Atomic-Scale Visualization of Spontaneous Bilayer MoS2 Nanoribbon Scrolling via Coupled Rotation-Bending within Carbon Nanotubes.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

We study the influence of mechanical deformations on the Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants from first principles. In particular, we perform symmetry-adapted density functional theory simulations with spin–orbit coupling to determine the variation in the electronic band structure splittings with axial and torsional deformations. We find significant effects in molybdenum and tungsten nanotubes, for which the Zeeman splitting decreases with increase in strain, going to zero for large enough tensile/shear strains, while the Rashba splitting coefficient increases linearly with shear strain, while being zero for all tensile strains, a consequence of the inversion symmetry remaining unbroken. In addition, the Zeeman splitting is relatively unaffected by nanotube diameter, whereas the Rashba coefficient decreases with increase in diameter. Overall, mechanical deformations represent a powerful tool for spintronics in nanotubes.

We calculate the torsional moduli of single-walled transition metal dichalcogenide (TMD) nanotubes using ab initio density functional theory (DFT). Specifically, considering forty-five select TMD nanotubes, we perform symmetry-adapted DFT calculations to calculate the torsional moduli for the armchair and zigzag variants of these materials in the low-twist regime and at practically relevant diameters. We find that the torsional moduli follow the trend: MS2 > MSe2 > MTe2. In addition, the moduli display a power law dependence on diameter, with the scaling generally close to cubic, as predicted by the isotropic elastic continuum model. In particular, the shear moduli so computed are in good agreement with those predicted by the isotropic relation in terms of the Young’s modulus and Poisson’s ratio, both of which are also calculated using symmetry-adapted DFT. Finally, we develop a linear regression model for the torsional moduli of TMD nanotubes based on the nature/characteristics of the metal-chalcogen bond, and show that it is capable of making reasonably accurate predictions.

We have utilized bright-field conventional transmission electron microscopy tomography and annular dark-field scanning transmission electron microscopy (ADF-STEM) tomography to characterize a well-defined carbon black (CB)-filled polymer nanocomposite with known CB volume concentration. For both imaging methods, contrast can be generated between the CB and the surrounding polymer matrix. The involved contrast mechanisms, in particular for ADF-STEM, will be discussed in detail. The obtained volume reconstructions were analysed and the CB volume concentrations were carefully determined from the reconstructed data. For both imaging modes, the measured CB volume concentrations are substantially different and only quantification based on the ADF-STEM data revealed about the same value as the known CB loading. Moreover, when applying low-convergence angles for imaging ADF-STEM tomography, data can be obtained of micrometre-thick samples.

Microstructures of epitaxial Ca0.33CoO2 thin films, which were grown on m plane and c(0001) plane of α–Al2O3 by the reactive solid-phase epitaxy (R-SPE) method and the subsequent ion-exchange treatment, were investigated in detail by using selected-area electron diffraction, high-resolution transmission electron microcopy, spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (Cs-corrected HAADF-STEM), and electron energy-loss spectroscopy (EELS). Detailed electron diffraction analyses reveal that the orientation relationships between Ca0.33CoO2 thin film and substrate are and , having an angle of about 43° with for the film deposited on m plane, and and for the film deposited on c(0001) plane though a Ca–Al–O amorphous layer formed between them. CoO seed layer near the interface and residual Co3O4 phase inside the films were observed and identified by HAADF-STEM and EELS in both samples. Such microstructural configuration indicates that the processes of film growth during R-SPE are (i) oxidation of CoO into Co3O4 with residual CoO layer near the interface and (ii) intercalation of Na+ layer into Co3O4 to achieve the layered NaxCoO2 film while forming Na–Al–O amorphous layer at the interface.

