HAADF Research Articles

Scalable faceted voids with luminescent enhanced edges in WS2 monolayers.

A scalable approach is needed in the formation of atomically flat edges with specific terminations to enhance local properties for optoelectronic, nanophotonic and energy applications. We demonstrate point defect clustering-driven faceted void formations with luminescent enhanced edges in WS2 monolayers during large-scale CVD growth and controlled annealing. With the aid of aberration-corrected scanning transmission electron microscopy (AC-STEM) high angle annular dark field (HAADF) imaging, we probed atomic terminations of S and W to explain observed luminescence enhancement in alternate edges. Faceted void formation in monolayer WS2 was found to be sensitive to annealing temperature, time, gas environment and precursor supply. Our observations of areal coverage evolution over time revealed competition between monolayer WS2 growth and void formation at 850 °C. While the initial stage was dominated by monolayer growth, defect generation and void growth dominated at later stages and provided an optimum processing window for monolayer WS2 as well as faceted void growth. Growth of faceted voids not only followed the geometry of monolayer facets but also showed similar atomic terminations at the edges and thus enabled local manipulation of photoluminescence enhancement with an order of magnitude increase in intensity. The developed CVD processing enabled multi-fold increase in the luminescent active edge length through the formation of faceted voids within the WS2 monolayer.

Facile silane functionalization of graphene oxide.

The facile silane functionalization of graphene oxide (GO) was achieved yielding vinyltrimethoxysilane-reduced graphene oxide (VTMOS-rGO) nanospheres located in the inter-layer spacing between rGO sheets via an acid-base reaction using aqueous media. The successful grafting of the silane agent with pendant vinyl groups to rGO was confirmed by a combination of Fourier-transform infrared (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The structure and speciation of the silane-graphene network (nanosphere) and, the presence of free vinyl groups was verified from solid-state magic angle spinning (MAS) and solution 13C and 29Si nuclear magnetic resonance (NMR) measurements. Evidence from Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM) and TEM-High-Angle Annular Dark-Field (TEM-HAADF) imaging showed that these silane networks aided the exfoliation of the rGO layers preventing agglomeration, the interlayer spacing increased by 10 Å. The thermal stability (TGA/DTA) of VTMOS-rGO was significantly improved relative to GO, displaying just one degradation process for the silane network some 300 °C higher than either VTMOS or GO alone. The reduction of GO to VTMOS-rGO induced sp2 hybridization and enhanced the electrical conductivity of GO by 105 S m-1.

In situ synthesis of metal clusters encapsulated within small-pore zeolites via a dry gel conversion method.

We report here a facile strategy for encapsulating Pt clusters into MFI crystals via a dry gel conversion (DGC) method, by introducing Pt-immobilized Schiff-SiO2 as the silicon resource. This protocol involves a solid phase transformation process for crystallization which occurs without significant deconstruction of the structure of the Pt-containing precursor. High-angle annular dark-field scanning transmission electron microscopy images indicate that most of the Pt clusters are less than 1 nm in size and located in defective channels or voids of MFI. Thanks to the encapsulation role, these Pt clusters are stable against sintering even after calcination at 500 °C and only grow up to ∼2.8 nm after calcination at 800 °C. The uniform pore structure of a zeolite endows the Pt@zeolite catalyst with shape-selectivity for hydrogenation and oxidation reactions over size differentiated analog substrates. This synthesis method is attainable for the embedment of stable metal clusters inside the small-pore MFI zeolite.

Alleviating luminescence concentration quenching in lanthanide doped CaF2 based nanoparticles through Na+ ion doping.

