bluuebunny/crossref_metadata_2025 huggingface dataset:
Conformational features of poly(1,1-dihydroperfluorooctyl acrylate) and poly(vinyl acetate) diblock oligomers in supercritical carbon dioxide
Authors: Baysal Canan , Erman Burak , Chu Benjamin
Published: July 2002

We report detailed molecular dynamics calculations of single chain diblocks of poly(1,1- dihydroperfluorooctyl acrylate) (PFOA) and poly(vinyl acetate) (PVAc) in supercritical carbon dioxide, SCCO2. At the critical micelle concentration, this system exhibits self-assembly into micellar structures due to the solvent specific selectivity of the blocks. Although the intermolecular factors determining micelle formation are well studied for this system, the intramolecular single chain conformational features of the molecules have not yet been investigated in the literature. The specific aim of the present work is to study the conformational properties of the single diblock chains in supercritical carbon dioxide at 65 °C, and at four different pressures by molecular dynamics simulations. Fluctuations in the shapes of the PVAc and PFOA blocks are observed to be strongly dependent on pressure. The rate of approach of an initially rodlike chain to its equilibrium conformational space is likewise found to depend strongly on pressure. Calculations of the radial distribution function for the solvent molecules around the chain show that a solvation shell is formed around the polymer. Important correlations between the results of single chain calculations and micelle behavior are observed and discussed.
Electron heating in atmospheric pressure glow discharges
Authors: Stark Robert H. , Schoenbach Karl H.
Published: July 2002

The application of nanosecond voltage pulses to weakly ionized atmospheric pressure plasmas allows heating the electrons without considerably increasing the gas temperature, provided that the duration of the pulses is less than the critical time for the development of glow-to-arc transitions. The shift in the electron energy distribution towards higher energies causes a temporary increase in the ionization rate, and consequently a strong rise in electron density. This increase in electron density is reflected in an increased decay time of the plasma after the pulse application. Experiments in atmospheric pressure air glow discharges with gas temperatures of approximately 2000 K have been performed to explore the electron heating effect. Measurements of the temporal development of the voltage across the discharge and the optical emission in the visible after applying a 10 ns high voltage pulse to a weakly ionized steady state plasma demonstrated increasing plasma decay times from tens of nanoseconds to microseconds when the pulsed electric field was raised from 10 to 40 kV/cm. Temporally resolved photographs of the discharge have shown that the plasma column expands during this process. The nonlinear electron heating effect can be used to reduce the power consumption in a repetitively operated air plasma considerably compared to a dc plasma operation. Besides allowing power reduction, pulsed electron heating also has the potential to enhance plasma processes, which require elevated electron energies, such as excimer generation for ultraviolet lamps.
Structural study of InGaAs/GaAs quantum dots grown by metalorganic chemical vapor deposition for optoelectronic applications at 1.3 μm
Authors: Passaseo A. , Rinaldi R. , Longo M. , Antonaci S. , Convertino A. L. , Cingolani R. , Taurino A. , Catalano M.
Published: July 2002

We have studied the influence of difference growth conditions on the two-dimensional to three-dimensional growth mode transition for a specific class of InGaAs/GaAs quantum dots (QDs) optimized for applications to optical devices operating around 1.3 μm (In content x≈0.5). The dots are grown by low-pressure metalorganic chemical vapor deposition on GaAs substrates. We demonstrate that the critical layer thickness corresponding to optimized single-QD layer structures (i.e., with reduced wetting layer thickness and high uniformity) can be controlled by kinetic effects. The optimized growth conditions allow us to grow six-layers stacked QD structures as active material for the fabrication of a light emitting devices operating around 1.3 μm at room temperature.
Magnetic micropatterning of FeNi/FeMn exchange bias bilayers by ion irradiation
Authors: Mougin A. , Poppe S. , Fassbender J. , Hillebrands B. , Faini G. , Ebels U. , Jung M. , Engel D. , Ehresmann A. , Schmoranzer H.
Published: July 2002

Ion irradiation is an excellent tool to modify magnetic properties on the submicrometer scale, without modification of the sample topography. We utilize this effect to magnetically pattern exchange bias double layers using resist masks patterned by electron-beam lithography. Ion irradiation through the masks leads to a lateral modification of the magnetization reversal behavior and allows one to study the magnetization reversal as a function of the exchange bias field strength on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and magnetic force microscopy, respectively.
Hydrogen depth-profiling in chemical-vapor-deposited diamond films by high-resolution elastic recoil detection
Authors: Kimura Kenji , Nakajima Kaoru , Yamanaka Sadanori , Hasegawa Masataka , Okushi Hideyo
Published: July 2002

We have measured the hydrogen depth profiles in chemical-vapor-deposited diamond films by elastic recoil detection. Depth resolution of ∼0.23 nm is achieved using a high-resolution magnetic spectrometer. The hydrogen depth profile shows a sharp peak at surface, and the hydrogen coverage is estimated to be 1±0.3 ML, indicating formation of the monohydride structure. The surface peak has a small tail toward deeper region, which is ascribed to hydrogen atoms incorporated in a subsurface region. These subsurface hydrogen atoms might be the origin of the surface conductivity.
Dispersion of the ordinary refractive-index change in a proton-exchanged LiNbO3 waveguide
Authors: Ramponi R. , Marangoni M. , Osellame R.
Published: July 2002

