bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of torsion on O–H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide
Authors: Haynes Laura M. , Vogelhuber Kristen M. , Pippen Jessica L. , Hsieh Shizuka
Published: December 2005

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O–H stretch excitation (3νOH–6νOH) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O–O bond. Experimental spectral profiles (3νOH–6νOH) and cross sections (3νOH–5νOH) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298K. A Birge-Sponer analysis yields experimental values for the O–H stretch frequency (ω=3773±15cm−1) and anharmonicity (ωx=94±3cm−1). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4νOH and 5νOH. While the spectral profile at 5νOH mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4νOH, reflecting the fact that the 4νOH excitation energy is below the literature dissociation energy (D0=42.6±1kcalmol−1) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.
Bulk magnetization and nuclear magnetic resonance of magnetically purified layered silicates and their polymer-based nanocomposites
Authors: Levin E. M. , Rawal A. , Bud’ko S. L. , Kracher A. , Schmidt-Rohr K.
Published: December 2005

The bulk magnetization and the H1 and Si29 nuclear-magnetic-resonance (NMR) spectra of two layered silicates, montmorillonite (MMT) and hectorite (HCT), purified by high-gradient magnetic separation, and of HCT-polymer nanocomposites have been measured. At 300K, the magnetization of MMT as received shows a behavior typical of paramagnets and does not change significantly even after ∼100h of magnetic separation. The magnetization of HCT as received is typical of a weak ferromagnet but it changes drastically after magnetic separation. The extracted particles have sizes varying from a few to about 150μm and show ferromagneticlike properties at 300K. While the magnetization/magnetic-field ratio, M∕H, of HCT is reduced 50-fold after ∼15min of magnetic separation and shows predominantly diamagnetic properties at 300K after ∼30h, we have also found that HCT contains ∼0.2mass% of paramagnetic Fe ions, with an effective magnetic moment of ∼5.2μB per Fe ion, as a regular element of its lattice. This is much smaller than the ∼3.8mass% in MMT. Spinning sidebands in H1 NMR spectra of HCT as received are reduced by magnetic separation. This indicates that dipolar interactions between nuclei and paramagnetic ions even in separated phases can affect the NMR spectra. Magnetically purified hectorite was used for preparing polymer-HCT nanocomposites where the NMR spectra show increased signal-to-noise ratios.
Defect energy levels in hydrogen-implanted and electron-irradiated n-type 4H silicon carbide
Authors: Alfieri G. , Monakhov E. V. , Svensson B. G. , Hallén A.
Published: December 2005

Using deep level transient spectroscopy (DLTS), we have studied the energy position and thermal stability of deep levels in nitrogen doped 4H–SiC epitaxial layers after 1.2MeV proton implantation and 15MeV electron irradiation. Isochronal annealing was performed at temperatures from 100to1200°C in steps of 50°C. The DLTS measurements, which were carried out in the temperature range from 120to630K after each annealing step, reveal the presence of ten electron traps located in the energy range of 0.45–1.6eV below the conduction band edge (Ec). Of these ten levels, three traps at 0.69, 0.73, and 1.03eV below Ec, respectively, are observed only after proton implantation. Dose dependence and depth profiling studies of these levels have been performed. Comparing the experimental data with computer simulations of the implantation and defects profiles, it is suggested that these three new levels, not previously reported in the literature, are hydrogen related. In particular, the Ec−0.73eV level displays a very narrow depth distribution, confined within the implantation profile, and it originates most likely from a defect involving only one H atom.
Transmission electron microscopy observations of Cu-induced directional crystallization of amorphous silicon
Authors: Lee Sung Bo , Choi Duck-Kyun , Lee Dong Nyung
Published: December 2005

By annealing at 500°C for 1h with an electric field of 180V∕cm, a Cu-deposited a-Si/glass sample undergoes a partial crystallization of amorphous Si, whose microstructure is characterized by conventional and high-resolution transmission electron microscopy. The Si crystallites grow in the ⟨111⟩ and ⟨211⟩ directions with their {011} planes parallel to the amorphous Si film surface, assuming a needlelike shape. Copper silicides are not observed at the leading edges of the crystalline Si needles. The growth directions of ⟨111⟩ and the {011} surface plane orientations are accepted to arise from elastic anisotropy of c-Si and can be explained by considering the strain energy under the uniaxial stress state and the plane stress, equibiaxial strain state. Crystallization behavior of a-Si in the Ni∕a-Si layer is also briefly discussed.
Observation of injection and pinning of domain walls in magnetic nanowires using photoemission electron microscopy
Authors: Thomas Luc , Rettner Charles , Hayashi Masamitsu , Samant Mahesh G. , Parkin Stuart S. P. , Doran Andrew , Scholl Andreas
Published: December 2005

