bluuebunny/crossref_metadata_2025 huggingface dataset:
Triple excitation effects in coupled-cluster calculations of indirect spin–spin coupling constants
Authors: Auer Alexander A. , Gauss Jürgen
Published: July 2002

The effect of triple excitations in coupled-cluster calculations of indirect spin-spin coupling constants is investigated in coupled-cluster singles and doubles (CCSD) calculations augmented by a perturbative treatment of triples [CCSD(T)], in calculations based on the CC3 model as well as in coupled-cluster singles, doubles, and triples (CCSDT) calculations. Though triple excitation effects are in most cases not particularly pronounced, it is demonstrated that among the approximate schemes for handling triples only the CC3 model with no orbital relaxation included (unrelaxed CC3) provides an adequate description. The otherwise successful CCSD(T) aproach appears to either significantly overestimate triple excitation effects or to yield corrections with the wrong sign in comparison to CCSDT.
Ultraviolet–visible near-field microscopy of phase-separated blends of polyfluorene-based conjugated semiconductors
Authors: Stevenson R. , Riehn R. , Milner R. G. , Richards D. , Moons E. , Kang D.-J. , Blamire M. , Morgado J. , Cacialli F.
Published: July 2002

We have used fluorescence scanning near-field microscopy to characterize polymer blends for electroluminescent applications, and thereby identify compositional nonhomogeneities. In particular, we have focused on the binary system constituted by poly(9,9′-dioctylfluorenealt-benzothiadiazole) and poly(9,9′-dioctylfluorene) (PFO), known to give efficiencies of up to 22 cd/A in light-emitting devices with suitable electrodes. Our primary aim was the assignment of the morphological features revealed in shear-force and atomic-force images of spin-coated films, and suggestive of phase separation on a 300-nm-length scale. From analysis of the fluorescence images (325 and 488 nm excitation), and quantitative correlation of optical and topographic data, we identify the raised features with PFO-rich regions. However, the limited variation in fluorescence intensity reveals a high extent of mixing within each phase on the length scale accessible in our experiment, approximately 100 nm for our focused-ion-beam-processed probe apertures.
Electron field emission from polymer films treated by a pulsed ultraviolet laser
Authors: Huang S. M. , Sun Z. , An C. W. , Lu Y. F. , Hong M. H.
Published: July 2002

The poly(phenylcarbyne) polymer films were coated on silicon substrates and then irradiated by a pulsed ultraviolet laser (λ=248 nm) with various fluences (1–60 mJ/cm2) at an atmospheric pressure of nitrogen. The structures of the resulted films were investigated by Raman spectroscopy. The morphologies of the films were examined by scanning electron microscopy (SEM). The electron field emission properties of the films as cathodes were studied. Raman spectrum analysis and SEM results indicate that the polymer film is converted to nanoparticle carbon film with the laser fluence from 10 to 60 mJ/cm2. The conversion mechanism from the polymer to nanoparticle carbon and electron field emission mechanism from the converted carbon film is discussed. The converted carbon film showed better field emission properties, i.e., lower turn-on threshold emission field, higher emission current density, and higher emission light spot density with increasing laser fluence from 10 to 50 mJ/cm2. By increasing the laser fluence from 50 to 60 mJ/cm2, the emission property showed a slight degradation. A turn-on threshold emission field of 1.8 V/μm (at 1 nA/cm2) and emission current density of 30 mA/cm2 with an emission light spot density of 104/cm2 (at 14 V/μm) were observed for the polymer film irradiated with a fluence of 50 mJ/cm2. Circular and multiple-line field emission patterns are demonstrated using the laser irradiation of the poly(phenylcarbyne) polymer.
Carbon nanotube films prepared by thermal chemical vapor deposition at low temperature for field emission applications
Authors: Li Y. J. , Sun Z. , Lau S. P. , Chen G. Y. , Tay B. K.
Published: July 2002

Mirror-like smooth carbon nanotube (CNT) films were grown by thermal chemical vapor deposition on glass substrate at a relatively low temperature of 570 °C. Cobalt-containing amorphous carbon composite films were employed as a catalyst layer for the growth of CNTs by decomposing acetylene in a tube furnace. The diameter of the CNTs was around 10 nm and the root-mean-square roughness of the film was about 12 nm, indicating a relatively smooth surface. A high emission current density of 12 mA/cm2 and relatively uniform emission sites were obtained from this type of CNT films. The relatively smooth CNT films, which can be deposited at low temperature on glass substrates and compatible with current semiconductor processes, have a potential for fabricating high-density gated CNT field emitter array.
Collective emission degradation behavior of carbon nanotube thin-film electron emitters
Authors: Nilsson L. , Groening O. , Groening P. , Schlapbach L.
Published: July 2002

The current-induced emission degradation of a carbon nanotube (CNT) thin-film electron emitter is studied under constant emission current for different current levels, using a scanning anode field emission microscope. A permanent emission degradation is observed for emission currents higher than 300 nA per CNT and is associated with resistive heating at the CNT–substrate interface for the sample under investigation. A second field-induced emission degradation mechanism, associated with the removal of CNTs from the substrate, is also reported.
Compact coarse approach mechanism for scanning tunneling microscope
Authors: Gupta Anjan K. , Ng K.-W.
Published: July 2002

