bluuebunny/crossref_metadata_2025 huggingface dataset:
The roles of apex dipoles and field penetration in the physics of charged, field emitting, single-walled carbon nanotubes
Authors: Peng Jie , Li Zhibing , He Chunshan , Chen Guihua , Wang Weiliang , Deng Shaozhi , Xu Ningsheng , Zheng Xiao , Chen GuanHua , Edgcombe Chris J. , Forbes Richard G.
Published: July 2008

A 1 μm long, field emitting, (5, 5) single-walled carbon nanotube (SWCNT) closed with a fullerene cap, and a similar open nanotube with hydrogen-atom termination, have been simulated using the modified neglect of diatomic overlap quantum-mechanical method. Both contain about 80 000 atoms. It is found that field penetration and band bending, and various forms of chemically and electrically induced apex dipole play roles. Field penetration may help explain electroluminescence associated with field emitting CNTs. Charge-density oscillations, induced by the hydrogen adsorption, are also found. Many of the effects can be related to known effects that occur with metallic or semiconductor field emitters; this helps both to explain the effects and to unify our knowledge about FE emitters. However, it is currently unclear how best to treat correlation-and-exchange effects when defining the CNT emission barrier. A new form of definition for the field enhancement factor (FEF) is used. Predicted FEF values for these SWCNTs are significantly less than values predicted by simple classical formulae. The FEF for the closed SWCNT decreases with applied field; the FEF for the H-terminated open SWCNT is less than the FEF for the closed SWCNT but increases with applied field. Physical explanations for this behavior are proposed but the concept of FEF is clearly problematical for CNTs. Curved Fowler–Nordheim plots are predicted. Overall, the predicted field emission performance of the H-terminated open SWCNT is slightly better than that of the closed SWCNT, essentially because a C–H dipole is formed that reduces the height of the tunneling barrier. In general, the physics of a charged SWCNT seems much more complex than hitherto realized.
Electronic structures of organic molecule encapsulated BN nanotubes under transverse electric field
Authors: He Wei , Li Zhenyu , Yang Jinlong , Hou J. G.
Published: July 2008

The electronic structures of boron nitride nanotubes (BNNTs) doped by different organic molecules under a transverse electric field were investigated via first-principles calculations. The external field reduces the energy gap of BNNT, thus makes the molecular bands closer to the BNNT band edges and enhances the charge transfers between BNNT and molecules. The effects of the electric field direction on the band structure are negligible. The electric field shielding effect of BNNT to the inside organic molecules is discussed. Organic molecule doping strongly modifies the optical property of BNNT, and the absorption edge is redshifted under static transverse electric field.
Orientation of dipole molecules and clusters upon adiabatic entry into an external field
Authors: Bulthuis J. , Becker J. A. , Moro R. , Kresin V. V.
Published: July 2008

The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin–Debye susceptibility. This distinction, which is important for the interpretation of deflection and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their “adiabatic-entry” orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and “adiabatic-entry” ensembles of prolate and oblate tops, and of symmetric top and linear rotators, are illustrated and identified.
Characterization of intermittency of impurity turbulent transport in tokamak edge plasmas
Authors: Futatani S. , Benkadda S. , Nakamura Y. , Kondo K.
Published: July 2008

The statistical properties of impurity transport of a tokamak edge plasma embedded in a dissipative drift-wave turbulence are investigated using structure function analysis. The impurities are considered as a passive scalar advected by the plasma flow. Two cases of impurity advection are studied and compared: A decaying impurities case (given by a diffusion-advection equation) and a driven case (forced by a mean scalar gradient). The use of extended self-similarity enables us to show that the relative scaling exponent of structure functions of impurity density and vorticity exhibit similar multifractal scaling in the decaying case and follows the She–Lévêque model. However, this property is invalidated for the impurity driven advection case. For both cases, potential fluctuations are self-similar and exhibit a monofractal scaling in agreement with Kolmogorov–Kraichnan theory for two-dimensional turbulence. These results obtained with a passive scalar model agree also with test-particle simulations.
Helicon mode formation and radio frequency power deposition in a helicon-produced plasma
Authors: Niemi K. , Krämer M.
Published: July 2008

Time- and space-resolved magnetic (B-dot) probe measurements in combination with measurements of the plasma parameters were carried out to investigate the relationship between the formation and propagation of helicon modes and the radio frequency (rf) power deposition in the core of a helicon plasma. The Poynting flux and the absorbed power density are deduced from the measured rf magnetic field distribution in amplitude and phase. Special attention is devoted to the helicon absorption under linear and nonlinear conditions. The present investigations are attached to recent observations in which the nonlinear nature of the helicon wave absorption has been demonstrated by showing that the strong absorption of helicon waves is correlated with parametric excitation of electrostatic fluctuations.
An analytic model of rotationally inelastic collisions of polar molecules in electric fields
Authors: Lemeshko Mikhail , Friedrich Bretislav
Published: July 2008

