Dark-field Transmission Electron Microscopy Research Articles

Synthesis and Characterization of W80Ni10Mo10 alloy produced by mechanical alloying

The present study aims at synthesis and characterization of nanostructured W80Ni10Mo10 (wt. %) alloy produced by mechanical alloying (MA). Elemental powders of tungsten (W), nickel (Ni), molybdenum (Mo) were subjected to mechanical attrition in a high energy planetary ball-mill using chrome steel as grinding media and toluene as a process control agent. The crystallite size and lattice strain of the nanostructured powders at different stages of milling (0 h to 20h) was calculated from the X-ray diffraction patterns (XRD). The crystallite size of W in W80Ni10Mo10 powder was reduced from 100 μm to 55 nm at 10 h and farther reduction to 40 nm at 20 h of milling with increase in lattice strain of 0.25% at 20 h of milling. The lattice parameter of tungsten showed initial expansion upto 0.03% at 10 h of milling and then contraction upto 0.04% at 20 h of milling. The scanning electron microscopy (SEM) also showed mixed morphology of W80Ni10Mo10 powders consisting spherical and elongated particles after 20 h of milling. SEM analysis also revealed that particle size reduced from 100 μm to 2 μm with an increase in the milling time from 0 to 20 hours. The dark-field Transmission Electron Microscopy (TEM) observations revealed that the crystallite size of W in milled W80Ni10Mo10 alloy is in good agreement with calculated crystallite size from XRD.

Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia)

Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the \(\left\langle {0 10} \right\rangle_{R} \left\{ { 10 1} \right\}_{R} ||\left\langle {00 1} \right\rangle_{H} \left\{ { 1 10} \right\}_{H}\) orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact \(\left\langle {0 10} \right\rangle_{R} \left\{ { 40 1} \right\}_{R} ||\left\langle {00 1} \right\rangle_{H} \left\{ { 1 70} \right\}_{H}\). The intergrowth is dictated by the formation of \(\left\{ { 1 70} \right\}_{H} |\left\{ { 40 1} \right\}_{R}\) equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile–rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga samples. Using a HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian–ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier–Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form \(\left\{ { 30 1} \right\}_{R} |\left\{ {0 30} \right\}_{H}\) equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian–ilmenite crystals was close to Ilm67Hem33, typical for Fe–Ti-rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian–ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO2.

An Experimental Protocol Development of Three-Dimensional Transmission Electron Microscopy Methods for Ferrous Alloys: Towards Quantitative Microstructural Characterization in Three Dimensions

The majority of engineering steels are ferromagnetic and structurally inhomogeneous on scales ranging from nanometers to micrometers, and their physical properties depend on the three-dimensional (3D) features in their microstructures. Thus, obtaining a 3D image with a large field of view is desirable for transmission electron microscopy (TEM) based microstructure characterization in order to establish the relationship between the microstructure and the physical properties with a reasonable statistical relevancy. Here, we use a conventional sample preparation process, i.e., mechanical polishing followed by electropolishing, and optimizing experimental protocols for electron tomography (ET) of ferromagnetic materials, to carry out microstructural characterization of engineering steel. We determined that the sample thickness after the mechanical polishing step is a critical experimental parameter affecting the success rate of tilt-series image acquisitions. For example, for ferritic heat-resistant 9Cr steel, mechanical thinning down to 30 μm or less was necessary to acquire an adequate tilt-series image of the carbide precipitates in the annular dark-field scanning TEM (ADF-STEM) mode. However, acquiring tilt-series images of dislocation structures remains a challenge due to an unavoidable, significant electron beam deflection during specimen tilt, even with a thinned sample. To overcome the electron beam deflection problem, we evaluated several relatively accessible approaches including the “Low-Mag STEM and Lorentz TEM” modes. Although rarely used for ET, both modes reduce or even zero the objective lens current, likely weakening the magnetic interference between the ferromagnetic specimen and the objective lens magnetic field. The advantages and disadvantages of these experimental components are discussed.

