Contrast Scanning Transmission Electron Microscopy Research Articles

Diffraction contrast of ferroelectric domains in DPC STEM images.

Differential phase contrast scanning transmission electron microscopy (DPC STEM) is a powerful technique for directly visualizing electromagnetic fields inside materials at high spatial resolution. Electric field observation within ferroelectric materials is potentially possible by DPC STEM, but concomitant diffraction contrast hinders the quantitative electric field evaluation. Diffraction contrast is basically caused by the diffraction-condition variation inside a field-of-view, but in the case of ferroelectric materials, the diffraction conditions can also change with respect to the polarization orientations. To quantitatively observe electric field distribution inside ferroelectric domains, the formation mechanism of diffraction contrast should be clarified in detail. In this study, we systematically simulated diffraction contrast of ferroelectric domains in DPC STEM images based on the dynamical diffraction theory, and clarify the issues for quantitatively observing electric fields inside ferroelectric domains. Furthermore, we conducted experimental DPC STEM observations for a ferroelectric material to confirm the influence of diffraction contrast predicted by the simulations.

One-Pot Synthesis of CHA/ERI-Type Zeolite Intergrowth from a Single Multiselective Organic Structure-Directing Agent.

We report the one-pot synthesis of a chabazite (CHA)/erionite (ERI)-type zeolite intergrowth structure characterized by adjustable extents of intergrowth enrichment and Si/Al molar ratios. This method utilizes readily synthesizable 6-azaspiro[5.6]dodecan-6-ium as the exclusive organic structure-directing agent (OSDA) within a potassium-dominant environment. High-throughput simulations were used to accurately determine the templating energy and molecular shape, facilitating the selection of an optimally biselective OSDA from among thousands of prospective candidates. The coexistence of the crystal phases, forming a distinct structure comprising disk-like CHA regions bridged by ERI-rich pillars, was corroborated via rigorous powder X-ray diffraction and integrated differential-phase contrast scanning transmission electron microscopy (iDPC S/TEM) analyses. iDPC S/TEM imaging further revealed the presence of single offretite layers dispersed within the ERI phase. The ratio of crystal phases between CHA and ERI in this type of intergrowth could be varied systematically by changing both the OSDA/Si and K/Si ratios. Two intergrown zeolite samples with different Si/Al molar ratios were tested for the selective catalytic reduction (SCR) of NOx with NH3, showing competitive catalytic performance and hydrothermal stability compared to that of the industry-standard commercial NH3-SCR catalyst, Cu-SSZ-13, prevalent in automotive applications. Collectively, this work underscores the potential of our approach for the synthesis and optimization of adjustable intergrown zeolite structures, offering competitive alternatives for key industrial processes.

Atomic Imaging of the Pb Precipitation in Lead-Halide Perovskites.

The lead iodide (PbI2 ) in lead-halide perovskite (LHP) is both a positive additive for material properties and a site for the formation of device defects. Therefore, atomic-level detection of PbI2 and its derivedPbstructures are crucial for understanding the performance and stability of the LHP material. In this work, the atomic imaging of the LHP, PbI2 , and Pb lattices is achieved using low-dose integrated differential phase contrast (iDPC) scanning transmission electron microscopy (STEM). Combining it with the traditional high-angle annular dark field (HAADF)-STEM, the Pb precipitation in different LHPs (CsPbI3 , CsPbBr3, and FAPbI3 ) and under different conditions (light, air, and heat) can be investigated in real space. Then, the features of Pb precipitation (positions and sizes) are visually revealed under different conditions and the stabilities of different LHPs. Meanwhile, the pathway of Pb precipitation is directly imaged and confirmed by the iDPC-STEM during an in situ heating process, supporting the detailed mechanism of Pb precipitation. These results provide the visual evidence for analyzing atomic Pb precipitation in LHPs, which helps better understand the structure-property relation induced by Pb impurity.

Magnetic Field Observation in a Magnetic Tunnel Junction by Scanning Transmission Electron Microscopy.

A magnetic tunnel junction (MTJ) consists of two ferromagnetic layers separated by a thin insulating layer. MTJs show tunnel magnetoresistance effect, where the resistance in the direction perpendicular to the insulator layer drastically changes depending on the magnetization directions (parallel or antiparallel) in the ferromagnetic layers. However, direct observation of local magnetizations inside MTJs have been challenging. In this study, we demonstrate direct observation of magnetic flux density distribution inside epitaxially grown Fe/MgO/Fe layers using differential phase contrast scanning transmission electron microscopy. By utilizing newly-developed tilt-scan averaging system for suppressing diffraction contrasts, we clearly visualize parallel and antiparallel states of ferromagnetic layers at nanometer resolution.