Photoemission electron microscopy is used to explore the injection and pinning of magnetic domain walls in 250-nm-wide, 20-nm-thick Permalloy nanowires. Domain walls are injected from a micron-sized elliptical nucleation pad at one end of the nanowire. A vortex-like structure is readily nucleated in this pad at low magnetic fields (<15Oe), whereas injection of a domain wall into the nanowire requires significantly larger fields (∼60Oe). Domain walls are pinned in the nanowire at notches patterned along the wire’s edges. The domain walls are observed to have vortex-like structures with chiralities that vary in successive experiments.
Statistics and geometry of passive scalars in turbulence
Authors: Schumacher Jörg , Sreenivasan Katepalli R.
Published: December 2005

We present direct numerical simulations of the mixing of the passive scalar at modest Taylor microscale (10⩽Rλ⩽42) and Schmidt numbers larger than unity (2⩽Sc⩽32). The simulations resolve below the Batchelor scale up to a factor of 4. The advecting turbulence is homogeneous and isotropic, and is maintained stationary by stochastic forcing at low wave numbers. The passive scalar is rendered stationary by a mean scalar gradient in one direction. The relation between geometrical and statistical properties of scalar field and its gradients is examined. The Reynolds numbers and Schmidt numbers are not large enough for either the Kolmogorov scaling or the Batchelor scaling to develop and, not surprisingly, we find no fractal scaling of scalar level sets, or isosurfaces, in the intermediate viscous range. The area-to-volume ratio of isosurfaces reflects the nearly Gaussian statistics of the scalar fluctuations. The scalar flux across the isosurfaces, which is determined by the conditional probability density function (PDF) of the scalar gradient magnitude, has a stretched exponential distribution towards the tails. The PDF of the scalar dissipation departs distinctly, for both small and large amplitudes, from the log-normal distribution for all cases considered. The joint statistics of the scalar and its dissipation rate, and the mean conditional moment of the scalar dissipation, are studied as well. We examine the effects of coarse-graining on the probability density to simulate the effects of poor probe-resolution in measurements.
Systematic uncertainties in shock-wave impedance-match analysis and the high-pressure equation of state of Al
Authors: Celliers P. M. , Collins G. W. , Hicks D. G. , Eggert J. H.
Published: December 2005

A method for producing quantitative estimates of systematic uncertainties generated in the analysis of impedance-match shock-wave data is described. Central to the method is an analytic representation of the principal Hugoniot of the standard which incorporates a description of data-dependent uncertainties of the principal Hugoniot and model-dependent uncertainties of the off-Hugoniot states. Expressions for the sound speed and Grüneisen coefficient along the principal Hugoniot are also derived with uncertainties. An accurate impedance-match shock-wave equation of state for Al to shock pressure of 3TPa is given and is used to estimate the systematic uncertainties of several previously published experimental results.
Measurements of the structures of nanoparticles in flames by in situ detection of scattered x-ray radiation
Authors: Ossler Frederik , Larsson Jörgen
Published: December 2005

The angular pattern of scattered synchrotron x-ray radiation has been used to measure the composition of molecules and nanometer-sized particles in flames. The measured patterns were compared with patterns obtained from calculations for different species compositions. After ensuring that the calculations could reproduce the experiments for air and for ethylene flames under two different combustion conditions flames under special particle producing conditions were studied. In one case, the patterns showed a strong presence of spherical or fullerenelike structures with very little presence of graphitelike particles and little soot production on a cooling plate. In the other case, under soot producing conditions, the scattering showed a clear presence of nanometer-sized graphite particles. The results show that high concentrations of particles with the size around 1nm can be produced in flames. These particles either remain free or condense into larger particles, depending on the combustion conditions. This technique opens up for experimental studies of molecular and particle dynamics in combustion processes and other processes where nucleation and structural transformations of particles occur.
Low-field magnetoresistance in nanocrystalline La0.7Sr0.3MnO3 films
Authors: Cheng S. L. , Lin J. G.
Published: December 2005