We present an analytic model of thermal state-to-state rotationally inelastic collisions of polar molecules in electric fields. The model is based on the Fraunhofer scattering of matter waves and requires Legendre moments characterizing the “shape” of the target in the body-fixed frame as its input. The electric field orients the target in the space-fixed frame and thereby effects a striking alteration of the dynamical observables: both the phase and amplitude of the oscillations in the partial differential cross sections undergo characteristic field-dependent changes that transgress into the partial integral cross sections. As the cross sections can be evaluated for a field applied parallel or perpendicular to the relative velocity, the model also offers predictions about steric asymmetry. We exemplify the field-dependent quantum collision dynamics with the behavior of the Ne–OCS(Σ1) and Ar–NO(Π2) systems. A comparison with the close-coupling calculations available for the latter system [Chem. Phys. Lett. 313, 491 (1999)] demonstrates the model’s ability to qualitatively explain the field dependence of all the scattering features observed.
Algorithmic decoherence time for decay-of-mixing non–Born–Oppenheimer dynamics
Authors: Cheng Shu Chun , Zhu Chaoyuan , Liang Kuo Kan , Lin Sheng Hsien , Truhlar Donald G.
Published: July 2008

The performance of an analytical expression for algorithmic decoherence time is investigated for non–Born–Oppenheimer molecular dynamics. There are two terms in the function that represents the dependence of the decoherence time on the system parameters; one represents decoherence due to the quantum time-energy uncertainty principle and the other represents a back reaction from the decoherent force on the classical trajectory. We particularly examine the question of whether the first term should dominate. Five one-dimensional two-state model systems that represent limits of multidimensional nonadiabatic dynamics are designed for testing mixed quantum-classical methods and for comparing semiclassical calculations with exact quantum calculations. Simulations are carried out with the semiclassical Ehrenfest method (SE), Tully’s fewest switch version (TFS) of the trajectory surface hopping method, and the decay-of-mixing method with natural switching, coherent switching (CSDM), and coherent switching with reinitiation (CSDM-D). The CSDM method is demonstrated to be the most accurate method, and it has several desirable features: (i) It behaves like the representation-independent SE method in the strong nonadiabatic coupling regions; (ii) it behaves physically like the TFS method in noninteractive region; and (iii) the trajectories are continuous with continuous momenta. The CSDM method is also demonstrated to balance coherence well with decoherence, and the results are nearly independent of whether one uses the adiabatic or diabatic representation. The present results provide new insight into the formulation of a physically correct decoherence time to be used with the CSDM method for non–Born–Oppenheimer molecular dynamic simulations.
Optimal protocols for minimal work processes in underdamped stochastic thermodynamics
Authors: Gomez-Marin Alex , Schmiedl Tim , Seifert Udo
Published: July 2008

For systems in an externally controllable time-dependent potential, the optimal protocol minimizes the mean work spent in a finite-time transition between two given equilibrium states. For overdamped dynamics which ignores inertia effects, the optimal protocol has been found to involve jumps of the control parameter at the beginning and end of the process. Including the inertia term, we show that this feature not only persists but that even delta-peak-like changes of the control parameter at both boundaries make the process optimal. These results are obtained by analyzing two simple paradigmatic cases: First, a Brownian particle dragged by a harmonic optical trap through a viscous fluid and, second, a Brownian particle subject to an optical trap with time-dependent stiffness. These insights could be used to improve free energy calculations via either thermodynamic integration or “fast growth” methods using Jarzynski’s equality.
Hard-sphere perturbation theory for a model of liquid Ga
Authors: Tsai K. H. , Wu Ten-Ming
Published: July 2008

Investigating thermodynamic properties of a model for liquid Ga, we have extended the application of the hard-sphere (HS) perturbation theory to an interatomic pair potential that possesses a soft repulsive core and a long-range oscillatory part. The model is interesting for displaying a discontinuous jump on the main-peak position of the radial distribution function at some critical density. At densities less than this critical value, the effective HS diameter of the model, estimated by the variational HS perturbation theory, has a substantial reduction with increasing density. Thus, the density dependence of the packing fraction of the HS reference fluid has an anomalous behavior, with a negative slope, within a density region below the critical density. By adding a correction term originally proposed by Mon to remedy the inherent deficiency of the HS perturbation theory, the extended Mansoori–Canfield/Rasaiah–Stell theory [J. Chem. Phys. 120, 4844 (2004)] very accurately predicts the Helmholtz free energy and entropy of the model, including an excess entropy anomaly. Almost occurring in the same density region, the excess entropy anomaly is found to be associated with the anomalous packing faction of the HS fluid.
The Wolf method applied to the liquid-vapor interface of water
Authors: Mendoza Francisco Noé , López-Lemus Jorge , Chapela Gustavo A. , Alejandre José
Published: July 2008