We report on the design and fabrication of a coarse approach mechanism, called a piezotube walker, based on a piezoelectric tube moving inside a triangular prism shaped cavity. This walker walks like a six legged insect moving its legs one by one and then the belly following. The walker works in any orientation from horizontal to vertical and its motion is found to be linear with the applied voltage above a threshold voltage. A compact scanning tunneling microscope (STM) was fabricated using this approach mechanism. The scanner tube of the STM is mounted on the inside of the walker tube, reducing the size of the STM considerably. Topographical images with atomic resolution were obtained for layered materials like graphite and NbSe2.
Coarse-grained interactions in polymer melts: A variational approach
Authors: Akkermans Reinier L. C. , Briels W. J.
Published: July 2002

We investigate the modeling of a polymer melt on large length scale by averaging out fast fluctuating degrees of freedom in the microscopic model. We determine pair interactions in the coarse-grained system that give the best representation of the fine-grained system in a variational sense. Starting from the Gibbs-Bogoliubov inequality we derive a correction to a trial potential that minimizes the variational free energy of the coarse-grained system. By applying this correction repeatedly, pair interactions that are optimal in variational sense are obtained self-consistently. To calculate the potential of mean force in the polymer system, we consult the replica approach. The effective potential results in a radial distribution function for the coarse-grained sites that is less structured than that of the microscopic system. We also found that the soft effective interaction is unable to reproduce the virial distribution of the fine-grained system.
Uniform patterned growth of carbon nanotubes without surface carbon
Authors: Teo K. B. K. , Chhowalla M. , Amaratunga G. A. J. , Milne W. I. , Hasko D. G. , Pirio G. , Legagneux P. , Wyczisk F. , Pribat D.
Published: July 2002

In order to utilize the unique properties of carbon nanotubes in microelectronic devices, it is necessary to develop a technology which enables high yield, uniform, and preferential growth of perfectly aligned nanotubes. We demonstrate such a technology by using plasma-enhanced chemical-vapor deposition (PECVD) of carbon nanotubes. By patterning the nickel catalyst, we have deposited uniform arrays of nanotubes and single free-standing aligned nanotubes at precise locations. In the PECVD process, however, detrimental amorphous carbon (a-C) is also deposited over regions of the substrate surface where the catalyst is absent. Here, we show, using depth-resolved Auger electron spectroscopy, that by employing a suitable deposition (acetylene, C2H2) to etching (ammonia, NH3) gas ratio, it is possible to obtain nanotube growth without the presence of a-C on the substrate surface.
Formation of radially and azimuthally polarized light using space-variant subwavelength metal stripe gratings
Authors: Bomzon Ze’ev , Kleiner Vladimir , Hasman Erez
Published: July 2002

We present a unique method for converting circularly polarized light into radially and azimuthally polarized beams. The method is based on the use of computer-generated space-variant subwavelength metal stripe gratings. The elements were realized on GaAs substrates and produced beams with high polarization purity at a wavelength of 10.6 μm. As a result of the conversion, the beams also undergo space-variant phase modification, which has an effect on their propagation. We demonstrate the effect experimentally, and calculate it using full space-variant polarization and phase analysis based on rigorous coupled-wave analysis and Jones calculus.
Approach to study the noise properties in nanoscale electronic devices
Authors: Oriols Xavier , Martı́n Ferran , Suñé Jordi
Published: July 2002

An approach to study the noise characteristics in mesoscopic devices is presented. It extends, via quantum trajectories, the classical particle Monte Carlo techniques to devices where quantum nonlocal effects are important. As a numerical example, the fluctuations of the electron current through single-tunnel barriers are compared with the standard Landauer–Buttiker results, showing an excellent agreement.
Theoretical modeling and experimental characterization of a rod-pinch diode
Authors: Cooperstein G. , Boller J. R. , Commisso R. J. , Hinshelwood D. D. , Mosher D. , Ottinger P. F. , Schumer J. W. , Stephanakis S. J. , Swanekamp S. B. , Weber B. V. , Young F. C.
Published: July 2002

The rod-pinch diode consists of an annular cathode and a small-diameter anode rod that extends through the hole in the cathode. With high-atomic-number material at the tip of the anode rod, the diode provides a small-area, high-yield x-ray source for pulsed radiography. The diode is operated in positive polarity at peak voltages of 1 to 2 MV with peak total electrical currents of 30–70 kA. Anode rod diameters as small as 0.5 mm are used. When electrode plasma motion is properly included, analysis shows that the diode impedance is determined by space-charge-limited current scaling at low voltage and self-magnetically limited critical current scaling at high voltage. As the current approaches the critical current, the electron beam pinches. When anode plasma forms and ions are produced, a strong pinch occurs at the tip of the rod with current densities exceeding 106 A/cm2. Under these conditions, pinch propagation speeds as high as 0.8 cm/ns are observed along a rod extending well beyond the cathode. Even faster pinch propagation is observed when the rod is replaced with a hollow tube whose wall thickness is much less than an electron range, although the propagation mechanism may be different. The diode displays well-behaved electrical characteristics for aspect ratios of cathode to anode radii that are less than 16. New physics understanding and important properties of the rod-pinch diode are described, and a theoretical diode current model is developed and shown to agree with the experimental results. Results from numerical simulations are consistent with this understanding and support the important role that ions play. In particular, it is shown that, as the ratio of the cathode radius to the anode radius increases, both the Langmuir–Blodgett space-charge-limited current and the magnetically limited critical current increase above previously predicted values.
Density-functional theory of the crystallization of hard polymeric chains
Authors: Sushko Nazar , van der Schoot Paul , Michels M. A. J.
Published: July 2002

