Scanning Transmission Electron Microscopy Experiments Research Articles

Real Structure, Magnetism and Chemical Bonding of SmSi3‐x

New metastable SmSi3‐x (x = 0‐0.05) is obtained by high‐pressure high‐temperature synthesis (9.5 GPa, 870‐1270 K). Powder diffraction data refinements reveal that the crystal structure of SmSi3 is isotypic to that of YbSi3 (space group I4/mmm, a = 7.23634(5) Å, c = 11.0854(1) Å). In the crystal structure, two types of Si2 dumbbells agglomerate into layers, which embed the samarium atoms. At ambient pressure, SmSi3 decomposes exothermally upon heating into Si and SmSi2‐x. Single‐crystal structure refinements of a specimen SmSi3‐x (x=0.05) reveal considerable electron density, which is not accounted for by the YbSi3 model. The additional maxima can be assigned to disorder which affects the samarium positions and induces silicon vacancies. Scanning transmission electron microscopy experiments evidence that the disorder can be attributed to extended defects. Magnetic measurements on SmSi3‐x reveal van Vleck paramagnetic behavior and antiferromagnetic ordering at low temperatures. Computations within the local spin density approximation (LSDA and LSDA+U) on the crystal structure of SmSi3 reproduce the anti­ferro­magnetic coupling as the favored long‐range order. Quantum chemical analysis of the chemical bonding in SmSi3 reveals two‐center two‐electron bonds within the Si2 dumbbells plus a total of a little less than four electrons in lone pairs at each silicon atom.

Designing better 4D scanning transmission electron microscopy experiments

Designing better 4D scanning transmission electron microscopy experiments

Thermodynamic Understanding of Impurity Phase Segregation in a PdCrO2/CuCrO2 Heterostructure

AbstractPdCrO2 films are synthesized on CuCrO2 buffer layers on Al2O3 substrates. This synthesis is accompanied by impurity phase segregation, which hampers the synthesis of high quality PdCrO2 films. Potential causes ofth impurity phase segregation are studied by using a combination of experiments and ab initio calculations. X‐ray diffraction and scanning transmission electron microscopy experiments reveal impurity phases of CuxPd1 − x alloy and chromium oxides, Cr2O3 and Cr3O4, in PdCrO2. Calculations determine that oxygen deficiency can cause the impurity phase segregation. Therefore, preventing oxygen release from delafossites can suppress the impurity phase segregation. The amounts of Cr2O3 and Cr3O4 depend on temperature and oxygen partial pressure. A reasonable theory‐based explanation for this experimental observation is provided.

In-situ STEM study on thermally induced phase transformation of magnetic (Nd0.75Ce0.25)2Fe14B ribbons

Studying the phase transformation and microstructure evolution of magnetic Nd-Ce-Fe-B nanocrystalline melt-spun ribbon is of paramount significance in developing this important class of magnetic materials. In this work, the thermal-induced phase transformation of (Nd0.75Ce0.25)2Fe14B nanocrystalline sample was revealed via in-situ heating scanning transmission electron microscopy (STEM) experiment (RT-750 ℃). Starting from a single-phase (Nd0.75Ce0.25)2Fe14B nanocrystalline matrix at room temperature (RT), grain boundary (GB) diffusion behavior was investigated at an atomic scale. Our result shows that the rare earth elements accumulate at GB area from 200 °C and form a uniform thin amorphous layer between grains. The decomposition of the primary phase was observed at about 350 ℃ and crystalized as BCC-Fe, Fe2B, and (Nd0.75Ce0.25)2O3 phases driven by thermal activation at a higher temperature. This work provides the essential information of phase transformation and microstructure evolution of NdCeFeB-based magnets at high temperatures and new insights into thermal treatment designing and thermal stability considerations.

Unveiling transport properties of Co2MnSi Heusler epitaxial thin films with ultra-low magnetic damping

The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 - 10-3 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5 – 300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

Brittle-to-ductile transition in Ti–Pt intermetallic compounds

Phase transformation changes numerous properties of materials. Ti–Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications, especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-to-ductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.