The Wolf method for the calculation of electrostatic interactions is applied in a liquid phase and at the liquid-vapor interface of water and its results are compared with those from the Ewald sums method. Molecular dynamics simulations are performed to calculate the radial distribution functions at room temperature. The interface simulations are used to obtain the coexisting densities and surface tension along the coexistence curve. The water model is a flexible version of the extended simple point charge model. The Wolf method gives good structural results, fair coexistence densities, and poor surface tensions as compared with those obtained using the Ewald sums method.
Pseudocritical or hysteresis temperature versus pore size for simple fluids confined in cylindrical nanopores
Authors: Puibasset Joël
Published: July 2008

The adsorption/desorption isotherms measured in nanoporous materials generally present a hysteresis. The hysteresis shrinks upon increasing the temperature (for a given pore size) or decreasing the pore size (for a given temperature), until it finally disappears at the so-called hysteresis (or pseudocritical) temperature Th or hysteresis (or pseudocritical) pore size Rh, not to be confused with a true critical point. In this paper, a Monte Carlo approach allowed calculating the surface free energy of confined fluid along the adsorption/desorption isotherms for various cylindrical pore sizes and temperatures. A simple phenomenological model then allowed exploiting these results to determine the relation between Th and Rh. The prediction is compared to various literature models and experimental data, showing agreement within uncertainties. On the other hand, the simulations cannot be used directly to predict Th and Rh since they significantly overestimate the hysteresis width. The model predicts a nonlinear relation between the reduced hysteresis temperature and the inverse pore radius.
The underlying terahertz vibrational spectrum of explosives solids
Authors: Melinger Joseph S. , Laman N. , Grischkowsky D.
Published: July 2008

Using waveguide terahertz time-domain spectroscopy, we demonstrate the measurement of the underlying terahertz vibrational spectrum of the explosive solids hexahydro-1,3,5-trinitro-1,3,5-triazine, and 2,4,6-trinitrotoluene. Each explosive is cast as a polycrystalline thin film with planar ordering on the inner surface of metal parallel plate waveguide. For measurements near 10K, each explosive reveals a complex spectrum of approximately 20 vibrational modes between 0.5 and 3.5THz. The explosive films are of sufficient quality to produce vibrational linewidths as much as an order of magnitude sharper compared to conventional terahertz measurements of corresponding pellet samples of explosives.
Laser-induced spallation of aluminum and Al alloys at strain rates above 2×106s−1
Authors: Dalton D. A. , Brewer J. L. , Bernstein A. C. , Grigsby W. , Milathianaki D. , Jackson E. D. , Adams R. G. , Rambo P. , Schwarz J. , Edens A. , Geissel M. , Smith I. , Taleff E. M. , Ditmire T.
Published: July 2008

Material microstructure is a significant determinant of the tensile stress at which materials fail. Using a high-energy laser to drive shocks in thin slabs, we have explored the role material microstructure plays on the spall strength of high-purity and alloyed aluminum at strain rates of (2–7.5)×106s−1. Slabs of pure recrystallized Al and recrystallized or cold worked Al+3wt% Mg were shock driven using the Z-Beamlet Laser at Sandia National Laboratories. Velocity interferometer measurements determined the spall strength of the materials, and postshot target analysis explored the microscopic fracture morphology. We observed the greatest spall strength for large-grained, recrystallized high-purity aluminum, with the dominant failure mode being ductile and transgranular. We observe for the first time at these strain rates fracture features for a fine-grained Al+3wt% Mg that were a combination of brittle intergranular and ductile transgranular fracture types. Postshot analysis of target cross sections and hydrocode simulations indicate that this mixed-mode failure results from spall dynamics occurring on spatial scales on the order of the grain size. Differences in spall strength between these Al samples were experimentally significant and correlate with the damage morphologies observed.
The orientation of semidilute rigid fiber suspensions in a linearly contracting channel
Authors: Krochak P. J. , Olson J. A. , Martinez D. M.
Published: July 2008

We measure the spatial evolution of the orientation distribution of a semidilute rigid fiber suspension in the contraction plane of a linear contracting channel. We fit these data with a Fokker–Plank type equation using equations for rotary diffusion coefficient advanced by either (i) Folgar and Tucker [J. Reinf. Plast. Compas. 3, 98 (1984)] or (ii) Koch [Phys. Fluids 7, 2086 (1995)]. Both models of the diffusion coefficient were found to first increase with increasing suspension concentration up to a maximum and then decrease with concentrations above this point. This nonmonotonic behavior was attributed to fiber clumping or flocculation, a mechanism not considered in the closure relationships.
Solution-processible high-permittivity nanocomposite gate insulators for organic field-effect transistors
Authors: Kim P. , Zhang X.-H. , Domercq B. , Jones S. C. , Hotchkiss P. J. , Marder S. R. , Kippelen B. , Perry J. W.
Published: July 2008

