HAADF STEM Research Articles

Microstructural evolution of rhenium Part II: Tension

The microstructural evolution of Re under tension was investigated using EBSD, in-situ and ex-situ TEM. The samples had a texture that suppressed possible {112̅1}〈112̅6〉 twinning, while favoring <a> type 〈112̅0〉 basal dislocations during tensile straining. In-situ tensile straining was performed on rhenium both at room temperature and at 920 °C to investigate the low strain region of deformation, where it was found that <a> type b⃗=[112̅0] screw dislocations operated on the basal planes in loosely aligned slip bands. Through Schmidt factor analysis, EBSD had shown that pyramidal slip activated appreciably only at strain values above half of the failure strain. At failure stresses, <a> type b⃗=[112̅0] basal screw dislocations observed in dislocation slip bands and <c+a> type b⃗=[112̅3] pyramidal screw dislocations formed dislocation nets interfering with <a> type glide. HAADF STEM imaging was used to view the morphology of tension-induced {112̅1}〈112̅6〉 twins at higher strain values. Twin transmission with {112̅1}〈112̅6〉 twins changing twin planes between parent and matrix orientations was observed. The twin structure observed in tension were representative of classical twin structures, contrary to the {112̅1}〈112̅6〉 type twins seen in compression which consisted of twin aggregates. Our results suggest that the compression-tension asymmetry in Re is likely due to the twin favorability of the microstructure forcing the creation of many more twins during compression as compared to tension. The exceptionally-high work hardening rate in Re was found to be mainly due to large scale twin formation coupled with basal slip activity during initial straining.

Cluster structure in amorphous Ti-Ni-Cu alloys subjected to high-pressure torsion deformation

The features of the internal structure's evolution of melt-spun (MS) amorphous Ti50Ni25Cu25 alloy samples subjected to high-pressure torsion (HPT) at 20 and 150 °C were revealed by TEM and STEM – in the planar direction and the cross-sectional direction of the HPT-processed disc-shaped samples.In STEM HAADF observations in the planar direction, areas with brightness contrast with a size of 20 nm are revealed. We assume them to be “amorphous nanoclusters”. In dark-field TEM images, in many areas nanocrystals (with sizes about 5 nm) are revealed, resulting from strain-induced nanocrystallization. In STEM HAADF images, there can be seen nanoclusters (with sizes about 20 nm) separated by bright boundaries. Groups of shear bands (SBs) are revealed on the cross-sectional directions on lamellae cut from the HPT-processed samples. Nanocrystals form directly in the shear bands as a result of strain-induced nanocrystallization. Amorphous nanoclusters are also observed in the amorphous phase surrounding the SBs. Formation of the amorphous nanocluster in the areas surrounding the shear bands has been discovered for the first time. Atomic force microscopy (AFM) was used to study the surface morphology of foils prepared by ion polishing from the initial amorphous ribbons and HPT-processed samples. A hollows-like surface morphology in the HPT-processed state is revealed by AFM, which is reflected as nanocluster contrast in TEM images. The hollows-like surface morphology could be explained by the specifics of etching processes in the HPT-processed state. Brittle fracture of melt-spun Ti50Ni25Cu25 in the as-cast state and after HPT processing is determined during tensile tests. However, observable changes in the fracture surfaces correspond to an increased local ductility in samples subjected to HPT, in comparison with the initial state.

Shock-tube study of the formation of iron, carbon, and iron–carbon binary nanoparticles: experiment and detailed kinetic simulations

ABSTRACTAn experimental and computational study of the formation of pure iron nanoparticles, carbon nanoparticles (soot), and binary carbon-coated iron nanoparticles during the pyrolysis of iron pentacarbonyl–argon, ethylene–argon, and iron pentacarbonyl–ethylene–argon mixtures, respectively, behind reflected shock waves is carried out. The shape and size distribution of these nanoparticles are examined on a Zeiss Ultra plus ultrahigh-resolution field-emission scanning electron microscope. The binary iron–carbon particles were also investigated by high-resolution transmission electron microscopy and high-angle annular dark-field imaging (HAADF STEM) on a FEI Osiris transmission electron microscope equipped with a Bruker SuperX detector. Detailed kinetic simulations of the formation of these three types of particles are performed, which predict the concentration, average size, and size distribution of particles.

