High-angle Annular Dark Field Detector Research Articles

Origin of spin glass magnetic behavior and phase stability of flux-grown RE(Al, Cr, Fe)B4 (RE = Ho and Er) single crystals

The YCrB4 type (α-phase, orthorhombic with space group Pbam) α-HoAlB4 and α-ErAlB4 single crystals which were self-flux grown exhibit the spin glass magnetic behavior, which is anomalous compared to the other reported rare earth system phases of α-REAlB4. Local atomic arrangements with the atomic-resolution for α-HoAlB4 and α-ErAlB4 compounds have been investigated by spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM) with high-angle annular detector dark-field (HAADF) and annular bright-field (ABF), and STEM-energy-dispersive X-ray spectroscopy (EDS) techniques to elucidate the origin of the exotic magnetic behavior. Observed HAADF- and ABF-STEM images and STEM-EDS chemical maps taken with the incident beam parallel to the [001] direction clearly indicate the existence of substitutions for Al atom sites. The phase stability of RAlB4 (R = Ho ∼ Lu) compounds was discussed from the viewpoint of crystallographic features with lattice parameters and compositions. Based on the presented results, it could be concluded that the origin of the spin glass behavior for self-flux grown α-HoAlB4 and α-ErAlB4 single crystals is the unexpected substitution of Cr/Fe atoms for Al ones. Simultaneously, the substitution by Cr/Fe atoms for Al atoms is indicated to play a significant role to stabilize the YCrB4-type α-HoAlB4 and α-ErAlB4 compounds.

Structure of an Al64Cu22Co14 decagonal quasicrystal studied by Cs-corrected STEM

During the last several decades, since the discovery of a decagonal quasicrystal, a 2 nm cluster model has been widely accepted as its basic quasi-unit-cell (QUC). Instead of the traditional 2 nm QUC, a 3.2 nm QUC is proposed in this paper. The 3.2 nm QUC can fill all the blank areas. The 3.2 nm QUC consists of 251 atoms. The element type and position of each atom are determined using high-angle annular detector dark-field (HAADF) images taken along three projection directions, i.e., one along the ten-fold symmetry and the other two along the two-fold symmetry with an intersection angle of 18 degrees. The proposed model opens an avenue for further investigation of the aperiodic atomic structure of other quasicrystals.

Structural developments during the low-temperature oxidation of Nb7W10O47

The oxidation of Nb7W10O47 at relatively low temperature TOX = 500 °C leads to the formation of domains of the Nb8W9O47 structure between which coherently embedded planar defects appear. The basic Nb8W9O47 structure was identified by powder X-ray diffraction (XRD) and subsequent Rietveld refinement. The structure of the planar defects building up the boundaries was determined by scanning transmission electron microscopy (STEM) images recorded with a high-angle annular dark field (HAADF) detector and an annular bright field (ABF) detector. We compare STEM images of the well-known Nb8W9O47 structure with images of the oxidation product of Nb7W10O47 and show that domains of Nb8W9O47 are also present while additional coherently embedded planar defects appear. HAADF-STEM images reveal that these defects often comprise arrays of three hexagons of octahedra that are locally arranged in a one-dimensional periodic way. ABF-STEM images reveal that two pairs of octahedra in one of the hexagons are actually connected by edge-sharing instead of the usual and hitherto assumed corner-sharing.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Intergrowth of niobium tungsten oxides of the tetragonal tungsten bronze type

Abstract Since the 1970s, high-resolution transmission electron microscopy (HRTEM) is well established as the most appropriate method to explore the structural complexity of niobium tungsten oxides. Today, scanning transmission electron microscopy (STEM) represents an important alternative for performing the structural characterization of such oxides. STEM images recorded with a high-angle annular dark field (HAADF) detector provide not only information about the cation positions but also about the distribution of niobium and tungsten as the intensity is directly correlated to the local scattering potential. The applicability of this method is demonstrated here for the characterization of the real structure of Nb7W10O47.5. This sample contains well-ordered domains of Nb8W9O47 and Nb4W7O31 besides little ordered areas according to HRTEM results. Structural models for Nb4W7O31 and twinning occurring in this phase have been derived from the interpretation of HAADF-STEM images. A remarkable grain boundary between well-ordered domains of Nb4W7O31 and Nb8W9O47 has been found that contains one-dimensionally periodic features. Furthermore, short-range order observed in less ordered areas could be attributed to an intimate intergrowth of small sections of different tetragonal tungsten bronze (TTB) based structures.

