bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron field emission from carbon nanoparticles prepared by microwave-plasma chemical-vapor deposition
Authors: Yu J. , Wang E. G. , Bai X. D.
Published: July 2002

Carbon nanoparticles were prepared from H2 and CH4 at various temperatures. The carbon nanoparticles were well graphitized, and the degree of graphitization increased with increasing growth temperature. Field-emission measurements showed that the carbon nanoparticles were excellent electron field emitters, comparable to carbon nanotubes. The field-emission properties became better with increasing growth temperature, and the threshold fields of the carbon nanoparticles deposited at 400, 500, 670 °C were 3.2, 3, and 1 V/μm, respectively. The low-threshold field of the carbon nanoparticles is attributed to the field-enhancement effect and the higher degree of graphitization.
Effect of microstructures on the microwave dielectric properties of ZrTiO4 thin films
Authors: Kim Yongjo , Oh Jeongmin , Kim Tae-Gon , Park Byungwoo
Published: July 2002

To obtain various paraelectric ZrTiO4 thin-film microstructures, the films were synthesized at different deposition temperatures using rf magnetron sputtering. Both the dielectric losses (tan δ) and dielectric constants (ε) of the ZrTiO4 thin films were measured up to 6 GHz using a circular-patch capacitor geometry. The films showed enhanced crystallinity with increasing deposition temperature, as determined from the x-ray diffraction peak widths at various scattering vectors. The microwave dielectric losses correlated very well with the level of crystallinity or strain, while the dielectric constants did not alter significantly.
Evidence for electric field-assisted back-electron transfer through a methylene bond in a linked compound of phenanthrene and phthalimide in a polymer film
Authors: Kawabata Hiroshi , Ohta Nobuhiro , Arakawa Hideaki , Ashida Masatada , Kohtani Shigeru , Nakagaki Ryoichi
Published: July 2002

Fluorescence emitted from phenanthrene linked to phthalimide by a short methylene chain is unusually enhanced by an electric field in a polymer film. Back-electron transfer which produces the fluorescent state of phenanthrene occurs through a short methylene bond following photoinduced electron transfer from phenanthrene to phthalimide, and its efficiency is enhanced by an electric field.
Lateral instabilities in cubic autocatalytic reaction fronts: The effect of autocatalyst decay
Authors: Tóth Ágota , Horváth Dezsö , Jakab Éva , Merkin John H. , Scott Stephen K.
Published: July 2002

The conditions are derived for the onset of lateral instabilities in planar waves propagating in a chemical system based on cubic autocatalysis coupled with autocatalyst decay, represented by the parameter κ. The results from a linear stability analysis of the corresponding traveling wave equations are presented and compared with predictions obtained from a “thin front” analysis. Instabilities arise when δ, the ratio of the diffusion coefficients of the reactant and autocatalyst, is greater than some critical value δcrit, with δcrit being found to depend strongly on κ. Numerical simulations of the full initial-value problem are also determined and confirm the theoretical predictions.
Efficient multiple time step method for use with Ewald and particle mesh Ewald for large biomolecular systems
Authors: Zhou Ruhong , Harder Edward , Xu Huafeng , Berne B. J.
Published: July 2002

The particle–particle particle–mesh (P3M) method for calculating long-range electrostatic forces in molecular simulations is modified and combined with the reversible reference system propagator algorithm (RESPA) for treating the multiple time scale problems in the molecular dynamics of complex systems with multiple time scales and long-range forces. The resulting particle–particle particle–mesh Ewald RESPA (P3ME/RESPA) method provides a fast and accurate representation of the long-range electrostatic interactions for biomolecular systems such as protein solutions. The method presented here uses a different breakup of the electrostatic forces than was used by other authors when they combined the Particle Mesh Ewald method with RESPA. The usual breakup is inefficient because it treats the reciprocal space forces in an outer loop even though they contain a part that changes rapidly in time. This does not allow use of a large time step for the outer loop. Here, we capture the short-range contributions in the reciprocal space forces and include them in the inner loop, thereby allowing for larger outer loop time steps and thus for a much more efficient RESPA implementation. The new approach has been applied to both regular Ewald and P3ME. The timings of Ewald/RESPA and P3ME/RESPA are compared in detail with the previous approach for protein water solutions as a function of number of atoms in the system, and significant speedups are reported.
Nano-oxidation of silicon surfaces: Comparison of noncontact and contact atomic-force microscopy methods
Authors: Tello Marta , Garcı́a Ricardo
Published: July 2002

Local oxidation lithography by atomic-force microscopy is emerging as a powerful method for nanometer-scale patterning of surfaces. Here, we perform a comparative study of contact and noncontact atomic-force microscopy (AFM) oxidation experiments. The comparison of height and width dependencies on voltage and pulse duration allows establishing noncontact AFM as the optimum local oxidation method. For the same electrical conditions, noncontact AFM oxides exhibit higher aspect ratios (0.04 vs 0.02). The smallness of the liquid meniscus in noncontact AFM oxidation produces smaller oxide widths. We also report a slower oxidation rate in contact AFM oxidation. We explain this result by introducing an effective energy barrier (∼0.14 eV) that includes the mechanical work done by the growing oxide against the cantilever (∼0.01 eV).
Analytical and numerical studies of noise-induced synchronization of chaotic systems
Authors: Toral Raúl , Mirasso Claudio R. , Hernández-Garcı́a Emilio , Piro Oreste
Published: July 2002