We study the effect of torsional deformations on the electronic properties of single-walled transition metal dichalcogenide (TMD) nanotubes. In particular, considering forty-five select armchair and zigzag TMD nanotubes, we perform symmetry-adapted Kohn–Sham density functional theory calculations to determine the variation in bandgap and effective mass of charge carriers with twist. We find that metallic nanotubes remain so even after deformation, whereas semiconducting nanotubes experience a decrease in bandgap with twist—originally direct bandgaps become indirect—resulting in semiconductor to metal transitions. In addition, the effective mass of holes and electrons continuously decrease and increase with twist, respectively, resulting in n-type to p-type semiconductor transitions. We find that this behavior is likely due to rehybridization of orbitals in the metal and chalcogen atoms, rather than charge transfer between them. Overall, torsional deformations represent a powerful avenue to engineer the electronic properties of semiconducting TMD nanotubes, with applications to devices like sensors and semiconductor switches.

HRTEM and ADF-STEM of precipitates at peak-ageing in cast A356 aluminium alloy

Comparison of intensity distributions in tomograms from BF TEM, ADF STEM, HAADF STEM, and calculated tilt series

With the rapid development of electronic devices and hybrid-electric/electric vehicles (H/EVs), lithium-ion batteries have become the most promising electrical storage system due to its high volumetric and gravimetric energy density. However, the high energy density of lithium-ion batteries is difficult to maintain after extended cycles due to capacity fading, which can be attributed to multiple possible reasons including SEI formation,[1] electrolyte decomposition,[2] and structural changes in electrode materials.[3] These issues generate important concerns to the stability and lifetime of the battery. Layered lithium-transition metal oxides represent a major type of cathode materials that are widely used in the commercial market. However, these materials are suggested to suffer structural transformation during electrochemical cycling. Previous studies suggest the possible surface structural transitions from the layered structure to spinel[4], and/or rock-salt structures,[5] or possible further decomposition of the transformed phase.[6]A clear explanation of these surface phenomenon is still under debate. An in-depth investigation of the structural reconstruction is necessary to elucidate the degradation mechanisms of the layered cathode materials. In the present study, we investigated the chemical evolution and structural transformation of a prominent layered cathode material for lithium-ion batteries, LiNi1/3Mn1/3Co1/3O2 (NMC). The redox reaction of NMC cathode during charge-discharge process was analyzed in detail using high-resolution electron-energy loss spectroscopy (EELS) and 7Li magic-angle spinning (MAS) NMR. Our results suggest that the charge compensation of the NMC cathode during the delithiation-lithiation process is mainly achieved by the oxidation and reduction of Ni2+↔Ni4+, whereas Mn4+ and Co3+ remain mostly unchanged. This is consistent with the NMR findings, from which a trend to lower chemical shift upon lithium extraction is well-correlated with the oxidation change from paramagnetic Ni2+ to diamagnetic Ni4+. Furthermore, the electronic structure of the NMC cathode during initial charging process is found to be inhomogeneous from the particle surface to the bulk using spatially resolved STEM-EELS technique (Figure 1a-1d). It is revealed that the particle surface is at lower oxidation state compared with the bulk region, indicating that the surface evolution of NMC cathode material occurs during the initial cycle. This surface evolution is further analyzed at different numbers of cycles using atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) imaging combined with EELS and nano-beam electron diffraction (NBED). Figure 1e shows a HAADF-STEM image of the NMC particle after 50 cycles. It can be clearly seen from the image that the bulk of the cycled NMC particle shows identical layered structure symmetry, the NBED (Figure 1h) obtained from the bulk can be indexed to the R-3m space group. In contrast, the Li layers are occupied by transition metal ions at the particle surface, and this surface reconstruction layer thickens with extended cycles. When probing to the outer-most surface, the corresponding NBED can be indexed to an Fm-3m rock-salt structure, as shown in Figure 1f. The valence change of the transition metal cations is analyzed using STEM-EELS. The results indicate that the transition metal ions are reduced to divalent states at the surface. In addition, the TM:O ratio increases almost twice at the surface than the bulk. The results indicating that the surface layer is a MO-type rock-salt phase with small amount of residual Li ions and/or vacancies. In addition, a transition zone is observed (Figure 1e) between the bulk layered region and the surface rock-salt layer, where the Li sites are partially occupied by the transition metal ions (some are marked by the red arrows), and the diffraction spots corresponding to the alternating arrangement of Li-containing layers and transition metal layers (some are marked with arrows) become weaker (Figure 1g). 7Li NMR studies of the local Li-environments, following ten cycles and ending at the top of charge, indicate that the order of the TM layers remains unchanged in the bulk of the material. This highlights the importance of complementary studies that are sensitive to different length scales. Further data, including EELS and NMR of the NMC cathode at different cycling state and electrolyte effect will be discussed in the presentation. [1] K. Edström, T. Gustafsson, J. Thomas, Electrochim. Acta 2004, 50, 397. [2] L. Terborg, S. Weber, F. Blaske, et al. J. Power Sources 2013, 242, 832. [3] B. Xu, C.R. Fell, M. Chi, et al. Energy Environ. Sci. 2011, 4, 2223. [4] A. Boulineau, L. Simonin, J.F. Colin, et al. Chem. Mater. 2012, 24, 3558. [5] F. Lin, I. M. Markus, D. Nordlund, et al. Nat. Commun. 2014, 5, 3529. [6] J. Zheng, M. Gu, J. Xiao, et al. Nano Lett. 2013, 13, 3824. Figure 1

