Atomic Resolution Imaging Research Articles

Li-based layered nickel-tin oxide obtained through electrochemically-driven cation exchange.

The Li-based layered nickel-tin oxide Li0.35Na0.07Ni0.5Sn0.5O2 has been synthesized via electrochemically-driven Li+ for Na+ exchange in O3-NaNi0.5Sn0.5O2. The crystal structure of Li0.35Na0.07Ni0.5Sn0.5O2 was Rietveld-refined from powder X-ray diffraction data (a = 3.03431(7) Å, c = 14.7491(8) Å, S. G. R3̄m). It preserves the O3 stacking sequence of the parent compound, but with ∼13% lower unit cell volume. Electron diffraction and atomic-resolution scanning transmission electron microscopy imaging revealed short-range Ni/Sn ordering in both the pristine and Li-exchanged materials that is similar to the “honeycomb” Li/M ordering in Li2MO3 oxides. As supported by bond-valence sum and density functional theory calculations, this ordering is driven by charge difference between Ni2+ and Sn4+ and the necessity to maintain balanced bonding for the oxygen anions. Li0.35Na0.07Ni0.5Sn0.5O2 demonstrates reversible electrochemical (de)intercalation of ∼0.21 Li+ in the 2.8–4.3 V vs. Li/Li+ potential range. Limited electrochemical activity is attributed to a formation of the surface Li/Ni disordered rock-salt barrier layer as the Li+ for Na+ exchange drastically reduces the energy barrier for the Li/Ni antisite disorder.

Electron Microscopy: Liquid‐Flowing Graphene Chip‐Based High‐Resolution Electron Microscopy (Adv. Mater. 2/2021)

In article number 2005468, Jong Min Yuk and co-workers develop a novel operando imaging platform, “liquid-flowing graphene-chip transmission electron microscopy”. Featured with graphene viewing windows and anti-bulging supports, this novel method provides controllable liquid thickness and structural integrity under a high degree of pressure gradient and fast liquid circulation. These “liquid-flowing graphene chips” provide atomic-resolution imaging and molecular-level information for inorganic and biological samples, and can be applicable to in situ heating and biasing experiments.

Growth of faceted, monolayer-coated nanovoids in aluminium

Voids can cause structural and electrical failure in materials but also show promising properties for use in plasmonics and photonics. Key to understanding the mechanical, optoelectronic and thermal properties of voids is an accurate characterisation of their structure and evolution, in particular of their surfaces. Here we report the formation of voids, coated with a monolayer of tin, in two aluminium alloys. Amongst such voids, those with high aspect ratios (“tubular voids”) have been found to grow to hundreds of nanometres in length under certain heat treatment conditions. Using spectroscopy and atomic-resolution imaging in scanning transmission electron microscopy (STEM), we reveal that the voids are covered by a single-atomic-layer tin shell, which is continuous over the entire void surface and has the same atomic structure as the Al matrix. Tubular voids are invariably attached to Sn particles that have a specific orientation relationship with the Al matrix, whilst equiaxed voids are generally not attached to Sn particles exhibiting such an orientation relationship. The aspect ratios of tubular voids show a strong correlation with the coherence between the tin particle and the arrangement of the tin atoms in the void coating, along the growth directions of the tubular voids. These tubular voids could be considered as single-walled nanotubes that are embedded in the aluminium matrix and also as “anti-nanorods”. They are of great research interest because they are more likely to cause mechanical or electrical failure than equiaxed voids with the same volume. The coated voids are highly reproducible, controllable and free from contamination, and are therefore ideal for future studies of localized surface plasmon resonances (LSPRs).

Atomic-Resolution Imaging of Fast Nanoscale Dynamics with Bright Microsecond Electron Pulses.

