bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation study of intra- and intermicellar ordering in triblock-copolymer systems
Authors: Wijmans C. M. , Eiser E. , Frenkel D.
Published: March 2004

We report a numerical study of the structure and phase behavior of a model for a triblock-copolymer solution. The aim of this study is to investigate the nature of the dense micellar phase that can form in such systems. The simulations were performed on a lattice model for PEO (poly(ethylene-oxide))–PPO (poly(propylene-oxide))–PEO polymers. At high volume fractions, the structure factor of the amphiphile–solvent system can be mapped onto that of a monodisperse hard-sphere fluid. Yet, a low-density hard-sphere model cannot account for the properties of the dilute micellar solution. Moreover, direct inspection of the snapshots of the suspension show that these model triblock-copolymer micelles are neither hard, nor spherical, nor monodisperse.
The unphysical pinning of the domain growth during the separation of homopolymer blends near the spinodal
Authors: Fiałkowski Marcin , Hołyst Robert
Published: March 2004

We simulate model B of mesoscopic dynamic with the Flory–Huggins free energy for the homopolymer blend. We concentrate the study on the rescaling of the spatial coordinates in the model. We show that the commonly used rescaling of the spatial coordinates, by the function vanishing at the spinodal, leads to the unphysical freezing of the domains. We study the evolution of the system in two different processes: One is the growth process induced by the temperature quench to the metastable or unstable region of the phase diagram and the second is the quench-jump process in which we first allow the system to separate below the spinodal and next heat it up below or above the spinodal. The proper rescaling avoiding the unphysical pinning of the domain growth at the spinodal is proposed.
Microstructure and electronic properties of Cu/Mo multilayers and three-dimensional arrays of nanocrystalline Cu precipitates embedded in a Mo matrix
Authors: Zhang X. , Hundley M. F. , Malinowski A. , Misra A. , Wang H. , Nastasi M.
Published: March 2004

Nanostructured Cu/Mo multilayers were prepared by magnetron sputtering. The thickness of the Cu layer was kept constant at 0.6 nm, while the thickness of the Mo layers varied from 2.5 to 20 nm for different specimens. The Cu layers exhibit a body centered cubic (bcc) structure and the interface between Cu and Mo remains sharp and planar in all specimens. Annealing of a Cu 0.6 nm/Mo 20 nm multilayer produced three-dimensional arrays of Cu nanoparticles lying along the previous interface. These Cu nanoparticles have an average particle size of roughly 2 nm with a bcc structure. Temperature-dependent resistivity measurements in as-deposited and annealed samples are reported. These data indicate that carrier scattering changes markedly as the system evolves from one that consists predominantly of planar interfaces to one dominated by spherical scattering centers.
Chemistry on the low field anodes and cathodes of air corona gaps
Authors: Séverac C. , Jouve G. , Goldman A. , Goldman M.
Published: March 2004

Surface effects of electrical coronas have been studied on the low field electrodes of point-to-plane air gaps with discharges burning in negative and positive glow regimes, i.e., mainly producing chemical effects without significant sputtering or thermal effects. Most of these chemical effects are due to oxidation and reduction processes taking place in aqueous microdroplets formed by the discharge where oxides, nitrates, and cyanides, are mainly found. Generally oxidation prevails providing very high oxidation states, but using preoxidized gold samples as cathodes enabled the separate observation of reduction phenomena.
Synthesis and spectroscopic characterization of CaF2:Er3+ single crystal for highly efficient 1.53 μm amplification
Authors: Kumar G. A. , Riman Richard , Chae S. C. , Jang Y. N. , Bae I. K. , Moon H. S.
Published: March 2004

A CaF2:Er3+ single crystal was grown by the Tammann–Stober method. The potential of this material as a laser crystal for 1530 nm emission was established by quantitative analysis of the optical absorption and emission spectra. Assuming the tetragonal symmetry of the Er3+ sites, the Bkq crystal field parameters, Racah parameters, spin–orbit interaction parameters, and configuration interaction parameters were derived by fitting the experimental absorption band positions with the model energy Hamiltonian. Judd–Ofelt parametrization was done to compute the radiative decay time and fluorescence branching ratio of various meta stable transitions. Using the measured fluorescence decay time and computed radiative decay time, 100% quantum efficiency is obtained for the 1530 nm band, which is reasonable due to the low multiphonon relaxation, and absence of nonradiative energy transfer processes at the 0.01 at.% Er3+ concentration. The narrow bandwidth (13 nm) and high stimulated emission cross section (3.2×10−20 cm2) support the suitability of CaF2:Er3+ for use in high gain optical amplifiers.
Spectroscopy and modeling of solid state lanthanide lasers: Application to trivalent Tm3+ and Ho3+ in YLiF4 and LuLiF4
Authors: Walsh Brian M. , Barnes Norman P. , Petros Mulugeta , Yu Jirong , Singh Upendra N.
Published: March 2004