The wavelength dependence of the ordinary refractive-index change of a proton exchanged LiNbO3 waveguide is determined by applying a recently proposed method based on radiation modes. A peculiar behavior is observed since the absolute value of the ordinary index change increases with the wavelength, while the extraordinary one decreases. The Sellmeier curves for the dispersion of both the index changes are provided in the wavelength range from λ=0.514 μm to λ=1.554 μm.
Electrical resistivity measurements on fragile organic single crystals in the diamond anvil cell
Authors: Adachi T. , Tanaka H. , Kobayashi H. , Miyazaki T.
Published: July 2002

A method of sample assembly for four-probe resistivity measurements on fragile organic single crystals using a diamond anvil cell is presented. A procedure to keep insulation between the metal gasket and four leads of thin gold wires bonded to the sample crystal by gold paint is described in detail. The resistivity measurements performed on a single crystal of an organic semiconductor and that of neutral molecules up to 15 GPa and down to 4.2 K showed that this new procedure of four-probe diamond anvil resistivity measurements enables us to obtain sufficiently accurate resistivity data of organic crystals.
Angular momentum and energy transferred through ferromagnetic resonance
Authors: Jander Albrecht , Moreland John , Kabos Pavel
Published: July 2002

We show that ferromagnetic resonance (FMR) selectively transfers angular momentum and energy from a microwave field to the lattice as measurable torque and heat. The expected torque and absorbed power are derived classically in terms of Landau–Lifshitz dynamics, including demagnetizing field effects. The torque is also described as a photon absorption process, in which the absorbed photons carry both energy and angular momentum. FMR data are shown for a thin NiFe film deposited on a micromechanical cantilever detector that measures both torque and heat under nearly identical conditions.
Stress dependent activation entropy for dynamic fatigue of pristine silica optical fibers
Authors: Shiue Y. S. , Matthewson M. J.
Published: July 2002

Subcritical crack growth in fused silica is treated as a stress assisted chemical reaction between water and strained bonds at the crack tip. In this work, the kinetics of the reaction is modeled by assuming the stress reduces the energy barrier of the activated complex by affecting both the activation enthalpy and entropy, where the stress dependence can take different forms. This theory is compared with dynamic fatigue data obtained for pristine fused silica optical fiber. The experiments were conducted in both distilled water and pH 7 buffer solution, and the results are found to be similar. The fatigue parameters were found by fitting to three different forms for the stress dependence; the activation enthalpy and entropy were then determined from the fatigue parameters. It is found that stress increases the activation entropy, whichever kinetic form is used, and thereby reduces the activation energy barrier height. The activation enthalpy is also stress dependent, but stress tends to increase the enthalpy contribution to the barrier height. The results show subcritical crack growth in high strength silica is dominated by entropy effects.
Electrode and interfacial polarization in broadband dielectric spectroscopy measurements
Authors: Pizzitutti Francesco , Bruni Fabio
Published: July 2002

The accumulation of charge carriers at the sample-electrode interface in broadband dielectric spectroscopy results in large electrode polarization masking the frequency response of the sample. Additionally, heterogeneous samples exhibit frequency-dependent dielectric and conductive properties that differ from those of the constituent components, yielding so-called Maxwell–Wagner relaxation. We present here an experimental approach to extract true sample dielectric properties from the total frequency response of the electrode–sample assembly. A test case (i.e., ice) is shown to prove the correctness of the proposed method. Results also confirm that polarization due to blocking electrodes is not negligible even in the MHz frequency range.
High-performance InAs/GaAs quantum-dot infrared photodetectors with a single-sided Al0.3Ga0.7As blocking layer
Authors: Lin Shih-Yen , Tsai Yau-Ren , Lee Si-Chen
Published: July 2002

Ten-stacked InAs/GaAs quantum-dot infrared photodetectors with single Al0.3Ga0.7As blocking layers at either side of the stacked dots are investigated. With peak responsivity 214 mA/W and specific detectivity 1.17×1010 cm Hz1/2/W at 6 μm, quantum-dot infrared photodetectors with single-sided blocking layers are superior in responsivity with compatible detectivity as compared to those without blocking layers. Enhancement of the photoelectron avalanche process and the absence of negative differential conductance are observed. The devices exhibit two different infrared absorption regions at 2–6 and 6–10 μm, which indicates a wide detection window of the device.
The trace formula for Sturm–Liouville operator with operator coefficient
Authors: Adigüzelov Ehliman , Avci Hamit , Gül Erdal
Published: July 2002

We will obtain a formula for the trace of the Sturm–Liouville operator with operator coefficient.
Extending Wertheim’s perturbation theory to the solid phase: The freezing of the pearl-necklace model
Authors: Vega C. , MacDowell L. G.
Published: July 2002

An extension of Wertheim’s first order perturbation theory [M. S. Wertheim, J. Chem. Phys. 87, 7323 (1987)] for chain molecules in the solid phase is proposed. In particular the solid phase of a model of freely jointed chains of tangent hard spheres has been considered. The equation of state and free energies from theory are compared with simulation results obtained previously by Malanoski and Monson [A. P. Malanoski and P. A. Monson, J. Chem. Phys. 107, 6899 (1997)] for chains with up to eight monomer units. The agreement between theory and simulation is good for the densities close to the melting, and deteriorates for densities close to close-packing. We also evaluated theoretically the fluid-solid equilibrium of the pearl-necklace model and compare with simulation results. The theory reproduces reasonably well the trends observed in the simulation results.
Physical mechanisms of thermal-diffusivity depth-profile generation in a hardened low-alloy Mn, Si, Cr, Mo steel reconstructed by photothermal radiometry
Authors: Nicolaides Lena , Mandelis Andreas , Beingessner Clare J.
Published: July 2002