Atomic-resolution electron microscopy is a crucial tool to elucidate the structure of matter. Recently, fast electron cameras have added the time domain to high-resolution imaging, allowing static images to be acquired as movies from which sample drift can later be removed computationally and enabling real-time observations of atomic-scale dynamics on the millisecond time scale. Even higher time resolution can be achieved with short electron pulses, yet their potential for atomic-resolution imaging remains unexplored. Here, we generate high-brightness microsecond electron pulses from a Schottky emitter whose current we briefly drive to near its limit. We demonstrate that drift-corrected imaging with such pulses can achieve atomic resolution in the presence of much larger amounts of drift than with a continuous electron beam. Moreover, such pulses enable atomic-resolution observations on the microsecond time scale, which we employ to elucidate the crystallization pathways of individual metal nanoparticles as well as the high-temperature transformation of perovskite nanocrystals.

In-plane and vertical heterostructures from 1T′/2H transition-metal dichalcogenides

Abstract An avalanche of research has been carried out on two-dimensional (2D) transition metal dichalcogenides (TMDs) due to their potential applications in advanced electronics and flexible devices. To take full use of the emerging 2D TMDs materials, their in-plane/vertical heterostructures have been explored, enabling effective tuning of their physical and chemical properties. However, structural differences between the various phases impede the formation of functional heterostructures. Therefore, robust synthesis strategies for heterostructures with different phases have been explored in this study. A chemical vapor deposition process has been proposed in which the key parameters like reaction sources and deposition sites have been carefully adjusted, trying to achieve simultaneous synthesis of 1T′/2H in-plane and vertical heterostructures. Consequently, 2D in-plane RexMo1−xS2/MoS2 and vertical ReS2/MoS2 heterostructures have been produced in different regions at the same time. Atomic-resolution Z-contrast images reveal the detailed atomic structure of the 1T′/2H interfaces. The lateral interface is found to contain Mo atoms with only 5-fold coordination with S due to the phase mismatch. This work demonstrates a route to exploit heterostructures of different phases and opens the possibility to build more complicated 2D heterostructures using CVD.

(Invited) Electrolyte and Interphase Revealed By Cryogenic Electron Microscopy and Solvation Measurement

Obtaining atomic and molecular level understanding of electrolyte and electrode-electrolyte interphase is critical to the battery developments. In this talk, I will present a multimodal approach combining the powerful cryogenic electron microscopy technique, electrochemical technique and solvation entropy and free energy measurements. I will discover including: 1) atom-resolved images of solid electrolyte interphase for Li metal and Si anodes; 2) Insight on the debate on the existence of cathode electrolyte interphase; 3) the solvation structure and related thermodynamic parameters and their correlation with electrochemical performance.

Cathode-Electrolyte Interphase in Lithium Batteries Revealed by Cryogenic Electron Microscopy

Summary Cathode electrolyte interphase (CEI), the intimate coating layer formed on the positive electrode, has been thought to be critical. However, many aspects of CEI remain unclear. This originates from the lack of effective tools to characterize structural and chemical properties of these sensitive interphases at nanoscale. Here, we develop a protocol to preserve the native state and directly visualize the interface on the positive electrode using cryogenic electron microscopy. We find that under normal operation conditions, there does not exist an intimate coating layer at the single-particle level in carbonate-based electrolyte. However, upon brief external electrical shorting, a solid-electrolyte interphase, which usually forms on anodes, could form on cathodes and be electrochemically converted into a stable, conformal CEI in situ. The conformal CEI helps improve Coulombic efficiency and overall capacity retention of the battery. This generates a different perspective of CEI in commercial carbonate-based electrolytes than previously understood.

Interface-induced ferromagnetism in μ-Fe2O3/β-Ga2O3 superlattices

Superlattices of antiferromagnetic μ-Fe2O3 and diamagnetic β-Ga2O3 are grown by plasma-assisted molecular beam epitaxy on (010) oriented β-Ga2O3 substrates in which ferromagnetism emerges above room temperature. To investigate the suspected interface origin of the ferromagnetic phase, identical superlattice structures are grown at various substrate temperatures and beam fluxes. Atomic-resolution scanning transmission electron microscopy images confirm the registry of μ-Fe2O3 to the β-Ga2O3 layers in these superlattices. Atomic force microscopy and high-resolution x-ray diffraction are used to examine the growth morphology and characterize the superlattice interface roughness. The saturation magnetization of the ferromagnetic phase is observed to increase strongly with the interface roughness. Conversely, smoother superlattices exhibit a weaker ferromagnetic response and a higher density of paramagnetic moments along with evidence of superparamagnetic clusters. These findings are consistent with the interface origin for the ferromagnetic response in these superlattices. The demonstration of an interface magnetic phase in nearly lattice-matched monoclinic Fe2O3/Ga2O3 opens the door to ultrawide bandgap heterostructure-engineered magnetoelectronic devices, where ferromagnetic switching of the interface phase can be incorporated into high-field devices.

