Highlights from

Techniques and Material Applications T ickness and Stacking Sequence Determination of Exfoliated Dichalcogenides (1T-TaS2, 2H-MoS2) Using Scanning Transmission Electron Microscopy by R Hovden, P Liu, N Schnitzer, AW Tsen, Y Liu, W Lu, Y Sun, and LF Kourkoutis, Microsc Microanal | doi: 10.1017/S1431927618012436

Layered transition metal dichalcogenides (TMDs) have accrued considerable interest due to their promise for future electronic and optoelectronic technologies. Approaching the 2D limit, thickness and local topology greatly influence macroscopic material properties. Toward developing an understanding of the unique behavior of TMDs, it is necessary to characterize the number of atomic layers and their stacking sequence in a sample. Pairing experimentally recorded high-angle annular dark-field (HAADF) STEM images and position averaged convergent beam electron diffraction (CBED) patterns with quantum mechanical multislice scattering simulations, the thickness and stacking of TMDs can be measured directly. Notably, CBED measurements are insensitive to amorphous surface material and do not require the lattice to be resolved in real space, enabling accurate and high throughput structural characterization. We demonstrate the determination from CBED of crystal thickness in exfoliated 1T-TaS 2 and 2H-MoS 2 to within a single layer for ultrathin <~9 layers and ±1 atomic layer (or better) in thicker specimens, while also revealing information about stacking order.

Techniques and Biological Applications

Biological Applications at the Cutting Edge of Cryo-Electron Microscopy by RS Dillard, CM Hampton, JD Strauss, Z Ke, D Altomara, RC Guerrero-Ferreira, G Kiss, and ER Wright, Microsc Microanal | doi: 10.1017/S1431927618012382

Cryo-electron microscopy (cryo-EM) is a powerful tool for macromo- lecular to near-atomic resolution structure determination in the biological sciences. T e specimen is maintained in a near-native environment within a thin fi lm of vitreous ice and imaged in a transmission electron microscope at cryogenic temperatures. T e images can then be processed by a number of computational methods to produce three-dimensional information. Recent advances in sample preparation, imaging, and data processing have led to tremendous growth in the fi eld of cryo-EM by providing higher-resolution structures and the ability to investigate macromolecules within the context of the cell. We review developments in grid preparation methods and substrates, such as affi nity capture systems, vitrifi cation devices, and gold grids; improvements in detectors that have signifi cantly contributed to the expansion of cryo-EM resolution capabil- ities; contrast enhancement using phase plates; and cryo-correlative light and electron microscopy, which allows us to combine spatio-temporal information from fl uorescence microscopy with structural information from cryo-EM. We have included specifi c biological applications to illustrate these advances.

46 doi: 10.1017/S1551929518000810

CBED patterns ( left ) and HAADF STEM image ( right ) reveal thickness and stacking order in regions of 1T-TaS 2 crystal. The 6-fold symmetry of A..A.. stacking is refl ected in the CBED pattern (orange arrows). Sample thickness is 35 ± 1 u.c as determined by comparison to simulations.

Three-dimensional segmentation of ϕ CbK bacteriophages assembling within a lysing Caulobacter crescentus cell. Cells were infected with ϕ CbK, plunge frozen, and imaged by cryo-electron tomography using Zernike Phase Contrast phase plates, which provide higher contrast to reveal internal features. The bacterial hexagonal surface layer is shown in green, outer membrane in gold, inner membrane in red, and assembling ϕ CbK phages in magenta. Scale bar is 100 nm. • 2018 September

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