Defect Density and Atomic Defect Recognition in the Middle Layer of a Trilayer MoS2 Stack.

Molybdenum disulfide (MoS2) is one of the most intriguing two-dimensional materials, and moreover, its single atomic defects can significantly alter the properties. These defects can be both imaged and engineered using spherical and chromatic aberration-corrected high-resolution transmission electron microscopy (CC/CS-corrected HRTEM). In a few-layer stack, several atoms are vertically aligned in one atomic column. Therefore, it is challenging to determine the positions of missing atoms and the damage cross-section, particularly in the not directly accessible middle layers. In this study, we introduce a technique for extracting subtle intensity differences in CC/CS-corrected HRTEM images. By exploiting the crystal structure of the material, our method discerns chalcogen vacancies even in the middle layer of trilayer MoS2. We found that in trilayer MoS2 the middle layer's damage cross-section is about ten times lower than that in the monolayer. Our findings could be essential for the application of few-layer MoS2 in nanodevices.

Reversible superdense ordering of lithium between two graphene sheets.

Many carbon allotropes can act as host materials for reversible lithium uptake1,2, thereby laying the foundations for existing and future electrochemical energy storage. However, insight into how lithium is arranged within these hosts is difficult to obtain from a working system. For example, the use of in situ transmission electron microscopy3-5 to probe light elements (especially lithium)6,7 is severely hampered by their low scattering cross-section for impinging electrons and their susceptibility to knock-on damage8. Here we study the reversible intercalation of lithium into bilayer graphene by in situ low-voltage transmission electron microscopy, using both spherical and chromatic aberration correction9 to enhance contrast and resolution to the required levels. The microscopy is supported by electron energy-loss spectroscopy and density functional theory calculations. On their remote insertion from an electrochemical cell covering one end of the long but narrow bilayer, we observe lithium atoms to assume multi-layered close-packed order between the two carbon sheets. The lithium storage capacity associated with this superdense phase far exceeds that expected from formation of LiC6, which is the densest configuration known under normal conditions for lithium intercalation within bulk graphitic carbon10. Our findings thus point to the possible existence of distinct storage arrangements of ions in two-dimensional layered materials as compared to their bulk parent compounds.

An Efficient Electron Ptychography Method for Retrieving the Object Spectrum from Only a Few Iterations.

We present an efficient approach for electron ptychography based on a mathematical relationship that differs from that underlying the established algorithms of the ptychography iterative engine or the noniterative algorithms like the Wigner-distribution-deconvolution or the single-side-band method. Three variables are handled in this method-the transfer function of the objective lens, the object spectrum, and the diffraction wave whose phase is unknown. In the case of an aberration-corrected electron microscope, one is able to obtain a well-estimated transfer function of the lens. After reducing the number of three variables down to two, we construct an iterative loop between the object spectrum and the diffraction wave, which retrieves the object spectrum within a small number of iterations. We tested this object spectrum retrieval method on both a calculated and an experimental 4D-STEM datasets. By applying this method, we explore the influence of sampling, dose, and the size of illumination aperture on the reconstructed phase images.

Simultaneous Inside and Outside Functionalization of Single-Walled Carbon Nanotubes.

Functionalizing single-walled carbon nanotubes (SWCNTs) in a robust way that does not affect the sp2 carbon framework is a considerable research challenge. Here we describe how triiodide salts of positively charged macrocycles can be used not only to functionalize SWCNTs from the outside, but simultaneously from the inside. We employed disulfide exchange in aqueous solvent to maximize the solvophobic effect and therefore achieve a high degree of macrocycle immobilization. Characterization by Raman spectroscopy, EDX-STEM and HR-TEM clearly showed that serendipitously this wet-chemical functionalization procedure also led to the encapsulation of polyiodide chains inside the nanotubes. The resulting three-shell composite materials are redox-active and experience an intriguing interplay of electrostatic, solvophobic and mechanical effects that could be of interest for applications in energy storage.