Lanthanide series ions are considered in the context of acquiring spectroscopic parameters and their application to modelling of quasifour-level lasers. Tm:Ho codoped crystals of YLiF4 (YLF) and the isomorphs LuLiF4 (LuLF) and GdLiF4 (GdLF) as 2.0 μm lasers are used for illustration of the experimental and theoretical techniques presented here. While these materials have similar physical properties, they differ in the strength of the crystal field at the site of optically active lanthanide dopant ions such as Tm3+ and Ho3+. This is due in part to the size of the Lu3+, Y3+, and Gd3+ ions, which comprise part of the host lattice, but ionicity plays a role as well. This selection of lanthanide: host materials provides a useful basis on which to assess laser materials with regards to changes in the strength of the crystal field at the dopant ion site. It is demonstrated that Tm:Ho:LuLF has a larger crystal field splitting than Tm:Ho:YLF and Tm:Ho:GdLF, leading to smaller thermal populations in the Ho lower laser level. To assess this effect quantitatively, the energy levels of the first ten manifolds in Ho:LuLF have been determined. Measurement of Ho:XLiF4 (X=Y,Lu,Gd) emission cross sections at 2.0 μm, Tm:XLiF4 pump absorption cross sections around 0.78 μm, manifold to manifold decay times and energy transfer parameters in Tm:Ho:XLiF4 systems are also determined to provide a consistent set of parameters to use in laser modeling. The techniques presented here are applicable to any lanthanide series ion in a crystalline host. A theoretical laser model has been developed that is easily adapted to any lanthanide ion in a crystal host. The model is used to predict diode side-pumped laser performance of Tm:Ho:LuLF and Tm:Ho:YLF using input parameters determined from the spectroscopy presented here. An explanation is presented for the improved performance of Tm:Ho:LuLF over Tm:Ho:YLF by modeling the laser. A demonstration that small changes in lower laser thermal population can substantially alter laser performance is noted, an effect that has not been fully appreciated previously.
Role of marble microstructure in near-infrared laser-induced damage during laser cleaning
Authors: Rodriguez-Navarro Carlos , Rodriguez-Navarro Alejandro , Elert Kerstin , Sebastian Eduardo
Published: March 2004

When marble is cleaned by nanosecond neodymium yttrium–aluminum–garnet lasers (1064 nm), strongly absorbing surface contaminants are removed at fluences substantially below the damage threshold for the much less absorptive marble substrate. Recent studies have shown, however, that unacceptable roughening of the marble surface also may occur at low fluences due to removal of individual grains. In order to elucidate this effect, we have compared the low-fluence response of marbles with two different grain sizes and single-crystal calcite, in the fluence range 0.12–1.25 J cm−2. Damage was greater in fine-grained than coarse-grained marble, and did not occur in the single-crystal calcite at these fluences. The temperature rise following defect-mediated absorption triggers thermal plasma emission and generates shock waves; the concomitant surface damage depends on the size and crystallographic orientation of the crystals. Laser irradiation anneals the defects and increases “crystallite size.” The implications for the laser-assisted cleaning of marble artworks are outlined.
Exchange bias and coercivity in oxygen-exposed cobalt clusters
Authors: Morel Robert , Brenac Ariel , Portemont Céline
Published: March 2004

Exchange bias and coercivity have been studied in nanometric-size cobalt clusters as a function of oxygen exposure under high vacuum. This technique allows looking at the early stages of oxidation, as well as studying exchange bias in small clusters before they are completely transformed into oxide. It is found that oxygen induces at first an increase in the coercive field and in the superparamagnetic blocking temperature, before any substantial loss in the magnetization, which indicates that this effect is due to adsorbed oxygen rather than to exchange coupling with an oxide overlayer.
Dynamic contact angles under evaporation
Authors: Ybarra Robert M. , Neogi P.
Published: March 2004

Wetting kinetics in the presence of heat transfer, evaporation, and Marangoni effect has been explored using a method used by Joanny and de Gennes [C. R. Acad. Sci. Ser. II 299, 279 (1984)]. The method solves for the dynamic contact angle α by equating the rate of surface work to the rate of dissipation. The result in the form of dynamic contact angle α as a function of capillary number Ca agrees well with existing experimental data. Qualitative comparisons have been made in cases where quantitative data do not exist. Finally, a form of contact line instability is predicted, for which some experimental justifications are seen.
Relaxation dynamics of a polymer in a 2D confinement
Authors: Schwartz Gustavo A. , Bergman Rikard , Swenson Jan
Published: March 2004

The molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (MW=1000, 2000, and 4000 g/mol) confined in a two-dimensional layer-structured Na–vermiculite clay has been studied by broadband dielectric spectroscopy. The α-relaxation and the normal mode relaxation processes were studied for all samples in bulk and confinement. The most prominent experimental observation was that for the normal mode process: the relaxation rate in the clay is drastically shifted to lower frequencies compared to that of the bulk material. This slowing down is probably caused by the strongly reduced number of accessible chain conformations in two dimensions. Also the temperature dependence of the relaxation time for the normal mode process is strongly affected by the confinement. In contrast, for the α-relaxation of the confined polymers we observed only a slight increase of the relaxation rate at high temperatures compared to the corresponding bulk samples, and a decrease of its relaxation strength relative to the β relaxation. Thus, the glass transition is unaffected by the 2D confinement, suggesting that the underlying phenomena responsible for the glass transition is the same as in bulk. Moreover, in the clay the intensity of the normal mode is stronger than that of the α-process, in contrast to the bulk samples where the opposite behavior is observed.
Interactions of D2O with methane and fluoromethane surfaces
Authors: Souda R. , Kawanowa H. , Kondo M. , Gotoh Y.
Published: March 2004