Nano-architectural complexity of zinc oxide nanowall hollow microspheres and their structural properties

Zinc oxide (ZnO) hollow spheres with defined morphology and micro-/nanostructure are prepared by a hydrothermal synthesis approach. The materials possess fine-leaved structures at their particle surface (nanowall hollow micro spheres). Morphology control is achieved by citric acid used as an additive in variable relative quantities during the synthesis. The structure formation is studied by various time-dependent ex situ methods, such as scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The fine-leaved surface structure is characterized by high-resolution transmission electron microscopy techniques (HRTEM, STEM), using a high-angle annular dark field detector, as well as by differential phase contrast analysis. In-depth structural characterization of the nanowalls by drop-by-drop ex situ FE-SEM analysis provides insight into possible structure formation mechanisms. Further investigation addresses the thermal stability of the particle morphology and the enhancement of the surface-to-volume ratio by heat treatment (examined by N2 physisorption).

Characterization of Palladium Nanoparticles Produced by Healthy and Microwave-Injured Cells of Desulfovibrio desulfuricans and Escherichia coli.

Numerous studies have focused on the bacterial synthesis of palladium nanoparticles (bio-Pd NPs), via uptake of Pd (II) ions and their enzymatically-mediated reduction to Pd (0). Cells of Desulfovibrio desulfuricans (obligate anaerobe) and Escherichia coli (facultative anaerobe, grown anaerobically) were exposed to low-dose radiofrequency (RF) radiation(microwave (MW) energy) and the biosynthesized Pd NPs were compared. Resting cells were exposed to microwave energy before Pd (II)-challenge. MW-injured Pd (II)-treated cells (and non MW-treated controls) were contacted with H2 to promote Pd(II) reduction. By using scanning transmission electron microscopy (STEM) associated with a high-angle annular dark field (HAADF) detector and energy dispersive X-ray (EDX) spectrometry, the respective Pd NPs were compared with respect to their mean sizes, size distribution, location, composition, and structure. Differences were observed following MWinjury prior to Pd(II) exposure versus uninjured controls. With D. desulfuricans the bio-Pd NPs formed post-injury showed two NP populations with different sizes and morphologies. The first, mainly periplasmically-located, showed polycrystalline Pd nano-branches with different crystal orientations and sizes ranging between 20 and 30 nm. The second NPpopulation, mainly located intracellularly, comprised single crystals with sizes between 1 and 5 nm. Bio-Pd NPs were produced mainly intracellularly by injured cells of E. coli and comprised single crystals with a size distribution between 1 and 3 nm. The polydispersity index was reduced in the bio-Pd made by injured cells of E. coli and D. desulfuricans to 32% and 39%, respectively, of the values of uninjured controls, indicating an increase in NP homogeneity of 30–40% as a result of the prior MWinjury. The observations are discussed with respect to the different locations of Pd(II)-reducing hydrogenases in the two organisms and with respect to potential implications for the catalytic activity of the produced NPs following injury-associated altered NP patterning.

Formation of hollow and solid carbon spheres in thermally stressed jet fuel in the low temperature autoxidation regime

We report the formation of hollow and solid carbon spheres and spherical nanostructures by self-assembly through thermal stressing of jet fuel (Jet A-1) in the autoxidation temperature range. The characterization of Jet A-1 deposits was conducted by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS). Additionally, high resolution mass spectrometry with electro spray ionization source (ESI-MS) have been used to identify large molecular weight compounds in the mass range 350 Da to 1000 Da in the thermally stressed fuel. Fourier transform infrared (FTIR) spectroscopy analysis revealed oxygenated functional groups in the jet fuel treated by flask tests. Compositional analysis of deposit by TEM EDS and scanning transmission electron microscopy (STEM) and a high-angle annular dark field (HAADF) detector identified multiple trace metallic elements such as Cu, Fe, Zn, Sn, Mg, Al and heteroatoms S, N, O. Hollow carbon sphere formation was due to the catalytic activity of trace metals. Spherical particle growth in this method also appeared to follow Ostwald ripening mechanism. Simple flask static tests in the low temperature regime with jet fuel as hydrocarbon source without externally added catalysts thus is a novel method for hollow and solid carbon sphere synthesis and presented in this paper.

