bluuebunny/crossref_metadata_2025_split huggingface dataset:
Superparamagnetism and Microstructural Properties of Carbon Encapsulated Ni nanoparticle Assemblies
Authors: Sun Xiang-Cheng , Dong Xinglong , Toledo J. A. , Yacaman M. J. 
Published: April 2011

ABSTRACTCarbon encapsulated Ni nanoparticles (Ni(C)) were synthesized by modified arc-discharge reactor under methane atmosphere. The presence of carbon encapsulation is confirmed by HR-TEM imaging, and Nano-diffraction. The average particle radius is typically 10.5 nm with spherical shape. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers.Superparamagnetic property studies were performed using SQUID magnetometer for the assemblies of Ni(C) nanoparticles. The blocking temperature (TB) is determined to around 115K at 1000Oe applied field. Above TB, the magnetization M (H, T) can be described by the classical Langevin function L using the relation, M/Ms(T=0) = coth(μH/kT)- kT/μH. The particle radius can be inferred from Langevin fit (particle moment μ) and blocking temperature theory (TB), which values are a little bigger than HR-TEM observations. It is suggested, these assemblies of carbon encapsulated Ni nanoparticles have been showed typical single-domain, field-dependent superparamagnetic relaxation properties.
Optical And Structural properties of Vertical Aligned Self-Assembled InAs Quantum Dots Multilayers
Authors: González J. C. , da Silva M. I. N. , Rodrigues W. N. , Matinaga F. M. , Magalhaes-Paniago R. , Moreira M. V. , Oliveira A. G. de , Ugarte D. 
Published: April 2011

ABSTRACTIn this work, we report optical and structural properties of vertical aligned self-assembled InAs quantum dots multilayers. The InAs quantum dots samples were grown by Molecular Beam Epitaxy. Employing Atomic Force Microscopy, Transmission Electron Microscopy, and Gracing Incident X-ray Diffraction we have studied the structural properties of samples with different number of periods of the multiplayer structure, as well as different InAs coverage. The optical properties were studied using Photoluminescence spectroscopy.
Biomedical application of ferrofluids containing magnetite nanoparticles
Authors: Kim D. K. , Voit W. , Zapka W. , Bjelke B. , Muhammed M. , Rao K.V. 
Published: April 2011

ABSTRACTFerrofluids containing superparamagnetic Fe3O4 nanoparticles have been prepared by a controlled co-precipitation method.The aggregation of the particles was prevented by using a polymeric starch network as a coating agent. Structural and magnetic measurements reveal a particle size of around 6 nm, with a clear evidence of a uniform particle coating. The ferrofluids have been used as a contrast agent for MR imaging in biological tissue.
Direct Electronic Control of Biomolecular Systems: Using Nanocrystals as Antennas for Regulation of Biological Activity
Authors: Hamad-Schifferli Kimberly , Schwartz John J. , Santos Aaron T. , Zhang Shuguang , Jacobson Joseph M. 
Published: April 2011

ABSTRACTWe report a means of directly controlling DNA dehybridization by radio frequency magnetic field coupling to a nanometer scale antenna covalently linked to the DNA. The method of control relies on induction heating of an Au nanocrystal, which raises the temperature of a biomolecule to which it is covalently bound, while leaving surrounding molecules relatively unaffected. Because heat dissipation in biomolecules in solution is rapid(<50 picoseconds[1]) this switching is reversible. This technique is specific, reversible, and non-optical. Since it can be used in solution, it has the potential to be extended to systems in vivo. The ability to differentially control local temperature forms the basis of control of properties such as hybridization and enzyme activity, and has the potential of controlling many biological processes.
A Combined Density Functional and Monte Carlo Study of Polycarbonate
Authors: Jones R. O. , Ballone P. 
Published: April 2011

ABSTRACTDensity functional computations have been performed to investigate the structure, potential en- ergy surface and reactivity for organic systems closely related to bisphenol-A-polycarbonate(BPA- PC). The results provide the basis for the construction of two different empirical models, the first extending the atomistic simulations into the mesoscopic range (104–105 atoms and 0.01–0.1 μs), the second providing an idealized description of polymerization in BPA-PC. The combination of models and computational techniques focusing on different length and time scales provides a route to determine mechanical and thermal properties of materials without experimental input.
Electronic and Optical Properties of Si/SiO2 Superlattices from First Principles: Role of Interfaces.
Authors: Carrier Pierre , Abramovici Gilles , Lewis Laurent J. , Dharma-wardana M. W. C. 
Published: April 2011

