HAADF-STEM Images Research Articles

The role of MOFs in Thin-Film Nanocomposite (TFN) membranes

Incorporation of MOFs in interfacially polymerized Thin-Film Nanocomposite (TFN) membranes has widely been shown to result in increased membrane performance. However, the exact functioning of these membranes is poorly understood as large variability in permeance increase, filler incorporation and rejection changes can be observed in literature. The synthesis and functioning of TFN membranes (herein exemplified by ZIF-8 filled polyamide (PA) membranes prepared via the EFP method) was investigated via targeted membrane synthesis and thorough characterization via STEM-EDX, XRD and PALS. It is hypothesized that the acid generated during the interfacial polymerization (IP) at least partially degrades the crystalline, acid-sensitive ZIF-8 and that this influences the membrane formation (through so-called secondary effects, i.e. not strictly linked to the pore morphology of the MOF). Nanoscale HAADF-STEM imaging and STEM-EDX Zn-mapping revealed no ZIF-8 particles but rather the presence of randomly shaped regions with elevated Zn-content. Also XRD failed to show the presence of crystalline areas in the composite PA films. As the addition of the acid-quenching TEA led to an increase in the diffraction signal observed in XRD, the role of the acid was confirmed. The separate addition of dissolved Zn2+ to the synthesis of regular TFC membranes showed an increase in permeance while losing some salt retention, similar to observations regularly made for TFN membranes. While the addition of a porous material to a TFC membrane is a straightforward concept, all obtained results indicate that the synthesis and performance of such composite membranes is often more complex than commonly accepted.

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.

Utilising unit-cell twinning operators to reduce lattice thermal conductivity in modular structures: Structure and thermoelectric properties of Ga2O3(ZnO)9

The Ga2O3(ZnO)m family of homologous compounds have been identified as potential thermoelectric materials, but properties are often limited due to low densification. By use of B2O3 as an effective liquid phase sintering aid, high density, high quality ceramic samples of Ga2O3(ZnO)9 have been synthesised. The atomic structure and local chemical composition of Ga2O3(ZnO)9 have been determined by means of high resolution X-ray diffraction and atomic resolution STEM-HAADF, EDS and EELS measurements. X-ray analysis showed that the compound crystalizes in the Cmcm orthorhombic symmetry. Atomically resolved HAADF-STEM images unambiguously showed the presence of nano-sized, wedge-shaped twin boundaries, parallel to the b-axis. These nano-scale structural features were chemically investigated, for the first time, revealing the exact distributions of Zn and Ga; it was found that Ga ions occupy sites at the junction of twin boundaries and inversion boundaries. HAADF-EDS analysis showed that the calcination step has a significant impact on crystal structure homogeneity. By use of a sintering aid and optimization of processing parameters the ceramics achieved a low thermal conductivity of 1.5–2.2 W/m.K (for the temperature range 300–900 K), a power factor of 40–90 μW/K.m2, leading to a ZT of 0.06 at 900 K. The work shows a route to exploit nanoscale interface features to reduce the thermal conductivity and thereby enhance the thermoelectric figure of merit in complex thermoelectric materials.

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

Summary The most striking difference between the trivalent rare-earth cations and their alkali and alkaline-earth peers is the presence of localized 4 f electrons. Placing trivalent rare-earth cations inside fullerene molecules or in between blocks of intermetallics has given rise to a plethora of fascinating properties and materials. A long-sought-after yet undiscovered material is a semiconducting compound with rare-earth cations situated inside the oversized polyhedral cages of a three-dimensional framework. In this work, we present a synthesis of unconventional clathrates, Ba 8−x R x Cu 16 P 30 , with the smallest pentagonal dodecahedral cages encapsulating La 3+ and Ce 3+ . Their composition and crystal structure were unambiguously determined by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances hole mobility and reduces hole concentration, resulting in a drastic (700%) increase in the thermoelectric performance.

