bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ x-ray diffraction studies of alkyl quaternary ammonium montmorillonite in a CO2 environment
Authors: Thompson M. R. , Balogh M. P. , Speer R. L. , Fasulo P. D. , Rodgers W. R.
Published: January 2009

An in situ study was conducted using an x-ray diffractometer and a specially designed high pressure cell to examine the effects of carbon dioxide at different pressures and temperatures on three different modified montmorillonite species. These organoclays possessed organic pillars of quaternary ammonium surfactant with either one, two, or three long (C16–C18) alkyl chains attached to expand the galleries of the clay mineral. The three clay species were tested between 50 °C and 200 °C with carbon dioxide pressures between 0.1 and 8.4 MPa. Under these conditions, the three organoclays exhibited marked differences in their basal spacing depending on the surfactant used. The physical state of the intercalated surfactant was found to be of critical importance, with no changes in basal spacing noted until the organic component began to melt, in spite of CO2 being present. A pressure effect was also noted which delayed melting of the surfactants as the pressure of the system increased. In all cases, further cation exchange with residual sodium cation present in the galleries was observed in the presence of CO2 and above the melting state of the surfactant. The study included examination of the effect of rapid depressurization on the clay structure, which produced only a small change in basal spacing.
Photodissociation of vinyl cyanide at 193 nm: Nascent product distributions of the molecular elimination channels
Authors: Wilhelm Michael J. , Nikow Matthew , Letendre Laura , Dai Hai-Lung
Published: January 2009

The photodissociation dynamics of vinyl cyanide (H2CCHCN, acrylonitrile) and deuterated vinyl cyanide (D2CCDCN) at 193 nm are examined using time-resolved Fourier transform infrared emission spectroscopy. Prior photofragment translational spectroscopy studies [D. A. Blank et al., J. Chem. Phys. 108, 5784 (1998)] of the dissociation have observed the presence of four main dissociation channels; two molecular and two radical in nature. However, with the exception of a<0.01 quantum yield determined for the CN radical loss channel, the branching ratios of the remaining three elimination channels were not measured. The time-resolved emission spectra, including those from the deuterated samples, revealed the presence of acetylene, hydrogen cyanide (HCN), as well as the energetically less stable isomer hydrogen isocyanide (HNC). Acetylene is found in two distinct energetic distributions, suggesting that both three- and four-centered elimination reactions are occurring significantly in the dissociation. In contrast to prior ab initio studies that have suggested the dominant nature of the three-center elimination of molecular hydrogen (H2) and cyanovinylidene (:CCHCN), we find this reaction channel to be of little importance as there is no evidence to support any significant presence of rovibrationally excited cyanoacetylene. Spectral modeling of the product distributions allows for the first experimental determination of the relative occurrence of the three-centered (resulting in HCN+vinylidene) versus four-centered (HNC+acetylene) elimination channels as 3.34 to 1.00, in contrast to the previously calculated value of 126:1. Rice–Ramsperger–Kassel–Marcus analysis depicts that the transition state energy of the four-centered reaction should be about 10 kcal mole−1 lower than the three-centered reaction.
Linear stability analysis for the onset of convection during microwave heating of oil confined within horizontal plates
Authors: Bhattacharya Madhuchhanda , Basak Tanmay
Published: January 2009

This work performs linear stability analysis to investigate the onset of convection due to microwave incidences in a horizontal oil layer. The probability of onset has been characterized in terms of critical Rayleigh number (Rac), while the microwave power required to initiate convection has been quantified in terms of modified Rayleigh number (Ra¯c). The associated convection cell size has been illustrated by critical wave number (kc). These critical numbers (Rac, Ra¯c, and kc) have been determined for the entire spectrum of oil thicknesses ranging from thin to thick samples and various possible microwave source configurations starting from one sided to distributed incidences. It has been shown that the critical numbers oscillate with oil thickness in intermediate regime indicating preferential onset for specific oil thicknesses. It has also been shown that microwave source configuration plays an important role in controlling the onset of convection. Finally, the variations of Rac and kc have been correlated to the spatial patterns of absorbed microwave power within oil, while modified Rayleigh numbers are shown to follow the qualitative trends of ΔTs, where ΔTs is the base state temperature difference between top and bottom plate. The results are useful not only for forecasting the onset but also for controlling onset of convection by selecting suitable experimental parameters.
Accurate modeling of split ring metamaterial lenses for magnetic resonance imaging applications
Authors: Jelinek L. , Marqués R. , Freire M. J.
Published: January 2009

The usefulness of thin split ring metamaterial slabs for imaging applications, including magnetic resonance imaging applications, has attracted some attention in the past years. However, the small number of unit cells across these thin slabs prevents the direct application of continuous medium models for its characterization. The main aim of this contribution is to provide a rigorous model for these structures, also clarifying the usefulness of continuous medium approach for their characterization. The proposed model is a generalization of the classical Lorentz procedure to two dimensions and is able to deal with electrically thin slabs made of small resonant closed current loops. The obtained results are validated by full-wave electromagnetic simulations and compared with the continuous model approximation of the slab.
Obliquely propagating ion acoustic solitary waves in magnetized dusty plasma in the presence of nonthermal electrons
Authors: Saha Taraknath , Chatterjee Prasanta
Published: January 2009

