bluuebunny/crossref_metadata_2025 huggingface dataset:
Subpicosecond ultraviolet laser ablation of diamond: Nonlinear properties at 248 nm and time-resolved characterization of ablation dynamics
Authors: Preuss S. , Stuke M.
Published: July 2002

Ablation of diamond by short UV pulses is reported. The ablation threshold for 500 fs pulses at 248 nm was found to be 0.6 J/cm2 (equal to a peak intensity of 1.1 TW/cm2). Intensity-dependent transmission measurements from low intensities to 100 GW/cm2 showed that two-photon absorption is the dominant nonlinearity in this range and the two-photon absorption coefficient was measured to be (1.6±0.3) cm/GW. In the intensity regime above threshold (≳1 TW/cm2), however, low ablation rates indicate strong absorption by laser induced free carriers. The dynamics of diamond ablation was investigated time-resolved using pump probe transmission and pulse pair ablation measurements. Ablation with short pulses at 248 nm is also compared to ablation with 15 ns pulses at 193 nm.
Free energy barrier to nucleation of amorphous-to-crystalline transformation selects the scale of microstructure of crystallized materials
Authors: Shi F. G. , Tong H. Y. , Ayers J. D.
Published: July 2002

Experimental determination of the free energy barrier to nucleation of amorphous-to-crystalline transformation reveals a selection principle for the average scale of crystalline microstructure of partially or fully crystallized materials prepared by crystallization of the amorphous precursors: it is determined by the free energy barrier to nucleation, and minimization of the free energy barrier leads to the formation of the finest crystalline microstructure. It is found that the nucleation at the temperature at which an amorphous alloy is crystallized with the finest crystalline microstructure is an isoenergetic process, the free energy barrier to the nucleation of crystallites at that temperature is contributed only by the entropic change. A model is developed to explain these observations that may provide a unified principle for designing materials with desirable scale of microstructure.
Growth of fluorocarbon polymer thin films with high CF2 fractions and low dangling bond concentrations by thermal chemical vapor deposition
Authors: Limb Scott J. , Labelle Catherine B. , Gleason Karen K. , Edell David J. , Gleason Edward F.
Published: July 2002

Thermal chemical vapor deposition was used to deposit fluorocarbon films with chemical resemblance to bulk polytetrafluoroethylene. X-ray photoelectron spectroscopy revealed that the films deposited from thermal decomposition of hexafluoropropylene oxide had fluorine to carbon ratios of 2.0 and CF2 fractions of 90% along with 10% of CF3 and CF moieties. Electron spin resonance results found the dangling bond density to be 1.2×1018 spins/cm3, low compared to conventional plasma polymerized films. Low dangling bond densities were achieved by using a clean source of CF2 in the absence of plasma source.
Recombination velocity at oxide–GaAs interfaces fabricated by in situ molecular beam epitaxy
Authors: Passlack M. , Hong M. , Mannaerts J. P. , Kwo J. R. , Tu L. W.
Published: July 2002

The recombination velocity at oxide–GaAs interfaces fabricated by in situ multiple-chamber molecular beam epitaxy has been investigated. Ga2O3, Al2O3, SiO2, and MgO films have been deposited on clean, atomically ordered n- and p-type (100) GaAs surfaces using molecular beams of Ga–, Al–, Si–, and Mg oxide, respectively. Based on the internal quantum efficiency measured for incident light power densities 1≤P0≤104 W/cm2, the interface recombination velocity S has been inferred using a self-consistent numerical heterostructure device model. While Al2O3–, SiO2–, and MgO–GaAs structures are characterized by an interface recombination velocity which is comparable to that of a bare GaAs surface (≂ 107 cm/s), S observed at Ga2O3–GaAs interfaces is as low as 4000–5000 cm/s. The excellent Ga2O3–GaAs interface recombination velocity is consistent with the previously reported low interface state density in the mid 1010 cm−2 eV−1 range.
The relationship between the spatially resolved field emission characteristics and the raman spectra of a nanocrystalline diamond cold cathode
Authors: Talin A. A. , Pan L. S. , McCarty K. F. , Felter T. E. , Doerr H. J. , Bunshah R. F.
Published: July 2002

Spatially resolved electron field emission measurements from a nanocrystalline diamond film grown by plasma-enhanced chemical transport deposition have been obtained using a scanning probe apparatus with micrometer resolution. Macroscopic regions with a high emission site density, and turn-on fields below 3 V/μm, comprised approximately 1/2 of the total sample area. The emitting and the nonemitting regions of the specimen are differentiated distinctly by Raman spectra and subtly by morphologies. Both areas are largely sp3-bonded, but only the nonemitting regions exhibit a sharp line at 1332 cm−1, a well-known signature of diamond in larger crystallites.
Photoluminescence spectroscopy of self-assembled InAs quantum dots in strong magnetic field and under high pressure
Authors: Itskevich I. E. , Henini M. , Carmona H. A. , Eaves L. , Main P. C. , Maude D. K. , Portal J. C.
Published: July 2002

We have investigated the photoluminescence spectrum of self-assembled InAs quantum dots embedded in a GaAs matrix in magnetic field B up to 23 T and under hydrostatic pressure up to 8 kbar. A strong anisotropy in the diamagnetic shift is found depending on whether B is applied parallel or perpendicular to the growth direction. In the former case, the spatial extent of the carrier wave function in the dot is estimated to be 60 Å. The pressure coefficient for the dot emission line is (9.1±0.2) meV/kbar, about 20% smaller than for the Γ-point band gap in bulk GaAs.
On the use of dust plasma acoustic waves for the diagnostic of nanometer-sized contaminant particles in plasmas
Authors: Kortshagen U.
Published: July 2002