ABSTRACTThe observation of intense luminescence in Si/SiO2 superlattices (SLs) has lead to new theoretical research on silicon-based materials. We have performed first-principles calculations using three Si/SiO2 SL models in order to examine the role of interfaces on the electronic structure and optical properties. The first two models are derived directly from crystalline structures and have simple interfaces. These models have been studied using the full-potential, linearized-augmented-plane-wave method, in the local-density-approximation (LDA). The optical absorption within the interband transition theory (excluding excitonic effects) have been deduced. The Si(001)-SiO2 interface structure is shown to affect the optical behaviour. Following these observations, we have considered a more realistic, fully-relaxed model. The projector-augmented-wave method under the LDA is used to perform the structural relaxation as well as band structure and optical calculations. The role of confinement on the energy gap is studied by inserting additional silicon slabs into the supercell. Direct energy gaps are observed and the energy gap is found to decrease with increasing silicon slab thickness, as observed experimentally. The role of the interface has been considered in more details by studying the contribution to the energy gap of Si atoms having different oxidation patterns; partially oxidized Si atoms at the interface, as well as Si atoms inside the Si layer, are shown to contribute to the transitions at the energy gap.
New Polaronic-Type Excitons in Ferroelectric Oxides: INDO-Calculations and Experimental Manifestation
Authors: Vikhnin Valentin S. , Eglitis Roberts I. , Kotomin Eugene A. , Kapphan Siegmar , Borstel Gunnar 
Published: April 2011

ABSTRACTThe current experimental and theoretical knowledge of new polaronic-type excitons in ferroelectric oxides – charge transfer vibronic excitons (CTVE) – is discussed. It is shown that Hartree-Fock-type INDO calculations as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE-concept. Single CTVE as well as a new phase of strongly correlated CTVEs are analysed.
Structural and vibrational properties of silicon dioxide thin films densified by medium-energy particles bombardment
Authors: Lefèvre Alexis , Lewis Laurent J. , Martinu Ludvik , Wertheimer Michael R. 
Published: April 2011

ABSTRACTUsing classical molecular-dynamics simulations, we worked out a simple model of Ion Beam Assisted Deposition (IBAD) of silicon dioxide on an amorphous silica substrate, in view to in- vestigate the modifications of the structural and vibrational properties induced by medium-energy bombardment. Atoms are assumed to interact via the two- and three-body potential developed by Nakano et al.[1]. Analysis of the films grown with increasing ratio, R, of medium-(30 eV) to low- (1 eV) kinetic energy SiO2 particles shows that the density rises rapidly, from 1.3 g/cm3 for R =0 to about 2.3 g/cm3 for R = 0.7. This effect can be associated primarily with structural changes occurring at an intermediate length scale (4-10 Å), as it manifests itself by changes in the so-called first sharp diffraction peak (FSDP), the finger-print of medium range order (MRO) in a-SiO2 glass [2, 3]. We found also that the densification results in a significant decrease of the number of “soft” vibrational modes, occurring in the 0.5-3.5 THz frequency range.
Deep levels of antisite defects clusters in ZnGeP2
Authors: Voevodin Valeriy G. , Grinyaev Sergey N. 
Published: April 2011

ABSTRACTOn the basis of the methods of pseudopotential and large unit cell the research of deep lev- els of point and cluster defects in ZnGeP2 (ZGP) was carried out. It was shown that owing to lowering of the crystal lattice symmetry localised states of point defects in the ternary compound essentially differ from such states in its binary analogue GaP. Small quasicubic antisite defects clusters reduce the value of band gap, but render weak influence on the probability of optical transitions. But both single and the cluster antisite defects cause large modifications in deep levels of another point defects closed to them. On an example of phosphorus vacancy it was shown that intensive optical transitions with small energies can appear and cause an absorption in near IR – range because of antisite defects effect.
Applications of Real-Time Multiresolution Analysis for Molecular Dynamics Simulations of Infrequent Events
Authors: Richie David A. , Kim Jeongnim , Wilkins John W. 
Published: April 2011

