Probing plasmonic excitation mechanisms and far-field radiation of single-crystalline gold tapers with electrons.

Conical metallic tapers represent an intriguing subclass of metallic nanostructures, as their plasmonic properties show interesting characteristics in strong correlation to their geometrical properties. This is important for possible applications such as in the field of scanning optical microscopy, as favourable plasmonic resonance behaviour can be tailored by optimizing structural parameters like surface roughness or opening angle. Here, we review our recent studies, where single-crystalline gold tapers were investigated experimentally by means of electron energy-loss and cathodoluminescence spectroscopy techniques inside electron microscopes, supported by theoretical finite-difference time-domain calculations. Through the study of tapers with various opening angles, the underlying resonance mechanisms are discussed. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Atomic-scale investigation of the heterogeneous structure and ionic distribution in an ionic liquid using scanning transmission electron microscopy.

Ionic liquids show characteristic properties derived from them being composed of only molecular ions, and have recently been used as solvents for chemical reactions and as electrolytes for electrochemical devices. The liquid structures, i.e., ionic distributions, form when solutes are dissolved in ionic liquids and fundamentally affect the reactions and transfer efficiency in such solutions. In this study, we directly observe the liquid structure in a solution of the long-chain ionic liquid 1-octyl-3-methylimidazolium bromide (C8mim Br) and barium stearate (Ba(C17H35COO)2) using the annular dark-field method of scanning transmission electron microscopy (ADF-STEM). The ADF image shows a 10 nm-scale heterogeneity in the image intensity, which reflects the heterogeneous ionic distribution in the solution. The number density distributions of all the component ions (C8mim+, Br-, Ba2+, and C17H35COO-) were estimated from the ADF image intensity and then visualized. These ionic distribution maps depicted the spatial relationships between the ions at the sub-nanometer scale and revealed that the heterogeneity is largely derived from the large differences in size, charge distributions, and van der Waals interactions.

Material structure, properties, and dynamics through scanning transmission electron microscopy.

Scanning transmission electron microscopy (STEM) has advanced rapidly in the last decade thanks to the ability to correct the major aberrations of the probe-forming lens. Now, atomic-sized beams are routine, even at accelerating voltages as low as 40 kV, allowing knock-on damage to be minimized in beam sensitive materials. The aberration-corrected probes can contain sufficient current for high-quality, simultaneous, imaging and analysis in multiple modes. Atomic positions can be mapped with picometer precision, revealing ferroelectric domain structures, composition can be mapped by energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS), and charge transfer can be tracked unit cell by unit cell using the EELS fine structure. Furthermore, dynamics of point defects can be investigated through rapid acquisition of multiple image scans. Today STEM has become an indispensable tool for analytical science at the atomic level, providing a whole new level of insights into the complex interplays that control material properties.

Strain-Dependent Edge Structures in MoS2 Layers.

Edge structures are low-dimensional defects unavoidable in layered materials of the transition metal dichalcogenides (TMD) family. Among the various types of such structures, the armchair (AC) and zigzag (ZZ) edge types are the most common. It has been predicted that the presence of intrinsic strain localized along these edges structures can have direct implications for the customization of their electronic properties. However, pinning down the relation between local structure and electronic properties at these edges is challenging. Here, we quantify the local strain field that arises at the edges of MoS2 flakes by combining aberration-corrected transmission electron microscopy (TEM) with the geometrical-phase analysis (GPA) method. We also provide further insight on the possible effects of such edge strain on the resulting electronic behavior by means of electron energy loss spectroscopy (EELS) measurements. Our results reveal that the two-dominant edge structures, ZZ and AC, induce the formation of different amounts of localized strain fields. We also show that by varying the free edge curvature from concave to convex, compressive strain turns into tensile strain. These results pave the way toward the customization of edge structures in MoS2, which can be used to engineer the properties of layered materials and thus contribute to the optimization of the next generation of atomic-scale electronic devices built upon them.

Enhancement of noisy EDX HRSTEM spectrum-images by combination of filtering and PCA.

STEM spectrum-imaging with collecting EDX signal is considered in view of the extraction of maximum information from very noisy data. It is emphasized that spectrum-images with weak EDX signal often suffer from information loss in the course of PCA treatment. The loss occurs when the level of random noise exceeds a certain threshold. Weighted PCA, though potentially helpful in isolation of meaningful variations from noise, might provoke the complete loss of information in the situation of weak EDX signal. Filtering datasets prior PCA can improve the situation and recover the lost information. In particular, Gaussian kernel filters are found to be efficient. A new filter useful in the case of sparse atomic-resolution EDX spectrum-images is suggested.

Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt.

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

Development of a monochromator for aberration-corrected scanning transmission electron microscopy.