A Novel Room‐Temperature Multiferroic System of Hexagonal Lu1−xInxFeO3

AbstractIn the present work, h‐RFeO3 multiferroic ceramics are designed and created by introducing chemical pressure (In‐substitution for Lu) in LuFeO3. Lu1−xInxFeO3 (x = 0‐0.75) ceramics are prepared by the standard solid‐state reaction process. The crystal structure of the present ceramics is tuned from centrosymmetric Pbnm (x = 0) to non‐centrosymmetric P63cm (x = 0.4–0.6), and subsequently to centrosymmetric P63/mmc (x = 0.75), while the Pbnm and P63cm biphase structure is detected for x = 0.25. The Curie temperature for the polar P63cm (x = 0.4–0.6) phase decreases from &gt;1000 to ≈550 K with increasing x. Cloverleaf ferroelectric domain structures are determined in polar Lu0.5In0.5FeO3 samples, and the ferroelectric domain walls at atomic scale are evaluated by the aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM), where the spontaneous polarization of 1.73 µC cm−2 is determined for x = 0.5. The spontaneous polarization is also confirmed by calculating the site displacement from the centrosymmetric phase based on the X‐ray diffraction (XRD) data. Meanwhile, two magnetic transitions are determined for all compositions, that is, paramagnetic to antiferromagnetic transition at Néel temperature TN (≈350 K for x = 0.4–0.6), and antiferromagnetic to weak‐ferromagnetic transition at spin‐reorientation temperature TSR. The co‐presence of ferroelectric and antiferromagnetic orders confirms the present ceramics as promising room‐temperature multiferroic materials.

Synthesis of nanometer-sized fayalite and magnesium-iron(II) mixture olivines

Olivines are divalent orthosilicates with important geologic, biological, and industrial significance and are typically comprised of mixtures of Mg2+ and Fe2+ ranging from forsterite (Mg2SiO4) to fayalite (Fe2SiO4). Investigating the role of Fe(II) in olivine reactivity requires the ability to synthesize olivines that are nanometer-sized, have different percentages of Mg2+ and Fe2+, and have good bulk and surface purity. This article demonstrates a new method for synthesizing nanosized fayalite and Mg-Fe mixture olivines.First, carbonaceous precursors are generated from sucrose, PVA, colloidal silica, Mg2+, and Fe3+. Second, these precursors are calcined in air to burn carbon and create mixtures of Fe(III)-oxides, forsterite, and SiO2. Finally, calcination in reducing CO-CO2 gas buffer leads to Fe(II)-rich olivines. XRD, Mössbauer, and IR analyses verify good bulk purity and composition. XPS indicates that surface iron is in its reduced Fe(II) form, and surface Si is consistent with olivine. SEM shows particle sizes predominately between 50 and 450 nm, and BET surface areas are 2.8–4.2 m2/g. STEM HAADF analysis demonstrates even distributions of Mg and Fe among the available M1 and M2 sites of the olivine crystals. These nanosized Fe(II)-rich olivines are suitable for laboratory studies with in situ probes that require mineral samples with high reactivity at short timescales.

Crystallization of amorphous Cu50Ti50 alloy prepared by high-energy ball milling

Amorphous Cu50Ti50 alloy was prepared by short-term (20 min) high-energy ball milling (HEBM). According to TEM, electron diffraction, and HAADF STEM results, the material was 93% amorphous with an admixture of nanocrystalline particles 2–8 nm in their size. Thermal stability and crystallization of amorphous Cu50Ti50 was studied by differential scanning calorimetry (DSC) and time-resolved XRD (TRXRD). Depending on heating rate, the crystallization of Cu50Ti50 took place within the temperature range 298–397 °C. The activation energy (199 ± 4 kJ/mol) was found by using the Kissinger equation. Time-resolved diffraction patterns have revealed that the phase transition proceeds without melting within a time period of 20–30 s and the onset of crystallization gets started around 300 °C which is markedly lower than the melting temperature in the Cu–Ti system (982 °C). Based on our TRXRD results, a mechanism of the phase transition under study was suggested.