We study how connectivity influences the crystallization of fully flexible model polymers by applying a recently advanced amalgamation of the Green-function description of polymers, and the density-functional theory of simple liquids. Our calculations show that the model polymers only crystallize if the effective Kuhn length of the chains is sufficiently large compared with the range of the hard-core interaction between the segments. Also shown is the importance of bond-length fluctuations for the stability of the crystal phase.
High efficiency solid-state photovoltaic device due to inhibition of interface charge recombination
Authors: Krüger Jessica , Plass Robert , Cevey Le , Piccirelli Marco , Grätzel Michael , Bach Udo
Published: July 2002

The performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD was considerably improved by controlling charge recombination across the interface of the heterojunction. This was achieved by blending the hole conductor matrix with a combination of 4-tert-butylpyridine (tBP) and Li[CF3SO2]2N. Open circuit voltages Uoc over 900 mV and short circuit currents Isc up to 5.1 mA were obtained, yielding an overall efficiency of 2.56% at AM1.5 illumination. These values have been fully confirmed at the National Renewable Energy Laboratories for a device with an active area of 1.07 cm2, signifying a dramatic improvement compared to previously reported values for a similar device.
Properties of amorphous carbon–silicon alloys deposited by a high plasma density source
Authors: Racine B. , Ferrari A. C. , Morrison N. A. , Hutchings I. , Milne W. I. , Robertson J.
Published: July 2002

The addition of silicon to hydrogenated amorphous carbon can have the advantageous effect of lowering the compressive stress, improving the thermal stability of its hydrogen, and maintaining a low friction coefficient up to high humidity. Most experiments to date have been on hydrogenated amorphous carbon–silicon alloys (a-C1−xSix:H) deposited by rf plasma enhanced chemical vapor deposition. This method gives alloys with sizeable hydrogen content and only moderate hardness. Here we use a high plasma density source known as the electron cyclotron wave resonance source to prepare films with higher sp3 content and lower hydrogen content. The composition and bonding in the alloys is determined by x-ray photoelectron spectroscopy, Rutherford backscattering, elastic recoil detection analysis, visible and ultraviolet (UV) Raman spectroscopy, infrared spectroscopy, and x-ray reflectivity. We find that it is possible to produce relatively hard, low stress, low friction, almost humidity insensitive a-C1−xSix:H alloys with a good optical transparency and a band gap well over 2.5 eV. The friction behavior and friction mechanism of these alloys are studied and compared with that of a-C:H, ta-C:H, and ta-C. We show how UV Raman spectroscopy allows the direct detection of Si–C, Si–Hx, and C–Hx vibrations, not seen in visible Raman spectra.
Effect of Al-induced gettering and back surface field on the efficiency of Si solar cells
Authors: Plekhanov P. S. , Negoita M. D. , Tan T. Y.
Published: July 2002

In silicon solar cell fabrication, impurity gettering from Si by an aluminum layer and indiffusion of Al for creating the back surface field (BSF) are inherently carried out in the same process. We have modeled these two processes and analyzed their impact on solar cell efficiency. The output of gettering and Al indiffusion modeling is used as an input for calculation of solar cell efficiency. The cell efficiency gain is obtained as a function of the processes duration. To check the relative contributions of gettering and BSF in improving the cell efficiency, their effects are evaluated together as well as separately. It is found that, for solar cells fabricated from low quality, multicrystalline Si, the efficiency gain is solely due to gettering. In solar cells made of high quality Si, the efficiency gain is primarily due to gettering, but the BSF may play a significant role if the cell thickness is less than about 200 μm. The two effects are found to be synergetic. The model provides a means for optimization of the temperature regime for both processes, as well as for maximization of solar cell efficiency.
Direct observation of irradiation-induced nanocavity shrinkage in Si
Authors: Zhu X. F. , Williams J. S. , Conway M. J. , Ridgway M. C. , Fortuna F. , Ruault M.-O. , Bernas H.
Published: July 2002

Nanocavities in Si substrates, formed by conventional H implantation and thermal annealing, are shown to evolve in size during subsequent Si irradiation. Both ex situ and in situ analytical techniques were used to demonstrate that the mean nanocavity diameter decreases as a function of Si irradiation dose in both the crystalline and amorphous phases. Potential mechanisms for this irradiation-induced nanocavity evolution are discussed. In the crystalline phase, the observed decrease in diameter is attributed to the gettering of interstitials. When the matrix surrounding the cavities is amorphized, cavity shrinkage may be mediated by one of two processes: nanocavities can supply vacancies into the amorphous phase and/or the amorphous phase may flow plastically into the nanocavities. Both processes yield the necessary decrease in density of the amorphous phase relative to crystalline material.
Visualization of single-walled carbon nanotubes electrical networks by scanning force microspy
Authors: de Pablo P. J. , Gómez-Navarro C. , Gil A. , Colchero J. , Martinez M. T. , Benito A. M. , Maser W. K. , Gómez-Herrero J. , Baró A. M.
Published: July 2002