Luminescence concentration quenching, mainly due to a cross relaxation (CR) process between lanthanide ions (Ln3+), widely occurs in Ln3+ doped luminescent materials, setting a limit in the dopant content of Ln3+ emitters to withhold the brightness. Here, we introduced Na+ ions into the CaF2 host lattice codoped with Nd3+ emitters that alleviates concentration quenching greatly. And we show that the optimal dopant concentration of Nd3+ in colloidal CaF2:Nd nanoparticles increased from 10 to 30 mol%, resulting in an ∼32 times near-infrared (NIR) (1052 nm) brightness under 800 nm laser irradiation. Our mechanistic investigation suggests that the enhancement of NIR photo-luminescence (PL) could be attributed to not only the increasing crystallinity of nanoparticles but also the reducing concentration quenching of Nd3+ by improving the dopant distribution of Nd3+ ions in the CaF2 lattice, as evidenced by the high angle annular dark field images. These result in the optimal concentration increase to produce brightness enhancement greatly. This strategy can be utilized for other Ln3+ doped CaF2 based nanomaterials for bio-imaging.

Thermally induced alloying processes in a bimetallic system at the nanoscale: AgAu sub-5 nm core-shell particles studied at atomic resolution.

Alloying processes in nanometre-sized Ag@Au and Au@Ag core@shell particles with average radii of 2 nm are studied via high resolution Transmission Electron Microscopy (TEM) imaging on in situ heatable carbon substrates. The bimetallic clusters are synthesized in small droplets of superfluid helium under fully inert conditions. After deposition, they are monitored during a heating cycle to 600 K and subsequent cooling. The core-shell structure, a sharply defined feature of the TEM High-Angle Annular Dark-Field images taken at room temperature, begins to blur with increasing temperature and transforms into a fully mixed alloy around 573 K. This transition is studied at atomic resolution, giving insights into the alloying process with unprecedented precision. A new image-processing method is presented, which allows a measurement of the temperature-dependent diffusion constant at the nanoscale. The first quantification of this property for a bimetallic structure <5 nm sheds light on the thermodynamics of finite systems and provides new input for current theoretical models derived from bulk data.

Thin-Film Nanocomposite Membrane with the Minimum Amount of MOF by the Langmuir-Schaefer Technique for Nanofiltration.

An innovative procedure for positioning a monolayer of hydrophilic metal organic framework (MOF) MIL-101(Cr) (MIL, Materials of Institute Lavoisier) nanoparticles (NPs) in thin-film nanocomposite (TFN) membranes has been implemented by transferring a Langmuir-Schaefer (LS) film of the MOF in between the polyamide thin layer at the top and the cross-linked asymmetric polyimide (P84) support at the bottom. The presence and layout of the LS-MIL-101(Cr) monolayer in the TFN membrane was confirmed by scanning transmission electron microscopy imaging with a high-angle annular dark-field detector images and X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and atomic force microscopy analyses. This methodology requires the smallest amount of MOF reported to date, 3.8 μg cm-2, and promotes the formation of a defect-free ultrathin MOF film. Although conventional TFN membranes tend to show MOF agglomerates that could contribute to the formation of unselective defects, LS-TFN membranes, characterized by a homogeneous and continuous MOF coating, exhibit an optimal membrane performance, without a significant decrease in selectivity. Outstanding methanol permeances, one of the best results reported to date, of 10.1 ± 0.5 L m-2 h-1 bar-1 when filtering sunset yellow and of 9.5 ± 2.1 L m-2 h-1 bar-1 when filtering rose bengal have been achieved in LS-TFN membranes with a rejection higher than 90% in all cases. Methanol permeates through the polyamide and the LS-MIL-101(Cr) monolayer, greatly enhanced by the MOF pore system, in comparison to thin-film composite and conventional TFN membranes (7.5 ± 0.7 and 7.7 ± 1.1 L m-2 h-1 bar-1 when filtering sunset yellow), respectively, in which polyamide areas free of MOF NPs are present.