ABSTRACTThe simulation of defect dynamics (e.g., transient enhanced diffusion of boron in the presence of silicon interstitials) is a technologically relevant challenge for computational materials science. The dynamics of defect structures in bulk unfolds as a sequence of thermally induced structural transitions. Identifying and characterizing reaction paths, as well as extracting dynamical quantities (e.g., diffusion constants) is important for modeling the macroscopic properties of real materials. Applying real-time multiresolution analysis (RTMRA) to various dynamical quantities using simple Haar wavelets, we have developed a computationally cheap data compression scheme to handle the massive data sets generated in molecular dynamics (MD) simulations; data storage has been reduced hundredfold with no loss of relevant information. More importantly, the same RTMRA techniques are developed into a sophisticated event detection scheme capable of solving three major challenges to multiscale MD simulations, specifically, (1) identifying meta-stable structures against the background of thermal vibrations, (2) detecting infrequent events, e.g., structural transitions, in the presence of thermal noise, and (3) accurately identifying transition times to further enhance recently emerging MD acceleration techniques.
Temperature and Strain-Rate Dependent Plastic Deformation of Carbon Nanotube
Authors: Wei Chengyu , Cho Kyeongjae 
Published: April 2011

ABSTRACTIn this work we use classical molecular dynamics to study strain rate and temperature dependent plasticity of carbon nanotube (CNT) under compressive strain. We focus on two types of defects: sp3 bond formation and bond rotation. Our simulation shows that thermal fluctuations help the strained CNT to overcome the local energy barrier to obtain plastic deformation. The yielding strain of a compressed CNT found to be strain-rate and temperature dependent, and low strain rate limit of the yielding strain is estimated to be less that 6%.
Percolation/Diffusion through the Void Space of a Bed of Randomly Packed Particles of Different Sizes
Authors: Riley Daniel P , Belova Irina V , Murch Graeme E 
Published: April 2011

ABSTRACTIn this paper a Monte Carlo method is introduced for determining the diffusivity in the void space of porous media constructed from spheres. We studied ordered and random arrangements of spheres, the latter with different sizes. It is found that the effect of structure on the diffusivity is fairly slight. The empirical curve of Neale and Nader is shown to provide a convenient upper bound. The result of Bruggemann provides a convenient lower bound.
Structure of crystallographically challenged materials by profile analysis of atomic pair distribution functions: study of LiMoS2 and mesostructured MnGe4S10.
Authors: Petkov V. , Rangan K. K. , Kanatzidis M. G. , Billinge S.J.L. 
Published: April 2011

AbstractThe approach of the atomic pair distribution function (PDF) technique to study the structure of materials with significant disorder is considered and successfully applied to LiMoS2 and mesostructured MnGe4S10. We find that LiMoS2 is built of layers of distorted MoS6 octahedra stacked along the c axis of a triclinic unit cell with well-defined Mo-Mo bonding. Mesostructured MnGe4S10 is a three-dimensional framework of “adamantane-like” [Ge4S10] units bridged by Mn atoms.
Comparison of Crack Geometry Determined with Phase Contrast Radiography and with Microtomography
Authors: Stock S.R. , Ignatiev K. , Lee W.K. , Fezzaa K. , Davis G.R. , Elliott J.C. 
Published: April 2011

AbstractX-ray microtomography of the three-dimensional spatial distribution of crack opening as a function of applied load has helped clarify important processes of roughness-induced fatigue crack closure. Fracture mechanics dictates sample geometries which limit the crack opening sensitivity that can be obtained with microtomography, and there is a real need to be able to quantify smaller openings. Synchrotron phase contrast radiographs of an AA 2090 T8E41 (central 2 mm of a 12.7 mm thick plate) compact tension sample show the fatigue crack geometry clearly while absorption contrast radiographs of the same sample have difficulty even detecting the crack. The phase contrast expected in the projection images is determined from the microtomography-derived, three-dimensional crack geometry and compared to that observed experimentally. Good agreement is obtained between the phase contrast radiographs and the expected pattern of contrast.
Correlated Local Atomic Displacements: The Microscopic Origins for Macroscopic Phenomena.
Authors: Bridges Frank , Cao Daliang , Booth Corwin H. 
Published: April 2011

