High-angle Annular Dark-field Technique Research Articles

Atomic segregation at twin boundaries in a Mg-Ag alloy

Segregation of solute atoms at twin boundaries (TBs) plays a critical role in mechanical properties and thermal stability of magnesium alloys. Here, segregation structures at {101¯1}, {101¯2} and {101¯3} TBs are characterized in a Mg-Ag alloy by means of the atomic resolution high-angle annular dark-field technique based on scanning transmission electron microscopy. Of particular finding is the unique complex segregation at {101¯3} TBs, where Ag atoms occupy both substitutional and interstitial sites. By contrast, Ag atoms only substitutionally segregate at {101¯1} and {101¯2} TBs. Calculation simulation of segregation energy and three-dimensional structure of TBs helps understanding of hybrid segregation.

Microstructural and Compositional Characterisation of Electronic Materials

<p class="p1"> </p><p class="p2"><span class="s1"> </span>Microstructural and compositional characterisation of electronic materials in support of the development of GaAs, GaN, and GaSb based multilayer device structures is described. Electron microscopy techniques employing nanometer and sub-nanometer scale imaging capability of structure and chemistry have been widely used to characterise various aspects of electronic and optoelectronic device structures such as InGaAs quantum dots, InGaAs pseudomorphic (pHEMT), and metamorphic (mHEMT) layers and the ohmic metallisation of GaAs and GaN high electron mobility transistors, nichrome thin film resistors, GaN heteroepitaxy on sapphire and silicon substrates, as well as InAs and GaN nanowires. They also established convergent beam electron diffraction techniques for determination of lattice distortions in III-V compound semiconductors, EBSD for crystalline misorientation studies of GaN epilayers and high-angle annular dark field techniques coupled with digital image analysis for the mapping of composition and strain in the nanometric layered structures. Also, <em>in-situ </em>SEM experiments were performed on ohmic metallisation of pHEMT device structures. The established electron microscopy expertise for electronic materials with demonstrated examples is presented.</p>

Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: Nanometer-Scale Compositional and Morphological Changes

Electrochemical measurements showed an Pt surface area loss and an specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated “” catalyst particles in a proton exchange membrane fuel cell after voltage cycling between 0.65 and vs reversible hydrogen electrode. Transmission electron microscopy, scanning transmission electron microscopy, associated X-ray energy dispersive spectroscopy, and high angle annular dark-field techniques were used to probe the microstructural changes of the MEA cathode and the compositional changes along the MEA cathode thickness and within individual nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell particles was largely responsible for the reduction in the specific activity of nanoparticle after potential cycling. The Pt weight loss associated with the formation of Pt crystallites near the cathode/membrane interface largely contributed to the measured electrochemical surface area loss, while particle growth of the particles via Ostwald ripening played a lesser role.

Relation between inhomogeneous structure and transport properties for superconductor-insulator transitions

In the study of superconductor-insulator transitions of high-Tc cuprates complicated behaviors in transport properties were observed for example quasi-reentrant behaviors. In this study we report that the relation between microscopic structure and transport properties for Bi2Sr2CaCu2O8+y crystal with substitution of Ca by Y. From the energy dispersive X-ray spectroscopy it was estimated that samples contain almost 45% Y when the starting composition was only 10% Y. Also the values of c-axis length were almost constant and shorter than those of Y free samples. However there were various ρ(T) behaviors dependent on sample: localized, quasi-reentrant, broad superconducting transition etc. All sheet resistances defined per CuO2 bilayer in the normal state were much larger than the quantum resistance h/4e2 or 6450 Ω. We were able to observe the inhomogeneous distribution of Y using a combination of electron energy loss spectroscopy and a high-angle annular dark-field technique in a scanning transmission electron microscope. The Y rich region formed the belt with about 20 nm width. This inhomogeneous structure seems to be the origin of various transport properties.

