bluuebunny/crossref_metadata_2025 huggingface dataset:
Advanced core/multishell germanium/silicon nanowire heterostructures: Morphology and transport
Authors: Dayeh S. A. , Gin A. V. , Picraux S. T.
Published: April 2011

A precise level of control over morphology and transport in germanium/silicon core/multishell semiconductor nanowires is attained by interface engineering. Epitaxial in situ growth of such advanced heterostructures is achieved, enabling smooth and crystalline shell quality without ex situ thermal or chemical treatment. Transport simulation predicts such heterostructures with engineered energy band-edges will exhibit enhanced on-currents and transconductances over traditional device designs. Based on this synthesis approach, a 2× improvement in experimental hole mobility, transconductance, and on-currents is demonstrated for heterostructures with smooth surface morphologies compared to those with rough surface morphologies and record normalized on-currents for p-type field effect transistors are achieved.
p-type K-doped ZnO nanorods for optoelectronic applications
Authors: Gupta Manoj K. , Sinha Nidhi , Kumar Binay
Published: April 2011

Single crystalline p-type K-doped ZnO nanorods (NR) have been synthesized by convenient and low-cost solution technique. X-ray diffraction analysis confirmed the hexagonal system of K-doped ZnO nanorods and a preferred a axis orientation. The diameter and length were found to be 30–50 nm and 150–200 nm, respectively, in electron microscopic studies. The p-type nature and high hole density of 2.36×1019 per cm3 was demonstrated by Hall studies. A sharp lower cutoff and reduction in the bandgap was observed in UV-Vis study which is linked to its smaller size and p-type conduction in NR. In Photoluminescence studies, a near band emission in the UV range and a yellow emission with a blueshift were detected in the visible region in K-doped ZnO NR. The variation in intensity of peaks is correlated with the dimensions of NR. The redshift observed in Raman peaks is explained in terms of the stress developed in ZnO nanostructure due to lattice mismatch by K doping. These results provide better insight of K-doped ZnO nanostructures for both fundamental research and technological applications in optoelectronic devices.
Low electric field, easily reversible electrical set and reset processes in a Ge15Te83Si2 glass for phase change memory applications
Authors: Anbarasu M. , Asokan S.
Published: April 2011

We report here an easily reversible set–reset process in a new Ge15Te83Si2 glass that could be a promising candidate for phase change random access memory applications. The I-V characteristics of the studied sample show a comparatively low threshold electric field (Eth) of 7.3 kV/cm. Distinct differences in the type of switching behavior are achieved by means of controlling the on state current. It enables the observation of a threshold type for less than 0.7 mA beyond memory type (set) switching. The set and reset processes have been achieved with a similar magnitude of 1 mA, and with a triangular current pulse for the set process and a short duration rectangular pulse of 10 msec width for the reset operation. Further, a self-resetting effect is seen in this material upon excitation with a saw-tooth/square pulse, and their response of leading and trailing edges are discussed. About 6.5 × 104 set–reset cycles have been undertaken without any damage to the device.
Influence of reactive atmosphere on properties of cobalt ferrite thin films prepared using pulsed-laser deposition
Authors: Raghunathan A. , Jiles D. C. , Snyder J. E.
Published: April 2011

A series of cobalt ferrite (CFO) thin films were grown on SiO2/Si(100) substrates using pulsed-laser deposition (PLD) at substrate temperature (TDEP) of 250 °C and oxygen pressures (PO2) from 0.67 to 6.7 Pa. The influence of PO2 on crystal structure, phase mixture, deposition rate, and magnetic properties was investigated. It is shown in this study that there is a window of PO2 for optimized growth of nanograined CFO films at low TDEP and that either higher or lower values of PO2 produce undesirable multiphase mixtures. CFO films grown at such low substrate temperature and optimized oxygen pressure on thermal expansion matched substrates can be applied in multilayer sensors or MEMS devices.
Physical model of plastic deformation on magnetization in ferromagnetic materials
Authors: Wang Z. D. , Deng B. , Yao K.
Published: April 2011

Traditional magnetic/stress coupling models are based on the magnetoelastic theory, which cannot explain magnetic phenomena when ferromagnetic materials are subjected to a plastic deformation. To promote the development of magnetomechanical theories, a new physical-based magnetic/stress coupling model was proposed in this paper, where the effects of the applied magnetic field, elastic and plastic deformations on the magnetization are, respectively, considered in terms of different acting mechanisms. Moreover, the influences of variants on the strength in different magnetization stages are also calculated.
Characteristics of turbulence-driven plasma flow and origin of experimental empirical scalings of intrinsic rotation
Authors: Wang W. X. , Hahm T. S. , Ethier S. , Diamond P. H. , Rewoldt G. , Tang W. M. , Lee W. W.
Published: April 2011