We study the effect that the injection of a common source of noise has on the trajectories of chaotic systems, addressing some contradictory results present in the literature. We present particular examples of one-dimensional maps and the Lorenz system, both in the chaotic region, and give numerical evidence showing that the addition of a common noise to different trajectories, which start from different initial conditions, leads eventually to their perfect synchronization. When synchronization occurs, the largest Lyapunov exponent becomes negative. For a simple map we are able to show this phenomenon analytically. Finally, we analyze the structural stability of the phenomenon.
Molecular dynamics simulation of NMR relaxation rates and slow dynamics in lipid bilayers
Authors: Lindahl Erik , Edholm Olle
Published: July 2002

By performing a 100 ns molecular dynamics simulation of a dipalmitoylphosphatidylcholine lipid bilayer we are able to calculate the full rotational correlation functions of the hydrocarbon chain C–H vectors and determine rotational diffusion of entire lipid molecules with high accuracy. The simulated relaxation is strongly nonexponential already on time scales from 0.1 ps. Fourier transformation of the correlation functions yields data that in the relatively narrow frequency range accessible to H2 and C13 NMR experiments are consistent with the reported 1/ω dependence of spin-lattice relaxation rates. The simulated relaxation dynamics is found to be slightly faster than experimental, which we suggest is explained by the limited accuracy in dihedral potentials of present force fields. By introducing a local frame of reference, the chain motion is separated into local dihedral transitions and overall lipid reorientation. The internal chain isomerization dominates the relaxation and is well-described by power laws. The small molecular reorientation contribution to the decay is exponential with separate time scales for motions of the lipid long axis (D⊥=2.9×107 s−1) and spinning rotation around it (D∥=3.8×108 s−1). The mean square lateral displacement over 100 ns, corrected for the relative motions of the layers, corresponds to a long-time translational diffusion coefficient of Dlat=1.2×10−7 cm2 s−1 at 323 K.
Si/6H–SiC(0001): An unexpected cubic 4×3 Si phase overlayer
Authors: Amy F. , Enriquez H. , Soukiassian P. , Brylinski C. , Mayne A. , Dujardin G.
Published: July 2002

We investigate Si deposition on the 6H–SiC(0001) 3×3 surface reconstruction by atom-resolved scanning tunneling microscopy. Upon thermal annealing, the Si thin film forms an unexpected structure having dimer rows in a cubic 4×3 surface array. Such a 4×3 Si phase has a very open surface very likely being at the origin of the exceptionally high reactivity to oxygen of the Si/6H–SiC(0001) system. These findings are relevant in silicon carbide oxidation.
Bond contraction and lone pair interaction at nitride surfaces
Authors: Sun C. Q. , Tay B. K. , Lau S. P. , Sun X. W. , Zeng X. T. , Li S. , Bai H. L. , Liu H. , Liu Z. H. , Jiang E. Y.
Published: July 2002

It is shown that bond contraction and nonbonding lone-pair interaction dominate at nitride surfaces. The maximum elastic recovery of a nitride surface was found to be 100% under a relatively lower nanoindentation load (<1.0 mN) and the hardness of the surface was found to be 100% higher than the bulk value. It is interpreted that the spontaneous bond contraction, estimated at 12%–14%, strengthens the binding energy and hence the hardness and Young’s modulus at the surface. The lone-pair weak interaction claims the responsibility for (i) the high elastic recovery, (ii) the lower Raman frequencies of vibration, and (iii) the existence of critical loads for slide friction or lone-pair broken.
Dissipative wave-packet dynamics and electron transfer
Authors: Lucke Andreas , Ankerhold Joachim
Published: July 2002

Coherent wave-packet dynamics on coupled electronic surfaces within a condensed–phase environment is studied. Based on a path integral approach, exact expressions for the case of a one-dimensional reaction coordinate in a donor–acceptor complex are derived. The path integrals over the electronic degree of freedom are further evaluated by applying the golden rule approximation. This procedure allows for an exact treatment of the coupling between solvent or residual molecular degrees of freedom and the reaction coordinate. A detailed picture of the intimate relation between dissipative wave-packet motion and electron transfer is gained also for lower temperatures, strong coupling and slow bath modes, where Redfield type of equations cannot be used. The electron transfer triggered by the coherent vibronic motion leads to a stepwise decay of the population in the donor state. On the acceptor surface the wave-packet dynamics displays a complex interference pattern. Further, it turns out that for a reaction coordinate fast compared to the environmental modes the electronic population dynamics is very sensitive to the initial correlations between bath and reactive mode.
Nanospot welding of carbon nanotubes
Authors: Hirayama H. , Kawamoto Y. , Ohshima Y. , Takayanagi K.
Published: July 2002

Single wall carbon nanotube (SWNT) bundles protruding from the SWNT layers on self-aligned Sn apexes were brought to a distance of 30 nm by a scanning tunneling microscope inside a transmission electron microscope. A straight bundle on the tip could be observed in situ in contact electrostatically with a looped bundle on the sample by applying tip bias voltages above 2.0 V. The bundles were welded at the nanometer size contact area by local Joule heating.
Effect of the dust grain size-distribution on the nonlinear dispersion relation for parametric instabilities in dusty plasmas
Authors: Boushaki M. Ishak , Annou R. , Ferhat B.
Published: July 2002

