Electron beam surface melting of model 1200 Al alloys

Electron beam surface melting has been used to characterise the phase content formed in a number of model 1200 series Al alloys with increasing solidification velocity in the range 2–50 mm s−1, typical of that experienced during continuous strip casting. Phases were extracted from the Al matrix and analysed by X-ray diffraction. A qualitative solidification microstructure selection map has been produced, showing that, for a given Fe content of 0.55 wt.%: with increasing solidification velocity the metastable aluminides FeAl6 and FeAlm displace equilibrium Fe4Al13 at Si contents 0.15 wt.%, and that α-AlFeSi is an equilibrium phase at a Si content ≥0.50 wt.%.

High contrast imaging and thickness determination of graphene with in-column secondary electron microscopy

We report a new method for quantitative estimation of graphene layer thicknesses using high contrast imaging of graphene films on insulating substrates with a scanning electron microscope. By detecting the attenuation of secondary electrons emitted from the substrate with an in-column low-energy electron detector, we have achieved very high thickness-dependent contrast that allows quantitative estimation of thickness up to several graphene layers. The nanometer scale spatial resolution of the electron micrographs also allows a simple structural characterization scheme for graphene, which has been applied to identify faults, wrinkles, voids, and patches of multilayer growth in large-area chemical vapor deposited graphene. We have discussed the factors, such as differential surface charging and electron beam induced current, that affect the contrast of graphene images in detail. (C) 2011 American Institute of Physics. doi:10.1063/1.3608062]

Transmission electron microscopy study of domain structures in ferroelectric SrBi2Nb2O9 ceramics

A transmission electron microscopy study has been carried out on the domain structures of SrBi2Nb2O9 (SBN) ferroelectric ceramics which belong to the Aurivillius family of bismuth layered perovskite oxides. SBN is a potential candidate for Ferroelectric Random access memory (FeRAM) applications. The 90° ferroelectric domains and antiphase boundaries (APBs) were identified with dark field imaging techniques using different superlattice reflections which arise as a consequence of octahedral rotations and cationic shifts. The 90° domain walls are irregular in shape without any faceting. The antiphase boundaries are less dense compared to that of SrBi2Ta2O9(SBT). The electron microscopy observations are correlated with the polarization fatigue nature of the ceramic where the domain structures possibly play a key role in the fatigue- free behavior of the Aurivillius family of ferroelectric oxides.

Electron Cyclotron Resonance in Strained Si and Si0.94Ge0.06 Channels on Relaxed Si0.62Ge0.38 Buffers Grown by Rapid Thermal Chemical Vapor Deposition

We report the far-infrared measurements of the electron cyclotron resonance absorption in n-type Si/Si0. 62Ge0.38 and Si0.94Ge0.06 /Si0. 62Ge0.38 modulation- doped heterostructures grown by rapid thermal chemical vapor deposition. The strained Si and Si0.94Ge0.06 channels were grown on relaxed Si0.62Ge0.38 buffer layers, which consist of 0.6 μm uniform Si0.62Ge0.38 layers and 0.5 μm compositionally graded relaxed SiGe layers from 0% Ge to 38 % Ge. The buffer layers were annealed at 800 °C for 1 hr to obtain complete relaxation. The samples had 100 Å spacers and 300 Å 2×1019 cm-3 n-type supply layers on the tops of the 75 Å channels. The far-infrared measurements of electron cyclotron resonance were performed at 4K with the magnetic field of 4 – 8 Tesla. The effective masses determined from the slope of center frequency of absorption peak vs applied magnetic field plot are 0.20 mo and 0.19 mo for the two dimensional electron gases in the Si and Si0.94Ge0.06 channels, respectively. The Si effective mass is very close to that of two dimensional electron gas in Si MOSFET (0.198mo). The electron effective mass of Si0.94Ge0.06 is reported for the first time and about 5 % lower than that of pure Si.

Microstructure and mechanical properties of electron beam weld joints of a Zr41Ti14Cu12Ni10Be23 bulk metallic glass with Zr

The electron beam welding technique was used to join Zr41Ti14Cu12Ni10Be23 bulk metallic glass (BMG) to crystalline pure Zr. Compositional, microstructural, and mechanical property variations across the welded interface were evaluated. It is shown that a crystalline layer develops close to the welding interface. Transmission electron microscopy of this layer indicates the crystalline phase to be tetragonal with lattice parameters close to that reported for Zr2Ni. However, the composition of this phase is different as it contains other alloying additions. The interface layer close to the bulk metallic glass side contains nanocrystalline Zr2Cu phase embedded in the glassy matrix. Nanoindentation experiments indicate that the hardness of the crystalline layer, although less than the bulk metallic glass, is more than the Zr itself. Commensurately, tensile tests indicate that the failure of the welded samples occurs at the Zr side rather than at the weld joint.