TOF-SIMS is used to investigate the interactions between D2O and hydrophobic molecules, such as CH4, CH3F, CH2F2, CHF3, and CF4, at cryogenic temperatures (15 K). By irradiation with a 1.5-keV He+ beam, the D+(D2O)n ions are ejected efficiently from the D2O nanoclusters physisorbed on the CF4 layer due to Coulomb explosion: the ion yields are by about two orders of magnitude higher than those from a thick D2O layer via the kinetic sputtering. The D+(D2O)n yields decrease on the CHnF4−n layer with increasing the number of the C–H group. This is because the Coulombic fission is quenched due to the delocalization of valence holes through the C–H⋯H–C and C–H⋯D2O contacts. A pure D2O film is hardly grown on the CH4 layer as a consequence of intermixing whereas the D2O molecules basically adsorb on the surfaces of fluoromethanes, suggesting the attractive (water-repellent) interactions in the C–H⋯D2O (C–F⋯D2O) contacts. The C–H⋯O bond behaves like a conventional O–H⋯O hydrogen bond as far as the collision-induced proton transfer reaction is concerned.
Molecular binding at gold transport interfaces. III. Field dependence of electronic properties
Authors: Basch Harold , Ratner Mark A.
Published: March 2004

The behavior of the electronic structure in a metal/molecular/metal junction as a function of the applied electric field is studied using density functional methods. Although the calculations reported here do not include the electrode bulk, or intermolecular interactions, and do not permit actual transport to occur, nevertheless they illuminate the charging, energy shift, polarization and orbital occupation changes in the molecular junction upon the application of a static electric field. Specifically, external electric fields generally induce polarization localization on the two cluster ends. The HOMO/LUMO gap usually decreases and, for large enough fields, energy levels can cross, which presages a change of electronic state and, if found in molecular electronic circuits, a change in transmission. The calculations also show changes in the geometry both of the molecule and the molecule/cluster interface upon application of the electric field. These effects should be anticipated in whole circuit studies.
Mixing rules for multicomponent mixture mass diffusion coefficients and thermal diffusion factors
Authors: Harstad K. G. , Bellan J.
Published: March 2004

Mixing rules are derived for mass diffusion coefficient and thermal diffusion factor matrices by developing compatibility conditions between the fluid mixture equations obtained from nonequilibrium thermodynamics and Grad’s 13-moment kinetic theory. The mixing rules are shown to be in terms of the species mole fractions and binary processes. In particular, the thermal diffusion factors for binary mixtures obtained by the Chapman–Enskog expansion procedure are suitably generalized for many-component mixtures. Some practical aspects of the results are discussed including the utilization of these mixing rules for high pressure situations.
Reduction of variable range hopping conduction in low-temperature molecular-beam epitaxy GaAs
Authors: Albuquerque H. A. , de Oliveira A. G. , Ribeiro G. M. , da Silva R. L. , Rodrigues W. N. , Rubinger R. M.
Published: March 2004

Studying the transport properties via Hall and resistivity measurements of low-temperature molecular-beam epitaxy (LT-MBE) GaAs samples, the optimal conditions for fabricating high-resistivity material are found. We present results on three LT-MBE GaAs samples grown at 215, 265, and 315 °C. The measurements were carried out at temperatures ranging from 130 to 300 K, and the hopping conduction mechanism in this range is identified as variable range hopping. The sample grown at 315 °C presents the highest hopping parameter; this appears to be due to a reduction in the density of hopping centers. The mechanisms responsible for this are discussed.
Control of bond formation, electron transport, and interference in a biased asymmetric parallel double-dot system
Authors: Mourokh Lev G. , Kovalev Vadim M. , Horing Norman J. M.
Published: March 2004

We analyze electron transport through a biased asymmetric double-dot system in a parallel arrangement between leads. We show that the imposition of a dot-to-dot bias induces changes in the symmetry of the wave functions associated with the double-dot energy levels, resulting in a dip in the dependence of the lead-to-lead current on dot-to-dot bias. We also demonstrate that when the dot-to-dot bias compensates (energetically) the structure asymmetry, the lead-to-lead current will be an oscillating function of an applied magnetic field in the case in which only one of the double-dot levels is conductive. However, destructive interference between different parts of the same dot gives rise to suppression of current at high magnetic fields.
Near-field optics simulation of a solid immersion lens combining with a conical probe and a highly efficient solid immersion lens-probe system
Authors: Chau Yuan-Fong , Yang Tzong-Jer , Tsai Din Ping
Published: March 2004

A solid immersion lens (SIL) combined with near-field probes which are the conic dielectric probe and the local metallic coating on the SIL probe is designed for optical recording by means of a three-dimensional rather than two-dimensional finite difference time domain method to gain more insight in the near-field distribution. Our computation model is realistic to match the real SIL-probe system. The optical properties of near-field distributions between the SIL probe and recording layers are investigated. A promising idea for fabricating a type of the SIL-probe system is proposed. Results of our designation of an optimal SIL-probe system can achieve higher performance (in terms of high throughput and low reflection of the light beam) for optical data storage and microscopy than the conventional metal NSOM probe or SIL used alone.
Attogram detection using nanoelectromechanical oscillators
Authors: Ilic B. , Craighead H. G. , Krylov S. , Senaratne W. , Ober C. , Neuzil P.
Published: March 2004