Direct Synthesis of 7 nm-Thick Zinc(II)–Benzimidazole–Acetate Metal–Organic Framework Nanosheets

Two-dimensional metal-organic frameworks (MOFs) are promising candidates for high performance gas separation membranes. Currently, MOF nanosheets are mostly fabricated through delamination of layered MOFs, which often results in a low yield of intact free-standing nanosheets. In this work, we present a direct synthesis method for zinc(II)-benzimidazole-acetate (Zn(Bim)OAc) MOF nanosheets. The obtained nanosheets have a lateral dimension of 600 nm when synthesized at room temperature. By adjusting the synthesis temperature, the morphology of obtained nanosheets can be readily tuned from nanosheets to nanobelts. Using zeolite nanosheets of known thickness as internal standard, we introduce a method based on high-angle annular dark-field scanning transmission electron microscopy images to determine nanosheet thickness. The thickness of Zn(Bim)OAc nanosheets is determined to be 7 nm, which combined with their estimated pore opening makes them promising building blocks for gas separation membranes.

Precipitation priority in 5083 Al-alloys containing small amounts of Cu or Zn

ABSTRACTThis study investigated the precipitation priority in Al–Mg alloys modified by the addition of 0.8 wt-% Cu or Zn, revealing that both these alloys exhibit increased hardness and decreased corrosion resistance. Scanning electron microscopy, transmission electron microscopy, and high-angle annular dark field imaging show that the precipitation priority changed the main second-phase particles of Cu- and Zn-containing alloys. Additionally, thermodynamic data of these phases were calculated using a semi-empirical model, showing good agreement with the experimental results.This paper is part of a thematic issue on Light Alloys.

Elemental preference and atomic scale site recognition in a Co-Al-W-base superalloy

Using state-of-the-art atomic scale super energy dispersive X-ray spectroscopy and high angle annular dark field imaging this study reveals the elemental partitioning preference between the γ′ and γ phases in a Co-Al-W-Ti-Ta superalloy and the site preference of its alloying elements in the ordered L12 γ′ phase. A semi-quantitative analysis of atomic column compositions in the ordered L12 γ′ structure is provided. Co atoms were found to occupy the {1/2, 1/2, 0} face-center positions whereas Al, W, Ti and Ta atoms prefer to occupy the {0, 0, 0} cube corner positions in the L12 γ phase. These findings agree well with predictions from first principles simulations in the literature.

Microstructures in landslides in northwest China – Implications for creeping displacements?

Microstructures, mineralogical composition and texture of selected landslide samples from three landslides in the southern part of the Gansu Province (China) were examined with optical microscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD) and synchrotron x-ray diffraction measurements. Common sheet silicates are chlorite, illite, muscovite, kaolinite, pyrophyllite and dickite. Other minerals are quartz, calcite, dolomite and albite. In one sample, graphite and amorphous carbon were detected by TEM-EDX analyses and TEM high-angle annular dark-field images. The occurrence of graphite and pyrophyllite with very low friction coefficients in the gouge material of the Suoertou and Xieliupo landslides is particularly significant for reducing the frictional strength of the landslides. It is proposed that the landslides underwent comparable deformation processes as fault zones. The low friction coefficients provide strong evidence that slow-moving landsliding is controlled by the presence of weak minerals. In addition, TEM observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion. For calcite and quartz, grain size reduction was attributed to both pressure solution and cataclasis. Therefore, besides landslide composition, the occurrence of ultrafine-grained slip zone material may also contribute to weakening processes of landslides. TEM images of slip-zone samples show both locally aligned clay particles, as well as kinked and folded sheet silicates, which are widely disseminated in the whole matrix. Small, newly formed clay particles have random orientations. Based on synchrotron x-ray diffraction measurements, the degree of preferred orientation of constituent sheet silicates in local shear zones of the Suoertou and Duang-He-Ba landslide is strong. This work is the first reported observation of well-oriented clay fabrics in landslides.

Formation and mosaicity of coccolith segment calcite of the marine algae Emiliania huxleyi.