Measurement of Electron Spin Lifetime and Optical Orientation Efficiency in Germanium Using Electrical Detection of Radio Frequency Modulated Spin Polarization

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pickup coil and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of 4.8% could be determined for Ge at 1342 nm excitation wavelength. In the presence of a small external magnetic field, the signal decayed according to the Hanle effect, from which a spin lifetime of 4.6 +/- 1.0 ns for electrons in bulk Ge at 127 K was extracted.

Self-Assembly of Metallamacrocycles Using a Dinuclear Organometallic Acceptor: Synthesis, Characterization, and Sensing Study

A dinuclear organometallic acceptor 4,4'-bis[trans-Pt(PEt(3))(2)(O(3)SCF(3))(ethynyl)]biphenyl (1) containing Pt-ethynyl functionality is synthesized. Multinuclear NMR ((1)H, (31)P, and (13)C), infrared (IR), and electrospray ionization mass spectrometry (ESI-MS) including single-crystal X-ray diffraction analysis established the formation of 1. Equimolar treatment of acceptor 1 separately with three different ``clip'' type ditopic donors (L(a)-L(c)) yielded [2 + 2] self-assembled three metallamacrocycles 2a-2c, respectively. These macrocycles were characterized by various spectroscopic techniques, and their sizes/shapes were obtained through geometry optimization using molecular mechanics universal force field (MMUFF) simulations. Attachment of unsaturated ethynyl functionality to biphenyl building unit helped to make the macrocycles (2a-2c) pi-electron rich and thereby fluorescent in nature. Furthermore, 2c in solution has been examined to be suitable for sensing electron-deficient nitroaromatic like picric acid, which is often considered as a secondary chemical explosive. The fluorescence study of 2c showed a marked quenching of initial emission intensity upon titrating with picric acid (PA), and it exhibited the largest fluorescence quenching response with high selectivity among various other electron deficient aromatic compounds tested.

Electron paramagnetic resonance study of porous silicon

Electron paramagnetic resonance studies under ambient conditions of boron‐doped porous silicon show anisotropic Zeeman (g) and hyperfine (A) tensors, signaling localization of the charge carriers due to quantum confinement.

Preliminary X-ray crystallographic studies on acetate kinase (AckA) from Salmonella typhimurium in two crystal forms

Acetate kinase (AckA) catalyzes the reversible transfer of a phosphate group from acetyl phosphate to ADP, generating acetate and ATP, and plays a central role in carbon metabolism. In the present work, the gene corresponding to AckA from Salmonella typhimurium (StAckA) was cloned in the IPTG-inducible pRSET C vector, resulting in the attachment of a hexahistidine tag to the N-terminus of the expressed enzyme. The recombinant protein was overexpressed, purified and crystallized in two different crystal forms using the microbatch-under-oil method. Form I crystals diffracted to 2.70 angstrom resolution when examined using X-rays from a rotating-anode X-ray generator and belonged to the monoclinic space group C2, with unit-cell parameters a = 283.16, b = 62.17, c = 91.69 angstrom, beta = 93.57 degrees. Form II crystals, which diffracted to a higher resolution of 2.35 angstrom on the rotating-anode X-ray generator and to 1.90 angstrom on beamline BM14 of the ESRF, Grenoble, also belonged to space group C2 but with smaller unit-cell parameters (a = 151.01, b = 78.50, c = 97.48 angstrom, beta = 116.37 degrees). Calculation of Matthews coefficients for the two crystal forms suggested the presence of four and two protomers of StAckA in the asymmetric units of forms I and II, respectively. Initial phases for the form I diffraction data were obtained by molecular replacement using the coordinates of Thermotoga maritima AckA (TmAckA) as the search model. The form II structure was phased using a monomer of form I as the phasing model. Inspection of the initial electron-density maps suggests dramatic conformational differences between residues 230 and 300 of the two crystal forms and warrants further investigation.

Growth of silicon nanowires by electron beam evaporation using indium catalyst

For the first time silicon nanowires have been grown on indium (In) coated Si (100) substrates using e-beam evaporation at a low substrate temperature of 300 degrees C. Standard spectroscopic and microscopic techniques have been employed for the structural, morphological and compositional properties of as grown Si nanowires. The as grown Si nanowires have randomly oriented with an average length of 600 nm for a deposition time of 15 min. As grown Si nanowires have shown indium nanoparticle (capped) on top of it confirming the Vapor Liquid Solid (VLS) growth mechanism. Transmission Electron Microscope (TEM) measurements have revealed pure and single crystalline nature of Si nanowires. The obtained results have indicated good progress towards finding alternative catalyst to gold for the synthesis of Si nanowires. (C) 2011 Elsevier B.V. All rights reserved.