AbstractIn many systems for which there are several atoms in the unit cell, the displacements of the atoms may be locally correlated even though there is no long range coherence. Such displacements can play an important role in determining various macroscopic properties. We consider several examples to demonstrate this phenomena - the local distortions in the colossal magneto-resistive (CMR) and charge-ordered manganites, magnetic field induced distortions in CMR materials that are connected to macroscopic magnetostriction, and correlated displacements of tetrahedral units within the negative thermal expansion material ZrW2O8. The distortions in the CMR materials observed using XAFS change rapidly just below Tc and are attributed to the formation of polarons as the temperature is increased through Tc. In the ferromagnetic state, the lattice is more ordered for CMR systems; consequently applying a magnetic field for T ∼ Tc should decrease the local distortions. Such an effect has been observed and is a much larger effect than the measured macroscopic magnetostriction. Finally, in ZrW2O8 the tetrahedral and octahedral units are found to be very rigid as expected. More surprising is that the width of the pair-distance distribution for the W-Zr atom-pair is also quite small, indicating that the W-Zr linkage is stiff. In contrast, for the W-W pair, the distribution width grows rapidly with T, indicating correlated displacements of two WO4 tetrahedral units.
Structural Distortions in the Paramagnetic Insulating Phase of La0.7Ca0.3MnO3
Authors: Nelson C.S. , Kim Y.J. , Hill J.P. , Gibbs Doon , Kiryukhin V. , Koo T.Y. , Cheong S-W. 
Published: April 2011

AbstractWe report x-ray scattering studies of polarons and tilt ordering in the high-temperature, paramagnetic insulating phase of La0.7Ca0.3MnO3. The temperature dependence of scattering due to individual and correlated polarons was measured, and the latter was found to disappear at ∼400 K. The scattering due to tilt ordering, which was observed by tuning the incident photon energy near the La L1 edge, was also measured as a function of temperature. The destruction of tilt ordering at ∼690 K is believed to be associated with the orthorhombic-rhombohedral structural transition.
A silicon structure for electrical characterisation of nanoscale elements
Authors: Sazio P.J.A. , Berg J. , See P. , Ford C.J.B. , Lundgren P. , Greenham N.C. , Ginger D.S. , Bengtsson S. , Chin S.N. 
Published: April 2011

ABSTRACTThe problem of mass manufacturing electrode structures suitable for contacting nanoscale elements lies primarily in the difficulty of fabricating a nanometre-scale gap between two electrodes in a well controlled, highly parallel manner. In ULSI circuit production, the gate and substrate in MOSFETs are routinely fabricated with a precise vertical spacing of 3 nm between them. In this work, we have investigated a number of highly parallel methods for the generation of nanogaps, including reconfiguration of the ubiquitous MOS device structure. The silicon dioxide layer that provides vertical separation and electrical insulation between two regions of silicon (the crystalline substrate and the poly-crystalline gate) gives a leakage current of 1 nA μm−2 at 1 V for an oxide thickness of 2 nm [1]. This will enable objects the size of single molecules that are held across this layer to be detected electrically if they provide currents on the nanoampere scale, assuming a parasitic area for leakage between gate and substrate of order 1 µm2. In the future this kind of device has the potential to provide a bolt-on technology for the fabrication of ULSI circuits in which conventional CMOS devices are directly hybridised with functional nanoscale elements.
MOCVD Growth of Ga(Al)N/InGaN/Ga(Al)N-Heterostructures: Influence of the Buffer Layer Al-Concentration and Growth Duration on the In-Incorporation in InGaN
Authors: Schowalter Marco , Neubauer Brigitte , Rosenauer Andreas , Gerthsen Dagmar , Schön Oliver , Heuken Michael 
Published: April 2011