Effect of Storage Conditions on the Long-Term Stability of Bactericidal Effects for Laser Generated Silver Nanoparticles.

Silver nanoparticles (AgNPs) are widely used as antibacterial agents, but their antibacterial durability and the influence by storage conditions have not been thoroughly investigated. In this study, AgNPs were produced using a picosecond laser and stored under three different conditions: daylight, dark and cold (4 °C). The antibacterial effects of the laser AgNPs were examined against Escherichia coli in either a 14-day interval (frequent air exposure) or a 45-day interval (less frequent air exposure) using a well-diffusion method until the antibacterial effects disappeared. Results showed that the antibacterial activity of the laser generated AgNPs lasted 266 to 405 days. Frequent air exposure increased particle oxidation as measured by high-angle annular dark-field detector for scanning transmission electron microscopy (HAADF-STEM) and X-ray energy dispersive (EDX) spectroscopy, and reduced the antibacterial duration by about 13 weeks. Compared to the chemically produced AgNPs, the antibacterial effect of the laser AgNPs lasted over 100 days longer when tested in the 45-day interval, but was susceptible to oxidation when frequently exposed to the air. The laser generated AgNPs had lower antibacterial activity when stored in cold compared to that stored at room temperature. This study demonstrated the long lasting antibacterial durability of the laser generated AgNPs. Such information could help design future medical applications for the AgNPs.

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.

Pumping Metallic Nanoparticles with Spatial Precision within Magnetic Mesoporous Platforms: 3D Characterization and Catalytic Application.

The present work shows an efficient strategy to assemble two types of functional nanoparticles onto mesoporous MCM-41 silica nanospheres with a high degree of spatial precision. In a first stage, magnetite nanoparticles are synthesized with a size larger than the support pores and grafted covalently through a peptide-like bonding onto their external surface. This endowed the silica nanoparticles with a strong superparamagnetic response, while preserving the highly ordered interior space for the encapsulation of other functional guest species. Second, we report the finely controlled pumping of preformed Pt nanoparticles (1.5 nm) within the channels of the magnetic MCM-41 nanospheres to confer an additional catalytic functionality to the multiassembled nanoplatform. The penetration depth of the metallic nanoparticles can be explained as a result of the interplay between the particle-wall electrostatic attraction and the repulsive forces between neighboring Pt nanoparticles. A detailed transmission electron microscopy and a 3D high-resolution high-angle annular dark-field detector electron tomography study were carried out to characterize the material and to explain the assembly mechanism. Finally, the performance of these multifunctional nanohybrids as magnetically recoverable catalysts has been evaluated in the selective hydrogenation of p-nitrophenol, a well-known pollutant and intermediate in multiple industrial processes.

Microstructure evolution during heat treatment of Mg–Gd–Y–Zn–Zr alloy and its low-cycle fatigue behavior at 573 K

In as-cast Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr (mole fraction, %) alloy, lamellar microstructures that extend from grain boundaries to the interior of α-Mg grains are identified as clusters of γ′ using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector. Under a total strain-controlled low-cyclic loading at 573 K, the mechanical response and failure mechanism of Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr alloy (T6 peak-aging heat treatment) were investigated. Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and 573 K. The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures and α-Mg matrix and extend along the basal plane of α-Mg. The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.

Determination of three-dimensional atomic positions from tomographic reconstruction using ensemble empirical mode decomposition

Tomographic reconstruction from a tilt series of electron micrographs has raised great interest in materials, chemical, and condensed matters science because of its capability of revealing 3D local atomic structures within nanomaterials. Previous breakthroughs have demonstrated that the positions of individual atoms can not only be visualized but also determined by combining a scanning transmission electron microscope with a high-angle annular dark-field detector, equally sloped tomography, and the filtering/denoising method. However, the filtering/denoising approach—whether imposed on 2D projections or 3D reconstruction—raises concerns regarding the robustness of image processing, the accuracy of atomic positions, and the artificial atoms introduced during filtering. In this article, we report a general method that overcomes these limitations. By removing unphysical oscillations in 2D projections through ensemble empirical mode decomposition and applying a standard Wiener filter to the 3D reconstruction, we are able to determine atomic structures with higher accuracy.