Toroidal plasma flow driven by turbulent torque associated with nonlinear residual stress generation is shown to recover the observed key features of intrinsic rotation in experiments. Specifically, the turbulence-driven intrinsic rotation scales close to linearly with plasma gradients and the inverse of the plasma current, qualitatively reproducing empirical scalings obtained from a large experimental data base. The effect of magnetic shear on the symmetry breaking in the parallel wavenumber spectrum is identified. The origin of the current scaling is found to be the enhanced k∥ symmetry breaking induced by increased radial variation of the safety factor as the current decreases. The physics origin for the linear dependence of intrinsic rotation on the pressure gradient comes from the fact that both turbulence intensity and the zonal flow shear, which are two key ingredients for driving the residual stress, are increased with the strength of the turbulence drives, which are R/LTe and R/Lne for the collisionless trapped electron mode (CTEM). Highlighted results also include robust radial pinches in toroidal flow, heat and particle transport driven by CTEM turbulence, which emerge “in phase,” and are shown to play important roles in determining plasma profiles. Also discussed are the experimental tests proposed to validate findings from these gyrokinetic simulations.
Adjusting exchange bias and coercivity of magnetic layered systems with varying anisotropies
Authors: Tillmanns A. Ehrmann née , Blachowicz T.
Published: April 2011

The impact of a variation of anisotropy constants on the resulting coercivity and exchange bias has been analyzed modeling the total energy density in thin layered ferromagnetic/antiferromagnetic in-plane systems. For a broad range of fourfold, uniaxial, and unidirectional anisotropies, our results illustrate that the exchange bias can grow significantly for a sample rotation off the cooling field direction, while for other combinations of anisotropies, a positive exchange bias can be found near or even in the cooling field direction. These findings allow identification of anisotropies based on superconducting quantum interference device or magneto-optical Kerr effect measurements as well as tailoring desired angular dependencies for magnetoelectronic applications.
Abrasive rolling effects on material removal and surface finish in chemical mechanical polishing analyzed by molecular dynamics simulation
Authors: Si Lina , Guo Dan , Luo Jianbin , Lu Xinchun , Xie Guoxin
Published: April 2011

In an abrasive chemical mechanical polishing (CMP) process, materials were considered to be removed by abrasive sliding and rolling. Abrasive sliding has been investigated by many molecular dynamics (MD) studies; while abrasive rolling was usually considered to be negligible and therefore was rarely investigated. In this paper, an MD simulation was used to study the effects of abrasive rolling on material removal and surface finish in the CMP process. As the silica particle rolled across the silicon substrate, some atoms of the substrate were dragged out from their original positions and adhered to the silica particle, leaving some atomic vacancies on the substrate surface. Meanwhile, a high quality surface could be obtained. During the abrasive rolling process, the influencing factors of material removal, e.g., external down force and driving force, were also discussed. Finally, MD simulations were carried out to examine the effects of abrasive sliding on material removal under the same external down force as abrasive rolling. The results showed that the ability of abrasive rolling to remove material on the atomic scale was not notably inferior to that of abrasive sliding. Therefore, it can be proposed that both abrasive sliding and rolling play important roles in material removal in the abrasive CMP of the silicon substrate.
Nitrogen doped-ZnO/n-GaN heterojunctions
Authors: Chen Xin Yi , Fang Fang , Ng Alan M. C. , Djurišič Aleksandra B. , Cheah Kok Wai , Ling Chi Chung , Chan Wai Kin , Fong Patrick W. K. , Lui Hsian Fei , Surya Charles
Published: April 2011

Nitrogen-doped ZnO nanorods were prepared by electrodeposition using two different Zn precursors (zinc nitrate and zinc acetate), while all other growth conditions (dopant precursor, concentration, growth temperature, and bias) were identical. We have shown that the precursor used affects the properties of the ZnO nanorods, and that the presence of rectifying properties in n-GaN/N:ZnO heterojunctions is strongly related to the use of nitrate precursor for ZnO growth. The difference in the properties of ZnO obtained from two precursors is attributed to the differences in native defect and impurity concentrations, which could affect the electronic properties of the samples.
Nonlinear restoring forces and geometry influence on stability in near-field acoustic levitation
Authors: Li Jin , Liu Pinkuan , Ding Han , Cao Wenwu
Published: April 2011

Stability is a key factor in near-field acoustic levitation (NFAL), which is a popular method for noncontact transportation of surface-sensitive objects. Since the physical principle of NFAL is based on nonlinear vibration and nonuniform pressure distribution of a plate resonator, traditional linearized stability analysis cannot address this problem correctly. We have performed a theoretical analysis on the levitation stability using a nonlinear squeeze film model including inertia effects and entrance pressure drop, and obtained nonlinear effective restoring force and moment. It was found that the nonuniform pressure distribution is mode-dependent, which determines the stability of the levitation system. Based on the theoretical understanding, we have designed a NFAL resonator with tapered cross section, which can provide higher stability for the levitating object than the rectangular cross-section resonator.
Theoretical investigation of rotationally inelastic collisions of CH2(ã) with helium
Authors: Ma Lifang , Alexander Millard H. , Dagdigian Paul J.
Published: April 2011

Rotationally inelastic collisions of the CH2 molecule in its $\tilde a^1 A_1$ã1A1 electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH2(ã), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RSSCD(T)]. The PES is quite anisotropic, due to lack of electron density in the unoccupied CH2 non-bonding orbital perpendicular to the molecular plane. Quantum scattering calculations have been carried out to compute state-to-state rotational energy transfer and elastic depolarization cross sections at collision energies up to 2400 cm−1. These cross sections were thermally averaged to derive room-temperature rate constants. The total removal and elastic depolarization rate constants for the ortho ka = 1 levels agree well with recent experimental measurements by Hall, Sears, and their co-workers. We observe a strong even–odd alternation in the magnitude of the total rate constants which we attribute to the asymmetry splitting of the ka = 1 levels.
Magnetic and electronic structure properties of Co-doped SnO2 nanoparticles synthesized by the sol-gel-hydrothermal technique
Authors: Chen Weibing , Li Jingbo
Published: April 2011

