Aberration-corrected High-resolution Transmission Electron Microscopy Research Articles

Phase transitions in Pb0.96La0.04(Zr0.95Ti0.05)O3 capacitors by in-situ AC-HRTEM

Antiferroelectrics (AFEs) undergo reversible antiferroelectric-ferroelectric (AFE-FE) phase transitions under external electric fields, and show great promise in modern electronic devices. The real-time phase transition behavior of pure PZO has been detected, while the dynamic studies of complex incommensurately modulated structure have never been addressed. Here, with the strong support of in-situ aberration corrected high resolution transmission electron microscopy (AC-HRTEM), the structural evolutionary behaviors in Pb0.96La0.04(Zr0.95Ti0.05)O3 (PLZT) film excited by energetic electron beams is decoded. The AFE-to-FE phase transition occurs via a 90° reorientation and a rapid expansion of domain boundary region at the expense of the initial AFE matrix. At the same time, the modulation periods experience a monotonic increase in the reorientation region. The observation is of certain significance to establish the deep-seated phase transition mechanism, which will facilitate the development of antiferroelectric-based nanoelectronics.

Defect Density and Atomic Defect Recognition in the Middle Layer of a Trilayer MoS2 Stack.

Molybdenum disulfide (MoS2) is one of the most intriguing two-dimensional materials, and moreover, its single atomic defects can significantly alter the properties. These defects can be both imaged and engineered using spherical and chromatic aberration-corrected high-resolution transmission electron microscopy (CC/CS-corrected HRTEM). In a few-layer stack, several atoms are vertically aligned in one atomic column. Therefore, it is challenging to determine the positions of missing atoms and the damage cross-section, particularly in the not directly accessible middle layers. In this study, we introduce a technique for extracting subtle intensity differences in CC/CS-corrected HRTEM images. By exploiting the crystal structure of the material, our method discerns chalcogen vacancies even in the middle layer of trilayer MoS2. We found that in trilayer MoS2 the middle layer's damage cross-section is about ten times lower than that in the monolayer. Our findings could be essential for the application of few-layer MoS2 in nanodevices.

Temperature-dependence of beam-driven dynamics in graphene-fullerene sandwiches

C60 fullerenes encapsulated between graphene sheets were investigated by aberration-corrected high-resolution transmission electron microscopy at different temperatures, namely about 93 K, 293 K and 733 K, and by molecular dynamics simulations. We studied beam-induced dynamics of the C60 fullerenes and the encapsulating graphene, measured the critical doses for the initial damage to the fullerenes and followed the beam-induced polymerization. We find that, while the doses for the initial damage do not strongly depend on temperature, the clusters formed by the subsequent polymerization are more tubular at lower temperatures, while sheet-like structures are generated at higher temperatures. The experimental findings are supported by the results of first-principles and analytical potential molecular dynamics simulations. The merging of curved carbon sheets is clearly promoted at higher temperatures and proceeds at once over few-nm segments.

Atomic scale formation mechanism of the Amorphous-Nanocrystalline biphase structure in TiSiN Coating: Phase field crystal simulation and experimental characterization

TiSiN nanocomposite coatings have exceptional strength, toughness, and corrosion resistance due to their unique biphase amorphous/nanocrystalline structure with tiny TiN grains embedded in an amorphous Si3N4 phase. However, the unclear formation process makes microstructural control challenging. In this study the phase field crystal method has been employed to analyze the microstructure evolution in TiSiN coatings, uncovering a four-step formation mechanism for the amorphous-nanocrystalline structure. The initial stage results in appearance of concentration heterogeneities and the development of the triangular-like short-range order (Stage I). This stage is followed by the nucleation of nanocrystals in Ti-rich zones and the transformation of triangular short-range to the long-range order with a square symmetry (Stage II). Then, during coarsening stage, Ti-rich polycrystalline nanocrystals are formed (Stage Ⅲ) followed by the vacancy migration from the polycrystalline nanocrystals to amorphous phase and formation of monocrystalline Ti nanocrystals embedded in amorphous phase (Stage Ⅳ). The simulation results are compared qualitatively with aberration-corrected High-Resolution Scanning Transmission Electron Microscopy and High-Resolution Transmission Electron Microscopy images from our study.