Synergistic polarization engineering on BaTiO3 bulk and surface for boosting redox kinetics of polysulfides in lithium–sulfur batteries

The notorious shuttle effect of intermediate lithium polysulfides and the sluggish kinetics of sulfur redox reactions restrict the further development of high-energy-density lithium-sulfur (Li–S) batteries. Herein, a strategy of synergistic polarization engineering on ferroelectric barium titanate (BaTiO3) bulk and surface is reported, aiming to collectively improve the adsorption-catalytic ability of ferroelectric BaTiO3 to polysulfides. Differential phase contrast-scanning transmission electron microscopy (DPC-STEM) demonstrates that the introduction of ultrathin heteroepitaxial TiOx surface can give rise to the surface local electric field differing from bulk internal polarization field. Experimental and theoretical results further reveal that synergistic polarization engineering by integrating the enhanced bulk internal polarization field with the surface local electric field can collaboratively realize excellent adsorption-catalytic activity resulting from rapid electrons transfer and optimized active sites. As a result, employing BaTiO3@TiOx modified separator can endow the batteries with favorable rate performance (the discharge specific capacity achieves 710 mAh g−1 at 4 C) and substantial enhancement on cyclic performance (64.5 % capacity retention at 2 C over 500 cycles). Impressively, even at high sulfur loading of 2.5 mg cm−2, the BaTiO3@TiOx-based cell still stabilizes 78 % capacity retention with at 1 C over 150 cycles, showing the practical potential of BaTiO3@TiOx in optimizing catalytic efficiency.

Evolution of local misorientations in the γ/γ'-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method.

The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the γ- and γ'-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850°C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at γ/γ'-interfaces and the filling of γ-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented γ/γ'-neighborhoods, on a size scale above characteristic γ/γ'-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (<200 μm). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature. RESEARCH HIGHLIGHTS: Microstructure evolution during [001] tensile creep of Ni-based single-crystalline alloy. Application of RVB-EBSD technique, focused on angular misorientations between γ/γ' phases, with accuracy of 0.01°. Separation of γ/γ' phases using experimental post-processing of raw EBSD data.

Anisotropic Amorphization and Phase Transition in Na2 Ti3 O7 Anode Caused by Electron Beam Irradiation.

Na2 Ti3 O7 is considered one of the most promising anode materials for sodium ion batteries due to its superior safety, environmental friendliness, and low manufacturing cost. However, its structural stability and reaction mechanism still have not been fully explored. As the electron beam irradiation introduces a similar impact on the Na2 Ti3 O7 anode as the extraction of Na+ ions during the battery discharge process, the microstructure evolution of the materials is investigated by advanced electron microscopy techniques at the atomic scale. Anisotropic amorphization is successfully observed. Through the integrated differential phase contrast-scanning transmission electron microscopy technique and density functional theory calculation, a phase transition pathway involving a new phase, Na2 Ti24 O49 , is proposed with the reduction of Na atoms. Additionally, it is found that the amorphization is dominated by the surface energy and electron dose rate. These findings will deepen the understanding of structural stability and deintercalation mechanism of the Na2 Ti3 O7 anode, providing new insight into exploring the failure mechanism of electrode materials.

A metal-free photoactive nitrogen-doped carbon nanosolenoid with broad absorption in visible region for efficient photocatalysis

Riemann surfaces inspired chemists to design and synthesize such multidimensional curved carbon architectures. It has been predicted that carbon nanosolenoid materials with Riemann surfaces have unique structures and novel physical properties. Here we report the first synthesis of a nitrogen-doped carbon nanosolenoid (N-CNS) using bottom-up approach with a well-defined structure. N-CNS was obtained by a rational Suzuki polymerization, followed by oxidative cyclodehydrogenation. The successful synthesis of N-CNS was fully characterized by GPC, FTIR, solid-state 13C NMR and Raman techniques. The intrinsic single-strand molecular structures of N-CNS helices can be clearly resolved using low-dose integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) technique. Possessing unique structural and physical properties, this long π-extended polymer N-CNS can provide new insight towards bottom-up syntheses of curved nanoribbons and potential applications as a metal-free photocatalyst for visible-light-driven H2 evolution and highly efficient photocatalyst for photoredox organic transformations.