Investigating the Doping Effect of Single Transition Metal Atoms on Basal Planes of MoS2 Monolayer Nanosheets for Electrochemical Hydrogen Evolution Reaction

Surface sites of extensive exposed basal planes of MoS2 nanosheets for the first time have been doped with isolated transition metal atoms as 2D monolayer catalysts for the electrochemical hydrogen evolution reaction (HER). Our HAADF-STEM images clearly show that single transition metal atoms are situated on these multiple surface basal sites that subtly modify the electro-catalytic activities of the monolayer MoS2 dependent on the electronic and stereospecific properties. From the results of DFT calculations, it is found that these dopers play important roles in tuning the hydrogen adsorption enthalpies on exposed surface S atoms and Mo atoms in HER. Our electrochemical testing, characterization and computational modelling demonstrate that Co can significantly enhance the HER activity by the dominant Co-S interaction whereas Ni substantially lowers the HER rate due to the Ni-Mo interaction on the same basal site despite the two transition metal elements are neighbored to each other in the periodic table. We believe that both the geometric and electronic factors exerted by transition metal dopants are important parameters in further tuning the 2D MoS2 structure for rational design of the composite materials for more efficient electrochemical hydrogen production from water.

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.

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.

Composition determination of multinary III/V semiconductors via STEM HAADF multislice simulations

Knowledge of the microscopic elemental composition of multinary III/V semiconductor materials is crucial to the development of functionalized opto-electronic devices. Well-proven composition analysis methods, such as high resolution X-ray diffraction (HRXRD), fail to determine the elemental composition when more than three atomic species are involved. In this work we propose a procedure for the composition analysis of multinary III/V semiconductors at atomic resolution using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image simulations. Our method exploits the dependence of HAADF-STEM image intensities on the atomic number and static atomic displacements (SAD) at different detector inner angles. Here, we describe the proposed method in detail using Ga(NAsP) as an example multinary material.

Topotactic Reduction toward a Noncentrosymmetric Deficient Perovskite Tb0.50Ca0.50Mn0.96O2.37 with Ordered Mn Vacancies and Piezoelectric Behavior

Low-temperature reduction of perovskite Tb0.5Ca0.5MnO3–x yields novel crystal-structured noncentrosymmetric compound Tb0.50Ca0.50Mn0.962.33+O2.37, which unusually crystallizes in cubic lattice I23 (a ∼ 15.27 A) based on a 4ap × 4ap × 4ap expansion relative to the simple cubic perovskite unit cell. Rietveld refinements and HAADF-STEM images are used for the structure determination, revealing a rare-typed metal-anion coordination framework which consists of corner-shared tetrahedra and pyramids, and edge-shared bipyramids and octahedra. 2/64 B-site Mn-ordered vacancies are observed for the first time acting as the apex and body center of the I lattice in reduced systems. Room-temperature piezoelectricity is detected, with a quasistatic d33 value of ∼0.32 pC N–1 and inverse d33 value of ∼10.5 pm V–1. This phase primarily exhibits antiferromagnetic ordering below TN ∼ 70 K, with ferromagnetic responses resulted from spin-canting below 40 K. This work provides a new way toward synthesizing unconventional acent...

Cs-corrected HAADF-STEM observations on the structural modulations caused by charge density wave and Te-vacancy ordering in LaTe&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;minus;&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;&amp;delta;&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;

