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Janus monolayers of transition metal dichalcogenides (TMDCs) are promising building blocks for moiré superlattices because of their built‐in electric field and clean fabrication process. In particular, Janus TMDC monolayers can be chemically converted from conventional TMDC monolayers by atomic substitution, enabling the direct formation of lattice‐mismatched heterobilayers from TMDC bilayers. However, the moiré superlattices of Janus heterobilayers have not been studied experimentally. Herein, this work reports the fabrication and characterization of semiconductor moiré superlattices in chemically tailored Janus heterobilayers. The MoSSe/MoSe2 (or WSSe/WSe2) Janus heterobilayers are prepared by replacing the top layer Se atoms with S atoms in MoSe2 (or WSe2) bilayer using H2 plasma treatment. Scanning transmission electron microscopy reveals that an average moiré period of about 14 nm formed due to lattice mismatch resulting from the chalcogen substitutions. The cryogenic photoluminescence spectra show sharp, near‐infrared emissions, which are attributed to excitons trapped by moiré potentials based on comparison with theoretical calculations. The Janus‐based heterostructures provide a long‐period moiré system with built‐in potential even for nontwisted heterobilayers, allowing the functionalization of confined and correlated electron systems.

Abstract The physical and chemical properties of nanoribbon edges are important for characterizing nanoribbons and applying them in electronic devices, sensors, and catalysts. The mechanical response of molybdenum disulfide nanoribbons, which is an important issue for their application in thin resonators, is expected to be affected by the edge structure, albeit this result is not yet being reported. In this work, the width‐dependent Young's modulus is precisely measured in single‐layer molybdenum disulfide nanoribbons with armchair edges using the developed nanomechanical measurement based on a transmission electron microscope. The Young's modulus remains constant at ≈166 GPa above 3 nm width, but is inversely proportional to the width below 3 nm, suggesting a higher bond stiffness for the armchair edges. Supporting the experimental results, the density functional theory calculations show that buckling causes electron transfer from the Mo atoms at the edges to the S atoms on both sides to increase the Coulomb attraction.

Surface modification of high-capacity lithium-rich layered oxides for improved capacity retention is an active area of battery materials research. Here authors demonstrate lithium-rich layered surfaces with a framework matching the host's, but with nickel atoms regularly arranged between layers.

We fabricated and investigated an InAs/high-k/low-k structure in comparison with an InAs/low-k structure, where the former and the latter are respectively obtained by bonding of InAs/Al2O3/AlN and InAs on low-k flexible substrates (FS). The InAs/high-k/low-k (InAs/Al2O3/AlN/FS) exhibits electron mobilities immune to interface fluctuation scattering, whereas this scattering is serious for the InAs/low-k (InAs/FS). Moreover, we find that electron sheet concentrations in the InAs/high-k/low-k are significantly higher than those in the InAs/low-k. From InAs/Al2O3 interface analysis by energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy, we find that the higher electron concentrations can be attributed to natural modulation doping from Al2O3 to InAs.

Focused ion beam technology with light gas ions has recently gained attention with the commercial helium and neon ion beam systems. These ions are atomic and thus, the beam/ sample interaction is well understood. In the case of the nitrogen ion beam, several questions remain due to the molecular nature of the source gas and in particular, if and when the molecular bond is split. Here, the authors report a cross-sectional scanning transmission electron microscopy (STEM) study of irradiated single crystalline silicon by various doses and energies of nitrogen ionized in a gas field ion source. The shape and dimensions of the subsurface damage is compared to Monte Carlo simulations and show very good agreement with atomic nitrogen with half the initial energy. Thus, it is shown that the nitrogen molecule is ionized as such and splits upon impact and proceeds as two independent atoms with half of the total beam energy. This observation is substantiated by molecular dynamics calculations. High resolution STEM images show that the interface between amorphous and crystalline silicon is well defined to few tens of nanometers. [ABSTRACT FROM AUTHOR]

