the crystal structure, including defects and (surface) strain, is equally essential, since it will directly affect plasmonic or catalytic properties.9,10 Being able to perform electron tomography with atomic resolution is therefore crucial. Although this is not yet a standard possibility for all structures, significant progress for samples that are relatively stable under the electron beam has recently been achieved using different approaches, which will be further explained in this article.
Visualizing atoms in 3D electron tomography First reports in which the 3D atomic structure of a nanoparticle was visualized were based on a single 2D projection image. Li et al. were able to extract a thickness profile and proposed a 3D model11 through quantitative analysis of the projected intensities in atomically resolved high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images, acquired from an isolated Au nanocluster. However, it is likely there are different 3D models matching a specific 2D image.
Sara Bals, Electron Microscopy for Materials Research Laboratory, University of Antwerp, Belgium; [email protected] Bart Goris, Electron Microscopy for Materials Research Laboratory, University of Antwerp, Belgium; [email protected] Annick De Backer, Electron Microscopy for Materials Research Laboratory, University of Antwerp, Belgium; [email protected] Sandra Van Aert, Electron Microscopy for Materials R
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