Scanning Transmission Electron Microscopy Mode Research Articles

Designing complex radial heterostructures of Te/Bi2Te3 and Te/Bi2-x Pb x Te3 nanowires: fundamental mechanistic insights into nanowire growth and evolution

Metal telluride/Te heterostructure nanowires are important thermoelectric materials and it is important to be able to tune these materials according to the requirement of the application. In order to do so, a good understanding of the reaction mechanism and critical observation of the evolution of the nanowire heterostructure during the course of reaction is essential. Here, single crystalline, anisotropic Te core/Bi2Te3 shell nanowires have been synthesized by a facile template-based wet chemical synthesis method. The formation and evolution mechanism of the heterostructure has been elucidated by several control reactions, detailed transmission electron microscopy imaging and composition analysis using energy dispersive spectroscopy in scanning transmission electron microscopy mode of the products of the reactions. Fundamental understanding of the formation mechanism and time-dependent evolution of the core–shell structure in the nanowire have led to successful designing of higher order heterostructures involving Te/Bi2-x Pb x Te3. Through this study, interesting insights into the crystal structure evolution, crystal growth and miscibility of PbTe and Bi2Te3 into each other is obtained.

In Situ Thermal-Stage Fitted-STEM Characterization of Spherical-Shaped Co/MoS2 Nanoparticles for Conversion of Heavy Crude Oils

We report the thermal stability of spherically shaped cobalt-promoted molybdenum disulfide (Co/MoS2) nano-catalysts from in-situ heating under electron irradiation in the scanning transmission electron microscope (STEM) from room temperature to 550 °C ± 50 °C with aid of Fusion® holder (Protochip©, Inc.). The catalytic nanoparticles were synthesized via a hydrothermal method using sodium molybdate (Na2MoO4·2H2O) with thioacetamide (CH3CSNH2) and cobalt chloride (CoCl2) as promoter agent. The results indicate that the layered molybdenum disulfide structure with interplanar distance of ~0.62 nm remains stable even at temperatures of 550 °C, as observed in STEM mode. Subsequently, the samples were subjected to catalytic tests in a Robinson Mahoney Reactor using 30 g of Heavy Crude Oil (AGT-72) from the golden lane (Mexico’s east coast) at 50 atm using (ultrahigh purity) UHP hydrogen under 1000 rpm stirring at 350 °C for 8 h. It was found that there is no damage on the laminar stacking of Co/MoS2 with temperature, with interlayer spacing remaining at 0.62 nm; these sulfided catalytic materials led to aromatics rise of 22.65% and diminution of asphaltenes and resins by 15.87 and 3.53%, respectively.

Highly crystalline MAPbI3 perovskite grain formation by irreversible poor-solvent diffusion aggregation, for efficient solar cell fabrication

Energy efficient synthesis providing high quality crystalline thin films are highly desired in many applications. Here we devise a non-toxic solvent approach for production of highly crystalline MAPbI 3 perovskite by exploiting diffusion aggregation processes. Isopropanol solution based methylammonium lead triiodide (MAPbI 3 ) is used in this context, where the crystal growth initiation starts in an unstable suspension far from equilibrium and the subsequent crystallization is driven by the solubility parameters. The crystal formation is monitored by scanning transmission electron microscope (STEM), observing small crystallization centers growing as time evolves to large grains with high crystal purity. Energy dispersive X-ray spectroscopy (EDS) in STEM mode revealed a Pb rich core-shell structure in newly formed grains. Nano-beam Electron Diffraction (NBED) scan defined PbI 2 crystallites in the Pb rich shell with a single crystal MAPbI 3 core in newly formed grains. After a week stirring, the same aggregated suspension exhibited grains with only single crystal MAPbI 3 structure. The NBED analysis shows a kinetically slow transition from a core shell structure to a single crystal grain. This research presents an impactful insight on the factors that may cause sub-stoichiometric grain boundary effects which can influence the solar cell performance. In addition, the structure, morphology and optical properties of the perovskite grains have been presented. A powder of highly crystalline particles was subsequently prepared by evaporation of the solvent in a low-vacuum oven. Thin film MAPbI 3 solar cells were fabricated by dissolving the powder and applying it in a classical fabrication route. The MAPbI 3 solar cells gave a champion efficiency of 20% (19.9%) and an average efficiency at approximately 17% with low hysteresis effects. Here a strategy to manufacture the material structure without toxic solvents is highlighted. The single-crystal growth devised here opens both for shelf storage of materials as well as a more flexible manufacturing of devices. The process can likely be extended to other fields, where the intermediate porous framework and large surface area would be beneficial for battery or super capacitor materials. • A non-toxic solvent approach for production of high quality MAPbI 3 perovskite grains. • Nano-beam electron diffraction scan on PbI 2 crystallites and single crystal MAPbI 3 grains. • The porous framework of grains and large surface area would be beneficial for energy storage. • Low hysteresis effects and high efficiency for the perovskite solar cells.

Atom column detection from simultaneously acquired ABF and ADF STEM images

In electron microscopy, the maximum a posteriori (MAP) probability rule has been introduced as a tool to determine the most probable atomic structure from high-resolution annular dark-field (ADF) scanning transmission electron microscopy (STEM) images exhibiting low contrast-to-noise ratio (CNR). Besides ADF imaging, STEM can also be applied in the annular bright-field (ABF) regime. The ABF STEM mode allows to directly visualize light-element atomic columns in the presence of heavy columns. Typically, light-element nanomaterials are sensitive to the electron beam, limiting the incoming electron dose in order to avoid beam damage and leading to images exhibiting low CNR. Therefore, it is of interest to apply the MAP probability rule not only to ADF STEM images, but to ABF STEM images as well. In this work, the methodology of the MAP rule, which combines statistical parameter estimation theory and model-order selection, is extended to be applied to simultaneously acquired ABF and ADF STEM images. For this, an extension of the commonly used parametric models in STEM is proposed. Hereby, the effect of specimen tilt has been taken into account, since small tilts from the crystal zone axis affect, especially, ABF STEM intensities. Using simulations as well as experimental data, it is shown that the proposed methodology can be successfully used to detect light elements in the presence of heavy elements.

Comparative study on the specimen thickness measurement using EELS and CBED methods

Two thickness measurement methods using an electron energy loss spectroscopy (EELS) and 10a convergent beam electron diffraction (CBED) were compared in an Fe-18Mn-0.7C alloy. The thin foil specimen was firstly tilted to satisfy 10a two-beam condition. Low loss spectra of EELS and CBED patterns were acquired in scanning transmission electron microscopy (STEM) and TEM-CBED modes under the two-beam condition. The log-ratio method was used for measuring the thin foil thickness. Kossel-Möllenstedt (K-M) fringe of the mathbf{13}overline{mathbf{1}} diffracted disk of austenite was analyzed to evaluate the thickness. The results prove the good coherency between both methods in the thickness range of 72 ~ 113 nm with a difference of less than 5%.

(Invited) Fabrication and Characterization of B- or P- Doped Silicon Nanoparticles Embedded in SiOxNy Transparent Matrix

Silicon nanocrystals (Si-Ncs)-based devices are some of the most studied nanomaterials and could be of great interest in many fields such as electronics1, optical sensing2, photovoltaics3 and biomedicine4. First of all, it is important to control their size, shape and aggregation which are parameters related to their potential applications. In our case, these Si-Ncs are embedded in an insulating dielectric matrix of SiOxNy, resulting in a composite material with a tunable effective bandgap. Doping Si-Ncs can enlarge their applicability by modifying their electrical properties5, the effective bandgap5, the light absorption range and the nanoparticles photoluminescence.In this study, we used a magnetron sputtering deposition system to deposit silicon rich silicon oxynitride (SiOxNy) thin films. Then, n- and p-type dopants such as boron, arsenic and phosphorus were introduced in the thin films using ion implantation which was carried out at different doses and accelerating energies5. Thanks to a thermal treatment at high temperature, silicon atoms diffuse and nucleate to form silicon nanostructures with size varying between 1.5 nm and several dozen nanometers. This step is necessary for the phase separation leading to the nanoparticles nucleation.Atom probe tomography (APT) and energy dispersive X-ray mapping in scanning TEM mode, coupled with spectrum imaging on silicon plasmon, were performed to localize the dopant impurities versus the Si-Ncs. The results have showed that n-type (P and As) dopants were energetically favored to be placed into the Si-Ncs core, whereas p-type dopants are preferentially located at the Si-NCs/SiOxNy interface.The effect of composition and post-deposition annealing temperature on the Si-Ncs light-emitting properties was investigated by the examination of photoluminescence (PL) evolution. The dopants distribution profiles within the host matrix were determined by Rutherford Backscattering Spectroscopy (RBS). EFTEM combined with X Rays mapping analysis have exhibited that the incorporation of arsenic at a high doping level (dose of 5x 1016 As/cm2) leads to an increase of the Si-Ncs size by a factor around 4, precisely from 4 nm to more than 20 nm (Figure 1). On the other hand, the EFTEM-X Rays analysis was unable to detect boron dopants in the SiOxNy films. At this doping level, it could be possible to observe a localized surface plasmon resonance (LSPR) 7.For both n- and p-type dopants, the silicon nanoparticles crystalline fraction was found to be much higher than the undoped structures. The dopants facilitate the nucleation and growth of crystalline nanoparticles. This behavior is quite similar to that observed for thin amorphous silicon films. The results have showed that the nanostructures shape have different behaviors depending on the dopants.Finally Schottky diodes were prepared. The I-V characteristic curves show that the electrical conduction in the -doped structures increases with the dopant doses and we obtained a rectifier behavior. The results have showed that the heavily As-doped silicon structures are limited by the tunnel oxide thickness between the large Si-Ncs. These results are very encouraging when aiming at implementing such nanocomposite materials in photovoltaic solar cells.

Spinodal decomposition of reactively sputtered (V0.64Al0.36)0.49N0.51 thin films

We investigate the decomposition mechanisms of metastable cubic (c-)(V0.64Al0.36)0.49N0.51 thin films, grown by reactive high power pulsed magnetron sputtering, by combination of structural and compositional characterization at the nanometer scale with density functional theory (DFT) calculations. Based on thermodynamic considerations of ∂2∆G/∂x2 < 0, spinodal decomposition is expected for c-V1-xAlxN with x ≥ 0.35. While no indications for spinodal decomposition are observable from laboratory and synchroton diffraction data after annealing in Ar atmosphere at 1300 °C, the formation of wurtzite (w-)AlN is evident after annealing at 900 °C by utilizing high energy synchrotron X-ray diffraction. However, the complementary nature of elemental V and Al maps, obtained by energy dispersive X-ray spectroscopy in scanning transmission electron microscopy mode, imply spinodal decomposition of c-(V0.64Al0.36)0.49N0.51 into V- and Al-rich cubic nitride phases after annealing at 900 °C. These chemical modulations are quantified by atom probe tomography and maximum variations of x in V1-xAlxN are in the range of 0.36 to 0.50. The magnitude of the compositional modulations is enhanced after annealing at 1100 °C as x varies on average between 0.30 and 0.61, while the modulation wavelength remains unchanged at approximately 8 nm. Based on DFT data, the local x variation from 0.30 to 0.61 would cause lattice parameter variations from 4.111 to 4.099 Å. This difference corresponds to a shift of the (200) peak from 44.0 to 44.1°. As the maximum decomposition-induced peak separation magnitude of 0.1° is significantly smaller than the measured full width at half maximum of 0.4°, spinodal decomposition cannot be unravelled by diffraction data. However, consistent with DFT predictions, spinodal decomposition in c-(V0.64Al0.36)0.49N0.51 is revealed by chemical composition characterization at the nanometer scale.

Methodology for observation of maraging tool steel after 3D printing using FIB and STEM mode

The components of the maraging tool steel produced by 3D printing are further heat-treated after printing. It includes a stress relief annealing and precipitation hardening. During this process precipitates of Ni3Ti and Fe2Mo or Fe7Mo are formed, which significantly contribute to an increase in ultimate tensile strength, hardness, and toughness. To understand the precipitation processes, it is necessary to describe the particles in detail. However, they are very small in size and cannot be examined properly by a light microscope (LM) or by scanning electron microscope (SEM). A possible way to investigate and describe these precipitates is to produce a thin lamella using a focused ion beam (FIB) in the electron microscope chamber, where scanning transmission electron microscopy (STEM) is subsequently used for observation. A lamella was prepared from the DMLS (Direct Metal Laser Sintering) printed part by an ion beam for observation in STEM mode. The experiment took place at the Zeiss AURIGA scanning electron microscope, equipped with an ion gun and also with STEM capabilities for thin samples. The lamella preparation methodology was gradually optimized to achieve sufficient resolution for observation of these very fine microstructures produced by 3D printing.

Quantitative 3D Characterization of Elemental Diffusion Dynamics in Individual Ag@Au Nanoparticles with Different Shapes

Anisotropic bimetallic nanoparticles are promising candidates for plasmonic and catalytic applications. Their catalytic performance and plasmonic properties are closely linked to the distribution of the two metals, which can change during applications in which the particles are exposed to heat. Due to this fact, correlating the thermal stability of complex heterogeneous nanoparticles to their microstructural properties is of high interest for the practical applications of such materials. Here, we employ quantitative electron tomography in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode to measure the 3D elemental diffusion dynamics in individual anisotropic Au-Ag nanoparticles upon heating in situ. This approach allows us to study the elemental redistribution in complex, asymmetric nanoparticles on a single particle level, which has been inaccessible to other techniques so far. In this work, we apply the proposed method to compare the alloying dynamics of Au-Ag nanoparticles with different shapes and compositions and find that the shape of the nanoparticle does not exhibit a significant effect on the alloying speed whereas the composition does. Finally, comparing the experimental results to diffusion simulations allows us to estimate the diffusion coefficients of the metals for individual nanoparticles.

Influence of combinatory effects of STEM setups on the sensitivity of differential phase contrast imaging

Differential phase-contrast (DPC) imaging in the scanning transmission electron microscopy (STEM) mode has been suggested as a new method to visualize the nanoscale electromagnetic features of materials. However, the quality of the DPC image is very sensitive to the electron-beam alignment, microscope setup, and specimen conditions. Unlike normal STEM imaging, the microscope setup variables in the DPC mode are not independent; rather, they are correlated factors decisive for field sensitivity. Here, we systematically investigated the independent and combinatory effects of microscope setups on the sensitivity of the DPC image in a hard magnet, Nd2Fe14B alloy. To improve sensitivity, a smaller overlap of the electron beam with annular detectors and a greater camera length were required. However, these factors cannot be controlled independently in the two-condenser-lens system. In this linked system, the effect of the camera length on the DPC sensitivity was slightly more predominant than the overlap. Furthermore, the DPC signal was noisy and scattered at a small overlap of less than 11%. The electron-beam current does not evidently affect the sensitivity. In addition, the DPC sensitivity was examined with respect to the sample thickness, and the optimum thickness for high sensitivity was approximately 65 nm for the hard magnetic material Nd2Fe14B. This practical approach to the STEM setup and sample thickness may provide experimental guidelines for further application of the DPC analysis method.

Synthesis of Curved CuIn1–xGax(S1–ySey)2 Nanocrystals and Complete Characterization of Their Diffraction Contrast Effects

Herein, we report the synthesis and complete structural characterization of curved CuIn1–xGax(S1–ySey)2 (x = y = 0.5) nanocrystals, which adopt a two-dimensional (2D) nanoplate morphology and crystallize in the hexagonal wurtzite phase. A detailed transmission electron microscopy (TEM) analysis reveals that the 2D nanoplates are predominately [001]-oriented and exhibit a unique bend contour pattern, because of curvature on the (001) basal plane. The contrast behavior of bend contours in this system is further supported through specimen-tilting experiments inside TEM, in combination with dark-field imaging in both TEM and scanning TEM modes. This bending is believed to originate from the proximity effect of the top and bottom (001) surfaces.