Obliquely propagating dust ion acoustic solitary waves subjected to an external magnetic field are studied using Sagdeev’s pseudopotential technique. Nonthermal distribution for electrons is considered. The pseudopotential is derived without considering the quasineutrality condition. It is obtained from the Poisson equation instead. Using Sagdeev’s technique, the ranges of parameters for which solitary waves exist are studied in detail.
Propagation of the three-dimensional dust acoustic solitons in magnetized quantum plasmas with dust polarity effect
Authors: Sadiq M. , Ali S. , Sabry R.
Published: January 2009

The quantum hydrodynamical model is employed to investigate the nonlinear properties of the quantum dust acoustic waves in a magnetized dusty plasma composed of inertialess electrons, ions, and mobile positive/negative charged dust particles. For this purpose, a quantum Zakharov–Kuznetsov equation is derived and the basic features of the electrostatic excitations are investigated by applying the direct method. It is found that positive and negative bell-shaped solitary pulses become explosive pulses depending mainly upon the angles of propagation and dust polarity. Furthermore, the effects due to nondimensional quantum parameter and the external magnetic field are examined on the width of the quantum dust acoustic solitary pulses. The relevance of the present results to semiconductor quantum wells is mentioned.
Coupled drift-Alfvén–Shukla–Varma modes in quantum dusty plasmas
Authors: Misra A. P. , Bhowmik C.
Published: January 2009

A set of nonlinear equations that governs the dynamics of quantum drift-Alfvén waves (QDAWs) in a nonuniform quantum dusty plasma, which consists of electrons, positive ions, and stationary negatively charged dust grains, is derived using a quantum magnetohydrodynamic model. In the linear limit, Fourier transformation of these equations yields the dispersion relation that exhibits a coupling between QDAWs and the Shukla–Varma mode. The dispersion relation is then analyzed numerically to ascertain the real wave frequencies and the growth/damping rates.
Geometric transition in friction for flow over a bubble mattress
Authors: Davis Anthony M. J. , Lauga Eric
Published: January 2009

Laminar flow over a bubble mattress is expected to experience a significant reduction in friction since the individual surfaces of the bubbles are shear-free. However, if the bubbles are sufficiently curved, their protrusion into the fluid and along the flow direction can lead to an increase in friction as was recently demonstrated experimentally and computationally. We provide in this paper a simple model for this result. We consider a shear flow at low Reynolds number past a two-dimensional array of bubbles and calculate analytically the effective slip length of the surface as a function of the bubble geometry in the dilute limit. Our model is able to reproduce quantitatively the relationship between effective friction and bubble geometry obtained in numerical computations and, in particular, (a) the asymmetry in friction between convex and concave bubbles and (b) the existence of a geometric transition from reduced to enhanced friction at a critical bubble protrusion angle.
Control of photodissociation and photoionization of the NaI molecule by dynamic Stark effect
Authors: Han Yong-Chang , Yuan Kai-Jun , Hu Wen-Hui , Cong Shu-Lin
Published: January 2009

The diabatic photodissociation and photoionization processes of the NaI molecule are studied theoretically using the quantum wave packet method. A pump laser pulse is used to prepare a dissociation wave packet that propagates through both the ionic channel (NaI→Na++I−) and the covalent channel (NaI→Na+I). A Stark pulse is used to control the diabatic dissociation dynamics and a probe pulse is employed to ionize the products from the two channels. Based on the first order nonresonant nonperturbative dynamic Stark effect, the dissociation probabilities and the branching ratio of the products from the two channels can be controlled. Moreover the final photoelectron kinetic energy distribution can also be affected by the Stark pulse. The influences of the delay time, intensity, frequency, and carrier-envelope phase of the Stark pulse on the dissociation and ionization dynamics of the NaI molecule are discussed in detail.
Control of liquid crystal pretilt angle by anchoring competition of the stacked alignment layers
Authors: Lee You-Jin , Gwag Jin Seog , Kim Young-Ki , Jo Soo In , Kang Seung-Gon , Park Young Ran , Kim Jae-Hoon
Published: January 2009

We proposed a method to control the pretilt angle of liquid crystals by stacking of a vertical alignment layer on a planar alignment layer. The pretilt angle can be controlled over the full range (0°–90°) depending on the thickness of the vertical alignment layer. We also proposed a numerical model to describe the physical mechanism based on the anchoring competition between liquid crystal, planar, and vertical polyimide alignment layers.
Effect of oxygen concentration on the structural and magnetic properties of LaRh1/2Mn1/2O3 thin films
Authors: Sheets W. C. , Smith A. E. , Subramanian M. A. , Prellier W.
Published: January 2009

Epitaxial LaRh1/2Mn1/2O3 thin films have been grown on (001)-oriented LaAlO3 and SrTiO3 substrates using pulsed laser deposition. The optimized thin film samples are semiconducting and ferromagnetic with a Curie temperature close to 100 K, a coercive field of 1200 Oe, and a saturation magnetization of 1.7μB per formula unit. The surface texture, structural, electrical, and magnetic properties of the LaRh1/2Mn1/2O3 films was examined as a function of the oxygen concentration during deposition. While an elevated oxygen concentration yields thin films with optimal magnetic properties, slightly lower oxygen concentrations result in films with improved texture and crystallinity.
Conducting channel formation and annihilation in organic field-effect structures
Authors: Liang Yan , Frisbie C. Daniel , Chang Hsiu-Chuang , Ruden P. Paul
Published: January 2009

