Annular Bright-field Scanning Transmission Research Articles

On the shear strain associated with T1 precipitation in Al-Cu-Li(-Mg-Ag) alloys

The formation mechanism and the distribution arrangements of T1 precipitate plates in peak-aged samples of an Al-4Cu-1.2Li-0.3Mg-0.4Ag (wt.%) alloy are studied using atomic-resolution high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM). The formation of a T1 plate of a single unit-cell thickness from the α-Al matrix is found to be associated with a large shear strain that is equivalent to the formation and glide of two different Shockley partial dislocations on two alternate {111}α planes. The T1 precipitate plates tend to cluster into various configurations, including T-shaped and V-shaped configurations comprising two plates of two different T1 variants, and X-shaped configurations consisting of three or four plates of two different variants. The signature of the shear strain is clearly visible in these configurations. A single T1 plate of one unit-cell thickness is elastically distorted in a T-shaped configuration, and the two T1 plates of the same variant in an X-shaped configuration are displaced with respect to each other. Different types of stair-rods are also formed in the intersection of the paired T1 plates in the V-shaped configurations.

A P2/P3 Biphasic Layered Oxide Composite as a High-Energy and Long-Cycle-Life Cathode for Potassium-Ion Batteries.

Layered transition metal oxides are extensively considered as appealing cathode candidates for potassium-ion batteries (PIBs) due to their abundant raw materials and low cost, but their further implementations are limited by slow dynamics and impoverished structural stability. Herein, a layered composite having a P2 and P3 symbiotic structure is designed and synthesized to realize PIBs with large energy density and long-term cycling stability. The unique intergrowth of P2 and P3 phases in the obtained layered oxide is plainly characterized by X-ray diffraction refinement, high-angle annular dark field and annular bright field-scanning transmission electron microscopy at atomic resolution, and Fourier transformation images. The synergistic effect of the two phases of this layered P2/P3 composite is well demonstrated in K+ intercalation/extraction process. The as-prepared layered composite can present a large discharge capacity with the remarkable energy density of 321 Wh kg-1 and also manifest excellent capacity preservation after 600 cycles of K+ uptake/removal.

Demonstration and STEM Analysis of Ferroelectric Switching in MOCVD‐Grown Single Crystalline Al0.85Sc0.15N

AbstractWurtzite‐type Al1−xScxN solid solutions grown by metal organic chemical vapor deposition are for the first time confirmed to be ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive field of 5.5 MV cm−1 at a measurement frequency of 1.5 kHz. The single crystal quality and homogeneous chemical composition of the film are confirmed by X‐ray diffraction and spectroscopic methods such as time of flight secondary ion mass spectrometry. Annular bright field scanning transmission electron microscopy serves to prove the ferroelectric polarization inversion at the unit cell level. The single crystal quality further allows to image the large‐scale domain pattern of a wurtzite‐type ferroelectric for the first time, revealing a predominantly cone‐like domain shape along the c‐axis of the material. As in previous work, this again implies the presence of strong polarization discontinuities along this crystallographic axis, which can be suitable for current transport. The domains are separated by narrow domain walls, for which an upper thickness limit of 3 nm is deduced but which can potentially be atomically sharp. The authors are confident that these results will advance the commencement of the integration of wurtzite‐type ferroelectrics to GaN as well as generally III‐N‐based heterostructures and devices.

Unusual structural rearrangement and superconductivity in infinite layer cuprate superlattices