Structural, magnetic and electronic properties of Co-doped SnO2 nanoparticles sample with the concentration of 3.37% synthesized by the sol-gel-hydrothermal technique are studied in detail with experiments and ab initio calculations. Room temperature ferromagnetism is observed in this dilute magnetic semiconductor. Secondary phases and clusters of cobalt are excluded according to XRD, HRTEM, Raman, and XPS analyses. Magnetic measurements demonstrate the coexistence of ferromagnetism and paramagnetism in this sample, which is explained reasonably through our ab initio calculations in Co-doped SnO2 system.
Green luminescence of InGaN nanowires grown on silicon substrates by molecular beam epitaxy
Authors: Goodman Kevin D. , Protasenko Vladimir V. , Verma Jai , Kosel Thomas H. , Xing Huili G. , Jena Debdeep
Published: April 2011

Indium gallium nitride nanowires show promise as being prime candidates for optical devices since they can be grown with band gaps spanning the visible spectra, while at the same time can be composed of stress free material. The goal of the work presented here was to obtain InGaN nanowires producing green emission at room temperature. Two growth recipes were found to yield InGaN nanowire growth on silicon substrates using plasma-assisted molecular beam epitaxy. At room temperature the photoluminescence (PL) of wire ensembles indeed peaked at 530 nm but, in addition, it was discovered that at low temperatures the emission often covered a broader (360–700 nm) spectrum. This broad optical range indicated indium content fluctuations in individual wires, wire-to-wire fluctuations, or a combination of the two. EDX measurements performed on single wires confirmed this hypothesis and correlated well with PL data. Low temperature PL studies of InGaN individual wires also revealed interwire and intrawire inhomogeneity of emission spectra stemming from a nonuniform indium distribution. The emission quantum yield for bright single wires was extracted to be more than 50% at 4 K. The findings suggest that the wire surfaces do not efficiently quench optical emission at low temperatures. These defect-free wires offer not only a potential path for green emitters, but also as integrated phosphors for broad spectral emission.
Phonon-assisted tunneling in charge transport of polypyrrole thin films and nanofibers
Authors: Ohlckers Per , Pipinys Povilas
Published: April 2011

This article reinterprets the recent experimental results for temperature and field dependence of conductivity in polypyrrole thin films and nanofibers in the framework of phonon-assisted tunneling electrons from traps into conduction band theories. Our proposed model describes well not only the conductivity dependence with temperature but also the temperature-dependent I-V data using the same set of parameters for characterizing the material. The values of active phonon energy are estimated from the fit of the conductivity dependence on temperature with the theory.
Pressure-dependent Hugoniot elastic limit of Gd3Ga5O12 single crystals
Authors: Zhou Xianming , Li Jun , Nellis Williams J. , Wang Xiang , Li Jiabo , He Hongliang , Wu Qiang
Published: April 2011

Single-crystal Gd3Ga5O12 has been studied at high dynamic pressures generated with plate impacts. Shock-wave profiles and Hugoniot points were measured with a picosecond time-resolved Doppler Pin System. For final shock pressures in the range 8.52-113 GPa, a two-wave structure is observed below 59.3 GPa, a three-wave structure at ∼88.5 GPa, and a single shock wave is observed at ∼113 GPa. Our data show that the Hugoniot elastic limit (HEL) of single-crystal Gd3Ga5O12 is strongly dependent on final shock pressure. The HEL increases from 7.65 to 24.2 GPa as final pressure increases from 8.52 to 88.5 GPa. A shock-induced phase transformation is observed at a pressure of ∼75.9 GPa, which is a little higher than the value reported previously (Mashimo et al., Phys. Rev. Lett. 96, 105504, 2006), but is consistent with previous DAC work (Mao et al., Phys. Rev. B 83, 054114, 2011).
Implementation of faceted dendrite growth on floating cast method to realize high-quality multicrsytalline Si ingot for solar cells
Authors: Usami Noritaka , Takahashi Isao , Kutsukake Kentaro , Fujiwara Kozo , Nakajima Kazuo
Published: April 2011

We carried out small-scale crystal growth experiment to apply faceted dendrite growth at the top of the melt to floating cast method in order to grow high-quality multicrystalline Si (mc-Si) ingot for solar cells. By appropriate cooling at the initial stage, the most part of the top surface was covered by a single faceted dendrite crystal followed by directional solidification from the top to the bottom. As a consequence, the cross-section of the ingot was dominated by a single crystal grain. The crystal grain was found to be almost free from subgrain boundaries as evidenced by spatially resolved single-peak x-ray rocking curve profiles. The minority carrier diffusion length was found to be much longer than that in a control sample grown from the bottom to the top presumably due to the minimized contact with the crucible wall. This shows that the floating cast method combined with formation of faceted dendrite crystals at the initial stage is a promising route to realize high-quality mc-Si ingot for high-efficiency solar cells.
Thickness dependence of switching time and coercive field in ferroelectric thin films
Authors: Musleh Alrub Ahmad , Ong Lye-Hock
Published: April 2011