Microstructural features play a critical role for the understanding of the essential properties of novel functional materials and new devices. Atomic-resolution scanning transmission electron microscopy (STEM) is invaluable for determining the average atomic structure and local structural defects. For strongly correlated electron materials, STEM can be applied to distinguish the structural modulation caused by charge density wave (CDW), chemical ordering (vacancy or impurity atoms). RTe2− δ (R=La, Ce) compounds have attracted recent attention due to their effective low dimensionality. The materials play host to a CDW state above room temperature and can be described in terms of a modulated Cu2Sb-type structure (P4/ nmm ) based on alternating layers of square-planar Te sheets and a corrugated RTe slab. Furthermore, pressure-induced superconductivity in CeTe1.82 with T C of 2.7 K has been reported, suggesting that the nonstoichiometric Te defects are correlated to superconductivity in this material system. Here, we report the study of the structural modulations in LaTe2− δ using STEM. The LaTe2− δ single crystal was grown by self-flux technique. The TEM samples used in the present study were prepared by crushing the well- characterized single crystal, and then the resultant suspensions were dispersed on a holey carbon-covered Cu grid. Electron diffraction experiments were performed in the FEI Tecnai F20 microscope, and HRTEM and high angle annular dark field (HAADF) STEM were performed in the JEOL ARM200F equipped with double aberration correctors and cold field emission gun at room temperature. The experiment result revealing the charge density wave in LaTe2− δ can be tuned by the Te content; the structural modulation correlated with charge density wave can be characterized by a modulation wave vector of q CDW=(1/2− α ) a ∗, where α is the incommensurate parameter determined by the chemical composition. Our experiment data demonstrate that the Cs-corrected HAADF-STEM image can directly reveal the atomic displacements in the Te plane due to electron-phonon coupling. Detailed analysis suggests that the Te atomic displacements adopt an incommensurate wave-pocket structure along each Te-chain with a long periodicity determined by the CDW incommensurability. In addition to the q CDW=(1/2− α ) a ∗ CDW modulation, a superstructure with the vector q 2=1/5(3 a ∗+ b ∗) has been also observed in some regions. In previous study, this modulation was proposed to be correlated with CDW instability, however, recent experimental and theoretical analysis on the electronic structure (FS) shows that there is no such nesting wave vector could be identified. In this paper, our Cs-STEM observations directly demonstrated that the q 2 superstructure actually originates from an imperfect stoichiometry in this layered system. We proposed a 5 × 5 supercell associated with the Te vacancy ordering with the chemical composition of LaTe1.85 based on detailed STEM data analysis. From HRTEM images, it is also noted that the CDW modulation ( q CDW) totally disappears in the region with Te vacancy ordering, suggesting that the CDW can not coexist in the crystals with the Te vacancy ordering. The possible correlation between the ordering of the nonstoichiometric Te defects and the superconductivity in this material system needs to be further investigated.

Quantitative HAADF STEM of SiGe in presence of amorphous surface layers from FIB preparation

The chemical composition of four Si1−xGex layers grown on silicon was determined from quantitative scanning transmission electron microscopy (STEM). The chemical analysis was performed by a comparison of the high-angle annular dark field (HAADF) intensity with multislice simulations. It could be shown that amorphous surface layers originating from the preparation process by focused-ion beam (FIB) at 30 kV have a strong influence on the quantification: the local specimen thickness is overestimated by approximately a factor of two, and the germanium concentration is substantially underestimated. By means of simulations, the effect of amorphous surface layers on the HAADF intensity of crystalline silicon and germanium is investigated. Based on these simulations, a method is developed to analyze the experimental HAADF-STEM images by taking the influence of the amorphous layers into account which is done by a reduction of the intensities by multiplication with a constant factor. This suggested modified HAADF analysis gives germanium concentrations which are in agreement with the nominal values. The same TEM lamella was treated with low-voltage ion milling which removed the amorphous surface layers completely. The results from subsequent quantitative HAADF analyses are in agreement with the nominal concentrations which validates the applicability of the used frozen-lattice based multislice simulations to describe the HAADF scattering of Si1−xGex in STEM.

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.

Direct observation of charged domain walls in hybrid improper ferroelectric (Ca,Sr)3Ti2O7

We investigated ferroelectric (FE) domain wall structures including “charged domain walls” of hybrid improper FE (Ca,Sr)3Ti2O7 at the subatomic resolution by dark-field transmission electron microscopy (TEM) and high-resolution state-of-the-art aberration-corrected high-angle annular-dark-field (HAADF) scanning transmission electron microscopy (STEM). Dark-field TEM and high-resolution HAADF-STEM images obtained in the FE phase of single crystals of Ca2.46Sr0.54Ti2O7 revealed the formation of abundant charged domain walls with the head-to-head and tail-to-tail configurations in the FE domain structure, in addition to the FE 180° domain structure. The charged domain walls with the head-to-head and tail-to-tail FE polarizations exist stably and can be characterized as the unique double arc-type displacement of Ca/Sr ions in a unit cell without charge accumulation.