Understanding CeO2‐Based Nanostructures through Advanced Electron Microscopy in 2D and 3D

AbstractEngineering morphology and size of CeO2‐based nanostructures on a (sub)nanometer scale will greatly influence their performance; this is because of their high oxygen storage capacity and unique redox properties, which allow faster switching of the oxidation state between Ce4+ and Ce3+. Although tremendous research has been carried out on the shape‐controlled synthesis of CeO2, the characterization of these nanostructures at the atomic scale remains a major challenge and the origin of debate. The rapid developments of aberration‐corrected transmission electron microscopy (AC‐TEM) have pushed the resolution below 1 Å, both in TEM and in scanning transmission electron microscopy (STEM) mode. At present, not only morphology and structure, but also composition and electronic structure can be analyzed at an atomic scale, even in 3D. This review summarizes recent significant achievements using TEM/STEM and associated spectroscopic techniques to study CeO2‐based nanostructures and related catalytic phenomena. Recent results have shed light on the understanding of the different mechanisms. The potential and limitations, including future needs of various techniques, are discussed with recommendations to facilitate further developments of new and highly efficient CeO2‐based nanostructures.

Three-dimensional scanning transmission electron microscopy of dislocation loops in tungsten

Scanning transmission electron microscopy (STEM) imaging using diffraction contrast is a powerful technique to assess crystal defects. In this work it is used to assess the spatial distribution of radiation induced defect in tungsten. In effect, its irradiation leads to the formation of nanometric dislocation loops that under certain conditions may form intriguing 3-D rafts. In this study, we have irradiated thin tungsten samples in situ in a TEM with 1.2 MeV W ions to 0.017 dpa at room temperature (RT) and at 700 °C. Besides the Burgers vector analysis, the number density and size of the dislocation loops with their spatial arrangement were quantitatively characterized by stereo imaging in STEM mode. Most of the loops have a Burgers vector ½ a0 〈111〉, with some a0 〈100〉 at room temperature. Loops are located mainly in the simulated damage profile but there is also a significant portion in deeper regions of the sample, indicating that loops in W diffuse easily, even at RT. At 700 °C, loops form elongated rafts that contain dislocation segments having a Burgers vector ½ a0 〈111〉. The rafts are narrow and reside on {111} planes; they are elongated along 〈110〉 directions, which correspond, when combined to the rafts’ Burgers vector, to the lines of edge dislocations. Compared to conventional TEM, 3-D analysis in STEM appears thus as a powerful technique for quantitative analyses of defects in tungsten, as it allows reducing the background diffraction contrast and reaching thicker areas of the electron transparent foil, here 0.5 μm of tungsten at 200 kV.

Effect of chemical inhomogeneity on domains and ferroelectric properties of Fe‐modified 0.77Bi0.5Na0.5TiO3–0.23SrTiO3

AbstractLead‐free 0.77(Bi0.5Na0.5)TiO3–0.23Sr(Ti1−xFex)O3 (x = 0, 0.04) (BNT–23STFx) was prepared using a conventional solid‐state reaction route. The effects of Fe‐modification on the chemical homogeneity from a μm scale perspective, the core‐shell domains structures, and the ferroelectric properties were investigated. The chemical homogeneity was analyzed using energy dispersive X‐ray mapping in scanning transmission electron microscopy mode, and the field‐dependent behaviors of strain and polarization were obtained to determine the ferroelectric properties. Substituting Fe3+ for Ti4+ resulted in completely different electrical behavior and properties, despite similar XRD patterns and microstructures. The Fe‐substitution promoted the mobility of Sr2+ ions in the BNT phase and, as a consequence, the chemical homogeneity increased and the core‐domains collapsed. Extending the ceramic processing, such as milling time and sintering time, affected domain distribution and compositional inhomogeneity, which led to a gradual transformation from ferroelectric to relaxor.