Importance of Defective and Nonsymmetric Structures in Silver Nanoparticles.

Scanning transmission electron microscopy experiments indicate that face-centered cubic (FCC) is the predominant ordered structure for Ag309 ± 7 nanoclusters, synthesized in vacuum. Historically, experiments do not present a consensus on the morphology at these sizes, whereas theoretical studies find the icosahedral symmetry for Ag309 and the decahedral shape for nearby sizes. We employ density functional theory calculations to rationalize these observations, considering both regular and defective Ag nanoparticles (281-321 atoms). The change of stability induced by the presence of defects, symmetry loss, and change of number of atoms is evaluated by the nanoparticle surface energy, which was measured previously. FCC and decahedral symmetries are found to be more favorable than icosahedral, consistent with our measurements of clusters protected from extended atmospheric exposure. In addition, an energy-free descriptor, surface atomic density, is proposed and qualitatively reproduces the surface energy data. Nonsymmetric and defective structures may be preferred over perfectly regular ones within a given size range.

Step‐By‐Step Atomic Insights into Structural Reordering from 2D to 3D MoS2

AbstractVertically stacked low‐dimensional heterostructures are outstanding systems both for exploring fundamental physics and creating new devices. Due to nanometer‐scale building blocks, atomic scale phenomena become for them of fundamental importance, including during device operation. These can be accessed in situ in aberration‐corrected scanning transmission electron microscopy (STEM) experiments. Here, the dynamics of a graphene‐MoS2 heterostructure are studied under Joule heating, where the graphene serves as a high temperature atomically thin and electron transparent “hot plate” for the MoS2. Structural dynamics and evolution of the system are shown at the atomic scale, demonstrating that at the highest temperatures (estimated to exceed 2000 K), the continuous 2D MoS2 transforms into separated 3D nanocrystals, initiated by sulfur vacancy creation and migration followed by formation of voids and clustering at their edges. The resulting nanocrystals exhibit predominantly hexagonal shapes with the 2H and hybrid (2H/3R, 3R/TZ) polytypes. The observed morphology of the crystals is further discussed during and after the transformation, as well as their different edge configurations and stability under electron irradiation. These observations of MoS2 at extreme temperatures provide insights into the operation of devices based on graphene/MoS2 heterostructures and ultimately may help device fabrication techniques to create MoS2‐based nanostructures, for example, in hydrogen evolution reaction applications.

De-alloyed PtCu/C catalysts with enhanced electrocatalytic performance for the oxygen reduction reaction.

In electrochemical reactions, interactions between reaction intermediates and catalytic surfaces control the catalytic activity, and thereby require to be optimized. Electrochemical de-alloying of mixed-metal nanoparticles is a promising strategy to modify catalysts' surface chemistry and/or induce lattice strain to alter their electronic structure. Perfect design of the electrochemical de-alloying strategy to modify the catalyst's d-band center position can yield significant improvement on the catalytic performance of the oxygen reduction reaction (ORR). Herein, carbon supported PtCu catalysts are prepared by a simple polyol method followed by an electrochemical de-alloying treatment to form PtCu/C catalysts with a Pt-enriched porous shell with improved catalytic activity. Although the pristine PtCu/C catalyst exhibits a mass activity of 0.64 A mg-1Pt, the dissolution of Cu atoms from the catalyst surface after electrochemical de-alloying cycling leads to a significant enhancement in mass activity (1.19 A mg-1Pt), which is 400% better than that of state-of-the-art commercial Pt/C (0.24 A mg-1Pt). Furthermore, the de-alloyed PtCu/C-10 catalyst with a Pt-enriched shell delivers prolonged stability (loss of only 28.6% after 30 000 cycles), which is much better than that of Pt/C with a loss of 45.8%. By virtue of scanning transmission electron microscopy and elemental mapping experiments, the morphology and composition evolution of the catalysts could clearly be elucidated. This work helps in drawing a roadmap to design highly active and stable catalyst platforms for the ORR and relevant proton exchange membrane fuel cell applications.

