Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Symmetry Engineering in Twisted Bilayer WTe2.

The fabrication of artificial structures using a twisted van der Waals assembly has been a key technique for recent advancements in the research of two-dimensional (2D) materials. To date, various exotic phenomena have been observed thanks to the modified electron correlation or moiré structure controlled by the twist angle. However, the twisted van der Waals assembly has further potential to modulate the physical properties by controlling the symmetry. In this study, we fabricated twisted bilayer WTe2 and demonstrated that the twist angle successfully controls the spatial inversion symmetry and hence the spin splitting in the band structure. Our results reveal the further potential of a twisted van der Waals assembly, suggesting the feasibility of pursuing new physical phenomena in 2D materials based on the control of symmetry.

Stiffer Bonding of Armchair Edge in Single-Layer Molybdenum Disulfide Nanoribbons.

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.

Singular behaviour of atomic ordering in Pt-Co nanocubes starting from core-shell configurations.

The ordered structure of platinum-cobalt (Pt-Co) alloy nanoparticles has been studied actively because the structure influences their magnetic and catalytic properties. On the Pt-Co alloy's surface, Pt atoms preferentially segregate during annealing to reduce the surface energy. Such surface segregation has been shown to promote the formation of an ordered structure near the surface of Pt-Co thin films. Although this phenomenon seems also useful to control the nanoparticle structure, this has not been observed. Here, we have studied the ordered structure in annealed Pt@Co core-shell nanoparticles using a scanning transmission electron microscope. The nanoparticles were chemically synthesized, and their structural changes after annealing at 600 °C, 700 °C, and 800 °C for 3 h were observed. After being annealed at 600 °C and 800 °C, the particles contained the L12-Pt3Co ordered structure. The structure seems reasonable considering an initial Pt : Co ratio of ∼4 : 1. However, we found that the L10-PtCo structure was formed near the nanoparticle surface after annealing at 700 °C. The L10-PtCo structure was thought to be formed from the surface segregation of Pt atoms and insufficient diffusion of Pt and Co atoms to mix them in the particle overall.

Surface Effect on Young's Modulus of Sub-Two-Nanometer Gold [111] Nanocontacts.

The surface bond nature of face centered cubic metals has been controversial between hardening and softening theoretically because of the lack of precise measurement. Here, we precisely measured the size dependence of Young's modulus of gold [111] nanocontacts with a clean surface by our in situ TEM-frequency modulation force sensing method in ultrahigh vacuum at room temperature. Young's modulus gradually decreased from ca. 80 to 30 GPa, as the nanocontact width decreased below 2 nm, which could be explained by surface softening; Young's modulus of the outermost atomic layer was estimated to be approximately 22 GPa, while that of the other part was almost the same with the bulk.

Mechanical Robustness of Metal Nanocomposites Rendered by Graphene Functionalization.

Nanocarbon materials, such as graphene, carbon nanotubes, and their derivatives, are considered highly effective reinforcing agents in metals. Copious experimental and computational observations suggest that the nature of the interfaces may significantly affect the mechanical behavior of nanocarbon-metal composites, while the exact correlation between the interfacial structure and the deformation and failure mechanisms of the composite remains elusive. Using a nanolaminated graphene-aluminum (Al) composite as the model material, we designed and created composites with distinct interfacial structures and bonding states via graphene functionalization. The mechanical behavior of the composites was strongly affected by the structure of the functionalized graphene (FG)/Al interface, and the optimum strength-ductility synergy came from the composite with the intermediate extent of functionalization. Complementing experimental results with molecular dynamics and phase-field simulation efforts, we interpreted these results by the combined effects of the intrinsic strength of FG nanosheets and the FG/Al interfacial bonding state.

Subpercent Local Strains Due to the Shapes of Gold Nanorods Revealed by Data-Driven Analysis.

Analysis of subpercent local strain is important for a deeper understanding of nanomaterials, whose properties often depend on the strain. Conventional strain analysis has been performed by measuring interatomic distances from scanning transmission electron microscopy (STEM) images. However, measuring subpercent strain remains a challenge because the peak positions in STEM images do not precisely correspond to the real atomic positions due to disturbing influences, such as random noise and image distortion. Here, we utilized an advanced data-driven analysis method, Gaussian process regression, to predict the true strain distribution by reconstructing the true atomic positions. As a result, a precision of 0.2% was achieved in strain measurement at the atomic scale. The method was applied to gold nanoparticles of different shapes to reveal the shape dependence of the strain distribution. A spherical gold nanoparticle showed a symmetric strain distribution with a contraction of ∼1% near the surface owing to surface relaxation. By contrast, a gold nanorod, which is a cylinder terminated by hemispherical caps on both sides, showed nonuniform strain distributions with lattice expansions of ∼0.5% along the longitudinal axis around the caps except for the contraction at the surface. Our results indicate that the strain distribution depends on the shape of the nanomaterials. The proposed data-driven analysis is a convenient and powerful tool to measure the strain distribution with high precision at the atomic scale.