We report on the fabrication of nanometer-scale mass sensors with subattogram sensitivity. Surface micromachined polycrystalline silicon and silicon nitride nanomechanical oscillators were used to detect the presence of well-defined mass loading. Controlled deposition of thiolate self-assembled monolayers on lithographically defined gold dots were used for calibrated mass loading. We used a dinitrophenyl poly(ethylene glycol) undecanthiol-based molecule (DNP-PEG4-C11thiol) as a model ligand for this study. Due to the fact that the gold mass is attached at the distance l0 from the end x=l of the cantilever beam, an additional moment evolves in the boundary condition of the oscillator, which was taken into consideration through the rotational inertia of the attached mass. We showed that the corresponding correction of the frequency is on the order of γ(l0/l), where γ is the attached mass normalized to the mass of the beam. The rotational inertia correction to the frequency is on the order of γ(l0/l)2. The adopted approach permits accurate determination of the eigenfrequency in the framework of the Euler–Bernoulli beam when rotational inertia of the attached mass is included. From the resonant frequency shift, the mass of the adsorbed species was determined and compared to results obtained by other techniques. Utilizing vacuum encapsulation, we demonstrate sensing capability in the attogram regime of the adsorbed self-assembled monolayer.
Simulation of confined and enhanced optical near-fields for an I-shaped aperture in a pyramidal structure on a thick metallic screen
Authors: Tanaka Kazuo , Tanaka Masahiro
Published: March 2004

A modification of the I-shaped aperture proposed previously by the authors—that is, an I-shaped aperture in a pyramidal structure on a thick metallic screen—is proposed and simulated. This structure is useful in the application of the I-shaped aperture to practical near-field optical systems. Scattering of optical waves by this structure in a thick metallic screen is solved numerically using a volume integral equation by generalized conjugate residual iteration and fast Fourier transformation. It is shown that this aperture structure simultaneously provides high emission intensity and small spot size through excitation of the surface plasmon polariton on the sidewalls of the aperture in the pyramid structure.
Stoichiometry and infrared absorption of amorphous a-C1−xNx:H carbon nitride films
Authors: Bouchet-Fabre B. , Godet C. , Lacerda M. , Charvet S. , Zellama K. , Ballutaud D.
Published: March 2004

Stoichiometry, bonding configurations and structural properties of plasma-deposited hydrogen-rich amorphous carbon nitride a-C1−xNx:H (0<x<0.20) films have been investigated using infrared and Raman vibrational spectroscopies, along with x-ray photoemission spectroscopy at the C and N K-edges. With increasing N incorporation in a-C1−xNx:H, the total H content decreases slightly and C–H vibrations are progressively replaced by N–H vibrations. The dominant bonding configuration is C=N throughout the composition range, although C=N and C–N saturate above 10 N at. %, and C≡N configurations steadily increase throughout the stoichiometry range. A strong conjugation of imine (C=N) and nitrile (C≡N) groups with aromatic rings is evidenced while the Raman signature of the sp2 C phase indicates an increase in the D to G peak intensity ratio, related to some ordering of the sp2 clustered phase.
Characterization of a silicon-on-insulator based thin film resistor in electrolyte solutions for sensor applications
Authors: Nikolaides M. G. , Rauschenbach S. , Bausch A. R.
Published: March 2004

We characterize the recently introduced silicon-on-insulator based thin film resistor in electrolyte solutions and demonstrate its use as a pH sensing device. The sensor’s response function can be tuned by a back gate potential, which is demonstrated by employing known changes of the pH of the solution. The highest sensitivity to pH changes is obtained when the charge carrier concentration at the back interface of the thin Si-film is low compared to the front interface. Calibration measurements with a reference electrode are used to relate the obtained resistance to the surface potential. Applying the site binding model to fit the measured data for variations of the pH gives excellent agreement. The sensors response can be related to a surface potential change of −50 mV/pH and from the obtained signal–to–noise ratio, the detection limit can be estimated to be 0.03 pH. For a (bio-)molecular use of the sensor element, a passivation of the silicon oxide surface against this pH response can be achieved by depositing an organic layer of poly- methyl-methacrylate (PMMA) onto the devices by spin coating. As expected, the pH response of the surface disappears after the deposition of PMMA. This passivation technique provides an easy and reliable way to obtain a biocompatible interface, which can be further functionalized for the detection of specific molecular recognition events.
Effect of diffraction and film-thickness gradients on wafer-curvature measurements of thin-film stress
Authors: Breiland W. G. , Lee S. R. , Koleske D. D.
Published: March 2004

When optical measurements of wafer curvature are used to determine thin-film stress, the laser beams that probe the sample are usually assumed to reflect specularly from the curved surface of the film and substrate. Yet, real films are not uniformly thick, and unintended thickness gradients produce optical diffraction effects that steer the laser away from the ideal specular condition. As a result, the deflection of the laser in wafer-curvature measurements is actually sensitive to both the film stress and the film-thickness gradient. We present a Fresnel–Kirchhoff optical diffraction model of wafer-curvature measurements that provides a unified description of these combined effects. The model accurately simulates real-time wafer-curvature measurements of nonuniform GaN films grown on sapphire substrates by vapor-phase epitaxy. During thin-film growth, thickness gradients cause the reflected beam to oscillate asymmetrically about the ideal position defined by the stress-induced wafer curvature. This oscillating deflection has the same periodicity as the reflectance of the growing film, and the deflection amplitude is a function of the film-thickness gradient, the mean film thickness, the wavelength distribution of the light source, the illuminated spot size, and the refractive indices of the film and substrate. For typical GaN films grown on sapphire, misinterpretation of these gradient-induced oscillations can cause stress-measurement errors that approach 10% of the stress-thickness product; much greater errors occur in highly nonuniform films. Only transparent films can exhibit substantial gradient-induced deflections; strongly absorbing films are immune.
InP nanocrystals via surfactant-aided hydrothermal synthesis
Authors: Wei Shuo , Lu Jun , Yu Weichao , Qian Yitai
Published: March 2004