We report on solution-processible high permittivity nanocomposite gate insulators based on BaTiO3 nanoparticles, surface-modified with a phosphonic acid, in poly(4-vinylphenol) for organic field-effect transistors. The use of surface-modified BaTiO3 nanoparticles affords high quality nanocomposite thin films at large nanoparticle volume fractions (up to 37vol%) with a large capacitance density and a low leakage current (10−8A∕cm2). The fabricated pentacene field-effect transistors using these nanocomposites show a large on/off current ratio (Ion∕off 104–106) due to the high capacitance density and small leakage current of the gate insulator.
Micro-four-point probe Hall effect measurement method
Authors: Petersen Dirch H. , Hansen Ole , Lin Rong , Nielsen Peter F.
Published: July 2008

We report a new microscale Hall effect measurement method for characterization of semiconductor thin films without need for conventional Hall effect geometries and metal contact pads. We derive the electrostatic potential resulting from current flow in a conductive filamentary sheet with insulating barriers and with a magnetic field applied normal to the plane of the sheet. Based on this potential, analytical expressions for the measured four-point resistance in presence of a magnetic field are derived for several simple sample geometries. We show how the sheet resistance and Hall effect contributions may be separated using dual configuration measurements. The method differs from conventional van der Pauw measurements since the probe pins are placed in the interior of the sample region, not just on the perimeter. We experimentally verify the method by micro-four-point probe measurements on ultrashallow junctions in silicon and germanium. On a cleaved silicon ultrashallow junction sample we determine carrier mobility, sheet carrier density, and sheet resistance from micro-four-point probe measurements under various experimental conditions, and show with these conditions reproducibility within less than 1.5%.
Diffusion trapping times and dynamic percolation in an Ising system
Authors: Chen C.-L. , Shapir Y. , Chimowitz E. H.
Published: July 2008

We address the problem of diffusion through dynamic Ising network structures using random walkers (RWs) whose net displacements are partitioned into two contributions, arising from (1) transport through neighboring “conducting” clusters and (2) self-diffusion of the site on which the RW finds itself, respectively. At finite temperatures, the conducting clusters in the network exhibit correlated dynamic behavior, making our model system different to most prior published work, which has largely been at the random percolation limit. We also present a novel heuristic scaling analysis for this system that utilizes a new scaling exponent θz for representing RW trapping time as a function of “distance” from the dynamic percolation transition. Simulation results in two-dimensional networks show that when θz=2, a value found from independent physical arguments, our scaling equations appear to capture universal behavior in the system, at both the random percolation (infinite temperature) and finite temperature conditions studied. This study suggests that the model and the scaling approach given here should prove useful for studying transport in physical systems showing dynamic disorder.
Dark channels in resonant tunneling transport through artificial atoms
Authors: Vaz Eduardo , Kyriakidis Jordan
Published: July 2008

We investigate sequential tunneling through a multilevel quantum dot confining multiple electrons in the regime where several channels are available for transport within the bias window. By analyzing solutions to the master equations of the reduced density matrix, we give general conditions on when the presence of a second transport channel in the bias window quenches transport through the quantum dot. These conditions are in terms of distinct tunneling anisotropies which may aid in explaining the occurrence of negative differential conductance in quantum dots in the nonlinear regime.
An efficient and accurate decomposition of the Fermi operator
Authors: Ceriotti Michele , Kühne Thomas D. , Parrinello Michele
Published: July 2008

We present a method to compute the Fermi function of the Hamiltonian for a system of independent fermions based on an exact decomposition of the grand-canonical potential. This scheme does not rely on the localization of the orbitals and is insensitive to ill-conditioned Hamiltonians. It lends itself naturally to linear scaling as soon as the sparsity of the system’s density matrix is exploited. By using a combination of polynomial expansion and Newton-like iterative techniques, an arbitrarily large number of terms can be employed in the expansion, overcoming some of the difficulties encountered in previous papers. Moreover, this hybrid approach allows us to obtain a very favorable scaling of the computational cost with increasing inverse temperature, which makes the method competitive with other Fermi operator expansion techniques. After performing an in-depth theoretical analysis of computational cost and accuracy, we test our approach on the density functional theory Hamiltonian for the metallic phase of the LiAl alloy.
Phase behavior of a simple lattice model with a two-scale repulsive interaction
Authors: Høye J. S. , Lomba E.
Published: July 2008

The properties of a simple one-dimensional lattice model with two repulsive ranges are studied in terms of its analytic solution. Its phase behavior is characterized by the presence of a disorder-order-disorder transition (or a fluid-solid-fluid transition in lattice gas language). A similar situation was discussed by Hemmer and Stell [Phys. Rev. Lett. 24, 1284 (1970)] when considering the purely repulsive version of their ramp potential. The melting of the solid phase, when pressure is increased along an isotherm, is a feature common to both models and one of the characteristic features of water.
Tunneling dynamics with a mixed quantum-classical method: Quantum corrected propagator combined with frozen Gaussian wave packets
Authors: Gelman David , Schwartz Steven D.
Published: July 2008