During the phase of dust nucleation in plasma enhanced chemical vapor deposition systems, nanometer-sized dust particles appear in large concentrations. The dispersion characteristics of dust acoustic waves in such a plasma during the nucleation phase is analyzed. It is shown that the dispersion properties of the dust acoustic wave depend sensitively on the particle size. It is proposed to use dust acoustic waves as a novel particle size diagnostic for nanometer-sized particles.
Optical anisotropy of dispersed carbon nanotubes induced by an electric field
Authors: Bubke Karsten , Gnewuch Harald , Hempstead Martin , Hammer Jens , Green Malcom L. H.
Published: July 2002

Carbon nanotubes dispersed in ethanol are aligned by an electric field. Due to the orientation of these elongated particles, the dispersion exhibits anisotropic behavior. Transmission experiments show rotation of the linear polarization of an incident laser beam. Alignment and relaxation times and the influence of the magnitude of the electric field have been measured.
Self-organized chain of crystalline-silicon nanospheres
Authors: Kohno Hideo , Takeda Seiji
Published: July 2002

We have fabricated a self-organized chain of crystalline-silicon nanospheres via an extension of the vapor-liquid-solid mechanism. Transmission electron microscopy, electron energy-loss spectroscopy, and electron-induced x-ray fluorescence analyses have proved that the crystalline silicon nanospheres, of about 10 nm in diameter and at a nearly equal spacing, are supported in amorphous silica and carbon. The novel self-organized phenomenon is attributed to the periodic instability of catalysts and spontaneous oxidization during the growth of nanowhiskers.
Transmission electron microscopy observations of fracture of single-wall carbon nanotubes under axial tension
Authors: Lourie O. , Wagner H. D.
Published: July 2002

Well-aligned bundles of single-wall carbon nanotubes under tensile stresses were observed to fracture in real-time by transmission electron microscopy. The expansion of elliptical holes in the polymer matrix results in a tensile force in bridging nanotubes. The polymer matrix at both ends of the bundles deforms extensively under the tension force, and fracture of the nanotubes occurs in tension within the polymer hole region rather than in shear within the gripping polymer region at the ends of the bundles. This provides evidence of significant polymer-nanotube wetting and interfacial adhesion.
Dense arrays of well-aligned carbon nanotubes completely filled with single crystalline titanium carbide wires on titanium substrates
Authors: Gao Y. , Liu J. , Shi M. , Elder S. H. , Virden J. W.
Published: July 2002

We report the synthesis of dense and uniform arrays of well-aligned carbon nanotubes on titanium substrates over large areas, in which all the tubes are simultaneously and completely filled with single crystals of titanium carbide. The carbon nanotubes were synthesized by thermal chemical vapor deposition of ethylene on iron-coated substrates, while the titanium carbide was simultaneously formed inside the nanotubes through a simultaneous solid state reaction. We propose a base dissolution and precipitation mechanism for the growth of titanium carbide filled carbon nanotubes. The same method can be used to fabricate oriented nanotube arrays filled with other carbides on a variety of substrates over large scale. Such well-aligned and densely packed uniform carbon nanotubes, completely filled with nanowires on conducting substrates, will have great potential in many applications.
Microstructural study of a C–Fe alloy synthesized by ion-beam sputtering co-deposition
Authors: Babonneau David , Cabioc’h Thierry , Denanot Marie-Françoise , Naudon André
Published: July 2002

A carbon–iron thin film, composition 46 at. % Fe and thickness 26 nm, was synthesized by ion-beam sputtering with a substrate temperature of 573 K. The microstructure of the film was characterized by transmission electron microscopy and small-angle x-ray scattering under grazing incidence. It consisted of iron-rich particles, with an average in-plane diameter of 3.2 nm, uniformly dispersed in a graphite-like carbon matrix. These particles were elongated along the in-depth direction, that of the thin-film growth. After annealing at 623 K for 1 h, no modification in the microstructure of the film was observed. The present study shows that the co-sputtering of graphite and iron performed at low temperature in comparison with the conventional arc discharge method, is a useful synthesis method to obtain thin films of encapsulated nanoparticles which have a good thermal stability.
Electrical transport in pure and boron-doped carbon nanotubes
Authors: Wei Bingqing , Spolenak Ralph , Kohler-Redlich Philipp , Rühle Manfred , Arzt Eduard
Published: July 2002

The resistivities of individual multiwalled pure and boron-doped carbon nanotubes have been measured in the temperature range from 25 to 300 °C. The connection patterns were formed by depositing two-terminal tungsten wires on a nanotube using focused-ion-beam lithography. A decrease of the resistivity with increasing temperature, i.e., a semiconductor-like behavior, was found for both B-doped and pure carbon nanotubes. B-doped nanotubes have a reduced room-temperature resistivity (7.4×10−7–7.7×10−6 Ωm) as compared to pure nanotubes (5.3×10−6–1.9×10−5 Ωm), making the resistivity of the doped tubes comparable to those along the basal plane of graphite. The activation energy derived from the resistivity versus temperature Arrhenius plots was found to be smaller for the B-doped (55–70 meV) than for the pure multiwalled nanotubes (190–290 meV).
Resonantly enhanced Raman scattering and high-order Raman spectra of single-walled carbon nanotubes
Authors: Tan PingHeng , Tang Yan , Deng Yuan Ming , Li Feng , Wei Yong Liang , Cheng Hui Ming
Published: July 2002