In the presence of surfactant, potassium stearate, quantum-confined InP nanocrystals (NCs) were hydrothermally synthesized in aqueous ammonia. Powder x-ray diffraction (XRD) patterns give the zinc blende phase of InP with lattice constant a=5.8377±8×10−4 Å. Transmission electron microscopy (TEM) micrographs show that the as-prepared InP NCs are spherical secondary particles (120 nm) consisting of spherical nanocrystals and rod-like nanocrystals grown in the direction perpendicular to the [111] direction, which is different from those grown by solution–liquid–solid process. X-ray photoelectron spectra indicate that the nanocrystals have a stoichiometric ratio of In:P=1.2:1 and their surfaces are capped with stearate ions. Powder XRD, TEM images, Raman spectra, optical absorption and photoluminescence spectra of InP NCs grown with surfactant were compared with those of the InP NCs grown without surfactant, and it indicated that the as-prepared InP NCs via surfactant-aided synthesis are quantum confined: their transverse optical and longitudinal optical vibration modes exhibit frequency redshifting and asymmetric broadening which is consistent with the phonon confinement model of nanoparticles. Compared with bulk values, InP NCs exhibit a band-edge emission band centered at 1.81 eV (685 nm) with a blueshift of 0.44 eV at 300 K. A wide distribution in size and the two-morphology nature of the NCs resulted in featureless absorption spectra and broadening of the emission band. The formation mechanism of the InP NCs is discussed and attributed to an in situ decomposition process of an indium polyphosphide amorphous precursor.
Predictions for weak mechanical ignition of strain hardened granular explosive
Authors: Gonthier Keith A.
Published: March 2004

Predictions are given for the coupled bulk and grain scale response of initially unstressed, strain hardened granular HMX (C4H8N8O8) due to mild piston impact (impact speeds <100 m/s). Importantly, this response depends on the material’s strain history as the stress necessary for bulk inelastic compaction (crush up) increases with the solid volume fraction. Although the quasistatic compaction behavior of HMX is well characterized, the influence of strain history on the bulk and grain scale dynamic loading response has largely been unexplored. In this study, the initial solid volume fraction of the unstressed material is varied over the range of φf⩽φ0⩽1, where φf=0.655 is its free pour value. A Hugoniot analysis for the bulk material identifies three dispersed compaction wave structures that depend on the impact speed and initial solid volume fraction, and are analogous to elastic-plastic waves in dynamically loaded solids. For increasing impact speed, these structures consist of (1) a single viscoelastic wave; (2) a leading viscoelastic wave and a trailing viscoplastic wave; and (3) a single viscoplastic wave. It is shown that the resulting localized heating near intergranular contact surfaces can trigger sustained combustion of the material. Predictions for the grain scale thermochemical response indicate that significant bulk viscoplastic heating is required for ignition of materials with φf⩽φ0⩽0.88, whereas bulk viscoelastic heating leads to the ignition of denser materials (φ0>0.91). Both viscoelastic and viscoplastic heating are predicted to be important for ignition of materials having 0.88⩽φ0⩽0.91. Within this transition range there is predicted a sharp increase in impact sensitivity as the power input needed for ignition rapidly decreases to a value close to that for the free pour density (0.40 MW/cm2) before increasing again. This result is important for assessing the impact sensitivity and deflagration-to-detonation transition of damaged explosives.
Electron transport in Si/SiGe modulation-doped heterostructures using Monte Carlo simulation
Authors: Monsef Florian , Dollfus Philippe , Galdin-Retailleau Sylvie , Herzog Hans-Joest , Hackbarth Thomas
Published: March 2004

The electron transport in two-dimensional gas formed in tensile-strained Si1−xGex/Si/Si1−xGex heterostructures is investigated using Monte Carlo simulation. First the electron mobility is studied in ungated modulation-doped structures. Calculation matches the experimental results very well over a wide range of electron densities. The mobility typically varies between 1100 cm2/V s in highly-doped structures and 2800 cm2/V s at low electron density. The mobility is shown to be significantly influenced by the thickness of the spacer layer separating the strained Si channel from the pulse-doped supply layers. Then the electron transport is investigated in a gated modulation-doped structure in which the contribution of parasitic paths is negligible. The mobility is shown to be higher than in comparable ungated structures and dependent on the gate voltage as a result of the electron density dependence of remote impurity screening.
Effects of substrate temperature on the device properties of pentacene-based thin film transistors using Al2O3+x gate dielectric
Authors: Lee Jiyoul , Kim J. H. , Im Seongil
Published: March 2004