(Invited) Achievement of Ultralow Contact Resistivity of Metal/n+-Ge Contacts with Zr-N-Ge Amorphous Interlayer

1) Introduction Ge is of great interest as a channel material in future CMOS devices. A bottleneck for achieving Ge-CMOS is the Fermi-level pinning (FLP) phenomenon. Recently, the S/D formation for the Ge n-MOSFET has remarkably progressed, in which several group have reported the low contact resistance for metal/n+-Ge contacts. Manik et al. have demonstrated the low contact resistivity (ρ C) of 1.4×10−7 Ω·cm2 on epitaxial n+-Ge (2.5×1019 cm−3) using a thin ZnO interlayer capped with Ti [1]. Chen et al. have reported a low ρ C of 3×10−7 Ω·cm2 on epitaxial n+-GeSn (1019 cm−3) with the O2 plasma treatment [2].We also reported a low ρ C of 4×10−7 Ω·cm2 on n+-doped Ge (3.9×1019 cm−3) using a thin Zr-N-Ge amorphous interlayer capped with Ti and with appropriate annealing [3]. In this presentation, we focus on the ZrN/Ge contact and demonstrate the unique feature of this contact. First, we show that thin amorphous interlayer (a-IL) is formed between ZrN and Ge during the sputter deposition, which induces the strong FLP alleviation. Second, we show that only the a-IL can be retained on Ge surface, which enables us to fabricate metal/a-IL/Ge contacts with different metals. We demonstrate electrical properties of metal/a-IL/Ge and metal/a-IL/n+-Ge contacts with post metallization annealing (PMA) and the a-IL remarkably modulate the barrier height and ρ C. 2) Electrical properties of ZrN/Ge contacts ZrN films were directly deposited on (100) Ge by rf magnetron sputtering. The contacts with no-PMA and 450°C-PMA showed hole barrier heights (ΦBP) of 0.40 and 0.56 eV, respectively. The HAADF-STEM images showed a-ILs with thicknesses of 1.4 and 1.8 nm exist at interfaces in ZrN/Ge contacts with no-PMA and 450°C-PMA, respectively, leading to large FLP alleviation. The ρ C of ZrN contact on n+-Ge (3.9×1019 cm-3) was as high as 6.3×10-4 Ω·cm2[4], suggesting that the series resistance of the a-IL is high. Fortunately, a ZrN film is easily etched by dilute HF solution, and the a-IL can be retained on Ge surface. This enables us to fabricate metal/a-IL/Ge contacts with different metals. 3) Electrical properties of metal/a-IL/Ge contacts We tried to fabricate the low ρ C contact on the n+-Ge using this chemical feature. For ρ c measurements, we used the circular transmission line measurement (CTLM). The substrate was p-Ge (100) with a resistivity of 0.2 Ω·cm. We used the thermal diffusion for fabricating the n+-Ge. The surface P concentration (N D) and the P diffused depth were 3.9×1019 cm-3 and 0.5 μm from SIMS measurements, respectively. Then, a ZrN film was deposited on the substrate with ring-patterned photoresist using a ZrN target, followed by removal by dilute HF solution. Then Zr, Ag, Al, Ti, or Ni was deposited on the a-IL/n+-Ge. After the metal deposition, the PMA was carried out in the temperature range of 350-450°C for 10 min. It was confirmed from a dark field-STEM image of a Ag/a-IL/Ge contact without PMA that an a-IL was clearly observed between the Ag film and the Ge substrate. In this study, metal/n+-Ge contacts without a-IL were also fabricated. For electron barrier height (ΦBN) estimation, the metal/n-Ge contacts and metal/a-IL/n-Ge contacts were also formed. All metal/Ge contacts without a-IL showed the rectified features, which are the feature of usual metal/Ge contacts. On the other hand, Ag/a-IL/n-Ge contact showed ohmic behavior, and Ti/a-IL/n-Ge showed weak rectifying feature. These results suggest that a-IL contributes the low ΦBN. All contacts without a-IL showed higher ρ c (>10-5 Ω·cm2). On the other hand, contacts with a-IL showed lower ρ c. It was found that Ag and Ti contacts on n+-Ge represent extremely low ρ c (4.4 – 7.2×10-7 Ω·cm2). In particular, the ρ c of Ti/a-IL/n+-Ge contact was three orders of magnitude lower than that without a-IL. These results suggest that the reaction between metal and a-IL causes to be thinning the a-IL, and a result, the series resistance of a-IL is decreased while maintaining the low ΦBN. Acknowledgement This work was supported by (JSPS) KAKENHI (grant No. 26289090) and was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