Randomly oriented twin domains in electrodeposited silver dendrites

Silver dendrites were prepared by electrochemical deposition. The structures of Ag dendrites, the type of twins and their distribution were investigated by scanning electron microscopy (SEM), Z-contrast high angle annular dark field transmission electron microscopy (HAADF), and crystallografically sensitive orientation imaging microscopy (OIM). The results revealed that silver dendrites are characterized by the presence of randomly distributed 180? rotational twin domains. The broad surface of dendrites was of the {111} type. Growth directions of the main dendrite stem and all branches were of <112> type.

Phase change properties of ZnSb-doped Ge2Sb2Te5 films

ZnSb-doped Ge2Sb2Te5 films have been deposited by magnetron co-sputtering using separated ZnSb and Ge2Sb2Te5 alloy targets. The concentrations of ZnSb dopant in the ZnSb-added Ge2Sb2Te5 films, measured by using energy dispersive spectroscopy (EDS), are identified to be 5.4, 9.9, 18.7 and 24.3 at. %, respectively. X-ray diffraction (XRD), in situ sheet resistance measurements, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), are used to analyze the relationships among the composition, structures and properties of the films. The sheet resistance as a function of the temperature (R-T) is in situ measured using the four-probe method in a home-made vacuum chamber. It is found that the crystallization temperature of ZnSb-doped Ge2Sb2Te5 films are much higher than that of conventional Ge2Sb2Te5 (～168℃). The higher crystallization temperature is helpful to improve the amorphous thermal stability. Data retention can be obtained by the extrapolated fitting curve based on the Arrhenius equation. It is shown that the values of 10-yr data retention for ZnSb-doped Ge2Sb2Te5 films are higher than that of conventional Ge2Sb2Te5 film (～ 88.9℃). XRD patterns of the as-deposited films when annealed at 200℃, 250℃, 300℃, and 350℃ show that ZnSb-doping can suppress the phase transition from fcc phase to hex phase. XPS spectra are further used to investigate the binding state of (ZnSb)18.7(Ge2Sb2Te5)81.3, suggesting that the Zn–Sb and Zn–Te bonds may exist in an amorphous state. In addition, we have measured the dark-field TEM images, selected area electron diffraction patterns, and high-resolution transmission electron microscopy images of the (ZnSb)18.7(Ge2Sb2Te5)81.3 films. Apparently, the films show a uniform distribution of crystalline phase with the dark areas surrounded by bright ones (Zn–Te or Zn–Sb domain). A static tester using pulsed laser irradiation is employed to investigate the phase transition behavior in nanoseconds. Results show that the ZnSb-doped Ge2Sb2Te5 films exhibit a faster crystallization speed. Among these samples, the (ZnSb)24.3(Ge2Sb2Te5)75.7 film exhibits a higher crystallization temperature of 250℃ and the 10 years data retention is 130.1℃. The duration of time for crystallization of (ZnSb)24.3(Ge2Sb2Te5)75.7 is revealed to be as short as ～64 ns at a given proper laser power 70 mW. A reversible repetitive optical switching behavior can be observed in (ZnSb)24.3(Ge2Sb2Te5)75.7, confirming that the ZnSb doping is responsible for a fast switching and the compound is stable with cycling. These excellent properties indicate that the (ZnSb)24.3(Ge2Sb2Te5)75.7 film is a potential candidate as the high-performance phase change material.

Structural Heterogeneity of the Melt-spun (Fe, Co)-Si-B-P-Cu Alloy with Excellent Soft Magnetic Properties

A structural heterogeneity of a newly developed soft magnetic Fe81.2Co4Si0.5B9.5P4Cu0.8 melt-spun alloy has been studied by transmission electron microscopy (TEM) and electron diffraction. Hollow-cone dark-field TEM imaging revealed that the density of coherent scattering regions in the as-quenched Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy is as high as that in the Fe85.2Si2B8P4Cu0.8 hetero-amorphous alloy (NANOMET®). According to the aberration-corrected high-resolution TEM, crystalline atomic clusters, typically of ∼1nm in diameter, are densely distributed in an amorphous matrix of Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy, while nanobeam electron diffraction detected weak Bragg reflections from Fe clusters with the body centered cubic structure. These results unambiguously reveal structural heterogeneity of the as-quenched Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy, which is similar to that observed in the NANOMET®.