The Raman spectra of single-walled carbon nanotubes (SWNTs) produced by the catalytic decomposition of hydrocarbons have been measured in the range of 100–7500 cm−1. The tangential C–C stretching modes show an unusual resonant enhancement process that results from the one-dimensional quantum confinement of the electrons in the SWNT, and its intensity of the tangential C–C stretching mode is about 100 times as strong as that of highly oriented pyrolytic graphite. Because of the resonant enhancement, high-order Raman bands up to fifth order have been revealed and assigned to the corresponding fundamentals.
Boron ion-implanted C60 heterojunction photovoltaic devices
Authors: Narayanan K. L. , Yamaguchi M.
Published: July 2002

An attempt has been made on the device fabrication with boron ion-implanted C60 thin films grown on n-type Si (100) by molecular-beam epitaxy. The studies on the films implanted to a dose of 1×1014 ions/cm2 with multiple B ion energies in the range of 50–80 keV reveal p-type conduction in the B ion-implanted C60 thin films as a result of amorphous carbon layer formation by B ion implantation. In addition, p-type B-implanted C60/n-type Si heterojunction solar cells with a conversion efficiency as high as 0.02% higher than previous highest value ever reported have been fabricated. The photovoltaic properties of the solar-cell structure are discussed along with the dark and illuminated I–V characteristics.
Room-temperature oxide field-effect transistor with buried channel
Authors: Misewich J. A. , Schrott A. G.
Published: July 2002

In this letter, we introduce an architecture for a room-temperature oxide channel field-effect transistor where the oxide channel material is buried below the gate oxide layer. This architecture has several significant advantages over the surface channel architecture [D. M. Newns, J. A. Misewich, C. C. Tseui, A. Gupta, B. A. Scott, and A. Schrott, Appl. Phys. Lett. 73, 780 (1998).] in coupling capacitance, channel mobility, and channel stability. Although the transconductance in the devices has been improved to 45 μS (at Vd=1 V and Vg=2 V for a channel length of 1 μm and width=150 μm), capacitance measurements show that the surface charge density is still below the optimal theoretical value.
Structural anisotropy of magnetically aligned single wall carbon nanotube films
Authors: Smith B. W. , Benes Z. , Luzzi D. E. , Fischer J. E. , Walters D. A. , Casavant M. J. , Schmidt J. , Smalley R. E.
Published: July 2002

Thick films of aligned single wall carbon nanotubes and ropes have been produced by filtration/deposition from suspension in strong magnetic fields. We measured mosaic distributions of rope orientations in the film plane, for samples of different thicknesses. For an ∼1 μm film the full width at half maximum (FWHM) derived from electron diffraction is 25°–28°. The FWHM of a thicker film (∼7 μm) measured by x-ray diffraction is slightly broader, 35±3°. Aligned films are denser than ordinary filter-deposited ones, and much denser than as-grown material. Optimization of the process is expected to yield smaller FWHMs and higher densities.
Fibers of aligned single-walled carbon nanotubes: Polarized Raman spectroscopy
Authors: Gommans H. H. , Alldredge J. W. , Tashiro H. , Park J. , Magnuson J. , Rinzler A. G.
Published: July 2002

To probe the one-dimensional nature of single-wall carbon nanotubes (SWNTs) in bulk samples, we have devised a simple method for generating fibers of aligned SWNTs. We measured polarization-dependent Raman spectra on the oriented fibers. Contrary to what is expected from their theoretically assigned vibration-mode symmetries, all the Raman line intensities are observed to decrease in nearly equal amounts for the 647.1 nm laser excitation polarized perpendicular to the fiber axis versus that polarized parallel to the fiber axis. The effect is explained as a loss of resonance Raman scattering for the perpendicular polarization case.
Cluster beam microfabrication of patterns of three-dimensional nanostructured objects
Authors: Barborini E. , Piseri P. , Podesta’ A. , Milani P.
Published: July 2002

This letter describes the use of supersonic cluster beam deposition (SCBD) through a stencil mask for the fabrication of patterns of cluster-assembled objects. Using carbon cluster beams, micrometer-size pillars and tips have been produced on a variety of substrates. SCBD is characterized by high deposition rates, high lateral resolution, and low temperature processing. Nanostructured objects can be produced with high aspect ratio and controlled shapes. Micropatterning with SCBD can be of interest for applications requiring the integration of cluster-assembled structures with microelectronic or micromechanical devices.
Elastic moduli renormalization in self-interacting stretchable polyelectrolytes
Authors: Podgornik Rudi , Hansen Per Lyngs , Parsegian V. Adrian
Published: July 2002

We study the effect of intersegment interactions on the effective bending and stretching moduli of a semiflexible polymer chain with a finite stretching modulus. For an interaction potential of a screened Debye–Hückel type, renormalization of the stretching modulus is derived on the same level of approximation as the celebrated Odijk–Skolnick–Fixman result for the bending modulus. The presence of mesoscopic intersegment interaction potentials couples the bending and stretching moduli in a manner different from that predicted by the macroscopic elasticity theory. We thus advocate a fundamental change in the perspective regarding the dependence of elastic moduli of a flexible polyelectrolyte on the ionic conditions: stretchability. The theory presented here and its consequences compare favorably with recent experiments on DNA bending and stretching at not too low salt conditions.
Nano-tube-like surface structure in graphite particles and its formation mechanism: A role in anodes of lithium-ion secondary batteries
Authors: Moriguchi K. , Munetoh S. , Abe M. , Yonemura M. , Kamei K. , Shintani A. , Maehara Y. , Omaru A. , Nagamine M.
Published: July 2002