We report measurements of displacement current to study the transient effects of conducting channel formation and annihilation at organic semiconductor/dielectric interfaces. The device structure resembled a typical organic thin-film transistor with either source or drain electrode removed. However, the channel length was very long (1–6 mm) in order to increase the transit time and enhance the displacement current. The devices consisted of a gold electrode contacting a 30 nm thick pentacene thin film, thermally deposited on SiO2 dielectric, with a heavily doped p-type Si substrate serving as a bottom electrode. Electrical measurements were performed by measuring the displacement current running through the grounded gold contact while linearly sweeping the voltage bias applied to the bottom electrode. The processes of conducting channel formation and annihilation were observed as transients in the I-V characteristics. By integrating the displacement current with respect to time, the concentrations of carriers injected into, extracted from, and trapped inside the pentacene film during the voltage sweep cycle were determined. The results are analyzed in terms of a device model that includes the effects of traps.
An arbitrary order Douglas–Kroll method with polynomial cost
Authors: Peng Daoling , Hirao Kimihiko
Published: January 2009

A new Douglas–Kroll transformation scheme up to arbitrary order is presented to study the convergence behavior of the Douglas–Kroll series and the influence of different choices of parametrization for the unitary transformation. The standard approach for evaluating the Douglas–Kroll Hamiltonian suffers from computational difficulties due to the huge number of matrix multiplications, which increase exponentially with respect to the order of truncation. This makes it prohibitively expensive to obtain results for very high order Douglas–Kroll Hamiltonians. The highest order previously presented is 14th order, but it is not enough to obtain accurate results for systems containing heavy elements, where the Douglas–Kroll series converges very slowly. In contrast, our approach dramatically reduces the number of matrix multiplications, which only increase with a polynomial scaling. With the new method, orders greater than 100 and machine accuracy are possible. This fast method is achieved by employing a special transformation to all Douglas–Kroll operators and our algorithm is very simple. We demonstrate the performance of our implementation with calculations on one-electron systems and many-electron atoms. All results show very good convergence behavior of the Douglas–Kroll series. Very small differences are found between the different parametrizations, and therefore the exponential form, which is the simplest and fastest, is recommended.
Domain switching mechanisms in polycrystalline ferroelectrics with asymmetric hysteretic behavior
Authors: Anton Eva-Maria , García R. Edwin , Key Thomas S. , Blendell John E. , Bowman Keith J.
Published: January 2009

A numerical method is presented to predict the effect of microstructure on the local polarization switching of bulk ferroelectric ceramics. The model shows that a built-in electromechanical field develops in a ferroelectric material as a result of the spatial coupling of the grains and the direct physical coupling between the thermomechanical and electromechanical properties of a bulk ceramic material. The built-in fields that result from the thermomechanically induced grain-grain electromechanical interactions result in the appearance of four microstructural switching mechanisms: (1) simple switching, where the c-axes of ferroelectric domains will align with the direction of the applied macroscopic electric field by starting from the core of each grain; (2) grain boundary induced switching, where the domain’s switching response will initiate at grain corners and boundaries as a result of the polarization and stress that is locally generated from the strong anisotropy of the dielectric permittivity and the local piezoelectric contributions to polarization from the surrounding material; (3) negative poling, where abutting ferroelectric domains of opposite polarity actively oppose domain switching by increasing their degree of tetragonality by interacting with the surrounding domains that have already switched to align with the applied electrostatic field. Finally, (4) domain reswitching mechanism is observed at very large applied electric fields, and is characterized by the appearance of polarization domain reversals events in the direction of their originally unswitched state. This mechanism is a consequence of the competition between the macroscopic applied electric field, and the induced electric field that results from the neighboring domains (or grains) interactions. The model shows that these built-in electromechanical fields and mesoscale mechanisms contribute to the asymmetry of the macroscopic hysteretic behavior in poled samples. Furthermore, below a material-dependent operating temperature, the predicted built-in electric fields can potentially drive the aging and electrical fatigue of the system to further skew the shape of the hysteresis loops.
Quantum simulation of noise in silicon nanowire transistors with electron-phonon interactions
Authors: Park Hong-Hyun , Jin Seonghoon , Park Young June , Min Hong Shick
Published: January 2009

We present a quantum mechanical approach to the calculation of electronic noise in the presence of a continuous distribution of phase-breaking processes based on the nonequilibrium Green’s function formalism. Using this approach, we investigate the drain current and its fluctuations in silicon nanowire transistors taking into account the intravalley elastic and intervalley inelastic electron-phonon scattering mechanisms. The calculated results are compared against the results of phase-coherent transport at various bias conditions, which shows the role of electron-phonon scattering on the drain current noise.
Photoresponse and saturation behavior of organic thin film transistors
Authors: Wasapinyokul Kamol , Milne W. I. , Chu D. P.
Published: January 2009