Short-Range Stacking Disorder in Mixed-Layer Compounds: A HAADF STEM Study of Bastnäsite-Parisite Intergrowths

Atomic-scale high angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and electron diffractions are used to address the complexity of lattice-scale intergrowths of REE-fluorocarbonates from an occurrence adjacent to the Olympic Dam deposit, South Australia. The aims are to define the species present within the intergrowths and also assess the value of the HAADF STEM technique in resolving stacking sequences within mixed-layer compounds. Results provide insights into the definition of species and crystal-structural modularity. Lattice-scale intergrowths account for the compositional range between bastnäsite and parasite, as measured by electron probe microanalysis (at the µm-scale throughout the entire area of the intergrowths). These comprise rhythmic intervals of parisite and bastnäsite, or stacking sequences with gradational changes in the slab stacking between B, BBS and BS types (B—bastnäsite, S—synchysite). An additional occurrence of an unnamed B2S phase [CaCe3(CO3)4F3], up to 11 unit cells in width, is identified among sequences of parisite and bastnäsite within the studied lamellar intergrowths. Both B2S and associated parisite show hexagonal lattices, interpreted as 2H polytypes with c = 28 and 38 Å, respectively. 2H parisite is a new, short hexagonal polytype that can be added to the 14 previously reported polytypes (both hexagonal and rhombohedral) for this mineral. The correlation between satellite reflections and the number of layers along the stacking direction (c*) can be written empirically as: Nsat = [(m × 2) + (n × 4)] − 1 for all BmSn compounds with S ≠ 0. The present study shows intergrowths characterised by short-range stacking disorder and coherent changes in stacking along perpendicular directions. Knowing that the same compositional range can be expressed as long-period stacking compounds in the group, the present intergrowths are interpreted as being related to disequilibrium crystallisation followed by replacement. HAADF STEM imaging is found to be efficient for depiction of stacking sequences and their changes in mixed-layer compounds, particularly those in which heavy atoms, such as rare-earth elements, are essential components.

Automatic correction of nonlinear damping effects in HAADF–STEM tomography for nanomaterials of discrete compositions

HAADF-STEM tomography is a common technique for characterizing the three-dimensional morphology of nanomaterials. In conventional tomographic reconstruction algorithms, the image intensity is assumed to be a linear projection of a physical property of the specimen. However, this assumption of linearity is not completely valid due to the nonlinear damping of signal intensities. The nonlinear damping effects increase w.r.t the specimen thickness and lead to so-called “cupping artifacts”, due to a mismatch with the linear model used in the reconstruction algorithm. Moreover, nonlinear damping effects can strongly limit the applicability of advanced reconstruction approaches such as Total Variation Minimization and discrete tomography.In this paper, we propose an algorithm for automatically correcting the nonlinear effects and the subsequent cupping artifacts. It is applicable to samples in which chemical compositions can be segmented based on image gray levels. The correction is realized by iteratively estimating the nonlinear relationship between projection intensity and sample thickness, based on which the projections are linearized. The correction and reconstruction algorithms are tested on simulated and experimental data.

Atomic structure of nano voids in irradiated 3C-SiC

It is important to understand the atomic structure of defect clusters in SiC, a promising material for nuclear application. In this study, we have directly observed and identified nano voids in ion irradiated 3C-SiC at 800 °C, 20 dpa through ABF and HAADF STEM images. A quantitative method was used to analyze HAADF images in which atomic columns with a difference in the number of atoms could be identified and scattered intensities can be computed. Our result shows that these voids are composed of atomic vacancies in an octahedral arrangement. The density of the void was measured by STEM to be 9.2 × 1019m−3 and the size was ∼1.5 nm.

Interfacial tuning for the nucleation of liquid AlCu alloy

Heterogeneous nucleation interface structure of Al-Cu alloys was tuned through solute composition and substrate lattice structure in this study to achieve a sophisticated nucleation control. The tuning mechanism at atomic level was systematically investigated using high angle annular dark field scanning transmission electron microscopy (HAADF STEM) and electron energy loss spectroscopy (EELs) techniques. The results show that the solutal element and substrate both can be used to modify the lattice matching of the nucleation interface, where alloying element Cu was found to adsorb at the nucleation interface tuning the lattice matching between new crystal and substrate rather than embed in the matrix distorting the lattice structure of the matrix. The substrate structure can also affect the preferred crystal orientation of new crystals instead. A relationship between the nucleation potency (undercooling) of the alloys and the lattice misfit was further proved, which may lead to a quantitative approach for the nucleation control of Al.

Towards production of novel catalyst powders from supported size-selected clusters by multilayer deposition and dicing

A multilayer deposition method has been developed with the potential to capture and process atomic clusters generated by a high flux cluster beam source. In this deposition mode a series of sandwich structures each consisting of three layers—a carbon support layer, cluster layer and polymer release layer—is sequentially deposited to form a stack of isolated cluster layers, as confirmed by through-focal aberration-corrected HAADF STEM analysis. The stack can then be diced into small pieces by a mechanical saw. The diced pieces are immersed in solvent to dissolve the polymer release layer and form small platelets of supported clusters.