Imaging the dynamic influence of functional groups on metal-organic frameworks

Metal-organic frameworks (MOFs) with different functional groups have wide applications, while the understanding of functionalization influences remains insufficient. Previous researches focused on the static changes in electronic structure or chemical environment, while it is unclear in the aspect of dynamic influence, especially in the direct imaging of dynamic changes after functionalization. Here we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to directly ‘see’ the rotation properties of benzene rings in the linkers of UiO-66, and observe the high correlation between local rigidity and the functional groups on the organic linkers. The rigidity is then correlated to the macroscopic properties of CO2 uptake, indicating that functionalization can change the capability through not only static electronic effects, but also dynamic rotation properties. To the best of our knowledge this is the first example of a technique to directly image the rotation properties of linkers in MOFs, which provides an approach to study the local flexibility and paves the way for potential applications in capturing, separation and molecular machine.

Atomically Unraveling the Structural Evolution of Surfaces and Interfaces in Metal Halide Perovskite Quantum Dots.

Revealing the local structural change of metal halide perovskites (MHPs) induced by external conditions is important to understand its performance and stability in optoelectronic applications. However, previous studies on the properties and structures of MHPs are usually limited by the spatial resolution of the probe, and it is still challenging to obtain its atomic structural information in real space. In this work, the integrated differential-phase-contrast scanning transmission electron microscopy is applied to the low-dose imaging of CsPbI3 quantum dots (QDs). In particular, the local structures in QDs, such as surfaces and interfaces, can be atomically resolved. Then, the structural evolution of CsPbI3 QDs under various external conditions can be unraveled during in situ heating or ex situ treatments, where it lose cubic shapes and fuse to larger particles. The changes in surfaces and interfaces with missing Cs ions and PbI6 octahedrons can be semi-quantitatively studied by profile analysis and bond-length measurement in images. Finally, density functional theory calculations are performed to illustrate the properties and stabilities of the different structures that are observed. These results provide atomic-scale insights into the structural evolution of QDs, which is of great importance to modify the performance of perovskite materials and devices.

On the faulting and twinning mediated strengthening and plasticity in a γʹ strengthened CoNi-based superalloy at room temperature

Microscopic defects and their mutual interactions are fundamentally important in deciding the load carrying capacity of structural materials. In this context, the present work reports an unprecedented precipitate shearing mechanism in CoNi-based superalloys at room temperature where L12 ordered γʹ precipitates are embedded within a disordered CoCrNi-rich matrix. A combined electron channelling contrast imaging (ECCI) and scanning transmission electron microscopy (STEM) analysis revealed stacking fault (SF) and SF coupled nano-twin formation that rendered a significant work hardening rate. The observed twinning induced plasticity (TWIP) effect correlates with an excellent combination of yield strength, ultimate tensile strength (UTS) and plasticity of 910 ± 30 MPa, 1360 ± 40 MPa and ∼ 18.5 (%), respectively, as exhibited by the peak aged Co-30Ni-12Cr-7Al-4Ti-2Nb-0.006B alloy at room temperature. Rigorous center of symmetry (COS) analysis on atomic resolution STEM images from the deformed samples unambiguously establishes precipitate shearing via single a/6⟨112⟩{111} partial dislocations resulting in twin formation, succeeding the intrinsic stacking fault (ISF) and complex stacking fault (CSF) of γ and γʹ, respectively. Through thermodynamic and first principles density functional theory calculations, it is demonstrated that the experimentally observed deformation mechanism is directly correlated with the negative stacking fault energy of γ matrix and low CSF energy of the γʹ precipitates. Based on these new findings, the present work demonstrates the flexibility of tuning the composition of CoCrNi containing superalloys to widen the alloy spectrum for designing alloys with improved properties.

Real-space observation of a two-dimensional electron gas at semiconductor heterointerfaces.

Mobile charge carriers are essential components in high-performance, nano-engineered semiconductor devices. Employing charge carriers confined to heterointerfaces, the so-called two-dimensional electron gas, is essential for improving device performance. The real-space visualization of a two-dimensional electron gas at the nanometre scale is desirable. However, it is challenging to accomplish by means of electron microscopy due to an unavoidable strong diffraction contrast formation at the heterointerfaces. We performed direct, nanoscale electric field imaging across a GaN-based semiconductor heterointerface using differential phase contrast scanning transmission electron microscopy by suppressing diffraction contrasts. For both nearly the lattice-matched GaN/Al0.81In0.19N interface and pseudomorphic GaN/Al0.88In0.12N interface, the extracted quantitative electric field profiles show excellent agreement with profiles predicted using Poisson simulation. Furthermore, we used the electric field profiles to quantify the density and distribution of the two-dimensional electron gas across the heterointerfaces with nanometre precision. This study is expected to guide the real-space characterization of local charge carrier density and distribution in semiconductor devices.