In this letter, a method is presented to qualitatively investigate the electrical connection between single-walled carbon nanotubes and gold electrodes. By applying a time independent bias voltage to the scanning force microscope tip, there is a clear apparent growth effect in the topography of the electrically connected carbon nanotubes. This effect is induced by the electrostatic interaction of the tip with the free charges located at the conducting areas of the surface. This is a general method that could be applied to any molecular conducting nanowire.
Phases of neon, xenon, and methane adsorbed on nanotube bundles
Authors: Calbi M. Mercedes , Gatica Silvina M. , Bojan Mary J. , Cole Milton W.
Published: July 2002

We explore the behavior of neon, xenon, and methane films adsorbed on the external surface of a bundle of carbon nanotubes. The methods used are classical: a ground state calculation, by grand potential energy minimization, and the grand canonical Monte Carlo (GCMC) method of simulation. Our results are similar to those found recently in a GCMC study of Ar and Kr. At low chemical potential (pressure) the particles form a quasi-one-dimensional phase within the groove formed by two contiguous tubes. At higher chemical potential, there occurs a “three-stripe” phase aligned parallel to the groove (except for xenon). This is followed by monolayer and bilayer phases. The low temperature monolayer phase is striped; the number of stripes per nanotube is a quantized function of the adatom size. In the neon case, the bilayer regime also includes a second layer groove phase. Our results are compared with recent thermal and diffraction experiments. We find no evidence of a zigzag phase reported recently.
Direct quantitative measurement of compositional enrichment and variations in InyGa1−yAs quantum dots
Authors: Crozier P. A. , Catalano M. , Cingolani R. , Passaseo A.
Published: July 2002

Assessment of the composition of quantum dots on the nanoscale is crucial for a deeper understanding of both the growth mechanisms and the properties of these materials. In this letter, we discuss a direct method to obtain a quantitative evaluation of the In variation across nanometer-sized InGaAs quantum dots embedded in a GaAs matrix, by means of electron energy-loss spectroscopy in a scanning transmission electron microscope.
Promising ferromagnetic Ni–Co–Al shape memory alloy system
Authors: Oikawa K. , Wulff L. , Iijima T. , Gejima F. , Ohmori T. , Fujita A. , Fukamichi K. , Kainuma R. , Ishida K.
Published: July 2002

A system of ferromagnetic β phase Ni–Co–Al alloys with an ordered B2 structure that exhibits the shape memory effect has been developed. The alloys of this system within the composition range Ni (30–45 at. %) Co–(27–32 at. %) Al, undergo a paramagnetic/ferromagnetic transition as well as a thermoelastic martensitic transformation from the β to the β′(L10) phase. The Curie and the martensitic start temperatures in the β phase can be controlled independently to fall within the range of 120–420 K. The specimens from some of the alloys undergoing martensitic transformation from ferromagnetic β phase to ferromagnetic β′ phase are accompanied by the shape memory effect. These ferromagnetic shape memory alloys hold great promise as new smart materials.
Constant pressure hybrid Molecular Dynamics–Monte Carlo simulations
Authors: Faller Roland , de Pablo Juan J.
Published: July 2002

New hybrid Molecular Dynamics-Monte Carlo methods are proposed to increase the efficiency of constant-pressure simulations. Two variations of the isobaric Molecular Dynamics component of the algorithms are considered. In the first, we use the extended-ensemble method of Andersen [H. C. Andersen, J. Chem. Phys. 72, 2384 (1980)]. In the second, we arrive at a new constant-pressure Monte Carlo technique based on the reversible generalization of the weak-coupling barostat [H. J. C. Berendsen et al., J. Chem. Phys. 81, 3684 (1984)]. This latter technique turns out to be highly effective in equilibrating and maintaining a target pressure. It is superior to the extended-ensemble method, which in turn is superior to simple volume-rescaling algorithms. The efficiency of the proposed methods is demonstrated by studying two systems. The first is a simple Lennard-Jones fluid. The second is a mixture of polyethylene chains of 200 monomers.
Beam model for calculating magnetostriction strains in thin films and multilayers
Authors: Wetherhold Robert C. , Chopra Harsh Deep
Published: July 2002

A standard technique to measure the magnetostriction strain in thin films involves measurement of the end deflection or slope of cantilever beams using optical deflectometry or capacitive methods. This article presents a general beam model for inferring magnetostriction strain from the end deflection or slope data. This model greatly extends the range of applicability over existing shell models, allowing for inference of magnetostriction strains for practical film/substrate thickness ratios that are important in microelectromechanical systems (MEMS) and bio-microelectromechanical systems (bio-MEMS). If the properties of individual layers are known, the beam theory can be used to design multilayer MEMS or bio-MEMs beams over a full range of thickness ratios.
Time resolved synthetic aperture terahertz impulse imaging
Authors: McClatchey K. , Reiten M. T. , Cheville R. A.
Published: July 2002