High-sensitivity mapping of magnetic induction fields with nanometer-scale resolution: comparison of off-axis electron holography and pixelated differential phase contrast

We compare two transmission electron microscopy (TEM) based techniques that can provide highly spatially resolved quantitative measurements of magnetic induction fields at high sensitivity. To this end, the magnetic induction of a ferromagnetic NiFe nanowire has been measured and compared to micromagnetic modeling. State-of-the-art off-axis electron holography has been performed using the averaging of large series of holograms to improve the sensitivity of the measurements. These results are then compared to those obtained from pixelated differential phase contrast, a technique that belongs to pixelated (or 4D) scanning transmission electron microscopy (STEM) experiments. This emerging technique uses a pixelated detector to image the local diffraction patterns as the beam is scanned over the sample. For each diffraction pattern, the deflection of the beam is measured and converted into magnetic induction, while scanning the beam allows a map to be generated. Aberration corrected Lorentz (field-free) configurations of the TEM and STEM were used for an improved spatial resolution. We show that the pixelated STEM approach, even when performed using an old generation of charge-coupled device camera, provides better sensitivity at the expense of spatial resolution. A more general comparison of the two quantitative techniques is given.

Fully resolved strain field of the [formula omitted] precipitate calculated by density functional theory

The β″ precipitate is the main hardening phase in age hardenable Al-Mg-Si alloys, and it is therefore of major scientific and industrial importance. A full model of the β″ precipitate cross-section embedded in an aluminium host lattice is created for a range of precipitate sizes, and relaxed by first principle calculations. The influence of periodic images is avoided by applying a cluster based model with fixed boundary conditions, where the surface is corrected by a displacement field calculated by linear elasticity theory. The calculated misfit values between the precipitate and the host lattice vectors are consistent with experimental scanning transmission electron microscopy results. The misfit area increases proportionally with the cross sectional area, suggesting that the lattice parameters of β″ do not change as the size increases. Both the displacement field and the strain field are in agreement with experimental results. The strain field calculated by density functional theory shows a local zone close to the precipitate where the chemical contribution to the strain field is dominant. The strong correspondence between the experimental and the modelling results supports the methodology to be used in general to study other phases.

Bond Dissociation and Reactivity of HF and H2O in a Nano Test Tube.

Molecular motion and bond dissociation are two of the most fundamental phenomena underpinning the properties of molecular materials. We entrapped HF and H2O molecules within the fullerene C60 cage, encapsulated within a single-walled carbon nanotube (X@C60)@SWNT, where X = HF or H2O. (X@C60)@SWNT represents a class of molecular nanomaterial composed of a guest within a molecular host within a nanoscale host, enabling investigations of the interactions of isolated single di- or triatomic molecules with the electron beam. The use of the electron beam simultaneously as a stimulus of chemical reactions in molecules and as a sub-angstrom resolution imaging probe allows investigations of the molecular dynamics and reactivity in real time and at the atomic scale, which are probed directly by chromatic and spherical aberration-corrected high-resolution transmission electron microscopy imaging, or indirectly by vibrational electron energy loss spectroscopy in situ during scanning transmission electron microscopy experiments. Experimental measurements indicate that the electron beam triggers homolytic dissociation of the H-F or H-O bonds, respectively, causing the expulsion of the hydrogen atoms from the fullerene cage, leaving fluorine or oxygen behind. Because of a difference in the mechanisms of penetration through the carbon lattice available for F or O atoms, atomic fluorine inside the fullerene cage appears to be more stable than the atomic oxygen under the same conditions. The use of (X@C60)@SWNT, where each molecule X is "packaged" separately from each other, in combination with the electron microscopy methods and density functional theory modeling in this work, enable bond dynamics and reactivity of individual atoms to be probed directly at the single-molecule level.