POM@ZIF Derived Mixed Metal Oxide Catalysts for Sustained Electrocatalytic Oxygen Evolution.

The design of efficient and stable oxygen evolution reaction (OER) catalysts based on noble-metal-free materials is crucial for energy conversion and storage. In this work, it was demonstrated how polyoxometalate (POM)-doped ZIF-67 can be converted into a stable oxygen evolution electrocatalyst by chemical etching, cation exchange, and thermal annealing steps. Characterization by X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy indicate that POM-doped ZIF-67 derived carbon-supported metal oxides were synthesized. The resulting composite shows structural and compositional advantages which lead to low overpotential (306 mV at j=10 mA ⋅ cm-2 ) and long-term stability under harsh OER conditions (1.0 M aqueous KOH).

Integrated Differential Phase Contrast (IDPC)-STEM Utilizing a Multi-Sector Detector for Imaging Thick Samples.

The integrated differential phase contrast (IDPC) method is useful for generating the potential map of a thin sample. We evaluate theoretically the potential of IDPC imaging for thick samples by varying the focus at different sample thicknesses. Our calculations show that high defocus values result in enhanced anisotropy of the contrast transfer function (CTF) and uninterpretable images, if a quadrant detector is applied. We further show that applying a multi-sector detector can result in an almost isotropic CTF. By sector number-dependent calculations for both Cc/C3-corrected and C3-corrected scanning transmission electron microscopy (STEM), we show that the increase of detector sectors not only removes the anisotropy of the CTF, but also improves image contrast and resolution. For a proof-of-principle IDPC-STEM (uncorrected) experiment, we realize the functionality of a 12-sector detector from a physical quadrant detector and demonstrate the improvement in contrast and resolution on the example of InGaN/GaN quantum well structure.

Computationally Efficient Handling of Partially Coherent Electron Sources in (S)TEM Image Simulations via Matrix Diagonalization.

We introduce a novel method to improve the computational efficiency for (S)TEM image simulation by employing matrix diagonalization of the mixed envelope function (MEF). The MEF is derived by taking the finite size and the energy spread of the effective electron source into account, and is a component of the transmission cross-coefficient that accounts for the correlation between partially coherent waves. Since the MEF is a four-dimensional array and its application in image calculations is time-consuming, we reduce the computation time by using its eigenvectors. By incorporating the aperture function into the matrix diagonalization, only a small number of eigenvectors are required to approximate the original matrix with high accuracy. The diagonalization enables for each eigenvector the calculation of the corresponding image by employing the coherent model. The individual images are weighted by the corresponding eigenvalues and then summed up, resulting in the total partially coherent image.

Bottom-up Design of Bimetallic Cobalt-Molybdenum Carbides/Oxides for Overall Water Splitting.

Earth-abundant transition-metal-based catalysts for electrochemical water splitting are critical for sustainable energy schemes. In this work, we use a rational design method for the synthesis of ultrasmall and highly dispersed bimetallic CoMo carbide/oxide particles deposited on graphene oxide. Thermal conversion of the molecular precursors [H3 PMo12 O40 ], Co(OAc)2 ⋅4 H2 O and melamine in the presence of graphene oxide gives the mixed carbide/oxide (Co6 Mo6 C2 /Co2 Mo3 O8 ) nanoparticle composite deposited on highly dispersed, N,P-doped carbon. The resulting composite shows outstanding electrocatalytic water-splitting activity for both the oxygen evolution and hydrogen evolution reaction, and superior performance to reference samples including commercial 20 % Pt/C & IrO2 . Electrochemical and other materials analyses indicate that Co6 Mo6 C2 is the main active phase in the composite, and the N,P-doping of the carbon matrix increases the catalytic activity. The facile design could in principle be extended to multiple bimetallic catalyst classes by tuning of the molecular metal oxide precursor.

Direct Imaging of Atomic Permeation Through a Vacancy Defect in the Carbon Lattice.