Giant piezoresponse in nanoporous (Ba,Ca)(Ti,Zr)O3 thin film.

Lattice strain effects on the piezoelectric properties of crystalline ferroelectrics have been extensively studied for decades; however, the strain dependence of the piezoelectric properties at nano-level has yet to be investigated. Herein, a new overview of the super-strain of nanoporous polycrystalline ferroelectrics is reported for the first time using a nanoengineered barium calcium zirconium titanate composition (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCZT). Atomic-level investigations show that the controlled pore wall thickness contributes to highly strained lattice structures that also retain the crystal size at the optimal value (<30 nm), which is the primary contributor to high piezoelectricity. The strain field derived from geometric phase analysis at the atomic level and aberration-corrected high-resolution scanning transmission electron microscopy (STEM) yields of over 30% clearly show theoretical agreement with high piezoelectric properties. The uniqueness of this work is the simplicity of the synthesis; moreover the piezoresponse d 33 becomes giant, at around 7500 pm V-1. This response is an order of magnitude greater than that of lead zirconate titanate (PZT), which is known to be the most successful ferroelectric over the past 50 years. This concept utilizing nanoporous BCZT will be highly useful for a promising high-density electrolyte-free dielectric capacitor and generator for energy harvesting in the future.

Defect-Rich PdIr Bimetallene Nanoribbons with Interatomic Charge Localization for Isopropanol-Assisted Seawater Splitting.

Metallene with outstanding physicochemical properties is an efficient two-dimensional electrocatalysts for sustainable hydrogen (H2 ) production applications. However, the controllable fabrication of extended atomically thin metallene nanoribbons remains a formidable challenge. Herein, this work proposes a controllable preparation strategy for atomically thin defect-rich PdIr bimetallene nanoribbons (PdIr BNRs) with a thickness of only 1.5nm for the efficient and stable isopropanol-assisted seawater electrolytic H2 production. When using PdIr BNRs as catalyst to build an isopropanol-assisted seawater electrolysis system, a voltage of only 0.38V is required at @10mA cm-2 to achieve energy-saving H2 production, while producing high value-added acetone at the anode. The aberration-corrected high-resolution transmission electron microscopy (HRTEM) clearly reveals that the PdIr BNRs possess abundant structural defects, which can additionally serve as highly catalytically active sites. Density functional theory (DFT) calculations combined with X-ray absorption spectroscopy studies reveal that the introduction of Ir atoms can induce the formation of a localized charge region and shift the d-band center of Pd down, thereby reducing the adsorption energy on the catalyst in favor of the rapid desorption of H2 . This work opens the way for the controllable design and construction of defect-rich atomically thin metallene nanoribbons for efficient electrocatalytic applications.

Hierarchical domain structures associated with oxygen octahedra tilting patterns in lead-free (Bi1/2Na1/2)TiO3

Hierarchical domain structures associated with oxygen octahedra tilting patterns were observed in lead-free (Bi1/2Na1/2)TiO3 ceramics using aberration-corrected high-resolution transmission electron microscopy (HRTEM). Three types of domains are induced by distinct mechanisms: the ‘orientation-domain’ is induced at micrometer scale formed by different tilting orientations of the oxygen octahedra, the ‘meso-chemical-domain’ occurs at a few tens of nanometer scale by chemical composition variation on the A-site in the ABO3 perovskite structure, and the ‘nano-cluster-region’ runs across several unit-cells with apparent A-site cation segregation with oxygen vacancies clustering around Na cations. Based on HRTEM amplitude contrast imaging (ACI), the correlation between the oxygen octahedral tilting pattern and compositional non-stoichiometry was established. The role of the hierarchical domain structure associated with the tilting patterns of the oxygen octahedra on the ferroelectric behavior of (Bi1/2Na1/2)TiO3 is also discussed.