The switching time and the coercive field in the polarization reversal of ferroelectric (FE) films have been investigated theoretically using the Landau Devonshire free energy expression and the Landau Khalatnikov dynamic equation in this work. Our numerical data show that the switching time is an exponential function of the applied field, and the function implies that there is a definite coercive field in the switching of a FE film. The effects of the thickness and the surface parameter δ on the switching time and the coercive field have been studied, and we found that the coercive field and the switching time could either decrease or increase with decreasing film thickness in film with surface conditions of positive δ or negative δ, respectively. These results are consistent with the switching phenomena reported in experiments regarding the film thickness dependence of the coercive field and the switching time.
Remarkable synergistic effects in a mixed porphyrin dye-sensitized TiO2 film
Authors: Griffith Matthew J. , Mozer Attila J. , Tsekouras George , Dong Ying , Wagner Pawel , Wagner Klaudia , Wallace Gordon G. , Mori Shogo , Officer David. L.
Published: April 2011

A remarkable 300% efficiency enhancement driven by a matching increase in the short circuit current was observed in a mixed porphyrin dye-sensitized solar cell constructed from two dyes in a 3:1 ratio. Absorbed photon-to-current conversion efficiency measurements indicate an improved charge injection yield for both dyes in the mixture. Several possible origins for the observed performance enhancement are discussed.
Orientation dependence of the elastic instability on strained SiGe films
Authors: Aqua J.-N. , Gouyé A. , Auphan T. , Frisch T. , Ronda A. , Berbezier I.
Published: April 2011

At low strain, SiGe films on Si substrates undergo the Asaro–Tiller–Grinfeld morphological instability. We demonstrate experimentally that this instability develops on Si(001) but not on Si(111) even after long annealing. Using a continuum description of surface diffusion in presence of wetting, elasticity, and surface energy anisotropy, we find a retardation of the instability on Si(111) due to a strong dependence of the instability onset as function of the surface stiffness. This retardation is at the origin of the inhibition of the instability on experimental time scales even after long annealing.
Ordering of amphiphilic Janus particles at planar walls: A density functional study
Authors: Rosenthal Gerald , Klapp Sabine H. L.
Published: April 2011

We investigate the structure formation of amphiphilic molecules at planar walls using density functional theory. The molecules are modeled as (hard) spheres composed of a hydrophilic and hydrophobic part. The orientation of the resulting Janus particles is described as a vector representing an internal degree of freedom. Our density functional approach involves fundamental measure theory combined with a mean-field approximation for the anisotropic interaction. Considering neutral, hydrophilic, and hydrophobic walls, we study the adsorption of the particles, focusing on the competition between the surface field and the interaction-induced ordering phenomena. Finally, we consider systems confined between two planar walls. It is shown that the anisotropic Janus interaction yields pronounced frustration effects at low temperatures.
In situ, noninvasive characterization of superhydrophobic coatings
Authors: Samaha Mohamed A. , Ochanda Fredrick O. , Tafreshi Hooman Vahedi , Tepper Gary C. , Gad-el-Hak Mohamed
Published: April 2011

Light scattering was used to measure the time-dependent loss of air entrapped within a submerged microporous hydrophobic surface subjected to different environmental conditions. The loss of trapped air resulted in a measurable decrease in surface reflectivity and the kinetics of the process was determined in real time and compared to surface properties, such as porosity and morphology. The light-scattering results were compared with measurements of skin-friction drag, static contact angle, and contact-angle hysteresis. The in situ, noninvasive optical technique was shown to correlate well with the more conventional methods for quantifying surface hydrophobicity, such as flow slip and contact angle.
Split Hopkinson resonant bar test for sonic-frequency acoustic velocity and attenuation measurements of small, isotropic geological samples
Authors: Nakagawa Seiji
Published: April 2011

Mechanical properties (seismic velocities and attenuation) of geological materials are often frequency dependent, which necessitates measurements of the properties at frequencies relevant to a problem at hand. Conventional acoustic resonant bar tests allow measuring seismic properties of rocks and sediments at sonic frequencies (several kilohertz) that are close to the frequencies employed for geophysical exploration of oil and gas resources. However, the tests require a long, slender sample, which is often difficult to obtain from the deep subsurface or from weak and fractured geological formations. In this paper, an alternative measurement technique to conventional resonant bar tests is presented. This technique uses only a small, jacketed rock or sediment core sample mediating a pair of long, metal extension bars with attached seismic source and receiver—the same geometry as the split Hopkinson pressure bar test for large-strain, dynamic impact experiments. Because of the length and mass added to the sample, the resonance frequency of the entire system can be lowered significantly, compared to the sample alone. The experiment can be conducted under elevated confining pressures up to tens of MPa and temperatures above 100 ºC, and concurrently with x-ray CT imaging. The described split Hopkinson resonant bar test is applied in two steps. First, extension and torsion-mode resonance frequencies and attenuation of the entire system are measured. Next, numerical inversions for the complex Young's and shear moduli of the sample are performed. One particularly important step is the correction of the inverted Young's moduli for the effect of sample–rod interfaces. Examples of the application are given for homogeneous, isotropic polymer samples, and a natural rock sample.
Integral representation of first kind Kapteyn series
Authors: Baricz Árpád , Jankov Dragana , Pogány Tibor K.
Published: April 2011

