Practical aspects of monochromators developed for transmission electron microscopy

A novel ground-potential monochromator design

An electron monochromator design is presented as an instrumental development for electron energy loss spectroscopy (EELS) and imaging in (scanning) transmission electron microscopy ((S)TEM). The main purpose of this development is enhancing the energy resolving power in spectroscopy and filtering. In addition, it helps reducing the effect of the objective lens' chromatic aberration Cc in imaging and therefore, enhancing the spatial resolving power of electron microscopes. General estimates for the performance of a monochromator in energy distribution and the resulting usable beam currents are given. The special monochromator design presented is a ground-potential monochromator based on magnetic sector fields. The monochromator generates a spatially and angular un-dispersed spot and has no mechanically actuated parts in the filter sections. The optics can be operated at electron acceleration voltages from 30kV to 300kV and shows an energy resolving power of better than 2⋅10-7 relative to the primary electron energy. The actual device is designed to be retro-fittable to microscopes from various manufacturers.

Construction of sp2/sp3 Hybrid Carbon Thin Layers on Silicon Substrates Using Nonequilibrium Excitation Reaction Fields

The authors have developed a crystal growth process that utilizes electron beams from field emission (FE) to grow materials bottom-up by a method other than the transfer of thermal energy. In this study, highly crystalline single-walled carbon nanotubes were used as a field emission electron source. Electron beams with high resolution energy emitted from the source were irradiated onto acetylene gas as a nonequilibrium reaction field to induce acetylene dissociation. The generated carbon ions were then irradiated onto a [100] silicon substrate, resulting in the irradiation of the silicon substrate surface with graphene. Moreover, the crystal growth of sp2/sp3 hybrid carbon thin layers, which is different from the crystal structures of graphite, diamond, and diamond-like carbon, proceeded on the surface of the silicon substrate. Carbon layers on periodic crystal structures whose growth depends at least on the morphology of the substrate are formed through bridging with the binding site of the substrate. The authors have succeeded in developing a nonthermal technique of crystal bridging between different elements. The substrate on which the carbon layer is formed is not limited to silicon; other substrates with various crystal structures and periodicities are expected to be used.

Alignment-invariant signal reality reconstruction in hyperspectral imaging using a deep convolutional neural network architecture

The energy resolution in hyperspectral imaging techniques has always been an important matter in data interpretation. In many cases, spectral information is distorted by elements such as instruments' broad optical transfer function, and electronic high frequency noises. In the past decades, advances in artificial intelligence methods have provided robust tools to better study sophisticated system artifacts in spectral data and take steps towards removing these artifacts from the experimentally obtained data. This study evaluates the capability of a recently developed deep convolutional neural network script, EELSpecNet, in restoring the reality of a spectral data. The particular strength of the deep neural networks is to remove multiple instrumental artifacts such as random energy jitters of the source, signal convolution by the optical transfer function and high frequency noise at once using a single training data set. Here, EELSpecNet performance in reducing noise, and restoring the original reality of the spectra is evaluated for near zero-loss electron energy loss spectroscopy signals in Scanning Transmission Electron Microscopy. EELSpecNet demonstrates to be more efficient and more robust than the currently widely used Bayesian statistical method, even in harsh conditions (e.g. high signal broadening, intense high frequency noise).

Advances and Applications of Atomic-Resolution Scanning Transmission Electron Microscopy

Although scanning transmission electron microscopy (STEM) images of individual heavy atoms were reported 50 years ago, the applications of atomic-resolution STEM imaging became wide spread only after the practical realization of aberration correctors on field-emission STEM/TEM instruments to form sub-Ångstrom electron probes. The innovative designs and advances of electron optical systems, the fundamental understanding of electron–specimen interaction processes, and the advances in detector technology all played a major role in achieving the goal of atomic-resolution STEM imaging of practical materials. It is clear that tremendous advances in computer technology and electronics, image acquisition and processing algorithms, image simulations, and precision machining synergistically made atomic-resolution STEM imaging routinely accessible. It is anticipated that further hardware/software development is needed to achieve three-dimensional atomic-resolution STEM imaging with single-atom chemical sensitivity, even for electron-beam-sensitive materials. Artificial intelligence, machine learning, and big-data science are expected to significantly enhance the impact of STEM and associated techniques on many research fields such as materials science and engineering, quantum and nanoscale science, physics and chemistry, and biology and medicine. This review focuses on advances of STEM imaging from the invention of the field-emission electron gun to the realization of aberration-corrected and monochromated atomic-resolution STEM and its broad applications.

Nonthermal and selective crystal bridging of ZnO grains by irradiation with electron beam as nonequilibrium reaction field

Ceramic particles, such as titanium oxide and indium tin oxide, are expected to be used as electric or catalytic materials for various applications. In this work, we progressed to employ the irradiation with an electron beam as the nonequilibrium reaction field for ceramic composition, and we successfully obtained the basic technology for a ceramic thin-film fabrication using a field emission (FE) electron beam with low energy resolution having a half width under 100 meV that had a homogeneous planar electron emission as the nonequilibrium reaction field. In particular, ZnO particles synthesized by electron beam irradiation show selective crystal bridging along the c-axis during FE electron beam irradiation, which is important for synthesizing poly-ZnO crystals without a heating process, because the energy fluctuations of FE electron beams are small and affect the directionality of ZnO crystal growth along the c-axis. This accomplishment may make a significant contribution to the analysis of the formation mechanism of ZnO particles with a uniform morphology and crystal structure by the FE electron beam during the crystallization. Moreover, we will be able to provide basic elements for next-generation nanodevices with highly functional properties by controlling each terminal crystal interface of metals, ceramics, and semiconductors with this technique.