Nanocrystalline La0.7Sr0.3MnO3 films with thickness t=10-60nm were grown on LaAlO3(100) substrates by radio-frequency magnetron sputtering. Their electrical resistivity and low-field magnetoresistance (MR) were measured. Metal-insulator transitions occur above 275K for films with t=20-60nm, but the electron localization prevails in the 10nm thick film. Furthermore, only the 10nm thick film has an MR that depends on the inverse of temperature, consistent with the model of spin-polarized tunneling. This relationship may reflect a critical aspect of the structure of grain∕grain-boundaries. Accordingly, the tunneling MR in this film is 27% at 75K.
Nonlinear instability and saturation of linearly stable current-carrying pair plasmas
Authors: Luque A. , Schamel H. , Eliasson B. , Shukla P. K.
Published: December 2005

The nonlinear instability of current-carrying pair plasmas is investigated with a Vlasov–Poisson model for the two-particle species. It is shown that linearly stable configurations are unstable against small incoherent perturbations of the particle distribution functions. The instability gives rise to a self-acceleration and growth of phase-space holes. After the growth of the phase-space holes, the instability reaches a chaotic saturation where the finite-amplitude holes interact and merge, and after a long time, the system attains a stable equilibrium state with a smaller drift and a larger temperature than the initial one, and where a few stable phase-space holes are present.
Development of a simulator of a satellite-to-satellite interferometer for determination of the Earth’s gravity field
Authors: Nagano Shigeo , Hosokawa Mizuhiko , Kunimori Hiroo , Yoshino Taizoh , Kawamura Seiji , Ohkawa Masashi , Sato Takashi
Published: December 2005

A satellite-to-satellite laser interferometer is essential for a future gravity-field mission to improve the accuracy and spatial resolution of the Earth’s gravity model. We have designed a ground-based simulator of a satellite-to-satellite interferometer, which aims to evaluate instrument error reflecting on the accuracy of the gravity-field retrieval and achieve the essential technologies required to measure the intersatellite range rate. The developed simulator was successfully operated with a range-rate sensitivity of 40nm∕s∕Hz in the measurement band of 10−2–1Hz. From a primary demonstration, it was confirmed to be available for experimental simulation of the satellite-to-satellite interferometer in the laboratory. The simulator will be a powerful tool for establishing guidelines for the development of future gravity missions.
Characterization system for resonant micro- and nanocantilevers
Authors: Sandberg Rasmus , Boisen Anja , Svendsen Winnie
Published: December 2005

We present a system for characterization of the resonant properties of micro- and nanocantilever sensors. The system has been constructed as a vacuum chamber with capabilities for controlling environmental conditions such as pressure, temperature, and chemical constituents. Characterization can be achieved either electrically or using a specialized laser-optical detection system. The system has been used to characterize the resonant properties of SiO2 cantilevers as well as other resonant structures. We present experimental results of a SiO2 resonant cantilever, showing an exceptional accuracy in resonant frequency determination, and demonstrating the importance of resonance characterization in a controlled environment.
Microscopic four-point probe based on SU-8 cantilevers
Authors: Keller S. , Mouaziz S. , Boero G. , Brugger J.
Published: December 2005

A microscopic four-point probe (μ4PP) for resistivity measurements on thin films was designed and fabricated using the negative photoresist SU-8 as base material. The device consists of four microscopic cantilevers, each of them supporting a probe tip at the extremity. The high flexibility of SU-8 ensures a stable electrical point contact between samples and probe tip with all four electrodes even on rough surfaces. With the presented surface micromachining process, μ4PPs with a probe-to-probe spacing of 10–20μm were fabricated. Resistivity measurements on thin Au, Al, and Pt films were performed successfully. The measured sheet resistances differ by less than 5% from those obtained by a commercial macroscopic resistivity meter. Due to the low contact forces (Fcont<10−4N), the μ4PP is suitable to be applied also to fragile materials such as conducting polymers. Here the authors demonstrate the possibility of performing resistivity measurements on 100-nm-thick pentacene (C22H14) films with a sheet resistance Rs>106Ω∕◻.
A microbeam wavelength-dispersive x-ray fluorescence system and its application for thin-film analysis
Authors: Gao Ning , Chen Zewu , Xiao Qifan
Published: December 2005