HAADF STEM and PL Characterization of Monolayer-Thick GaN/(Al,Ga)N Quantum Wells for Deep UV Optoelectronics Applications

HAADF STEM and PL Characterization of Monolayer-Thick GaN/(Al,Ga)N Quantum Wells for Deep UV Optoelectronics Applications

Spatial Distribution of the Vanadium Atomic Species in MoVTeO and MoVTeNbO Oxide Catalysts as Revealed by High‐Angle Annular Dark‐Field Scanning Transmission Electron Microscopy

AbstractHigh‐angle annular dark‐field scanning transmission electron microscopy (STEM–HAADF) was used to characterize a light‐alkane mild‐oxidation catalyst corresponding to two different MoVTe(Nb)O M1 phases. For that purpose a semiautomatic integration routine was developed, which allows the analysis of several hundred atomic columns with a normalization procedure to quantitatively determine the vanadium content of mixed (V, Mo) crystallographic sites with a low statistical variability. The obtained results were compared with data previously obtained on other M1 phases with different compositions. The comparison showed that the V distribution over the sites was not strongly influenced by the presence or absence of niobium, the method of synthesis used, and the nature of the Te or Sb cations in the hexagonal channels, but depended mainly on the total V content of the phases. Extrapolation to low V content enabled a discussion on the possible location of the vanadium active sites.

Correlation of Reduced Interlayer Charge Transfer with Antiphase Boundary Formation in BixSn1–xSe–NbSe2 Heterostructures

Heterostructures of alloyed BixSn1–xSe layers, 0 ≤ x ≤ 1.0, interleaved with NbSe2 monolayers, were prepared by using the modulated elemental reactants technique to investigate the occurrence of antiphase boundaries as a function of Bi concentration. A Rietveld refinement of the c‐axis structure of the x = 0.50 compound revealed a reduced gap distance between the Bi plane in the BixSn1–xSe layers and the Se plane in the NbSe2 layers and an increased internal Se–Se plane spacing within the BixSn1–xSe layers relative to the end member compounds, suggesting increased interaction between the layers at this composition. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM) images showed no antiphase boundary formation in the BixSn1–xSe layers for x = 0.24, some antiphase boundary formation when x = 0.50, and increased antiphase boundary formation for x = 0.73. Correlated with the absence of antiphase boundary formation for values of x &lt; 0.50 and the increasing presence of antiphase boundaries for x ≥ 0.5, electrical transport measurements showed interlayer charge transfer initially increased with x, then decreased as x was increased to 0.50 and beyond.

High-surface-area, iron-oxide films prepared by atomic layer deposition on γ-Al2O3

High-surface-area iron oxides were prepared by Atomic Layer Deposition (ALD) on 130-m2/g γ-Al2O3 for use as a catalyst support. Measurements of the sample mass, surface area, and pore-size distribution as a function of the number of ferrocene-O2 ALD cycles at 623K suggested that the iron oxide grew as a dense, conformal film with a growth rate similar to 0.016-nm per cycle. While films with 20 ALD cycles (20Fe2O3-Al2O3, 0.25g Fe2O3/g Al2O3) were difficult to distinguish by HAADF STEM, EDS mapping indicated the Al2O3 was uniformly coated. Raman Spectroscopy showed the films were in the form of Fe2O3; but XRD measurements on samples with as many as 100 ALD cycles (100Fe2O3-Al2O3, 0.84g Fe2O3/g Al2O3) showed no evidence for crystalline iron-oxide phases, even after calcination at 1073K. Specific rates for the water-gas-shift (WGS) reaction on the ALD-coated samples were significantly lower than those on bulk Fe2O3. However, addition of 1wt.% Pd to Fe2O3/Al2O3 supports prepared by ALD exhibited specific rates that were much higher than that observed when 1wt.% Pd was added to Fe2O3/Al2O3 prepared by conventional impregnation of Fe salts, suggesting more uniform contact between the Pd and FeOx phases on samples prepared by ALD.

HAADF-STEM characterization and simulation of nanoparticle distributions in an inhomogeneous matrix.