Nanometer-Thick Crystalline Carbon Films Having a Spinel Structure Grown on ZnO Substrates: Implications for New Ceramic-Carbon Composition

I developed a bottom-up process of crystal growth using a field emission (FE) electron beam without transfer of heat energy. In this study, highly crystalline single-walled carbon nanotubes were used as the FE electron source. Acetylene was irradiated with an electron beam of high-resolution energy emitted from the electron source. Then, zinc oxide (ZnO) was irradiated with the carbon-based ions dissociated from the acetylene and electron beam, which formed a nonequilibrium excitation reaction field. As a result, a crystalline carbon thin film with a spinel-like structure different from the structures of graphite and diamond was grown on the ZnO surface. It is considered that the carbon film can be formed on substrates with a periodic crystal structure, not only ZnO. I confirmed that a carbon film with a periodic crystal structure independent of the crystal structure of the underlying substrate was grown, which bridged with the substrate. Thus, I have established a technique of crystal bridging between a ceramic and carbon for the first time to the best of our knowledge.

Imaging Beam-Sensitive Materials by Electron Microscopy

Electron microscopy allows the extraction of multidimensional spatiotemporally correlated structural information of diverse materials down to atomic resolution, which is essential for figuring out their structure-property relationships. Unfortunately, the high-energy electrons that carry this important information can cause damage by modulating the structures of the materials. This has become a significant problem concerning the recent boost in materials science applications of a wide range of beam-sensitive materials, including metal-organic frameworks, covalent-organic frameworks, organic-inorganic hybrid materials, 2D materials, and zeolites. To this end, developing electron microscopy techniques that minimize the electron beam damage for the extraction of intrinsic structural information turns out to be a compelling but challenging need. This article provides a comprehensive review on the revolutionary strategies toward the electron microscopic imaging of beam-sensitive materials and associated materials science discoveries, based on the principles of electron-matter interaction and mechanisms of electron beam damage. Finally, perspectives and future trends in this field are put forward.

Improving the sensitivity of X-ray microanalysis in the analytical electron microscope

A study of the influence of experimental parameters on the sensitivity of x-ray energy dispersive spectroscopy in the analytical electron microscope from 20-200 kV is conducted. Optimization of conditions in the next generation of aberration corrected AEM instrument coupled with an array configuration of SDD detectors can potentially yield a 10-20 fold improvement over older Si(Li) systems still in use today.

Atomic and nanoscale imaging of a cellulose nanofiber and Pd nanoparticles composite using lower-voltage high-resolution TEM

We have examined the advanced application of transmission electron microscopy (TEM) for the structural characterization of a composite of cellulose nanofiber (CNF) and palladium (Pd) nanoparticles. In the present study, we focused on electron-irradiation damage and optimization of high-resolution TEM imaging of the composite. The investigation indicates that the CNF breaks even under low-electron-dose conditions at an acceleration voltage of 200 kV. We then applied lower-voltage TEM at 60 kV using a spherical aberration corrector and a monochromator, in order to reduce electron-irradiation damage and improve the spatial resolution. The TEM observation achieved high-resolution imaging and revealed the existence of small Pd nanoparticles, around 2 nm in diameter, supported on the CNF. It is considered that the use of a monochromator in combination with spherical aberration correction contributed to the atomic and nanoscale imaging of the composite, owing to the improvement of the information limit under a lower-acceleration voltage.

Basics and applications of ELNES calculations

The electron energy loss near edge structures (ELNES) appearing in an electron energy loss spectrum obtained through transmission electron microscopy (TEM) have the potential to unravel atomic and electronic structures with sub-nano meter resolution. For this reason, TEM-ELNES has become one of the most powerful analytical methods in materials research. On the other hand, theoretical calculations are indispensable in interpreting the ELNES spectrum. Here, the basics and applications of one-particle, two-particle and multi-particle ELNES calculations are reviewed. A key point for the ELNES calculation is the proper introduction of the core-hole effect. Some applications of one-particle ELNES calculations to huge systems of more than 1000 atoms, and complex systems, such as liquids, are reported. In the two-particle calculations, the importance of the correct treatment of the excitonic interaction is demonstrated in calculating the low-energy ELNES, for example at the Li-K edge. In addition, an unusually strong excitonic interactions in the O-K edge of perovskite oxides is identified. The multi-particle calculations are necessary to reproduce the multiplet structures appearing at the transition metal L2,3-edges and rare-earth M4,5-edges. Applications to dilute magnetic semiconductors and Li-ion battery materials are presented. Furthermore, beyond the 'conventional' ELNES calculations, theoretical calculations of electron/X-ray magnetic circular dichroism (MCD) and the vibrational information in ELNES, are reported.