Aberration-corrected Microscopy Research Articles

Improved Method for Electron Powder Diffraction-Based Rietveld Analysis of Nanomaterials.

Multiphase nanomaterials are of increasing importance in material science. Providing reliable and statistically meaningful information on their average nanostructure is essential for synthesis control and applications. In this paper, we propose a novel procedure that simplifies and makes more effective the electron powder diffraction-based Rietveld analysis of nanomaterials. Our single step in-TEM method allows to obtain the instrumental broadening function of the TEM directly from a single measurement without the need for an additional X-ray diffraction measurement. Using a multilayer graphene calibration standard and applying properly controlled acquisition conditions on a spherical aberration-corrected microscope, we achieved the instrumental broadening of ±0.01 Å in terms of interplanar spacing. The shape of the diffraction peaks is modeled as a function of the scattering angle using the Caglioti relation, and the obtained parameters for instrumental broadening can be directly applied in the Rietveld analysis of electron diffraction data of the analyzed specimen. During peak shape analysis, the instrumental broadening parameters of the TEM are controlled separately from nanostructure-related peak broadening effects, which contribute to the higher reliability of nanostructure information extracted from electron diffraction patterns. The potential of the proposed procedure is demonstrated through the Rietveld analysis of hematite nanopowder and two-component Cu-Ni nanocrystalline thin film specimens.

Design Strategies for Perovskite-Type High-Entropy Oxides with Applications in Optics.

It is essential and challenging to develop advanced ceramic materials with thermal stability and high reflectivity for optical fields. Encouragingly, recent breakthroughs and significant advances in high-entropy ceramics have made high-entropy oxides a potential candidate material for optical applications. Therefore, in this study, we analyzed the effect of lattice distortion on the design of high-reflectivity, high-entropy oxides using first-principles calculations and aberration-corrected microscopy. In order to optimize the optical properties of the materials, a series of novel perovskite-type high-entropy oxides, (LaxK0.4-xCa0.2Sr0.2Ba0.2)TiO3+δ (x = 0.1, 0.15, 0.2, 0.25, 0.3), were designed and synthesized using solid-state sintering based on the charge conservation principle and bond energy principle. When the content of La in the A-site element was 30%, the optical reflectivity reached 94% by suppressing the oxygen vacancy. Furthermore, we have successfully prepared a series of coatings by air spraying based on the regulation of the mass ratio of resin and powder. Compared to the uncoated substrate, the backside temperature can be reduced by 41%. This work provides a feasible design route with the first clear guidelines for highly reflective high-entropy ceramic materials and enables highly stable material design in multielement spaces.

Electron-Beam Source with a Superconducting Niobium Tip

Electron-beam sources are foundational in high-resolution electron microscopy and spectroscopy, but applications have been limited due to their relatively large energy spread. The authors fabricate a monocrystalline niobium nanotip electron field emitter and characterize it in superconducting and normal-conducting regimes. This bright, stable, coherent electron beam source features an exceptionally narrow energy spread. The authors also study the role of xenon adsorption and two-electron correlations. This work may improve aberration-corrected microscopy and electron energy-loss spectroscopy and enable high-resolution vibrational spectroscopy or quantum electron microscopy.

Aberration-corrected scanning transmission electron microscopy: the potential for nano- and interface science

Abstract The sub-Ångström probe of an aberration-corrected scanning transmission electron microscope will enable imaging and analysis of nanostructures and interfaces with unprecedented resolution and sensitivity. In conjunction with first-principles theory, new insights are anticipated into the atomistic processes of growth and the subtle link between structure and functionality. We present initial results from the aberration-corrected microscopes at Oak Ridge National Laboratory that indicate the kinds of studies that will become feasible in the near future. Examples include (1) the three-dimensional location and identification of individual dopant and impurity atoms in semiconductor interfaces, and their effect on local electronic structure; (2) the accurate reconstruction of surface atomic and electronic structure on nanocrystals, and the effect on optical properties; and (3) the ability to distinguish which configurations of catalyst atoms are active, and why.

Surface Functionalization of a γ-Graphyne-like Carbon Material via Click Chemistry.

Surface functionalization of carbon materials is of interest in many research fields, such as electrocatalysis, interfacial engineering, and supercapacitors. As an emerging carbon material, γ-graphyne has attracted broad attention. Herein, we report that the surface functionalization of a γ-graphyne-like carbon material (γ-G1) is achieved by immobilizing functional groups via the click chemistry. Texture analysis of aberration-corrected microscopy, X-ray photoelectron spectroscopy, and electrochemistry confirm the successful surface modification of γ-G1 through a strong covalent linkage 1,2,3-triazole. The direct linkage of functional groups on γ-G1 via the click chemistry represents a general method for preparing other functional materials by using γ-graphyne-like materials as a skeleton.