Interaction between Single Metal Atoms and UiO-66 Framework Revealed by Low-Dose Imaging.

Atomically dispersed metals encapsulated in metal-organic frameworks (MOFs) have attracted extensive attention in catalysis and energy fields. Amino groups were considered conducive to the formation of single atom catalysts (SACs) due to the strong metal-linker interactions. Here, atomic details of Pt1@UiO-66 and Pd1@UiO-66-NH2 are revealed using low-dose integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). Single Pt atoms locate on the benzene ring of p-benzenedicarboxylic acid (BDC) linkers in Pt@UiO-66, while single Pd atoms are adsorbed by the amino groups in Pd@UiO-66-NH2. However, Pt@UiO-66-NH2 and Pd@UiO-66 show obvious clusters. Therefore, amino groups do not always favor the formation of SACs, and density functional theory (DFT) calculations indicate that a moderate binding strength between metals and MOFs is preferred. These results directly reveal the adsorption sites of single metal atoms in UiO-66 family, paving the way for understanding the interaction between single metal atoms and the MOFs.

Interfacial Characteristics and Optical Properties of InAs/InAsSb Type II Superlattices for the Mid‐Infrared Operation

Identifying interfacial properties and discussing optical properties of antimony‐based type II superlattices are the key factors for developing high performance of infrared optoelectronic devices. Herein, the multi‐epitaxy‐layered structure of a mid‐wavelength‐responsive infrared detector with an InAs/InAsSb superlattice as an active area layer is grown and investigated. High‐resolution X‐ray diffraction, high‐resolution transmission electron microscopy, and the geometric phase analysis indicate epitaxial layers of high crystalline quality and small lattice mismatches. The electric field results obtained by differential phase contrast scanning transmission electron microscopy further confirm the sharp interface states and good periodic structure of the as‐grown samples. The photoluminescence spectrum shows that the photoluminescence signal center is 5.4 μm at 75 K, even at 320 K, the sample still maintains a photoluminescence signal of 6.2 μm.

Optimizing experimental parameters of integrated differential phase contrast (iDPC) for atomic resolution imaging

Integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) technique has been well developed for studying atomic structures at sub-Å resolution with the capability of simultaneously imaging heavy and light atoms even at an extremely low electron dose. As a direct phase contrast imaging technique, atomic resolution iDPC-STEM is sensitive to the imaging conditions. Although great achievements have been made both in aspect of theory and experiments, the influence of experimental parameters on the contrast of atomic resolution iDPC-STEM images has not been systematically investigated. Here, we perform the iDPC-STEM simulations on the prototypical example of SrTiO3 with respect to the routine experimental factors, including the defocus, specimen thickness, accelerating voltage, convergence angle, collection angle, sample tilt and electron dose. Through the evaluation of image contrast and atom column intensity, the parameters are discussed to improve the image contrast and the visibility of light elements. Moreover, the dose-dependent simulations demonstrate the advantage of low dose iDPC-STEM imaging over other conventional STEM modes. Our results provide a practical guideline to experimentally obtain accessible atomic resolution iDPC-STEM images.

Direct Visualization of Atomic Structure in Multivariate Metal-Organic Frameworks (MOFs) for Guiding Electrocatalysts Design.

The direct utilization of metal-organic frameworks (MOFs) for electrocatalytic oxygen evolution reaction (OER) has attracted increasing interests. Herein, we employ the low-dose integrated differential phase contrast-scanning transmission electron microscopy (iDPC-STEM) technique to visualize the atomic structure of multivariate MOFs (MTV-MOFs) for guiding the structural design of bulk MOFs for efficient OER. The iDPC-STEM images revealed that incorporating Fe3+ or 2-aminoterephthalate (ATA) into Ni-BDC (BDC: benzenedicarboxylate) can introduce inhomogeneous lattice strain that weaken the coordination bonds, which can be selectively cleaved via a mild heat treatment to simultaneously generate coordinatively unsaturated metal sites, conductive Ni@C and hierarchical porous structure. Thus, excellent OER activity with current densities of 10 and 100 mA cm-2 are achieved over the defective MOFs at small overpotentials of 286 mV and 365 mV, respectively, which is superior to the commercial RuO2 catalyst and most of the bulk MOFs.