The main aim of this research note is to establish two different types of integral representation formulae for the Kapteyn series of the first kind. The first one is a double definite integral expression, while the second type includes indefinite integral representation formula.
Crossover between surface field and photo-Dember effect induced terahertz emission
Authors: Reklaitis Antanas
Published: April 2011

Pulsed terahertz emission from semiconductor surface illuminated by femtosecond laser pulse is analyzed. The analysis is based on the Boltzmann transport equation. It is shown that terahertz emission is induced by the surface field if the photon energy is close to the band-gap energy. The terahertz emission is controlled by the photo-Dember effect when the photon energy significantly exceeds the band-gap energy. A compact criterion for the crossover between the two mechanisms of terahertz emission is obtained. The analytical results are compared to the results of detailed Monte Carlo simulations.
Michaelis–Menten relations for complex enzymatic networks
Authors: Kolomeisky Anatoly B.
Published: April 2011

Most biological processes are controlled by complex systems of enzymatic chemical reactions. Although the majority of enzymatic networks have very elaborate structures, there are many experimental observations indicating that some turnover rates still follow a simple Michaelis–Menten relation with a hyperbolic dependence on a substrate concentration. The original Michaelis–Menten mechanism has been derived as a steady-state approximation for a single-pathway enzymatic chain. The validity of this mechanism for many complex enzymatic systems is surprising. To determine general conditions when this relation might be observed in experiments, enzymatic networks consisting of coupled parallel pathways are investigated theoretically. It is found that the Michaelis–Menten equation is satisfied for specific relations between chemical rates, and it also corresponds to a situation with no fluxes between parallel pathways. Our results are illustrated for a simple model. The importance of the Michaelis–Menten relationship and derived criteria for single-molecule experimental studies of enzymatic processes are discussed.
Second harmonic generation with zero phase velocity waves
Authors: Somerville Walter R. C. , Powell David A. , Shadrivov Ilya V.
Published: April 2011

We design a dual-band nonlinear composite right-left handed transmission line with phase-matching achieved between the fundamental frequency and second harmonic when both interacting waves have zero phase velocity. Additionally, we show that such a transmission line supports a new regime where the generation of backward second harmonic waves is achieved from a backward fundamental frequency wave.
Vibrational solvatochromism and electrochromism of infrared probe molecules containing C≡O, C≡N, C=O, or C−F vibrational chromophore
Authors: Choi Jun-Ho , Cho Minhaeng
Published: April 2011

Solvatochromic vibrational frequency shifts of a few different infrared (IR) probe molecules have been studied by carrying out quantum chemistry calculations for a number of their water clusters. We are particularly focused on the vibrational solvatochromic and electrochromic effects on the CO, CN, and CF stretch modes in carbon monoxide, acetone, 4-cyanopyridine, p-tolunitrile, fluorobenzene, and 3-fluoropyridine. Using multiple interaction site antenna model, we show that their solvatochromic vibrational frequency shifts can be successfully described by considering spatially nonuniform electrostatic potential generated by the surrounding water molecules. It turns out that the CO and CF stretch mode frequencies are approximately proportional to the solvent electric field projected onto the bond axes, whereas the vibrational frequencies of the nitrile stretch mode in 4-cyanopyridine and p-tolunitrile are not. Consequently, it is confirmed that the vibrational Stark tuning rates of the CO and CF stretching modes can be directly used to describe their solvatochromic frequency shifts in condensed phases. However, the nitrile stretch mode frequency shift induced by solvent electrostatic potential appears to be more complicated than its electrochromic phenomenon. To examine the validity of the distributed interaction site model for solvatochromic frequency shifts of these vibrational chromophores, we thus calculated the vibrational Stark tuning rates of the CO, CN, and CF stretch modes and found that they are in good agreement with the experimental results found in literatures. This confirms that a collection of properly chosen distributed interaction sites can be an excellent electric antenna sensing local electrostatics that affects on vibrational frequencies of IR probe modes.
Effect of dislocation density on thermal boundary conductance across GaSb/GaAs interfaces
Authors: Hopkins Patrick E. , Duda John C. , Clark Stephen P. , Hains Christopher P. , Rotter Thomas J. , Phinney Leslie M. , Balakrishnan Ganesh
Published: April 2011

We report on the thermal boundary conductance across structurally-variant GaSb/GaAs interfaces characterized by different dislocations densities, as well as variably-rough Al/GaSb interfaces. The GaSb/GaAs structures are epitaxially grown using both interfacial misfit (IMF) and non-IMF techniques. We measure the thermal boundary conductance from 100 to 450 K with time-domain thermoreflectance. The thermal boundary conductance across the GaSb/GaAs interfaces decreases with increasing strain dislocation density. We develop a model for interfacial transport at structurally-variant interfaces in which phonon propagation and scattering parallels photon attenuation. We find that this model describes the measured thermal boundary conductances well.
Fundamental design paradigms for systems of three interacting magnetic nanodiscs
Authors: Forrester D. M. , Kürten K. E. , Kusmartsev F. V.
Published: April 2011