Unravelling orientation distribution and merging behavior of monolayer MoS2 domains on sapphire.

Monolayer MoS2 prepared by chemical vapor deposition (CVD) has a highly polycrystalline nature largely because of the coalescence of misoriented domains, which severely hinders its future applications. Identifying and even controlling the orientations of individual domains and understanding their merging behavior therefore hold fundamental significance. In this work, by using single-crystalline sapphire (0001) substrates, we designed the CVD growth of monolayer MoS2 triangles and their polycrystalline aggregates for such purposes. The obtained triangular MoS2 domains on sapphire were found to distributively align in two directions, which, as supported by density functional theory calculations, should be attributed to the relatively small fluctuations of the interface binding energy around the two primary orientations. Using dark-field transmission electron microscopy, we further imaged the grain boundaries of the aggregating domains and determined their prevalent armchair crystallographic orientations with respect to the adjacent MoS2 lattice. The coalescence of individual triangular flakes governed by unique kinetic processes is proposed for the polycrystal formation. These findings are expected to shed light on the controlled MoS2 growth toward predefined domain orientation and large domain size, thus enabling its versatile applications in next-generation nanoelectronics and optoelectronics.

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

Grains of pyrochlore and secondary phases from tailings of Silver Crater Mine in Bancroft, Ontario (Canada) have been studied to understand the alteration processes, redox conditions, and retention of pyrochlore-derived species (U, Ti, Nb, Pb, Ta, REE) in near-field environments. Alteration processes are documented by the formation of two types of co-existing secondary veins associated with primary apatite and calcite: (i) amorphous Fe-rich veins, 46–75wt.% of FeO, and ∼500ppm of UO2, and (ii) crystalline calcite-rich veins, found in fractures and penetrating the pyrochlore. Based on electron microprobe analysis (EMPA), the chemical composition of the pyrochlore is: (Ca0.84U0.35Fe0.20Na0.09Pb0.04Ln0.04Mn0.03Sr0.01Th0.01Mg0.01)1.62 (Nb1.00Ti0.87Ta0.10Si0.02)2.0O6.5F0.14. Elemental mapping revealed that migration of liberated U, Pb, Nb, Ta, Ti, and REE, is confined to the secondary veins of Fe-rich and calcite-rich compositions. Transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM), energy dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS) results showed that pyrochlore contains nanoparticulate inclusions of uraninite, galena, and magnetite, while secondary veins host betafite, magnetite, Pb0, cerusite, and 10Å mica nanoparticles (NPs). Randomly oriented uraninite NPs, 15nm in size, concentrate around pores, 50–100nm in size, in the pyrochlore. In the Fe-rich veins, HAADF-STEM images revealed that U, Pb, Nb, and Ti were sequestered in the form of spherical betafite NPs, <800nm in size, with composition: (Ca1.1Fe0.35Pb0.28U0.09)1.83(Ti1.56Nb0.44)2.0O6.1. The association of betafite NPs, magnetite, and Pb0 NPs in Fe-rich and calcite-rich veins indicates reducing conditions during alteration of pyrochlore and immobilization of pyrochlore derived elements. This observation combined with identification of nanoscale galena and magnetite in pyrochlore, and the association of Pb0 and Fe3O4 in veins, indicate relatively low fS2 and fO2 conditions during pyrochlore alteration. In spite of prolonged exposure (⩾20years) to atmospheric conditions, pyrochlore and betafite NPs retained <25wt.% and <6wt.% of UO2, respectively; and no secondary uranyl phases were observed. The alteration of pyrochlore most likely began with metamictization, followed by volume swelling, fracturing and surface interactions with fluids that caused mobilization of major and minor elements. The occurrence of amorphous Fe-rich material on the surface of the pyrochlore suggests that amorphous gels could form in Fe-rich environments as an alteration product of crystalline waste forms. The nano-geochemical complexity of the samples investigated here suggests that there is a significant nano-scale component to the sequestration of actinides during the alteration of natural and likely synthetic materials.