P-CoOx/n-SnO2 nanostructures: New highly selective materials for H2S detection

Nanostructures p-CoOx/n-SnO2 based on tin oxide nanowires have been prepared by two step CVD technique and characterized in detail by XRD, XRF, XPS, HAADF-STEM imaging and EDX-STEM mapping. Depending on the temperature of decomposition of cobalt complex during the second step of CVD synthesis of nanostructures cobalt oxide forms a coating and/or isolated nanoparticles on SnO2 nanowire surface. It was found that cobalt presents in +2 and +3 oxidation states. The measurements of gas sensor properties have been carried out during exposure to CO (14ppm), NH3 (21ppm), and H2S (2ppm) in dry air. The opposite trends were observed in the effect of cobalt oxide on the SnO2 gas sensitivity when detecting CO or NH3 in comparison to H2S. The decrease of sensor signal toward CO and NH3 was attributed to high catalytic activity of Co3O4 in oxidation of these gases. Contrary, the significant increase of sensor signal in the presence of H2S was attributed to the formation of metallic cobalt sulfide and removal of the barrier between p-CoOx and n-SnO2. This effect provides an excellent selectivity of p-CoOx/n-SnO2 nanostructures in H2S detection.

Coordination-Supported Imidazolate Networks: Water- and Heat-Stable Mesoporous Polymers for Catalysis.

The poor water stability of most porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) is widely recognized as a barrier hampering their practical applications. Here, a facile and scalable route to prepare metal-containing polymers with a good stability in boiling water (100 °C, 24 h) and air (up to 390 °C) is presented. The bifunctional 1-vinylimidazole (VIm) with a coordinating site and a polymerizable organic group is introduced as the building block. This core strategy includes the synthesis of a rigid monomer with four VIm branches through a coordination process at room temperature, followed by a radical polymerization. We refer to this material as coordination-supported imidazolate networks (CINs). Interestingly, CINs are composed of rich mesopores from 2-15 nm, as characterized by low-energy (60 kV) STEM-HAADF images. In particular, the stable CINs illustrate a high turnover frequency (TOF) of 779 h-1 in the catalytic oxidation of phenol with H2 O as the green solvent.

Deactivation of electrically supersaturated Te-doped InGaAs grown by MOCVD

Achieving and sustaining the highest doping level possible in InGaAs is critical for the reduction of contact resistance in future microelectronic applications. Tellurium (Te) is a very promising n-type dopant with high reported n-type doping densities. However, the stability of this dopant during post-growth thermal processing is unknown. Supersaturated Te-doped InGaAs layers were grown by MOCVD at 500 °C. The electrically active concentration of Te doping was 4.4 × 1019 cm−3 as grown. The thermal stability of the Te was investigated by studying the effect of post-growth annealing between 550 and 700 °C on the electrical activation. At all temperatures, the electrical activation decreased from its starting electron concentration of 4.4 × 1019 cm−3 down to 6–7 × 1018 cm−3. The rate of deactivation was measured at each temperature, and the activation energy for the deactivation process was determined to be 2.6 eV. The deactivation could be caused by either Te–Te clustering or a Te-point defect reaction. HAADF-STEM images showed no visible clustering or precipitation after deactivation. Based on previous ab initio calculations that suggest the VIII population increases as the Fermi level moves toward the conduction band, it is proposed that formation of isolated point defect complexes, possibly a Te–VIII complex, is associated with the deactivation process.

Cobalt location in p-CoOx/n-SnO2 nanocomposites: Correlation with gas sensor performances

Nanocomposites CoOx/SnO2 based on tin oxide powders with different crystallinity have been prepared by wet chemical synthesis and characterized in detail by ICP-MS, XPS, EPR, XRD, HAADF-STEM imaging and EDX-STEM mapping. It was shown that cobalt is distributed differently between the bulk and surface of SnO2 nanocrystals, which depends on the crystallinity of the SnO2 matrix. The measurements of gas sensor properties have been carried out during exposure to CO (10 ppm), and H2S (2 ppm) in dry air. The decrease of sensor signal toward CO was attributed to high catalytic activity of Co3O4 leading to oxidation of carbon monoxide entirely on the surface of catalyst particles. The formation of a p-CoOx/n-SnO2 heterojunction results in high sensitivity of nanocomposites in H2S detection. The conductance significantly changed in the presence of H2S, which was attributed to the formation of metallic cobalt sulfide and removal of the p – n junction.