The magnetic properties of a system of three interacting magnetic elliptical disks are examined. For the various levels of uniaxial anisotropy investigated a complicated series of phase transitions exist. These are marked by the critical lines of stability that are demonstrated in an applied magnetic field plane diagram.
Guidelines for choosing molecular “alligator clip” binding motifs in electron transport devices
Authors: Reuter Matthew G. , Seideman Tamar , Ratner Mark A.
Published: April 2011

We employ a one-electron, tight-binding model of an electrode–molecule–electrode junction to explore the fundamental relationship between adsorption geometry and electron transport, producing exact results (within this model). By varying the chemisorption location (e.g., atop a surface atom or in a hollow site between surface atoms) and the molecule–electrode coupling, we find that the largest currents are realized when the molecule (i) is highly coordinated by the surface and (ii) has favorable overlap with electrode states near the Fermi level. We also show the importance of electrode-induced molecular level shifting for certain adsorption geometries, which can cause molecular levels far from the Fermi level to conduct better than those near the Fermi level. Since all of these factors are greatly influenced by the chemical moiety used to link the molecule to an electrode, these results present a set of guidelines to help choose “alligator clips” for molecular electronic devices.
High-sensitivity Q-band electron spin resonance imaging system with submicron resolution
Authors: Shtirberg Lazar , Twig Ygal , Dikarov Ekaterina , Halevy Revital , Levit Michael , Blank Aharon
Published: April 2011

A pulsed electron spin resonance (ESR) microimaging system operating at the Q-band frequency range is presented. The system includes a pulsed ESR spectrometer, gradient drivers, and a unique high-sensitivity imaging probe. The pulsed gradient drivers are capable of producing peak currents ranging from ∼9 A for short 150 ns pulses up to more than 94 A for long 1400 ns gradient pulses. Under optimal conditions, the imaging probe provides spin sensitivity of ∼1.6 × 108 spins/√Hz or ∼2.7 × 106 spins for 1 h of acquisition. This combination of high gradients and high spin sensitivity enables the acquisition of ESR images with a resolution down to ∼440 nm for a high spin concentration solid sample (∼108 spins/μm3) and ∼6.7 μm for a low spin concentration liquid sample (∼6 × 105 spins/μm3). Potential applications of this system range from the imaging of point defects in crystals and semiconductors to measurements of oxygen concentration in biological samples.
Communication: Tailoring the optical gap in light-harvesting molecules
Authors: Karolewski A. , Stein T. , Baer R. , Kümmel S.
Published: April 2011

Systematically varying the optical gap that is associated with charge-transfer excitations is an important step in the design of light-harvesting molecules. So far the guidance that time-dependent density functional theory could give in this process was limited by the traditional functionals’ inability to describe charge-transfer excitations. We show that a nonempirical range-separated hybrid approach allows to reliably predict charge-transfer excitations for molecules of practically relevant complexity. Calculated absorption energies agree with measured ones. We predict from theory that by varying the number of thiophenes in donor-acceptor-donor molecules, the energy of the lowest optical absorption can be tuned to the lower end of the visible spectrum. Saturation sets in at about five thiophene rings.
Coordinate space translation technique for simulation of electronic process in the ion–atom collision
Authors: Wang Feng , Hong Xuhai , Wang Jian , Kim Kwang S.
Published: April 2011

Recently we developed a theoretical model of ion–atom collisions, which was made on the basis of a time-dependent density functional theory description of the electron dynamics and a classical treatment of the heavy particle motion. Taking advantage of the real-space grid method, we introduce a “coordinate space translation” technique to allow one to focus on a certain space of interest such as the region around the projectile or the target. Benchmark calculations are given for collisions between proton and oxygen over a wide range of impact energy. To extract the probability of charge transfer, the formulation of Lüdde and Dreizler [J. Phys. B 16, 3973 (1983)] has been generalized to ensemble-averaging application in the particular case of O(3P). Charge transfer total cross sections are calculated, showing fairly good agreements between experimental data and present theoretical results.
Study of relaxation process of dipalmitoyl phosphatidylcholine monolayers at air–water interface: Effect of electrostatic energy
Authors: Ou-Yang Wei , Weis Martin , Manaka Takaaki , Iwamoto Mitsumasa
Published: April 2011

The instability of organic monolayer composed of polar molecules at the air–water interface has been a spotlight in interface science for many decades. However, the effect of electrostatic energy contribution to the free energy in the system is still not understood. Herein, we investigate the mechanical and electrical properties by studying the isobaric relaxation process of a dipalmitoyl phosphatidylcholine monolayer on water subphase with various concentrations of divalent ions to reveal the effect of electrostatic energy on thermodynamics and kinetics of the collapse mechanism. Our results demonstrate that electrical energy among the dipolar molecules plays an important role in the stability of monolayer and enhances the formation of micelles into subphase under high pressure. In addition, to confirm the electrostatic energy contribution, the well-known thermal effect on the stability of the film is compared. Hence, the general description of the monolayer free energy with contribution of electrostatic energy is suggested to describe the phase transition.
Thermophotonic radar imaging: An emissivity-normalized modality with advantages over phase lock-in thermography
Authors: Tabatabaei Nima , Mandelis Andreas , Amaechi Bennett T.
Published: April 2011