Clarification of the ordering of intercalated Fe atoms in FexTiS2 and its effect on the magnetic properties.

Even though there has been a lot of studies on the magnetic properties of FexTiS2 and their corresponding atomic structures at different Fe concentrations, the dependency of the properties on the Fe atomic arrangement has not been fully clarified yet. In this study, FexTiS2 structures, synthesized by chemical vapor transport technique at Fe concentrations of 0.05, 0.10, 0.15, 0.20 0.25 and 0.33, were observed three-dimensionally using a transmission electron microscope and their corresponding magnetization values were measured using a superconducting quantum interference device. The results show a switch from local in-plane two-dimensional (2D) ordering of \sqrt 3 a and 2a at concentrations below 0.15 to three-dimensional (3D) ordering of 2a × 2a × 2c at x = 0.20 and 0.25, as well as \sqrt 3 a × \sqrt 3 a × 2c superstructures at x = 0.33, although it should be noted that the x = 0.20 sample only had partial ordering of Fe atoms. The type of Fe ordering present in FexTiS2 could be explained by the balance of cohesive energy of neighboring Fe atoms and local strain energy imposed on the host structure due to the formation of Fe clusters. It is also found that the switch from 2D to 3D Fe order coincides with the magnetic measurements, which reveal spin-glass behavior below x = 0.15 and ferromagnetic behavior above x = 0.20. This suggests that the magnetic properties of the FexTiS2 structure are highly influenced by the ordering of Fe atoms between planes.

Peculiar Atomic Bond Nature in Platinum Monatomic Chains.

Metal atomic chains have been reported to change their electronic or magnetic properties by slight mechanical stimulus. However, the mechanical response has been veiled because of lack of information on the bond nature. Here, we clarify the bond nature in platinum (Pt) monatomic chains by our in situ transmission electron microscope method. The stiffness is measured with sub-N/m precision by quartz length-extension resonator. The bond stiffnesses at the middle of the chain and at the connection to the base are estimated to be 25 and 23 N/m, respectively, which are higher than the bulk counterpart. Interestingly, the bond length of 0.25 nm is found to be elastically stretched to 0.31 nm, corresponding to a 24% strain. Such peculiar bond nature could be explained by a novel concept of "string tension". This study is a milestone that will significantly change the way we think about atomic bonds in one-dimension.

Quantitative estimation of atom-scaled ripple structure using transmission electron microscopy images.

Atom-scaled ripple structure can be intrinsically formed because of thermal instability or induced stress in graphene or two-dimensional (2D) materials. However, it is difficult to estimate the period, amplitude, and shape of such a ripple structure. In this study, by applying the geometrical phase analysis method to atomically resolved transmission electron microscopy images, we demonstrate that the atom-scaled ripple structure of MoS2 nanosheet can be quantitatively analyzed at the subnanometer scale. Furthermore, by analyzing the observed ripple structure of the MoS2 nanosheet, we established that it is inclined by approximately 7.1° from the plane perpendicular to the incident electron beam; it had 5.5 and 0.3 nm in period and amplitude, respectively. For quantitative estimation of ripple structure, our results provide an effective method that contributes to a better understanding of 2D materials in the sub-nanometre scale.

In-situ electrical conductance measurement of suspended ultra-narrow graphene nanoribbons observed via transmission electron microscopy.

Graphene nanoribbon is an attractive material for nano-electronic devices, as their electrical transport performance can be controlled by their edge structures. However, in most cases, the electrical transport has been investigated only for graphene nanoribbons fabricated on a substrate, which hinders the appearance of intrinsic electrical transport due to screening effects. In this study, we developed special devices based on silicon chips for transmission electron microscopy to observe a monolayer graphene nanoribbon suspended between two gold electrodes. Moreover, with the development of an in-situ transmission electron microscopy holder, the current-voltage characteristics were achieved simultaneously with observing and modifying the structure. We found that the current-voltage characteristics differed between 1.5 nm-wide graphene nanoribbons with armchair and zigzag edge structures. The energy gap of the zigzag edge was more than two-fold larger than that of the armchair edge and exhibited an abrupt jump above a critical bias voltage in the differential conductance curve. Thus, our in-situ transmission electron microscopy method is promising for elucidating the structural dependence of electrical conduction in two-dimensional materials.

One-Pot Synthesis of PtNi Alloy Nanoparticle-Supported Multiwalled Carbon Nanotubes in an Ionic Liquid Using a Staircase Heating Process.

High-performance PtNi alloy nanoparticle-supported multiwalled carbon nanotube composite (PtNi/MWCNT) electrocatalysts can be prepared via one-pot preparation for oxygen reduction reaction. This route of preparation utilizes the pyrolytic decomposition of metal precursors, such as Pt(acac)2 with Ni precursors, nickel bis(trifluoromethanesulfonyl)amide (Ni[Tf2N]2) or nickel acetylacetonate (Ni(acac)2), in an ionic liquid (IL), N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide ([N1,1,1,3][Tf2N]). Currently, there is insufficient information concerning the effect of difference in preparation conditions on the formation mechanism and catalytic activity of PtNi/MWCNT. In this article, a staircase heating process was used to investigate the PtNi alloy nanoparticle formation mechanism and catalytic activity of the resulting PtNi/MWCNT. We found that the alloy formation process, composition, and crystal structure, which directly affect the electrocatalytic activity, strongly depended on the Ni precursor species and heating process. The catalytic performance of certain PtNi/MWCNTs collected during the staircase heating process was better than that of PtNi/MWCNTs produced via the conventional heating process.