Nano-structures on the surface of graphite based carbon particles have been investigated by means of high resolution transmission electron microscopy. The surfaces consist of “closed-edge” structures in a similar manner as carbon nano-tube. That is, they are composed of coaxial carbon tubes consisting of adequate coupling of graphite layer edges. These graphite particles are chemically stable and, therefore, applicable for lithium-ion secondary battery anodes. Molecular dynamics simulations based on the Tersoff potential reveal that the vibrations of the graphite layers at the free edges play an important role in the formation of the closed-edge structures. In lithium-ion secondary batteries, Li ions can intrude into bulk carbon anodes through these closed-edge structures. In order to clarify this intrusion mechanism, we have studied the barrier potentials of Li intrusion through these closed edges using the first-principles cluster calculations. From electrochemical measurements, the carbon anodes composed of these closed-edge structures show actually high battery performance with a large discharge capacity and a small irreversible capacity. This article also implies that we can control these surface structures by choosing some suitable heat treatment conditions and/or pulverization conditions before the final heat treatment process.
Model simulations of DNA denaturation dynamics
Authors: Drukker Karen , Wu Guosheng , Schatz George C.
Published: July 2002

We present a model of DNA for use in computer simulations. This model is simple enough to allow long-time large-scale dynamics simulations, while, on the other hand, it is sophisticated enough to describe both double stranded and single stranded DNA and the transition between the two. We employed our simple model in the simulation of denaturation of double stranded DNA helices using Langevin dynamics. These are the first simulations of its kind of DNA denaturation. We have studied the melting behavior for several short double-stranded sequences of different composition. Duplexes of different lengths were considered, and also base pair mismatches were included in the study. Results are in good agreement with experimental data.
Investigations of the plume accompanying pulsed ultraviolet laser ablation of graphite in vacuum
Authors: Claeyssens Frederik , Lade Robert J. , Rosser Keith N. , Ashfold Michael N. R.
Published: July 2002

The plume accompanying 193 nm pulsed laser ablation of graphite in vacuum has been studied using wavelength, time and spatially resolved optical emission spectroscopy and by complementary Faraday cup measurements of the positively charged ions. The temporal and spatial extent of the optical emissions are taken as evidence that the emitting species result from electron–ion recombination processes, and subsequent radiative cascade from the high n,l Rydberg states that result. The distribution of C neutral emission is symmetric about the surface normal, while the observed C+ emission appears localized in the solid angle between the laser propagation axis and the surface normal. However, Faraday cup measurements of the ion yield and velocity distributions, taken as a function of scattering angle and incident pulse energy, indicate that the total ion flux distribution is peaked along the surface normal. The derived ion velocity distributions are used as input for a two-dimensional model which explains the observed anisotropy of the C+ emission in terms of preferential multiphoton excitation and ionization of C species in the leading part of the expanding plasma ball that are exposed to the greatest incident 193 nm photon flux, prior to electron–ion recombination and subsequent radiative decay.
A computational method to discover the existence of promoting vibrations for chemical reactions in condensed phases
Authors: Caratzoulas Stavros , Schwartz Steven D.
Published: July 2002

A computational method to discover the existence of vibrations (promoting vibrations) symmetrically coupled to the reaction coordinate is presented in the context of chemical reactions in condensed phases. Using the Zwanzig Hamiltonian as a theoretical model and molecular dynamics simulations of a model, linear triatomic in a Lennard–Jones liquid, it is shown that such a coupled motion leaves a unique signature on the spectral density computed from the autocorrelated force on the reaction coordinate. The spectral density is shown to have a peak at the effective frequency of the promoting vibration whose height increases with the reaction coordinate and vanishes at the transition state.
A dielectric continuum molecular dynamics method
Authors: Marchi Massimo , Borgis Daniel , Levy Nicolas , Ballone Pietro
Published: July 2002

We introduce a novel method to simulate hydrated macromolecules with a dielectric continuum representation of the surrounding solvent. In our approach, the interaction between the solvent and the molecular degrees of freedom is described by means of a polarization density free energy functional which is minimum at electrostatic equilibrium. After a pseudospectral expansion of the polarization and a discretization of the functional, we construct the equations of motion for the system based on a Car–Parrinello technique. In the limit of the adiabatic evolution of the polarization field variables, our method provides the solution of the dielectric continuum problem “on the fly,” while the molecular coordinates are propagated. In this first study, we show how our dielectric continuum molecular dynamics method can be successfully applied to hydrated biomolecules, with low cost compared to free energy simulations with explicit solvent. To our knowledge, this is the first time that stable and conservative molecular dynamic simulations of solutes can be performed for a dielectric continuum model of the solvent.
Electron attachment in HBr and HCl
Authors: Speck T. , Le Garrec J-L. , Le Picard S. , Canosa A. , Mitchell J. B. A. , Rowe B. R.
Published: July 2002