A low-power wavelength dispersive x-ray fluorescence (WDXRF) system with a spatial resolution of 25μm has been built to measure thin-film thickness. By using advanced x-ray optics and a 50-W air-cooled x-ray tube, the system achieved a detection limit equivalent to that of a conventional WDXRF system that employs a 4-kW x-ray tube and a spatial resolution of 10mm in diameter. For a 25-nm-thick tantalum underlayer, a detection limit of 0.18nm was achieved and measurement precision of 2% was obtained with a measurement time of 100s.
Compact high-resolution spectral phase shaper
Authors: Postma S. , van der Walle P. , Offerhaus H. L. , van Hulst N. F.
Published: December 2005

The design and operation of a high-resolution spectral phase shaper with a footprint of only 7×10cm2 is presented. The liquid-crystal modulator has 4096 elements. More than 600 independent degrees of freedom can be positioned with a relative accuracy of 1pixel. The spectral shaping of pulses from a broadband Ti:sapphire laser is verified by a hybrid cross-frequency-resolved optical gating/Grenouille measurement and intensity autocorrelation. We demonstrate the ability to split one pulse into two or more pulses with a programmable delay of more than 8.5ps. To our knowledge this represents the most compact high resolution device in liquid-crystal modulator-based shaping to this date.
Compact 300keV electron gun for radiation processing
Authors: Auditore L. , Barnà R. C. , De Pasquale D. , Interdonato S. , Italiano A. , Trifiró A. , Trimarchi M.
Published: December 2005

A new self-shielded system for surface radiation treatments has been developed, based on a 300keV electron gun, able to irradiate a 100mm×10mm area. The compact and durable system described in this paper provides the required dose of treatment for several industrial or scientific research applications, with a good reproducibility of the parameters.
Calibration of a thermocouple for measurement of oscillating temperature
Authors: Tashiro Yusuke , Biwa Tetushi , Yazaki Taichi
Published: December 2005

We report on the dynamic calibration of a thermocouple for the measurement of the oscillating temperature. Temperature oscillation is induced in a gas-filled tube by a periodically forced pressure oscillation and measured by a thermocouple. The radial profile of the measured temperature oscillations is compared with the theoretical one, which is determined from the simultaneously measured pressure. A response function of the thermocouple is obtained from the difference in amplitude and phase angle between them by varying the diameter of the thermocouple, oscillating frequency, tube radius, and working gas. We can obtain a true temperature oscillation by using the response function given in this experiment.
Pulsed laser heating and temperature determination in a diamond anvil cell
Authors: Deemyad S. , Sterer E. , Barthel C. , Rekhi S. , Tempere J. , Silvera Isaac F.
Published: December 2005

Pulsed laser heating in a diamond anvil cell has several advantages over cw heating: it can suppress thermally activated diffusion of the pressurization medium into the gasket and diamond, suppress chemical reactions of impurities with the sample and environment, requires far less average power and thus less heating of the sample environment, and can attain very high temperatures. It was recently shown that it is possible to accurately measure the melting point of platinum and other materials at ambient conditions using a pulsed laser and a simple ungated CCD detector, measuring the time-averaged irradiance. We show that this same technique can be used for high-pressure samples in a diamond anvil cell. As a demonstration, the high-pressure melting curve of iron is reproduced and compared to literature values.
Dynamics of low capillary number interfaces moving through sharp features
Authors: Reddy Shravanthi , Schunk P. Randall , Bonnecaze Roger T.
Published: December 2005

