Phase Contrast Scanning Transmission Electron 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.

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.

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.

Polarization Switching and Correlated Phase Transitions in Fluorite-Structure ZrO2 Nanocrystals.

Unconventional ferroelectricity in fluorite-structure oxides enables tremendous opportunities in nanoelectronics owing to their superior scalability and silicon compatibility. However, their polarization order and switching process remain elusive due to the challenges of visualizing oxygen ions in nanocrystalline films. In this work, the oxygen shifting during polarization switching and correlated polar-nonpolar phase transitions are directly captured among multiple metastable phases in freestanding ZrO2 thin films by low-dose integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM). Bidirectional transitions between antiferroelectric and ferroelectric orders and interfacial polarization relaxation are clarified at unit-cell scale. Meanwhile, polarization switching is strongly correlated with Zr-O displacement in reversible martensitic transformation between monoclinic and orthorhombic phases and two-step tetrahedral-to-orthorhombic phase transition. These findings provide atomic insights into the transition pathways between metastable polymorphs and unravel the evolution of polarization orders in (anti)ferroelectric fluorite oxides.

Microscopic investigation of crack and strain of LiCoO2 cathode cycled under high voltage

The structural stability of LiCoO2 at high voltages is especially important when enhancing the cutoff voltage used as an effective method to boost the energy density. we trace the origin of different crack formation mechanisms during fast and slow charging rates using a combination of qualitative atomic-scale differential phase-contrast scanning transmission electron microscope (STEM) images and electron energy loss spectroscopy (EELS) analysis of LiCoO2 (LCO). We demonstrate that fast charging induces large heterogenous Li flux insertion into the cathode, resulting in large strain and big crack formation; in contrast, low cycling rates trigger phase transformations that only induce micro-cracks. To clarify the mechanisms, we mapped out local strain analysis for the LCO particles and detailed phase transition routes. The comprehensive characterization results are meaningful for researchers to understand the failure mechanisms under high voltage and inspire innovative solutions to capture higher capacity out of LCO.

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.

Dislocation-twin interaction in medium entropy alloy containing a high density of oxygen interstitials

Dislocation-twin interactions play important roles in tuning the plastic deformation of materials. Here, we report the great influence of interstitial oxygen atoms on dislocation-twin interactions in NiCoCrO medium entropy alloy through in-situ transmission electron microscopy mechanical testing, atomic resolution energy-dispersive X-ray spectroscopy characterizations, and integrated differential phase-contrast scanning transmission electron microscopy imaging. The spreading lattice distortion in medium-entropy alloy was found to enable large oxygen interstitial solubility. Oxygen atoms occupied both the tetrahedral and octahedral interstitial sites, and tended to be enriched at defects such as twin boundaries and stacking faults. Oxygen-enriched regions near the twin boundary hindered dislocations from direct transmitting or cross-slip into another twin plane during plastic deformation, resulting in significant dislocation entanglement, partial dissociation of dislocations, and the deviation of slip directions. Our results give insight into the strengthening mechanisms associated with the synergetic effects of twin boundary and interstitial atoms.

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.

First direct observation of the built-in electric field and oxygen vacancy migration in ferroelectric Hf0.5Zr0.5O2 film during electrical cycling.

The wake-up and fatigue effects exhibited by ferroelectric hafnium oxide (HfO2) during electrical cycling are two of the most significant obstacles limiting its development and application. Despite a mainstream theory relating these phenomena to the migration of oxygen vacancies and the evolution of the built-in field, no supportive experimental observations from a nanoscale perspective have been reported so far. By combining differential phase contrast scanning transmission electron microscopy (DPC-STEM) and energy dispersive spectroscopy (EDS) analysis, we directly observe the migration of oxygen vacancies and the evolution of the built-in field in ferroelectric HfO2 for the first time. These solid results indicate that the wake-up effect is caused by the homogenization of oxygen vacancy distribution and weakening of the vertical built-in field whereas the fatigue effect is related to charge injection and transverse local electric field enhancement. In addition, using a low-amplitude electrical cycling scheme, we exclude field-induced phase transition from the root cause of the wake-up and fatigue in Hf0.5Zr0.5O2. With direct experimental evidence, this work clarifies the core mechanism of the wake-up and fatigue effects, which is important for the optimization of ferroelectric memory devices.

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 %.