Decagonal structure of Al 72Ni 20Co 8 studied by atomic-resolution electron microscopy

The milling profiles of single-crystal gallium nitride (GaN) when subjected to focused ion beams (FIBs) using gallium (Ga), xenon (Xe), and helium (He) ion sources were investigated. An experimental analysis via annular dark-field scanning transmission electron microscopy (ADF-STEM) and high-resolution transmission electron microscopy (HRTEM) revealed that Ga-FIB milling yields trenches with higher aspect ratios compared to Xe-FIB milling for the selected ion beam parameters (30 kV, 42 pA), while He-FIB induces local lattice disorder. Molecular dynamics (MD) simulations were employed to investigate the milling process, confirming that probe size critically influences trench aspect ratios. Interestingly, the MD simulations also showed that Xe-FIB generates higher aspect ratios than Ga-FIB with the same probe size, indicating that Xe-FIB could also be an effective option for nanoscale patterning. Atomic defects such as vacancies and interstitials in GaN from He-FIB milling were suggested by the MD simulations, supporting the lattice disorder observed via HRTEM. This combined experimental and simulation approach has enhanced our understanding of FIB milling dynamics and will benefit the fabrication of nanostructures via the FIB technique.

Electron tomography has become a cornerstone technique for the visualization of nanoparticle morphology in three dimensions. However, to obtain in-depth information about a nanoparticle beyond surface faceting and morphology, different electron microscopy signals must be combined. The most notable examples of these combined signals include annular dark-field scanning transmission electron microscopy (ADF-STEM) with different collection angles and the combination of ADF-STEM with energy-dispersive X-ray or electron energy loss spectroscopies. Here, the experimental and computational development of various multimode tomography techniques in connection to the fundamental materials science challenges that multimode tomography has been instrumental to overcoming are summarized. Although the techniques can be applied to a wide variety of compositions, the study is restricted to metal and metal oxide nanoparticles for the sake of simplicity. Current challenges and future directions of multimode tomography are additionally discussed.

Liquid-cell annular dark-field scanning transmission electron microscopy imaging of single crystal samples on a low-index zone-axis incidence condition.

Chapter 4 - Electron Microscopy Studies on Magnetic L1 0-Type FePd Nanoparticles

High-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging was applied for structural observations of organic crystals so as to achieve quantitative atomically resolved images. In particular, a low angle annular dark-field method (LAADF-STEM) was found to be highly advantageous for radiation sensitive organic materials, because stronger thermal diffuse scatterings can be utilized for imaging. So called quantitative Z-contrast images of organic molecules at atomic resolution were demonstrated in several organic crystals.