Organic thin film transistors based on poly(3,3‴-didodecylquarter-thiophene) were characterized under illumination with a fixed wavelength but various intensities from dark to 1100 μW cm−2. Typically the illumination process should increase the drain current through the increase in the number of charge carriers in the channel in the form of polarons, as a result of generation and dissociation of excitons or electron-hole pairs. However, the rate of the current increase was found to decrease as the light intensity was increased, and eventually the level of drain current reached a maximum before declining. We suggest that the physics behind this oversaturation behavior is related to the increasing number of electron-hole recombination events associated with the increase in polaron density in the channel. When the polaron density goes above a threshold value at high light intensity, the number of polarons cannot increase further as they are already closely packed and the recombination overtakes generation, resulting in a decrease in the drain current from its peak value. We show that quantitative analysis agreed well with our model, and in our device the polaron diameter and mean free path are 19 and 2 nm, respectively.
Analysis of electron mobility in HfO2/TiN gate metal-oxide-semiconductor field effect transistors: The influence of HfO2 thickness, temperature, and oxide charge
Authors: Negara M. A. , Cherkaoui K. , Hurley P. K. , Young C. D. , Majhi P. , Tsai W. , Bauza D. , Ghibaudo G.
Published: January 2009

We report a new analysis of electron mobility in HfO2/TiN gate metal-oxide-semiconductor field effect transistors (MOSFETs) by investigating the influence of HfO2 thickness (1.6–3 nm), temperature (50–350 K), and oxide charge (∼1×1011–8×1012 cm−2) in the high inversion charge region. The fixed oxide charge and interface state densities are deliberately increased using negative-bias-temperature stress, allowing the determination of the Coulomb scattering term as a function of temperature for various oxide charge levels. The temperature dependence of the Coulomb scattering term is consistent with the case of a strongly screened Coulomb potential. Using the experimentally determined temperature dependence of Coulomb scattering term, a model is developed for the electron mobility, including the effects oxide charge (μC), high-k phonon (μPh-Hk), silicon phonon (μPh-Si), and surface roughness scattering (μSR). The model provides an accurate description of the experimental data for variations in HfO2 thickness, temperature, and oxide charge. Using the model the relative contributions of each mobility component are presented for varying oxide charge and high-k thickness. Scaling of the HfO2 physical thickness provided a reduction in the oxide charge and high-k phonon scattering mechanisms, leading to an increase in electron mobility in HfO2/TiN gate MOSFETs.
Stokes shear flow over a grating: Implications for superhydrophobic slip
Authors: Ng Chiu-On , Wang C. Y.
Published: January 2009

A semianalytical model based on the method of eigenfunction expansions and domain decomposition is developed for Stokes shear flow over a grating composed of a periodic array of parallel slats, with finite slippage on solid surfaces and infinite slippage on the bottom of troughs mimicking a no-shear liquid-gas interface penetrating into the space between slats. The model gives the macroscopic slip lengths for flow parallel or normal to the slats in terms of the microscopic slip length of the liquid-solid interface, area fraction of the no-shear liquid-gas interface, and depth of the liquid-gas interface in the grooves. When the no-shear interface lies flat on the top of the slats, the macroscopic slip lengths are the maximum and can be estimated with reasonably good accuracy by simple formulas. However, the slip lengths, particularly the transverse one, are very sensitive to penetration of the no-shear interface into the grooves. They can be reduced by a large factor when the interface just slightly gets into the grooves. On comparing with some molecular-dynamics simulation measures, it is pointed out that the applied pressure, which has to be less than the capillary pressure in the superhydrophobic state, can be correlated with the penetration depth of the no-shear interface.
The duals of Gabor frames on discrete periodic sets
Authors: Lian Qiao-Fang , Li Yun-Zhang
Published: January 2009

Due to its good potential for digital signal processing analysis, the discrete version of Gabor analysis has interested many mathematicians. In this paper, we focus on Gabor analysis on discrete periodic sets, which can model a signal to appear periodically but intermittently. For a Gabor frame on a discrete periodic set, three types of dual frames with Gabor structure are introduced and investigated in terms of discrete Zak transform. Given a Gabor frame on a discrete periodic set, a characterization of a Gabor system being separately its dual of types I–III is obtained. A method to construct these three types of duals is presented. An explicit expression of its canonical dual is established. The uniqueness of these three types of duals is characterized. Some examples are also provided to illustrate the generality of the theory.
A new explicit multisymplectic scheme for the regularized long-wave equation
Authors: Cai Jiaxiang
Published: January 2009

In this paper, we derive a new ten-point multisymplectic scheme for the regularized long-wave equation from its Bridges’ multisymplectic form. The new scheme is an explicit scheme in the sense that it does not need iteration. We discuss some properties of the new scheme. The performance and the efficiency of the new scheme are illustrated by solving several test examples. The obtained results are presented and compared with previous methods. Numerical results indicate that the multisymplectic scheme cannot only obtain satisfied solutions for the regularized long-wave equation but also keep three invariants of motion which are evaluated to determine the conservation properties of the algorithm very well.
Rayleigh, Mie, and Tyndall scatterings of polystyrene microspheres in water: Wavelength, size, and angle dependences
Authors: He Guang S. , Qin Hai-Yan , Zheng Qingdong
Published: January 2009