Nanostabilization of tetragonal distorted FeCo variants in ultra-thin FeCo/TiN multilayer films

Composites of ultra-thin nanolayers of FeCo/TiN with individual layer thickness of a few unit cells offer stability at elevated temperatures and exhibit defined magnetic properties which can be tuned by variying the layers thickness making them interesting candiates as components for magnetoelectric (ME) sensors. Initial studies describing the nanostructure of such thin films, however, left uncertainty about their exact crystal structure. With aberration-corrected microscopes the characterization of the local structure down to the atomic scale is possible to address the real structure with high precision. Here, by means of atomic resolution TEM imaging together with X-ray diffraction analysis, we were able to describe the nanostructure to be composed of individual layers of FeCo and TiN forming a superlattice with a defined cube-on-cube orientation relationship. The simulation of electron diffraction pattern featuring superlattice reflections on basis of structural modeling together with geometric phase analysis verified a defined orientation relationship resolving in the nanostabilization of a tetragonal distorted FeCo unit cell with c/a < 1.

Spiers Memorial Lecture: Understanding reaction mechanisms in heterogeneously catalysed reactions.

Heterogeneous catalysis lies at the heart of the chemical and fuel manufacturing industries and hence is a cornerstone of many economies. Many of the commercially operated heterogeneous catalysts have remained basically unchanged for decades, undergoing small but important optimisation of their formulations. Yet we all acknowledge that there is a continuous drive towards improved catalysts or designing new ones. At the heart of these studies has been the need to gain an improved understanding of the reaction mechanism for these important reactions since this can unlock new ways to improve catalyst design and, of course, the ultimate aim is to design catalysts based on the detailed understanding of the reaction mechanism. These advanced studies have been aided in the last decade by two key factors, namely: (a) access to advanced characterisation techniques based on synchrotron methods and aberration-corrected microscopy that can probe the nature of the active site, and (b) the application of high-level computational methods to understand how the reactants and products interact at the active site. In this paper this theme will be explored using two examples to bring out the complexity in gaining an understanding of a reaction mechanism. Using the zeolite H-ZSM-5 as an example of a single site catalyst, the mechanism of the conversion of methanol to the first hydrocarbon carbon-carbon bond will be discussed. In this section the use of model reactants and reaction probes will be used to try to differentiate between different mechanistic proposals. The second example explores the use of gold catalysts for CO oxidation and acetylene hydrochlorination. In both these examples the importance of advanced characterisation and theory will be highlighted.

Misorientation dependence grain boundary complexions in symmetric tilt Al grain boundaries

Since polycrystalline alloys consist of a complex network of various types of grain boundaries (GBs), detailed atomic-scale analysis of how some impurities are distributed at every type of GBs is necessary to fully understand the implications of GB segregation on material's performance. In this study, we present the atomic-scale structural combined with a chemical analysis of segregation induced GB complexions across the various types of Al alloy 7075 GBs using aberration-corrected microscopy and crystal orientation mapping assisted with precession electron diffraction. The result shows multilayer Cu GB segregation containing non-uniformly segregated mixed atomic columns across the interfaces. Two distinct types of Cu GB segregation behavior were observed, point and parallel array, analyzed by means of a displacement field obtained from the dichromatic pattern. Atomistic simulations were performed to test the energetic feasibility of the observed segregation behavior. As per the knowledge of the authors, this is the first report on experimental analysis of segregation induced periodic ordered structured GB complexions on Al alloy system. Furthermore, every GBs of the films were segregated uniquely forming ordered structures along the interface. The distance between two consecutive high segregated units was periodic for the point segregated GBs and followed a trend of a theoretical model of dislocation spacing. Based on the distance between two high segregated units, it is inferred that highly misorientated GBs are more segregated than low misoriented GBs. This study demonstrates that the misorientation between the neighboring grains significantly influences the segregation behavior across the interface and consequently, the structure of segregation-induced GB complexions.

Engineering the In-Plane Structure of Metallic Phase Molybdenum Disulfide via Co and O Dopants toward Efficient Alkaline Hydrogen Evolution.