Probing One-Dimensional Oxygen Vacancy Channels Driven by Cation-Anion Double Ordering in Perovskites.

Visualizing the oxygen vacancy distributions is highly desirable for understanding the atomistic oxygen diffusion mechanisms in perovskites. In particular, the direct observation of the one-dimensional oxygen vacancy channels has not yet been achieved in perovskites with dual ion (i.e., cation and anion) ordering. Here, we perform atomic-resolution imaging of the one-dimensional oxygen vacancy channels and their structural dynamics in a NdBaCo2O5.5 double perovskite oxide. An in situ heating transmission electron microscopy investigation reveals the disordering of oxygen vacancy channels by local rearrangement of oxygen vacancies at the specific temperature. A density functional theory calculation suggests that the possible pathway of oxygen vacancy migration is a multistep route via Co-O and Nd-Ov (oxygen vacancy) sites. These findings could provide robust guidance for understanding the static and dynamic behaviors of oxygen vacancies in perovskite oxides.

Unravelling the room-temperature atomic structure and growth kinetics of lithium metal

Alkali metals are widely studied in various fields such as medicine and battery. However, limited by the chemical reactivity and electron/ion beam sensitivity, the intrinsic atomic structure of alkali metals and its fundamental properties are difficult to be revealed. Here, a simple and versatile method is proposed to form the alkali metals in situ inside the transmission electron microscope. Taking alkali salts as the starting materials and electron beam as the trigger, alkali metals can be obtained directly. With this method, atomic resolution imaging of lithium and sodium metal is achieved at room temperature, and the growth of alkali metals is visualized at atomic-scale with millisecond temporal resolution. Furthermore, our observations unravel the ambiguities in lithium metal growth on garnet-type solid electrolytes for lithium-metal batteries. Finally, our method enables a direct study of physical contact property of lithium metal as well as its surface passivation oxide layer, which may contribute to better understanding of lithium dendrite and solid electrolyte interphase issues in lithium ion batteries.

A symmetry-derived mechanism for atomic resolution imaging

We introduce an image-contrast mechanism for scanning transmission electron microscopy (STEM) that derives from the local symmetry within the specimen. For a given position of the electron probe on the specimen, the image intensity is determined by the degree of similarity between the exit electron-intensity distribution and a chosen symmetry operation applied to that distribution. The contrast mechanism detects both light and heavy atomic columns and is robust with respect to specimen thickness, electron-probe energy, and defocus. Atomic columns appear as sharp peaks that can be significantly narrower than for STEM images using conventional disk and annular detectors. This fundamentally different contrast mechanism complements conventional imaging modes and can be acquired simultaneously with them, expanding the power of STEM for materials characterization.

Point defects and interstitial climb of 90° partial dislocations in brown type IIa natural diamond

Multiple electron microscopy techniques have been used to study a brown type IIa natural diamond. Electron backscatter diffraction shows evidence of plastic deformation in the form of slip bands, while cathodoluminescence reveals a network of low-angle grain boundaries, also observed in transmission electron microscopy together with long straight dislocations and dislocation dipoles. Aberration-corrected scanning transmission electron microscopy shows interstitial absorption on the 90° partial of both dissociated dislocations and Z-type faulted vacancy dipoles, forming structures similar to that observed in other fcc materials. The observations indicate an interstitial concentration of 1017 to 1019 cm−3 and calculations of point defect concentrations produced by plastic deformation show that this can be produced by strains of the order of 1%. Brown coloration in diamond has been previously attributed to vacancies and vacancy clusters with concentrations around 1018 cm−3, which suggests that roughly equal numbers of interstitials and vacancies are generated in diamond via plastic deformation. Atomic resolution images of Z-type faulted dipoles allowed a stacking fault energy of 472 ± 38 mJ m−2 to be determined.