The wavelength-, size-, and angle-dependent light scattering properties of polystyrene microspheres dispersed in water are systematically investigated in a spectral range from 300 to 1300 nm and a sphere-diameter range from 20 nm to 85 μm. According to the wavelength dependence of the scattering cross section on the value of δ=λ1/2r, where λ1 and 2r are the wavelength in the surrounding medium and the diameter of the microsphere, the light scattering behaviors can be roughly classified by three major categories. Under our considered conditions, when δ≥10, it is termed Rayleigh scattering, characterized by a minus fourth-power dependence on the wavelength; when 10−2≤δ≤10, it is termed Mie scattering, characterized by a power factor varying within a range roughly −4 to +2; finally when δ≤10−2, it is termed Tyndall scattering, characterized by an almost constant scattering cross section that is actually independent of the wavelength. All of our experimental results on 16 scattering samples of different sizes are in good agreement with the theoretical predictions within an experimental uncertainty of ±5%.
Transmission spectra changes produced by decreasing compactness of opal-like structures
Authors: Andueza A. , Echeverría R. , Morales P. , Sevilla J.
Published: January 2009

Artificial opal-like structures based on spheres and colloidal particles have been fabricated in a controlled way, presenting optical band-gap properties in the optical frequency range. Nonclose packed artificial opals have also been fabricated and studied recently. In order to gain a better understanding of these phenomena, we have studied macroscopic models of nonclose packed fcc lattices using glass spheres (ε=7) of 8 mm diameter, and measuring in the microwave region (from 10 to 30 GHz). The results have shown a Bragg resonance tunable with filling factor of the opal, and a strong rejected band similar, also present in close packed samples, much less affected by compactness. The relation of this high order band with spheres single layer behavior is also discussed.
Nanocrystal characterization by ellipsometry in porous silicon using model dielectric function
Authors: Petrik Peter , Fried Miklos , Vazsonyi Eva , Basa Peter , Lohner Tivadar , Kozma Peter , Makkai Zsolt
Published: January 2009

Porous silicon layers were prepared by electrochemical etching of p-type single-crystal Si (c-Si) of varying dopant concentration resulting in gradually changing morphology and nanocrystal (wall) sizes in the range of 2–25nm. We used the model dielectric function (MDF) of Adachi to characterize these porous silicon thin films of systematically changing nanocrystal size. In the optical model both the surface and interface roughnesses have to be taken into account, and the E0, E1, and E2 critical point (CP) features are all described by a combination of several lineshapes (two-dimensional CP, excitonic, damped harmonic oscillator). This results in using numerous parameters, so the number of fitted parameters were reduced by parameter coupling and neglecting insensitive parameters. Because of the large number of fitted parameters, cross correlations have to be investigated thoroughly. The broadening parameters of the interband transitions in the measured photon energy range correlate with the long-range order in the crystal. The advantage of this method over the robust and simple effective medium approximation (EMA) using a composition of voids and c-Si with a nanocrystalline Si reference [Petrik et al., Appl. Surf. Sci. 253, 200 (2006)] is that the combined EMA+MDF multilayer method of this work provides a more detailed description of the material and layer structure.
The perfect lens on a finite bandwidth
Authors: Lind-Johansen Øyvind , Seip Kristian , Skaar Johannes
Published: January 2009

The resolution associated with the so-called perfect lens of thickness d is −2πd/ln(|χ+2|/2). Here the susceptibility χ is a Hermitian function in H2 of the upper half-plane, i.e., a H2 function satisfying χ(−ω)=χ(ω)¯. An additional requirement is that the imaginary part of χ be non-negative for non-negative arguments. Given an interval I on the positive half-axis, we compute the distance in L∞(I) from a negative constant to this class of functions. This result gives a surprisingly simple and explicit formula for the optimal resolution of the perfect lens on a finite bandwidth.
High frequency magnetoimpedance in Ni81Fe19/Fe50Mn50 exchange biased multilayer
Authors: da Silva R. B. , Viegas A. D. C. , Nascimento V. P. , Corrêa M. A. , Schelp L. F. , Baggio-Saitovitch E. , Sommer R. L.
Published: January 2009

Gigahertz magnetoimpedance (MI) curves obtained in an exchange biased Ni81Fe19/Fe50Mn50 multilayer are reported. Experimental MI curves are shifted by the exchange bias field (HEB), following the features presented by the hysteresis curves. Theoretical MI curves, calculated using the classical expression for the impedance of a planar magnetic conductor, describe well the experimental data. The results open possibilities for application of exchanged biased MI multilayered materials for the development of autobiased linear magnetic field sensors.
Flexible and transparent supercapacitor based on In2O3 nanowire/carbon nanotube heterogeneous films
Authors: Chen Po-Chiang , Shen Guozhen , Sukcharoenchoke Saowalak , Zhou Chongwu
Published: January 2009

In this paper, a supercapacitor with the features of optical transparency and mechanical flexibility has been fabricated using metal oxide nanowire/carbon nanotube heterogeneous film, and studies found that the power density can reach 7.48 kW/kg after galvanostatic measurements. In addition, to study the stability of flexible and transparent supercapacitor, the device was examined for a large number of cycles and showed a good retention of capacity (∼88%). This approach could work as the platform for future transparent and flexible nanoelectronics.
Electronic structures of β-Si3N4(0001)/Si(111) interfaces: Perfect bonding and dangling bond effects
Authors: Yang M. , Wu R. Q. , Deng W. S. , Shen L. , Sha Z. D. , Cai Y. Q. , Feng Y. P. , Wang S. J.
Published: January 2009