Coccolithophores belong to the most abundant calcium carbonate mineralizing organisms. Coccolithophore biomineralization is a complex and highly regulated process, resulting in a product that strongly differs in its intricate morphology from the abiogenically produced mineral equivalent. Moreover, unlike extracellularly formed biological carbonate hard tissues, coccolith calcite is neither a hybrid composite, nor is it distinguished by a hierarchical microstructure. This is remarkable as the key to optimizing crystalline biomaterials for mechanical strength and toughness lies in the composite nature of the biological hard tissue and the utilization of specific microstructures. To obtain insight into the pathway of biomineralization of Emiliania huxleyi coccoliths, we examine intracrystalline nanostructural features of the coccolith calcite in combination with cell ultrastructural observations related to the formation of the calcite in the coccolith vesicle within the cell. With TEM diffraction and annular dark-field imaging, we prove the presence of planar imperfections in the calcite crystals such as planar mosaic block boundaries. As only minor misorientations occur, we attribute them to dislocation networks creating small-angle boundaries. Intracrystalline occluded biopolymers are not observed. Hence, in E.huxleyi calcite mosaicity is not caused by occluded biopolymers, as it is the case in extracellularly formed hard tissues of marine invertebrates, but by planar defects and dislocations which are typical for crystals formed by classical ion-by-ion growth mechanisms. Using cryo-preparation techniques for SEM and TEM, we found that the membrane of the coccolith vesicle and the outer membrane of the nuclear envelope are in tight proximity, with a well-controlled constant gap of ~4nm between them. We describe this conspicuous connection as a not yet described interorganelle junction, the "nuclear envelope junction". The narrow gap of this junction likely facilitates transport of Ca2+ ions from the nuclear envelope tothe coccolith vesicle. On the basis of our observations, we propose that formation of the coccolith utilizes the nuclear envelope-endoplasmic reticulum Ca2+ -store of the cell for the transport of Ca2+ ions from the external medium to the coccolith vesicle and that E.huxleyi calcite forms by ion-by-ion growth rather than by a nanoparticle accretion mechanism.

Insights into Water-Induced Phase Separation in Itraconazole-Hydroxypropylmethyl Cellulose Spin Coated and Spray Dried Dispersions.

For amorphous solid dispersions, understanding the phase behavior of a given drug-polymer blend and factors that influence miscibility is crucial to designing an optimally performing formulation. However, it can be challenging to fully map the phase behavior of some systems, especially those produced using a cosolvent system. In this study, a comprehensive investigation of phase separation in itraconazole-hydroxypropylmethylcellulose (ITZ-HPMC) blends fabricated using solvent evaporation processes, including spin coating and spray drying, has been carried out. Phase separation was found to be driven by the presence of water, either acquired from the environment or from the solvent system. ITZ nanospecies were observed during the solvent evaporation process prior to solidification. The use of high resolution imaging techniques such as transmission electron microscopy including bright field and high angle annular dark field imaging, enabled detailed characterization of the microstructure of phase separated systems. Spectroscopic investigations suggested that drug domains contain supramolecular drug aggregates in which the nematic assembly of ITZ molecules results in the coupling of the optical transitions of ITZ monomers. Importantly, a similar pattern of behavior between drug-polymer phase in spin coated and spray dried dispersions was observed. The presence of as little as 1% water in the solvent was found to induce phase separation in the spray dried particles, which was detected using the unique photophysical properties of ITZ and fluorescence spectroscopy. The study highlights the complexity of drug-polymer phase behavior and the influence of solvent properties.

PL-1Precise Structural Observation and State Analysis

Functional properties found in perovskite oxides are related to the octahedral distortion as well as the constituent elements. Heteroepitaxial thin films grown on different oxide are a good platform to control their functionalities. In order to elucidate the correlation between structural distortions induced by the lattice mismatch and properties of thin films, it is required to reveal the local structure in the vicinity of the interface precisely. High-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging in aberration-corrected scanning transmission electron microscopy allows us for visualizing not only constituent cations but also oxygen atoms. It has been demonstrated that measuring atomic positions precisely from HAADF and ABF images is a good tool for analysis of structural distortions at oxide heterointerfaces [1]. Through such structural analysis we have recognized that the interfacial octahedral connection is an important factor controlling the properties of thin films. We have found that the octahedral connection can be engineered by A-site cation size [2] and the thickness of buffer layer inserted between the thin film and substrate [3].