It is well established that in hardened steels thermal-diffusivity broadly anticorrelates with microhardness, allowing thermal-wave depth profilometry to be used as a tool to measure microhardness profiles. Nevertheless, the physical mechanisms for this anticorrelation have not been well understood. In this work, the thermal-diffusivity profiles of rough, hardened industrial steels were reconstructed after the elimination of roughness effects from the experimental data. Carburizing and quenching are widely used for the heat treatment of steel components, and it is important to understand their effects on thermal-diffusivity profiles. A thorough examination of the actual mechanism by which thermal-diffusivity depth profiles are affected by first carburizing and then quenching AISI-8620 steels was performed. It was concluded that the variation of thermal diffusivity with depth is dominated by the carbon concentration profile, whereas the absolute value of the thermal diffusivity is a function of microstructure.
Molecular random access memory cell
Authors: Reed M. A. , Chen J. , Rawlett A. M. , Price D. W. , Tour J. M.
Published: July 2002

Electronically programmable memory devices utilizing molecular self-assembled monolayers are reported. The devices exhibit electronically programmable and erasable memory bits compatible with conventional threshold levels and a memory cell applicable to a random access memory is demonstrated. Bit retention times >15 min have been observed.
Spectroscopy and photodissociation of ClF in rare gas solids
Authors: Bargheer M. , Dietrich P. , Schwentner N.
Published: July 2002

Vibrational progressions in the A′→X(v″) fluorescence of ClF in Ar and Kr matrices with 1<v″<9 are observed in the spectral range from 600 to 900 nm with lifetimes of 141 ms. Two-photon excitation leads to emission from the ionic D′ state to the A′, 2 Π23, and 2 Δ23 valence states in the range from 390 to 470 nm with 50 ns lifetime. Ground state parameters ωe and ωexe as well as Te values for the A′ and D′ states are derived. Matrix and Stokes shifts are observed and the large linewidth of the D′ emission is related to the strong coupling of the ionic state to the lattice. Dissociation quantum efficiencies of 5% and 4.8% are determined for kinetic energies of the F fragment of 0.92 eV and 0.39 eV in Ar (19 K), respectively.
Structural and optical evidence of island correlation in CdTe/ZnTe superlattices
Authors: Maćkowski S. , Karczewski G. , Wojtowicz T. , Kossut J. , Kret S. , Szczepańska A. , Dl̸użewski P. , Prechtl G. , Heiss W.
Published: July 2002

The properties of superlattices consisting of 2 monolayer wide CdTe insertions into ZnTe spacer barriers with thickness ranging from 3 to 75 monolayers are investigated by means of transmission electron microscopy and photoluminescence spectroscopy. We show that quasi zero-dimensional CdTe islands form in this highly lattice-mismatched system. For spacer thickness smaller than 25 monolayers, the islands are vertically correlated along the axis tilted by 40° with respect to the growth direction, while for thicker ZnTe spacers no correlation is observed. The electronic coupling between the correlated islands manifests itself by the appearing of an additional emission band at energies lower to those corresponding to uncorrelated dots. The optical spectroscopy data reveal zero-dimensional localization of excitons by the electronically coupled islands. The decay time of the excitonic recombination is found to be over an order of magnitude longer in the case of the coupled islands than in the case of isolated ones.
Characterization of near-surface traps in semiconductors: GaN
Authors: Look D. C. , Fang Z.-Q.
Published: July 2002

We present a simple a criterion, based on deep-level transient spectroscopy peak heights S(Vr) at two or more values of reverse bias Vr, to unequivocally determine whether or not a particular semiconductor trap is of bulk or near-surface nature. Moreover, we present an expression for S(Vr) with fitting parameters φB, the Schottky barrier height; δ, the trap penetration depth; and NT, the trap density. Application of the method to a thick, high-quality, epitaxial GaN layer, reveals two common traps which penetrate only 2700±300 Å into the layer.
A new combination of replica exchange Monte Carlo and histogram analysis for protein folding and thermodynamics
Authors: Gront Dominik , Kolinski Andrzej , Skolnick Jeffrey
Published: July 2002

A novel combination of replica exchange Monte Carlo sampling techniques with a histogram analysis approach is developed and applied to study the thermodynamics of the folding transition in a face-centered cubic lattice chain protein model. Sequences of hydrophobic (H) and polar (P) topology residues were designed to fold into various β-barrel type proteins. The interaction scheme includes the short-range propensity to form extended conformations, residue-dependent long-range contact potentials, and orientation-dependent hydrogen bonds. Weakly cooperative folding transitions could be observed for properly designed HP. (Hydrophobic and polar residue sequences with cooperative long-range interaction methods were proposed and tested.) Based on the study of these systems, the computational cost of such an approach is many times less than the cost of other Monte Carlo procedures. This opens up the possibility for efficient studies of the folding thermodynamics of more detailed protein models.
Efficient electron injection in blue-emitting polymer light-emitting diodes with LiF/Ca/Al cathodes
Authors: Brown T. M. , Friend R. H. , Millard I. S. , Lacey D. J. , Burroughes J. H. , Cacialli F.
Published: July 2002