Epitaxial stabilization of thermodynamically metastable phases and advances in atomic control of complex oxide thin-film growth can be used effectively for realizing novel phenomena and as an alternative for bulk synthesis under extreme thermodynamic conditions. Here, we investigate infinite layer (IL) based cuprate superlattices, where 7--8 unit cells of ${\mathrm{Sr}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Cu}{\mathrm{O}}_{2}$ (SCCO) are sandwiched between ultrathin spacer layers of $\mathrm{SrTi}{\mathrm{O}}_{3}$ (STO), $\mathrm{SrRu}{\mathrm{O}}_{3}$, or $\mathrm{BaCu}{\mathrm{O}}_{2}$ (BCO) and only observe superconductivity in the pure [SCCO/BCO] superlattice (SL) without spacer layers. Apparently, the insertion of an additional STO spacer layer in the latter SL prevents the occurrence of superconductivity. The observed superconductivity in [SCCO/BCO] SL is discussed in terms of a structural model involving the interplay between the $\mathrm{Cu}{\mathrm{O}}_{2}$ plane and the CuO chain similar to the bulk $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ superconductor. The structural origin was found by the identification of a metastable IL-$\mathrm{BaCu}{\mathrm{O}}_{2}$ variant, which deviates highly from its parent bulk crystal structure and exhibits a relatively larger out-of-plane lattice parameter (around $7\phantom{\rule{0.16em}{0ex}}\AA{}$) when sandwiched with SCCO in the form of [SCCO/BCO] SL. However, this variant is absent when STO spacer layers are introduced between SCCO and BCO layers. X-ray absorption spectra of the Cu $L$ edge for BCO exhibits a slightly higher energy satellite peak as compared to the $3{d}^{9}L$ Zhang-Rice character observed in SCCO. This result indicates the existence of contrasting plane and chain-type Cu-O blocks in SCCO and BCO, respectively, which is further corroborated using annular bright field scanning transmission electron microscopy imaging. This work unravels an unexpected structure of BCO which helps in realizing superconductivity in [SCCO/BCO] SL and provides a wider perspective in the growth and design of cuprate-based hybrid structures.

Low-energy scanning transmission electron microscopy applied to ice-embedded biological macromolecules.

In this report, we applied annular bright-field and annular dark-field low-energy (30 keV) scanning transmission electron microscopy imaging to a vitreous ice-embedded biological macromolecule, T4 phage, to investigate the applicability of these methods for morphological investigation and sample screening. Multiple camera lengths were examined to find the optimal acceptance angle for both modes. Image clarity differed substantially between the modes, with the presence of ice also strongly influencing the quality of acquired micrographs. In annular dark-field mode, the proper discrimination of electrons scattered by the specimen from those scattered by the background ice was found to be difficult due to the severe overlap of the scattered electrons. The resulting micrographs lacked clarity, and the ice-embedded phage particles could only be discerned after post-processing image adjustment. However, in annular bright-field mode, despite similar overlapping of the scattered electrons, it was possible to assess the morphology and intactness of the specimen in the embedding ice, suggesting that this mode may find utility in low-energy cryo-scanning transmission electron microscopy imaging methods.

Large (anti)ferrodistortive NaNbO3-based lead-free relaxors: Polar nanoregions embedded in ordered oxygen octahedral tilt matrix

Relaxor ferroelectric perovskites have experienced a revival of interest in recent years accompanying the deeply exploration of the fine local structure of polar nanoregions, which are thought to be the structure root for unique electrical properties. Interestingly, a new type of low-tolerance relaxor ferroelectric, which exhibits both cation displacement and large oxygen octahedral tilt, exhibiting anomalous microdomains were found in NaNbO3-based lead-free perovskites, breaking through the limits of the relaxor ferroelectric models proposed from classical relaxors. Atomic-scale annular bright-field scanning transmission electron microscopy images indicate that the microdomains are formed by long-range ordered oxygen octahedral tilt. At the same time, substructure of randomly embedded nanoregions with inhomogeneous polarization vectors can be mapped, being the main reason for relaxor behavior. Based on this special ferroelectric-improper ferroelastic state, delayed polarization saturation can be obtained under strong electric field, providing an opportunity for generating excellent energy storage properties. The results found in this work not only open up a new kind of relaxor ferroelectrics but also will lead to the reconstruction of relaxor models and theories built for more than half a century.

Atomic-scale mapper for superlattice photodetectors analysis.