First-principles calculations based on density-functional theory and the generalized gradient approximations have been carried out to investigate interface properties of β-Si3N4/Si(111) systemically. An interface structure without dangling bonds at the interface was proposed, and this interface structure was found energetically more favorable than the existing model. Perfect bonding structure and strong Si–N bonds at the interface due to the charge transfer from Si atoms to N atoms result in this stable interface structure. The calculated band offsets of this interface structure are in agreement with previous theoretical estimations and experimental results. Besides, we also studied the effects of dangling bonds at the interface on electronic properties of β-Si3N4/Si(111). Dangling bonds would slightly decrease the valence band offset and generate gap states at the interface. The hydrogen saturated interface shows better electronic properties but the low dissociation energy of Si–H bonds would be a problem in applications.
Evaluation of the third-order nonlinear optical properties of tellurite glasses by thermally managed eclipse Z-scan
Authors: dos Santos F. Eroni. P. , Fávero F. C. , Gomes A. S. L. , Xing J. , Chen Q. , Fokine M. , Carvalho I. C. S.
Published: January 2009

The third-order nonlinearity is measured for TeO2–ZnO–Na2O (TZN) glasses codoped with BaO, Nb2O5, and La2O3. The results for the sign and magnitude of the nonlinearity were obtained using a combination of the eclipse Z-scan with thermal nonlinearity managed Z-scan, whereas the Kerr shutter technique was employed to obtain the electronic time response of the nonlinearity, all performed with 76 MHz repetition rate 150 fs pulses at 800 nm. The results show a fast response (<200 fs) and a nonlinear refractive index varying from 1.5 to 3.5×10−15 cm2/W, depending upon glass composition. At the peak power level employed, nonlinear absorption was negligible. The results obtained are in good agreement with other tellurite compositions reported, confirming the electronic origin of the nonlinearity
Local chemical environment of Nd3+, Eu3+, and Er3+ luminescent centers in lead germanate glasses
Authors: Ghigna Paolo , Tomasi Corrado , Speghini Adolfo , Bettinelli Marco , Scavini Marco
Published: January 2009

The local chemical environment of Nd3+, Eu3+, and Er3+ luminescent centers in various lead germanate glasses has been investigated by means of extended x-ray absorption fine structure at the Ln LIII edges. The main result of this investigation is that for all the Ln3+ ions the first coordination sphere is more symmetrical in glasses with high lead concentration than in glasses with low Pb content. This result is compatible with differential scanning calorimetry measurements and enlightens previous luminescence investigations carried out by some of the present authors.
Silicon threshold displacement energy determined by photoluminescence in electron-irradiated cubic silicon carbide
Authors: Lefèvre Jérémie , Costantini Jean-Marc , Esnouf Stéphane , Petite Guillaume
Published: January 2009

In view of the potential use of silicon carbide (SiC) in the nuclear industry, it is of major interest to understand point defect formation in this material. This work is a contribution to the determination of the silicon threshold displacement energy in the cubic polytype of SiC using electron irradiations with increasing energies from 275 to 680 keV. The photoluminescence signal of the silicon vacancy was related to the number of displacements per atom in the silicon sublattice. This quantity was calculated taking into account the energy loss and angular dispersion of electrons in the target. A best fit of experimental data was obtained for a displacement cross section using a threshold displacement energy of 25 eV along the [100] lattice direction. We checked the relevance of this result by comparing the experimental concentration of silicon single vacancies measured by electron paramagnetic resonance spectroscopy with the theoretical number of displaced silicon atoms.
Evidence of two plastic regimes controlled by dislocation nucleation in silicon nanostructures
Authors: Godet Julien , Hirel Pierre , Brochard Sandrine , Pizzagalli Laurent
Published: January 2009

We performed molecular dynamics simulations of silicon nanostructures submitted to various stresses and temperatures. For a given stress orientation, a transition in the onset of silicon plasticity is revealed depending on the temperature and stress magnitude. At high temperature and low stress, partial dislocation loops are nucleated in the {111} glide set planes. But at low temperature and very high stress, perfect dislocation loops are formed in the other set of {111} planes called shuffle. This result confirmed by three different classical potentials suggests that plasticity in silicon nanostructures could be controlled by dislocation nucleation.
Microwave resonant absorption of viruses through dipolar coupling with confined acoustic vibrations
Authors: Liu Tzu-Ming , Chen Hung-Ping , Wang Li-Tzu , Wang Jen-Ren , Luo Tang-Nian , Chen Yi-Jan , Liu Shen-Iuan , Sun Chi-Kuang
Published: January 2009

In this letter, with an electric double layer on the surface of spherical viruses, we confirm that one of the microwave resonant absorption (MRA) mechanisms of viruses is through dipolar coupling with confined acoustic vibrations. By treating spherical virions as free homogeneous nanoparticles, we found that the MRA frequencies agree well with that of l=1 dipolar modes predicted by the elastic continuum theory. The magnitude of MRA was also found to change with the amount of adsorbed charges on the surface of virions. Our results provide a method to observe three-dimensionally confined acoustic vibrations in biological systems.
Temperature dependence of critical current fluctuations in Nb/AlOx/Nb Josephson junctions
Authors: Pottorf Shawn , Patel Vijay , Lukens James E.
Published: January 2009