Polybenzimidazole copolymer derived lacey carbon film for graphene transfer and contamination removal strategies for imaging graphene nanopores

The study of the nanometer-scale vacancy defects (nanopores) in graphene by transmission electron microscopy (TEM) is severely hindered by the presence of polymeric residues originating from the graphene-transfer-step to the TEM grid. The state-of-the-art transfer strategies yield contamination-free pristine graphene specimens but do not work well for the nanoporous graphene. This is because of the relatively high energy of the vacant nanopores which makes it difficult to remove the residues without altering the structure of nanoporous graphene. Herein, we present a novel strategy to fabricate a sub-100-nm-thick lacey polymer film hosting see-through windows (10–900 nm) by using a facile nonsolvent-induced phase separation (NIPS). The polymer film is transformed into a lacey carbon film that reinforces graphene and allows residue-free transfer to the TEM grid as one avoids the direct contact between the polymer and the nanopores within a window. Finally, atmospheric-, graphene-synthesis-, and transfer-bath-related contaminants are removed by annealing the specimen inside an activated carbon bed at 900 °C in a reducing atmosphere. The method results in samples with large contamination-free areas which are easy to find during aberration-corrected high-resolution TEM (AC-HRTEM) imaging, enabling high throughput structural analysis of graphene nanopores.

Structures and Structural Evolution of Sublayer Surfaces of Metal-Organic Frameworks.

The structural characterization of sublayer surfaces of MIL-101 is reported by low-dose spherical aberration-corrected high-resolution transmission electron microscopy (HRTEM). The state-of-the-art microscopy directly images atomic/molecular configurations in thin crystals from charge density projections, and uncovers the structures of sublayer surfaces and their evolution to stable surfaces regulated by inorganic Cr3 (μ3 -O) trimers. This study provides compelling evidence of metal-organic frameworks (MOFs) crystal growth via the assembly of sublayer surfaces and has important implications in understanding the crystal growth and surface-related properties of MOFs.

Bond Dissociation and Reactivity of HF and H2O in a Nano Test Tube.

Molecular motion and bond dissociation are two of the most fundamental phenomena underpinning the properties of molecular materials. We entrapped HF and H2O molecules within the fullerene C60 cage, encapsulated within a single-walled carbon nanotube (X@C60)@SWNT, where X = HF or H2O. (X@C60)@SWNT represents a class of molecular nanomaterial composed of a guest within a molecular host within a nanoscale host, enabling investigations of the interactions of isolated single di- or triatomic molecules with the electron beam. The use of the electron beam simultaneously as a stimulus of chemical reactions in molecules and as a sub-angstrom resolution imaging probe allows investigations of the molecular dynamics and reactivity in real time and at the atomic scale, which are probed directly by chromatic and spherical aberration-corrected high-resolution transmission electron microscopy imaging, or indirectly by vibrational electron energy loss spectroscopy in situ during scanning transmission electron microscopy experiments. Experimental measurements indicate that the electron beam triggers homolytic dissociation of the H-F or H-O bonds, respectively, causing the expulsion of the hydrogen atoms from the fullerene cage, leaving fluorine or oxygen behind. Because of a difference in the mechanisms of penetration through the carbon lattice available for F or O atoms, atomic fluorine inside the fullerene cage appears to be more stable than the atomic oxygen under the same conditions. The use of (X@C60)@SWNT, where each molecule X is "packaged" separately from each other, in combination with the electron microscopy methods and density functional theory modeling in this work, enable bond dynamics and reactivity of individual atoms to be probed directly at the single-molecule level.

Near-atomic-scale observation of grain boundaries in a layer-stacked two-dimensional polymer.