Atomic scale mechanics explored by in situ transmission electron microscopy with a quartz length-extension resonator as a force sensor.

An in situ transmission electron microscopy (TEM) holder equipped with a quartz length-extension resonator (LER) as a force sensor was developed to examine the elastic properties of atomic-scale materials. This holder is a useful means of studying the effects of size and crystal orientation on the properties of nanomaterials via measurements of mechanical responses while simultaneously observing atomic structures. The spring constants of nanocontacts (NCs) were determined based on shifts in the resonance frequency of the LER during TEM observations. The LER spring constant and sensitivity (the ratio of the LER induced charge to its oscillation amplitude), both of which are crucial to mechanical evaluation of NCs, were precisely calibrated from an analysis of TEM images along with the output of the electronics attached to the holder. The mechanical stability of the newly developed TEM holder was sufficient to allow chains of Pt atoms in the NC to be maintained for at least several seconds. The minimum measurable NC spring constant was on the order of 1 N m-1, comparable to that associated with a single atomic bond. The spring constant of a NC composed of a single-bonded chain of two Pt atoms was found to be 13.2 N m-1. This holder therefore has significant potential with regard to the characterization of nanoscale mechanical properties.

Transmission electron diffraction study of a uniaxially-ordered high-mobility polymeric semiconductor.

Thin films of the polymeric semiconductor 2,5-bis (3-hexadecylthiophen-2-yl) thieno (3,2-b) thiophene obtained by compression with a glass blade exhibit high-carrier mobility. This enhanced performance has been attributed to the uniaxial alignment of the polymer molecules, although such alignment has not yet been demonstrated experimentally. In the present study, the local alignment of the polymer was estimated using transmission electron diffraction. The diffraction spots corresponding to π-π stacking layers (along the b-axis) showed an arc-like intensity distribution along with angles of 11.6°, 15.8° and 25.4° for selected areas having diameters of 70, 140 and 280 nm, respectively. This variation of the arc angle with diameter indicates that the polymer chains were arranged in a gentle winding state, with an average radius of curvature of ~630 nm. We conclude that, as expected, the high-carrier mobility of the polymer is related to its uniaxial alignment.

Follow-up review: recent progress in lithium detection.

Recently, visualization of lithium ions has brought great benefit for understanding lithium movement in the cathode, electrolyte and anode materials of lithium ion battery. It has been achieved by several methods such as spherical aberration corrected scanning transmission electron microscopy, multivariable analysis, advanced electron holography and so on, which was reviewed by the special issue, 'Challenges for Lithium Detection' in Microscopy (Vol. 66, No. 1, 2017). In this paper, recent research progress in lithium detection is introduced as a follow-up for the special issue.

Template Conversion of Covalent Organic Frameworks into 2D Conducting Nanocarbons for Catalyzing Oxygen Reduction Reaction.

Progress over the past decades in porous materials has exerted great effect on the design of metal-free carbon electrochemical catalysts in fuel cells. The carbon material must combine three functions, i.e., electrical conductivity for electron transport, optimal pores for ion motion, and abundant heteroatom sites for catalysis. Here, an ideal carbon catalyst is achieved by combining two strategies-the use of a 2D covalent organic framework (COF) and the development of a suitable template to guide the pyrolysis. The COF produces nanosized carbon sheets that combine high conductivity, hierarchical porosity, and abundant heteroatom catalytic edges. The catalysts achieve superior performance to authentic Pt/C with exceptional onset potential (0 V vs -0.03 V), half-wave potentials (-0.11 V vs -0.16 V), high limit current density (7.2 mA cm-2 vs 6.0 mA cm-2 ), low Tafel slope (110 mV decade-1 vs 121 mV decade-1 ), long-time stability, and methanol tolerance. These results reveal a novel material platform based on 2D COFs for designing novel 2D carbon materials.

Drying-Induced Self-Similar Assembly of Megamolecular Polysaccharides through Nano and Submicron Layering.

We propose a self-similar assembly to generate planar orientation of megamolecular polysaccharides on the nanometer scale and submicron scale. Evaporating the aqueous liquid crystalline (LC) solution on a planar air-LC interface induces polymer layering by self-assembly and rational action of macroscopic capillary forces between the layers. To clarify the mechanisms of nanometer- and submicron-scale layering, the polymer films are investigated by electron microscopy.

Three-dimensional characterization of Guinier-Preston zones in an Al-Cu alloy using depth-sectioning technique.

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 depth-sectioning technique can be used to investigate three-dimensional configurations of GP zones in the sample.

Visualization of lithium ions by annular bright field imaging.

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.