Measuring with a high accuracy the size distribution of small metallic nanoparticles loaded in a mesoporous metal oxide matrix is of particular interest for many studies related to new generations of interesting metamaterials. Transmission electron microscopy (TEM) is a powerful tool to determine the nature and morphology of very small particles, but their reliable and automatic identification in an inhomogeneous environment where the nanoparticle/background contrast locally varies is not straightforward. Here, we present how a quantitative analysis of high-angle annular dark field scanning TEM (HAADF STEM) images, accounting for the chemical sensitivity of the technique, can improve the accuracy of semiautomatic segmentation methods based on morphological processing to calculate size histograms. The paper also provides an estimate of the reliability of this method through the analysis of numerically synthesized images. The latter are based on the simulation of HAADF STEM projections of a volume filled with titania, pores and silver particles, whose morphological features, such as dimensions, shapes and densities are evaluated from experimental measurements of real samples. The results obtained with synthesized images prove the performances of the quantitative analysis to suppress nonsilver nanoparticles from the statistics and allow to infer empirical rules to determine imaging parameters that ensure a good reliability of histograms.

Modulation Doping in Metastable Heterostructures via Kinetically Controlled Substitution

Controlling carrier concentration is critical in many device applications, and both chemical substitution and modulation doping have been used in industry. For most inorganic materials, very low doping efficiencies are observed as site occupancies depend on both thermodynamic and kinetic factors. We demonstrate that we can make kinetically controlled site-specific substitutions in a series of (BixSn1–xSe)1+δTiSe2 compounds using the modulated elemental reactants method. These compounds were characterized using a combination of X-ray diffraction, resistivity and Hall coefficient measurements, and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). For small x, the doping efficiency is 0.7, close to that observed for B in silicon. For higher x values, a structural distortion is observed in X-ray diffraction data in which the symmetry of the in-plane unit cell decreases. HAADF STEM data reveals the presence of antiphase boundaries (Bi–Bi pairs) in the BixSn1–xSe layers, whic...

Silica subnanometer-sized nodes, nanoclusters and aggregates in Cyanate Ester Resin-based networks: Structure and properties of hybrid subnano- and nanocomposites

A series of the densely cross-linked Cyanate Ester Resins (CER)-based composites with 0.01–10wt.% silica, introduced via sol-gel process, were synthesized using dicyanate ester of bisphenol E (DCBE), tetraethoxysilane (TEOS) and γ-aminopropyltrimethoxysilane (APTMS). Their structure, molecular dynamics and properties are investigated by HAADF STEM, EDXS, DMA, DSC, and FTIR and Far-IR spectroscopies. The extremely high “constrained dynamics” effect and the most enhanced thermal, relaxation and elastic properties are revealed for the composites with ultralow silica contents, e.g., up to 0.1wt.%, in the absence of silica nano- or microclusters: increasing Tg by 50°C and dynamic modulus by 60% at 20°C and as much as forty times at 260°C are observed. EDXS analysis including getting the histograms of Si content shows that chemically embedded silica units are distributed quasi-regularly in nanovolumes of amorphous CER matrix in this case. The results obtained lead to conclusion that at ultralow silica contents only subnanometer-sized silica nodes may be generated within the CER network, i.e., the hybrid polymer subnanocomposites are formed. Formation of silica nanoclusters and, especially, their aggregates of hundreds nanometers in size at silica contents of 2–10wt.% results in their negative impact on the matrix properties.

Locating light and heavy atomic column positions with picometer precision using ISTEM

Recently, imaging scanning transmission electron microscopy (ISTEM) has been proposed as a promising new technique combining the advantages of conventional TEM (CTEM) and STEM (Rosenauer et al., 2014 [1]). The ability to visualize light and heavy elements together makes it a particularly interesting new, spatially incoherent imaging mode. Here, we evaluate this technique in term of precision with which atomic column locations can be measured. By using statistical parameter estimation theory, we will show that these locations can be accurately measured with a precision in the picometer range. Furthermore, a quantitative comparison is made with HAADF STEM imaging to investigate the advantages of ISTEM.

Corrigendum: Carbon Nanotubes as Support in the Platinum-Catalyzed Hydrolytic Dehydrogenation of Ammonia Borane.

In the Full Paper by Duan, Chen et al., incorrect units are shown for the scale bars of the transmission electron microscopy (TEM) images in Figure 1. In each case, the correct unit is nanometer (nm), instead of centimeter (cm). The figure is shown here with correct units. Typical HAADF–STEM images of fresh catalysts as well as deactivated ones after 5 cycles: (a) Pt/CNT-P, (b) Pt/CNT-O, and (c) Pt/CNT-D. The authors and the editorial office apologize for this oversight, and for any inconvenience caused.