Characterization of intracellular palladium nanoparticles synthesized by Desulfovibrio desulfuricans and Bacillus benzeovorans.

Early studies have focused on the synthesis of palladium nanoparticles within the periplasmic layer or on the outer membrane of Desulfovibrio desulfuricans and on the S-layer protein of Bacillus sphaericus. However, it has remained unclear whether the synthesis of palladium nanoparticles also takes place in the bacterial cell cytoplasm. This study reports the use of high-resolution scanning transmission electron microscopy with a high-angle annular dark field detector and energy dispersive X-ray spectrometry attachment to investigate the intracellular synthesis of palladium nanoparticles (Pd NPs). We show the intracellular synthesis of Pd NPs within cells of two anaerobic strains of D. desulfuricans and an aerobic strain of B. benzeovorans using hydrogen and formate as electron donors. The Pd nanoparticles were small and largely monodispersed, between 0.2 and 8 nm, occasionally from 9 to 12 nm with occasional larger nanoparticles. With D. desulfuricans NCIMB 8307 (but not D. desulfuricans NCIMB 8326) and with B. benzeovorans NCIMB 12555, the NPs were larger when made at the expense of formate, co-localizing with phosphate in the latter, and were crystalline, but were amorphous when made with H2, with no phosphorus association. The intracellular Pd nanoparticles were mainly icosahedrons with surfaces comprising {111} facets and about 5 % distortion when compared with that of bulk palladium. The particles were more concentrated in the cell cytoplasm than the cell wall, outer membrane, or periplasm. We provide new evidence for synthesis of palladium nanoparticles within the cytoplasm of bacteria, which were confirmed to maintain cellular integrity during this synthesis.Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-015-3067-5) contains supplementary material, which is available to authorized users.

The structure of an Al–Rh–Cu decagonal quasicrystal studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy

The structure of an Al–Rh–Cu decagonal quasicrystal formed with two quasiperiodic planes along the periodic axis in an Al63Rh18.5Cu18.5 alloy has been studied by spherical aberration (Cs)-corrected high-angle annular detector dark-field (HAADF)- and annular bright-field (ABF)-scanning transmission electron microscopy (STEM). Heavy atoms of Rh and mixed sites (MSs) of Al and Cu atoms projected along the periodic axis can be clearly represented as separate bright dots in observed HAADF-STEM images, and consequently arrangements of Rh atoms and MSs on the two quasiperiodic planes can be directly determined from those of bright dots in the observed HAADF-STEM image. The Rh atoms are arranged in pentagonal tiling formed with pentagonal and star-shaped pentagonal tiles with an edge-length of 0.76 nm, and also MSs with a pentagonal arrangement are located in the pentagonal tiles with definite orientations. The star-shaped pentagonal tiles in the pentagonal tiling are arranged in τ2(τ: golden ratio)-inflated pentagonal tiling with a bond-length of 2 nm. From arrangements of Rh atoms placed in pentagonal tilings with a bond-length of 2 nm, which are generated by the projection of a five-dimensional hyper-cubic lattice, occupation domains in the perpendicular space are derived. Al atoms as well as Rh atoms and MSs are represented as dark dots in an observed ABF-STEM image, and arrangements of Al atoms in well-symmetric regions are discussed.

Scanning Transmission Electron Tomography of Blood Platelets in Thick Sections

Electron tomography in the scanning transmission electron microscope (STEM) can be performed on sections of stained plastic-embedded tissues or cells of 1 to 2 micrometer thickness without effects of chromatic aberration because there are no imaging lenses after the specimen. By using a small STEM probe convergence angle of 1-2 mrad the geometrical broadening of the probe is restricted, which enables a spatial resolution of a few nanometers. Furthermore, by using an axial bright-field detector instead of the standard high-angle annular dark-field detector, image blurring due to multiple elastic scattering can be reduced in the lower part of the specimen. Here, we have applied STEM tomography to elucidate the 3D ultrastructure of human blood platelets, which are small anucleate blood cells that aggregate to seal leaks at sites of vascular injury and are important in the pathology of atherosclerosis and other diseases. Of particular interest are the morphological changes that occur in alpha-granules, which contain important proteins released when platelets are activated. Axial bright-field STEM electron tomographic tilt series were acquired at an accelerating voltage of 300 kV from 1.5-micrometer thick sections of platelets that had been prepared by rapid freezing and freeze-substitution; and the tomograms were reconstructed from dual-axis tilt series. The tomographic reconstructions revealed changes in ultrastructure that occurred on platelet activation including release of alpha granules through channels connecting to the plasma membrane. The research was supported by the intramural program of the National Institute of Biomedical Imaging and Bioengineering, and the research in the Storrie laboratory was supported in part by NIH grant R01 HL119393.