The nonlinear dispersion relation for parametric instabilities in dusty plasmas where the dust grain size distribution is taken into account, is addressed. The grains that are assumed spheres of different radii, are considered size-distributed according to power law, log-normal and normal distributions. The growth rate for the stimulated Brillouin scattering is deduced for each case.
Corresponding states law and molecular dynamics simulations of the Lennard-Jones fluid
Authors: Dunikov D. O. , Malyshenko S. P. , Zhakhovskii V. V.
Published: July 2002

Phase diagram of a Lennard-Jones fluid at liquid-gas equilibrium is studied by molecular dynamics simulations. The problem of potential cut-off influence on the properties of the model system is investigated. For several values of the cut-off radius, the physical properties of coexisting phases and critical parameters are calculated. It is shown that the results obtained for various cut-offs scaled by the critical temperature and density coincide, which means that fluids described by different modifications of the Lennard-Jones potential are thermodynamically similar, i.e., obey a corresponding states law.
Electronic structure and properties of rhombohedrally polymerized C60
Authors: Okotrub A. V. , Belavin V. V. , Bulusheva L. G. , Davydov V. A. , Makarova T. L. , Tománek D.
Published: July 2002

The valence electronic structure of rhombohedrally polymerized C60, synthesized at 6 GPa and 725 °C, has been studied using x-ray emission spectroscopy. The C Kα spectrum of the polymer was found to differ from that of the C60 fullerite especially in the high-energy region. The observed spectra were compared to densities of states, calculated using a tight-binding approximation for three two-dimensional hexagonal C60 networks, distinguished by the nature of intermolecular bonding. Theoretical spectra of the polymers agree well with experiment and indicate that changes in the valence band of C60 upon polymerization are mainly due to the formation of intermolecular bonds, and only to a negligible degree due to a distortion of the C60 cage. Rotation of C60 within the hexagonal layer changes dramatically the electronic properties of the polymer from a semiconducting behavior for the structure with 66/66 connections to a metallic behavior for a layer containing C60 molecules that are linked through 56/65 bonds. The occurrence of the latter configuration may explain the observed metalliclike in-plane conductivity of the rhombohedral phase of C60.
Double ratio fluorescence imaging for the detection of early superficial cancers
Authors: Bogaards A. , Aalders M. C. G. , Jongen A. J. L. , Dekker E. , Sterenborg H. J. C. M.
Published: July 2002

We have developed a fluorescence imaging device for the detection of superficial cancer based on the double ratio technique. In practice this device resembles an operation microscope and can be used in a clinical environment. This device acquires four different fluorescence images excited at two wavelengths and detected at two wavelengths. From these images it calculates, displays, and stores double ratio images at a maximum speed of 1 Hz. The double ratio image gives the distribution of the amount of fluorophore present in tissue and is not affected by local variations in tissue optics, i.e., tissue absorption and tissue scattering. The validity of the technique was confirmed by ex vivo tissue equivalent phantom experiments using hematoporphyrin and in vivo experiments on normal pigmented moles on Caucasian human skin using δ-aminolevulinic acid induced Protoporhyrin IX.
Dynamics of the response to microwave radiation in YBa2Cu3O7−x hot-electron bolometer mixers
Authors: Harnack Oliver , Il’in Konstantin S. , Siegel Michael , Karasik Boris S. , McGrath William R. , de Lange Gert
Published: July 2002

We present a systematic study of the intermediate frequency (IF) bandwidth of hot-electron bolometer mixers made from YBa2Cu3O7−x high-temperature superconductive thin films fabricated on substrates with high thermal conductivity (MgO and sapphire). At a local oscillator frequency of 100 GHz, a strong dependence of the IF bandwidth on the bias point and temperature has been experimentally found. Moreover, an unexpected IF bandwidth variation has been detected in a broad range of operating frequencies (1–480 GHz). Besides conventional bolometric mixing mechanisms associated with thermalization of electrons and phonons, the contribution of direct interaction between radiation and magnetic vortices in the YBa2Cu3O7−x film may be responsible for the observed effects.
Direct numerical simulation of particle dispersion in homogeneous turbulent shear flows
Authors: Ahmed A. M. , Elghobashi S.
Published: July 2002

In a recent paper [Phys. Fluids 12, 2906 (2000)] we presented the results of our direct numerical simulations (DNS) study of the two-way coupling effects of solid particles dispersed in a homogeneous turbulent shear flow. The objective of that study was to describe the physical mechanism associated with two-way coupling. Here, we present a DNS study of the dispersion of small spherical particles and fluid points in a homogeneous turbulent shear flow with one-way coupling. The effects of varying shear number, particle inertia, gravity magnitude, and direction on particle dispersion were studied. Our results show that the particle mean square displacement in the streamwise direction 〈xp,12〉∝t2 for short times, whereas for large times, the time exponent exceeds 3. The larger the particle inertia, the larger is the exponent. 〈xp,12〉 is at least one order of magnitude larger than the mean dispersion in the two lateral directions. This dispersion anisotropy is due to the dominance of the spanwise vortex layers which are elongated and inclined toward the streamwise direction, thus suppressing the velocity fluctuations in the lateral directions. These layers are generated from the inclined longitudinal vortex tubes by the action of the mean shear [Kida and Tanaka (1994)]. Preferential accumulation of particles in homogeneous shear flows at zero gravity is maximum when τp equals the Kolmogorov time scale.
Sphingomyelin at the air–water interface
Authors: Vaknin David , Kelley Michael S. , Ocko Benjamin M.
Published: July 2002