Tungsten-Based Mesoporous Silicates W-MMM-E as Heterogeneous Catalysts for Liquid-Phase Oxidations with Aqueous H2O2

Mesoporous tungsten-silicates, W-MMM-E, have been prepared following evaporation-induced self-assembly methodology and characterized by elemental analysis, XRD, N2 adsorption, STEM-HAADF (high angle annular dark field in scanning-TEM mode), DRS UV-vis, and Raman techniques. DRS UV-vis showed the presence of two types of tungsten oxo-species in W-MMM-E samples: isolated tetrahedrally and oligomeric octahedrally coordinated ones, with the ratio depending on the content of tungsten in the catalyst. Materials with lower W loading have a higher contribution from isolated species, regardless of the preparation method. W-MMM-E catalyzes selectively oxidize of a range of alkenes and organic sulfides, including bulky terpene or thianthrene molecules, using green aqueous H2O2. The selectivity of corresponding epoxides reached 85–99% in up to 80% alkene conversions, while sulfoxides formed with 84–90% selectivity in almost complete sulfide conversions and a 90–100% H2O2 utilization efficiency. The true heterogeneity of catalysis over W-MMM-E was proved by hot filtration tests. Leaching of inactive W species depended on the reaction conditions and initial W loading in the catalyst. After optimization of the catalyst system, it did not exceed 20 ppm and 3 ppm for epoxidation and sulfoxidation reactions, respectively. Elaborated catalysts could be easily retrieved by filtration and reused several times with maintenance of the catalytic behavior.

Magnetic properties of Co/Ni grain boundaries after annealing

Magnetic and microstructural properties of 111 textured Cu/N×[Co/Ni] films are studied as a function of the number of bilayer repeats N and annealing temperature. M(H) loop measurements show that coercivity, Hc, increases with annealing temperature and that the slope of the saturation curve at Hc has a larger reduction for smaller N. An increase of the magnetic anisotropy (Ku) to saturation magnetization (Ms) ratio after annealing N×[Co/Ni] with N &amp;lt; 15 only partially describes the increase to Hc. Energy-dispersive X-ray spectroscopy analyses performed in scanning transmission electron microscopy mode across cross-sections of as-deposited and annealed Cu/16×[Co/Ni] films show that Cu diffuses from the seed layer into grain boundaries of Co/Ni. Diffusion of Cu reduces exchange coupling (Hex) between the magnetic grains and explains the increase in Hc. Additionally, the difference in the slope of the M(H) curves at Hc between the thick (N = 16) and thin (N = 4) magnetic multilayers is due to Cu diffusion more effectively decoupling magnetic grains in the thinner multilayer.

A Macroscopically Relevant 3D‐Metrology Approach for Nanocatalysis Research

AbstractA 3D nanometrological approach, which considers as an unbiased validation criterion the quantitative match between values of properties determined by macroscopic characterization techniques and those determined from the nanoscopic results, is developed to unveil the details of complex nanocatalysts. This approach takes into account both the peculiar characteristics of this type of materials and the large influence of noise in the tilt series. It combines, in an optimized way, the latest experimental developments in high angle annular dark field scanning transmission electron microscopy mode (HAADF‐STEM) tomography, such as batch tomography, image denoising by undecimated wavelet transforms, improved reconstructions by total variation minimization and a more efficient, user‐independent, segmentation scheme. To illustrate the use of this novel approach, the 3D structural characterization of a model nanocatalyst comprising gold nanoparticles dispersed on the surface of CeO2 nanocubes is performed, and the obtained results used to compute the values of different macroscopic chemical and textural properties. Comparison with values obtained by macroscopic characterization techniques match very closely those obtained by 3D nanometrology. Importantly, the new approach described in this work also illustrates a pipeline for nearly fully automated HAADF‐STEM tomography studies, guaranteeing reliable correlations between nanoscopic and macroscopic properties.