We report on the electrical properties of pentacene-based organic thin film transistors (OTFTs), where the active pentacene channel layers have been deposited at fixed deposition rate of 1 Å/s on a 250-nm-thick Al2O3+x gate dielectric film at various substrate temperatures: room temperature, 60 °C, and 90 °C. The grain size of the pentacene layer was found to increase with the substrate temperature, accompanied by a phase transition. The highest saturation current of 20 μA (under a gate bias of −40 V) was obtained with a high field-effect hole mobility of ∼0.21 cm2/V s from an OTFT prepared at 90 °C despite a relatively low on/off current ratio of 2×105. It is concluded that the pentacene channel deposited at a high substrate temperature contains not only large grains but also high density traps.
Effects of failure criteria on the lifetime distribution of dual-damascene Cu line/via on W
Authors: Liu R. F. , Hu C.-K. , Gignac L. , Harper J. M. E. , Lloyd J. , Liu X.-H. , Stamper A. K.
Published: March 2004

Electromigration lifetime was studied on 0.23 μm wide Cu dual-damascene lines connected to underlying W lines with a TaN/Ta liner and SiNx/SiO2 insulator. The failure criteria used to study electromigration lifetime were based on the increase in test line resistance. Over 900 samples were tested to observe early electromigration Cu lifetime. Experimental results show that the lifetime distribution of samples tested using a failure criterion of 1% (3 Ω) resistance increase at the test temperature is best described by a trimodal function. However, the lifetime distribution of samples tested using a failure criterion of 50% is best described by a bimodal function. This difference in lifetime distribution is due to three types of observed deviant line resistance behaviors (plateau, fluctuating, and uphill) of certain samples which are caused by different types of void growth and a variation in liner stability.
Two-dimensional magnetic resonance tomographic microscopy using ferromagnetic probes
Authors: Barbic Mladen , Scherer Axel
Published: March 2004

We introduce the concept of computerized tomographic microscopy in magnetic resonance imaging using the magnetic fields and field gradients from a ferromagnetic probe. We investigate a configuration where a two-dimensional sample is under the influence of a large static polarizing field, a small perpendicular radio-frequency field, and a magnetic field from a ferromagnetic sphere. We demonstrate that, despite the nonuniform and nonlinear nature of the fields from a microscopic magnetic sphere, the concepts of computerized tomography can be applied to obtain proper image reconstruction from the original spectral data by sequentially varying the relative sample-sphere angular orientation. The analysis shows that the recent proposal for atomic resolution magnetic resonance imaging of discrete periodic crystal lattice planes using ferromagnetic probes can also be extended to two-dimensional imaging of noncrystalline samples with resolution ranging from micrometer to angstrom scales.
Kinetics of oxygen removal from ceria
Authors: Stan M. , Zhu Y. T. , Jiang H. , Butt D. P.
Published: March 2004

The kinetics of oxygen removal from CeO2 were investigated using thermogravimetric analysis, at high temperatures, under a reducing atmosphere of Ar–6%H2. A chemical diffusivity model was developed that takes into account the composition dependence of both the pre-exponential factor and the activation energy. In this model, the pre-exponential factor is given by D0=22.32 exp(−0.22x) (cm2/s), and the activation energy is ΔE=195.4−173.9x (kJ/mol), where x is the nonstoichiometry coefficient in CeO2−x. The model describes well the oxygen removal kinetics for the entire time range (100 min), demonstrating its superiority over two popular mathematical models that can only fit part of the experimental data.
Electric field effect on the rhombohedral–rhombohedral phase transformation in tin modified lead zirconate titanate ceramics
Authors: Yang Pin , Rodriguez Mark A. , Burns George R. , Stavig Mark E. , Moore Roger H.
Published: March 2004

The ferroelectric–to–ferroelectic phase transformation between the high temperature (FERH) and the low temperature (FERL) rhomobohedral phases in lead based perovskite under the dc bias conditions was investigated. Dielectric measurements show that an external electric field stabilizes the FERL phase and moves the phase transformation to a higher temperature. The observation has been further verified by an in situ microfocused x-ray study where an external field can effectively induce the oxygen octahedral tilting in the crystalline lattice and extends the thermal stability region of the FERL phase to a higher temperature. An analysis based on the combination of the Clausius–Clapeyron relationship with lattice dynamic principles suggests that the transformation from FERH to FERL is driven by a short-range interaction in the crystalline lattice. The origin of this short-range interaction is proposed, based on the structural evolution during the phase transformation. Experimental evidence suggests that such interaction driving the structural instability can be exploited by an external electric field near the phase transformation temperature and leads to an unusual, transient field-enhanced deformation near the FERH/FERL phase transformation.
Temperature and pressure behavior of the emission bands from Mn-, Cu-, and Eu-doped ZnS nanocrystals
Authors: Su F. H. , Fang Z. L. , Ma B. S. , Ding K. , Li G. H. , Xu S. J.
Published: March 2004