Molybdenum disulfide (MoS2) has attracted much attention as a promising alternative to Pt-based catalysts for highly efficient hydrogen generation. However, it suffers sluggish kinetics for driving the hydrogen evolution reaction (HER) process because of inert basal planes, especially in alkaline solution. Here, we show a combination of heteroatom doping and phase transformation strategies to engineer the in-plane structure of MoS2, that trigger their catalytic activities. Systematic characterizations are performed with advanced aberration-corrected microscopy and X-ray techniques, indicating that an as-designed MoS2 catalyst has a distorted zigzag-chain superlattice in metallic phase, while its in-plane structure was engineered via the incorporation of cobalt and oxygen species. The optimal Co, O dual-doped metallic phase molybdenum disulfide (1T-MoS2) electrocatalyst shows a significantly enhanced HER activity with a low overpotential of 113 mV at 10 mA cm-2 and corresponding small Tafel slope of 50 mV dec-1, accompanied by the robust stability in alkaline media. The calculated turnover frequency is higher than 6.65 H2 s-1 at an overpotential of 200 mV. More in-depth insights from the first-principle calculations illustrate that the water dissociation as a rate-determining step was largely accelerated by the in-plane Co-O-Mo species and fast electron transfer of the catalyst. Benefiting from ingenious design and fine identifications, this work provides a fundamental understanding of the relationships among heteroatom doping, phase transformation, and performance for MoS2-based catalysts.

Misorientation Dependence Grain Boundary Complexions in Al Alloy Thin Films

Since polycrystalline alloys consist of a complex network of various types of grain boundaries(GBs), detailed atomic-scale analysis of how some impurities are distributed at every type of GBs is necessary to fully understand the implications of GB segregation on material’s performance. In this study, we present the atomic-scale structural analysis combined with a chemical segregation induced GB complexions across the various types of Al alloy 7075 GBs using aberration-corrected microscopy and crystal orientation mapping assisted with precession electron diffraction. The result shows multilayer Cu GB segregation containing non-uniformly segregated mixed atomic columns across the interfaces. Two distinct types of Cu GB segregation behavior were observed, point and parallel array, analyzed by means of a displacement field obtained from the dichromatic pattern. Molecular dynamics simulations were performed to test the energetic feasibility of the observed segregation behavior. As per the knowledge of the authors, this is the first report on GB complexions on Al alloy system. Furthermore, every GBs of the films were segregated uniquely forming ordered structures along the interface. The distance between two consecutive high segregated units was periodic for the point segregated GBs and followed a trend of a theoretical model of dislocation spacing. Based on the distance between two high segregated units, it is inferred that highly misorientated GBs are more segregated than low misoriented GBs. This study demonstrates that the misorientation between the neighboring grains significantly influences the segregation behavior across the interface and consequently, the structure of segregation-induced GB complexions.

Tuning the Nanoarea Interfacial Properties for the Improved Performance of Li-Rich Polycrystalline Li-Mn-O Spinel.

The nontoxicity and low cost make LiMn2O4 a competitive cathode material for lithium-ion batteries. LiMn2O4 has a high theoretical capacity (296 mAh g-1) when cycled in the 3 and 4 V regions. However, it displays a low practical capacity (∼120 mAh g-1) because of the unavailability of the 3 V region caused by severe Jahn-Teller distortion. The present work investigated the full utilization of LiMn2O4 in both 3 and 4 V by tuning the nanoscale interfacial properties. Li-rich structures at the surface and interface of the spinel material and nanograin strain were introduced to improve the performances and were achieved by grinding LiMn2O4 and Li2O at 700 rpm for 10 h under an argon atmosphere. The product shows a high initial discharge capacity of 287.9 mAh g-1 at 0.05 C between 1.2 and 4.6 V and retains 83.2% of the capacity after 50 cycles. The nanoscale interfacial structure was clarified by spherical aberration-corrected microscopy and XRD refinement, and complex occupancies of Li and Mn were found at the interface. A correlation between the interfacial properties and electrochemical performance was established, and the improved performance could be attributed to the polycrystalline nature of the material, the unique Li-rich interfacial structure, and the slightly elevated valence state of Mn. The present results may provide insight for further evaluating the complex mechanism of controlling the electrochemical performance of LiMn2O4.

Sample preparation for aberration-corrected microscopy of high-quality TEM specimens of advanced integrated circuits

Sample preparation for aberration-corrected microscopy of high-quality TEM specimens of advanced integrated circuits

Real-space observation of nanoscale magnetic phase separation in dysprosium by aberration-corrected Lorentz microscopy

Real-space observation of nanoscale magnetic phase separation in dysprosium by aberration-corrected Lorentz microscopy

Quantitative comparison of bright field and annular bright field imaging modes for characterization of oxygen octahedral tilts

Octahedral tilt behavior is increasingly recognized as an important contributing factor to the physical behavior of perovskite oxide materials and especially their interfaces, necessitating the development of high-resolution methods of tilt mapping. There are currently two major approaches for quantitative imaging of tilts in scanning transmission electron microscopy (STEM), bright field (BF) and annular bright field (ABF). In this paper, we show that BF STEM can be reliably used for measurements of oxygen octahedral tilts. While optimal conditions for BF imaging are more restricted with respect to sample thickness and defocus, we find that BF imaging with an aberration-corrected microscope with the accelerating voltage of 300kV gives us the most accurate quantitative measurement of the oxygen column positions. Using the tilted perovskite structure of BiFeO3 (BFO) as our test sample, we simulate BF and ABF images in a wide range of conditions, identifying the optimal imaging conditions for each mode. We show that unlike ABF imaging, BF imaging remains directly quantitatively interpretable for a wide range of the specimen mistilt, suggesting that it should be preferable to the ABF STEM imaging for quantitative structure determination.