The success of any nanoimprint process depends upon its ability to exactly reproduce the template pattern. Thus, complete filling of recessed features in the template is an important issue that is controlled by the dynamics of the flow through these sharp structures. At these small scales, capillary forces are large and must be included in the fluid flow model. The mechanism of interface advancement at low capillary number through sharp rectangular features is useful for understanding how and why features fill or trap air. In this study we present a two-dimensional simulation of this feature filling to capture the details of the process, including the viscous and capillary effects. Fluid is injected into the channel between the template and substrate, where the fluid–air interface soon encounters a rectangular feature with some height greater than the channel gap. As the fluid advances through the channel, the shape of the interface is a circular arc due to the strong capillary forces. The interface maintains this circular arc as it negotiates the first sharp corner of the feature; the upper contact line effectively pins to the initial corner of the feature as it moves around this corner, during which time the lower contact line continues to advance forward along the substrate surface, causing the interface to stretch. For sufficiently wide or shallow features, once the upper contact line has negotiated the first corner and has moved vertically up the inner wall of the feature, it must move through the top corner of the feature. At this point the interface undergoes a rapid reconfiguration from a high surface area circular arc to a lower surface area circular arc inside the feature. Alternatively, for narrow or high features, the stretched interface can catch on the far, final corner of the feature, trapping air inside the feature and preventing filling. The conditions for filling are studied parametrically for a variety of wetting contact angles and feature dimensions with both the simulation and a simpler, successful geometric model. The dynamics of the feature filling suggest an effective boundary condition for a macroscopic lubrication model of the imprint lithography process in which a critical pressure is required to move fluid through a feature.
Ellipsoidal statistical Bhatnagar–Gross–Krook model with velocity-dependent collision frequency
Authors: Zheng Yingsong , Struchtrup Henning
Published: December 2005

In this paper, an ellipsoidal statistical (ES) Bhatnagar–Gross–Krook (BGK)-type kinetic model with velocity-dependent collision frequency is proposed and further numerically tested for one-dimensional shock waves and planar Couette flow at steady state for hard sphere molecules. In this new kinetic model, a physically meaningful expression for the velocity-dependent collision frequency derived from the Boltzmann equation is used, while the important properties for a kinetic model are retained at the same time. This kinetic model can be simplified to the classical ES-BGK model and the BGK model with velocity-dependent collision frequency for suitable choices of parameters. The H theorem for this new kinetic model has so far been proven only for small Knudsen numbers. The numerical method used here for kinetic models is based on Mieussens’s discrete velocity model [L. Mieussens, J. Comput. Phys. 162, 429 (2000)]. Computational results from the kinetic models (including the BGK model, the ES-BGK model, the BGK model with velocity-dependent collision frequency, and this new kinetic model) are compared to results obtained from the direct simulation Monte Carlo (DSMC) method. It is found that results obtained from this new kinetic model lie in between results from the ES-BGK model and results from the BGK model with velocity-dependent collision frequency. For one-dimensional shock waves, results from this new kinetic model fit best with results from the DSMC, while for planar Couette flow, the classical ES-BGK model is suggested.
Reynolds number dependency of the scalar flux spectrum in isotropic turbulence with a uniform scalar gradient
Authors: Bos W. J. T. , Touil H. , Bertoglio J.-P.
Published: December 2005

In this paper, the eddy-damped quasi-normal Markovian closure is used to study the behavior of the scalar flux spectrum in isotropic turbulence as the Reynolds number Reλ varies in a range between 30 and 107. The different contributions to the evolution equation of the scalar flux spectrum are studied. One-dimensional spectra are in good agreement with direct numerical simulation (DNS) and experiments at moderate Reλ. The closure shows that at high Reynolds numbers, a K−7∕3 scaling is found for the scalar flux spectrum, in agreement with Lumley’s prediction [Phys. Fluids 10, 855 (1967)], but enormous Reλ are needed before it can be clearly observed. In the range of wind tunnel experiments, the spectral exponent for the scalar flux is closer to −2 in agreement with existing measurements [Mydlarski and Warhaft, J. Fluid Mech. 358, 135 (1998)]. The results for the molecular dissipation of scalar flux are in agreement with the DNS results of Overholt and Pope [Phys. Fluids A 8, 3128 (1996)]. The large Reλ behavior of this quantity is also addressed.
Crystalline silicon surface passivation with amorphous SiCx:H films deposited by plasma-enhanced chemical-vapor deposition
Authors: Martín I. , Vetter M. , Garín M. , Orpella A. , Voz C. , Puigdollers J. , Alcubilla R.
Published: December 2005