Using a well characterized terahertz (THz) impulse ranging system we demonstrate broad bandwidth imaging at THz frequencies using an inverse synthetic aperture deconvolution technique. The system demonstrates millimeter and submillimeter resolutions along the cross range and range axes, respectively. The range resolution, determined by the THz pulse bandwidth is 0.12 mm, while the cross range resolution is 1.2 mm; both resolutions agree well with theoretical predictions. Through imaging of cylindrical targets we demonstrate quantitative measurement of target position in the image plane within the experimental error of 0.2 mm. Imaging of geometrically scaled complex targets is demonstrated for a 1:2400 scale model ship (1 mm=24 m) corresponding to a full scale frequency bandwidth of 83–625 MHz at a distance of 840 m.
Theoretical study of Cu–Au nanoalloy clusters using a genetic algorithm
Authors: Darby Sarah , Mortimer-Jones Thomas V. , Johnston Roy L. , Roberts Christopher
Published: July 2002

A study has been made of the structures and stabilities of copper and gold clusters and copper–gold nanoalloy clusters, with up to 56 atoms, modeled by the many-body Gupta potential. For pure copper clusters, the lowest energy structures are found to be based on icosahedral packing, while pure gold clusters tend to form less symmetrical (often amorphous) structures. In a number of cases, the replacement of a single gold atom by copper is found to be sufficient to convert the structure to that of the more symmetrical copper cluster. The lowest energy clusters are generally more difficult to find for the bimetallic clusters than for the pure metallic clusters, due to the presence of homotops (related by permuting Cu and Au atoms), as well as geometrical isomers. The structures of the lowest energy bimetallic clusters exhibit primarily icosahedral packing, with (CuAu)M and (CuAu3)M clusters tending to form layered structures and (Cu3Au)M clusters showing greater Cu–Au mixing.
In situ study on the reorientation of polymer chains in operating polymer diodes
Authors: Guo Tzung-Fang , Yang Yang
Published: July 2002

A reflection-absorption Fourier-transform infrared spectroscopy experiment has been designed to in situ monitor poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based polymer light-emitting diodes under stress test. This method enables the in situ study of the co-relation between device performance and the conformational transformation of a conjugated polymer. The experimental results indicate that the plane of the conjugated π-electron cloud in MEH-PPV tends to align parallel to the substrate. This rearrangement enhances the π–π electron coupling and lowers the device operating voltage under high current densities.
Powder neutron diffraction study of the antiferroelectric phase transition in Sr0.75Ca0.25TiO3
Authors: Mishra Sanjay Kumar , Ranjan Rajeev , Pandey Dhananjai , Kennedy Brendan J.
Published: July 2002

The structure of Sr0.75Ca0.25TiO3 has been studied using powder neutron diffraction and the Rietveld method in the temperature range 24–300 K. It is shown that new superlattice reflections characteristic of an antiferroelectric phase appear around T=185 K. The unit cell parameters change discontinuously around 185 K indicating first order nature of the antiferroelectric phase transition. Rietveld analysis of the diffraction data confirms the Pbcm space group proposed by Ranjan et al. [Phys. Rev. Lett. 84, 3726 (2000)] for the antiferroelectric phase. The most likely space group for the paraelectric phase of Sr0.75Ca0.25TiO3 is proposed to be Ibmm. The antiparallel arrangement of dipoles in the antiferroelectric phase is shown to be of very novel type mimicking long period modulated phases predicted by axial next nearest neighbor interaction type models.
Two-dimensional lateral superlattices of nanostructures: Nonlithographic formation by anodic membrane template
Authors: Liang Jianyu , Chik Hope , Yin Aijun , Xu Jimmy
Published: July 2002

A nonlithographic technique that utilizes highly ordered anodized aluminum oxide porous membrane as template is presented as a general fabrication means for the formation of an array of vastly different two-dimensional lateral superlattices structures. Hexagonal close-packed nanopore arrays were fabricated on Si, GaAs, and GaN substrates via reactive ion etching. Quantum dot arrays of various metals and semiconductors were formed through evaporation and subsequent etching. The two-dimensional lateral superlattice structures fabricated using this method are of a high level of ordering, uniformity, and packing density. The diameter and periodicity of the nanostructures are determined by the features of the original alumina membrane, which can be adjusted by varying the anodization conditions.
Effects of spatial separation on the growth of vertically aligned carbon nanofibers produced by plasma-enhanced chemical vapor deposition
Authors: Merkulov Vladimir I. , Melechko Anatoli V. , Guillorn Michael A. , Lowndes Douglas H. , Simpson Michael L.
Published: July 2002

Vertically aligned carbon nanofibers (VACNFs) with vastly different spacing were grown by catalytically controlled dc glow discharge chemical vapor deposition. Both densely packed VACNFs and essentially isolated VACNFs were studied using scanning electron microscopy and x-ray energy dispersive spectroscopy. The morphology and chemical composition of isolated VACNFs were found to have a strong dependence upon the growth conditions, in particular on the C2H2/NH3 gas mixture used. This is attributed to the sidewalls of isolated VACNFs being exposed to reactive species during growth. In contrast, the sidewalls of densely packed VACNFs were shielded by the neighboring VACNFs, so that their growth occurred mainly in the vertical direction, by diffusion of carbon through the catalyst nanoparticle and subsequent precipitation at the nanofiber/nanoparticle interface. These striking differences in the growth process result in the formation of flattened carbon nanostructures (carbon nanotriangles) and also are quite important for the realization of VACNF-based devices.
Largely tunable midinfrared (8–12 μm) difference frequency generation in isotropic semiconductors
Authors: Haı̈dar R. , Mustelier A. , Kupecek Ph. , Rosencher E. , Triboulet R. , Lemasson Ph. , Mennerat G.
Published: July 2002