We measured the low frequency critical current noise in Nb/AlOx/Nb Josephson junctions. Unshunted junctions biased above the gap voltage and resistively shunted junctions biased near the critical current Ic have been measured. For both, the spectral density of δIc/Ic, Sic(f), is proportional to 1/f, scales inversely as the area A and is independent of Jc≡Ic/A over a factor of nearly 20 in Jc. For all devices measured at 4.2 K, Sic(1 Hz)=2.0±0.4×10−12/Hz when scaled to A=1 μm2. We find that, from 4.2 to 0.46 K, Sic(f) decreases linearly with temperature.
Electrophotonic enhancement of bulk heterojunction organic solar cells through photonic crystal photoactive layer
Authors: Tumbleston John R. , Ko Doo-Hyun , Samulski Edward T. , Lopez Rene
Published: January 2009

We present one- (1D) and two-dimensional (2D) periodic nanostructured designs for organic photovoltaics where a photonic crystal is formed between blended poly-3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide. Absorption enhancements over the full absorption range of P3HT:PCBM of 20% (one polarization) and 14% are shown for the 1D and 2D structures, respectively. These improvements result in part from band edge excitation of quasiguided modes. The geometries are also shown to create excitons 26% (1D) and 11% (2D) closer to P3HT:PCBM exit interfaces indicating further photovoltaic improvement.
Dynamic scanning probe microscopy of adsorbed molecules on graphite
Authors: Berdunov N. , Pollard A. J. , Beton P. H.
Published: January 2009

We have used a combined dynamic scanning tunneling and atomic force microscope to study the organization of weakly bound adsorbed molecules on a graphite substrate. These weakly bound molecules may be imaged in dynamic scanning tunneling microscopy (STM) mode in which the probe is oscillated above the surface. We show that molecular resolution may be readily attained and that a similar mode of imaging may be realized using conventional STM arrangement. We also show, using tunneling spectroscopy, the presence of an energy gap for the adsorbed molecules confirming a weak molecule-substrate interaction.
Controlling magnetization reversal in Co/Pt nanostructures with perpendicular anisotropy
Authors: Rahman M. Tofizur , Dumas Randy K. , Eibagi Nasim , Shams Nazmun N. , Wu Yun-Chung , Liu Kai , Lai Chih-Huang
Published: January 2009

We demonstrate a simple method to tailor the magnetization reversal mechanisms of Co/Pt multilayers by depositing them onto large area nanoporous anodized alumina (AAO) with various aspect ratios, A=pore depth/diameter. Magnetization reversal in composite (Co/Pt)/AAO films with large A is governed by strong domain-wall pinning, which gradually transforms into a rotation-dominated reversal for samples with smaller A, as investigated by a first-order reversal curve method in conjunction with analysis of the angular dependent switching fields. The change in the magnetization reversal mode is attributed to topographical changes induced by the aspect ratio of the AAO templates.
Single-layer triplet white polymer light-emitting diodes incorporating polymer oxides: Effect of charge trapping at phosphorescent dopants
Authors: Chen Fang-Chung , Chien Shang-Chieh , Chen Yung-Shiuan
Published: January 2009

This paper describes the effects of charge trapping on the device performances of triplet polymer light-emitting diodes (PLEDs) after the cathode contact had been improved through the blending of poly(ethylene glycol) (PEG) into the active layer. The external quantum efficiency (EQE) was enhanced when the dopant tended to trap electrons. In contrast, we observed no EQE enhancement for the device featuring a hole-trapping dopant. Because PEG promoted electron injection, more electrons were trapped in the triplet molecules, thereby enhancing the probability of recombination. Finally, after incorporating PEG, we further achieved white PLEDs exhibiting both high EQE and high power efficiency.
Nanodiamonds in dusty low-pressure plasmas
Authors: Vandenbulcke L. , Gries T. , Rouzaud J. N.
Published: January 2009

Dusty plasmas composed of carbon, hydrogen, and oxygen have been evidenced by optical emission spectroscopy and microwave interferometry, due to the increase in electron energy and the decrease in electron density. These plasmas allow homogeneous synthesis of nanodiamond grains composed of either pure diamond nanocrystals only (2–10 nm in size) or of diamond nanocrystals and some sp2-hybridized carbon entities. The control of their size and their microstructure could open ways for a wide range of fields. Their formation from a plasma-activated gaseous phase is also attractive because the formation of nanodiamonds in the universe is still a matter of controversy.
Ohmic contacts and photoconductivity of individual ZnTe nanowires
Authors: Meng Q. F. , Jiang C. B. , Mao S. X.
Published: January 2009

Ohmic contacts to individual ZnTe nanowires were formed using Ni/Au multilayer electrodes. Measurements based on four terminals were carried out to test the current-voltage characteristics of contacts and nanowires. Specific contact resistivity of Ni/Au contacts is ∼5.2×10−2 Ω cm2 and the intrinsic resistivity of the ZnTe nanowire is ∼369.1 Ω cm. The photoconductivity behavior of individual ZnTe nanowires was observed, which was analyzed with theory of carrier generation, trapping, and recombination.
Uniform delivery of silicon nanoparticles on device quality substrates using spin coating from isopropyl alcohol colloids
Authors: Nayfeh Osama M. , Antoniadis Dimitri A. , Mantey Kevin , Nayfeh Munir H.
Published: January 2009