One major problem of frequency-domain photothermal radiometry, or alternatively in two-dimensional lock-in thermography, is the compromise one has to make between dynamic range (probing depth) and depth resolution. The thermal-wave radar incorporates chirped excitation through matched filtering to maintain good resolution and depth range inside a sample. This letter experimentally demonstrates the advantages of chirped modulation and introduces a thermophotonic modality of thermal-wave radar based on an emissivity-normalized, higher-dynamic-range contrast parameter known as cross-correlation phase. Finally, comparisons made on a biological (dental) sample show potential applications of the method.
The formulation and performance of a perturbative correction to the perfect quadruples model
Authors: Parkhill John A. , Azar Julian , Head-Gordon Martin
Published: April 2011

A recently published alternative hierarchy of coupled-cluster approximations is reformulated as a perturbative correction. A single variant, a model for the total electronic energy based on the perfect quadruples model, is explored in detail. The computational scaling of the method developed is the same as canonical second order Møller–Plesset perturbation theory (fifth order in the number of molecular orbitals), but its accuracy competes with the high-accuracy, high-cost standard CCSD(T), even when the latter is allowed to break spin-symmetry. The variation presented can be implemented without explicit calculation and storage of the most expensive energy contributions, thereby improving the range of systems which can be treated. The performance and scaling of the method are demonstrated with calculations on the water, fluorine, and oxirane molecules, and compared to the parent model.
Optical communication of spin information between light emitting diodes
Authors: Farshchi R. , Ramsteiner M. , Herfort J. , Tahraoui A. , Grahn H. T.
Published: April 2011

For the full implementation of spintronic circuits, it is necessary to transmit spin information from one device to another. Electrons in semiconductors often suffer from high spin relaxation rates, making electrical transport of spin information highly inefficient. Here, we propose optical transport of spin information as an alternative. We demonstrate that the spin information associated with electrons injected from Co2FeSi and Fe layers into the quantum wells of spin light emitting diodes (spin-LEDs) can be transported optically in the form of circularly polarized light and deciphered electrically via the magnetic field dependence of the photocurrent in a distant detector spin-LED.
Cyclotron resonance mass and Fermi energy pinning in the In(AsN) alloy
Authors: Drachenko O. , Patanè A. , Kozlova N. V. , Zhuang Q. D. , Krier A. , Eaves L. , Helm M.
Published: April 2011

We report cyclotron resonance (CR) experiments on the midinfrared alloy InAs1−xNx grown on GaAs with x from 0% to 1.9%. Using magnetic fields up to 60 T and terahertz photon sources from 3 to 30 THz, we determine the dependence on x of the electron density and CR mass. The increase in the carrier density with increasing x is accompanied by a redshift in the interband photoluminescence emission and is explained in terms of the pinning of the Fermi level to its value at x=0. The high carrier densities (∼1018 cm−3) at x∼1% give rise to a CR mass that is only weakly dependent on the excitation energy, significantly weaker than that in InAs.
An analytical model for the growth of quantum dots on ultrathin substrates
Authors: Gill S. P. A.
Published: April 2011

The self-assembly of heteroepitaxial quantum dots on ultrathin substrates is analyzed within the context of small perturbation theory. Analytical expressions are derived for the dependence of the quantum dot separation on the substrate thickness. It is shown that the substrate thickness is critical in determining this separation when it is below the intrinsic material length scale of the system. The model is extended to simultaneous dot growth on both sides of the substrate. It is shown that vertically anticorrelated structures are preferred with an increase in the dot separation of 15% above that found in the one-sided case.
Influence of growth temperature on AlGaN multiquantum well point defect incorporation and photoluminescence efficiency
Authors: Armstrong A. , Allerman A. A. , Henry T. A. , Crawford M. H.
Published: April 2011

The dependence of (Al)GaN/AlGaN multiquantum well (MQWs) optical efficiency and defect incorporation on the growth temperature (Tg) of the optically active region was investigated. Marked increase in MQW photoluminescence (PL) intensity was observed for increasing Tg. Correspondingly, increasing Tg also significantly reduced point defect incorporation under QW growth conditions, as determined by deep level optical spectroscopy. It is suggested that enhanced MQW PL with increasing Tg resulted from improved nonradiative lifetime through reduced nonradiative defect density in the MQW region.
Electroabsorption spectra of PbSe nanocrystal quantum dots
Authors: Liu Xiaoming , Iimori Toshifumi , Ohshima Ruriko , Nakabayashi Takakazu , Ohta Nobuhiro
Published: April 2011

The quantum-confined Stark effect of PbSe nanocrystal quantum dots (QDs) in a polymer film has been studied for the near-infrared absorption in the region of 1300–1600 nm (corresponding to diameters of 4.0–7.0 nm) by using electric field modulation spectroscopy. Electroabsorption spectra of QDs show the electric-field-induced energy shift in the exciton state, resulting in the spectral broadening given by the second derivative of the absorption spectra. It is shown that the magnitude of the change in electric dipole moment following excitation into the first exciton state increases with an increase in the size of QDs.
Science and the Arabs: opportunities and challenges
Authors: Zahlan A.B.
Published: April 2011