Largely tunable midinfrared (8–12 μm) generation is obtained in GaAs and ZnSe slabs by difference frequency mixing of optical parametric oscillator output waves (∼2 μm). The coherence lengths determined in semiconductor wedges are in excellent agreement with estimates from known Sellmeir formulas.
Memory device applications of a conjugated polymer: Role of space charges
Authors: Majumdar Himadri S. , Bandyopadhyay Anirban , Bolognesi Alberto , Pal Amlan J.
Published: July 2002

Conjugated polymers have been used in data-storage devices. A “state” has been written by applying a voltage pulse. The state of the device has been “read” from the current under a small probe voltage (0.2 V). The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current–voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was found to be independent of the density of the space charges or of the pace at which they were stored.
Temperature-gradient epitaxy under in situ growth mode diagnostics by scanning reflection high-energy electron diffraction
Authors: Koida T. , Komiyama D. , Koinuma H. , Ohtani M. , Lippmaa M. , Kawasaki M.
Published: July 2002

We have developed a parallel film growth method on a temperature-gradient substrate to quickly control and optimize the film growth mode. A continuous-wave neodymium-doped yttrium–aluminum–garnet laser heating was used to achieve a stable temperature gradient covering a 300 °C range of temperatures over a distance of 11 mm. The growth mode was determined by time-resolved scanning reflection high-energy electron diffraction. Transition from layer-by-layer to step-flow growth by the deposition temperature was observed during La0.5Sr0.5MnO3 film growth on a single SrTiO3 substrate, proving a powerful tool not only for investigating the growth dynamics but also for seeking the optimized deposition conditions in one run of experiment.
Improvement of critical current in MgB2/Fe superconducting wires by a ferromagnetic sheath
Authors: Horvat J. , Wang X. L. , Soltanian S. , Dou S. X.
Published: July 2002

Transport critical current (Ic) was measured for Fe-sheathed MgB2 round wires. A critical current density of 5.3×104 A/cm2 was obtained at 32 K. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6 T at 32 K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6 T. This was well beyond the maximum field for effective shielding of 0.2 T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least three times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between field and current showed that the transport current does not flow straight across the wire, but meanders between the grains.
Single-electron tunneling at room temperature in cobalt nanoparticles
Authors: Graf H. , Vancea J. , Hoffmann H.
Published: July 2002

We report on the observation of the Coulomb blockade with Coulomb staircases at room temperature in cobalt nanoparticles, with sizes ranging between 1 and 4 nm. A monolayer of these particles is supported by a thin 1–2 nm thick Al2O3 film, deposited on a smooth Au(111) surface. The local electrical transport on isolated Co clusters was investigated with a scanning tunneling microscope (STM). The tunnel contact of the STM tip allowed us to observe single-electron tunneling in the double barrier system STM-tip/Co/Al2O3/Au. Very high values of the Coulomb blockade of up to 1.0 V were reproducibly measured at room temperature on different particles with this setup. The current–voltage characteristics fit well by simulations based on the orthodox theory of single-electron tunneling.
Room-temperature magnetic anisotropy of lanthanide complexes: A model study for various coordination polyhedra
Authors: Mironov Vladimir S. , Galyametdinov Yury G. , Ceulemans Arnout , Görller-Walrand Christiane , Binnemans Koen
Published: July 2002

The dependence of the room-temperature magnetic anisotropy Δχ of lanthanide complexes on the type of the coordination polyhedron and on the nature of the lanthanide ion is quantitatively analyzed in terms of a model approach based on numerical calculations. The aim of this study is to establish general regularities in the variation of the sign and magnitude of the magnetic anisotropy of lanthanide complexes at room-temperature and to estimate its maximal value. Except for some special cases, the variation of the sign of the magnetic anisotropy over the series of isostructural lanthanide complexes is found to obey a general sign rule, according to which Ce(III), Pr(III), Nd(III), Sm(III), Tb(III), Dy(III), and Ho(III) complexes have one sign of Δχ and Eu(III), Er(III), Tm(III), and Yb(III) complexes have the opposite sign. Depending on the specific coordination polyhedron, a maximal magnetic anisotropy is observed for Tb(III), Dy(III), or Tm(III) complexes, and its absolute value can reach 50 000×10−6 cm3 mol−1 or more. Results of the present study can be helpful for the analysis of the orientational behavior of lanthanide-containing liquid crystals and lanthanide-doped bilayered micelles in an external magnetic field. The use of the Bleaney theory in the quantitative analysis of the magnetic anisotropy of lanthanide compounds is shown to have limitations because of a large ratio between the crystal-field splitting energy of the ground multiplet of the lanthanide ion and the thermal energy at room-temperature.
Reflection-mode operation of one-dimensional magnetophotonic crystals for use in film-based magneto-optical isolator devices
Authors: Kato Hideki , Inoue Mitsuteru
Published: July 2002