In this work, a new Python-based tool for atomic-scale mapping of high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM) images using the Z-contrast method is introduced, aimed to help in the analysis of superlattice layers' composition, and in the determination of material of interfaces. The operation principle of the program, as well as specific examples of use, are explained in many details. Good customization flexibility using the user-friendly graphical user interface (GUI), allows the processing of a wide range of images, demonstrating a decent accuracy of coordinates extraction and performance.

Robust Ruthenium Catalysts Supported on Mesoporous Cyclodextrin-Templated TiO2-SiO2 Mixed Oxides for the Hydrogenation of Levulinic Acid to γ-Valerolactone.

In this paper, we present a versatile template-directed colloidal self-assembly method for the fabrication in aqueous phase of composition-tuned mesoporous RuO2@TiO2-SiO2 catalysts. Randomly methylated β-cyclodextrin/Pluronic F127 supramolecular assemblies were used as soft templates, TiO2 colloids as building blocks, and tetraethyl orthosilicate as a silica source. Catalysts were characterized at different stages of their synthesis using dynamic light scattering, N2-adsorption analysis, powder X-ray diffraction, temperature programmed reduction, high-resolution transmission electron microscopy, high-angle annular bright-field and dark-field scanning transmission electron microscopy, together with EDS elemental mapping. Results revealed that both the supramolecular template and the silica loading had a strong impact on the pore characteristics and crystalline structure of the mixed oxides, as well as on the morphology of the RuO2 nanocrystals. Their catalytic performance was then evaluated in the aqueous phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) under mild conditions (50 °C, 50 bar H2). Results showed that the cyclodextrin-derived catalyst displayed almost quantitative LA conversion and 99% GVL yield in less than one hour. Moreover, this catalyst could be reused at least five times without loss of activity. This work offers an effective approach to the utilization of cyclodextrins for engineering the surface morphology of Ru nanocrystals and pore characteristics of TiO2-based materials for catalytic applications in hydrogenation reactions.

Epitaxy from a Periodic Y–O Monolayer: Growth of Single-Crystal Hexagonal YAlO3 Perovskite

The role of an atomic-layer thick periodic Y–O array in inducing the epitaxial growth of single-crystal hexagonal YAlO3 perovskite (H-YAP) films was studied using high-angle annular dark-field and annular bright-field scanning transmission electron microscopy in conjunction with a spherical aberration-corrected probe and in situ reflection high-energy electron diffraction. We observed the Y–O array at the interface of amorphous atomic layer deposition (ALD) sub-nano-laminated (snl) Al2O3/Y2O3 multilayers and GaAs(111)A, with the first film deposition being three cycles of ALD-Y2O3. This thin array was a seed layer for growing the H-YAP from the ALD snl multilayers with 900 °C rapid thermal annealing (RTA). The annealed film only contained H-YAP with an excellent crystallinity and an atomically sharp interface with the substrate. The initial Y–O array became the bottom layer of H-YAP, bonding with Ga, the top layer of GaAs. Using a similar ALD snl multilayer, but with the first film deposition of three ALD-Al2O3 cycles, there was no observation of a periodic atomic array at the interface. RTA of the sample to 900 °C resulted in a non-uniform film, mixing amorphous regions and island-like H-YAP domains. The results indicate that the epitaxial H-YAP was induced from the atomic-layer thick periodic Y–O array, rather than from GaAs(111)A.

High contrast STEM imaging for light elements by an annular segmented detector

Annular bright-field scanning transmission electron microscopy (ABF STEM) has been actively used to directly observe the light element atoms inside materials and devices. However, the detector angle condition for conventional ABF STEM has been empirically selected and thus is not always optimized for observing ultra-light element atoms such as hydrogen and lithium atoms. In this study, the detector conditions for ABF STEM were reexamined by calculating a new type of phase contrast transfer function (PCTF) for an annularly segmented detector to maximize the image contrast of ultra-light element atoms such as lithium. Using this new PCTF, an improved detector geometry for observing lithium atoms is demonstrated, which is confirmed by the image simulations and experiments in several types of lithium cathode materials.