In this article, we report the development of a new 200-kV analytical electron microscope equipped with a monochromator with an integrated double Wien-filter system. It enables us to study the electronic structures of materials in detail using electron energy-loss spectroscopy (EELS) analysis at an atomic scale. A highly monochromated and isotropically round electron probe is produced on the specimen plane. The ultimate energy resolutions with 0.1-s acquisition times are measured to be 36 meV at 200 kV and 30 meV at 60 kV. In an EELS mapping experiment performed on SrTiO3 with a monochromated electron probe whose energy resolution is 146 meV, an elemental map exhibits atomic resolution.

Magnified pseudo-elemental map of atomic column obtained by Moiré method in scanning transmission electron microscopy.

Moiré method in scanning transmission electron microscopy allows observing a magnified two-dimensional atomic column elemental map of a higher pixel resolution with a lower electron dose unlike conventional atomic column mapping. The magnification of the map is determined by the ratio between the pixel size and the lattice spacing. With proper ratios for the x and y directions, we could observe magnified elemental maps, homothetic to the atomic arrangement in the sample of SrTiO3 [0 0 1]. The map showed peaks at all expected oxygen sites in SrTiO3 [0 0 1].

Measurement of vibrational spectrum of liquid using monochromated scanning transmission electron microscopy-electron energy loss spectroscopy.

Investigations on the dynamic behavior of molecules in liquids at high spatial resolution are greatly desired because localized regions, such as solid-liquid interfaces or sites of reacting molecules, have assumed increasing importance with respect to improving material performance. In application to liquids, electron energy loss spectroscopy (EELS) observed with transmission electron microscopy (TEM) is a promising analytical technique with the appropriate resolutions. In this study, we obtained EELS spectra from an ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethyl-sulfonyl) imide (C2mim-TFSI), chosen as the sampled liquid, using monochromated scanning TEM (STEM). The molecular vibrational spectrum and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the liquid were investigated. The HOMO-LUMO gap measurement coincided with that obtained from the ultraviolet-visible spectrum. A shoulder in the spectrum observed ∼0.4 eV is believed to originate from the molecular vibration. From a separately performed infrared observation and first-principles calculations, we found that this shoulder coincided with the vibrational peak attributed to the C-H stretching vibration of the [C2mim(+)] cation. This study demonstrates that a vibrational peak for a liquid can be observed using monochromated STEM-EELS, and leads one to expect observations of chemical reactions or aids in the analysis of the dynamic behavior of molecules in liquid.

Unusual stacking variations in liquid-phase exfoliated transition metal dichalcogenides.

Liquid-phase exfoliation of layered materials offers a large-scale approach toward the synthesis of 2D nanostructures. Structural properties of materials can however change during transition from bulk to the 2D state. Any such changes must be examined and understood for successful implementation of 2D nanostructures. In this work, we demonstrate nonbulk stacking sequences in the few-layer MoS2 and WS2 nanoflakes produced by liquid-phase exfoliation. Our analysis shows that nonbulk stacking sequences can be derived from its bulk counterparts by translational shifts of the layers. No structural changes within the layers were observed. Twenty-seven MoS2 and five WS2 nanoflakes were imaged and analyzed. Nine MoS2 and four WS2 nanoflakes displayed nonbulk stacking. Such dominance of the nonbulk stacking suggests high possibility of unusual stacking sequences in other 2D nanostructures. Notably, the electronic structure of some non bulk stacked bilayers presents characteristics which are uncommon to either the bulk phase or the single monolayer, for instance, a spin-split conduction band bottom. Our main characterization technique was annular dark-field scanning transmission electron microscopy, which offers direct and reliable imaging of atomic columns. The stacking characterization approach employed here can be readily applied toward other few-layer transition metal chalcogenides and oxides.

Impurity induced non-bulk stacking in chemically exfoliated h-BN nanosheets.

Structural characterization of 2D nanomaterials is an important step towards their future applications. In this work we carried out imaging and structural analysis of 2D h-BN produced by chemical-exfoliation, emphasizing the stacking order in few-layer sheets. Our analysis, for the first time has shown conclusively that non-bulk stacking can exist in 2D h-BN.

Annular electron energy-loss spectroscopy in the scanning transmission electron microscope.

We study atomic-resolution annular electron energy-loss spectroscopy (AEELS) in scanning transmission electron microscopy (STEM) imaging with experiments and numerical simulations. In this technique the central part of the bright field disk is blocked by a beam stop, forming an annular entry aperture to the spectrometer. The EELS signal thus arises only from electrons scattered inelastically to angles defined by the aperture. It will be shown that this method is more robust than conventional EELS imaging to variations in specimen thickness and can also provide higher spatial resolution. This raises the possibility of lattice resolution imaging of lighter elements or ionization edges previously considered unsuitable for EELS imaging.

Identification of active atomic defects in a monolayered tungsten disulphide nanoribbon.