X-ray reflectivity (XR) and grazing incident-angle x-ray diffraction (GIXD) reveal that sphingomyelin, although forming homogeneous monomolecular films at air–water interfaces, does not develop long-range crystalline in-plane order at any in-plane density, most notably at high surface pressures where many other membrane lipids order as two-dimensional crystals. Studies were carried out on the monodisperse synthetic C18-sphingomyelin where both hydrocarbon chains are saturated and on natural sphingomyelin which contains a distribution of chain lengths, with saturated and unsaturated hydrocarbon chains. The surface pressure versus molecular area (π–A) isotherm for the natural sphingomyelin is similar to that of the synthetic, but has a somewhat higher surface pressure at intermediate areas. The absence of in-plane crystalline order is attributed to competing interactions in the head group region: hydrogen bonding in the erythro region, electrostatic interactions among zwitterions of neighboring head groups, and van der Waals among acyl chains.
Electrical transport through individual DNA molecules
Authors: Li Xin-Qi , Yan YiJing
Published: July 2002

A theoretical model is presented to describe electrical transport through individual DNA molecules. By contacting the proposed model with the experimentally measured data, a variety of valuable quantities are identified. The partially decoherent nature on the guanine–cytosine (GC) pairs of DNA is also elaborated in contrast to the completely incoherent hopping mechanism discussed in the context of charge transfer experiments.
Scanning tunneling microscope study of boron-doped highly oriented pyrolytic graphite
Authors: Endo Morinobu , Hayashi Takuya , Hong Seong-Hwa , Enoki Toshiaki , Dresselhaus Milred S.
Published: July 2002

Atomically resolved scanning tunneling microscopy (STM) results are shown for substitutionally doped boron atoms in the hexagonal carbon network of highly oriented pyrolytic graphite (HOPG). STM images of boron-doped HOPG reveal not only a clear change in the electronic structure of the surface graphene network, but also one that directly affects the electronic structure of the graphene layer from the HOPG surface which is very exceptional for STM measurements. The boron atom site in the graphene network appears as the brightest area in the image including the six adjacent carbon atoms which have relatively higher intensity than normal carbon atoms in the STM image. The average boron-to-boron distance in the basal plane is consistent with Raman spectroscopy results. These structural results suggest that the graphite planes can be tailor made both atomically and electronically, and that boron doping can contribute to controlling the properties of the hexagonal carbon network in order to modify its properties relative to those of ideal graphite.
Curvature dependent surface tension from a simulation of a cavity in a Lennard-Jones liquid close to coexistence
Authors: Moody M. P. , Attard P.
Published: July 2002

Monte Carlo simulations have been used to calculate the surface tension of a planar liquid–vapor interface of a Lennard-Jones fluid using a new cylindrical truncation correction procedure. A bubble was mimicked by simulating a Lennard-Jones liquid close to coexistence containing a single immobile hard-sphere cavity. From the density profile of this system the curvature dependent surface tension has been estimated via calculation of the Helmholtz free energy of the system and separately using a large cavity contact density expansion. Evidence is presented that the Tolman length becomes increasingly negative as the critical temperature is approached and possibly changes sign.
Lagrangian chaos and Eulerian chaos in shear flow dynamics
Authors: Finn John M. , del-Castillo-Negrete Diego
Published: July 2002

Shear flow dynamics described by the two-dimensional incompressible Navier–Stokes equations is studied for a one-dimensional equilibrium vorticity profile having two minima. These lead to two linear Kelvin–Helmholtz instabilities; the resulting nonlinear waves corresponding to the two minima have different phase velocities. The nonlinear behavior is studied as a function of two parameters, the Reynolds number and a parameter λ specifying the width of the minima in the vorticity profile. For parameters such that the instabilities grow to a sufficient level, there is Lagrangian chaos, leading to mixing of vorticity, i.e., momentum transport, between the chains of vortices or cat’s eyes. Lagrangian chaos is quantified by plotting the finite time Lyapunov exponents on a grid of initial points, and by the probability distribution of these exponents. For moderate values of λ, there is Lagrangian chaos everywhere except near the centers of the vortices and near the boundaries, and there are competing effects of homogenization of vorticity and formation of structures associated with secondary resonances. For smaller values of λ Lagrangian chaos occurs in the regions in the centers of the vortices, and the Eulerian behavior of the flow undergoes bifurcations leading to Eulerian chaos, as measured by the time series of several Galilean invariant quantities. A discussion of Lagrangian chaos and its relation to Eulerian chaos is given.
Controlled creation of a carbon nanotube diode by a scanned gate
Authors: Freitag Marcus , Radosavljevic Marko , Zhou Yangxin , Johnson A. T. , Smith Walter F.
Published: July 2002

We use scanning gate microscopy to precisely locate the gating response in field-effect transistors (FETs) made from semiconducting single-wall carbon nanotubes. A dramatic increase in transport current occurs when the device is electrostatically doped with holes near the positively biased electrode. We ascribe this behavior to the turn-on of a reverse biased Schottky barrier at the interface between the p-doped nanotube and the electrode. By positioning the gate near one of the contacts, we convert the nanotube FET into a rectifying nanotube diode. These experiments both clarify a longstanding debate over the gating mechanism for nanotube FETs and indicate a strategy for diode fabrication based on controlled placement of acceptor impurities near a contact.
Luminescence from erbium-doped silicon nanocrystals in silica: Excitation mechanisms
Authors: Kenyon A. J. , Chryssou C. E. , Pitt C. W. , Shimizu-Iwayama T. , Hole D. E. , Sharma N. , Humphreys C. J.
Published: July 2002