Influences of two-step growth and off-angle Ge substrate on crystalline quality of GaAs buffer layers grown by MOVPE

GaAs buffer layers were grown on off-angle Ge substrates with a two-step growth process by metalorganic vapor phase epitaxy in order to prepare a high quality buffer layer for the InGaAsN p–i–n solar-cell structure. In our contribution, we present results from the optimized two-step growth of GaAs buffer layers, namely with a low-temperature step at 470°C and a high-temperature step at 580°C, combined with the use of Ge substrates misoriented by 4° and 6° towards [110]. HRXRD results showed that by using the off-angle substrates, the FWHM of (004) rocking curves was decreased to 6.7sec, which is about 4 times the FWHM of the GaAs commercial substrate. A decreased FWHM indicates a narrower distribution of crystal orientation. Furthermore, a smooth surface with a RMS roughness of 0.7nm was clearly observed by AFM. Cross-sectional dark-field TEM images showed the GaAs buffer layer with 20–30nm diamond-shaped anti-phase domains (APDs) at the GaAs/Ge interface, followed by APDs-free GaAs regions on the 6° off-angle Ge substrate. However, anti-phase boundaries were generated along the [001] direction for the buffer layer on the on-axis substrate. Our results demonstrated that the use of 6° off-angle Ge substrate and a two-step growth process allow the suppression of APDs in the GaAs buffer layer.

Enhanced methanol oxidation activity of Au@Pd nanoparticles supported on MWCNTs functionalized by MB under ultraviolet irradiation

A successful approach to assemble Au core Pd shell (Au@Pd) nanoparticles on the surface of multi-walled carbon nanotubes functionalized by methylene blue (MB) (Au@Pd/fuv-MWCNTs) was reported. In this method, MWCNTs were functionalized under ultraviolet irradiation. UV–Vis analysis and high-angle annular dark-field transmission electron microscope (HAADF-TEM) image prove that core–shell structure of Au@Pd nanoparticles forms. TEM results indicate that Au@Pd nanoparticles (~5.2 nm) are well-dispersed on the surface of fuv-MWCNTs. X-ray photoelectron spectroscopy (XPS) reveals that ultraviolet irradiation can promote the interaction between Au@Pd nanoparticles and the functional groups on the surface of MWCNTs. Cyclic voltammograms (CV), chronoamperograms (CA), and electrochemical impedance spectroscopy (EIS) results demonstrate that the Au@Pd/fuv-MWCNTs catalysts show excellent electrocatalytic performance for methanol oxidation in alkaline media. The catalytic activity of the Au@Pd/fuv-MWCNTs is ~2 times higher than that of the commercial Pd/C catalysts. This is mostly attributed to that ultraviolet irradiation can make the moieties of MB provide a uniform surface with active and anchoring sites, and improves the functional effect of MB on the surface of MWCNTs. Especially, ultraviolet irradiation modifies electronic structure of Pd and is beneficial for the enhancement of catalytic activity.

Investigation of the effectiveness of SiNWs used as an antireflective layer in solar cells

Morphological, optical and electrical properties of SiNWs are studied under different preparation conditions. Despite an ultra-low reflectivity revealing a high light-trapping effect, we obtained a degradation of the resistivity and the minority carrier lifetime in SiNWs which are ascribed to a high electron-trapping. For solar cells application, we determine a trade-off between optical and electrical properties of SiNWs. However, even with this optimization, we obtained a degradation of the electrical parameters of the solar cell when SiNWs are used as compared to the cell without SiNWs. From the obtained results in this work, we put in evidence for the first time that despite the very low reflectivity that a SiNWs film has, its use in PV is faced to four major challenges; (1): an unbalancing between the length of SiNWs and the electron diffusion length, (2): a thick SiO2 layer covering silicon wires formed during the solar cell processing (observed using dark field transmission electron microscopy), (3) a high density of dangling bonds of porous-SiNWs when the SiO2 film is etched, and, (4) a poor metal/SiNWs front contact.