Investigation of the Structure of the Modulated Doubly Ordered Perovskite NaLaCoWO6 and Its Reversible Phase Transition with a Colossal Temperature Hysteresis.

Transmission electron microscopy, neutron diffraction, and synchrotron powder X-ray diffraction reveal a complex modulated structure on the doubly ordered perovskite NaLaCoWO6. Electron diffraction patterns as well as high-resolution transmission electron microscopy images clearly show a periodicity of 12 ap, where ap is the cell parameter of the generic perovskite, along either the [100]p or [010]p direction. Annular bright-field scanning transmission electron microscopy of slightly tilted samples shows that there is no chemical origin for the superstructure but that it is caused by geometric rearrangements. An atomic model of the superstructure is proposed on the basis of octahedral tilt twinning. At low temperature, NaLaCoWO6 undergoes a phase transition and the superstructure disappears. The compound takes on the more usual monoclinic P21 structure below the transition. Neutron powder diffraction reveals and electron diffraction confirms an unusually large temperature hysteresis, where the transition takes place at ∼180 K on cooling and at ∼320 K on heating. This hysteresis can be attributed to the necessity of rearranging the oxygen octahedra and the thus induced energy barrier for the transition.

2D AlN Layers Sandwiched Between Graphene and Si Substrates.

Due to the superior thickness-dependent properties, 2D materials have exhibited great potential for applications in next-generation optoelectronic devices. Despite the significant progress that has been achieved, the synthesis of 2D AlN remains challenging. This work reports on the epitaxial growth of 2D AlN layers via utilizing physically transferred graphene on Si substrates by metal-organic chemical vapor deposition. The 2D AlN layers sandwiched between graphene and Si substrates are confirmed by annular bright-field scanning transmission electron microscopy and the effect of hydrogenation on the formation of 2D AlN layers is clarified by theoretical calculations with first-principles calculations based on density functional theory. Moreover, the bandgap of as-grown 2D AlN layers is theoretically predicted to be ≈9.63 eV and is experimentally determined to be 9.20-9.60 eV. This ultrawide bandgap semiconductor shows great promise in deep-ultraviolet optoelectronic applications. These results are expected to support innovative and front-end development of optoelectronic devices.

Low-Dose Aberration-Free Imaging of Li-Rich Cathode Materials at Various States of Charge Using Electron Ptychography

Imaging the complete atomic structure of materials, including light elements, with minimal beam-induced damage of the sample is a long-standing challenge in electron microscopy. Annular bright-field scanning transmission electron microscopy is often used to image elements with low atomic numbers, but due to its low efficiency and high sensitivity to precise imaging parameters it comes at the price of potentially significant beam damage. In this paper, we show that electron ptychography is a powerful technique to retrieve reconstructed phase images that provide the full structure of beam-sensitive materials containing light and heavy elements. Due to its much higher efficiency, we can reduce the beam currents used down to the subpicoampere range. Electron ptychography also allows residual lens aberrations to be corrected at the postprocessing stage, which avoids the need for fine-tuning of the probe that would result in further beam damage and provides aberration-free reconstructed phase images. We have used electron ptychography to obtain structural information from aberration-free reconstructed phase images in the technologically relevant lithium-rich transition metal oxides at different states of charge. We can unambiguously determine the position of the lithium and oxygen atomic columnswhile amorphization of the surface, formation of beam-induced surface reconstruction layers, or migration of transition metals to the alkali layers are drastically reduced.