The recently developed mixed quantum-classical propagation method is extended to treat tunneling effects in multidimensional systems. Formulated for systems consisting of a quantum primary part and a classical bath of heavier particles, the method employs a frozen Gaussian description for the bath degrees of freedom, while the dynamics of the quantum subsystem is governed by a corrected propagator. The corrections are defined in terms of matrix elements of zeroth-order propagators. The method is applied to a model system of a double-well potential bilinearly coupled to a harmonic oscillator. The extension of the method, which includes nondiagonal elements of the correction propagator, enables an accurate treatment of tunneling in an antisymmetric double-well potential.
Intensity enhancement and selective detection of proximate solvent molecules by molecular near-field effect in resonance hyper-Raman scattering
Authors: Shimada Rintaro , Kano Hideaki , Hamaguchi Hiro-o
Published: July 2008

A new molecular phenomenon associated with resonance hyper-Raman (HR) scattering in solution has been discovered. Resonance HR spectra of all-trans-β-carotene and all-trans-lycopene in various solvents exhibited several extra bands that were not assignable to the solute but were unequivocally assigned to the solvents. Neat solvents did not show detectable HR signals under the same experimental conditions. Similar experiments with all-trans-retinal did not exhibit such enhancement either. All-trans-β-carotene and all-trans-lycopene have thus been shown to induce enhanced HR scattering of solvent molecules through a novel molecular effect that is not associated with all-trans-retinal. We call this new effect the “molecular near-field effect.” In order to explain this newly found effect, an extended vibronic theory of resonance HR scattering is developed where the vibronic interaction including the proximate solvent molecule (intermolecular vibronic coupling) is explicitly introduced in the solute hyperpolarizability tensor. The potential of “molecular near-field HR spectroscopy,” which selectively detects molecules existing in the close vicinity of a HR probe in complex chemical or biological systems, is discussed.
Metal-insulator transition and conduction mechanism in dysprosium doped Bi1.7Pb0.4Sr2Ca1.1Cu2.1O8+δ system
Authors: Shabna R. , Sarun P. M. , Vinu S. , Biju A. , Guruswamy P. , Syamaprasad U.
Published: July 2008

To explore the mechanisms behind the metal-insulator transition and charge transport in high temperature superconducting cuprates, a systematic study on the Bi1.7Pb0.4Sr2Ca1.1Cu2.1O8+δ compound was made by adding the rare-earth dysprosium at stoichiometric amounts (x) of 0.5≤x≤1. Phase analysis, determination of lattice parameters, microstructure analysis, and elemental analysis were carried out to evaluate the relative performance of the samples prepared by the solid state synthesis route. Charge transport in the insulating and superconducting samples were analyzed through resistivity measurements at (64–300 K). It is found that the x=0.5 sample is superconducting with a critical transition temperature of 94.8 K while for the samples with x>0.5 superconductivity is suppressed, along with an increase in their normal state resistivities. A metal-insulator transition is found to take place around 0.5<x≤6. A detailed analysis of the experimental data shows that the hole filling and disorder, induced by change in charge carrier concentration, lead to a metal-insulator transition in the present system. Also the conductivity of the semiconducting samples (x>0.5) at lower temperature is due to the two-dimensional variable range hopping mechanism of charge carriers between the spatially localized states.
Optical transmission in triple-film hetero-opals
Authors: Khunsin W. , Romanov S. G. , Sotomayor Torres C. M. , Ye J. , Zentel R.
Published: July 2008

Angle-resolved transmission of s-polarized light in triple-film hetero-opals has been investigated in the spectral range including high-order photonic band gaps, and compared to the transmission of its constituent single-film opals. The interfaces do not destroy the predominantly ballistic light propagation over the studied frequency and angular ranges, but heterostructuring leads to a smoothed angular distribution of intensity of the transmitted light and to the reconstruction of the transmission minima dispersion. The interface transmission function has been extracted by comparing the transmission of the hetero-opal and its components in order to demonstrate the difference. This deviation from the superposition principle was provisionally assigned to light refraction and reflection at the photonic crystal interfaces and to the mismatch between mode group velocities in hetero-opal components.
Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices
Authors: Meerheim Rico , Scholz Sebastian , Olthof Selina , Schwartz Gregor , Reineke Sebastian , Walzer Karsten , Leo Karl
Published: July 2008

We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)2(acac) doped in α-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m2 is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m2 is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an appropriate choice of the hole blocking material.
Effect of zirconium(IV) propoxide concentration on the thermophysical properties of hybrid organic-inorganic films
Authors: Wang Tao , Wang Xinwei , Zhang Yanwu , Liu Liying , Xu Lei , Liu Ying , Zhang Lijun , Luo Zhongyang , Cen Kefa
Published: July 2008