The temperature and pressure dependence of the photoluminescence from ZnS:Mn2+, ZnS:Cu2+, and ZnS:Eu2+ nanocrystals were investigated in the temperature range from 10 to 300 K and under hydrostatic pressure up to 6 GPa at room temperature. The orange emission (590 nm) from the T14-6A1 transition of Mn2+ ions, the green emission (518 nm) from the 4f65d1-4f7 transition of Eu2+ ions and the blue emission (460 nm) related to the transition from the conduction band of ZnS to the t2 level of Cu2+ ions were observed in the Mn-, Eu-, and Cu-doped samples, respectively. It was found that all of these emission bands decrease in intensity with increasing temperature. Among them the intensity of the Mn-orange emission dropped faster. The activation energies were estimated to be 58, 16, and 42 meV for the Mn-orange, Eu-green, and Cu-blue emissions, respectively. A negative pressure coefficient of −26 meV/GPa was obtained for the Mn-orange emission, which agrees with the value calculated from the crystal field theory. The pressure coefficient of the Cu-blue emission was found to be 62 meV/GPa, which is almost same as the value of the band gap of bulk ZnS. However, the pressure coefficient of the Eu-green emission is 23 meV/GPa, which is contrary to the predication by the crystal field theory. The strong interaction between the 4f65d1 state of the Eu2+ ions and the conduction band of ZnS may be the origin for the positive pressure coefficient and the small thermal activation energy of Eu-green emission.
Holographic scheme for independent component analysis
Authors: Anderson Dana Z.
Published: March 2004

An optoelectronic analog feedback circuit uses dynamic holography to perform independent component analysis on an array of input signals imposed on a set of optical carriers. The requisite nonlinearity for achieving high-order statistical analysis is established with optical phase modulation in the feedback loop. The dynamics of the system is shown to be driven by the statistical characteristic functions of the input signals and reveals winner-takes-all dynamics when at least one of the signals is sub-Gaussian. Signal correlation and circuit adaptation is carried out by the dynamic holographic medium and therefore takes place in a parallel fashion. The allowed signal bandwidth is limited by the delay time of the feedback loop, thus signal bandwidths exceeding 100 MHz should be possible using relatively standard electronic and optoelectronic technology. Moreover, the scheme can directly accommodate microwave carrier frequencies to several tens of gigahertz.
Current-voltage relation for thin tunnel barriers: Parabolic barrier model
Authors: Hansen Kim , Brandbyge Mads
Published: March 2004

We derive a simple analytic result for the current-voltage curve for tunneling of electrons through a thin uniform insulating layer modeled by a parabolic barrier. Our model, which goes beyond the Wentzel–Kramers–Brillouin approximation, is applicable also in the limit of highly transparant barriers subject to high voltages, and thus provides a more realistic description for this situation compared to the widely used rectangular barrier model.
Theoretical study of band edges in porous silicon
Authors: Yorikawa H. , Sato T. , Muramatsu S.
Published: March 2004

The shift of band edges in porous silicon is studied by simulation based on tight-binding band calculations for model structures mimicking porous silicon. The results are compared with recent measurements of band edge shifts, and the greater shift of the valence band edge compared to the conduction band edge in porous silicon is theoretically confirmed. It is found that the ratio of band edge shifts is related to the positions of the center of the electronic density of states for the first conduction and valence bands in bulk silicon.
Optical investigations on the annealing behavior of gallium- and nitrogen-implanted ZnO
Authors: Reuss F. , Kirchner C. , Gruber Th. , Kling R. , Maschek S. , Limmer W. , Waag A. , Ziemann P.
Published: March 2004

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy ED of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy EA of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved.
Dynamically stable gallium-induced 3×3-SiC (0001) surface for two-dimensional GaN nucleation by molecular-beam epitaxy
Authors: Jeganathan K. , Shimizu M. , Okumura H.
Published: March 2004

Two-dimensional (2D) GaN nucleation on a gallium (Ga) induced 3×3-SiC (0001) surface was achieved by molecular-beam epitaxy. The Ga adsorbate on SiC (0001) revealed well-ordered ∛×∛−R30° and 3×3 surface reconstructions with respect to Ga adatom coverage. The 3×3 surface was found to be highly reactive toward 2D GaN nuclei. This enhances the surface mobility of impinging adatoms during GaN growth, which promotes a step-flow growth mode. On the ∛×∛−R30° surface, polygonal Stranski–Krastanov islands formed during the initial stages, coalescing as thickness increased. The GaN growth kinetics was found to depend on the Ga atomistic process of the reconstructed SiC (0001) surfaces.
Charge carrier mobility in blue-green emitting fluorenyl-substituted poly(p-phenylene vinylene)s
Authors: Lee Sang Ho , Yasuda Takeshi , Tsutsui Tetsuo
Published: March 2004

The charge carrier mobility in three blue-green emitting conjugated polymers, poly[2-(9′,9′-dihexylfluorenyl)-1,4-phenylene vinylene] (DHF-PPV), poly[2-(7′-methoxy-9′,9′ -dihexylfluorenyl)-1,4-phenylene vinylene] (MDHF-PPV), and poly[2-(7′-cyano-9′,9′ -dihexylfluorenyl)-1,4-phenylene vinylene] (CNDHF-PPV) was measured using the time-of-flight technique. We observed dispersive charge carrier transport in DHF-PPV with a room-temperature hole mobility of 4.5×10−4 cm2/Vs at an electric field of 2.5×105 V/cm. This mobility is two orders of magnitude higher than that of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] at the same electric field. The introduction of an electron donating or an electron withdrawing group to fluorene unit in DHF-PPVs resulted in a decrease in the hole mobilities to 1.1×10−4 cm2/Vs for MDHF-PPV and 4.1×10−7 cm2/Vs for CNDHF-PPV at 298 K and an electric field of 2.5×105 V/cm, respectively. These results demonstrate that both the PPV backbone and the fluorene side unit in DHF-PPVs act as carrier hopping sites.
Simultaneous micro-Raman and photoluminescence study of spark-processed germanium: Report on the origin of the orange photoluminescence emission band
Authors: Kartopu G. , Bayliss S. C. , Hummel R. E. , Ekinci Y.
Published: March 2004