Atomic Level Engineering of Structural and Functional Properties of Transition Metal Oxides

Metal-oxygen bonds are responsible for a broad spectrum of functional properties of transition-metal oxides, and engineering the bonds is a key for exploring their properties. Artificial oxide heterostructures with chemically abrupt interfaces have provided a platform for engineering bonding geometries that could lead to emergent phenomena not seen in bulk oxides. In this paper, we demonstrate that interfacially engineering oxygen displacement is a good route for exploring structural and functional properties of oxides. Our high-resolution annular bright-field scanning transmission electron microscopy observations revealed that interfacial oxygen displacement determines metal-oxygen bond angles in entire heterostructures and the magnitude of the displacements can be adjusted by controlling propagations of the oxygen octahedral rotations across the heterointerface. We also show that by heterostructuring an itinerant ferromagnet SrRuO3 with Ca0.5Sr0.5TiO3 (0–4 monolayers thick) grown on a GdScO3 substrate, structural phase and magnetic anisotropy of SrRuO3 can be controlled through the interfacial engineering of its oxygen displacement.

Mechanistic Insight into the Photocatalytic Working of Fluorinated Anatase {001} Nanosheets

Anatase nanosheets with exposed {001} facets have gained increasing interest for photocatalytic applications. To fully understand the structure-to-activity relation, combined experimental and computational methods have been exploited. Anatase nanosheets were prepared under hydrothermal conditions in the presence of fluorine ions. High resolution scanning transmission electron microscopy was used to fully characterize the synthesized material, confirming the TiO2 nanosheet morphology. Moreover, the surface structure and composition of a single nanosheet could be determined by annular bright-field scanning transmission electron microscopy (ABF-STEM) and STEM electron energy loss spectroscopy (STEM-EELS). The photocatalytic activity was tested for the decomposition of organic dyes rhodamine 6G and methyl orange and compared to a reference TiO2 anatase sample. The anatase nanosheets with exposed {001} facets revealed a significantly lower photocatalytic activity compared to the reference. In order to understand the mechanism for the catalytic performance, and to investigate the role of the presence of F–, light-induced electron paramagnetic resonance (EPR) experiments were performed. The EPR results are in agreement with TEM, proving the presence of Ti3+ species close to the surface of the sample and allowing the analysis of the photoinduced formation of paramagnetic species. Further, ab initio calculations of the anisotropic effective mass of electrons and electron holes in anatase show a very high effective mass of electrons in the [001] direction, having a negative impact on the mobility of electrons toward the {001} surface and thus the photocatalysis. Finally, motivated by the experimental results that indicate the presence of fluorine atoms at the surface, we performed ab initio calculations to determine the position of the band edges in anatase slabs with different terminations of the {001} surface. The presence of fluorine atoms near the surface is shown to strongly shift down the band edges, which indicates another reason why it can be expected that the prepared samples with a large amount of {001} surface, but with fluorine atoms near the surface, show only a low photocatalytic activity.

Picometer-scale atom position analysis in annular bright-field STEM imaging

We study the effects of specimen mistilt on the picometer-scale measurement of local structure by combing experiment and simulation in annular bright-field scanning transmission electron microscopy (ABF-STEM). A relative distance measurement method is proposed to separate the tilt effects from the scan noise and sample drift induced image distortion. We find that under a typical experimental condition a small specimen tilt (∼6 mrad) in 25 nm thick SrTiO3 along [001] causes 11.9 pm artificial displacement between O and Sr/TiO columns in ABF image, which is more than 3 times of scan noise and sample drift induced image distortion ∼3.2 pm, suggesting the tilt effect could be dominant for the quantitative analysis of ABF images. The artifact depends on the crystal mistilt angle, specimen thickness, defocus, convergence angle and uncorrected aberration. Our study provides useful insights into detecting and correcting tilt effects during both experiment operation and data analysis to extract the real structure information and avoid mis-interpretations of atomic structure as well as the properties such as oxygen octahedral distortion/shift.