Studies of electron attachment in HBr and HCl gases at low temperatures have indicated that attachment to clusters of these gases can become efficient even though attachment to the monomers is endothermic and exhibits very low attachment rates. A complementary measurement of the reaction of OH radicals with HBr has enabled us to establish a lower limit for the rate of electron attachment to HBr clusters of approximately 10−8 cm3 s−1.
Large-scale well aligned carbon nitride nanotube films: Low temperature growth and electron field emission
Authors: Zhong Dingyong , Liu Shuang , Zhang Guangyu , Wang E. G.
Published: July 2002

Large-scale well aligned carbon nitride nanotube films (6 cm in diameter), which are easily processed and show potential for nanomanipulation, have been synthesized by microwave plasma enhanced chemical vapor deposition at a relatively low temperature of 550 °C. The characterization, using transmission electron microscopy and electron energy loss spectroscopy, shows that the nanotubes are polymerized by nanobells with nitrogen concentration of 10%. We propose a push-out growth mechanism for the formation of the special polymerized nanobell structure. A turn-on field of electron emission as low as 0.8 V/μm is obtained. Fowler–Nordheim, consisting of two straight lines with a gentle slope at low field and a steep one at relatively high field, are interpreted based on a top side emission mechanism related to the nanobell structures. No current saturation is found in the films.
Development of an automated gas adsorption apparatus for the characterization of the surface area, pore size distribution, and density of powdered materials
Authors: Badalyan Alexander , Pendleton Phillip , Wu Hua
Published: July 2002

An automated apparatus for the characterization of the surface area, pore size distribution, and density of powdered materials is described. A newly developed liquid nitrogen level control system maintains the nitrogen level around the tube containing the sample within ±0.2 mm of the initial predefined level. The dosing volume and pneumatically operated control valves are maintained at 25.00±0.05 °C. Equilibrium pressure is measured with the accuracy of ±0.05% of reading. The apparatus may be operated either in a manual or an automatic mode. The control software developed, using the VISIDAQ Version 3.11 software package, operates involving equilibrium and barometric pressure measurements, opening and closing pneumatically operated valves, and transferring experimental data to MS EXCEL spreadsheets. The subroutines developed within the spreadsheet program plot the adsorption and desorption isotherms, calculate the sample specific surface area and density, and provide αs- and t-plots and mesopore and micropore size distributions. Specific surface areas were defined using the Brunauer–Emmett–Teller method of analysis. The apparatus accuracy was tested via surface reference materials. These included garnet, kaolinite, and carbon black with quoted multiple-point surface areas of 3.00±0.30, 16.45±0.8, and 113±5 m2/g. Our values were 2.84±0.28, 16.02±0.8, and 110±5 m2/g. Specific surface areas as low as 0.2 m2/g have been reproducibly measured using this apparatus. Results of nitrogen adsorption on activated carbon B1 together with specific surface area determination, evaluation of α-plot, and micro-, and mesopore size distributions are reported. Automatic pressure and flow controllers allow adsorption and desorption isotherm definition either in continuous or in conventional static flow volumetric modes.
Straight carbon nanotube Y junctions
Authors: Li W. Z. , Wen J. G. , Ren Z. F.
Published: July 2002

Carbon nanotube junctions are of great interest in the fundamental research and nanoelectronic applications. Identical carbon nanotube Y junctions have been successfully synthesized by pyrolysis of methane over cobalt supported on magnesium oxide. The Y junctions have very straight arms with uniform diameters, and the angles between the three arms are close to 120°. Based on the analysis on the structure of the Y junctions the growth mechanism has been discussed. The fact that all the Y junctions grow in a similar way may be very important in the development of nanoelectronic devices.
Lithium storage in polymerized carbon nitride nanobells
Authors: Zhong D. Y. , Zhang G. Y. , Liu S. , Wang E. G. , Wang Q. , Li H. , Huang X. J.
Published: July 2002

Polymerized carbon nitride nanobells (CNNBs) have been intercalated by a large amount of Li ions using an electrochemical method. Li nanocrystals are observed on the inside surface of CNNB walls, which is direct evidence that a Li nanocrystal can exist at the micropore structure at a heavy intercalating level. Graphene layers are expanded and become partly disordered by Li intercalation, while after deintercalation, they are reordered to a certain degree, and Li nanocrystals disappear. The samples show a reversible Li storage capacity of 480 mAhg−1, much higher than 330 mAhg−1 of commercial carbon materials used for Li ion batteries.
Resolution of site-specific bonding properties of C60 adsorbed on Au(111)
Authors: Rogero C. , Pascual J. I. , Gómez-Herrero J. , Baró A. M.
Published: July 2002

We have performed a careful study of the adsorption of C60 molecules on a Au(111) surface by using scanning tunneling microscopy and spectroscopy at room temperature. In coincidence with results from other techniques, differential conductance spectra give a value of 2.3 eV for the HOMO–LUMO gap of a monomolecular layer, with the LUMO level located at 0.6 eV above the Fermi level as a consequence of electronic charge transfer from the substrate into the molecule. Small differences in position (and shape) of the LUMO-derived resonance, in the order of 0.1 eV, are found on molecules adsorbed at step edges. We consider the Smoluchowski effect, i.e., the interaction of the molecules with a charge-depleted region, to explain the observed differences in their bonding nature. On some molecules forming part of bidimensional fullerene islands, similar differences were also detected with spatially resolved scanning tunneling spectroscopy, giving rise to a 2×2 commensurate structure of the molecular adlayer with respect to the substrate. This finding is attributed to different electronic properties of the adsorption site, indicating that molecules adsorbed on the top position are less charged than those on bridge sites.
A pump–probe investigation of laser-droplet plasma dynamics
Authors: McNaught S. J. , Fan J. , Parra E. , Milchberg H. M.
Published: July 2002