Observation of metal nanoparticles at atomic resolution in Pt-based cancer chemotherapeutics.

The chemotherapeutics cisplatin and oxaliplatin are important tools in the fight against cancer. Both compounds are platinum complexes. Aberration-corrected scanning transmission electron microscopy using the annular dark-field imaging mode now routinely provides single-atom sensitivity with atomic number contrast. Here, this imaging mode is used to directly image the platinum within the two drugs in their dried form on an amorphous carbon support film. The oxaliplatin is found to have wetted the supporting amorphous carbon, forming disordered clusters suggesting that the platinum has remained within the complex. Conversely, the cisplatin sample reveals 1.8-nm-diameter metallic platinum clusters. The size and shape of the clusters do not appear to be dependent on drying rate nor formed by beam damage, which may suggest that they were present in the original drug solution.

Influence of alloying element Zn on the microstructural, mechanical and corrosion properties of binary Mg-Zn alloys after severe plastic deformation

This work presents an analysis of the microstructural, mechanical and corrosion properties of two binary Mg-Zn alloys. Mg-6 wt%Zn and Mg-12 wt%Zn cast alloys were subjected to annealing followed by quenching and processed via equal channel angular pressing with applied back-pressure (ECAP-BP). After ECAP-BP, both alloys were thoroughly examined and showed partially recrystallized and highly deformed areas. High-angle annular dark-field imaging revealed a difference in Zn content across the α-Mg matrix of the Mg-12 wt%Zn after ECAP-BP due to the growth of MgZn2 nanoparticles. Electron energy-loss spectroscopy (EELS) was carried out to qualify an average Zn content in these areas, and a variation in Zn content up to 2at.% was found. Compression tests revealed mechanical anisotropy and a significant increase in the strength of both alloys after ECAP-BP. The yield strength, σ02, was in the range from 269 to 385MPa depending on the composition and compression axis. The initial state alloys showed yield strengths, σ02, of only 75–150MPa but improved ductility. The corrosion rates of the Mg-Zn alloys in the initial state, evaluated using a hydrogen evolution method in NaCl solution, were higher for Mg-12 wt%Zn. The corrosion rates of both alloys after ECAP-BP were higher than those of the initial state. Light microscopy observations did not reveal any preference for corrosion propagation, including transcrystalline, intercrystalline or interphase corrosion, in any of the materials.

The structure of InAlGaN layers grown by metal organic vapour phase epitaxy: effects of threading dislocations and inversion domains from the GaN template.

Defects in quaternary InAlGaN barriers and their effects on crystalline quality and surface morphology have been studied. In addition to growth conditions, the quality of the GaN template may play an important role in the formation of defects in the barrier. Therefore, this work is focused on effects caused by threading dislocations (TDs) and inversion domains (IDs) originating from the underlying GaN. The effects are observed on the crystalline quality of the barrier and characteristic surface morphologies. Each type of TDs is shown to affect the surface morphology in a different way. Depending on the size of the corresponding hillock for a given pinhole, it was possible to determine the dislocation type. It is pointed out that the smallest pinholes are not connected to TDs whereas the large ones terminate either mixed type or edge type TDs. At sufficiently large layer thickness, the IDs originating from the GaN template lead to the formation of concentric trenches at the layer surface, and this is related to the change in growth kinetics on top and at the immediate surroundings of the ID.