We report electroabsorption and electroluminescence investigations of polymer light-emitting diodes featuring a LiF/Ca/Al cathode, for efficient electron injection into the electroluminescent polymer layer. Our measurement of the built-in potential gives direct evidence of a sizeable reduction of the cathodic barrier height not only with respect to Ca, but also versus LiF/Al or CsF/Al bilayer cathodes, currently amongst the most efficient electron injectors for low electron affinity polymers. In blue-emitting (∼2.7 at peak) polyfluorene-based LEDs, with poly(ethylenedioxythiophene)/poly(styrene sulphonic acid) anodes and LiF/Ca/Al cathodes, we measure a built-in potential of 2.7 V, a luminance of ∼1600 cd/m2 (the highest among the devices studied here) at a driving voltage of 5 V, and efficiencies as high as ∼3 lm/W. We also find that the turn-on voltage essentially coincides with the built-in potential within the experimental error.
Effect of surface roughness on field emission from chemical vapor deposited polycrystalline diamond
Authors: Koenigsfeld N. , Kalish R. , Cimmino A. , Hoxley D. , Prawer S. , Yamada I.
Published: July 2002

The effect of surface roughness on electron emission from hydrogenated polycrystalline diamond films is reported. Field emission measurements were performed with both millimeter and nanometer spatial resolution using scanning probe techniques. Surface asperities were removed by ion beam treatment, which resulted in a reduction of the rms roughness from 198 to 94 nm, leading to an increase in the threshold field required for electron emission by about a factor of 2. These results suggest that surface asperities, rather than grain boundaries, are the dominant influence on electron emission in polycrystalline diamond films.
Dielectric and ferroelectric properties of SrBi4Ti4O15 single crystals
Authors: Irie Hiroshi , Miyayama Masaru
Published: July 2002

SrBi 4 Ti 4 O 15 single crystals were grown, and their dielectric and ferroelectric properties were investigated along the a(b) axis and c axis, separately. The dielectric permittivity at 1 MHz was 1900 along the a(b) axis at the Curie temperature of 520 °C. This value was ten times higher than that along the c axis. With respect to the ferroelectricity, the saturated remanent polarization was 29 μC/cm2 and the saturated coercive field was 26 kV/cm along the a(b) axis under an electric field of 59 kV/cm, and ferroelectricity was not observed along the c axis.
Evanescent and propagating wave characteristics of the photonic quantum ring laser
Authors: Park B. H. , Ahn J. C. , Bae J. , Kim J. Y. , Kim M. S. , Baek S. D. , Kwon O’Dae
Published: July 2002

We present the wave propagation characteristics of photonic quantum ring lasers with clear evidence of the evanescence-propagation crossover behavior. The data obtained from the fiber probe measurements of 15–36 μm disk size lasers show that the Rayleigh’s whispering gallery modes always give rise to the propagation mode, with the crossover ranges of 55–80 μm. The three-dimensional but chiral and broken z-axis symmetry nature of the photonic quantum ring cavity appears to guarantee such a crossover to the propagation in general.
Surface passivation of p-type crystalline Si by plasma enhanced chemical vapor deposited amorphous SiCx:H films
Authors: Martı́n I. , Vetter M. , Orpella A. , Puigdollers J. , Cuevas A. , Alcubilla R.
Published: July 2002

Excellent passivation properties of intrinsic amorphous silicon carbide (a-SiCx:H) films deposited by plasma enhanced chemical vapor deposition on single-crystalline silicon (c-Si) wafers have been obtained. The dependence of the effective surface recombination velocity, Seff, on deposition temperature, total pressure and methane (CH4) to silane (SiH4) ratio has been studied for these films using lifetime measurements made with the quasi-steady-state photoconductance technique. The dependence of the effective lifetime, τeff, on the excess carrier density, Δn, has been measured and also simulated through a physical model based on Shockley–Read–Hall statistics and an insulator/semiconductor structure with fixed charges and band bending. A Seff at the a-SiCx:H/c-Si interface lower than 30 cm s−1 was achieved with optimized deposition conditions. This passivation quality was found to be three times better than that of noncarbonated amorphous silicon (a-Si:H) films deposited under equivalent conditions.
Crystallization kinetics of sputter-deposited amorphous AgInSbTe films
Authors: Njoroge Walter K. , Wuttig Matthias
Published: July 2002

AgInSbTe films have recently attracted considerable interest as advanced materials for phase change recording. For this application the determination of crystallization kinetics is of crucial importance. In this work the temperature dependence of structural and electrical properties of sputtered AgInSbTe films has been determined. Temperature dependent measurements of the electrical resistance have been employed to study the kinetics of structural changes of these films. Upon annealing a major resistivity drop is observed at around 160 °C which can be attributed to a structural change as corroborated by x-ray diffraction. X-ray diffraction shows an amorphous phase for as-deposited films, while crystalline films with hexagonal structure (a=4283 Å, c=16 995 Å) are obtained upon annealing above 160 °C. By applying Kissinger’s method, an activation energy of 3.03±0.17 eV is obtained for the crystallization. X-ray reflection measurements reveal a density increase of 5.2%±0.2% and a thickness decrease of 5.5%±0.2% upon crystallization.
Self-similar concentration profiles in buoyant mixing of miscible fluids in a vertical tube
Authors: Debacq M. , Fanguet V. , Hulin J. P. , Salin D. , Perrin B.
Published: July 2002