Surface-passivating properties of hydrogenated amorphous silicon carbide films (a-SiCx:H) deposited by plasma-enhanced chemical-vapor deposition on both p- and n-type crystalline silicon (c-Si) have been extensively studied by our research group in previous publications. We characterized surface recombination by measuring the dependence of the effective lifetime (τeff) on excess carrier density (Δn) through quasi-steady-state photoconductance technique. Additionally, we fitted the measured τeff(Δn) curves applying an insulator/semiconductor model which allows us to determine the surface recombination parameters. In this paper, this model is analyzed in detail focusing on the accuracy in the determination of the fitting parameters and revealing uncertainties not detected up to now. Taking advantage of this analysis, the dependence of surface passivation on film deposition conditions is revised including intrinsic a-SiCx:H films on both p- and n-type c-Si and phosphorus-doped a-SiCx:H films on p-type c-Si. As a consequence, a broad view of this passivation scheme is obtained.
Growth and characterization of epitaxial ferroelectric lanthanum-substituted bismuth titanate nanostructures with three different orientations
Authors: Lee Sung Kyun , Hesse Dietrich , Alexe Marin , Lee Woo , Nielsch Kornelius , Gösele Ulrich
Published: December 2005

Well-ordered large-area arrays of ferroelectric La-substituted Bi4Ti3O12 (BLT) nanostructures were prepared by pulsed laser deposition using gold nanotube membranes as shadow masks. By x-ray diffraction and transmission electron microscopy, it was found that well-defined (001)-, (118)∕(100)-, and (104)-oriented BLT nanostructures were obtained on (001)-, (011)-, and (111)-oriented SrTiO3 single crystal substrates covered with epitaxial SrRuO3 electrode layers, respectively, through ex situ crystallization at 700°C. Atomic force microscopy analyses revealed that the epitaxial BLT nanostructures maintain a height of about 100nm and a lateral size of about 150nm in spite of the postannealing process. The anisotropic ferroelectric properties of the BLT nanostructures of three different orientations were investigated by scanning force microscopy in piezoresponse mode. The highest effective remanent piezoresponse coefficient (2dzz) of about 38.0pm∕V was observed in (118)∕(100)-oriented BLT nanostructures, whereas (001)- and (104)-oriented nanostructures showed smaller 2dzz values of 5.0pm∕V and 1.4pm∕V, respectively.
Composition-dependent structural properties in ScGaN alloy films: A combined experimental and theoretical study
Authors: Constantin Costel , Haider Muhammad B. , Ingram David , Smith Arthur R. , Sandler Nancy , Sun Kai , Ordejón Pablo
Published: December 2005

Experimental and theoretical results are presented regarding the incorporation of scandium into wurtzite GaN. Variation of the a and c lattice constants with Sc fraction in the low Sc concentration regime (0%–17%) are found that can be well explained by the predictions of first-principles theory. The calculations allow a statistical analysis of the variations of the bond lengths and bond angles as functions of Sc concentration. The results are compared to predictions from both a prior experimental study [Constantin et al., Phys. Rev. B 70, 193309 (2004)] and a prior theoretical study [Farrer and Bellaiche et al. Phys. Rev. B 66, 201203(R) (2002)]. It is found that the ScGaN lattice can be very well modeled as being wurtzitelike but with local lattice distortions arising from the incorporation of the Sc atoms. Effects of the addition of Sc on the stacking order for a large Sc fraction is also studied by high resolution transmission electron microscopy. The results show the existence of stacking faults, and induced stacking disorder. The explanation for the lattice constant variations is based on the effects of local lattice distortions and not related to the stacking faults.
Two-dimensional simulation of a low-current dielectric barrier discharge in atmospheric helium
Authors: Zhang Yuan Tao , Wang De Zhen , Kong Michael G.
Published: December 2005

A two-dimensional computational study is presented to unravel radial structure of a dielectric barrier discharge in atmospheric helium when the gas voltage exceeds slightly the breakdown voltage and the discharge current is low to retain a repetitive dynamic pattern of one discharge event every half cycle of the applied voltage. Simulation results reveal that during each half cycle of the applied voltage gas breakdown occurs first in a central region around the electrode axis. After it is extinguished, a second breakdown is triggered in the boundary region near the radial edge of the two electrodes as confirmed by the dynamic evolution of the radial profile of the electric field, the current density and the charged particles. These predictions are consistent with relevant experimental observations in literature. It is also shown that an increase in the applied voltage or in the excitation frequency reduces the time delay between the two breakdown events and the difference between their corresponding current densities. This offers a route to improve the uniformity of atmospheric dielectric barrier discharges for their intended applications.
N incorporation into InGaAs cap layer in InAs self-assembled quantum dots
Authors: Chen J. F. , Hsiao R. S. , Hsieh P. C. , Chen Y. J. , Chen Y. P. , Wang J. S. , Chi J. Y.
Published: December 2005