In this work, micrometer-thick organic-inorganic hybrid films are fabricated. A photothermal experiment is designed and conducted to characterize the thermophysical properties of hybrid films, as well as the thermal contact resistance between the film and substrate. The molecular cagelike or nanopores, which can strongly enhance the phonon scattering, are considered to be formed inside the films during fabrication. The first order estimation of the volume fraction of cavities and its effect on thermophysical properties are obtained. The effect of zirconium(IV) propoxide (ZPO) concentration on the thermophysical properties of hybrid films is also studied. The effective (measured) thermal conductivity and thermal effusivity of hybrid films are close to those of polymethyl methacrylate (PMMA) films, and are not significantly affected by the added ZPO, which is used to adjust the optic properties of films. The extracted bulk thermal conductivity of the hybrid films is close or smaller than that of bulk PMMA, and shows certain thermal conductivity reduction by the ZPO addition. The thermal effusivity study indicates that the response of the surface temperature change to an abrupt heat flux across the surface of hybrid films will be similar to that of PMMA films.
Current through single conjugated molecules: Calculations versus measurements
Authors: Liang Y. Y. , Zhou Y. X. , Chen H. , Note R. , Mizuseki H. , Kawazoe Y.
Published: July 2008

We use density functional theory based nonequilibrium Green’s function to calculate the current through the different rodlike molecules at the finite temperatures self-consistently, which was compared to the experimental measurements presented by Reichert et al. [Phys. Rev. Lett. 88, 176804 (2002)] and by Mayor et al. [Angew. Chem. Int. Ed. 42, 5834 (2003)], respectively. Our results agree with the measurements very well, especially for the bias around ±1.0 V. The investigation of the topological effect for the symmetrical molecule reveals the fact that the para position compound provides a considerably larger conductance than the meta one.
Growth atmosphere dependence of transport properties of NiO epitaxial thin films
Authors: Oka Keisuke , Yanagida Takeshi , Nagashima Kazuki , Tanaka Hidekazu , Kawai Tomoji
Published: July 2008

Recent possible applications in nonvolatile resistive switching memory devices renewed the interests in the transport properties of NiO. The variation on the conductivities of NiO films was reported to strongly affect the resistive switching phenomena. The conduction mechanism of NiO has been interpreted in terms of the bulk p-type conduction mechanism via Ni deficiencies (Ni1−δO). Here we investigate the growth atmosphere dependence on the transport properties of NiO thin films epitaxially grown on MgO (001) substrate. The conductivities of NiO thin films showed completely an opposite tendency compared to the bulk p-type conduction mechanism. Microstructural analysis demonstrates that the conductivity of low temperature grown NiO thin films strongly correlates with tailing the band edge via the deterioration of entire film crystallinity rather than the grain boundaries including second phases.
In situ growth of aligned CdS nanowire arrays on Cd foil and their optical and electron field emission properties
Authors: Qian G. X. , Huo K. F. , Fu J. J. , Hung T. F. , Chu Paul K.
Published: July 2008

Aligned CdS nanowire arrays have been fabricated directly on a Cd foil via a simple solvothermal method. The metal Cd foil serves as both the Cd source and substrate during fabrication of the aligned CdS nanowire arrays. The morphology, structure, and composition of the samples are characterized by x-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy, and selected-area electron diffraction, and the results reveal the formation of aligned single-crystalline CdS nanowires with uniform diameters of 20–40 nm. The photoluminescence and Raman spectra disclose the optical properties of the products and the possible growth mechanism is suggested. The electron field emission properties are also investigated and analyzed. The screening effect is observed to play a vital role in the electron field emission properties due to the coalescent ends of the nanowires. The simple synthesis methodology in conjunction with the good field emission and optical properties makes the materials both scientifically and technologically interesting.
Theoretical modeling of dark current in quantum dot infrared photodetectors using nonequilibrium Green’s functions
Authors: Naser M. A. , Deen M. J. , Thompson D. A.
Published: July 2008

A theoretical model describing electron dynamics in quantum dot (QD) infrared photodetectors (QDIPs) is presented. The model is based on the nonequilibrium Green’s functions formalism which provides a general framework to study electron transport in a nonequilibrium quantum system and in the presence of interactions. A self-consistent solution of the charge density and the average potential energy through the device and satisfying Poisson’s equation has been obtained; hence, the Hamiltonian of the QDs is established. The self-energies due to coupling with the contact layers and due to internal electron interactions are calculated and then Green’s functions of the QDs are obtained by numerically solving their governing kinetic equations using the method of finite differences. A quantum transport equation using Green’s functions is formed to calculate the current. The model has been applied to simulate the dark current and to extract microscopic information about the density of states and carrier distribution in the quantum dot bound and continuum states. The simulated dark currents with this model are in good agreement with experimental results over a wide range of applied biases and temperatures. The model was also used to study the effect on the dark current and the average number of electrons occupying the QDs due to changing the QD doping density, the barrier separation between QD layers, and the number of QD layers. The model is general and can be applied to any QDIP structures as a tool in design and for predictions of their dark current characteristics.
Magnetization ground states and phase diagrams for a nanosized Co hollow sphere: An onion-type magnetization state
Authors: Kong Desheng , Wang Siming , Chen Chinping
Published: July 2008