Porous graphene has shown promise as a new generation of selective membrane for sieving atoms, ions and molecules. However, the atomistic mechanisms of permeation through defects in the graphenic lattice are still unclear and remain unobserved in action, at the atomic level. Here, the direct observation of palladium atoms from a nanoparticle passing through a defect in a single-walled carbon nanotube one-by-one has been achieved with atomic resolution in real time, revealing key stages of the atomic permeation. Bonding between the moving atom and dangling bonds around the orifice, immediately before and after passing through the subnano-pore, plays an important role in the process. Curvature of the graphenic lattice crucially defines the direction of permeation from concave to convex side due to a difference in metal-carbon bonding at the curved surfaces as confirmed by density functional theory calculations, demonstrating the potential of porous carbon nanotubes for atom sieving.

Water-Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Highly Selective Quasi-Homogeneous Photocatalysis.

Heptazine-based polymeric carbon nitrides (PCN) are promising photocatalysts for light-driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom-up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi-homogeneous conditions. The superior performance of water-soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2 O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4-methoxybenzyl alcohol and benzyl alcohol or lignocellulose-derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re-dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

Brodie's or Hummers' Method: Oxidation Conditions Determine the Structure of Graphene Oxide.

Synthesis and studies of graphite oxide started more than 150 years ago and turned into a boom by the measurements of the outstanding physical properties of graphene. A series of preparation protocols emanated trying to optimize the synthesis of graphene oxide in order to obtain a less defective material, as source for graphene. However, over-oxidation of the carbon framework hampered establishing structure-property relationships. Here, the fact that two different synthetic methods for graphene oxide preparation lead to very similar types of graphene oxide with a preserved graphene lattice is demonstrated. Either sodium chlorate in nitric acid (similar to Brodie's method) or potassium permanganate in sulfuric acid (similar to Hummers' method) treatment are possible; however, reaction conditions must be controlled. With a preserved carbon lattice analytical differences between the samples relate to the altered on-plane functionality. Consequently, terming preparation protocols "according to Brodie's/Hummers' method" is not sufficient.

Direct Correlation of Carbon Nanotube Nucleation and Growth with the Atomic Structure of Rhenium Nanocatalysts Stimulated and Imaged by the Electron Beam.

Subnanometer Re clusters confined in a single-walled carbon nanotube are activated by the 80 keV electron beam to promote the catalytic growth of a new carbon nanotube. Transmission electron microscopy images the entire process step-by-step, with atomic resolution in real time, revealing details of the initial nucleation followed by a two-stage growth. The atomic dynamics of the Re cluster correlate strongly with the nanotube formation process, with the growth accelerating when the catalyst becomes more ordered. In addition to the nanotube growth catalyzed by Re nanoclusters, individual atoms of Re released from the nanocluster play a role in the nanotube formation.

Modular Design of Noble-Metal-Free Mixed Metal Oxide Electrocatalysts for Complete Water Splitting.

Electrocatalytic water splitting into H2 and O2 is a key technology for carbon-neutral energy. Here, we report a modular materials design leading to noble metal-free composite electrocatalysts, which combine high electrical conductivity, high OER and HER reactivity and high durability. The scalable bottom-up fabrication allows the stable deposition of mixed metal oxide nanostructures with different functionalities on copper foam electrodes. The composite catalyst shows sustained OER and HER activity in 0.1 m aqueous KOH over prolonged periods (t>10 h) at low overpotentials (OER: ≈300 mV; HER: ≈100 mV) and high faradaic efficiencies (OER: ≈100 %, HER: ≈98 %). The new synthetic concept will enable the development of multifunctional, mixed metal oxide composites as high-performance electrocatalysts for challenging energy conversion and storage reactions.

Morphology-Engineered Highly Active and Stable Ru/TiO2 Catalysts for Selective CO Methanation.

Ru/TiO2 catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO2 - and H2 -rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non-deactivating Ru/TiO2 catalysts by engineering the morphology of the TiO2 support. Using anatase TiO2 nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO2 -{100} and Ru/TiO2 -{101} are very stable, while Ru/TiO2 -{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal-support interactions (MSIs). The stronger MSIs on the defect-rich TiO2 -{100} and TiO2 -{101} supports stabilize flat Ru nanoparticles, while on TiO2 -{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO2 -{001}.

