查看更多>>摘要：? 2021 Elsevier B.V.The author regrets the error in the last 2 lines of Table 3 and the erroneous conclusions on the benefits of higher energy Compton Scattering. The last 2 lines in Table 3 relating to 20 keV and 40 keV X-rays are are incorrect. Table 3 should be [Table presented] These incorrect values led to erroneous conclusions on the benefits of higher energy Compton scattering in the last paragraph of Section 4.1. “For Compton scattering it is not meaningful to use contrast factors similar to those defined by equations 25-27 as the low dose means that an impractically high fluence, 4.89 × 1010 X-rays/nm2 (See Table 3), would be needed to give an equivalent dose of 3 × 107 Gy. The pixel size would then be 0.4-0.5 nm, approaching atomic dimensions! In practice the limitation becomes the maximum fluence from the light source. The minimum pixel size is 20 nm similar to the other X-ray imaging methods, if one assumes that 10 counts are a minimum acceptable signal and the fluence is 104 X-rays/ ?2. However this could be considerably reduced for brighter sources.” no longer applies. A simple analysis from equations 24 and 26 shows that coherent elastic scattering is in practice always superior to Compton scattering in terms of dose limited spatial resolution. [Formula presented] The author would like to apologise for any inconvenience caused.
查看更多>>摘要：? 2021 The AuthorsSHELXL [1] which is based on kinematical scattering theory has been a popular software to perform the structural refinement of small molecules. However, it would not be able to distinguish between different enantiomers. The diffraction data has been reprocessed based on dynamical scattering theory, which considers the breakage of inversion symmetry of the diffraction to confirm our structural conclusion properly. In section 2, our original manuscript, we stated the following: “We tested the chirality by refining the same diffraction dataset against an inverted structural model in which the configuration of the amino stereocenter is reversed. By comparing our diffraction data to that expected from the enantiomer, D-serine, we confirmed our correct assignment of stereochemistry through this microED method. The R-factor of the refined model increased from 21.2% to 33.4% and Goodness of Fit (GooF) increased from 1.785 to 2.470, which both indicated an unfavourable refinement against a wrong chirality model. As expected, correct absolute structure would yield better R-factors and other refinement statistics.” We want to revise the above paragraph as following: “To further determine the chirality, we reprocessed the raw data frames based on a dynamic refinement approach [2,3] and tested the chirality by refining the diffraction dataset against an inverted structural model in which the configuration of the amino stereocenter is reversed. The refinement statistics shows that the correct enantiomer, L-serine, has R (obs) of 20.48% vs 21.79% in the incorrect one (D-serine). Similarly, wR (obs) increases from 20.20% to 21.46% from the correct to the incorrect one. As expected, correct absolute structure would yield better (lower) R-factors and other refinement statistics.” Accordingly in the section 3.4 in the original publication, we stated: “In this study, we used XDS to do indexing and integration, SHELXT for ab initio structure determination since it is ideal for such small molecule samples, and SHELXL for structure refinement.” The revised version would be: “In this study, we used XDS to do indexing and integration, SHELXT for ab initio structure determination and SHELXL for structure refinement. The additional processing based on dynamical scattering theory to determine the chirality was done in PETS2 (http://pets.fzu.cz/) and Jana2020 (http://jana.fzu.cz/) ”. This analysis demonstrates the feasibility of determining chirality from our diffraction data collected on a DE64 camera with a 200 KeV electron microscope. Lastly, one more citation error: in main text section 2, we stated: “……the duration of data collection as shown in the crystallographic statistics (Supplemental Table 1 and 2).” Here we meant Table 1 and 2 in main text. The authors would like to apologise for any inconvenience caused.
查看更多>>摘要：? 2022 The AuthorsSupport foils for (scanning) transmission electron microscopy ((S)TEM) samples are commonly amorphous carbon foils. State of the art (S)TEM high resolution imaging methods use ultra-thin carbon foils of only a few nm thickness, especially for imaging beam sensitive materials with low acceleration voltages and electron fluxes. In this study we analyze in situ the effect of chemical etching on a 2 nm amorphous carbon foil due to residual oxygen and by leaking in oxygen into the microscope column. We vary the vacuum level on a Nion UltraStem 100 between ultra high vacuum and that typical in TEM. This enables us to carry out a systematic investigation of chemical etching as function of both, oxygen pressure and electron flux. In addition the results of chemical etching are compared with those of sputtering from knock-on damage leading to the conclusion that chemical etching is the important cause for carbon removal from an amorphous foil at low oxygen pressures and low electron fluxes. We observe that the electron flux dependency using high resolution scanning conditions differs from the case of a resting electron beam. To interpret the results of chemical etching a scanning etching model is proposed that takes care of the specific conditions of STEM.