The temporal character of the plasma formed by the irradiation of micron-sized argon and krypton droplets with intense 100-ps laser pulses is investigated using a pump–probe scheme. The evolution of the droplet plasma is assessed by monitoring delay-dependent x-ray and extreme ultraviolet (EUV) emission, and by imaging frequency-doubled probe light scattered from the interaction region. Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective plasma lifetime can extend from a few hundred picoseconds to as long as several nanoseconds. Knowledge of the droplet plasma lifetime is important in optimizing the efficiency of this laser plasma as an EUV or x-ray source.
Stability of carbon nanotubes under electric field studied by scanning electron microscopy
Authors: Wei Yi , Xie Chenggang , Dean Kenneth A. , Coll Bernard F.
Published: July 2002

The influence of an applied electric field on carbon nanotubes protruding from a surface was investigated in situ using a high-resolution scanning electron microscopy. Under the applied electric field, the nanotubes flexed to orient themselves parallel to the electric field lines. For moderate field strengths below the electron field emission threshold, the flexed nanotubes relaxed back to their original shapes after the electric field was removed. However, when high electron field emission currents were extracted from the nanotubes, they were permanently deformed, leaving them aligned to the electric field direction after the electric field was removed. For high currents, the length of the carbon nanotubes were found to be shortened after field emission lasted for a period of time.
Alfvén instabilities driven by circulating ions in optimized stellarators and their possible consequences in a Helias reactor
Authors: Kolesnichenko Ya. I. , Lutsenko V. V. , Wobig H. , Yakovenko V.
Published: July 2002

This work investigates circulating-particle-induced Alfvén instabilities in optimized stellarators of the Wendelstein line [F. Wagner, Trans. Fusion Tech. 33, 67 (1998)]. A general expression for the growth rate of the instabilities is obtained and analyzed. It is shown that the absence of the axial symmetry makes it possible that various types of Alfvén eigenmodes will be destabilized; both the kind of destabilized Alfvén eigenmodes and the type of the resonances driving the instability may differ from those in tokamaks. In particular, an important role of the helicity-induced resonance is predicted. The discovered new resonances may considerably increase the instability growth rate of both the “gap” modes and the eigenmodes residing below cylindrical Alfvén continuum. The upper limits of the local energy losses of circulating α-particles caused by various Alfvén instabilities in a four-period Helias reactor [C. D. Beidler et al., in Fusion Energy 2000, 18th International Atomic Energy Agency Conference Proceedings, Sorrento, 2000 (International Atomic Energy Agency, Vienna, 2001), Report IAEA-CN-77/FT/4] are evaluated. It is found that certain destabilized Alfvén eigenmodes will affect only alphas with the energy well below 3.5 MeV, which seems to open a possibility to remove the helium ash by exciting the corresponding Alfvén eigenmodes by either energetic particles or an antenna system.
Velocity filtered density function for large eddy simulation of turbulent flows
Authors: Gicquel L. Y. M. , Givi P. , Jaberi F. A. , Pope S. B.
Published: July 2002

A methodology termed the “velocity filtered density function” (VFDF) is developed and implemented for large eddy simulation (LES) of turbulent flows. In this methodology, the effects of the unresolved subgrid scales (SGS) are taken into account by considering the joint probability density function of all of the components of the velocity vector. An exact transport equation is derived for the VFDF in which the effects of the SGS convection appear in closed form. The unclosed terms in this transport equation are modeled. A system of stochastic differential equations (SDEs) which yields statistically equivalent results to the modeled VFDF transport equation is constructed. These SDEs are solved numerically by a Lagrangian Monte Carlo procedure in which the Itô–Gikhman character of the SDEs is preserved. The consistency of the proposed SDEs and the convergence of the Monte Carlo solution are assessed by comparison with results obtained by an Eulerian LES procedure in which the corresponding transport equations for the first two SGS moments are solved. The VFDF results are compared with those obtained via several existing SGS closures. These results are also analyzed via a priori and a posteriori comparisons with results obtained by direct numerical simulation of an incompressible, three-dimensional, temporally developing mixing layer.
Low Stokes shift in thick and homogeneous InGaN epilayers
Authors: Srinivasan S. , Bertram F. , Bell A. , Ponce F. A. , Tanaka S. , Omiya H. , Nakagawa Y.
Published: July 2002

The optical properties of thick InxGa1−xN layers have been studied using optical absorption and cathodoluminescence techniques. The indium composition x of the layers ranged from 0.03 to 0.17 as determined by Rutherford backscattering measurements. The difference between the band gap and the peak emission energy (Stokes shift) was found to be considerably smaller than reported in the past for these alloys. Monochromatic images show that light emission from most of the film is homogeneous and is associated with a low Stokes shift. A second emission band at longer wavelengths is observed for x⩾0.08. This band originates from indium-rich regions in the vicinity of extended defects, and exhibits a larger Stokes shift. Our observations indicate that it is possible to grow InGaN epilayers with high indium composition, high homogeneity, and lower Stokes shift.
New mechanism of membrane fusion
Authors: Müller M. , Katsov K. , Schick M.
Published: July 2002