Microstructure and origin of the orange photoluminescence (PL) band of visible luminescing spark-processed germanium (sp-Ge) has been studied using simultaneous micro-Raman and -PL spectroscopy and scanning electron microscope. Instability of the sp-Ge film to energetic electron beams (⩾5 keV) during SEM measurements suggested that sp-Ge is not composed exclusively of purely Ge–Ge bonded material (elemental Ge) but also of other form(s) of material(s) of different chemistry. Indeed, micro-Raman spectra of the films showed that sp-Ge is composed of mainly substoichiometric germanium oxides (GeOxs) and elemental Ge (in the form of nano- or micro-crystals). Further, it is proposed that the Ge particles were embedded in thick GeOx (0<x<2) layers. Sizes of the Ge nanocrystals were estimated using a phonon confinement model and it was found that the spark processing method can generate Ge crystals with sizes as small as 6–8 nm. However, the micro-PL of sp-Ge indicated unambiguously that the orange PL band (near 1.9 eV) does not originate from or involve these Ge nanocrystals but the Ge–O bonded material, i.e., GeOxs.
Magnetic properties of Fe1−xMnx/Fe nanocomposites
Authors: Anhøj Thomas A. , Jacobsen Claus S. , Mørup Steen
Published: March 2004

We have prepared nanocomposites of mixtures of ferromagnetic α-Fe and antiferromagnetic γ-Fe50Mn50 nanoparticles, and studied their magnetic and structural properties by magnetization measurements, Mössbauer spectroscopy, and x-ray diffraction. A sample consisting of a 1:1 mixture of the two materials showed enhanced coercivity, but almost negligible exchange bias at room temperature after field cooling from 520 K. However, samples with higher content of γ-Fe50Mn50 showed significant exchange bias. The mechanisms for exchange bias and enhanced coercivity in the system are discussed.
Analysis of emission data from O2 plasmas used for microbe sterilization
Authors: Sharma S. P. , Cruden B. A. , Rao M. V. V. S. , Bolshakov A. A.
Published: March 2004

In order to study the sterilization capabilities of radio frequency driven low pressure oxygen plasmas, the radiative emission was recorded at various pressures and input powers. A distinct transition from the bright mode (primarily inductively coupled) to a dim mode (primarily capacitively coupled) was observed as the pressure was increased and/or the power decreased. The data was further analyzed to estimate the electron temperature, rotational and vibrational temperatures, and various species concentrations. Based on the diffusion and rovibrational relaxation times, it is concluded that the rotational temperatures can be assumed to be in equilibrium with the translational temperature. The ions are produced “hot” and have little time to get equilibrated with the translational temperature. It is further determined that in the bright mode, which is more effective in microbe sterilization, the translational/rotational temperatures are in the 650–850 K range, the electron temperatures are low (3.5–4.5 eV), and the concentrations of atomic O and atomic metastables are at 1 order of magnitude higher than in the dim mode.
X-ray photoemission spectroscopy of nonmetallic materials: Electronic structures of boron and BxOy
Authors: Ong C. W. , Huang H. , Zheng B. , Kwok R. W. M. , Hui Y. Y. , Lau W. M.
Published: March 2004

Although an increasing volume of x-ray photoemission spectroscopic (XPS) data has been accumulated on boron and boron-rich compounds because of their unusual properties, including a unique three-center, two-electron bonding configuration, their common nonmetallic nature has been overlooked. Typically, the measured energy-state data are not clarified by surface Fermi level positions of these nonmetallic samples, which compromises the scientific contents of the data. In the present study, we revisited the XPS studies of sputter-cleaned β-rhombohedral boron (βr-B), the oxidized surface of βr-B, B6O pellet, and polished B2O3, to illustrate the impact and resolution of this scientific issue. These samples were chosen because βr-B is the most thermodynamically stable polytype of pure boron, B2O3 is its fully oxidized form, and B6O is the best known superhard family member of boron-rich compounds. From our XPS measurements, including those from a sputter-cleaned gold as a metal reference, we deduced that our βr-B had a surface Fermi level located at 0.7±0.1 eV from its valence-band maximum (VBM) (referred as EFL) and a binding energy for its B 1s core level at 187.2 eV from VBM (Eb,VBM). The latter attribute, unlike typical XPS binding energy data that are referenced to a sample-dependent Fermi level (Eb,FL), is immune from any uncertainties and variations arising from sample doping and surface charging. For bulk B2O3, we found an Eb,VBM for its B 1s core level at 190.5 eV and an Eb,FL at 193.6 eV. For our βr-B subjected to a surface oxidation treatment, an overlayer structure of ∼1.2 nm B2O3/∼2 nm B2O/B was found. By comparing the data from this sample and those from βr-B and bulk B2O3, we infer that the oxide overlayer carried some negative fixed charge and this induced on the semiconducting βr-B sample an upward surface band bending of ∼0.6 eV. As for our B6O sample, we found an EFL of ∼1.7 eV and two different chemical states having Eb,VBM of 185.4 and 187.2 eV, with the former belonging to boron with no oxygen neighbor and the latter to boron with an oxygen neighbor. The methodology in this work is universally applicable to all nonmetallic samples.