This study presents the results of incorporating N into self-assembled InAs quantum dots (QDs) capped with an InGaAs cap layer. Experimental results indicate that such incorporation can redshift the QD ground state and decrease the energy spacing between the QD ground and first excited states. However, this incorporation reduces the potential barrier of the cap layer, increasing the electron escape from the QDs. Capacitance-voltage profiling shows that a broad shoulder corresponding to the electron emission from the QD ground to first-excited state cannot be resolved from the peak related to the electron emission from the excited states upon this incorporation. This finding implies that this incorporation reduces the energy spacing between the QD ground and first-excited states in the conduction band, thus correlating well with the photoluminescence data. In contrast, incorporating N directly into the InAs QD produces no redshift of the emission wavelength but introduces a deep trap at ∼0.21eV that depletes the electrons in the QDs.
Aging of electroluminescent ZnS:Mn thin films deposited by atomic layer deposition processes
Authors: Ihanus Jarkko , Lankinen Mikko P. , Kemell Marianna , Ritala Mikko , Leskelä Markku
Published: December 2005

Electroluminescent ZnS:Mn thin films were deposited by the atomic layer deposition (ALD) technique. The deposition processes were based on ZnI2 or ZnCl2 as the Zn source and Mn(thd)3 (thd=2,2,6,6-tetramethyl-3,5-heptanedionato) as the Mn source. The ZnI2 process was found to have a wide temperature range between 300 and 490°C where the growth rate was independent of the deposition temperature, which offers the possibility to select the deposition temperature according to the thermal stability of the dopant precursor without reducing growth of ZnS. The electro-optical measurements suggested that the amount of space charge was lower within the phosphors made with the iodide process, which resulted in higher efficiency of the “iodide” devices as compared to the “chloride” devices. Brightness and efficiency of the best iodide device after 64h aging were 378cd∕m2 and 2.7lm∕W, respectively, measured at 60Hz and at 40V above threshold voltage. Conversely, brightness and efficiency of the best chloride device after 64h aging were 355cd∕m2 and 1.6lm∕W, respectively. On the other hand, changes in the emission threshold voltages indicated that the chloride devices aged slower than the iodide devices. Though the samples were annealed later at high temperature, the deposition temperature was found to be a significant parameter affecting the grain size, luminance, and efficiency of the devices. Overall, the results of this study show that a relatively small change in the Zn precursor can have a clear impact on the electro-optical properties of the devices, and that a mixed halide/metalorganic ALD process can produce an electroluminescent device that ages relatively slowly.
Theoretical and experimental behavior of a compact magnetically insulated line oscillator electromagnetic structure
Authors: Cousin Richard , Larour Jean , Raymond Pierre , Wey Joseph , Gouard Philippe , Durand Alain Joseph
Published: December 2005

The magnetically insulated line oscillator (MILO) is a high power microwave source capable of delivering output powers greater than 1GW at a frequency of several gigahertz. The device is a crossed electric and magnetic field oscillator without any external structure to establish the static magnetic field, required to guide the electron beam, thus making it compact. We report on a compact MILO device based on the U.S. Air-Force design where all dimensions are reduced by a factor of 2. All the key points are reviewed using the Electromagnetic-PIC (Particle-In-Cell) code MAGIC in two-dimensional geometry. The operating frequency of such a device is 2.44GHz, leading to an output power slightly above 1GW for an operating voltage of 500kV and a maximum current of 45kA. The output power can be increased up to 2GW by optimizing the output coupling and reducing the beam loading effect. The analysis of the compact electromagnetic structure is based on the calculation of the external Q factor (Qext). The Qext coefficient is calculated for each resonant frequency of the four-cell coupled periodic resonator which accounts for the main part of the MILO slow wave structure. The optimized geometrical parameters such as the extractor vane radius (Rext=47mm) and the collector length (Lcol=92mm), which constitutes the output coupling section, lead to a maximum of simulated output power. A prototype has been built according to this geometry. The compact MILO structure is characterized by some cold tests (without electron beam) injecting a low rf power by a network analyzer; experimental results are presented and discussed in comparison with the simulation.
Wave propagation in carbon nanotubes via nonlocal continuum mechanics
Authors: Wang Q.
Published: December 2005

