Atomic electron tomography in three and four dimensions
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Introduction Recent years have witnessed an increasing demand for developing novel nanomaterials and nanostructures for applications in catalysis,1–5 electronics,6–8 energy conversion and storage,9–11 quantum materials,12–14 high-performance metals,15–17 biosensing, and targeted delivery.18–20 To customize and tailor functional properties, the three-dimensional (3D) atomic structures, including crystal defects and disorder, such as grain boundaries, dislocations, interfaces, and point defects, need to be determined. Furthermore, optimizing material synthesis and fabrication is essential for designing devices with desired properties. In order to achieve this, determination of just the 3D structure is not sufficient. Measuring atomic-scale dynamics during the sample fabrication process and under the working conditions of the device is a requirement. Transmission electron microscopy (TEM) is routinely capable of imaging atomic structures, but only provides twodimensional (2D) projection views of 3D crystalline samples. Scanning probe microscopy can image surface structures at atomic resolution, but not subsurface structures. Among several powerful 3D imaging and structural determination methods, including crystallography,21,22 coherent diffractive imaging,23–25 cryo-electron microscopy,26–28 and atom probe tomography,29,30 electron tomography has proven to be an important tool to image 3D structures of heterogeneous biological
and physical samples with nanometer resolution.31–33 By using crystallinity and other prior knowledge as constraints, electron tomography has been used to image the 3D structure of various nanostructures with atomic resolution from a single or a few projection images.34–40 However, the specimen-specific constraints or crystallinity assumption make this technique not generally applicable to determine 3D crystal defects and disordered structures. A major obstacle was overcome by the demonstration of atomic electron tomography (AET) in 2012, which achieved a 2.4 Å resolution without assuming crystallinity for the first time.41 In 2015, AET was further advanced to determine the 3D coordinates of individual atoms in materials with 19 picometer precision.42 The transformation from electron tomography at nanometer resolution43–52 to AET capable of identifying 3D atomic positions in materials represents a quantum leap from qualitative to quantitative material characterization. Subsequently, AET has been applied to study crystal defects such as grain boundaries, dislocations, stacking faults, point defects, and strain tensors with unprecedented 3D detail.41,42,53–56 The experimental atomic coordinates have also been used as direct input to ab initio calculations to correlate 3D atomic structures and the physical, chemical, and electronic properties of materials at the single-atom level.55
Jihan Zhou, Department of Physics and Astronomy, and California NanoSystems Institute, University of California, Los Angeles, USA; [email protected] Yongsoo Yang, Department of Physics, Korea Advanced Instit
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