The influence of the density contrast (characterized by the Atwood number At) on gravity-induced mixing between two miscible fluids in a long vertical tube has been studied experimentally. Cross-section averaged fluid concentration profiles along the tube are measured optically: for large enough At values, they display a self-similar dependence in a broad range of times and distances and verify a diffusion law with an effective diffusivity 105 times higher than for molecular diffusion. At lower At values, this diffusive domain is limited by a sharp front moving at a velocity increasing with At. Below a threshold At value the diffusive behavior disappears.
Tight-binding model and electronic structure of tetrahedral zirconium silicate
Authors: Jameson John R. , Harrison Walter , Griffin P. B.
Published: July 2002

We present a tight-binding model of Zr silicate in the limit that every Zr and Si atom is bonded to four O atoms. We view the material as being composed of small, relatively uncoupled collections of atoms, called bonding units, with stoichiometries SiSiO4 and ZrSiO4. The SiSiO4 bonding unit constitutes a model for pure SiO2, and the ZrSiO4 bonding unit represents the fundamental element distinguishing tetrahedral Zr silicate from pure SiO2. In the first part of this article we look at the electronic structure of “ideal” bonding units in which the O atoms are arranged in a perfect tetrahedron and the M–O–Si angle is 180° (M=Si or Zr). We find the valence levels of both bonding units to be dominated by O p states, and the lowest conduction levels of the ZrSiO4 bonding unit to derive primarily from Zr d states, whose energy depends sensitively upon the charge transfer to the O atoms. The energy gap of the ideal ZrSiO4 bonding unit is found to be 5.9 eV, compared to 8.0 eV for the SiSiO4 bonding unit. Finally, for the ZrSiO4 bonding unit, we present a simplification which allows the energy levels of the ZrSiO4 bonding unit to be obtained approximately in terms of decoupled Zr–O and Si–O interactions. In the next part of the article we investigate how bond angle and bond length distortions affect the electronic structure of the ZrSiO4 bonding unit. In particular, we note that significant distortions of the Zr–O–Si angle could produce Zr-based localized states that could act as traps for electrons tunneling through the material. In the last part of the article we discuss the basic principles governing band lineups for Si/silicate interfaces constructed by substituting Zr atoms for Si atoms on the SiO2 side of crystalline Si/SiO2 interfaces. We calculate the band lineups as a function of Zr concentration for one particular interface.
Explicit formulas for effective piezoelectric coefficients of ferroelectric 0-3 composites
Authors: Wong C. K. , Poon Y. M. , Shin F. G.
Published: July 2002

Explicit formulas have been found for the effective piezoelectric coefficients of a 0-3 composite of ferroelectric spherical particles in a ferroelectric matrix. Tensile loading and hydrostatic loading conditions were studied. Assuming that both phases are dielectrically and elastically isotropic, explicit expressions in simple closed form for the effective d33, d31 and dh coefficients were derived in terms of the constituents’ piezoelectric coefficients and the dielectric and elastic properties of the composite and constituents. Prediction of the piezoelectric coefficients for specific composite systems was compared with experimental values from published works, and good agreement with data was obtained. Goodness of fit is not limited to low volume fraction of inclusions.
Facile H–D exchange in adsorbed methylidyne on Pt{110}–(1×2) and deuteration to gaseous methane
Authors: Watson D. T. P. , Ge Q. , King D. A.
Published: July 2002

Hydrogen–deuterium exchange in adsorbed methylidyne, CHa, on Pt{110}–(1×2) has been studied for the first time using supersonic D2/H2 molecular beams, which provides new insights into the reversible hydrogenation of adsorbed hydrocarbon fragments. The exchange reaction is extremely facile at surface temperatures of 350–450 K and proceeds via a Langmuir–Hinshelwood reaction between Da and a CHa fragment to produce gas phase H2 and HD. The CDa+Ha (i.e., reverse) reaction was also studied and was found to proceed more slowly. Both exchange reactions were successfully modeled and the difference in reaction rates is explained using zero point energy differences alone. Finally, we demonstrate that with high incident D2 fluxes CHa can be completely deuterated to produce gaseous CHD3 and CD4.
Dynamics of photocreated positive holes in silver bromide microcrystals with adsorbed cyanine dyes
Authors: Hosoi Kazukuni , Hirano Akihiro , Tani Tadaaki
Published: July 2002

Techniques of radiowave photoconductivity (A) and microwave photoconductivity with two synchronized light pulses (B) were used to study the dynamics of photocreated positive holes in AgBr microcrystals with adsorbed cyanine dyes as spectral sensitizers. It was revealed by technique A that positive holes were captured by the dyes with oxidation potential (Eox) less positive than 1.40 V versus the saturated calomel electrode within a few microseconds. In the light of the fact that Eox corresponds to the height of the highest occupied molecular orbital (HOMO) of a cyanine dye, it was concluded that the height of the HOMO of the dye with Eox of 1.40 V was equivalent to that of the top of the valence band of AgBr. It was revealed by techniques A and B that positive holes decayed as a result of their encounter with such irreversible traps as Ag centers after repetition of trapping and detrapping by adsorbed dye molecules many times in a period ranging from 10−5 to 104 s, depending on the height of the HOMO and the amount of dyes.
Kinetic pathway of lamellar → gyroid transition: Pretransition and transient states
Authors: Imai Masayuki , Saeki Akira , Teramoto Takashi , Kawaguchi Ayako , Nakaya Kaori , Kato Tadashi , Ito Kazuki
Published: July 2002