Real-Space Imaging of the Node-Linker Coordination on the Interfaces between Self-Assembled Metal-Organic Frameworks.

Surface and interface, with unique local characteristics different from bulk structure, are of great significance in various applications of metal-organic frameworks (MOFs), which should be studied by real-space imaging methods, such as electron microscopy. However, it is still challenging to atomically resolve these local structures in MOFs, because they are even more sensitive to electron irradiation. Here, we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to achieve the atomic imaging of both the metal nodes and organic linkers in UiO-66 (Zr) nanocrystals and their assembly. After adding acetic acid, we modulate the whole process of MOF assembly and observe the organic linkers at both the surfaces and twin interfaces in the chemically assembled UiO-66 (Zr) crystals by the iDPC-STEM. These results bring us a deeper understanding on the role of acid modulators that promote the MOF assembly by generating the missing-linker defects on the crystal surface.

Microscopic Segregation Dominated Nano‐Interlayer Boosts 4.5 V Cyclability and Rate Performance for Sulfide‐Based All‐Solid‐State Lithium Batteries

AbstractTo implement the growing requirement for higher energy density all‐solid‐state lithium batteries (ASSLBs), further increasing the working voltage of LiCoO2 (LCO) is a key to breaking through the bottleneck. However, LiCoO2 severe structural degradation and side reactions at the cathode interface obstruct the development of high‐voltage sulfide‐based ASSLBs (≥4.5 V). Herein, a nano‐metric Li1.175Nb0.645Ti0.4O3 (LNTO) coated LCO cathode where microscopic Ti and Nb segregation at the interface during cycling potentially stabilizes the cathode lattice, and minimizes side reactions, simultaneously, is designed. Advanced transmission electron microscopy reveals that the stable spinel phase minimizes the micro stress at the cathode interface, avoids structure fragmentation, and hence significantly enhances the long‐term cyclic stability of LNTO@LCO @ 4.5 V. Moreover, the differential phase contrast scanning transmission electron microscopy (DPC‐STEM) visualizes the nano‐interlayer LNTO to boost Li+ migration at the cathode interface. Electrochemical impedance spectroscopy (EIS) reveals that sulfide‐based cells with the LNTO nano‐layer effectively reduce the interfacial resistance to 140 Ω compared to LiNbO3 (235 Ω) over 100 cycles. Therefore, 4.5 V sulfide‐based ASSLBs offer gratifying long‐cycle stability (0.5 C for 1000 cycles, 88.6%), better specific capacity, and rate performance (179.8 mAh g–1 at 0.1 C, 97 mAh g–1 at 2 C).

Heteroatoms Induce Localization of the Electric Field and Promote a Wide Potential-Window Selectivity Towards CO in the CO2 Electroreduction.

Carbon dioxide electroreduction (CO2 RR) is a sustainable way of producing carbon-neutral fuels. Product selectivity in CO2 RR is regulated by the adsorption energy of reaction-intermediates. Here, we employ differential phase contrast-scanning transmission electron microscopy (DPC-STEM) to demonstrate that Sn heteroatoms on a Ag catalyst generate very strong and atomically localized electric fields. In situ attenuated total reflection infrared spectroscopy (ATR-IR) results verified that the localized electric field enhances the adsorption of *COOH, thus favoring the production of CO during CO2 RR. The Ag/Sn catalyst exhibits an approximately 100 % CO selectivity at a very wide range of potentials (from -0.5 to -1.1 V, versus reversible hydrogen electrode), and with a remarkably high energy efficiency (EE) of 76.1 %.

A Three-Dimensional Reconstruction Algorithm for Scanning Transmission Electron Microscopy Data from a Single Sample Orientation

Abstract Increasing interest in three-dimensional nanostructures adds impetus to electron microscopy techniques capable of imaging at or below the nanoscale in three dimensions. We present a reconstruction algorithm that takes as input a focal series of four-dimensional scanning transmission electron microscopy (4D-STEM) data. We apply the approach to a lead iridate, PbIrO, and yttrium-stabilized zirconia, YZrO, heterostructure from data acquired with the specimen in a single plan-view orientation, with the epitaxial layers stacked along the beam direction. We demonstrate that Pb–Ir atomic columns are visible in the uppermost layers of the reconstructed volume. We compare this approach to the alternative techniques of depth sectioning using differential phase contrast scanning transmission electron microscopy (DPC-STEM) and multislice ptychographic reconstruction.