Edge structures and atomic defects can significantly affect the physical and chemical properties of low-dimensional materials, such as nanoribbons, and therefore merit a thorough investigation at the atomic scale. Here, we successfully discriminate single atoms on a monolayered tungsten disulphide nanoribbon by means of time-resolved annular dark-field imaging and spatially resolved electron energy-loss spectroscopy. We unambiguously identify and successfully visualize in motion atomic defects, such as vacancies and edge atoms, using scanning transmission electron microscopy. We also report a direct observation of slip deformation in the nanoribbons and present evidence demonstrating that the deformation process involves the migration of vacancies and rearrangement of tungsten atoms. Single-atom defects are successfully observed for the first time during plastic deformation.

Direct imaging of hydrogen-atom columns in a crystal by annular bright-field electron microscopy.

Enhancing the imaging power of microscopy to identify all chemical types of atom, from low- to high-atomic-number elements,would significantly contribute for a direct determination of material structures. Electron microscopes have successfully provided images of heavy-atom positions, particularly by the annular dark-field method, but detection of light atoms was difficult owing to their weak scattering power. Recent developments of aberration-correction electron optics have significantly advanced the microscope performance, enabling identification of individual light atoms such as oxygen, nitrogen, carbon, boron and lithium. However, the lightest hydrogen atom has not yet been observed directly, except in the specific condition of hydrogen adatoms on a graphene membrane. Here we show the first direct imaging of the hydrogen atom in a crystalline solid YH(2), based on a classic 'hollow-cone' illumination theory combined with state-of-the-art scanning transmission electronmicroscopy. The optimized hollow-cone condition derived from the aberration-corrected microscope parameters confirms that the information transfer can be extended to 22.5 nm(-1), which corresponds to a spatial resolution of about 44.4 pm. These experimental conditions can be readily realized with the annular bright-field imaging in scanning transmission electron microscopy according to reciprocity, revealing successfully the hydrogen-atom columns as dark dots, as anticipated from phase contrast of a weak-phase object.

Direct imaging of lithium atoms in LiV2O4 by spherical aberration-corrected electron microscopy.

We visualized lithium atom columns in LiV2O4 crystals by combining scanning transmission electron microscopy with annular bright field (ABF) imaging using a spherical aberration-corrected electron microscope (R005) viewed from the [110] direction. The incident electron beam was coherent with a convergent angle of 30 mrad (semi-angle), and the detector collected scattered electrons over 20-30 mrad (semi-angle). The ABF image showed dark dots corresponding to lithium, vanadium and oxygen columns.

Evidence of drug confinement into silica mesoporous matrices by STEM spherical aberration corrected microscopy.

For the first time it has been possible to detect drug molecules confined into the inner part of pore channels of ordered mesoporous materials. This has been possible using spherical aberration correctors incorporated to a STEM microscope, which allows illuminating an individual atom with the electron beam to identify an unknown substance.

Cs-corrected HAADF-STEM imaging of silicate minerals.

Significant improvement of the resolution for electron microscopy by the development of the spherical aberration corrector (Cs-corrector) brings valuable information to understand local atomic structures in silicate minerals. Cation columns separated by ca. 1.5 A, which originate from the ionic radius of oxygen anion or half of the closest oxygen-oxygen distance, are common in silicates and they are easily resolved by a Cs-corrected transmission electron microscope (TEM) or scanning TEM (STEM) with a resolution close to 1 A. High-angle annular dark-field (HAADF) imaging using Cs-corrected STEM was applied to orthopyroxene (Opx) with augite lamellae, and cronstedtite, an iron-bearing sheet silicate. Noisy contrast in the HAADF images was compensated by advanced noise filtering techniques. All cation columns in the pyroxene structure were resolved in the HAADF images from the c-axis. A sub-angstrom difference in the position of the M2 site between Opx and augite, which is caused by the occupation of the site by Fe (Opx) and Ca (augite), was clearly detected in the HAADF image as well as different contrasts of the cation columns related to occupying elements. A pair of tetrahedral cation columns separated by ca. 1.5 A in cronstedtite observed along [100] directions frequently show unequal contrast, suggesting a difference of the amounts of substituted Fe(3+) between the two columns. Comparison between the experimental contrast with a simple simulation suggests the distribution of Fe(3+) in a tetrahedral sheet avoiding linkage of Fe(3+) coordinating tetrahedrons.

HAADF-STEM study on the early stage of precipitation in aged Al-Ag alloys.

We report on the structure of Ag precipitates in aged Al-Ag alloys using transmission electron microscopy and high-angle annular detector darkfield scanning transmission electron microscopy (HAADF-STEM). Irregularly shaped small Ag particles of 1-2 nm dominate the alloy annealed at 140 degrees C for 10 h. These particles are present also within large precipitates (10-50 nm), which are often characterized by their {100} and {110} facets. In addition, atomic-resolution HAADF-STEM images revealed that Ag atoms tend to form {111} planar clusters, which criss-cross a colony of the irregularly shaped small Ag precipitates.