Image and atomic reconstruction
From the abstract:
Although atomic-resolution electron microscopy has been feasible for nearly four decades, neither electron tomography nor any other experimental technique has yet demonstrated atomic resolution in three dimensions. Here we report the 3D reconstruction of a complex crystalline nanoparticle at atomic resolution. To achieve this, we combined aberration-corrected scanning transmission electron microscopy, statistical parameter estimation theory, and discrete tomography, Unlike conventional electron tomography, only two images of the target—a silver nanoparticle embedded in an aluminium matrix—are sufficient for the reconstruction when combined with available knowledge about the particle’s crystallographic structure. Additional projections confirm the reliability of the result. The results we present help close the gap between the atomic resolution achievable in two-dimensional electron micrographs and the coarser resolution that has hitherto been obtained by conventional electron tomography. [Links added]
This computational reconstruction technique (imaging aided by inference) requires only two images, but it relies on prior knowledge (or assumptions) about the structure: for example, that the crystal is face-centered cubic structure, has no holes, and no deep grooves in its surface. The data analysis uses a stochastic method (simulated annealing), and 16 independent reconstruction runs gave a difference in the positioning of 41 (out of 784) atoms in the specimen, so the result isn’t fully determined.
When a structure results from what should be an atomically precise process, knowledge (or reasonable expectations) will typically be extensive and detailed, leaving only narrow questions to be answered by imaging — the orientation of parts across an interface, for example, or the nature of a defect in a mostly-correct structure. Imaging aided by inference should again be quite powerful, and helpful in debugging fabrication processes.
The specimen in the current study is a silver nanoparticle in an aluminum matrix, that is, an array of electron-dense atoms in a radiation-tolerant structure. Studies of biomolecular structures still face the problems of radiation damage to delicate structures that provide lower contrasts in electron density, but cryogenic electron tomography has been advancing in this domain, too. Here, covalent structures with local conformational constraints are typically known at the outset, and this has been used to provide a bridge from low-resolution images to inferred (almost) atomically precise structures.
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