Origin and effective reduction of inversion domains in aluminum nitride grown by a sublimation method

Owing to the large differences in the chemical properties between Al and N polarities in aluminum nitride (AlN), the choice of the polar direction for crystal growth strongly affects not only the quality but also the shape (facet formation) of the grown crystal. In particular, N-polar (000−1) has been considered to be a more preferable direction than Al-polar (0001) for sublimation growth because compared to Al-polar (0001), N-polar (000−1) exhibits better stability at high growth rate (high supersaturation) conditions and enables easier lateral enlargement of the crystal. However, some critical growth conditions induce polarity inversion and hinder stable N-polar growth. Furthermore, the origin of the polarity inversion in AlN growth by the sublimation method is still unclear. To ensure stable N-polar growth without polarity inversion, the formation mechanism of the inversion domain during AlN sublimation growth must be elucidated. Therefore, herein, we demonstrate homoepitaxial growth on an N-polar seed and carefully investigate the obtained crystal that shows polarity inversion. Annular bright-field scanning transmission electron microscopy reveals that polarity is completely converted to the Al polarity via the formation of a 30nm thick mixed polar layer (MPL) just above the seed. Moreover, three-dimensional atom probe tomography shows the segregation of the oxygen impurities in the MPL with a high concentration of about 3atom%. Finally, by avoiding the incorporation of oxygen impurity into the crystal at the initial stage of the growth, we demonstrate an effective reduction (seven orders of magnitude) of the inversion domain boundary formation.

Toward Switchable Photovoltaic Effect via Tailoring Mobile Oxygen Vacancies in Perovskite Oxide Films.

The defect chemistry of perovskite oxides involves the cause to most of their abundant functional properties, including interface magnetism, charge transport, ionic exchange, and catalytic activity. The possibility to achieve dynamic control over oxygen anion vacancies offers a unique opportunity for the development of appealing switchable devices, which at present are commonly based on ferroelectric materials. Herein, we report the discovery of a switchable photovoltaic effect, that the sign of the open voltage and the short circuit current can be reversed by inverting the polarity of the applied field, upon electrically tailoring the distribution of oxygen vacancies in perovskite oxide films. This phenomenon is demonstrated in lateral photovoltaic devices based on both ferroelectric BiFeO3 and paraelectric SrTiO3 films, under a reversed applied field whose magnitude is much smaller than the coercivity value of BiFeO3. The migration of oxygen vacancies was directly observed by employing an advanced annular bright-field scanning transmission electron microscopy technique with in situ biasing equipment. We conclude that the band bending induced by the motion of oxygen vacancies is the driving force for the reversible switching between two photovoltaic states. The present work can provide an active path for the design of novel switchable photovoltaic devices with a wide range of transition metal oxides in terms of the ionic degrees of freedom.

A new method to detect and correct sample tilt in scanning transmission electron microscopy bright-field imaging

Important properties of functional materials, such as ferroelectric shifts and octahedral distortions, are associated with displacements of the positions of lighter atoms in the unit cell. Annular bright-field scanning transmission electron microscopy is a good experimental method for investigating such phenomena due to its ability to image light and heavy atoms simultaneously. To map atomic positions at the required accuracy precise angular alignment of the sample with the microscope optical axis is necessary, since misalignment (tilt) of the specimen contributes to errors in position measurements of lighter elements in annular bright-field imaging. In this paper it is shown that it is possible to detect tilt with the aid of images recorded using a central bright-field detector placed within the inner radius of the annular bright-field detector. For a probe focus near the middle of the specimen the central bright-field image becomes especially sensitive to tilt and we demonstrate experimentally that misalignment can be detected with a precision of less than a milliradian, as we also confirm in simulation. Coma in the probe, an aberration that can be misidentified as tilt of the specimen, is also investigated and it is shown how the effects of coma and tilt can be differentiated. The effects of tilt may be offset to a large extent by shifting the diffraction plane detector an amount equivalent to the specimen tilt and we provide an experimental proof of principle of this using a segmented detector system.

Direct visualization of lithium via annular bright field scanning transmission electron microscopy: a review.

Annular bright field (ABF) scanning transmission electron microscopy has proven able to directly image lithium columns within crystalline environments, offering much insight into the structure and properties of lithium-ion battery materials. We summarize the image formation mechanisms underpinning ABF imaging, review the experimental application of this technique to imaging lithium in materials and overview the conditions that help maximize the visibility of lithium columns.