To investigate the potential of reflection type one-dimensional magnetophotonic crystals (1D–MPCs) for use in magneto-optical (MO) applications, theoretical studies were carried out. The calculations revealed that required MO isolator performance (reflectance R>99% and Kerr rotation angle θK=45°) can be fulfilled with the reflection-mode 1D–MPC having only one thin magnetic layer. Provided that a dielectric multilayer film and Al film are combinatory used as the reflection layer, the structure of 1D–MPC satisfying the isolator requirements become much simpler than that of the transmission-mode 1D–MPC. This is indeed very attractive for actual isolator fabrications.
Dissipative particle dynamics study of spontaneous vesicle formation of amphiphilic molecules
Authors: Yamamoto Satoru , Maruyama Yutaka , Hyodo Shi-aki
Published: July 2002

A dissipative particle dynamics (DPD) simulation has been used to study the spontaneous vesicle formation of amphiphilic molecules in aqueous solution. The amphiphilic molecule is represented by a coarse-grained model, which contains a hydrophilic head group and a hydrophobic tail. Water is also modeled by the same size particle as adopted in the amphiphile model, corresponding to a group of several H2O molecules. In the DPD simulation, from both a randomly dispersed system and a bilayer structure of the amphiphile for the initial condition, a spontaneous vesicle formation is observed through the intermediate state of an oblate micelle or a bilayer membrane. The membrane fluctuates and encapsulates water particles and then closes to form a vesicle. During the process of vesicle formation, the hydrophobic interaction energy between the amphiphile and water is diminishing. It is also recognized that the aggregation process is faster in two-tailed amphiphiles than those in the case of single-tailed ones.
Structural properties and electrical characteristics of electron-beam gun evaporated erbium oxide films
Authors: Mikhelashvili V. , Eisenstein G. , Edelmann F.
Published: July 2002

We report properties of Er2O3 films deposited on silicon using electron-beam gun evaporation. We describe the evolution with thickness and annealing temperature of the morphology, structure, and electrical characteristics. An effective relative dielectric constant in the range of 6–14, a minimum leakage current density of 1–2×10−8 A/cm2 at an electric field of 106 V/cm and breakdown electric field of 0.8–1.7×107 V/cm are demonstrated. Breakdown electric field and leakage current densities are correlated with the surface morphology. The obtained characteristics make the Er2O3 films a promising substitute for SiO2 as an ultrathin gate dielectric.
Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing
Authors: Arias A. C. , Corcoran N. , Banach M. , Friend R. H. , MacKenzie J. D. , Huck W. T. S.
Published: July 2002

Surface treatment and solvent evaporation control are used to promote vertical segregation in polyfluorene-blend thin films. This surface-mediated control of the compositional structure in the direction normal to the plane of the film has important implications for optimizing charge transport in solution-processed conjugated polymer-blend optoelectronics. Here, the surface energy of the hole-collector electrode of photovoltaic devices is modified by deposition of self-assembled monolayers to favor segregation of the hole-accepting component of the blend to the substrate. Devices fabricated with intentionally vertically segregated blends showed external quantum efficiencies of up to 14%, which is ten times higher than that of devices fabricated without surface modification.
Periodic poling of magnesium-oxide-doped lithium niobate
Authors: Nakamura Koichiro , Kurz Jonathan , Parameswaran Krishnan , Fejer M. M.
Published: July 2002

In this article, poling characteristics and periodic poling of magnesium-oxide-doped lithium niobate (MgLN) are described. Periodic poling was done by the electric field method, with which uniform gratings were produced. The domain wall velocity as a function of poling field was measured and used to determine conditions for self-terminating periodic poling. A computational model for the self-termination method is also discussed here. In the experiment, it was found that thermal history prior to poling has a significant influence on the poling quality.
Structure-based turbulence model: Application to a rotating pipe flow
Authors: Poroseva S. V. , Kassinos S. C. , Langer C. A. , Reynolds W. C.
Published: July 2002

A new approach for modeling the one-point turbulence statistics, which takes into account the information on turbulence structure, has been suggested in Kassinos and Reynolds (Report TF-61, Thermosciences Division, Department of Mechanical Engineering, Stanford University, 1994). In the present work, the structure-based model [Int. J. Heat Fluid Flow 21, 599 (2000)] (SBM) based on those ideas, was evaluated in a complex inhomogeneous turbulent flow in a cylindrical pipe rotating around its longitudinal axis. It was found that the SBM is able to predict the flow accurately at various Reynolds numbers and under stronger rotation than what is possible with the Reynolds stress transport models (RSTMs). In a fully developed rotating pipe flow, the SBM, being a linear model, slightly improves the profiles obtained with the nonlinear RSTM [J. Fluid Mech. 227, 245 (1991)]. However, if the standard equation for the dissipation rate is used, the SBM, as do the RSTMs, significantly overpredicts the turbulent kinetic energy level in this part of flow in comparison with the results of experiments.
Efficient multiple-time-step integrators with distance-based force splitting for particle-mesh-Ewald molecular dynamics simulations
Authors: Qian Xiaoliang , Schlick Tamar
Published: July 2002