ABSTRACTTransmission electron microscopy (TEM) has been applied to analyze the thickness and the In-concentration of InGaN layers in GaN/InGaN/GaN- and AlGaN/InGaN/AlGaN-quantum well (QW) structures. Two series of samples were grown by metal organic chemical vapor deposition varying either only the growth duration for the InGaN QW or by changing the Al- concentration in the buffer layers at unaltered InGaN growth conditions. A rising average In- concentration from 6.5 to 15.4 % and a decreasing growth rate are observed with increasing growth duration. The increase of the Al-concentration in the buffer layers from 0 to 36 % strongly affects the In-incorporation during the InGaN growth, which decreases from 17.5 to 2.5 %. All samples are characterized by an inhomogeneous In-distribution containing In-rich agglomerates with a size of only a few nanometers and less pronounced composition fluctuations on a scale of 100 nm.
Infrared absorption and annealing behavior of semi-insulating 4H SiC HTCVD substrates
Authors: Magnusson B. , Ellison A. , Storasta L. , Janzén E. 
Published: April 2011

ABSTRACTInfrared absorption and resistivity are studied in two different types of semi-insulating (SI) 4H SiC substrates. In type A the silicon vacancy dominates the infrared absorption spectra whereas in type B the UD-1 defect dominates. Both types of substrates are still SI (> 105 Ω cm at 200°C) after annealing at typical CVD growth temperatures (1600°C). The type A material shows a decreasing absorption of the silicon vacancy after 1600°C at the same time as the resistivity decreases from high 107 Ω cm to mid 106 Ω cm measured at 200°C. The thermal activation energy of the resistivity for the type A material is 0.85 eV for temperatures above 250°C both before and after annealing. The type B material does not change with annealing, besides that the absorption increases in intensity with a 1600°C anneal. The resistivity is > 1010 Ω cm at 200°C and the thermal activation energy is 1.4 eV.The absorption measurements on the silicon vacancy also show that it is still present in the as-grown material after a 1600°C anneal. We interpret this as follows: The silicon vacancy is stable even at 1600°C and does not get mobile at 750°C as earlier suggested.
Spectroscopic Investigation of Traps Producing Current Collapse In AlGaN/GaN Hemt Structures
Authors: Klein P.B. , Binari S.C. , Ikossi K. , Wickenden A.E. , Koleske D.D. , Henry R.L. 
Published: April 2011

ABSTRACTPhotoionization spectroscopy (PS) measurements, previously carried out for the GaN MESFET, have been extended to the more complicated AlGaN/GaN HEMT structures. In all cases, the spectra revealed that the same two traps causing current collapse in the high resistivity (HR) GaN buffer layer of the MESFET were also responsible for current collapse in the HEMT structures. The HR buffer layers supporting the HEMT structures were prepared by MOVPE at varying growth pressures, in order to vary the incorporation of deep trapping centers. Lower growth pressures were observed to correlate with more severe current collapse and with an enhanced incorporation of carbon. Detailed analysis of the PS data suggests that one of the two responsible traps is related to carbon, while the other may be associated with structural defects in the material.
Gadolinium Oxide Gate Dielectrics for GaN MOSFETs
Authors: Gila B.P. , Johnson J.W. , Lee K. N. , Krishnamoorthy V. , Bates S. , Abernathy C. R. , Ren F. , Pearton S. J. 
Published: April 2011

ABSTRACTSubstrate preparation of GaN, both in-situ and ex-situ, and the growth of gadolinium oxide, Gd2O3, by Gas source molecular beam epitaxy (GSMBE) have been investigated. Ex-situ cleaning techniques included wet chemical etching and UV-ozone treatments to remove surface contaminants and the native oxide. In-situ cleaning consisted of thermal treatment with and without exposure to an electron cyclotron resonance (ECR) oxygen plasma. A GaN (1x3) streaky RHEED pattern was the final product of this surface treatment study. Various growth initiation techniques were explored to produce Gd2O3 films with different microstructures as evidenced by RHEED, TEM, and XRD. Gd2O3 films planarized the initial GaN surface and stoichiometry was maintained over a range of substrate temperatures (300° to 650°C). Single crystal gadolinium oxide films were grown at substrate temperatures of 600-650°C. These films exhibited a breakdown field strength (EBD) of ∼1MV/cm, and showed high leakage current at high forward bias due to defects within the oxide. Single crystal oxide films were found to be thermally stable at annealing temperatures up to 1000°C. Quasi-amorphous films were grown at a substrate temperature of 100°C. These films exhibited a higher E BD of ∼3MV/cm and an interface state density of 3 × 1011 cm−2eV−1. However, the quasi-amorphous films were not thermally stable at 1000°C, showing evidence of re-crystallization in x-ray diffraction (XRD) scans.