The magnetization ground states (MGSs) for a nanosized Co hollow sphere, with the outer radius, R<50 nm, have been studied numerically by micromagnetic simulation using object oriented micromagnetic framework (OOMMF). In addition to the originally known single domain and vortex-curling states, a three dimensional “onion” state with a corresponding analytical expression is proposed and confirmed as one of the ground states. Two phase diagrams, one for a single crystalline and the other for a polycrystalline nanosphere, are obtained for the three MGSs. The result reveals that the magnetic anisotropy has a significant effect on the phase line in the diagrams. The finite temperature effect and the blocking properties of the nanosphere for the magnetization reversal are discussed.
Magnetization processes in single-layer and laminated CoFe films patterned into multiscale elements with write-head-like geometries
Authors: Kong X. , McVitie S. , Chapman J. N. , Weaver J. M. R. , O’Donnell D. O. , Johnston A. B.
Published: July 2008

Lorentz microscopy has been used to study the micromagnetic processes occurring during the reversal of multiscale single and multilayer CoFe elements. The elements, which have similar dimensions to write heads used in magnetic recording, typically have length scales varying from 10 μm in the element “core” down to 100 nm in the element “tip.” Marked differences both in the core and the tip regions are observed between single and multilayer elements. The results demonstrate clearly how the role played by shape anisotropy in complex elements can be effectively reduced by multilayering, allowing any anisotropy induced in the film during deposition to play a greater role in defining the resulting behavior.
Dielectric control of spin in semiconductor spherical quantum dots
Authors: Planelles J. , Rajadell F. , Royo M.
Published: July 2008

The ground state electronic configuration of semiconductor spherical quantum dots populated with different numbers of excess electrons, for different radii and dielectric constants of the embedding medium is calculated and the corresponding phase diagram drawn. To this end, an extension of the spin density functional theory to study systems with variable effective mass and dielectric constant is employed. Our results show that high/low spin configurations can be switched by appropriate changes in the quantum dot embedding environment and suggest the use of the quantum dot spin as a sensor of the dielectric response of media.
Oxygen defect induced photoluminescence of HfO2 thin films
Authors: Ni Jie , Zhou Qin , Li Zhengcao , Zhang Zhengjun
Published: July 2008

Amorphous HfO2 films prepared by e-beam deposition exhibited room-temperature photoluminescence (PL) in the visible range, i.e., at ∼620 and 700nm, due to oxygen vacancies involved during deposition. This PL can be enhanced by two orders in intensity by crystallizing the amorphous films in flowing argon, where a large amount of oxygen vacancies were introduced, and can be diminished by removal of the oxygen vacancies by annealing HfO2 films in oxygen. This study could help understand the defect-property relationship and provides ways to tune the PL property of HfO2 films.
Pulsed ultrasound-modulated optical tomography using spectral-hole burning as a narrowband spectral filter
Authors: Li Youzhi , Zhang Huiliang , Kim Chulhong , Wagner Kelvin H. , Hemmer Philip , Wang Lihong V.
Published: July 2008

We applied a submegahertz nonlinear optical filter afforded by a cryogenically cooled spectral-hole burning crystal to ultrasound-modulated optical tomography. Our experimental results show that this technique, having the largest etendue among all available ultrasound-modulated optical tomography techniques and being immune to speckle decorrelation, offers potential for imaging in vivo and forming high resolution optical tomograms in real time. It opens an opportunity for the development of a clinically applicable high resolution optical imaging modality.
Epitaxially grown flat MnSi ultrathin film on Si(111)
Authors: Higashi Shougo , Ikedo Yuichi , Kocán Pavel , Tochihara Hiroshi
Published: July 2008

Flat MnSi ultrathin films are epitaxially grown on the Si(111)-(7×7) surface by Mn deposition and subsequent annealing. Low-energy electron diffraction exhbits the brightest (3×3)R30° patterns when Mn atoms are deposited at ∼3 ML with subsequent annealing at 250°C. Scanning tunneling microscopy shows that atomically flat MnSi(111) surfaces with the (3×3)R30° periodicity are formed under above condition, and that the thickness of the flat MnSi films is ∼7Å. The results support a recent theoretical prediction of flat-film formation of the B20-type MnSi on the Si(111) surface.
Temperature-dependent parameters of band anticrossing in InGaPN alloys
Authors: Lin K. I. , Wang T. S. , Tsai J. T. , Hwang J. S.
Published: July 2008