The modification of MoO3 nanoparticles supported on mesoporous SBA-15: characterization using X-ray scattering, N2 physisorption, transmission electron microscopy, high-angle annular darkfield technique, Raman and XAFS spectroscopy

MoO3 was dispersed onto mesoporous SBA-15 by using ammonium heptamolybdate as MoO3 source. The formation of MoO3 was carried out by heating the loaded material to 500 °C for 3 h in air. Below 13 wt% Mo loading, no reflections of MoO3 occur in the X-ray powder patterns and even for high MoO3 contents, the intensities of the reflections are much lower than expected for fully crystalline material. A detailed XAFS analysis reveals that at low Mo contents, the metastable hexagonal modification of MoO3 is formed despite the high calcination temperature of 500 °C. It is highly likely that the nanosize of the particles and the interaction between MoO3 and SBA-15 stabilize the metastable modification of the material. Nitrogen physisorption experiments show the typical type-IV isotherms indicating that the mesoporosity of the materials is preserved despite the large amount of MoO3. Transmission electron micrographs demonstrate the presence of MoO3 inside the SBA-15 support. The Raman spectra display a remarkable size-dependent intensity loss and several features give evidences for a bond formation between nano-sized MoO3 particles and the silica support. Moreover, the spectroscopic details suggest the formation of (MoO3)n oligomers. MoO3 nanoparticles are successfully introduced into the pores of mesoporous SBA-15. Up to about 13 wt% Mo the material is amorphous and even for higher loadings a large amount of MoO3 is still not crystalline. Nitrogen physisorption and transmission electron microscopy evidences that the mesoporosity of the material is retained. At low Mo loading, the metastable hexagonal modification of MoO3 appears to be stabilized by the interaction with the SBA-15 support material (XAFS).

Structural, Mössbauer, and transport studies of the icosahedral quasicrystalsAl55Si7Cu25.5Fe12.5,Al62.5Cu24.5Fe13 and thecrystalline 1/1 approximant Al55Si7Cu25.5Fe12.5

As-quenched icosahedral Al55Si7Cu25.5Fe12.5,its 1/1 approximant of the same composition, and icosahedralAl62.5Cu24.5Fe13 alloys have been studied using x-ray diffraction, scanning transmission electronmicroscopy and the high-angle annular dark field technique, zero-field and in-field57Fe Mössbauer spectroscopy, and electrical resistivity. The crystal structure of the1/1 approximant Al55Si7Cu25.5Fe12.5 has been refined with the Rietveld method and shown to be compatiblewith the measured high-angle annular dark field images. The distributionof the principal component of the electric field gradient tensor has a bimodalcharacter with a dominant negative sign in the icosahedral Al–Cu–Fe system.The local order of the Fe structural environment is compared in icosahedralAl55Si7Cu25.5Fe12.5, its1/1 approximant,and icosahedral Al62.5Cu24.5Fe13.The average quadrupole splitting decreases with temperature asT3/2 for all alloys studied, and its value is significantly larger for the icosahedral alloys. Thevibrations of the Fe atoms in the alloys studied are well described by a Debye model, withcharacteristic Mössbauer temperatures of 468(25), 487(19), and 455(6) K for icosahedralAl55Si7Cu25.5Fe12.5, its1/1 approximant,and icosahedral Al62.5Cu24.5Fe13,respectively. The electrical resistivity is discussed in terms of quantum interference effectsand structural disorder.

Structural study of a superlattice Al-Ni-Ru decagonal quasicrystal using high-resolution electron microscopy and a high-angle annular dark-field technique

A high-quality superlattice Al-Ni-Ru decagonal quasicrystal with 0.4nm periodicity, formed in the conventionally solidified Al70Ni20Ru10 alloy, has been studied by high-resolution electron microscopy and a high-angle annular dark-field (Z-contrast) technique. It has been clearly revealed that its structure is characterized as an aperiodically ordered arrangement of decagon-shaped atomic columnar clusters which have a diameter of 2nm and show pentagonal symmetry. On the basis of high-resolution electron microscopy structure images, and the atom-resolution Z-contrast observations which highlight the transition-metal sites, the arrangement of atoms in the superlattice decagonal quasicrystal are proposed.