The kinetic pathway of a lamellar → gyroid transition has been investigated by means of a small angle x-ray scattering (SAXS) and a new computer simulation scheme developed by Saeki. Approaching temperature to a lamellar → gyroid transition temperature (TLG) from the lamellar phase, SAXS profiles reveal that modulation fluctuations with local hexagonal arrangement develop in the lamellar state. The modulation fluctuation layer (MFL) structure is an equilibrium structure and corresponds to the most unstable fluctuation modes of lamellar structure. The MFL transforms to a unique three dimensional network lattice having R3̄c symmetry, which agrees with the prediction of the computer simulation. This rhombohedral structure is a transient structure and transforms to the gyroid structure with elapse of time.
Role of the cesium antimonide layer in the Na2KSb/Cs3Sb photocathode
Authors: Natarajan Aravind , Kalghatgi A. T. , Bhat B. M. , Satyam M.
Published: July 2002

When radiation of sufficiently high energy is incident on the surface of a semiconductor photocathode, electrons are excited from the valence band to the conduction band and these may contribute to the photocurrent. The photocurrent in a single-layer cathode is found to be small, because of collisions within the cathode material, the electron affinity condition, etc. It is observed that when a thin layer of n-type cesium antimonide (Cs3Sb) is deposited over a p-type layer of sodium potassium antimonide (Na2KSb), there occurs a sharp rise in the photocurrent. The causes for the dramatic increase in the photocurrent obtainable from a sodium potassium antimonide cathode, by depositing a thin layer of cesium antimonide are analyzed in this article. It has been shown that the interface between sodium potassium antimonide and cesium antimonide can result in lowering of the electron affinity to a level below the bottom of the conduction band of sodium potassium antimonide. The drift field that arises at the heterointerface enables the electrons to reach the surface, leading to the emission of almost all the photogenerated electrons within the cathode. The processes involved in photoemission from such a double-layer cathode are examined from a theoretical point of view. The spectral response of the two-layer cathode is also found to be better than that of a single-layer cathode.
Electronic structure of the Fe-layer-catalyzed carbon nanotubes studied by x-ray-absorption spectroscopy
Authors: Yueh C. L. , Jan J. C. , Chiou J. W. , Pong W. F. , Tsai M.-H. , Chang Y. K. , Chen Y. Y. , Lee Y. F. , Tseng P. K. , Wei S. L. , Wen C. Y. , Chen L. C. , Chen K. H.
Published: July 2002

X-ray-absorption near edge structure (XANES) measurements have been performed to investigate the local electronic structures of the Fe-catalyzed and stabilized carbon nanotubes (CNT) with various diameters. The intensities of the π*- and σ*-band and the interlayer-state features in the C K-edge XANES spectra of these CNTs vary with the diameter of the CNT. The white-line features at the C K- and Fe L3-edges suggest a strong hybridization between the C 2p and Fe 3d orbitals, which lead to an enhancement of the C K- and reduction of the Fe L3-edge features, respectively, indicative of a charge transfer from C 2p to Fe 3d orbitals. The Fe K-edge spectra reveal a p–d rehybridization effect that reduces p-orbital occupation at the Fe site.
Thermal conductivity and interfacial thermal resistance of polymeric low k films
Authors: Hu Chuan , Kiene Michael , Ho Paul S.
Published: July 2002

The effective thermal conductivity of four polymeric thin films with distinct molecular morphologies has been measured as a function of film thickness down to 70 Å using a 3ω technique. Comparing to SiO2, the intrinsic thermal conductivity of polymers is about 5–10 times smaller while the interfacial thermal resistance is about 2–10 times larger. The interfacial thermal resistances are explained in terms of the mismatches of acoustic and mechanical properties between polymers and crystalline materials. Both elastic and inelastic scatterings at the interface are examined.
Role of metal–molecule chemistry and interdiffusion on the electrical properties of an organic interface: The Al–F16CuPc case
Authors: Shen Chongfei , Kahn Antoine , Schwartz Jeffrey
Published: July 2002

The chemistry and electronic structure of interfaces between Al and copper hexadecafluorophthalocyanine (F16CuPc) are studied via x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy (UPS), and current–voltage measurements. Electron injection barriers measured by UPS are reported. Analysis of the Al–F16CuPc reaction shows the formation of a phase of three-dimensional (F16CuPc)3Al species. The reacted region is extended at the Al-on-F16CuPc interface and narrow at the F16CuPc-on-Al interface. A series of metal/F16CuPc/metal structures is fabricated to study the impact of the interface chemistry and deposition sequence on device performances. It is found that (F16CuPc)3Al forms a low conductivity region, which has considerable bearing on the electron current. The interface fabrication sequence, which defines the thickness of the reacted region, therefore has a profound impact on device performance.
Formation of bubbles by high dose He implantation in 4H–SiC
Authors: Oliviero E. , David M. L. , Beaufort M. F. , Nomgaudyte J. , Pranevicius L. , Declémy A. , Barbot J. F.
Published: July 2002