We develop a model for the excitation of erbium ions in erbium-doped silicon nanocrystals via coupling from confined excitons generated within the silicon nanoclusters. The model provides a phenomenological picture of the exchange mechanism and allows us to evaluate an effective absorption cross section for erbium of up to 7.3×10−17 cm2: four orders of magnitude higher than in stoichiometric silica. We address the origin of the 1.6 eV emission band associated with the silicon nanoclusters and determine absorption cross sections and excitonic lifetimes for nanoclusters in silica which are of the order of 1.02×10−16 cm2 and 20–100 μs, respectively.
Silicon-based photonic crystal heterostructure
Authors: Jiang Z. M. , Shi B. , Zhao D. T. , Liu J. , Wang Xun
Published: July 2002

A silicon-based one-dimensional photonic crystal heterostructure is proposed, and its applications to an omnidirectional reflector and a transverse electric/transverse magnetic (TE/TM) splitter are illustrated. The photonic crystal heterostructure consists of two sets of multilayers, each has six periods of bilayers of poly-silicon and silicon dioxide. When the incident light comes from a medium with refractive index lower than 1.16, there is a frequency range over which of both TE and TM polarizations will be totally reflected at any incident angle. In such a case, the photonic crystal heterostructure can be used as a low loss omnidirectional dielectric reflector. However, when the refractive index of the medium is higher than 1.16, the heterostructure will serve as a good TE/TM splitter of very low crosstalk within certain ranges of light frequency and incident angle.
Tip growth model of carbon tubules grown on the glass substrate by plasma enhanced chemical vapor deposition
Authors: Han Jaehee , Yoo Ji-Beom , Park Chong Yun , Kim Ha-Jin , Park Gyeong Su , Yang Minho , Han In Taek , Lee Naesung , Yi, Whikun , Yu Se Gi , Kim J. M.
Published: July 2002

Morphology of Ni tips and microstructures of carbon tubules were investigated and a growth model for carbon tubules was proposed. Triangular shaped Ni tips and piled-cone structure of carbon tubules were observed at the growth temperature of 500 °C. At the high growth temperature of 620 °C, smoothened edge shaped Ni tips were found and graphite layers became parallel to the growth direction as they were close to the outer wall of carbon tubules. Formation of Ni nuclei for carbon tubule growth was explained by NH3 pre-etching and precipitation of carbon atoms diffused from the surface of the Ni layer. A preferential growth of carbon tubules was observed and explained with the Ni orientation and size effect. A modified tip growth model based on surface diffusion was proposed for the growth mechanism of carbon tubules.
Measurement and analysis of deposition-etch characteristics of BF3 plasma immersion ion implantation
Authors: Qin Shu , Bradley Michael P. , Kellerman Peter L. , Saadatmand Kourosh
Published: July 2002

The deposition-etch characteristics of BF3 plasmas were quantitatively measured and analyzed using a deposition monitor, and were correlated with plasma parameters. It was found that by controlling pressure and rf power, the source could be operated in regimes which were either deposition or etch dominant. This data was then applied to a −2 kV plasma immersion ion implantation BF3 process to explain retained dose characteristics. A good qualitative agreement between the deposition-etch data and implanted retained dose data was obtained.
Theory of direct tunneling current in metal–oxide–semiconductor structures
Authors: Clerc R. , Spinelli A. , Ghibaudo G. , Pananakakis G.
Published: July 2002

The physical bases of the most commonly used methods for the one-dimensional calculation of direct-tunneling current in metal–oxide–semiconductor (MOS) structures (i.e., Bardeen’s approach, the resonant transfer matrix method, and transparency-based approximations) are discussed. Each of them is presented in detail, underlining in a simple way the basic principles. In particular, an original derivation for Bardeen’s approach is proposed. A comparison of the different methods is then carried out for the simple case of two square quantum wells, where analytical solutions can be given, and for actual MOS structures, taking into account quantization effects. It is shown that all these methods, despite the very different formalisms, are based on similar physical approaches and provide very close results.
Multijunction carbon nanotube network
Authors: Ting Jyh-Ming , Chang Chi-Chih
Published: July 2002

Branching structure in carbon nanotubes (CNT) are of great importance in developing CNT-based mechanical or electrical applications. For this purpose, CNTs exhibiting two-dimensional and three-dimensional junctions are being sought. Among them, only two-dimensional branching structures of Y-junction and T-junction CNT have been synthesized. In this letter, we have grown CNT branching webs having two-dimensional H-junction and multiple Y-junctions, and three-dimensional multiple junctions using a thermal chemical vapor deposition method without the use of a template. The result not only shows the possibility of creating three-dimensional complex CNT structures but also provides the nanotechnology community with new base materials for the development of nanoscale mechanical or electrical devices.
Experimental determination of the internal quantum efficiency of AlGaInP microcavity light-emitting diodes
Authors: Royo P. , Stanley R. P. , Ilegems M. , Streubel K. , Gulden K. H.
Published: July 2002