Silicon nanoparticles, of predominately 2.9 nm in size, in isopropyl alcohol are spin coated directly on device quality silicon-dioxide layers across 150 mm substrates. Atomic force microscopy (AFM) is used to image the nanoparticle distributions and low levels of agglomeration with apparently regular internanoparticle distances are observed. AFM depth profiling of the nanoparticle size is in agreement with independent high resolution transmission electron microscopy measurements. Hartree–Fock based atomistic simulations confirm the possible formation of Si nanoparticle/isopropanol complexes with a calculated electrostatic binding energy of 30 meV, which is slightly larger than the room temperature thermal agitation energy. The low levels of agglomeration can be explained in terms of such complexes that may regulate the internanoparticle and nanoparticle-solvent interactions.
Influence of the measurement procedure on the field-effect dependent conductivity of ZnO nanorods
Authors: Weissenberger D. , Gerthsen D. , Reiser A. , Prinz G. M. , Feneberg M. , Thonke K. , Zhou H. , Sartor J. , Fallert J. , Klingshirn C. , Kalt H.
Published: January 2009

The electrical properties of field-effect transistors fabricated on the basis of single ZnO nanorods were analyzed under ambient conditions and in the chamber of a scanning electron microscope under high-vacuum conditions. Under ambient conditions, the threshold voltage and conductivity may depend strongly on the details of the measurement procedure as the chosen gate voltage range and gate voltage sweep direction. Electron irradiation in a scanning electron microscope under high-vacuum conditions at ∼10−5 mbar leads to desorption of oxygen and other electronegative molecules, which can increase the conductivity by more than two orders of magnitude.
The effect of magnetic irreversibility on estimating the magnetocaloric effect from magnetization measurements
Authors: Amaral J. S. , Amaral V. S.
Published: January 2009

We found that the anomalous magnetic entropy change peak obtained from magnetization measurements in some first-order magnetic phase transition materials may result from the usual data analysis procedure, which does not take into account magnetic irreversibility or mixed-phase regime. The deviations produced are comparable to anomalous effects discussed in the literature and may even exceed the theoretical limit. Our results show that this anomalous magnetic entropy change peak should not necessarily be interpreted as a consequence of the particular physics of the studied system. This also explains its absence in specific heat measurements.
High-efficiency staggered 530 nm InGaN/InGaN/GaN quantum-well light-emitting diodes
Authors: Park Seoung-Hwan , Ahn Doyeol , Kim Jong-Wook
Published: January 2009

Optical properties of staggered 530 nm InGaN/InGaN/GaN quantum-well (QW) light-emitting-diodes are investigated using the multiband effective mass theory. These results are compared with those of conventional 530 nm InGaN/GaN QW structures. A staggered InGaN/InGaN/GaN QW structure is shown to have much larger spontaneous emission than a conventional InGaN/GaN QW structure. This can be explained by the fact that a staggered QW structure has much larger matrix element than a conventional QW structure because a spatial separation between electron and hole wave functions is substantially reduced with the inclusion of a staggered InGaN layer. A staggered QW structure shows that the peak position at a high carrier density (530 nm) is similar to that at a noninjection level.
The role of p-doping in the gain dynamics of InAs/GaAs quantum dots at low temperature
Authors: Cesari Valentina , Langbein Wolfgang , Borri Paola
Published: January 2009

We measured the gain dynamics of the ground-state transition at 20 K in an undoped and identically fabricated p-doped InAs/GaAs quantum-dot amplifier. The dynamics in the doped device is dominated by a very short (∼0.1 ps) and a very long (∼300 ps) time constant. These were attributed to hole and electron dynamics, respectively, and quantitatively described by a microstate model. By comparing the dynamics for the same modal gain in the two devices, the gain recovery was initially faster in the p-doped sample, attributed to ultrafast hole-hole scattering, but slower at later times due to the lack of an electron reservoir.
Band gap enhancement and electrical properties of La2O3 films doped with Y2O3 as high-k gate insulators
Authors: Zhao Yi , Kita Koji , Kyuno Kentaro , Toriumi Akira
Published: January 2009

In this study, we prepare the well crystallized La2O3 films by doping Y2O3 with different contents (La2−xYxO3) and examine the dielectric and electrical properties of La2−xYxO3 films experimentally. It is found that the optical band gap of La2−xYxO3 film increases with the increase in Y content (x) monotonically. Furthermore, a low leakage current of about 10−5 A/cm2 (equivalent oxide thickness: 1 nm) when the gate voltage is 1 V larger than the flat band voltage, and good capacitance-voltage characteristics in Au/La2−xYxO3/Si metal-insulator-semiconductor capacitors are observed. Our results also indicate that Pt/La2−xYxO3/Au metal-insulator-metal capacitor shows a low leakage current and a small voltage dependence of the capacitance.
Evidence of the Ge nonreactivity during the initial stage of SiGe oxidation
Authors: Dkhissi A. , Upadhyay A. K. , Hemeryck A. , Estève A. , Landa G. , Pochet P. , Rouhani M. Djafari
Published: January 2009

Density functional theory calculations are used to identify preferential sites for oxygen adsorption on SiGe. It is shown that Ge atoms hinder O incorporation in their vicinity. Additionally, the silanone structure, known to be the key intermediate prior to silicon dioxide formation, is shown to be not favorable when close to a Ge site on the surface.