Chemical Reactions of Molecules Promoted and Simultaneously Imaged by the Electron Beam in Transmission Electron Microscopy.

The main objective of this Account is to assess the challenges of transmission electron microscopy (TEM) of molecules, based on over 15 years of our work in this field, and to outline the opportunities in studying chemical reactions under the electron beam (e-beam). During TEM imaging of an individual molecule adsorbed on an atomically thin substrate, such as graphene or a carbon nanotube, the e-beam transfers kinetic energy to atoms of the molecule, displacing them from equilibrium positions. Impact of the e-beam triggers bond dissociation and various chemical reactions which can be imaged concurrently with their activation by the e-beam and can be presented as stop-frame movies. This experimental approach, which we term ChemTEM, harnesses energy transferred from the e-beam to the molecule via direct interactions with the atomic nuclei, enabling accurate predictions of bond dissociation events and control of the type and rate of chemical reactions. Elemental composition and structure of the reactant molecules as well as the operating conditions of TEM (particularly the energy of the e-beam) determine the product formed in ChemTEM processes, while the e-beam dose rate controls the reaction rate. Because the e-beam of TEM acts simultaneously as a source of energy for the reaction and as an imaging tool monitoring the same reaction, ChemTEM reveals atomic-level chemical information, such as pathways of reactions imaged for individual molecules, step-by-step and in real time; structures of illusive reaction intermediates; and direct comparison of catalytic activity of different transition metals filmed with atomic resolution. Chemical transformations in ChemTEM often lead to previously unforeseen products, demonstrating the potential of this method to become not only an analytical tool for studying reactions, but also a powerful instrument for discovery of materials that can be synthesized on preparative scale.

Formation of Nickel Clusters Wrapped in Carbon Cages: Toward New Endohedral Metallofullerene Synthesis.

Despite the high potential of endohedral metallofullerenes (EMFs) for application in biology, medicine and molecular electronics, and recent efforts in EMF synthesis, the variety of EMFs accessible by conventional synthetic methods remains limited and does not include, for example, EMFs of late transition metals. We propose a method in which EMF formation is initiated by electron irradiation in aberration-corrected high-resolution transmission electron spectroscopy (AC-HRTEM) of a metal cluster surrounded by amorphous carbon inside a carbon nanotube serving as a nanoreactor and apply this method for synthesis of nickel EMFs. The use of AC-HRTEM makes it possible not only to synthesize new, previously unattainable nanoobjects but also to study in situ the mechanism of structural transformations. Molecular dynamics simulations using the state-of-the-art approach for modeling the effect of electron irradiation are performed to rationalize the experimental observations and to link the observed processes with conditions of bulk EMF synthesis.

Polyoxometalate-Based Bottom-Up Fabrication of Graphene Quantum Dot/Manganese Vanadate Composites as Lithium Ion Battery Anodes.

Lithium ion battery (LIB) electrodes require a stable connection between a redox-active metal oxide for charge storage and an electrically conductive (often carbon-based) material for charge transport. As charge transfer within the metal oxide is often a performance-limiting factor, one promising concept is the linking of charge transfer and charge storage components on the nanoscale. This would maximize the interfacial contact area and improve charging/discharging behavior. This work presents a one-step, room-temperature route giving nanostructured manganese vanadium oxide/graphene quantum dot (GQD) composite electrodes. Manganese vanadium oxide clusters are used as solution-processable precursors, which are deposited on GQDs using a sonication-driven conversion leading to electroactive, lightweight composites. Incorporation of the composites as anodes in LIBs shows high electrochemical performance featuring discharge capacities of 970 mAh g-1 over 100 cycles with coulombic efficiencies near 100 %. The study shows how 3d-metal oxide/GQD nanostructures can be accessed by a scalable sonication route starting from soluble, chemically tunable metal oxide clusters and graphene quantum dots.