Dark-field transmission electron microscopy of cortical bone reveals details of extrafibrillar crystals.

In a previous study we showed that most of the mineral in bone is present in the form of "mineral structures", 5-6nm-thick, elongated plates which surround and are oriented parallel to collagen fibrils. Using dark-field transmission electron microscopy, we viewed mineral structures in ion-milled sections of cortical human bone cut parallel to the collagen fibrils. Within the mineral structures we observe single crystals of apatite averaging 5.8±2.7nm in width and 28±19nm in length, their long axes oriented parallel to the fibril axis. Some appear to be composite, co-aligned crystals as thin as 2nm. From their similarity to TEM images of crystals liberated from deproteinated bone we infer that we are viewing sections through platy crystals of apatite that are assembled together to form the mineral structures.

Electrical properties and domain sizes of graphene films synthesized by microwave plasma treatment under a low carbon concentration

We performed Hall effect measurements and Raman mapping of graphene synthesized by microwave plasma treatment under a low carbon concentration. The Hall mobility and average domain size were estimated to be 100–1000cm2/Vs and around 30–100nm, respectively, which are much higher than those of graphene deposited by conventional plasma chemical vapor deposition. In addition, dark-field transmission electron microscopy images showed domain sizes of around 100nm. Thus, this method is expected to easily produce a large amount of high-quality graphene with high throughput.

Targeted nano analysis of water and ions in the nucleus using cryo-correlative microscopy.

The cell nucleus is a crowded volume in which the concentration of macromolecules is high. These macromolecules sequester most of the water molecules and ions which, together, are very important for stabilization and folding of proteins and nucleic acids. To better understand how the localization and quantity of water and ions vary with nuclear activity, it is necessary to study them simultaneously by using newly developed cell imaging approaches. Some years ago, we showed that dark-field cryo-Scanning Transmission Electron Microscopy (cryo-STEM) allows quantification of the mass percentages of water, dry matter, and elements (among which are ions) in freeze-dried ultrathin sections. To overcome the difficulty of clearly identifying nuclear subcompartments imaged by STEM in ultrathin cryo-sections, we developed a new cryo correlative light and STEM imaging procedure. This combines fluorescence imaging of nuclear GFP-tagged proteins to identify, within cryo ultrathin sections, regions of interest which are then analyzed by STEM for quantification of water and identification and quantification of ions. In this chapter we describe the new setup we have developed to perform this cryo-correlative light and STEM imaging approach, which allows a targeted nano analysis of water and ions in nuclear compartments.

Transmission electron microscopy analysis of the crystallography of precipitates in Mg–Sn alloys aged at high temperatures

The crystallographic orientation relationships (ORs) of precipitated β-Mg2Sn particles in Mg–9.76 wt% Sn alloy aged at 573 K for 5 h, corresponding to its peak hardness, were investigated by advanced transmission electron microscopy (TEM). OR-3 of (110)β//(0001)αand [\overline 111]β//[1\overline 210]αand OR-4 of (110)β//(0001)αand [001]β//[2\overline 1\overline 10]αare the key ORs of β-Mg2Sn particles in the alloy. The proportions of β-Mg2Sn particles exhibiting OR-3 and OR-4 were determined as 75.1 and 24.3%, respectively. Crystallographic factors determined the predominance of OR-3 in the precipitated β-Mg2Sn particles. This mechanism was analyzed by a three-dimensional invariant line model constructed using a transformation matrix in reciprocal space. Models of the interface of precipitated β-Mg2Sn and the α-Mg matrix were constructedviahigh-resolution TEM and atomic resolution high-angle annular dark-field scanning TEM.