Two-dimensional (2D) polymers hold great promise in the rational materials design tailored for next-generation applications. However, little is known about the grain boundaries in 2D polymers, not to mention their formation mechanisms and potential influences on the material's functionalities. Using aberration-corrected high-resolution transmission electron microscopy, we present a direct observation of the grain boundaries in a layer-stacked 2D polyimine with a resolution of 2.3 Å, shedding light on their formation mechanisms. We found that the polyimine growth followed a "birth-and-spread" mechanism. Antiphase boundaries implemented a self-correction to the missing-linker and missing-node defects, and tilt boundaries were formed via grain coalescence. Notably, we identified grain boundary reconstructions featuring closed rings at tilt boundaries. Quantum mechanical calculations revealed that boundary reconstruction is energetically allowed and can be generalized into different 2D polymer systems. We envisage that these results may open up the opportunity for future investigations on defect-property correlations in 2D polymers.

Insights into the Ti4+ doping in P2-type Na0.67Ni0.33Mn0.52Ti0.15O2 for enhanced performance of sodium-ion batteries

Due to the sodium abundance and availability, sodium-ion batteries (SIBs) have the potential to meet the worldwide growing demand of electrical energy storage. P2-type sodium transition-metal layer oxides with a high energy density are considered as the most promising cathode materials for SIBs. We present here a detailed study of the enhanced rate capability and cyclic stability of the Ti-doped Na0.67Ni0.33Mn0.67O2 cathode material. The combined analysis of ex-situ X-ray absorption fine structure (XAFS) spectroscopy, aberration-corrected high resolution transmission electron microscopy (AB-HRTEM) and X-ray diffraction (XRD) show that the strong Ti–O bond in the transition metal layers stabilizes the local structure, destroy the Na+-vacancy ordering and arrest the irreversible multiphase transformation that occurs during the intercalation/deintercalation process. Actually, Na0.67Ni0.33Mn0.52Ti0.15O2 exhibits a reversible capacity of 89.6 mA h g−1 even at 5 C, an excellent cyclability with 88.78 % capacity retention after 200 cycles at 0.5 C. This study provides a better understanding in optimization of the design of high-energy cathode materials based on titanium doped layered oxides for SIBs.

Interfacial chemistry and electronic structure of epitaxial lattice-matched TiN/Al0.72Sc0.28N metal/semiconductor superlattices determined with soft x-ray scattering

Epitaxial lattice-matched TiN/(Al,Sc)N metal/semiconductor superlattices have attracted significant interest in recent years for their potential applications in thermionic emission-based thermoelectric devices, optical hyperbolic metamaterials, and hot-electron-based solar-energy converters, as well as for the fundamental studies on the electron, photon, and phonon propagation in heterostructure materials. In order to achieve high efficiency devices and for the quest to discover new physics and device functionalities, it is extremely important that the superlattices exhibit atomically sharp and abrupt interfaces with minimal interface mixing and surface roughness. Moreover, as the energy transport across the cross-plane direction of these superlattices depends on the interface-properties, it is important to characterize the interfacial electronic structure and the chemistry of bond formation. Employing a combination of soft x-ray scattering techniques such as x-ray diffraction and synchrotron-based x-ray reflectivity, in this article, we demonstrate sharp and abrupt TiN/(Al,Sc)N superlattice interfaces with an asymmetric interface roughness ranging from two-to-three unit cells. Synchrotron-based soft x-ray absorption analysis revealed similar peak positions, line shapes, and absorption edges of different atoms in the individual thin films and in the superlattices, which demonstrate that the oxidation state of the atoms remains unchanged and rules-out the secondary structure or phase formation at the interfaces. The x-ray scattering results were further verified by aberration-corrected high-resolution scanning transmission electron microscopy imaging and energy dispersive x-ray spectroscopy mapping analysis. These results will be important for understanding of the transport properties of metal/semiconductor superlattices and for designing superlattice-based energy conversion devices.