We have carried out Monte Carlo simulation of the fusion of bilayers of single chain amphiphiles which show phase behavior similar to that of biological lipids. The fusion mechanism we observe is very different from the “stalk” hypothesis. Stalks do form on the first stage of fusion, but they do not grow radially to form a single bilayer diaphragm. Instead, stalk formation destabilizes the membranes and results in hole formation in the vicinity of the stalks. When holes in each bilayer nucleate spontaneously next to the same stalk, an incomplete fusion pore is formed. The fusion process is completed by propagation of the initial connection, the stalk, along the edges of the aligned holes.
The role of charge-transfer states of the metal-adsorbate complex in surface-enhanced Raman scattering
Authors: Arenas Juan F. , Soto Juan , Tocón Isabel López , Fernández David J. , Otero Juan C. , Marcos Juan I.
Published: July 2002

Surfaced-enhanced Ramon scattering (SERS) spectra of pyrazine are analyzed on the basis of the properties of the electronic states of the metal-adsorbate surface complex. Ab initio CIS calculations have been carried out for the Ag2-pyrazine complex, which have enabled us to find two excited singlets, namely CT0;1B1 and CT1;1A2, with properties quite similar to those of the pyrazine radical anion in its electronic B3u2 and Au2 states, respectively, and with energies falling in the range of the exciting photons usually employed in Raman spectroscopy. SERS spectra of pyrazine are compatible with a resonance Raman enhancement mechanism involving electronic transitions between the ground state S0;1A1 and both CT levels of the surface complex.
Effect of the substitutions of Ni3+, Co3+, and Fe3+ for Mn3+ on the ferromagnetic states of the La0.7Pb0.3MnO3 manganite
Authors: Young S. L. , Chen Y. C. , Chen H. Z. , Horng L. , Hsueh J. F.
Published: July 2002

The substitution at the Mn site of various transitional metal elements dramatically modifies the magnetic and electronic properties of the La0.7Pb0.3Mn0.8Me0.2O3 (Me=Ni, Co, and Fe) manganites. The powder x-ray diffraction pattern shows a single-phase rhombohedral distorted perovskite structure with space group R3̄c (No. 167) for all samples. The present article is focused on the effects induced by the partial substitution of Ni, Co, and Fe for the Mn site. The field cooled–zero field cooled curves indicate that the magnetic order is modified from a ferromagnetic long-range spin order for La0.7Pb0.3MnO3 to a spin cluster induced short-range spin order for all the doped compositions. The saturation magnetization, transition temperature, and resistance peak temperature decrease significantly from Ni-doped, Co-doped, to Fe-doped composition. In conclusion, the variation of the magnetic and electronic behaviors is due to the dissimilar 3d electron configuration of the substitutions.
Implementation of electronic ground states and singlet and triplet excitation energies in coupled cluster theory with approximate triples corrections
Authors: Hald Kasper , Jørgensen Poul , Christiansen Ove , Koch Henrik
Published: July 2002

An implementation of triples corrections for the calculation of the electronic ground states and for singlet and triplet excitation energies within the CC3 model is discussed. At most objects of size V2O2 and V3O are kept in memory and on disc, respectively (V is the number of virtual orbital and O is the number of occupied orbitals). The used strategy means that more terms that scales as V4O3 has to be calculated than if the triples amplitudes are kept on disc but it allows larger cases to be handled. Sample calculations are presented for the triplet excitation energies of benzene.
Dielectric spectra of mono- and disaccharide aqueous solutions
Authors: Fuchs K. , Kaatze U.
Published: July 2002

Complex (dielectric) permittivity spectra of aqueous solutions of the monosaccharides D-Xylose, D-Galactose, D-Glucose, D-Fructose, Methyl-β-D-xylose, and Methyl-β-D-glucose, as well as the disaccharides D-Lactose, D-Maltose, and D-Sucrose have been measured. In order to avoid any systematic errors, various experimental setups, each matched to a particular measuring range, have been used to cover the frequency domain from 300 kHz to 40 GHz. In some characteristic details our spectra for maltose solutions deviate from those recently reported by Weingärtner et al. [J. Chem. Phys. 115, 1463 (2001)]. The monosaccharide spectra at c=1 mol/l and the disaccharide spectra at c⩽0.5 mol/l reveal a rather homogeneous relaxation of the carbohydrate and water dipole moments, which can be well represented by a Cole–Cole relaxation time distribution. At higher solute concentration the dielectric spectra are favorably represented by two relaxation terms, though interpolation of the experimental data by the Cole–Cole spectral function is possible within the limits of errors. There are indications that these terms might be due to local fluctuations in the carbohydrate concentration, in conformity with previous ultrasonic spectra of monosaccharide solutions.
Raman scattering and thermogravimetric analysis of iodine-doped multiwall carbon nanotubes
Authors: Zhou Weiya , Xie Sishen , Sun Lianfeng , Tang Dongsheng , Li Yubao , Liu Zuqin , Ci Lijie , Zou Xiaoping , Wang Gang , Tan Pingheng , Dong Xiaoli , Xu Bo , Zhao Boru
Published: July 2002

Iodine-doped multiwall carbon nanotubes (I-MWNTs) were characterized by means of Raman scattering and thermogravimetric analysis. The results show that multiwall carbon nanotubes (MWNTs) can be effectively doped by iodine and exchange electrons with iodine. Iodine atoms form charged polyiodide chains inside tubes of different inner diameter, which is similar to the iodine-doped single-wall carbon nanotubes (I-SWNTs), but can not intercalate into the graphene walls of MWNTs. The Raman scattering behavior of I-MWNTs exhibits some differences from that of I-SWNTs and the low-dimensional conductive hydrocarbon-iodine complex “perylene⋅I2.92.”
Selective study of polymer/dielectric interfaces with vibrationally resonant sum frequency generation via thin-film interference
Authors: Wilson Philip T. , Briggman Kimberly A. , Wallace William E. , Stephenson John C. , Richter Lee J.
Published: July 2002