Study of PtPd Bimetallic Nanoparticles for Fuel Cell Applications

Bimetallic nanoparticles are of special interest for their potential applications for fuel cells, mainly for portable power applications. Among the bimetallic systems, Pt-Pd bimetallic nanoparticles have received great interest as they can be widely used as effective catalysts for various electrochemical reactions. In this work, Pt-Pd alloy bimetallic nanoparticles were synthesized through a chemical reduction method. The nanoparticles were characterized using aberration-corrected scanning/transmission electron microscopy (STEM). Also, parallel beam X-Ray diffraction analysis was carried out to evaluate the crystallographic structure. High-angle annular dark field (HAADF)-STEM images of the Pt-Pd bimetallic nanoparticles were obtained. The contrast of the images shows that the nanoparticles have an alloy structure with an average size of 7.15 nm. To understand the properties of the bimetallic nanoparticles, it is necessary to know the distribution of the elements in the nanostructure. We have used a semi-quantitative method to analyze the HAADF-STEM images, which allowed us to measure the total intensity of the scattered electrons for each atomic column. HAADF-STEM images of the Pt-Pd bimetallic nanoparticles were compared directly with image simulations, good agreement between simulation and experimental images was found. Cyclic voltammetry studies were carried out to analyze the electrochemical behavior of the bimetallic nanoparticles.

(Invited) The Evolution of W-Co-C Catalyst during the CVD Growth of Chirality-Selective Growth of Single-Walled Carbon Nanotubes

Bimetallic catalysts such as Fe-Co and Co-Mo have been used for efficient growth of bulk single-walled carbon nanotube (SWNTs) and vertically aligned SWNTs [1]. Recently, bimetallic catalysts such as Co-W [2] or Co-Cu [3] are employed for structure controlled growth of SWNTs. Here, roles of bimetallic catalysts are explored by newly proposed SiO2 grid based in-plane TEM technique, which enables a direct TEM characterization of catalysts on SiO2, CVD growth of SWNTs in normal furnace, and TEM characterization of catalysts and grown nanotubes after CVD. The in-plane TEM results of the traditional Co-Mo bimetallic catalysts were consist with our previous report [1]; metallic Co nanoparticle embedded in Co-Mo oxide is responsible for the nucleation of SWNTs. By using Co-Cu catalysts, we can synthesize vertically aligned SWNTs with subnanometer diameters on quartz (and SiO2/Si) substrates [3]. EDS-STEM and HAADF-STEM imaging of the Co-Cu bimetallic catalyst system showed that small Co catalysts were captured and anchored by adjacent Cu nanoparticles, which grew small diameter of SWNTs. High-melting point W6Co7 alloy produced from metal cluster precursors is reported to grow a single chirality (12,6) with over 90 % abundance [2]. Here, we show that a sputtered Co-W catalyst can selectively grow (12,6) SWNTs by CVD at lower reduction and growth temperatures [4]. Statistical Raman mapping analysis and optical absorption spectrum of the as-grown SWNTs reveal that the abundance of (12,6) is over 60 %. The morphology and structure of catalyst is investigated by the in-plane TEM before and after CVD growth. The catalyst alloy we produced was W6Co6C and the only metallic Co was left after the CVD of 5 min. By comparing the catalyst TEM images with variable CVD period, we can discuss the evolution of the alloy catalysts. The W rich initial catalyst is composed of bcc W bound with W6Co6C. Once ethanol is introduced, additional W will be gradually removed to the gas phase product of WO3 and the ratio of W decreases. It is speculated that after the formation of pure W6Co6C, further loss of W will result the precipitation of Co; nucleation of Co cluster which is primary responsible for growth of an SWNT. This work was supported by JSPS KAKENHI Grant Numbers JP25107002, JP15H05760, and IRENA Project by JST-EC DG RTD, Strategic International Collaborative Research Program, SICORP. Part of this work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). We also acknowledge supports from Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.