Mapping the layer count of few-layer hexagonal boron nitride at high lateral spatial resolutions

Layer count control and uniformity of two dimensional (2D) layered materials are critical to the investigation of their properties and to their electronic device applications, but methods to map 2D material layer count at nanometer-level lateral spatial resolutions have been lacking. Here, we demonstrate a method based on two complementary techniques widely available in transmission electron microscopes (TEMs) to map the layer count of multilayer hexagonal boron nitride (h-BN) films. The mass-thickness contrast in high-angle annular dark-field (HAADF) imaging in the scanning transmission electron microscope (STEM) mode allows for thickness determination in atomically clean regions with high spatial resolution (sub-nanometer), but is limited by surface contamination. To complement, another technique based on the boron K ionization edge in the electron energy loss spectroscopy spectrum (EELS) of h-BN is developed to quantify the layer count so that surface contamination does not cause an overestimate, albeit at a lower spatial resolution (nanometers). The two techniques agree remarkably well in atomically clean regions with discrepancies within ±1 layer. For the first time, the layer count uniformity on the scale of nanometers is quantified for a 2D material. The methodology is applicable to layer count mapping of other 2D layered materials, paving the way toward the synthesis of multilayer 2D materials with homogeneous layer count.

Qualifying ultrathin alumina film prepared by plasma-enhance atomic layer deposition under low temperature operation

Preparation of ultrathin alumina (Al2O3) films through Plasma-Enhanced Atomic Layer Deposition (PE-ALD) at low substrate temperature is discussed. The present work aims to investigate the physical mechanism of the PE-ALD deposition process and also the characteristics of the ultrathin alumina films on silicon 〈100〉 wafer deposited using the technique. The deposition was performed using trimethyl aluminum (Al (CH3)3) as the precursor and argon gas for purging. During deposition, the target temperature was kept constant at ~80, 100 and 150°C and the pressure was ~1.3×10−2Pa. Two deposition cycles were tested, 400 and 800 cycles. As for understanding the process, the films deposited with and without oxygen plasma were compared. Various thin film characterization techniques, including Atomic Force Microscope (AFM), ellipsometry, Raman spectrometry measurement, X-ray diffraction (XRD), and indentation technique, were applied for investigating the film properties. A transmission electron microscope (TEM) equipped with high-angle annular dark-field imaging line scan modes and energy-dispersive X-ray spectroscopy acquisition was used for imaging thin film cross-sections. We found that the number of deposition cycles did not affect the substrate surface roughness as evidenced by AFM images. The mechanical property, the hardness of the film deposited with 800 cycles and plasma was the best. Raman spectroscopy measurements showed that a Al-O-Si phase exists when the films were deposited at 100°C and 150°C for 400 and 800 cycles under oxygen plasma atmosphere. While no Al-O-Si phase existed after the same number of ALD deposition cycle without plasma. Results from XRD measurements indicated that the films deposited at 100°C and 150°C for 400 and 800 cycles under oxygen plasma atmosphere has an Al-O structure. TEM images clearly displayed the interface between the thin films, SiO2 interface layers and Si substrates. As for the sample deposited at 80°C, an Al2O3 film was hardly seen, but when increasing the deposition temperature to 100°C and 150°, films started to build on top of the substrate. However, for all deposition conditions, TEM revealed that the amounts of carbon atoms in the reaction site remained relatively high.

Observations of grain boundary chemistry variations in a boron carbide processed with oxide additives

Analytical electron microscopy was used to examine the grain boundary chemistry in boron carbide containing Al-O-rich phases at triple junctions. In this study, SiO2, Al2O3, and B2O3 additives were used to synthesize an aluminoborosilicate glass on the grain boundaries with the long-term goal to enhance fracture resistance. Nanolayer films were not observed using high-angle annular dark field imaging; however, energy-dispersive spectroscopy quantification revealed grain boundaries with varying excess of Si and Al. Overall, it was concluded variations in grain boundary chemistry depend upon the intrinsic grain boundary character and spatial heterogeneity of the oxide additives.

Small Nanosized Oxygen-Deficient Tungsten Oxide Particles: Mechanistic Investigation with Controlled Plasma Generation in Water for Their Preparation.

Production of oxygen-deficient tungsten oxide nanoparticles with a diameter of around 10 nm have been successfully developed using a microwave-induced plasma in liquid technique. The prepared blue-green nanoparticles exhibit strong absorption in the visible region; thus, these could be efficient visible-light photocatalysts. The high-angle annular dark-field images revealed the dislocation of tungsten, which causes oxygen deficiencies.