Interfaces in Heterogeneous Catalysts: Advancing Mechanistic Understanding through Atomic-Scale Measurements.

Grain boundaries (GBs) modulate the macroscopic properties in polycrystalline materials because they have different atomic and electronic structures from the bulk. Despite the progress on the understanding of GB atomic structures, knowledge of the localized electronic band structures is still lacking. Here, we experimentally characterized the atomic structures and the band gaps of four typical GBs in α-Al2O3 by scanning transmission electron microscopy and valence electron energy-loss spectroscopy (EELS). It was found that the band gaps of the GBs are narrowed by 0.5-2.1 eV compared with that of 8.8 eV in the bulk. By combing core-loss EELS with first-principles calculations, we elucidated that the band gap reductions directly correlate with the decrease of the coordination numbers of Al and O ions at the GBs. These results provide in-depth understanding between the local atomic and electronic band structures for GBs and demonstrate a novel electronic-structure analysis for crystalline defects.

The palladium (Pd)-catalyzed Suzuki reaction is widely applied in the pharmaceutical industry, where constructing highly active and low-cost Pd sites are impendent. Here, we report the fabrication ...

Density functional theory calculations are carried out to investigate the atomic and electronic structures of the $4H\text{-SiC}(0001)/{\mathrm{SiO}}_{2}$ interface. We find two characteristic interface atomic structures in scanning transmission electron microscopy images: One is an interface in which the density of atoms at the first interfacial SiC bilayer is greater than that in the SiC substrate, while the other is an interface where the density of atoms at the first interfacial SiC bilayer is lower. Density functional theory calculations reveal that the difference in the scanning transmission electron microscopy images is a reflection of the atomic structures of these two interfaces. In addition, it has been reported that the floating states, which appear at the conduction band edge of a $4H\text{-SiC}(0001)/{\mathrm{SiO}}_{2}$ interface, affect the electronic structure of the interface and cause marked scattering of the electrons flowing along the interface [S. Iwase, C. J. Kirkham, and T. Ono, Phys. Rev. B 95, 041302(R) (2017)]. Interestingly, we find that the floating states do not appear at the conduction band edge of one of the two interfaces. These results provide physical insights into understanding and controlling the electronic structure and carrier mobility of electronic devices using wide-band-gap semiconductors.

Here we review the scanning transmission electron microscopy (STEM) characterization technique and STEM imaging methods. We describe applications of STEM for studying inorganic nanoparticles, and other uses of STEM in biological and health sciences and discuss how to interpret STEM results. The STEM imaging mode has certain benefits compared with the broad-beam illumination mode; the main advantage is the collection of the information about the specimen using a high angular annular dark field (HAADF) detector, in which the images registered have different levels of contrast related to the chemical composition of the sample. Another advantage of its use in the analysis of biological samples is its contrast for thick stained sections, since HAADF images of samples with thickness of 100-120 nm have notoriously better contrast than those obtained by other techniques. Combining the HAADF-STEM imaging with the new aberration correction era, the STEM technique reaches a direct way to imaging the atomistic structure and composition of nanostructures at a sub-angstrom resolution. Thus, alloying in metallic nanoparticles is directly resolved at atomic scale by the HAADF-STEM imaging, and the comparison of the STEM images with results from simulations gives a very powerful way of analysis of structure and composition. The use of X-ray energy dispersive spectroscopy attached to the electron microscope for STEM mode is also described. In issues where characterization at the atomic scale of the interaction between metallic nanoparticles and biological systems is needed, all the associated techniques to STEM become powerful tools for the best understanding on how to use these particles in biomedical applications.

The atomic structure at the interface between two-dimensional (2D) and three-dimensional (3D) materials influences properties such as contact resistance, photo-response, and high-frequency electrical performance. Moiré engineering is yet to be utilized for tailoring this 2D/3D interface, despite its success in enabling correlated physics at 2D/2D interfaces. Using epitaxially aligned MoS2/Au{111} as a model system, we demonstrate the use of advanced scanning transmission electron microscopy (STEM) combined with a geometric convolution technique in imaging the crystallographic 32 Å moiré pattern at the 2D/3D interface. This moiré period is often hidden in conventional electron microscopy, where the Au structure is seen in projection. We show, via ab initio electronic structure calculations, that charge density is modulated according to the moiré period, illustrating the potential for (opto-)electronic moiré engineering at the 2D/3D interface. Our work presents a general pathway to directly image periodic modulation at interfaces using this combination of emerging microscopy techniques.

Single-Crystalline Mesoporous Palladium and Palladium-Copper Nanocubes for Highly Efficient Electrochemical CO ₂ Reduction

Understanding nanostructures down to the atomic level is the key to optimizing the design of advanced materials with revolutionary novel properties. This requires characterization methods capable of quantifying the three-dimensional (3D) atomic structure with the highest possible precision. A successful approach to reach this goal is to count the number of atoms in each atomic column from 2D annular dark field scanning transmission electron microscopy images. To count atoms with single atom sensitivity, a minimum electron dose has been shown to be necessary, while on the other hand beam damage, induced by the high energy electrons, puts a limit on the tolerable dose. An important challenge is therefore to develop experimental strategies to optimize the electron dose by balancing atom-counting fidelity vs the risk of knock-on damage. To achieve this goal, a statistical framework combined with physics-based modeling of the dose-dependent processes is here proposed and experimentally verified. This model enables an investigator to theoretically predict, in advance of an experimental measurement, the optimal electron dose resulting in an unambiguous quantification of nanostructures in their native state with the highest attainable precision.