Carbon Nanotubes as Electrically Active Nanoreactors for Multi-Step Inorganic Synthesis: Sequential Transformations of Molecules to Nanoclusters and Nanoclusters to Nanoribbons.

In organic synthesis, the composition and structure of products are predetermined by the reaction conditions; however, the synthesis of well-defined inorganic nanostructures often presents a significant challenge yielding nonstoichiometric or polymorphic products. In this study, confinement in the nanoscale cavities of single-walled carbon nanotubes (SWNTs) provides a new approach for multistep inorganic synthesis where sequential chemical transformations take place within the same nanotube. In the first step, SWNTs donate electrons to reactant iodine molecules (I2), transforming them to iodide anions (I(-)). These then react with metal hexacarbonyls (M(CO)6, M = Mo or W) in the next step, yielding anionic nanoclusters [M6I14](2-), the size and composition of which are strictly dictated by the nanotube cavity, as demonstrated by aberration-corrected high resolution transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy. Atoms in the nanoclusters [M6I14](2-) are arranged in a perfect octahedral geometry and can engage in further chemical reactions within the nanotube, either reacting with each other leading to a new polymeric phase of molybdenum iodide [Mo6I12]n or with hydrogen sulfide gas giving rise to nanoribbons of molybdenum/tungsten disulfide [MS2]n in the third step of the synthesis. Electron microscopy measurements demonstrate that the products of the multistep inorganic transformations are precisely controlled by the SWNT nanoreactor with complementary Raman spectroscopy revealing the remarkable property of SWNTs to act as a reservoir of electrons during the chemical transformation. The electron transfer from the host nanotube to the reacting guest molecules is essential for stabilizing the anionic metal iodide nanoclusters and for their further transformation to metal disulfide nanoribbons synthesized in the nanotubes in high yield.

Investigation of the Interactions and Bonding between Carbon and Group VIII Metals at the Atomic Scale.

The nature and dynamics of bonding between Fe, Ru, Os, and single-walled carbon nanotubes (SWNTs) is studied by aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). The metals catalyze a wide variety of different transformations ranging from ejection of carbon atoms from the nanotube sidewall to the formation of hollow carbon shells or metal carbide within the SWNT, depending on the nature of the metal. The electron beam of AC-HRTEM serves the dual purpose of providing energy to the specimen and simultaneously enabling imaging of chemical transformations. Careful control of the electron beam parameters, energy, flux, and dose allowed direct comparison between the metals, demonstrating that their chemical reactions with SWNTs are determined by a balance between the cohesive energy of the metal particles and the strength of the metal-carbon σ- or π-bonds. The pathways of transformations of a given metal can be drastically changed by applying different electron energies (80, 40, or 20 keV), thus demonstrating AC-HRTEM as a new tool to direct and study chemical reactions. The understanding of interactions and bonding between SWNT and metals revealed by AC-HRTEM at the atomic level has important implications for nanotube-based electronic devices and catalysis.

Chromatic Aberration Correction for Atomic Resolution TEM Imaging from 20 to 80 kV.

Atomic resolution in transmission electron microscopy of thin and light-atom materials requires a rigorous reduction of the beam energy to reduce knockon damage. However, at the same time, the chromatic aberration deteriorates the resolution of the TEM image dramatically. Within the framework of the SALVE project, we introduce a newly developed C_{c}/C_{s} corrector that is capable of correcting both the chromatic and the spherical aberration in the range of accelerating voltages from 20 to 80 kV. The corrector allows correcting axial aberrations up to fifth order as well as the dominating off-axial aberrations. Over the entire voltage range, optimum phase-contrast imaging conditions for weak signals from light atoms can be adjusted for an optical aperture of at least 55 mrad. The information transfer within this aperture is no longer limited by chromatic aberrations. We demonstrate the performance of the microscope using the examples of 30 kV phase-contrast TEM images of graphene and molybdenum disulfide, showing unprecedented contrast and resolution that matches image calculations.