A technique for selective characterization of the structure of free and buried thin-film interfaces by vibrationally resonant sum frequency generation spectroscopy is described. Manipulation of Fresnel coefficients by choice of film thickness on a reflecting substrate allows simultaneous optimization of the signal from the desired interface and minimization of the signal from other interfacial sources. This technique is demonstrated for the free polystyrene (PS)/air and the buried PS/spin-on glass interfaces. Our spectra show that the pendant phenyl group orientation is similar at the buried and free interfaces, with the phenyls pointing away from the bulk PS at each interface.
Probing single molecule kinetics by photon arrival trajectories
Authors: Barsegov Valeri , Mukamel Shaul
Published: July 2002

Multitime correlation functions of photon arrival times in single molecule fluorescence resonant energy transfer measurements are computed using a simple model representing slow conformational dynamics described by a collective stochastic Gaussian coordinate. The analogy with time domain nonlinear optical spectroscopy is explored. Various statistical measures of distributions of single photon arrival times and fluorescence lifetimes are employed to analyze non-Poissonian statistics.
Nonlinear time series analysis of electrocardiograms
Authors: Bezerianos A. , Bountis T. , Papaioannou G. , Polydoropoulos P.
Published: July 2002

In recent years there has been an increasing number of papers in the literature, applying the methods and techniques of Nonlinear Dynamics to the time series of electrical activity in normal electrocardiograms (ECGs) of various human subjects. Most of these studies are based primarily on correlation dimension estimates, and conclude that the dynamics of the ECG signal is deterministic and occurs on a chaotic attractor, whose dimension can distinguish between healthy and severely malfunctioning cases. In this paper, we first demonstrate that correlation dimension calculations must be used with care, as they do not always yield reliable estimates of the attractor’s ‘‘dimension.’’ We then carry out a number of additional tests (time differencing, smoothing, principal component analysis, surrogate data analysis, etc.) on the ECGs of three ‘‘normal’’ subjects and three ‘‘heavy smokers’’ at rest and after mild exercising, whose cardiac rhythms look very similar. Our main conclusion is that no major dynamical differences are evident in these signals. A preliminary estimate of three to four basic variables governing the dynamics (based on correlation dimension calculations) is updated to five to six, when temporal correlations between points are removed. Finally, in almost all cases, the transition between resting and mild exercising seems to imply a small increase in the complexity of cardiac dynamics.
Reaction-enhanced sintering of platinum thin films during ethylene oxidation
Authors: Wu Nae Lih , Phillips Jonathan
Published: July 2002

The sintering of sputter-produced, silica-supported platinum thin films was studied using electron microscopy and x-ray diffraction following treatments in both reactive (ethylene oxidation—O2+C2H4) and nonreactive atmospheres (vacuum, O2, N2, C2H4, CO2, H2O). Sintering behavior in nonreactive atmospheres (thermal sintering) was found to be insensitive to the gas phase and was a function of temperature only. Changes in film structure following thermal sintering were not dramatic and were readily explained by conventional models. In contrast, sintering in reactive atmospheres was found to be a strong function of temperature and the O2/C2H4 ratio. Over a relatively narrow temperature range (770–970 K), the surface was found to be dramatically restructured with plantinum completely stripped from some areas and large particles forming in others. This type of sintering (reaction-enhanced sintering) possibly results from the interaction of homogeneously formed radicals with the metal surface to form metastable mobile intermediates.
X-ray scattering studies of graphite fibers
Authors: Tang Ming-Ya , Rice G. G. , Fellers John F. , Lin J. S.
Published: July 2002

The structural features of three different graphite fibers were studied via small- and wide-angle x-ray techniques. The experimental evidence is consistent with a sheath/core fiber morphology. Graphitization, degree of orientation, crystallite size, and microporosity were analyzed. Samples included low (AS4) and high (HMS) modulus poly(acrylonitrile) (PAN) and melt-spun pitch-based (VSB-16) fibers. By wide-angle x-ray diffraction (WAXD) VSB-16 was found to have the highest degree of graphitization, the highest degree of orientation, and the largest crystallite regions, and AS4 the poorest graphitized structure. The void system in these graphite fibers was investigated by small-angle x-ray scattering (SAXS). SAXS from glycerin-soaked fibers indicates the scattering at very small angles (2θ<10 mrad) is dominated by total reflection of x rays at the fiber surface. The pores in HMS and VSB-16 fibers are inaccessible to glycerin and the pores in AS4 fiber are partially accessible. The pores in PAN-based HMS and AS4 fibers are of needlelike shape and those in VSB-16 are ellipsoidal. The porosity is 12.6%, 8.4%, and 4.5% in HMS, AS4, and VSB-16 fibers, respectively. Deviations from Porod’s law were observed at large angles and attributed to scattering from fractal aggregates of carbon atoms in the graphite crystallites and/or fractal boundaries of pores. The fractal dimension of the aggregates is 2.3±0.1, 2.8±0.2, and 3.0±0.2 for AS4, HMS, and VSB-16 fibers, respectively. Speculations about the fractal nature of aggregation may stimulate some new insight to the graphitization process, paracrystallinity, and the strength of graphite fibers.