Rapid measurement of low-order aberrations using Fourier transforms of crystalline Ronchigrams

The aberrations of the objective lens should be measured and adjusted to realize high spatial resolution in scanning transmission electron microscopy (STEM). Here we report a method of measuring low-order aberrations using the Fourier transforms of Ronchigrams of an arbitrary crystal such as a specimen of interest. We have applied this technique to measure first- and second-order geometrical aberrations using typical standard specimens. Focus and twofold astigmatism are measured using two Ronchigrams obtained under different foci. Axial coma and threefold astigmatism are evaluated using the Fourier transforms of small subareas of a Ronchigram. The time dependences of focus and twofold astigmatism are examined using this technique for an aberration-corrected microscope.

Prospects for electron beam aberration correction using sculpted phase masks

Technological advances in fabrication methods allowed the microscopy community to take incremental steps towards perfecting the electron microscope, and magnetic lens design in particular. Still, state of the art aberration-corrected microscopes are yet 20–30 times shy of the theoretical electron diffraction limit. Moreover, these microscopes consume significant physical space and are very expensive. Here, we show how a thin, sculpted membrane is used as a phase-mask to induce specific aberrations into an electron beam probe in a standard high resolution TEM. In particular, we experimentally demonstrate beam splitting, two-fold astigmatism, three-fold astigmatism, and spherical aberration.

Rapid aberration measurement with pixelated detectors.

Aberration-corrected microscopy in a scanning transmission electron microscope requires the fast and accurate measurement of lens aberrations to align or 'tune' the corrector. Here, we demonstrate a method to measure aberrations based on acquiring a 4D data set on a pixelated detector. Our method is compared to existing procedures and the choice of experimental parameters is examined. The accuracy is similar to existing methods, but in principle this procedure can be performed in a few seconds and extended to arbitrary order. This method allows rapid measurement of aberrations and represents a step towards more automated electron microscopy.

Higher Order Plasmonic Modes Excited in Ag Triangular Nanoplates by an Electron Beam

Ag triangular nanoplates are known to generate strong plasmonic resonances when excited by both light and electron beams. Experimental electron energy-loss spectra (EELS) and maps were acquired using an aberration-corrected JEOL-ARM microscope. The corner, edge and centre modes that are often observed in such structures were also observed in these measurements. In addition, novel higher order internal modes were observed and were found to be well reproduced by theoretical calculations using boundary element method (BEM). These modes are “dark modes” so are not observed in the optical extinction spectra. They are confined surface propagating modes and are analogous to laser cavity modes.

A high-resolution analytical scanning transmission electron microscopy study of the early stages of spinodal decomposition in binary Fe–Cr

Spinodal decomposition (SD) is an important phenomenon in materials science and engineering. For example, it is considered to be responsible for the 475°C embrittlement of stainless steels comprising the bcc (ferrite) or bct (martensite) phases. Structural characterization of the evolving minute nano-scale concentration fluctuations during SD in the Fe–Cr system is, however, a notable challenge, and has mainly been considered accessible via atom probe tomography (APT) and small-angle neutron scattering. The standard tool for nanostructure characterization, viz. transmission electron microscopy (TEM), has only been successfully applied to late stages of SD when embrittlement is already severe. However, we here demonstrate that the structural evolution in the early stages of SD in binary Fe–Cr, and alloys based on the binary, are accessible via analytical scanning TEM. An Fe–36wt% Cr alloy aged at 500°C for 1, 10 and 100h is investigated using an aberration-corrected microscope and it is found that highly coherent and interconnected Cr-rich regions develop. The wavelength of decomposition is rather insensitive to the sample thickness and it is quantified to 2, 3 and 6nm after ageing for 1, 10 and 100h, which is in reasonable agreement with prior APT analysis. The concentration amplitude is more sensitive to the sample thickness and acquisition parameters but the TEM analysis is in good agreement with APT analysis for the longest ageing time. These findings open up for combinatorial TEM studies where both local crystallography and chemistry is required.

Multidisciplinary Science at the Molecular Foundry

Multidisciplinary Science at the Molecular Foundry