查看更多>>摘要：? 2022Aberration correction combined with a pixelated detector enable atomic-resolution phase-contrast imaging in the scanning transmission electron microscope (STEM) using all elastically scattered electrons within the illumination cone. The review describes this possibility in detail revisiting the image formation in the STEM on a fundamental quantum-mechanical treatment of electron scattering within the object and the effect of the lenses on the electron wave. Describing electron scattering by means of scattering amplitudes enables a straightforward derivation of a) the reciprocity theorem, b) the optical theorem of electron scattering, and c) the precise formulation of the image intensity distribution in the STEM for different modes of operation. The second part of the review describes in detail a novel method for obtaining pure phase-contrast images in the STEM using the integrated differential phase-contrast (IDPC) procedure. The incorporation of a chromatic (Cc) and spherically (Cs) corrected objective lens and a pixelated detector in the STEM combined with numerical through-focusing enables optical sectioning with atomic 3D resolution of thick objects with about the same dose as that for a 2D object, at least in principle. Numerical simulations of the IDPC transfer function and the point spread function for the focal plane and several reconstructed defocused planes demonstrate the feasibility of the method.
查看更多>>摘要：? 2022In situ TEM utilizing windowed gas cells is a promising technique for studying catalytic processes, wherein temperature is one of the most important parameters to be controlled. Current gas cells are only capable of temperature measurement on a global (mm) scale, although the local temperature at the spot of observation (μm to nm scale) may significantly differ. Thus, local temperature fluctuations caused by gas flow and heat dissipation dynamics remain undetected when solely relying on the global device feedback. In this study, we overcome this limitation by measuring the specimen temperature in situ utilizing parallel-beam electron diffraction at gold nanoparticles. By combining this technique with an advanced data processing algorithm, we achieve sub-Kelvin precision in both, vacuum as well as gaseous environments. Mitigating charging effects is furthermore shown to minimize systematic errors. By utilizing this method, we characterize the local thermal stability of a state-of-the-art gas cell equipped with heating capability in vacuum and under various gas-flow conditions. Our findings provide crucial reference for in situ investigations into catalysis.
查看更多>>摘要：? 2022Defining analysis parameter space for reliable composition measurement of materials is of significance for atom probe tomography applications. This research carefully explores the influence of specimen temperature and ultraviolet laser energy on measured compositions of precipitates and the matrix in an Al-Mg-Si-Cu alloy with atom probe tomography using voltage pulsing and laser pulsing. Low specimen temperature and high laser energy are beneficial to reduce background noise and improve mass resolution. Under both voltage pulsing and laser pulsing, the detected compositions and Mg/Si ratios of precipitates are highly sensitive to specimen temperature in the range of 20 - 80 K. In contrast, at a fixed temperature of 20 K, laser energy variation from 40 to 80 pJ provides consistent measurements in composition and Mg/Si ratio of precipitates. Related field-induced preferential evaporation and surface migration of certain elements have been discussed.
查看更多>>摘要：? 2022 Elsevier B.V.Plan-view transmission electron microscopy (TEM) or electron diffraction imaging of a bulk or 2D material can provide detailed information about the structural or atomic arrangement in the material. A systematic and easily implementable approach to preparing site-specific plan-view TEM samples for 2D thin film materials using FIB is discussed that could be routinely used. The methodology has been successfully applied to prepare samples from 2D materials such as, MoS2 thin film, vertically oriented graphene film (VG), as well as heterostructure material SnTiS3. It is worth mentioning that in contrast to planar conventional graphene, VG grows vertically from the substrate and takes nanosheet arrays. Samples prepared using this methodology provide a simple, faster, and precise course in obtaining valuable structural information. The top-view imaging offers various information about the growth nature of the materials suggesting the efficiency of the sample preparation process.
查看更多>>摘要：? 2022The development of ultrafast electron microscopy (UEM), specifically stroboscopic imaging, has brought the study of structural dynamics to a new level by overcoming the spatial limitations of ultrafast spectroscopy and the temporal restrictions of traditional TEM simultaneously. Combining the concepts governing both techniques has enabled direct visualization of dynamics with spatiotemporal resolutions in the picosecond-nanometer regime. Here, we push the limits of imaging using a pulsed electron beam via RF induced transverse deflection based on the newly developed 200 keV frequency-tunable strip-line pulser. We demonstrate a 0.2 nm spatial resolution and elucidation of magnetic spin induction maps using the phase-microscopy method. We also present beam coherence measurements and expand our study using the breathing modes of a silicon interdigitated comb under RF excitation which achieves improved temporal synchronization between the electron pulse-train and electric field. A new RF holder has also been developed with impedance matching to the RF signal to minimize transmission power loss to samples and its performance is compared with a conventional sample holder.
查看更多>>摘要：? 2022Scanning transmission electron microscopy (STEM) tomography is widely used to reveal three-dimensional (3D) structure of the specimens from nanometers to atomic resolution. Advances in transmission electron microscope enable high resolution and low dose data acquisition for 3D reconstruction to resolve 3D coordinates of single atoms. Meanwhile, the developments of reconstruction algorithms increase the reconstruction quality. However, the as-acquired raw datasets often require alignments before they can be used as inputs to feed a reconstruction algorithm. The effect of post-acquisition data misalignments on the final reconstruction quality of STEM tomography is less discussed. Here, using phantom datasets with a focus on atomic features, we simulate a range of misalignments on each alignment step and correlate the level of misalignments to the final reconstruction quality. The study pinpoints that accurate alignments of tilt angles and stack positions and minimizing scan distortion are important to the reconstruction quality, while the sequence of alignment steps do not affect the reconstruction quality. The study not only emphasizes the most important alignment steps that should be pay attention to for a high-quality 3D reconstruction, but also summarizes post-acquisition data alignment procedures as a primer for new practitioners to use STEM tomography.
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