1.6 MeV He+ ions were implanted at room temperature into (0001)Si n-type 4H–SiC at a dose of 1×1017 cm−2. Cross-section transmission electron microscopy (TEM) and x-ray diffraction (XRD) were used to characterize the induced defects and the strain-induced effects before and after annealing. Infrared reflectivity was also used to localize changes in the microstructure. In the as-implanted samples, the TEM observations show a three layer damaged region consisting of a continuous amorphous layer surrounded with two buried crystalline zones. Bubbles of small diameter are readily formed in the as-implanted state. Only a few changes are observed after a 800 °C-30 min annealing. Voids formation and recrystallization of the amorphous state in different polytypes are observed for a 1500 °C-30 min annealing. Moreover, XRD shows that the dilatation of the c axis of the lattice observed after implantation completely disappears after the high temperature annealing implying the structural recovery of all the crystalline perturbed regions. An analytical study of atomic redistribution processes under ion implantation is also developed by introducing a frequency probability function of relocation between monolayers. The calculated distribution profiles of atoms are compared with the TEM results.
Organic heterojunction phototransistor
Authors: Zukawa Takehiro , Naka Shigeki , Okada Hiroyuki , Onnagawa Hiroyoshi
Published: July 2002

We propose an organic heterojunction phototransistor stacked with organic layers of electron and hole transport materials. The materials used in this study are perylene derivative (td-PTC) as the ultrathin base layer and triphenylamine derivative (TPD) as the emitter and collector layers. The current versus voltage characteristics as a function of incident optical intensity are typical of a phototransistor with current saturation. The bipolar transistor operation is confirmed by a comparison with the diode structure of the TPD/td-PTC device and because of the base thickness dependence of the current. External quantum efficiency is 2.9% under blue light-emitting diode illumination. In the current response, the initial displacement current is superimposed. Obtained response times are 600 μs for both transient turn-on and turn-off currents.
Optical gaps of ab initio generated random networks for a-SiNx alloys
Authors: Alvarez Fernando , Valladares Ariel A.
Published: July 2002

We report optical gaps for ab initio generated random networks of silicon–nitrogen alloys, a-SiNx, for thirteen values of x from 0 to x=1.29, a nearly stoichiometric composition. The random networks were constructed by amorphizing 64-atom periodically-continued diamond-like cells containing silicon and nitrogen, with a new thermal process and a Harris-functional based molecular dynamics code in the local density approximation. The electron energy levels were then calculated and the optical gaps obtained using a Tauc-like procedure that is not sensitive to gap states and band tails. Our results agree with experiment.
Two functions of the density matrix and their relation to the chemical bond
Authors: Schmider Hartmut L. , Becke Axel D.
Published: July 2002

We examine and compare two previously introduced functions of the one-particle density matrix that are suitable to represent its off-diagonal structure in a condensed form and that have illustrative connections to the nature of the chemical bond. One of them, the Localized-Orbital Locator (LOL) [J. Molec. Struct. (THEOCHEM) 527, 51 (2000)], is based only on the noninteracting kinetic-energy density τ and the charge density ρ at a point, and gives an intuitive measure of the relative speed of electrons in its vicinity. Alternatively, LOL focuses on regions that are dominated by single localized orbitals. The other one, the Parity Function P [J. Chem. Phys. 105, 11134 (1996)], is a section through the Wigner phase-space function at zero momentum, and contains information about the phase of the interference of atomiclike orbital contributions from bound centers. In this paper, we discuss the way in which these functions condense information in the density matrix, and illustrate on a variety of examples of unusual chemical bonds how they can help to understand the nature of “covalence.”
Structure formation in liquid crystalline polymers
Authors: Hamm M. , Goldbeck-Wood G. , Zvelindovsky A. V. , Sevink G. J. A. , Fraaije J. G. E. M.
Published: July 2002

We have developed a simulation technique, based on a dynamic mean field theory to describe the evolution of a melt of liquid crystalline polymers that is quenched below its isotropic–nematic transition temperature. The polymers are described by chains characterized by a bending energy depending on the angles between consecutive chain segments and a Maier Saupe term accounting for the effect of mutual alignment of the segments in the melt. The equations for the time evolution of the system depend on coarsened variables, which are the density ρ(r) and the orientation distribution S_(r). The density field ρ evolves according to a diffusion-type equation, whereas the orientation field S_ follows a growth equation.
Mechanism of atomic layer deposition of SiO2 on the silicon (100)-2×1 surface using SiCl4 and H2O as precursors
Authors: Kang Jeung Ku , Musgrave Charles B.
Published: July 2002

We use density functional theory to investigate atomic layer deposition (ALD) mechanism of silicon dioxide on the Si(100)-2×1 surface from the precursors SiCl4 and H2O. First, we explore the reaction mechanism of water with the bare Si(100)-2×1 surface to produce surface hydroxyl groups. We find that this reaction proceeds through a two-step pathway with an overall barrier of 33.3 kcal/mol. Next, we investigate the ALD mechanism for the binary reaction sequence: the SiCl4 half reaction and the H2O half reaction. For the SiCl4 half reaction, SiCl4 first forms a σ-bond with the oxygen of the surface OH group and then releases an HCl molecule. The predicted barrier for this process is 15.8 kcal/mol. Next, adsorbed SiCl3 reacts with a neighboring OH group to form bridged SiCl2 with a barrier of 22.6 kcal/mol. The H2O half reaction also proceeds through two sequential steps with an overall barrier of 19.1 kcal/mol for the reaction of H2O with bridged SiCl2 to form bridged Si(OH)2. The predicted barrier of 22.6 kcal/mol for the rate-limiting step of the ALD binary reaction mechanism is consistent with the experimental value of 22.0 kcal/mol. In addition, we find that the calculated frequencies are in good agreement with the experimentally measured IR spectra.
Nanoscale pore formation dynamics during aluminum anodization
Authors: Thamida Sunil Kumar , Chang Hsueh-Chia
Published: July 2002