Tunisians and Egyptians sought recently to transform their political culture. Their political economy has rendered it impossible to develop the scientific infrastructure needed to benefit from science. This paper shows that, by comparison with China and India, the Arabs in 1999 had a substantially higher level of university enrolment, per million inhabitants abroad and at home; and were ahead in per capita R&D output. Yet both China and India were able to take off with these resources while the Arab countries were left behind. The research output of the Arab countries increased about 50-fold over the period 1967–2010. Egypt, the GCC and the Maghreb are in the lead, and the leading countries are Egypt, Saudi Arabia and Tunisia. Yet Arab governments provide minimal support to R&D at 0.2% of GNP and few countries have recently increased support to R&D. The paper discusses their research output as well international collaboration.
Evaluation of an Infrared Thermal Detection System for Fever Recognition during the H1N1 Influenza Pandemic
Authors: Hewlett Angela L. , Kalil Andre C. , Strum Rahman A. , Zeger Wesley G. , Smith Philip W.
Published: April 2011

Infrared thermal detection systems (ITDSs) have been utilized in several countries to screen for fever in travelers. Since fever screening with an ITDS is rapid and noninvasive, this technology may be useful as an infection control measure in clinical settings during a pandemic.
Molecular Epidemiology and Risk Factors for Colonization by Vancomycin-ResistantEnterococcusin Patients with Hematologic Malignancies
Authors: Almyroudis N. G. , Lesse A. J. , Hahn T. , Samonis G. , Hazamy P. A. , Wongkittiroch K. , Wang E. S. , McCarthy P. L. , Wetzler M. , Segal B. H.
Published: April 2011

Objective.

To study the molecular epidemiology of vancomycin-resistantEnterococcus(VRE) colonization and to identify modifiable risk factors among patients with hematologic malignancies.

Setting.

A hematology-oncology unit with high prevalence of VRE colonization.

Participants.

Patients with hematologic malignancies and hematopoietic stem cell transplantation recipients admitted to the hospital.

Methods.

Patients underwent weekly surveillance by means of perianal swabs for VRE colonization and, if colonized, were placed in contact isolation. We studied the molecular epidemiology in fecal and blood isolates by pulsed-field gel electrophoresis over a 1-year period. We performed a retrospective case-control study over a 3-year period. Cases were defined as patients colonized by VRE, and controls were defined as patients negative for VRE colonization. Case patients and control patients were matched by admitting service and length of observation time.

Results.

Molecular genotyping demonstrated the primarily polyclonal nature of VRE isolates. Colonization occurred at a median of 14 days. Colonized patients were characterized by longer hospital admissions. Previous use of ceftazidime was associated with VRE colonization (P< .001), while use of intravenous vancomycin and antibiotics with anaerobic activity did not emerge as a risk factor. There was no association with neutropenia or presence of colonic mucosal disruption, and severity of illness was similar in both groups.

Conclusion.

Molecular studies showed that in the majority of VRE-colonized patients the strains were unique, arguing that VRE acquisition was sporadic rather than resulting from a common source of transmission. Patient-specific factors, including prior antibiotic exposure, rather than breaches in infection control likely predict for risk of fecal VRE colonization.
Evaluation of ATP Bioluminescence Assays for Potential Use in a Hospital Setting
Authors: Aiken Zoie A. , Wilson Michael , Pratten Jonathan
Published: April 2011

ATP bioluminescence is being applied in hospitals to measure surface contamination. We compared commercial luminometers for detecting the number Staphylococcus aureus associated with surfaces. The data showed that the ATP bioluminescence methods tested were not robust enough to generate quantitative data on bacterial numbers, especially at low concentrations.
Economic Value of Dispensing Home-Based Preoperative Chlorhexidine Bathing Cloths to Prevent Surgical Site Infection
Authors: Bailey Rachel R. , Stuckey Dianna R. , Norman Bryan A. , Duggan Andrew P. , Bacon Kristina M. , Connor Diana L. , Lee Ingi , Muder Robert R. , Lee Bruce Y.
Published: April 2011

Objective.

To estimate the economic value of dispensing preoperative home-based Chlorhexidine bathing cloth kits to orthopedic patients to prevent surgical site infection (SSI).

Methods.

A stochastic decision-analytic computer simulation model was developed from the hospital’s perspective depicting the decision of whether to dispense the kits preoperatively to orthopedic patients. We varied patient age, cloth cost, SSI-attributable excess length of stay, cost per bed-day, patient compliance with the regimen, and cloth antimicrobial efficacy to determine which variables were the most significant drivers of the model’s outcomes.

Results.

When all other variables remained at baseline and cloth efficacy was at least 50%, patient compliance only had to be half of baseline (baseline mean, 15.3%; range, 8.23%–20.0%) for Chlorhexidine cloths to remain the dominant strategy (ie, less costly and providing better health outcomes). When cloth efficacy fell to 10%, 1.5 times the baseline bathing compliance also afforded dominance of the preoperative bath.

Conclusions.

The results of our study favor the routine distribution of bathing kits. Even with low patient compliance and cloth efficacy values, distribution of bathing kits is an economically beneficial strategy for the prevention of SSI.