A hidden precipitation scenario of the θ′-phase in Al-Cu alloys

Al-Cu binary alloys are important and interesting industry materials. Up to date, the formation mechanisms of the key strengthening precipitates, named θ′-phase, in the alloys are still controversial. Here, we report that for non-deformed bulk Al-Cu alloys the θ′-phase actually has its own direct precursors that can form only at elevated aging temperature (> ca. 200 °C). These high-temperature precursors have the same plate-like morphology as the θ′-phase precipitates but rather different structures. Atomic-resolution imaging reveals that they have a tetragonal structure with a = 0.405 nm and c = 1.213 nm, and an average composition of Al5-xCu1+x (0 ≤ x <1), being fully coherent with the Al-lattice. This precursor phase may initiate with a composition of Al5Cu and evolve locally towards Al4Cu2 in composition, eventually leading to a consequent structural transformation into the θ′-phase (Al4Cu2=Al2Cu). There are evidences that because of their genetic links in structure, such a high-temperature precursor may transform to the θ′-phase without having to change their morphology and interface structure. Our study reveals a well-defined and previously hidden precipitation scenario for the θ′-phase to form in Al-Cu alloys at an elevated aging temperature.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Summary Grain boundaries (GBs) determine the properties of polycrystals, and tailoring the GB structure offers a promising method for the discovery and engineering of new materials. However, GB structures are far from well understood because of their structural complexity and limitations of conventional projection imaging methods. Here, we decipher three-dimensional atomic structure and crystallography of GBs in nanometals using atomic-resolution electron tomography. Unlike conventional descriptions, whereby they are either straight or curved planar planes with one-dimensional translational symmetry, we show that the high-angle GBs completely lose translational symmetry due to undulated curvature related to configurations of structural units. Moreover, we directly visualize kinks and jogs at the single-atom scale in dislocation-type GBs and investigate their mobilities. Our findings bring new insights to the conventional wisdom of GBs and show the importance of developing methods to include the non-planar nature of GBs to statistically evaluate the behavior of GBs in modeling studies.

Severe Dirac Mass Gap Suppression in Sb2Te3-Based Quantum Anomalous Hall Materials.

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr0.08(Bi0.1Sb0.9)1.92Te3 to that of its nonmagnetic parent (Bi0.1Sb0.9)2Te3, to explore the cause. In (Bi0.1Sb0.9)2Te3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr0.08(Bi0.1Sb0.9)1.92Te3 with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb2Te3-based FMTI materials to very low temperatures.

HAADF-STEM block-scanning strategy for local measurement of strain at the nanoscale

Lattice strain measurement of nanoscale semiconductor devices is crucial for the semiconductor industry as strain substantially improves the electrical performance of transistors. High resolution scanning transmission electron microscopy (HR-STEM) imaging is an excellent tool that provides spatial resolution at the atomic scale and strain information by applying Geometric Phase Analysis or image fitting procedures. However, HR-STEM images regularly suffer from scanning distortions and sample drift during image acquisition. In this paper, we propose a new scanning strategy that drastically reduces artefacts due to drift and scanning distortion, along with extending the field of view. It consists of the acquisition of a series of independent small subimages containing an atomic resolution image of the local lattice. All subimages are then analysed individually for strain by fitting a nonlinear model to the lattice images. The method allows flexible tuning of spatial resolution and the field of view within the limits of the dynamic range of the scan engine while maintaining atomic resolution sampling within the subimages. The obtained experimental strain maps are quantitatively benchmarked against the Bessel diffraction technique. We demonstrate that the proposed scanning strategy approaches the performance of the diffraction technique while having the advantage that it does not require specialized diffraction cameras.

Improvements in fundamental performance of in-liquid frequency modulation atomic force microscopy.