Guinier-Preston (GP) zones formed as nanometer-sized Cu-rich monolayers in α-Al matrix phase during aging process have been investigated using depth-sectioning technique, where through-focal high-angle annular dark-field (HAADF) images were recorded along a <001> zone axis of the α-Al phase using aberration-corrected scanning transmission electron microscopy (STEM). HAADF intensities of the GP zones in the through-focal series vary with defocus settings, depending on the depths of the GP zones in the sample. Determination of the depth of a GP zone is not straightforward because the electron wave function of the STEM probe in the aligned crystalline material is affected by electron channeling. The depths of GP zones were then estimated via comparison with multislice simulations where GP zones were arranged at various depths in a supercell, and HAADF intensities were simulated with various defocus values. We show that the depthsectioning technique can be used to investigate three-dimensional configurations of GP zones in the sample. [ABSTRACT FROM AUTHOR]

The detection of lithium ions is required for characterization of lithium ion batteries, since the movement of lithium ions in the battery is one of the key ways to improve the performance. Annular bright field (ABF) imaging enables us to visualize individual lithium atomic columns simultaneously with heavy elements. Furthermore, it has been found that the number of lithium ions at the column is countable when the specimen is thin. These results suggest that movement of lithium ions in the material can be observed by taking consecutive ABF images during operation or in situ ABF observation. Actually, the spinel structure of L2V4O crystals was directly observed to be transformed into the defective NaCl structure at the moment when lithium ions were extracted from the original position during electron beam irradiation. We clarify the features of ABF imaging by comparing it with HAADF imaging in order to understand what information can be obtained by ABF imaging directly. [ABSTRACT FROM AUTHOR]

An atomic resolution phase map, which enables us to observe charge distribution or magnetic properties at an atomic scale, has been pointed out to be retrieved by transport of intensity equation (TIE) when taking two atomic-resolved transmission electron microscope (TEM) images of small defocus difference. In this work, we firstly obtained the atomic-resolved phase maps of an exfoliated molybdenum disulfide sheet using spherical aberration-corrected transmission electron microscope. We successfully observed 60° grain boundary of mechanically exfoliated monolayer molybdenum disulfide sheet. The relative phase shift of a single molybdenum atomic column to the column consisting of two sulfur atoms was obtained to be about 0.01 rad on average, which was about half lower than the simulated TIE phase map, indicating that the individual atomic sites can be distinguished qualitatively. The appropriate condition for retrieving atomic-resolved TIE phase maps was briefly discussed. [ABSTRACT FROM AUTHOR]

The phase transition of layered manganese oxides to spinel phases is a well-known phenomenon in rechargeable batteries and is the main origin of the capacity fading in these materials. This spontaneous phase transition is associated with the intrinsic properties of manganese, such as its size, preferred crystal positions, and reaction characteristics, and it is therefore very difficult to avoid. The introduction of crystal water by an electrochemical process enables the inverse phase transition from spinel to a layered Birnessite structure. Scanning transmission electron microscopy can be used to directly visualize the rearrangement of lattice atoms, the simultaneous insertion of crystal water, the formation of a transient structure at the phase boundary, and layer-by-layer progression of the phase transition from the edge. This research indicates that crystal water intercalation can reverse phase transformation with thermodynamically favored directionality. [ABSTRACT FROM AUTHOR]

The transport of intensity equation (TIE) is a convenient method of obtaining a potential distribution, as it requires only three transmission electron microscopy images with different amounts of defocus. However, the spatial resolution of the TIE phase map has not yet been evaluated experimentally. In this study, we investigated the phase distribution of spherical gold nanoparticles and its dependence on the defocus difference and found that the spatial resolution was finer than 2 nm, even for a defocus difference of 4 µm. Theoretical calculations reproduced the experimental results well. [ABSTRACT FROM AUTHOR]