Temperature-dependent photoreflectance (PR) measurements are employed to characterize the conduction band structure of In0.54Ga0.46P1−yNy (y=0 and 0.02) grown on GaAs substrates. The band gap and the upper subband E+ transition are observed in InGaPN as predicted by the band anticrossing (BAC) model. To investigate the energetic positions of the features in the PR spectra, a Kramers–Kronig analysis is proposed. Based on the PR data and the BAC model, we find that the energy EN of isolated nitrogen states shifts significantly to higher energies with decreasing temperature. Simultaneously, the interaction potential V between the nitrogen states and the unperturbed conduction band also rises to higher values. At 293 K, EN=2.054 eV and V=1.513 eV are determined. The thermal shifts of EN and V are dEN/dT≈−0.43 meV/K and dV/dT≈−0.67 meV/K, respectively. The temperature-dependent EN level and interaction potential V are attributed to the lattice distortions, which can be affected by temperature-induced changes in deformation potential. This information is important for overall validity of the BAC model to dilute nitride InGaPN alloys.
Fourier transform infrared spectroscopy of azide and cyanate ion pairs in AOT reverse micelles
Authors: Owrutsky Jeffrey C. , Pomfret Michael B. , Barton David J. , Kidwell David A.
Published: July 2008

Evidence for ion pair formation in aqueous bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles (RMs) was obtained from infrared spectra of azide and cyanate with Li+, Na+, K+, and NH4+ counterions. The anions’ antisymmetric stretching bands near 2000 cm−1 are shifted to higher frequency (blueshifted) in LiAOT and to a lesser extent in NaAOT, but they are very similar to those in bulk water with K+ and NH4+ as the counterions. The shifts are largest for low values of wo=[water]/[AOT] and approach the bulk value with increasing wo. The blueshifts are attributed to ion pairing between the anions and the counterions. This interpretation is reinforced by the similar trend (Li+>Na+>K+) for producing contact ion pairs with the metal cations in bulk dimethyl sulfoxide (DMSO) solutions. We find no evidence of ion pairs being formed in NH4AOT RMs, whereas ammonium does form ion pairs with azide and cyanate in bulk DMSO. Studies are also reported for the anions in formamide-containing AOT RMs, in which blueshifts and ion pair formation are observed more than in the aqueous RMs. Ion pairs are preferentially formed in confined RM systems, consistent with the well established ideas that RMs exhibit reduced polarity and a disrupted hydrogen bonding network compared to bulk water and that ion-specific effects are involved in mediating the structure of species at interfaces.
Experimental investigation of ultraviolet laser induced plasma density and temperature evolution in air
Authors: Thiyagarajan Magesh , Scharer John
Published: July 2008

We present measurements and analysis of laser induced plasma neutral densities and temperatures in dry air by focusing 200 mJ, 10 MW high power, 193 nm ultraviolet ArF (argon fluoride) laser radiation to a 30 μm radius spot size. We examine these properties that result from multiphoton and collisional cascade processes for pressures ranging from 40 Torr to 5 atm. A laser shadowgraphy diagnostic technique is used to obtain the plasma electron temperature just after the shock front and this is compared with optical emission spectroscopic measurements of nitrogen rotational and vibrational temperatures. Two-color laser interferometry is employed to measure time resolved spatial electron and neutral density decay in initial local thermodynamic equilibrium (LTE) and non-LTE conditions. The radiating species and thermodynamic characteristics of the plasma are analyzed by means of optical emission spectroscopy (OES) supported by SPECAIR, a special OES program for air constituent plasmas. Core plasma rotational and vibrational temperatures are obtained from the emission spectra from the N2C-B(2+) transitions by matching the experimental spectrum results with the SPECAIR simulation results and the results are compared with the electron temperature just behind the shock wave. The plasma density decay measurements are compared with a simplified electron density decay model that illustrates the dominant three-and two-body recombination terms with good correlation.
Broadside coupling to long-range surface plasmons in metal stripes using prisms, particles, and an atomic force microscope probe
Authors: Charbonneau Robert , Berini Pierre
Published: July 2008

Techniques for broadside coupling to long-range surface plasmon waves propagating along metal stripes are investigated. The baseline technique consists of evanescently coupling an optical input beam originating from a polarization maintaining fiber to the plasmon wave via a right-angle prism positioned above the metal stripe, and providing an optical output some distance away through a mirror arrangement of identical elements. The technique is modeled theoretically using plane waves and implemented to measure the attenuation of the long-range plasmon wave propagating along a metal stripe supported by a thin freestanding dielectric membrane. An alternative technique for providing an output is proposed, whereby a tipless atomic force microscope probe physically contacts the metal stripe to generate out-of-plane scattering and a multimode fiber positioned nearby is used to capture a portion of the scattered light. This technique is easier to implement than the baseline technique, resulting in attenuation measurements of significantly better quality. The goodness of fit of the best fitting linear models to the measurements was significantly improved using this technique (0.93 and 0.99), and the measured attenuations were in very good agreement with the theoretical ones (6.01% and 0.27% error). This simple technique for optical probing and coupling could be applied to other surface plasmon waveguides and possibly to dielectric waveguides with modes having sufficient field strength in their evanescent tail. Output scattering using micron-sized particles located on the metal stripe was also investigated. The stability of the experimental setup was assessed and found to be about 0.01dB peak to peak over a few minutes at constant temperature using a reference optical signal.