Palladium assisted hetroepitaxial growth of an InAs nanowire by molecular beam epitaxy

The palladium (Pd) assisted epitaxial growth of technologically important InAs nanowires grown on GaAs{111}B substrates using molecular beam epitaxy is reported. The grown free-standing InAs nanowires adapted a vapor–liquid–solid growth mechanism. The impacts of the catalyst particle density, growth temperature and input V/III precursor ratio have been investigated to identify better growth conditions for getting high-density non-<111>-orientated InAs nanowires. We assert here that two kind of nanowires are observed, one having a pure zinc-blende crystalline structure free of stacking faults, and the other with a defect-free wurtzite crystalline structure. However, few of them have defect imperfections too. The L- and Y-shaped nanowires confirm similar surface free energies for possible <110> growth directions. These unusual growth directions are attributed to the effect of the catalyst material as well as the surface-induced strain at the interface between the grown nanowires with substrates. The structural features of the grown nanowires are studied by employing scanning and transmission electron microscopic techniques. The obtained TEM results confirm that the nanowire catalyst interface is not a straightforward zinc-blende structured nanowire. Energy dispersive x-ray (EDX) analysis reveals that the tip of the grown nanowires with the chemical composition of Pd and In have a nearly 50:50 ratio, while the nanowire body did not have any catalyst traces other than the composition of InAs for both type of nanowires. The obtained high angle annular dark field (HAADF) TEM image for both types of nanowires along with the intensity profile provided evidence for cubic as well as hexagonal facets.

Design of 3-Dimensionally Self-Assembled CeO2 Nanocube as a Breakthrough Catalyst for Efficient Alkylarene Oxidation in Water

This paper reports preparation of CeO2 nanocrystals by using oleic acid as a surfactant. A strong coordinating property of oleic acid toward (100) planes of the nanocrystals facilitated three-dimensional (3D) assembly via oriented attachment of the nanocrystal building blocks to form (100) surface exposed porous cubic ceria by sharing {111} facets. As-synthesized materials were characterized by HRTEM, SEM, XRD, FTIR, Raman spectroscopy, and BET surface area analyzer. 3D morphology was established by STEM-HAADF technique and dark-field TEM. As-synthesized CeO2 nanocubes showed breakthrough in catalytic performance, enabling quantitative pX conversion and absolute selectivity in terephthalic acid (>99%). The presence of higher concentrations of oxygen vacancies (1.42 × 1021 cm–3), owing to preferred exposed surfaces and lower individual crystallites size (6 nm), and larger surface area and pore size have been reasoned for high catalytic effectiveness. The catalyst recyclability experiments showed only 6% activity loss in TA yield in the fourth cycles.

Revealing Intricacies of Nano-Sized Intermetallic GaPd2Catalysts

The use of intermetallic compounds in catalysis is a promising means of achieving well-defined catalytic active sites with high selectivity. Simultaneously, it is desirable to nanostructure the catalyst to maximize activity. However in nanoparticulate form, structures and properties quite distinct from bulk systems may arise. Thorough characterization of the nano-sized catalyst is therefore required, for which the high-resolution analysis provided by transmission electron microscopy (TEM) can be invaluable. The directly interpretable imaging provided by aberration-corrected annular dark-field scanning TEM especially, can reveal atomic-scale intricacies and intrapopulation heterogeneities with clarity. Recently, a promising new class of selective hydrogenation catalyst has emerged based on intermetallic Ga-Pd compounds [1]. In foundational studies, use of bulk or powdered model systems led to insights into the relationship between structural and electronic properties and catalytic performance. Aiming for industrial applicability, attention is now being given to high-performance nanoparticulate Ga-Pd catalysts. Through high-resolution imaging, spectroscopic and 3D tomographic TEM studies, significant insight into the crystallographic status of unsupported Ga-Pd nanocatalysts, GaPd2 in particular, has been possible [2,3]. Further to direct verification of the distinctive intermetallic structure in nano-sized particles, catalytically significant and crystallographically intriguing deviations compared to the 'ideal' bulk crystal are revealed. These include strong surface segregation, lattice relaxation and particularly interesting morphologies of the small (&lt;10 nm) particles that comprise both nanocrystalline 'fcc-like' and 'non-crystallographic' five-fold twinned structures. The extent to which the intermetallic structure may be maintained in nanoscale morphologies and the nature of the resultant catalytically active sites are important aspects to be addressed.