Interfacial atomic structure analysis at sub-angstrom resolution using aberration-corrected STEM.

The atomic structure of a SiGe/Si epitaxial interface grown via molecular beam epitaxy on a single crystal silicon substrate was investigated using an aberration-corrected scanning transmittance electron microscope equipped with a high-angle annular dark-field detector and an energy-dispersive spectrometer. The accuracy required for compensation of the various residual aberration coefficients to achieve sub-angstrom resolution with the electron optics system was also evaluated. It was found that the interfacial layer was composed of a silicon single crystal, connected coherently to epitaxial SiGe nanolaminates. In addition, the distance between the dumbbell structures of the Si and Ge atoms was approximately 0.136 nm at the SiGe/Si interface in the [110] orientation. The corresponding fast Fourier transform exhibited a sub-angstrom scale point resolution of 0.78 Å. Furthermore, the relative positions of the atoms in the chemical composition line scan signals could be directly interpreted from the corresponding incoherent high-angle annular dark-field image.

Simultaneous Quantification of Indium and Nitrogen Concentration in InGaNAs Using HAADF-STEM

To unambiguously evaluate the indium and nitrogen concentrations in In(x)Ga(1-x)N(y)As(1-y), two independent sources of information must be obtained experimentally. Based on high-resolution scanning transmission electron microscopy (STEM) images taken with a high-angle annular dark-field (HAADF) detector the strain state of the InGaNAs quantum well is determined as well as its characteristic HAADF-scattering intensity. The strain state is evaluated by applying elasticity theory and the HAADF intensity is used for a comparison with multislice simulations. The combination of both allows for determination of the chemical composition where the results are in accordance with X-ray diffraction measurements, three-dimensional atom probe tomography, and further transmission electron microscopy analysis. The HAADF-STEM evaluation was used to investigate the influence of As-stabilized annealing on the InGaNAs/GaAs sample. Photoluminescence measurements show an annealing-induced blue shift of the emission wavelength. The chemical analysis precludes an elemental diffusion as origin of the energy shift--instead the results are in agreement with a model based on an annealing-induced redistribution of the atomic next-neighbor configuration.

Plan-view transmission electron microscopy investigation of GaAs/(In,Ga)As core-shell nanowires

Plan-view transmission electron microscopy in combination with electron energy-loss spectroscopy have been used to analyze the strain and the chemical composition of GaAs/(In,Ga)As core-shell nanowires. The samples consist of an GaAs core and a radially arranged (In,Ga)As layer as quantum well and GaAs outer-shell. The nominal parameters of the quantum well in the two samples under investigation are: an indium concentration of 25% and a quantum well thickness of 22 nm and 11 nm, respectively, while the core and the external shell dimensions are fixed. Scanning transmission electron microscopy using high-angle annular dark field detector was performed to verify the actual dimensions of the layers. Geometric phase analysis was carried out in order to examine the local strain of the radial (In,Ga)As quantum well, while the local chemical composition was determined by means of spatially resolved electron energy-loss spectroscopy. Finite elements calculations were carried out in order to simulate the multi-shell structure and extract the actual strain distribution. The results indicate that there is a uniform strain distribution at the coherent interfaces. In addition, based on calculations, we show that there is no region in the considered core-shell structure absolutely free of strain.

Interfacial Atomic Number Contrast in Thick Samples

A quantitative method to simulate the electron scattering at the interface of two material layers in thick samples is presented here. The samples are analyzed by Scanning Transmission Electron Microscopy (STEM) in a Tecnai F30 using a High Angle Annular Dark-Field (HAADF) detector with a collection range 55 mrad to 245 mrad. At the interface of a high-density and low-density material an increase in the HAADF signal for thick TEM samples can be observed. We report here the simulated HAADF detector intensity across the interface of two materials using Python programming language and its comparison with experimental results. The electron atomic scattering factor f e (s) for a complex potential is given by: