Atomic-resolution Scanning Transmission Electron Research Articles

Thermally Induced Irreversible Disorder in Interlayer Stacking of γ-GeSe.

The interlayer stacking shift in van der Waals (vdW) crystals represents an important degree of freedom to control various material properties, including magnetism, ferroelectricity, and electrical properties. On the other hand, the structural phase transitions driven by interlayer sliding in vdW crystals often exhibit thickness-dependent, sample-specific behaviors with significant hysteresis, complicating a clear understanding of their intrinsic nature. Here, the stacking configuration of the recently identified vdW crystal, γ-GeSe, is investigated, and the disordering manipulation of stacking sequence is demonstrated. It is observed that the well-ordered AB' stacking configuration in as-synthesized samples undergoes irreversible disordering upon Joule heating via electrical biasing or thermal treatment, as confirmed by atomic resolution scanning transmission electron microscopy (STEM). Statistical analysis of STEM data reveal the emergence of stacking disorder, with samples subjected to high electrical bias reaching the maximum levels of disorder. The energies of various stacking configurations and energy barriers for interlayer sliding are examined using first-principles calculation and a parameterized model to elucidate the key structural parameters related to stacking shift. The susceptibility of interlayer stacking to disorder through electrical or thermal treatments should be carefully considered to fully comprehend the structural and electrical properties of vdW crystals.

Atomic resolution scanning transmission electron microscopy at liquid helium temperatures for quantum materials

Fundamental quantum phenomena in condensed matter, ranging from correlated electron systems to quantum information processors, manifest their emergent characteristics and behaviors predominantly at low temperatures. This necessitates the use of liquid helium (LHe) cooling for experimental observation. Atomic resolution scanning transmission electron microscopy combined with LHe cooling (cryo-STEM) provides a powerful characterization technique to probe local atomic structural modulations and their coupling with charge, spin and orbital degrees-of-freedom in quantum materials. However, achieving atomic resolution in cryo-STEM is exceptionally challenging, primarily due to sample drifts arising from temperature changes and noises associated with LHe bubbling, turbulent gas flow, etc. In this work, we demonstrate atomic resolution cryo-STEM imaging at LHe temperatures using a commercial side-entry LHe cooling holder. Firstly, we examine STEM imaging performance as a function of He gas flow rate, identifying two primary noise sources: He-gas pulsing and He-gas bubbling. Secondly, we propose two strategies to achieve low noise conditions for atomic resolution STEM imaging: either by temporarily suppressing He gas flow rate using the needle valve or by acquiring images during the natural warming process. Lastly, we show the applications of image acquisition methods and image processing techniques in investigating structural phase transitions in Cr2Ge2Te6, CuIr2S4, and CrCl3. Our findings represent an advance in the field of atomic resolution electron microscopy imaging for quantum materials and devices at LHe temperatures, which can be applied to other commercial side-entry LHe cooling TEM holders.

Analysis of Defect Structures during the Early-Stages of PVT Growth of 4H-SiC Crystals

To better understand the effects of various growth parameters during the early-stages of PVT growth of 4H-SiC on resulting defect structures, multiple short duration growths have been carried out under varying conditions of seed quality, nucleation rate, thermal gradients, and N incorporation. Besides the replication of TSDs/TMDs and TEDs as well as the deflection of TSDs/TMDs into Frank dislocations, synchrotron monochromatic beam x-ray topography (SMBXT) studies also reveal the formation of stacking faults bounded by Frank dislocations. Using ray tracing simulations to characterize the Frank dislocations, three types of stacking faults are revealed: Type 1 stacking fault resulting from 2D nucleation of 6H polytype on terraces; Type 2 stacking fault resulting from macrostep overgrowth of the surface growth spiral steps of TSDs/TMDs which separate into c/2 or c/4 increments; Type 3 stacking fault resulting from vicinal step overgrowth of surface growth spiral steps of TSDs/TMDs which separate into c/4 and 3c/4 increments. Analysis of atomic resolution scanning transmission electron microscopy (STEM) images reveals the mechanism of the Type 3 fault.

Tuning Van Der Waals Heterostructure of WS2/SrTiO3 Via in Situ Transmission Electron Microscopy

Monolayer two-dimensional (2D) materials have been widely researched and discussed in recent years, while multi-layers have received comparatively less attention. Nowadays, combining different materials to create heterostructures has become mainstream. In semiconductor industry, heterostructure can form devices such as lasers, light-emitting diodes, fast transistors and so on. Heterostructures can create well electronic properties by combining two different materials together. Therefore, heterostructures become a great tool to manipulate the electronic structures. In traditional heterostructures, the interfaces between two different materials are formed by covalent bonds, hence it requires similar lattice constants and structures. On the other hand, novel van der Waal heterostructures (vdW-heterostructure), formed by 2D materials, do not require the same strict similarity as traditional ones. VdW-heterostructures, which create 3D structures through van der Waals forces, provide a new perspective. In this study, we created a vdW-heterostructures by stacking tungsten disulfide (WS2) with single-crystal strontium titanate, SrTiO3 (STO), shown in Figure 1a,b. The crystalline atomic-layer WS2 and STO were synthesized by chemical vapor deposition (CVD) and pulsed laser deposition (PLD), respectively. They are analyzed by high-resolution transmission electron microscope (HRTEM) shown in Figure 1c-h. Once the vdW-heterostructures are formed, we conducted in situ heating by TEM. Through the advanced in situ technique, we observed the change of the twisted stacking angle by heating the vdW-heterostructure. Ultimately, the lattices will shift and rotate, minimizing the free energy, so that the vdW-heterostructure will be aligned and formed epitaxially. In addition, we utilized atomic resolution scanning transmission electron microscope (STEM) to identify the stacking images and the moiré pattern of the vdW-heterostructure. During the rotation, we can observed different moiré patterns by various twisted stacking angles. These different moiré patterns can produce different electrical properties, which can be applied to advanced semiconductor devices. Keywords: high-resolution TEM/STEM, in situ heating, 2D materials, van der Waal heterostructures, moiré pattern Figure 1. The vdW-heterostructure of STO/WS2. a) The diagram showing that the formation process of WS2/STO vdW-heterostructures which have different twisted stacking angles. b) The low-magnification SEM image showing that the single crystal STO thin film is transferred onto the TEM heating chips. The circles in the middle are the observation windows for transmission electrons. c-e) HRTEM images. f-g) Diffraction patterns of HRTEM. c,f) Single crystal STO thin film with d-spacing (1−10) = 0.284 nm. d,g) Monolayer single crystal WS2 with d-spacing (−110) = 0.274 nm. e,h) WS2(red)/STO(yellow) vdW-heterostructure with stacking angle 3.6°. Figure 1

Field and Structure Imaging by Magnetic-field-free Atomic Resolution Scanning Transmission Electron Microscopy

Field and Structure Imaging by Magnetic-field-free Atomic Resolution Scanning Transmission Electron Microscopy

Large-Scale Direct Growth of Monolayer MoS2 on Patterned Graphene for van der Waals Ultrafast Photoactive Circuits.

Two-dimensional (2D) van der Waals heterostructures combine the distinct properties of individual 2D materials, resulting in metamaterials, ideal for emergent electronic, optoelectronic, and spintronic phenomena. A significant challenge in harnessing these properties for future hybrid circuits is their large-scale realization and integration into graphene interconnects. In this work, we demonstrate the direct growth of molybdenum disulfide (MoS2) crystals on patterned graphene channels. By enhancing control over vapor transport through a confined space chemical vapor deposition growth technique, we achieve the preferential deposition of monolayer MoS2 crystals on monolayer graphene. Atomic resolution scanning transmission electron microscopy reveals the high structural integrity of the heterostructures. Through in-depth spectroscopic characterization, we unveil charge transfer in Graphene/MoS2, with MoS2 introducing p-type doping to graphene, as confirmed by our electrical measurements. Photoconductivity characterization shows that photoactive regions can be locally created in graphene channels covered by MoS2 layers. Time-resolved ultrafast transient absorption (TA) spectroscopy reveals accelerated charge decay kinetics in Graphene/MoS2 heterostructures compared to standalone MoS2 and upconversion for below band gap excitation conditions. Our proof-of-concept results pave the way for the direct growth of van der Waals heterostructure circuits with significant implications for ultrafast photoactive nanoelectronics and optospintronic applications.

Atomic resolution transmission electron microscopy visualisation of channel occupancy in beryl in different crystallographic directions

The causes of colour in beryl have been a research topic for decades. For some varieties, such as emerald (green, coloured by Cr3+ and/or V3+), the main cause of colour is substitutions by metal atoms within the framework. However, the causes for the yellow and blue colours in heliodor, golden beryl and aquamarine are still debated. It is generally agreed that Fe ions are responsible for the colour, but there are differing conclusions about the valence states of these ions, the occupied positions and the colour-inducing processes involved. The colour of aquamarine is commonly attributed to intervalence charge transfer (IVCT) between Fe3+ and Fe2+. Various combinations of sites have been proposed to host the Fe ions engaging in this IVCT. Here we present a new approach to address the topic of colour generation: atomic resolution scanning transmission electron microscopy (STEM). For the first time, atomic resolution images of a beryl (natural aquamarine) are presented in the three crystallographic directions [0001], [1-210] and [1-100]. Ions are clearly resolved in the channels. From the ratio of channel occupation and the correlation of the atoms per formula unit (apfu) calculations we conclude that Fe resides in the framework, not in the channels. The projections in the [1-210] direction directly show that the cavity channel site 2a is occupied, most likely by Cs, in agreement with recent results in the literature.

Pt single-atom electrocatalysts at Cu2O nanowires for boosting electrochemical sensing toward glucose

In electrochemical biosensors, rational design and synthesis of high-performance electrochemical glucose sensors based on emerging single-atom catalysts (SACs) are paramount. Herein, a facile approach is proposed for dispersing single-atom doping of cuprous oxide (Cu2O) nanowires with Pt on a copper foam substrate (Pt1/Cu2O@CF) via the electrochemical deposition process. The specific nanostructure of the single-atom catalyst has been elaborately revealed with the aid of atomic resolution scanning transmission electron microscopy (STEM) and X-ray absorption fine structure spectroscopy (XAS). The as-fabricated Pt1/Cu2O@CF biosensor with satisfactory scalability exhibits a low limit of detection (1 μM), ultrahigh sensitivity (31.55 mA mM−1 cm−2), excellent selectivity, and robust reliability toward glucose. The first principles simulations reveal that Pt SAC is beneficial to the adsorption of glucose on the surface and further facilitates the electron transfer for the deprotonation process, resulting in high glucose sensing performance. This work sheds light on the applications of SACs for designing ultrasensitive electrochemical biosensors.

Application of the polyhedral template matching method for characterization of 2D atomic resolution electron microscopy images

High-throughput image segmentation of atomic resolution electron microscopy data poses an ongoing challenge for materials characterization. In this paper, we investigate the application of the polyhedral template matching (PTM) method, a technique widely employed for visualizing three-dimensional (3D) atomistic simulations, to the analysis of two-dimensional (2D) atomic resolution electron microscopy images. This technique is complementary with other atomic resolution data reduction techniques, such as the centrosymmetry parameter, that use the measured atomic peak positions as the starting input. Furthermore, since the template matching process also gives a measure of the local rotation, the method can be used to segment images based on local orientation. We begin by presenting a 2D implementation of the PTM method, suitable for atomic resolution images. We then demonstrate the technique's application to atomic resolution scanning transmission electron microscopy images from close-packed metals, providing examples of the analysis of twins and other grain boundaries in FCC gold and martensite phases in 304 L austenitic stainless steel. Finally, we discuss factors, such as positional errors in the image peak locations, that can affect the accuracy and sensitivity of the structural determinations.

Crystal structures of two new high-pressure oxynitrides with composition SnGe4N4O4, from single-crystal electron diffraction.

SnGe4N4O4 was synthesized at high pressure (16 and 20 GPa) and high temperature (1200 and 1500°C) in a large-volume press. Powder X-ray diffraction experiments using synchrotron radiation indicate that the derived samples are mixtures of known and unknown phases. However, the powder X-ray diffraction patterns are not sufficient for structural characterization. Transmission electron microscopy studies reveal crystals of several hundreds of nanometres in size with different chemical composition. Among them, crystals of a previously unknown phase with stoichiometry SnGe4N4O4 were detected and investigated using automated diffraction tomography (ADT), a three-dimensional electron diffraction method. Via ADT, the crystal structure could be determined from single nanocrystals in space group P63mc, exhibiting a nolanite-type structure. This was confirmed by density functional theory calculations and atomic resolution scanning transmission electron microscopy images. In one of the syntheses runs a rhombohedral 6R polytype of SnGe4N4O4 could be found together with the nolanite-type SnGe4N4O4. The structure of this polymorph was solved as well using ADT.

Oxidized structure and Compositional properties of 1144 phase FBS by analytical electron microscopy

The 1144 phase (Ae1A1Fe4As4) shows a strong advantage of engineering fabrication among Fe (Iron)-based superconductor (FBS) family due to the robustness of its superconducting properties with respect to chemical inhomogeneities, granted by its uniform crystalline-layered structure. This regularity is furthermore associated to crystalline defects capable of acting as efficient pinning centers, from which high critical currents can achieved at high fields. Like other FBS phases, its lossless current-carrying capability can be remarkably degraded by distractions at grain boundaries (GBs). GB oxidation is an issue of upmost importance to the realization of the practical FBS application for high field (> 20T) magnet. In this study, we explore oxidized grain boundary and intrinsic grain structural properties of 1144 polycrystalline samples by applying analytical electron microscopy such as atomic resolution scanning transmission electron microscopy and atom probe tomography. These structural properties of samples produced by a mechanochemically assisted synthesis are evaluated following the degradation of superconducting properties due to oxidation. We observe a strong correlation between the contamination at grain boundaries and the decrease of transport properties of the bulk sample, while the crystallin structure seems to be not affected by the oxidation.

Cr5.7Si2.3P8N24-A Chromium(+IV) Nitridosilicate Phosphate with Amphibole-Type Structure.

The first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr5.7Si2.3P8N24 was synthesized under high-pressure high-temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr2N, Si3N4 and P3N5, using NH4N3 and NH4F as additional nitrogen source and mineralizing agent, respectively. The crystal structure was elucidated by single-crystal X-ray diffraction with microfocused synchrotron radiation (C2/m, a=9.6002(19), b=17.107(3), c=4.8530(10) Å, β=109.65(3)°). The elemental composition was analyzed by energy dispersive X-ray spectroscopy. The structure consists of vertex-sharing PN4-tetrahedra forming zweier double chains and edge-sharing (Si,Cr)-centered octahedra forming separated ribbons. Atomic resolution scanning transmission electron microscopy shows ordered Si and Cr sites next to a disordered Si/Cr site. Optical spectroscopy indicates a band gap of 2.1 eV. Susceptibility measurements show paramagnetic behavior and support the oxidation state Cr+IV, which is confirmed by EPR. The comprehensive analysis expands the field of Cr-N chemistry and provides access to a nitride analog of one of the most prevalent silicate structures.

Cr5.7Si2.3P8N24—A Chromium(+IV) Nitridosilicate Phosphate with Amphibole‐Type Structure

AbstractThe first nitridic analog of an amphibole mineral, the quaternary nitridosilicate phosphate Cr5.7Si2.3P8N24 was synthesized under high‐pressure high‐temperature conditions at 1400 °C and 12 GPa from the binary nitrides Cr2N, Si3N4 and P3N5, using NH4N3 and NH4F as additional nitrogen source and mineralizing agent, respectively. The crystal structure was elucidated by single‐crystal X‐ray diffraction with microfocused synchrotron radiation (C2/m, a=9.6002(19), b=17.107(3), c=4.8530(10) Å, β=109.65(3)°). The elemental composition was analyzed by energy dispersive X‐ray spectroscopy. The structure consists of vertex‐sharing PN4‐tetrahedra forming zweier double chains and edge‐sharing (Si,Cr)‐centered octahedra forming separated ribbons. Atomic resolution scanning transmission electron microscopy shows ordered Si and Cr sites next to a disordered Si/Cr site. Optical spectroscopy indicates a band gap of 2.1 eV. Susceptibility measurements show paramagnetic behavior and support the oxidation state Cr+IV, which is confirmed by EPR. The comprehensive analysis expands the field of Cr−N chemistry and provides access to a nitride analog of one of the most prevalent silicate structures.

Tuning the atomic and electronic structures of mirror twin boundaries in molecular beam epitaxy grown MoSe2 monolayers via rhenium doping

Interplay between defects like mirror twin boundaries (MTBs) and dopants may provide additional opportunities for furthering the research on two-dimensional monolayer (ML) transition metal dichalcogenides. In this work, we successfully dope rhenium (Re) into molecular beam epitaxy grown ML MoSe2 and confirm the formation of a new type of MTBs, named 4|4E-M (M represents metal, Mo/Re) according to the configuration. Data from statistic atomic resolution scanning transmission electron microscopy also reveals a preferable MTB enrichment of Re dopants, rather than intra-domain. In conjunction with density functional theory calculation results, we propose the possible routes for Re doping induced formation of 4|4E-M MTBs. Electronic structures of Re doped MTBs in ML MoSe2 are also predicted theoretically and then preliminarily tested by scanning tunneling microscopy and spectroscopy.

Evidence of Xe-incorporation in the bubble superlattice in irradiated U-Mo fuel

For years, researchers have reported competing ideas on the formation of fission gas bubble superlattices in metallic fuels, which was postulated to be comprised of elemental xenon (Xe) atoms. However, the chemical and physical arrangement of these elements within this gas bubble superlattice had not been verified. In this contribution, Xe was chemically profiled using atomic resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT) techniques in irradiated uranium-molybdenum (U-Mo) fuels. These complementary techniques provide conclusive evidence of Xe presence in the bubble superlattice up to fission densities of 4.5 × 1021 fissions/cm3 and give a quantitative assessment of Xe content within the bubble superlattice. Based on the results of simultaneous imaging and spectroscopy using STEM, the chemical composition of the Xe bubble superlattice was measured and found to be up to 8±1.3 atomic %. APT data complemented STEM findings on Xe distribution in irradiated U-Mo fuel sample. This study provides conclusive evidence that Xe is not only present within an atomically arranged bubble superlattice but provides fundamental insight into the physical state of Xe in low enriched U-Mo monolithic fuel.

Coercivity modulation of FeCoCrMoTi films by artificial magnetic phase defects engineering based on multilayer structure

FeCoCr-based films have been considered as promising candidates for next-generation magnetic code disk materials used in high-accuracy magnetic rotary encoders. However, realizing high coercivity with sufficient remanent magnetization in FeCoCr-based films with in-plane anisotropy to ensure suitable disturbance-rejection performance and detectability of magnetic pole pairs is a critical issue, which hinders their practical applications. In this study, we demonstrated an in-plane anisotropic [Ta(20 nm)/FeCoCrMoTi(100 nm)]2/Ta(10 nm) multilayer film (M20 sample) with a high coercivity of 945 Oe and a sufficient remanent magnetization of 2890 Oe, which is 92.1% and 63.1% higher than FeCoCrMoTi(200 nm)/Ta(10 nm) film (SL sample), respectively. Detailed microstructure characterization using atomic resolution scanning transmission electron microscopy (STEM) indicates that the high coercivity originates from the domain-wall pinning effect induced by three factors including grain refinement and heterogeneous phases generation and composition fluctuation of modulated phases. X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) analyses reveal that the high remanent magnetization is ascribed to the higher relative content of Fe2Ta phases with strong ferromagnetism than that of Co2Ta phases with weak ferromagnetism.

Engineering fine grains, dislocations and precipitates for enhancing the strength of TiB2-modified CoCrFeMnNi high-entropy alloy using laser powder bed fusion

TiB2 nano-particle reinforced CoCrFeMnNi composite has been additively manufactured by using laser powder bed fusion (LPBF) technique. In comparison with the matrix CoCrFeMnNi sample, the average grain size of the TiB2 doped CoCrFeMnNi (CoCrFeMnNi + TiB2) sample reduces from 26.27 μm to 7.51 μm, appearing a morphological transformation from columnar grains to fine equiaxed grains and fine dendritic grains. Correspondingly, the compressive yield strength (σy) sharply increases from 484.41 ± 57.81 MPa of CoCrFeMnNi HEA to 952.62 ± 38.15 MPa of CoCrFeMnNi + TiB2, and the tensile yield strength increases from 480.27 ± 1.25 MPa to 834.21 ± 15.93 MPa, revealing a substantial enhancement of mechanical properties. The structural analysis unveils that the CoCrFeMnNi + TiB2 sample formed a tetragonal σ phase beside the FCC matrix and TiB2 phases compared with the single FCC phase of CoCrFeMnNi HEA. Especially, atomic resolution scanning transmission electron microscopy discloses that the TiB2 nanoparticles epitaxially grow onto the FCC matrix phase with [01–10]TiB2//[011]FCC orientation relationship. Theoretical calculations indicate that the enhancement mechanism of the CoCrFeMnNi + TiB2 sample should originate from a synergistic strengthening effect which is induced by grain refinement, TiB2 particles, σ phases and dislocations. Our work should provide a new view to utilize the rapid solidification process of LPBF and ceramic nano-particle reinforcement for the enhanced mechanical properties of HEAs with great potential.

Atomistic Understanding of the Coherent Interface Between Lead Iodide Perovskite and Lead Iodide

AbstractMetal halide perovskite semiconductors have shown great performance in solar cells, and including an excess of lead iodide (PbI2) in the thin films, either as mesoscopic particles or embedded domains, often leads to improved solar cell performance. Atomic resolution scanning transmission electron microscope micrographs of formamidinium lead iodide (FAPbI3) perovskite films reveal the FAPbI3:PbI2 interface to be remarkably coherent. It is demonstrated that such interface coherence is achieved by the PbI2 deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shifts in the stacking of the weakly van‐der‐Waals‐bonded layers containing the near‐octahedral units. The exact crystallographic interfacial relationship and lattice misfit are revealed. It is further shown that this 3R polytype of PbI2 has similar X‐ray diffraction (XRD) peaks to that of the perovskite, making XRD‐based quantification of the presence of PbI2 unreliable. Density functional theory demonstrates that this interface does not introduce additional electronic states in the bandgap, making it electronically benign. These findings explain why a slight PbI2 excess during perovskite film growth can help template perovskite crystal growth and passivate interfacial defects, improving solar cell performance.

Interlacing in Atomic Resolution Scanning Transmission Electron Microscopy.

Fast frame rates are desirable in scanning transmission electron microscopy for a number of reasons: controlling electron beam dose, capturing in situ events, or reducing the appearance of scan distortions. While several strategies exist for increasing frame rates, many impact image quality or require investment in advanced scan hardware. Here, we present an interlaced imaging approach to achieve minimal loss of image quality with faster frame rates that can be implemented on many existing scan controllers. We further demonstrate that our interlacing approach provides the best possible strain precision for a given electron dose compared with other contemporary approaches.

Role of dislocation climb on twin boundary and antiphase boundary formations in inverse-spinel MnAl2O4

The defects, e.g., stacking faults (SFs), antiphase boundaries (APBs), and twin, have been considered the critical components commonly introduced during the synthesis process of spinel oxides. In this study, employing advanced transmission electron microscopy, the formation mechanisms of twin and APBs in MnAl2O4 spinel oxide were clarified. Atomic resolution scanning transmission electron microscopy image and image simulation clarify that MnAl2O4 holds the inverse-spinel-typed structure. Detailed analysis of the twin structure reveals that the asymmetric twin structure in MnAl2O4 forms through an (Mn, Al)-containing plane missing by dislocation climb-up and then continuous shearing on {111} plane. APB forms by dislocation climb-down on an (Mn, Al)-containing {111} plane. The detailed lattice distortion induced by dislocation climb-down was analyzed by geometric phase analysis. The burgers vector for APB formation is identified as a/61̅1̅1+a/411̅0. Frank dislocation of a/61̅1̅1 climb-down and the lattice strain from introducing the extra plane brings a displacement of a/411̅0, which results in terrace-ledge-like morphology of APBs. This study provides a detailed understanding of twin and APB formation mechanisms in inverse-spinel-typed MnAl2O4.