Detailed study of external quantum efficiency ηQE is reported for AlGaInP-based microcavity light-emitting diodes (MCLEDs). Unlike conventional light-emitting diodes (LEDs) the extraction efficiency γext and far field profile depend on the linewidth of the intrinsic spontaneous emission and wavelength detuning between cavity mode and peak electroluminescence. This dependence makes it difficult to estimate the intrinsic spectrum, hence the performances of MCLEDs. By using a nondestructive deconvolution technique, the intrinsic spectra of a MCLED and a reference LED (with the same active regions) could be determined at different current densities. This allowed precise calculation of γext for both devices (values close to 11% were found for the MCLED), and hence of their apparent internal quantum efficiencies ηintapp. At 55 A/cm2, values of 90% and 40% were determined for the LED and MCLED, respectively. In order to explain this difference, we measured ηQE for devices with different sizes. From a fitting procedure based on a simple model taking into account the device size, we found that the radiative efficiencies of LEDs and MCLEDs were close to 90%. We concluded that the low ηintapp of MCLED was due to a bad current injection, and especially to electron leakage current, as confirmed by numerical simulations.
Cathodoluminescence identification of donor–acceptor related emissions in as-grown 4H–SiC layers
Authors: Kakanakova-Georgieva A. , Yakimova R. , Henry A. , Linnarsson M. K. , Syväjärvi M. , Janzén E.
Published: July 2002

A comparative analysis of cathodoluminescence spectra in 4H–SiC layers with different N, Al, and B content is reported. The layers were produced by sublimation epitaxy and residual impurity concentrations were determined by secondary ion mass spectrometry. Epilayers doped with B in a wide concentration range, 5×1015–3×1018 cm−3, were achieved. Evidence of N, Al, and B related emissions by cathodoluminescence experiments is presented. Differences in the luminescence emitted by the layers are established that are attributed to different B content and impurity cooperation. The characteristics of broad green emission, originating from B-related centers, at 4.6 K, 300 K, as well as in high temperature annealed layers are discussed. The experimental results suggest that boron is involved in more than one deep acceptor center.
Glass-forming ability of Pd42.5Cu30Ni7.5P20 alloy with a low critical cooling rate of 0.067 K/s
Authors: Nishiyama N. , Inoue A.
Published: July 2002

We investigated the glass-forming ability of three Pd–Cu–Ni–P alloys with different compositions (Pd40Cu30Ni10P20, Pd42.5Cu27.5Ni10P20 and Pd42.5Cu30Ni7.5P20) under a continuous cooling condition. The lowest critical cooling rate for glass formation was determined to be 0.067 K/s for the Pd42.5Cu30Ni7.5P20 alloy. The melting behavior indicates that the 7.5% Ni alloy has the nearest eutectic composition of the three alloys. The exceptionally low critical cooling rate of below 0.1 K/s is attributed to the elimination of the primary crystallization that occurs due to off-eutectic composition.
A model for the kinetics of protein folding: Kinetic Monte Carlo simulations and analytical results
Authors: Makarov Dmitrii E. , Metiu Horia
Published: July 2002

Makarov, Keller, Plaxco, and Metiu (MKPM) (to be published) have proposed a model that assumes that the kinetics of protein folding is controlled by the formation of the native contacts. An approximate solution of this model leads to a connection between the rate constant and the number of contacts in the folded state, which agrees with the measurements. In the present article, we perform Kinetic Monte Carlo simulations to test the assumptions and the results of MKPM theory. These simulations require us to know the rate constant for contact formation and dissociation. We show that these can be calculated for the case of a Gaussian chain and then used, in a Kinetic Monte Carlo program, to simulate folding kinetics. It is likely that a Gaussian chain is not a realistic model for the folding of a specific protein. However, it is reasonable to use it to test the assumptions made by MKPM, or by other kinetic models, or to derive generic features of folding rate, such as the dependence on the number of contacts. Another valuable feature of the simulation is that it provides information about the stochastic kinetics of a single chain, which is not yet available experimentally. The simulations find that the probability that a single chain folds at a time t has a maximum, at a time tm, and decays exponentially for longer times. We show that tm is shorter than the time resolution of the experiments and therefore the measurements will conclude that folding is a first-order rate process. This is what is observed experimentally. The folding rate constant produced by simulations agrees with the one obtained by the simplified MKPM theory. Moreover, the simulations were used to test directly some of the assumptions made by MKPM. For example, we show that the free energy of a chain that has made a fair number of contacts can be approximated by a linear function of the number of contacts. The paper also contains two simplified models, which do not assume that the chain is Gaussian but make various mean-field-type assumptions about kinetics. These are tested by simulations and found to be satisfactory.
Heating-mode transition in the capacitive mode of inductively coupled plasmas
Authors: Chung ChinWook , Chang Hong-Young
Published: July 2002