A theoretical analysis of nanoscale pore formation during anodization reveals its fundamental instability mechanism to be a field focusing phenomenon when perturbations on the minima of the two oxide interfaces are in phase. Lateral leakage of the layer potential at high wave number introduces a layer tension effect that balances the previous destabilizing effect to produce a long-wave instability and a selected pore separation that scales linearly with respect to voltage. At pH higher than 1.77, pores do not form due to a very thick barrier layer. A weakly nonlinear theory based on long-wave expansion of double free surface problem yields two coupled interface evolution equations that can be reduced to one without altering the dispersion relationship by assuming an equal and in-phase amplitude for the two interfaces. This interface evolution equation faithfully reproduces the initial pore ordering and their dynamics. A hodograph transformation technique is then used to determine the interior dimension of the well-developed pores in two dimensions. The ratio of pore diameter to pore separation is found to be a factor independent of voltage but varies with the pH of the electrolyte. Both the predicted pH range where pores are formed and the predicted pore dimensions are favorably compared to experimental data.
Fluid solid equilibrium for two dimensional tangent hard disk chains from Wertheim’s perturbation theory
Authors: McBride Carl , Vega Carlos
Published: July 2002

Wertheim’s first order thermodynamic perturbation theory [M. S. Wertheim, J. Chem. Phys. 87, 7323 (1987)] has been extended to the two-dimensional tangent hard disk dimer for both the solid and the fluid phases. This extension utilizes pre-existing equations of state for the fluid [C. F. Tejero and J. A. Cuesta, Phys. Rev. E 47, 490 (1993)] and for the solid phases [B. J. Alder, W. G. Hoover, and D. A. Young, J. Chem. Phys. 49, 3688 (1968)], of the two-dimensional hard disk monomer system. The theory has been compared to simulation results and a good agreement has been found between them. Results are also presented for the fluid–solid tie line for m=2 up to m=30. The results of this work may be of interest to those involved in the study of chainlike systems adsorbed on a two-dimensional surface.
Dielectric and ferroelectric properties of tetragonal tungsten bronze Sr2−xCaxNaNb5O15 (x=0.05–0.35) ceramics
Authors: Xie Rong-Jun , Akimune Yoshio , Matsuo Kazuo , Sugiyama Tatsuo , Hirosaki Naoto , Sekiya Tadshi
Published: July 2002

This letter reports the dielectric and ferroelectric properties of tungsten bronze Sr2−xCaxNaNb5O15 (SCNN, x=0.05–0.35) ceramics. Two dielectric anomalies and a diffuse ferroelectric transition behavior were appreciably observed in the compositions of x=0.05–0.25. The incorporation of smaller calcium cations into the crystal structure resulted in an increase in the Curie temperature, from 279 (x=0.05) to 297 (x=0.35), and a decrease in the permittivity, from 1353 to 543, at their respective Curie temperatures. Ferroelectricity was observed in the compositions with x=0.05–0.25, but absent in the compositions with x=0.30 and 0.35 at room temperature. The maximum spontaneous polarization Ps of 9.1 μC/cm2 and remanent polarization Pr of 3.0 μC/cm2 were achieved in the composition of x=0.15.
Observation of high junction electric fields in modulation-doped GaAs/AlGaAs heterostructures by room temperature photoreflectance spectroscopy
Authors: Estacio E. , Bailon M. , Somintac A. , Sarmiento R. , Salvador A.
Published: July 2002

Room temperature photoreflectance (PR) was utilized to determine the junction electric field in molecular-beam epitaxy (MBE)-grown modulation-doped GaAs/AlGaAs heterostructures (MDH) with varied spacer layer thickness and doping concentration. The PR spectra exhibited Franz–Keldysh oscillations (FKOs) at energies above the GaAs band gap. Linear regression from the energy plots of the FKO extrema allowed for the calculation of the junction electric field strengths. High junction electric fields of magnitude 270–430 kV/cm were obtained from the samples. These measured fields were almost a factor of two higher than what can be accounted for by the measured carrier concentration. The presence of impurities introduced during the growth process, which could account for the added field strength, was checked by photoluminescence (PL) spectroscopy at 10 K. The similar PL spectra of the samples with different electric field strength ruled out carbon as the source of added field observed. Deep level transient spectroscopy (DLTS) measurement performed on MBE-grown Si-doped GaAs revealed electron traps corresponding to M4 and EL2. M4 is an impurity-related trap involving As vacancies while EL2 is an As-antisite trap. Although DLTS was not performed directly on MDH samples, results in the doped GaAs indicate As-related electron traps as the possible cause of the added field observed by PR.
Waveguide structures using one-dimensional photonic crystal
Authors: Taniyama Hideaki
Published: July 2002

This article reports on the numerical analysis of the reflectivity of electromagnetic waves for several one-dimensional photonic crystal structures. A waveguide consisting of one-dimensional photonic crystal is shown to be promising for optical interconnection on a semiconductor wafer.