By exploiting characteristics such as negligible vapour pressure and ion-conductive nature of an ionic liquid (IL), we established an in situ scanning electron microscope (SEM) method to observe the electrode reaction in the IL-based Li-ion secondary battery (LIB). When 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) with lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) was used as the electrolyte, the Si negative electrode exhibited a clear morphology change during the charge process, without any solid electrolyte interphase (SEI) layer formation, while in the discharge process, the appearance was slightly changed, suggesting that a morphology change is irreversible in the charge-discharge process. On the other hand, the use of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][TFSA]) with Li[TFSA] did not induce a change in the Si negative electrode. It is interesting to note this distinct contrast, which could be attributed to SEI layer formation from the electrochemical breakdown of [C2mim]+ at the Si negative electrode|separator interface in the [C2mim][TFSA]- based LIB. This in situ SEM observation technique could reveal the effect of the IL species electron-microscopically on the Si negative electrode reaction. [ABSTRACT FROM AUTHOR]

A new type of electrochemical cell was developed for in situ transmission electron microscopy observation that enables the electrode materials to be conveniently changed. The electrochemical cell was used to observe the electrochemical growth or dissolution of copper islands on a gold film with simultaneous cyclic voltammetry measurements. The copper islands could be explained by three-dimensional growing model. [ABSTRACT FROM AUTHOR]

We present an innovative method for characterizing the strain field in three dimensions in a hetero interface. Largely defocused probe scanning transmission electron microscopy (LDP-STEM) was employed for imaging the inhomogeneous strain field in a germanium (Ge) layer deposited on a silicon (Si) substrate. In the LDP-STEM image, Ge-atomic columns that are relaxed or strained to the Si substrate in the Si/Ge hetero interface were observed to be distinguishable, allowing for the qualitative characterization of the coherency of the crystal growth. Our results revealed that the strain field is locally modulated along the in-plane direction in the Si/Ge hetero interface. [ABSTRACT FROM AUTHOR]

Gold (Au) nanocontacts (NCs) being stretched in the [110] direction were analyzed using a transmission electron microscopy (TEM) holder equipped with a force sensor to measure spring constants. The Au NCs showed similar change in spring constant with electrical conductance, regardless of the initial shape of NC formed during stretching of contacted two Au wires. The Young’s modulus of the Au NC was estimated to be close to the bulk value. This study demonstrated the performance of TEM-combined method to characterize the mechanical properties of nanomaterials via knowing their shape and size. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including lithium detection through electron microscopy, light elements of lithium-ion batteries (LIB), and energy loss spectroscopy.

Recently, the scanning transmission electron microscope-moiré fringe (SMF) method has attracted much interest because of its precise estimation of the local strain distribution of strained semiconducting devices. In this study, by SMF method, we found that the strain was not uniformly distributed in the InxGa1-xAs layer when it was sandwiched between two InP layers. The most notable point is that the In0.53Ga0.47As layer showed compressive strain at the interface with the InP buffer layer, and showed tensile strain at the interface with InP cap layer even though both InP and In0.53Ga0.47As had the same lattice constant. [ABSTRACT FROM AUTHOR]

Tetrahedrite (Cu12Sb4S13) is a new type of thermoelectric material with an extremely low thermal conductivity attributed to the anomalous large thermal vibration of specific Cu sites. The tetrahedrite crystal was observed from the [111] direction by high-angle annular dark-field (HAADF) imaging and the image intensity was found to be 64% lower at specific sites. This could be explained by the blurring of the intensity distribution owing to a large atomic displacement, suggesting that anomalous large thermal vibrations at specific sites in the crystal can be distinguished in HAADF images. [ABSTRACT FROM AUTHOR]

Density functional theory calculations are carried out to investigate the atomic and electronic structures of the 4H-SiC(0001)/SiO2 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-SiC(0001)/SiO2 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. [ABSTRACT FROM AUTHOR]