We develop an efficient multiple-time-step force splitting scheme for particle-mesh-Ewald molecular dynamics simulations. Our method exploits smooth switch functions effectively to regulate direct and reciprocal space terms for the electrostatic interactions. The reciprocal term with the near field contributions removed is assigned to the slow class; the van der Waals and regulated particle-mesh-Ewald direct-space terms, each associated with a tailored switch function, are assigned to the medium class. All other bonded terms are assigned to the fast class. This versatile protocol yields good stability and accuracy for Newtonian algorithms, with temperature and pressure coupling, as well as for Langevin dynamics. Since the van der Waals interactions need not be cut at short distances to achieve moderate speedup, this integrator represents an enhancement of our prior multiple-time-step implementation for microcanonical ensembles. Our work also tests more rigorously the stability of such splitting schemes, in combination with switching methodology. Performance of the algorithms is optimized and tested on liquid water, solvated DNA, and solvated protein systems over 400 ps or longer simulations. With a 6 fs outer time step, we find computational speedup ratios of over 6.5 for Newtonian dynamics, compared with 0.5 fs single-time-step simulations. With modest Langevin damping, an outer time step of up to 16 fs can be used with a speedup ratio of 7.5. Theoretical analyses in our appendices produce guidelines for choosing the Langevin damping constant and show the close relationship among the leapfrog Verlet, velocity Verlet, and position Verlet variants.
Radiative and nonradiative lifetimes in GaInN/GaN multiquantum wells
Authors: Minsky Milan S. , Watanabe Satoshi , Yamada Norihide
Published: July 2002

This paper analyzes a simplified rate equation model of localized exciton emission in GaInN. Expressions for temperature dependent photoluminescence (PL) efficiency and decay time are derived and compared with time integrated (TIPL) and time resolved photoluminescence (TRPL) data for a series of multiquantum well light emitting diodes with varying In composition in the active region. Time resolved photoluminescence is measured up to relatively high temperature (540 K) and a decreasing efficiency coupled with a peak energy decay time that is weakly dependent on temperature is observed. The decay time at peak emission energy begins to decrease at a temperature that depends on the In content in the quantum wells. The analysis developed here demonstrates that application of the expressions τr=τpl/η and τnr=τpl/(1−η) is not sufficient to determine radiative and nonradiative lifetimes from TRPL and TIPL data in the GaInN system. (Here τr is the radiative decay time, τnr is the nonradiative decay time, τpl is the measured PL decay time, and η is the measured TIPL intensity normalized to the low temperature intensity.) GaInN with even small amounts of In exhibits highly efficient luminescence due to recombination through localizing centers. As relaxation occurs into both defects and localizing states after initial generation with the above GaInN band gap excitation, the number of carriers arriving at localization centers can change with temperature. This temperature dependent change should be considered when calculating relevant decay times from TRPL and TIPL data. This mechanism is distinct from an increase in the intrinsic radiative decay time obtained by applying conventional analysis to extract radiative and nonradiative lifetimes from TRPL and TIPL data.
Field emission from dense, sparse, and patterned arrays of carbon nanofibers
Authors: Teo K. B. K. , Chhowalla M. , Amaratunga G. A. J. , Milne W. I. , Pirio G. , Legagneux P. , Wyczisk F. , Pribat D. , Hasko D. G.
Published: July 2002

We compare the field emission characteristics of dense (109 nanofibers/cm2), sparse (107 nanofibers/cm2), and patterned arrays (106 nanofibers/cm2) of vertically aligned carbon nanofibers on silicon substrates. The carbon nanofibers were prepared using plasma-enhanced chemical vapor deposition of acetylene and ammonia gases in the presence of a nickel catalyst. We demonstrate how the density of carbon nanofibers can be varied by reducing the deposition yield through nickel interaction with a diffusion layer or by direct lithographic patterning of the nickel catalyst to precisely position each nanofiber. The patterned array of individual vertically aligned nanofibers had the most desirable field emission characteristics, highest apparent field enhancement factor, and emission site density.
Comparison of a one-dimensional particle-in-cell–Monte Carlo model and a one-dimensional fluid model for a CH4/H2 capacitively coupled radio frequency discharge
Authors: Ivanov Vladimir , Proshina Olga , Rakhimova Tatyana , Rakhimov Alexander , Herrebout Dieter , Bogaerts Annemie
Published: July 2002

A one-dimensional particle-in-cell–Monte Carlo (PIC–MC) model was developed for a capacitively coupled rf discharge in a mixture of CH4 and H2. The electron behavior is kinetically simulated by solving Newton’s equations and treating the electron collisions with the Monte Carlo algorithm, whereas the behavior of the ions and radicals is treated by a set of continuity equations. The distinctive feature of this model is its self-consistency, i.e., the motion of the electrons is considered in the real electric field calculated from the Poisson equation, and not in the time-averaged electric field. The PIC–MC results were compared with the data calculated by means of a pure fluid model. In both models, exactly the same type of species, reactions, and cross sections are used. The results of both models, such as the electron energy distribution function, the average electron energy, and the densities of the various plasma species, are compared at a gas pressure of 0.14 Torr and a discharge frequency of 13.56 MHz, for the power ranging from 0.5 to 25 W. The nonstationary and nonlocal features of the electron energy distribution function are shown in the PIC–MC calculations. The effect of accumulation of low-energy electrons in the center of the discharge at higher input power P=25 W is observed in the PIC–MC model, in contrast with the fluid model. The mechanisms causing the accumulation of low-energy electrons, and the processes defining the stationary state of the discharge are analyzed. The applicability of the fluid model for the calculation of the density of different hydrocarbon radicals is discussed.