Combined STEM-EDS tomography of nanowire structures

The 3D spatial resolution, the material contrast and the evolution of the noise are analyzed in the reconstructed volume of a combined scanning transmission electron microscopy (HAADF-STEM) and energy dispersive x-ray spectroscopy (EDS) tomography experiment. Standard simultaneous iterative reconstruction technique and HAADF-EDS bimodal tomographic reconstruction are considered for the +/−90° tomography series of a pillar shaped sample embedding a full nanowire device. With a high number of iterations, a spatial resolution for both HAADF and EDS down to 5 nanometer can be reached for this volume. Best material’s contrast and minimum noise are obtained for medium number of iterations. Improvement of the signal-to-noise and contrast can be obtained by filtering the EDS data while the spatial resolution is not impacted. A fast and reliable preparation methodology for rectangularly shaped pillar samples for tomography analysis is discussed.

Epitaxial graphene on 6H–SiC(0001) : Defects in SiC investigated by STEM

The continuous improvement of the sublimation process of SiC allows using epitaxial graphene nowadays for quantum metrology. While it is known that the interface between graphene and the SiC surface is crucial for graphene's transport properties, almost no information about the composition of the SiC substrate after the sublimation process is available. In this study we present high resolution ${\mathrm{c}}_{\text{s}}$-corrected scanning transmission electron microscopy (STEM) experiments on $6H$-SiC(0001) samples after growth of graphene. A Si deficiency within the first three SiC bilayers was found by atomically resolved energy dispersive x-ray spectroscopy (EDX). The Si concentration within the first bilayer can be reduced up to 50%. In addition, as probed by electron energy loss spectroscopy, the hybridization state of C within the first five bilayers revealed a ${\mathrm{sp}}^{2}$ contribution, which we refer to as the precipitation of small carbon clusters. Our analysis clearly shows that the electronic interface of epitaxial graphene on $6H$-SiC(0001) is not atomically sharp.

Layer‐Switching Mechanisms in Sb2Te3

Interfacial phase‐change memory (iPCM) based on layer‐structured Ge‐Sb‐Te crystals has been recently proposed, offering an energy‐efficient implementation of nonvolatile memory cells and supplementing the development of Ge‐Sb‐Te‐based phase‐change random access memories (PRAMs). Although the working principle of iPCM is still under debate, it is believed that layer‐switching plays a role in the switching process between the low‐resistance and high‐resistance states of iPCM memory cells. However, the role of Ge in forming swapped bilayers—the key elements for layer‐switching—is not yet clarified. This work manages to achieve layer‐switching in Sb2Te3 thin films by manipulating the formation of bilayer defects using magnetron sputtering and post‐thermal annealing. By combining scanning transmission electron microscopy (STEM) experiments with density functional theory (DFT) calculations, the essential role of Sb‐Te intermixing is elucidated in stabilizing swapped bilayers at a low energy cost. In situ STEM experiments provide a real‐time and real‐space view of dynamical reconfiguration of van der Waals‐like gaps in Sb2Te3 thin films under electron‐beam irradiation. The results show that the Ge atoms are not necessary for the formation and motion of swapped bilayers, providing atomic insights on the layer‐switching mechanism in layer‐structured binary and ternary group V‐ and IV–V‐tellurides for memory applications.

Kinetics of Ni reaction with organic matter-ferrihydrite composites: Experiments and modeling

The interactions between organic matter (OM) and minerals are among the most important environmental processes, which significantly affect the fate of carbon and contaminants in the environment. How the interactions between OM and ferrihydrite (Fh), one of the most reactive iron oxides, affect the kinetic reactions of heavy metals with both OM and iron oxides, however, are still largely unknown. In this study, two typical OM, humic acids (HA) and fulvic acids (FA), were used to synthesize OM-Fh co-precipitation composites. We investigated the interactions between OM and ferrihydrite and their impact on kinetic reactions of nickel (Ni) with OM-Fh composites using spherical aberration corrected scanning transmission electron microscopy (Cs-STEM), batch and kinetic experiments, and X-ray adsorption spectroscopy (XAS). Cs-STEM results, at both nano and sub-nano scales, showed three typical distributions of OM on ferrihydrite, adsorbed on ferrihydrite surfaces, accumulated as OM nanoparticles, and being blocked in tiny pores in composites. Batch experiments and modeling quantified the effects of OM and ferrihydrite interactions, which was supported by XAS results. The kinetics model developed in this study integrated both WHAM 7 and CD-MUSIC models and successfully predicted the kinetics of Ni adsorption and desorption on OM-Fh composites under the impact of OM-Fh interactions. The model can quantitatively assess the dynamic changes of Ni speciation on different OM and Fh binding sites during the kinetic processes. Our results contribute to developing a comprehensive model for predicting the dynamic metal behavior in the environment when multiple adsorbents are present.