Additive‐Morphology Interplay and Loss Channels in “All‐Small‐Molecule” Bulk‐heterojunction (BHJ) Solar Cells with the Nonfullerene Acceptor IDTTBM

AbstractAchieving efficient bulk‐heterojunction (BHJ) solar cells from blends of solution‐processable small‐molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well‐defined SM donors—DR3TBDTT (DR3), SMPV1, and BTR—used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All‐SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution‐processing additive 1,8‐diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as‐cast” BHJ devices from which only modest PCEs &lt;1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal‐insulator‐semiconductor carrier extraction” methods, and systematic recombination examinations by light‐dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase‐separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall.

Microstructure Analysis of Neutron Absorber Al/B4C Metal Matrix Composites

The microstructure of Al/B4C metal matrix composites (MMCs) used as neutron absorbers in both dry storage casks and wet storage pools of spent nuclear fuel was analyzed by SEM and TEM. A polishing method of a focused Ga+ ion beam was used to obtain an ideal sample surface with very low roughness, which was used to statistically analyze the distribution characteristics and size factor of B4C particles in the aluminum matrix. The area of B4C particles mainly ranged from 0 to 0.5 μm2, which was the proportion of 64.29%, 86.99% and 76.86% of total statistical results for the Al-15%B4C, Al-25%B4C and Al-30%B4C MMCs, respectively. The average area of B4C particles in the Al-15%B4C, Al-25%B4C and Al-30%B4C MMCs were about 1.396, 0.528 and 1.183 μm2, respectively. The nanoscale precipitates were analyzed by the element mappings in scanning transmission electron microscopy (STEM) mode and electron energy loss spectroscopy (EELS) mode, which included elliptic alloy precipitates with elemental Cu, Cr, Fe and Si, except for Al, and B4C nanoparticles with polygonal shape. The interface characteristics showed that the (021) crystal plane of B4C particle and (111) crystal plane of aluminum matrix grew together. The lattice misfit was about 1.68% for (111)Al//(021)B4C. The corrosion properties and corresponding mechanism of Al/B4C MMCs were investigated in an aqueous solution with 5000 ppm boric acid at 100 °C and atmospheric pressure, which showed that the mass increment rate was first decreased with increasing corrosion time and then increased.

Analysis of InGaN nanodots grown by droplet heteroepitaxy using grazing incidence small-angle X-ray scattering and electron microscopy

We present a detailed structural investigation of self-assembled indium gallium nitride nanodots grown on c-plane aluminum nitride templates by the droplet heteroepitaxy technique in a plasma-assisted molecular beam epitaxy reactor. Various growth parameters, including the total coverage of the metal species, relative and total metal effusion fluxes, and nitridation temperature were investigated. Analyses of in situ reflection high-energy electron diffraction patterns and comparison with simulations showed that the resulting crystal structure was a mixture of wurtzite and twinned zinc blende phases, with the zinc blende phase increasingly dominant for lower metal coverages and lower nitridation temperatures, and the wurtzite phase increasingly dominant for higher nitridation temperature. Studies by field emission scanning electron microscopy and atomic force microscopy revealed that the nanodots exhibit trimodal size distributions, with the dot morphologies of the intermediate size mode often resembling aggregations of distinct clusters. Nanodots grown at higher nitridation temperatures had larger inter-dot spacings, with hexagonal in-plane ordering observable at a sufficiently high temperature. Using grazing incidence small angle X-ray scattering, we determined the nanodots to be approximately truncated cone shaped, and extracted the mean radius, height, and inter-dot distance for each distribution. Microstructural investigations of the nanodots by cross-sectional transmission electron microscopy indicated that the majority of the dots were formed in dislocation-free regions, and confirmed that the intermediate size dots were approximately truncated cone shaped and consisted of both zinc blende and wurtzite regions. Mapping of the elemental distributions by energy dispersive X-ray spectroscopy in scanning transmission electron microscopy mode indicated highly nonuniform indium distributions within both small and intermediate size dots which are potentially indicative of indium clustering and kinetically controlled nanoscale phase separation rather than the spinodal decomposition associated with bulk diffusion. The observed enrichment in indium concentration towards the tops of the nanodot layers could be ascribed to the compositional pulling effect.