The evolution of the electron energy distribution function (EEDF) against pressure is investigated in the capacitive mode of inductively coupled plasma (ICP). A significant change in the EEDFs is observed: a bi-Maxwellian EEDF at low pressure (⩽10 mTorr) evolves into a Druyvestein-like EEDF at high pressure (⩾50 mTorr) in the capacitive mode (low-density mode) while the EEDFs in the inductive mode (high-density mode) does not evolve like in the capacitive mode due to high electron–electron collisions. This EEDF transition in the capacitive mode of ICP is similar to that in the capacitive coupled plasma (CCP) reported in literature [V. A. Godyak and R. B. Piejak, Phys. Rev. Lett. 65, 996(1990)] as pressure increases. This observation directly shows that the electron heating mechanism of the capacitive mode in the ICP is the same as that in the CCP, as expected.
Effect of the structural disorder on the magnetic, transport, and optical properties of B2-phase Ni0.50Al0.50 alloy films
Authors: Lee Y. P. , Kudryavtsev Y. V. , Nemoshkalenko V. V. , Rhee J. Y. , Kim K. W.
Published: July 2002

The influence of structural disorder on the magnetic, transport, and optical properties of Ni0.50Al0.50 alloy films has been investigated. A significantly disordered state was obtained by using vapor-quenching deposition onto substrates cooled by liquid nitrogen. The loss of translational invariance in the disordered state leads to a prominent increase of the magnetic moment of alloy below 50 K. This growth can be explained by the appearance of antistructure Ni atoms (or their clusters) in the disordered state of alloy and their ferromagnetic coupling below 50 K. The temperature dependences of resistivity for both ordered and disordered states of the Ni0.50Al0.50 alloy films exhibit the resistivity minimum at 17–18 K, which has a nonmagnetic nature and can be related mainly to the quantum corrections to the electron–electron interactions in the presence of weak localization. It was shown that the resistivity of the ordered Ni0.50Al0.50 alloy films in 50–300 K temperature range originates mainly from the electron–phonon scattering, while in the disordered state the contribution from the electron-phonon-vibrating impurity scattering becomes dominant. The structural disordering also leads to a noticeable change in the optical properties of alloy, especially in the infrared region. The observed temperature and structural dependences of the resistivity as well as the optical properties in the intraband region confirm the thesis on partial localization of the electronic states near the Fermi level.
Temperature stability of sputtered niobium–oxide films
Authors: Venkataraj S. , Drese R. , Liesch Ch. , Kappertz O. , Jayavel R. , Wuttig M.
Published: July 2002

The effect of annealing temperature on the structural and optical properties of sputtered niobium–oxide films has been investigated. The temperature dependence of structure, density, and optical constants has been studied by Rutherford backscattering, x-ray diffraction, x-ray reflection, optical spectroscopy, and variable angle spectroscopic ellipsometry techniques. Rutherford backscattering measurements show no variation in the stoichiometry of the films upon annealing of amorphous Nb2O5 films, while amorphous NbO is oxidized to Nb2O5. X-ray diffraction studies show that as-deposited films are amorphous and only crystallize at around 500 °C. X-ray reflectivity studies reveal a continuous increase of film density with increasing annealing temperature. Optical spectroscopy and spectroscopic ellipsometry confirm that the refractive index n and the band gap Eg increase upon increasing annealing temperature. The variation of the refractive index with density of the niobium–oxide films is observed to follow the Clausius–Mossotti relation and the molecular electronic polarizability has been deduced from the straight line fit of Lorentz–Lorentz law.
Study of the field-screening effect of highly ordered carbon nanotube arrays
Authors: Suh Jung Sang , Jeong Kwang Seok , Lee Jin Seung , Han Intaek
Published: July 2002

We have studied the field-screening effect provoked by the proximity of neighboring tubes by changing the tube height of highly ordered carbon nanotubes fabricated on porous anodic aluminum oxide templates. The field emission was critically affected by the tube height that protruded from the surface. The field emission was optimal when the tube height was similar to the intertube distance. The intertube distance to the tube height for maximum field emission is about one half the intertube distance predicted by Nilsson et al. [Appl. Phys. Lett. 76, 2071 (2000)].
Does water need a new Tg?
Authors: Johari G. P.
Published: July 2002

The basis for the conjecture that water’s Tg may be 165±5 K [Velikov, Borick, and Angell, Science 294, 2335 (2001)] has been examined. It is shown that (i) differential scanning calorimetry (DSC) scans provided by Hallbrucker and Mayer [J. Phys. Chem. 91, 503 (1987)], and used as a basis for the conjecture, do not represent the heat capacity of the assumed, slow-cooled glassy water or of hyperquenched glassy water, and (ii) there is no fundamental requirement that the excess heat capacity show a peak at T<Tg—instead the peak may appear at TTg. On heating, the enthalpy of glasses produced by hyperquenching or rapid cooling begins to decrease at a much lower T than that of the glasses obtained by slow cooling. Annealing increases this temperature toward Tg, and the enthalpy decrease continues at T above Tg. In the enthalpy relaxation region, the diffusion coefficient of the hyperquenched glassy state is higher than that of a slow-cooled glassy state at a given T, and a local minimum in the DSC scan does not appear at T<Tg in several glasses. These findings remove the basis for the conjecture that water’s Tg may be ∼165 K. Several analyses confirm that the known sigmoid-shape endotherm of glassy water represents the glass-softening range with onset temperature of 136 K. The DSC scans of a glassy state similar to that of water have been simulated by using a nonlinear, nonexponential enthalpy relaxation formalism. These show that a peak in the difference scan of the simulated glass appears above its Tg of 136 K.
Controlled placement of an individual carbon nanotube onto a microelectromechanical structure
Authors: Williams P. A. , Papadakis S. J. , Falvo M. R. , Patel A. M. , Sinclair M. , Seeger A. , Helser A. , Taylor R. M. , Washburn S. , Superfine R.
Published: July 2002