Quantifying Morphology and Diffusion Properties of Mesoporous Carbon From High-Fidelity 3D Reconstructions.

A reliable quantitative analysis in electron tomography, which depends on the segmentation of the three-dimensional reconstruction, is challenging because of constraints during tilt-series acquisition (missing wedge) and reconstruction artifacts introduced by reconstruction algorithms such as the Simultaneous Iterative Reconstruction Technique (SIRT) and Discrete Algebraic Reconstruction Technique (DART). We have carefully evaluated the fidelity of segmented reconstructions analyzing a disordered mesoporous carbon used as support in catalysis. Using experimental scanning transmission electron microscopy (STEM) tomography data as well as realistic phantoms, we have quantitatively analyzed the effect on the morphological description as well as on diffusion properties (based on a random-walk particle-tracking simulation) to understand the role of porosity in catalysis. The morphological description of the pore structure can be obtained reliably both using SIRT and DART reconstructions even in the presence of a limited missing wedge. However, the measured pore volume is sensitive to the threshold settings, which are difficult to define globally for SIRT reconstructions. This leads to noticeable variations of the diffusion coefficients in the case of SIRT reconstructions, whereas DART reconstructions resulted in more reliable data. In addition, the anisotropy of the determined diffusion properties was evaluated, which was significant in the presence of a limited missing wedge for SIRT and strongly reduced for DART.

Quantum mechanical treatment of atomic-resolution differential phase contrast imaging of magnetic materials

Utilizing the Pauli equation based multislice method, introduced in Phys. Rev. Lett. 116, 127203 (2016), we study the atomic resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center of mass pattern in a scanning transmission electron microscopy (STEM) experiment carries information about both electric and magnetic fields. The momentum transfer remains curl-free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.

Point Defects and Localized Excitons in 2D WSe2.

Identifying the point defects in 2D materials is important for many applications. Recent studies have proposed that W vacancies are the predominant point defect in 2D WSe2, in contrast to theoretical studies, which predict that chalcogen vacancies are the most likely intrinsic point defects in transition metal dichalcogenide semiconductors. We show using first-principles calculations, scanning tunneling microscopy (STM), and scanning transmission electron microscopy experiments that W vacancies are not present in our CVD-grown 2D WSe2. We predict that O-passivated Se vacancies (OSe) and O interstitials (Oins) are present in 2D WSe2, because of facile O2 dissociation at Se vacancies or due to the presence of WO3 precursors in CVD growth. These defects give STM images in good agreement with experiment. The optical properties of point defects in 2D WSe2 are important because single-photon emission (SPE) from 2D WSe2 has been observed experimentally. While strain gradients funnel the exciton in real space, point defects are necessary for the localization of the exciton at length scales that enable photons to be emitted one at a time. Using state-of-the-art GW-Bethe-Salpeter-equation calculations, we predict that only Oins defects give localized excitons within the energy range of SPE in previous experiments, making them a likely source of previously observed SPE. No other point defects (OSe, Se vacancies, W vacancies, and SeW antisites) give localized excitons in the same energy range. Our predictions suggest ways to realize SPE in related 2D materials and point experimentalists toward other energy ranges for SPE in 2D WSe2.

Structure Retrieval at Atomic Resolution in the Presence of Multiple Scattering of the Electron Probe.

The projected electrostatic potential of a thick crystal is reconstructed at atomic resolution from experimental scanning transmission electron microscopy data recorded using a new generation fast-readout electron camera. This practical and deterministic inversion of the equations encapsulating multiple scattering that were written down by Bethe in 1928 removes the restriction of established methods to ultrathin (≲50 Å) samples. Instruments already coming on line can overcome the remaining resolution-limiting effects in this method due to finite probe-forming aperture size, spatial incoherence, and residual lens aberrations.