Amorphous Metal Oxide Nanosheets Featuring Reversible Structure Transformations as Sodium-Ion Battery Anodes

The size and shape control of amorphous nanomaterials has long been a bottleneck restricting their further development. Here, we present a general approach to synthesize more than twenty kinds of amorphous metal oxide nanosheets. The amorphous state of the nanosheets is determined using aberration-corrected high-resolution transmission electron microscopy and X-ray diffraction. By using extended X-ray absorption fine structure analysis, we demonstrate that amorphous nanosheets have a larger interatomic distance and a looser packing characteristic compared with crystalline counterparts. The as-prepared amorphous FeOx nanosheets are used as sodium-ion batteries anode materials, exhibiting notable performance with a specific capacity of 263.4 mAh g−1 at 100 mA g−1 and long-term cyclic stability. Significantly, a reversible amorphous-crystalline-amorphous structure transformation phenomenon during the cycling test is observed at the atomic scale.

Adsorption Site Regulation to Guide Atomic Design of Ni-Ga Catalysts for Acetylene Semi-Hydrogenation.

Atomic regulation of metal catalysts has emerged as an intriguing yet challenging strategy to boost product selectivity. Here, we report a density functional theory-guided atomic design strategy for the fabrication of a NiGa intermetallic catalyst with completely isolated Ni sites to optimize acetylene semi-hydrogenation processes. Such Ni sites show not only preferential acetylene π-adsorption, but also enhanced ethylene desorption. The characteristics of the Ni sites are confirmed by multiple characterization techniques, including aberration-corrected high-resolution scanning transmission electron microscopy and X-ray absorption spectrometry measurements. The superior performance is also confirmed experimentally against a Ni5 Ga3 intermetallic catalyst with partially isolated Ni sites and against a Ni catalyst with multi-atomic ensemble Ni sites. Accordingly, the NiGa intermetallic catalyst with the completely isolated Ni sites shows significantly enhanced selectivity to ethylene and suppressed coke formation.

Two-dimensional halide perovskite lateral epitaxial heterostructures.

Epitaxial heterostructures based on oxide perovskites and III-V, II-VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics1-7. Halide perovskites-an emerging family of tunable semiconductors with desirable properties-are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers8-15. Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration16,17. Atomically sharp epitaxial interfaces are necessary to improve performance and fordevice miniaturization.However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility, which leads to interdiffusion and large junction widths18-21, and owing to their poor chemical stability, which leads to decomposition of prior layers during the fabrication of subsequent layers. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance22-26. Here we report an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. We demonstrate highly stable and tunable lateral epitaxial heterostructures, multiheterostructures and superlattices. Near-atomically sharp interfaces and epitaxial growth are revealed by low-dose aberration-corrected high-resolution transmission electron microscopy. Molecular dynamics simulations confirm the reduced heterostructure disorder and larger vacancy formation energies of the two-dimensional perovskites in the presence of conjugated ligands. These findings provide insights into the immobilization and stabilization of halide perovskite semiconductors and demonstrate a materials platform for complex and molecularly thin superlattices, devices and integrated circuits.

Fabricating antimony sulfide Sb2S3 microbars using solvothermal synthesis: effect of the solvents used on the optical, structural, and morphological properties

Antimony trisulfide (Sb2S3) is a promising candidate for cell absorbers due to its appropriate band gap, abundant constituents, non-toxicity, simple composition, and long-term stability. In this study, highly oriented Sb2S3 microbars were prepared utilizing an easy, low-cost solvothermal approach. The effects of the solvents used on the morphological and the crystal structure and optical properties of the Sb2S3 films were inspected using X-ray powder direction (XRD), UV/Vis spectroscopy, and field emission scanning electron microscopy (FESEM). The XRD analysis demonstrated that the Sb2S3 crystals had an orthorhombic phase. The Sb2S3 nanorods/bars mostly grew along the (010) direction. Energy-dispersive X-ray analysis (EDX) peaks had an atomic ratio of 2:3 for Sb:S. UV/Vis absorption spectroscopy showed that the optical energy gap of the Sb2S3 microbars was 1.5 with ethylene glycol as the solvent. Aberration-corrected high-resolution transmission electron microscopy (HRTEM) micrographs demonstrated that the Sb2S3 was dendrite-like consisting of microbars with a standard a length of 8–15 µm and width of 250–400 nm.