We report on the precise placement of a single carbon nanotube (CNT) onto a microlectromechanial system (MEMS) structure. Using a hybrid atomic force microscope/scanning electron microscope (AFM/SEM) system, an individual multiwalled CNT was retrieved from a cartridge by the AFM tip, translated to a MEMS device, and then placed across a gap between an actuating and a stationary structure. Progress toward a resistance versus stress/strain measurement on a CNT will be discussed, including SEM images of a MEMS structure we have designed specifically for such a measurement.
The role of p-type doping and the density of states on the modulation response of quantum dot lasers
Authors: Shchekin O. B. , Deppe D. G.
Published: July 2002

The modulation response of quantum dot (QD) lasers is analyzed using a quasi-equilibrium approach. The model suggests that present QD lasers are limited due to hole levels that are closely spaced in energy, as well as inhomogeneous broadening. Significant improvements are predicted through p-type modulation doping. The results are consistent with present QD lasers being limited in their modulation response by their differential gain as opposed to carrier capture.
Effect of carbazole–oxadiazole excited-state complexes on the efficiency of dye-doped light-emitting diodes
Authors: Jiang Xuezhong , Register Richard A. , Killeen Kelly A. , Thompson Mark E. , Pschenitzka Florian , Hebner Thomas R. , Sturm James C.
Published: July 2002

Interactions between hole-transporting carbazole groups and electron-transporting 1,3,4-oxadiazole groups were studied by photoluminescence and electroluminescence (EL) spectroscopy, in blends of poly(N-vinylcarbazole) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD) and in random copolymers with carbazole and oxadiazole groups attached as side chains. Different excited-state complexes form in the blends, which exhibit exciplexes, and in the copolymers, which manifest electroplexes, due to topological constraints on the position of carbazole and oxadiazole units in the polymer. Both types of complex red-shift the EL spectra of the matrices compared with pure PVK homopolymer, although the shift is significantly greater for the electroplex. The presence of these complexes has a profound effect on the external quantum efficiency of dye-doped organic light-emitting diodes employing the blends or copolymers as matrices, as it strongly affects the efficiency of Förster energy transfer from the matrix to the dye. Single-layer devices doped with either coumarin 47 (C47), coumarin 6 (C6), or nile red (NR) were compared. Among the three dye-doped PVK:PBD devices, C6 doping yields the highest efficiency, while NR doping produced the most efficient copolymer devices, consistent with the degree of overlap between the EL spectrum of the matrix material and the absorption spectrum of the dye.
In situ four-point conductivity and Hall effect apparatus for vacuum and controlled atmosphere measurements of thin film materials
Authors: Moulzolf S. C. , Frankel D. J. , Lad R. J.
Published: July 2002

An ultrahigh vacuum (UHV) chamber equipped with a fixture for in situ four-point Van Der Pauw conductivity and Hall effect measurements has been constructed and attached to a multichamber thin film synthesis and characterization system. The combined systems allow for film synthesis and characterization of microstructure, chemical composition, morphology, and electronic transport properties without air exposure. The four-point measurement fixture features spring-loaded probes for electrical contacts and temperature measurement and a sample docking mechanism designed to minimize probe damage to the films. The electronics were designed for measurement of high resistance samples. Measurements can be made at sample temperatures from 25 to 450 °C in selected gas environments from UHV to atmospheric pressure. The design and performance of the system are reported, and representative results on the electronic transport properties of n-type Si (100) and tungsten oxide films on sapphire are presented.
Nanoporous-carbon films for microsensor preconcentrators
Authors: Siegal M. P. , Overmyer D. L. , Kottenstette R. J. , Tallant D. R. , Yelton W. G.
Published: July 2002

Nanoporous-carbon (NPC) films are grown using physical processes such as low-power pulsed-laser deposition with attenuation of the ablated carbon species kinetic energy attained by using an inert background gas. With room-temperature growth and negligible residual stress, NPC can coat nearly any substrate to any desired thickness. Control of the deposition energetics yields precise morphology, density, and hence, porosity, with no discernable variation in chemical bonding. We produce NPC films 8 μm thick with density <0.2 g/cm3. The well-controlled porosity, i.e., available surface area, is demonstrated by using films with different thicknesses as a preconcentrator for a nerve-gas simulant.
Contact-induced defect propagation in ZnO
Authors: Bradby J. E. , Kucheyev S. O. , Williams J. S. , Jagadish C. , Swain M. V. , Munroe P. , Phillips M. R.
Published: July 2002

Contact-induced damage has been studied in single-crystal (wurtzite) ZnO by cross-sectional transmission electron microscopy (XTEM) and scanning cathodoluminescence (CL) monochromatic imaging. XTEM reveals that the prime deformation mechanism in ZnO is the nucleation of slip on both the basal and pyramidal planes. Some indication of dislocation pinning was observed on the basal slip planes. No evidence of either a phase transformation or cracking was observed by XTEM in samples loaded up to 50 mN with an ∼4.2 μm radius spherical indenter. CL imaging reveals a quenching of near-gap emission by deformation-produced defects. Both XTEM and CL show that this comparatively soft material exhibits extensive deformation damage and that defects can propagate well beyond the deformed volume under contact. Results of this study have significant implications for the extent of contact-induced damage during fabrication of ZnO-based (opto)electronic devices.