In-liquid frequency modulation atomic force microscopy (FM-AFM) has been used for visualizing subnanometer-scale surface structures of minerals, organic thin films and biological systems. In addition, three-dimensional atomic force microscopy (3D-AFM) has been developed by combining it with a three-dimensional (3D) tip scanning method. This method enabled the visualization of 3D distributions of water (i.e. hydration structures) and flexible molecular chains at subnanometer-scale resolution. While these applications highlighted the unique capabilities of FM-AFM, its force resolution, speed and stability are not necessarily at a satisfactory level for practical applications. Recently, there have been significant advancements in these fundamental performances. The force resolution was dramatically improved by using a small cantilever, which enabled the imaging of a 3D hydration structure even in pure water and made it possible to directly compare experimental results with simulated ones. In addition, the improved force resolution allowed the enhancement of imaging speed without compromising spatial resolution. To achieve this goal, efforts have been made for improving bandwidth, resonance frequency and/or latency of various components, including a high-speed phase-locked loop (PLL) circuit. With these improvements, now atomic-resolution in-liquid FM-AFM imaging can be performed at ∼1 s/frame. Furthermore, a Si-coating method was found to improve stability and reproducibility of atomic-resolution imaging owing to formation of a stable hydration structure on a tip apex. These improvements have opened up new possibilities of atomic-scale studies on solid-liquid interfacial phenomena by in-liquid FM-AFM.

Atomic Resolution Imaging of CrBr3 Using Adhesion-Enhanced Grids.

Suspended specimens of 2D crystals and their heterostructures are required for a range of studies including transmission electron microscopy (TEM), optical transmission experiments, and nanomechanical testing. However, investigating the properties of laterally small 2D crystal specimens, including twisted bilayers and air-sensitive materials, has been held back by the difficulty of fabricating the necessary clean suspended samples. Here we present a scalable solution that allows clean free-standing specimens to be realized with 100% yield by dry-stamping atomically thin 2D stacks onto a specially developed adhesion-enhanced support grid. Using this new capability, we demonstrate atomic resolution imaging of defect structures in atomically thin CrBr3, a novel magnetic material that degrades in ambient conditions.

The effect of low-angle off-axis GaN substrate orientation on the surface morphology of Mg-doped GaN epilayers

The effect of low-angle off-axis GaN substrate orientation on the surface morphology of Mg-doped GaN epilayers has been studied using atomic force microscopy (AFM) and transmission electron microscopy. Undoped- and magnesium-doped GaN layers were grown on (0001) GaN surfaces tilted by 0.3°, 2°, and 4° toward a ⟨11¯00⟩ direction. AFM images show the presence of pinholes associated with threading screw dislocations originating from the substrate. Mg doping causes enhanced step-flow growth with well-defined periodic steps and a tendency to cover the pinholes. In regions far from defects, atomic-resolution imaging shows the coexistence of surface regions with different atomic step densities, i.e., with slightly different inclination, that depend on the substrate tilt angle. For low tilt (0.3°), the steps involve a single basal plane (with a height = c/2). At higher tilt, the steps involve two basal planes with a tendency toward step bunching. Cathodoluminescence spectroscopic imaging has been used to correlate the electronic properties with the thin film surface morphology, showing that step bunching reduces p-type doping efficiency.

Toward Controlled Thermal Energy Storage and Release in Organic Phase Change Materials

Download : Download high-res image (105KB) Download : Download full-size image Grace Han was born in South Korea and graduated from POSTECH with a BS in Chemistry. She obtained her PhD in Chemistry at MIT in 2015 and joined the Department of Materials Science and Engineering at MIT as a postdoctoral associate. Grace started her independent career as an assistant professor at the Department of Chemistry at Brandeis University in 2018. Her research interests span energy conversion and storage, molecular switch chemistry, phase transition of materials, and atomic-resolution molecular imaging. Download : Download high-res image (79KB) Download : Download full-size image Mihael Gerkman received his BS in Chemistry from SUNY Potsdam in 2016. Currently, he is pursuing his PhD at Brandies University in the Han group. His research is focused on the synthesis of novel molecular switches and the study of their reversible isomerization dynamics in condensed phases for thermal energy storage applications.