NbC Precipitation and Deformation of SS 347H Crept at 850°C

Niobium stabilized 347H stainless steel is a candidate alloy for fuel cladding of Generation IV Supercritical Water-Cooled Reactors (SCWR). The creep properties and microstructural evolution of SS 347H are vitally important for the proposed SCWR challenging design conditions. The high temperature mechanical properties and phase stability of stainless steels is linked to formation, dissolution and coarsening of precipitates [1]. In solution annealed and aged condition, type 347 stainless steel forms NbC precipitates which stabilise the steel against grain boundary precipitation of M23C6 to reduce sensitizati knowledge, microstructural investigation on crept specimens at 850°C is not available. In this study precipitation and deformation behaviours of a commercial SS 347H plate (17%Cr, 9%Ni, 0.36%Nb, 0.04%C) in the as received, aged at 850°C for 1244h (i.e., TEM sample taken from the head of creep specimen) and the creep conditions were examined using the FEI TEM -ray detection technology operating at 200kV. In Fig 1(A) typical NbC precipitates within the matrix in the as received condition are shown. Figure 1(B) is an HRTEM image of one of these precipitates that appears to be coherent with the matrix. An overlay of Nb EDX map on a high angle annular dark field (HAADF) image of this sample is shown in Fig 1(C), highlighting uniformly distributed precipitates. In the aged condition, Fig 2 presents a series of micrographs showing distribution of the NbC precipitates within the matrix and also at grain boundaries. In the crept sample, Fig. 3(A) is a STEM-HAADF image with CBED inset representing [011] zone axis and Fig. 3(B) shows the corresponding EDX elemental map highlighting the NbC precipitates of the crept specimen. These precipitates were found to be aligned towards slip direction (Fig. 3(B)). The slip lines visible in Fig 3(A) were analysed in HRTEM image of Fig 3(C), where the inset is the corresponding fast Fourier transform (FFT). From the FFT pattern the ( 11 1) spot was used to form the masked inversed FFT shown in Fig 3(D), illustrating the effect of slip in ( 11 1) planes. Figure 3(E) is a dark field TEM image formed using (1 11 ) reflection of a cubic NbC precipitate circled in Fig 3(B). Figures 3(F) and (G) compare the deformation in the matrix of the aged and the crept samples. In the high resolution STEM images, Moire fringes magnify the effect of defects within the matrix, and are indicative of the deformation. Arrows point to the image of the dislocations in Moire fringes and illustrate severe deformation in the crept sample as opposed to the aged sample. Figure 3(G) also implies that the NbC precipitate (circled in 3(B)) is oriented towards slip lines of creep deformation. In all samples larger NbC precipitates were also found in grain boundaries. It is interesting to note that no sigma, Z-phase (CrNbN) or M23C6 precipitates were observed in the aged specimens. Based on published data presence of the sigma phase in the aging regime we used is expected [3]. This study shows that NbC precipitates were oriented towards slip direction during creep at 850°C and can hinder the slip process. Study of longer creep test specimens is underway.

Carbon impurities on graphene synthesized by chemical vapor deposition on platinum

We report nanocrystalline carbon impurities coexisting with graphene synthesized via chemical vapor deposition on platinum. For certain growth conditions, we observe micron-size island-like impurity layers which can be mistaken for second graphene layers in optical microscopy or scanning electron microscopy. The island orientation depends on the crystalline orientation of the Pt, as shown by electron backscatter diffraction, indicating growth of carbon at the platinum surface below graphene. Dark-field transmission electron microscopy indicates that in addition to uniform single-crystal graphene, our sample is decorated with nanocrystalline carbon impurities with a spatially inhomogeneous distribution. The impurity concentration can be reduced significantly by lowering the growth temperature. Raman spectra show a large D peak, however, electrical characterization shows high mobility (∼8000 cm2/Vs), indicating a limitation for Raman spectroscopy in characterizing the electronic quality of graphene.