Imaging dislocations with an annular dark-field detector

High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has been recently the subject of active research because of its successful applications to the characterization of supported catalysts, interface problems in MBE grown semiconductors, superconductors and X-ray multilayers. The characteristics of HAADF images are different from those of the TEM images. For perfect single crystals the HAADF signal is mainly generated from thermal diffuse scattering. HAADF technique has also been used to study dopant contrast effects in semiconductors and dislocations have also been observed with an ADF detector. In this paper we report a study of dislocation contrasts and their dependence on the inner collection angle of the ADF detector.The STEM instrument used for the observations was the HB5 from VG Microscopes, Ld., modified by the addition of an ultra-high resolution pole piece (Cs = 0.8 mm) and a two-dimensional detector system. Post specimen lenses and various beam stops were used to change the inner (α) and outer (β) collection angles of the ADF detector.

High-angle annular dark-field microscopy of Mo/Si multilayer structures

The high-angle annular dark-field (HAADF) technique of scanning transmission electron microscopy (STEM) has been applied to the study of Mo/Si multilayer structures fabricated by e-beam evaporation and sputtering under various growth conditions. Comparison with bright-field STEM images confirms that the HAADF technique can give high-resolution imaging contrast which is sensitive to the atomic number of the scatterer and is free of the complications of dynamical scattering and coherent interference effects which are limitations of the bright-field electron microscopy. The HAADF images allow compositional distributions to be drawn concerning the sharpness of the interfaces between the layers. A method for quantifying the compositional variations across the layers is proposed.

Imaging single platinum atoms on zeolites in the STEM

High angle annular dark-field (HAAD) imaging in the scanning transmission electron microscope has been shown in recent years to be a very effective tool in characterizing materials in which there are large differences in atomic number. Supported metal catalysts, in particular, have been explored extremely successfully using this Z-contrast technique. HAAD has very good sensitivity to high atomic number clusters on low atomic number supports, due to the approximately Z2 relationship. Furthermore, since the image contrast is due primarily to amplitude contrast, the resulting images are maps of mass thickness. Owing to the linear proportionality between intensity and the number of atoms probed, the intensity values integrated over metal clusters can be used as a measure of the cluster size.High resolution bright-field imaging is better suited for resolving structure in periodic specimens, and can be used to obtain structure images of zeolites. However, even with contrast enhancements such as Fourier filtering available from image processing, bright-field images are ineffective for detecting clusters containing fewer than about 20 Pt atoms in supports thicker than about 100Å. In comparison, we have demonstrated that the HAAD technique can be used successfully to detect single atoms of platinum on a 200Å thick zeolite support.

Compositional observations of franckeite using high-angle annular dark-field microscopy

Franckeite (˜FeSn3Pb5Sb2Si14) has a complex modulated structure containing alternating layers of different composition. The complex composition and structure, lack of a well-defined unit cell, incommensurate character, and poor quality of crystals has made it difficult to study by standard X-ray and electron-diffraction methods. High-angle annular dark-field (HAADF) STEM imaging provides a way of mapping the compositional distributions directly and with atomic resolution. For thin regions of the sample, the intensity of the electrons scattered at high angles depends strongly on the atomic number of the scatterer. This unique property makes HAADF imaging powerful for studying compositional modulations on the atomic scale. By comparing the structure as viewed by high-resolution imaging to the compositional information provided by the HAADF technique, we have been able to obtain an improved understanding of crystal structures with displacive and compositional modulations, such as franckeite. We use the HAADF technique to test the assumption that in franckeite two kinds of layers (SnS2 and PbS) are stacked along the a axis with a 1.73 nm repeat, and that Sb and Fe produce structural modulations in the c direction.