A novel FeNi-based bulk metallic glass with high notch toughness over 70 MPa m1/2 combined with excellent soft magnetic properties

A novel Fe39Ni39B12.82Si2.75Nb2.3P4.13 bulk metallic glass (BMG) with high notch toughness of 72.3 MPa m1/2, large plastic strain of 9.8%, high yield strength of 2930 MPa, and a large critical diameter of 2.5 mm was successfully developed. This BMG also exhibits excellent soft magnetic properties, i.e., higher saturation magnetic flux density of 0.86 T, extremely low coercivity of 0.65 A/m, and high effective permeability of 23,250 with high frequency stability. Its toughness value is the highest among Fe-based BMG family. The origin of high toughness was also investigated in detail by using synchrotron X-ray diffraction and aberration-corrected high-resolution transmission electron microscopy. It was found that the high toughness are attributed to atomic-scale structural heterogeneity that resulted from large free volume, high content of metal-metal bonds and high structural disorder degree, which can promote the multiple shear bands and hinder the subsequently propagation. This work provides useful guidelines for developing tough FeNi-based BMGs with high strength and excellent soft magnetic properties from an atomic structural perspective.

Quantitative insights into the growth mechanisms of nanopores in hexagonal boron nitride

The formation of nanopores under electron irradiation is an ideal process to quantify chemical bonds in two-dimensional materials. Nowadays, high-resolution transmission electron microscopy (HRTEM) allows investigating such nucleation and growth phenomena with incomparable spatial and temporal resolution. Moreover, theoretical calculations are usually exploited to confirm characteristic features of these atomic-scale observations. Nevertheless, the full understanding of the ejection mechanisms of atoms requires a detailed investigation of the interplay between the very dynamic edge structure of expanding nanopores and the displacement energy of edge atoms $({E}_{D})$. Here, the dynamics of triangular nanopores in hexagonal boron nitride (h-BN) under various electron dose rates was followed by aberration-corrected HRTEM with high temporal resolution to provide new in situ insights into their growth processes. We reveal that the ejection of atomic pairs is an elemental mechanism that considerably speeds up the expansion of nanopores. Atomic-scale calculations were exploited to quantify the structure-dependent ${E}_{D}$ of all the ejected edge atoms. They revealed strong variations of this threshold energy during the growth processes. This quantitative study reconciles theoretical and experimental measurements of the ejection rate of atoms in h-BN under electron irradiation, which is essential for nanopore engineering in this atomically thin membrane.

Single iron atoms anchored on activated carbon as active centres for highly efficient oxygen reduction reaction in air-cathode microbial fuel cell

Abstracts Microbial fuel cells are an emerging bio-battery that has attracted much attention due to its integration of power generation and wastewater treatment. However, the sluggish oxygen reduction reaction hinders its practical application. It is necessary to develop a high-performance catalyst for oxygen reduction reaction. Here, we report single iron atoms loaded on activated carbon as active centres used in microbial fuel cell that shows a maximum power density of 2264 ± 46 mW m−2, which is 156% higher than that of the bare activated carbon and comparable to commercial Pt/C. In addition, the higher exchange current density (16.069 × 10−4 A cm−2) and the efficient 4-electron pathway also verify its remarkable catalytic property. Moreover, we employ the aberration-corrected high-resolution high-angle annular dark-field scanning transmission electron microscopy and scanning electron microscopy to directly identify the single iron atoms on the carbon material and observe the morphology of catalyst, respectively. Besides, its large specific surface area (339.8 m2 g−1) and co-existence of microporous and mesoporous can promote the oxygen mass transfer and stabilize the single Fe atoms. In summary, the activated carbon-supported single iron atoms material can be a promising and highly efficient catalyst for future application.