Atomic and electronic structure of grain boundaries in a-Al2O3: A combination of machine learning, first-principles calculation and electron microscopy

Structure analysis of beta dicalcium silicate via scanning transmission electron microscope (STEM)

Disentangling the limitations of dipole activity at the atomic scale in the dielectric materials is key to boosting the intrinsic polarization ability and electromagnetic (EM) energy dissipation. Herein, an atomic structure regulation in molybdenum disulfide (MoS 2 ) nanosheets (NSs) connected by multi‐wall carbon nanotubes (MoS 2 ‐MWCNTs) is studied to explore the correlation between the local atomic structure and polarization properties at the atomic scale. Induced by the phase transition engineering, 1T‐phase dominated MoS 2 NSs with centrosymmetric D3d point groups are evolved from the 2H‐phase structure. Atomic‐scale and visualized electric field distribution in the MoS 2 NSs are verified by the advanced integrated differential phase contrast technology and related scanning transmission electron microscopy (iDPC‐STEM) imaging. Periodic atomic structure in the 1T‐phase MoS 2 is constructed to enhance symmetrical electric field distribution and those phase interlaced regions promoted a significant increase in dielectric relaxation. Benefiting the boosted dielectric ability and polarization behaviors, 1T‐phase dominated MoS 2 @MWCNTs (TMC) exhibit superior EM wave energy dissipation with the minimum reflection loss (RL min ) value of −74.3 dB at only 1.7 mm thickness. Changes in atomic structure and electric field study in the MoS 2 NSs help to further elucidate their polarization mechanism and to explore the for studying the mixed‐phase 2D functional nanosheets.

Low-voltage single-atom electron microscopy with carbon-based nanomaterials

Three-Dimensional Visualization of Whole Synapses by Stem Tomography

STEM tomography for thick biological specimens

Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain‐boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic–structure–property relationships, well‐defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain‐boundary orientation relationship and grain‐boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current–voltage ( I–V ) characteristics across ZnO grain boundaries. High‐resolution transmission electron microscopy (HRTEM) observations and lattice‐statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I–V measurements of the undoped ZnO bicrystals showed linear I–V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I–V characteristics of ZnO ceramics. On the other hand, the nonlinear I–V characteristic appeared when doping the boundaries with Pr. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of Pr‐doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I–V characteristics, as the Pr existed in the three‐plus state and would not produce acceptor states in the boundary. First‐principles calculations revealed that Pr doping instead promotes the formations of acceptor‐like native defects, such as Zn vacancies. We believe that such acceptor‐like native defects are microscopic origin of the nonlinear I–V characteristics. Investigations of various types of grain boundaries in the Pr and Co‐codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I–V characteristics significantly depend on the grain‐boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I–V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I–V characteristics. Finally, a Pr and Co‐codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I–V characteristic. This result indicates that ZnO single‐grain‐boundary varistors can be designed by controlling grain‐boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I–V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single‐grain‐boundary varistors.

Historically, the scanning transmission electron microscope (STEM) has not been widely used for phase contrast imaging because the small bright‐field detector required makes use of only a small fraction of the incident electrons and is therefore inefficient with respect to dose. This limitation has hindered the efficient imaging of light elements in STEM. Alternative modes also have limitations. For example, annular dark‐field (ADF) imaging of graphene only makes use of a few percent of the incident electrons, and annular bright‐field imaging (ABF) requires lens aberrations to form an effective phase plate to get contrast from weakly scattering objects. Electron ptychography in the STEM was first demonstrated more than 20 years ago in the context of improving image resolution [1]. At that time, the image field of view was restricted by the limitations of the camera technology and data handling technology. Here we make use of the pnCCD (S)TEM camera, a direct electron pixelated detector from PNDetector, mounted on the JEOL ARM200‐CF aberration corrected microscope. The detector has a grid of 264x264 pixels and operates at a speed of 1000 frames‐per‐second (fps). The detector can achieve a speed of up to 20,000 fps through binning/windowing. ADF images can be recorded simultaneously, as shown by the schematic in Fig. 1. The resulting 4D data set is formed of a series of coherent convergent beam diffraction patterns recorded as a function of illuminating probe position. Here we explore how the bright‐field and dark‐field regions of scattering can be used to enhance the capabilities of STEM. We compare a range of methods that can be used to form the phase image from this data set, including single side‐band [2,3], Wigner distribution deconvolution [4] (used to produce Fig. 2) and ePIE [5]. Phase imaging using ptychography has a relatively simple transfer function [3] and also provides an inherent filter of image noise without reducing the signal strength to form high quality phase images (Fig. 2). Furthermore, the four‐dimensional data set is highly redundant and it is possible to detect and correct for residual aberrations in the image. The ability to deconvolve lens aberrations can further be used to extract three‐dimensional information from a single STEM image acquisition scan. This is achieved by reconstructing the phase image at a specific depth in the sample, which can be performed even though the microscope may not have been focused at that depth (Fig. 3). Finally, we explore the potential for using information outside the bright‐field disc to enhance STEM imaging.