Wave propagation in carbon nanotubes (CNTs) is studied with two nonlocal continuum mechanics models: elastic Euler-Bernoulli and Timoshenko beam models [Philos. Mag. 41, 744 (1921)]. The small-scale effect on CNTs wave propagation dispersion relation is explicitly revealed for different CNTs wave numbers and diameters by theoretical analyses and numerical simulations. The asymptotic phase velocities and frequency are also derived from nonlocal continuum mechanics. The scale coefficient in nonlocal continuum mechanics is roughly estimated for CNTs from the obtained asymptotic frequency. In addition, the applicability and comparison of the two nonlocal elastic beam models to CNTs wave propagation are explored through numerical simulations. The research findings are proved effective in predicting small-scale effect on CNTs wave propagation with a qualitative validation study based on the published experimental reports in this field.
Depth dependence of defect density and stress in GaN grown on SiC
Authors: Faleev N. , Temkin H. , Ahmad I. , Holtz M. , Melnik Yu.
Published: December 2005

We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29to30μm. High level of residual elastic strain was found in thin (0.29to0.73μm thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2×107cm−2, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.
Martensitic phase transformations in the nanostructured surface layers induced by mechanical attrition treatment
Authors: Ni Zhichun , Wang Xiaowei , Wu Erdong , Liu Gang
Published: December 2005

Conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction (XRD) analysis have been used to investigate the relationship between characteristics of phase transformation and the treatment time in surface nanocrystallized 316L stainless steel induced by surface mechanical attrition treatment (SMAT). A similar trend of development of the martensitic phase upon the treatment time has been observed from both CEMS and XRD measurements. However, in the CEMS measurement, two types of martensite phase with different magnetic hyperfine fields are revealed. Based on a random distribution of the non-iron coordinating atoms, a three-element theoretical model is developed to illustrate the difference of two types of martensite phase. The calculated results indicate the segregation of the non-iron atoms associated with SMAT treatment.
Electromagnetic fluid drift turbulence in static ergodic magnetic fields
Authors: Reiser D. , Scott B.
Published: December 2005

Numerical simulations of three-dimensional nonlinear electromagnetic fluid drift turbulence in a tokamak plasma with externally applied stochastic magnetic-field perturbations are presented. The contributions to the radial particle transport due to nonlinearities arising from E×B advection and magnetic flutter are investigated for perturbation fields of varying strengths in the cases of low and high collisionalities. The perturbation strength is varied to study the physics for Chirikov parameters above 1. In all the cases considered a significant increase of E×B transport is found. A static contribution in the density and velocity perturbations contributes significantly to the total radial E×B transport. For low collisionality, the external perturbation leads to enhanced density and velocity fluctuations over a broad range in the toroidal wave-number spectrum, resulting in an enhanced turbulent flux. For high collisionality, the density fluctuations stay roughly the same and the velocity fluctuations are increased in an intermediate range of the toroidal wave number spectrum, separated from the maximum of the density fluctuations, thus leaving the turbulent flux almost unchanged.
Ion heating during magnetic relaxation in the helicity injected torus-II experiment
Authors: O’Neill R. G. , Redd A. J. , Hamp W. T. , Smith R. J. , Jarboe T. R.
Published: December 2005

Ion doppler spectroscopy (IDS) is applied to the helicity injected torus (HIT-II) spherical torus to measure impurity ion temperature and flows. [A. J. Redd et al., Phys. Plasmas 9, 2006 (2002)] The IDS instrument employs a 16-channel photomultiplier and can track temperature and velocity continuously through a discharge. Data for the coaxial helicity injection (CHI), transformer, and combined current drive configurations are presented. Ion temperatures for transformer-driven discharges are typically equal to or somewhat lower than electron temperatures measured by Thomson scattering. Internal reconnection events in transformer-driven discharges cause rapid ion heating. The CHI discharges exhibit anomalously high ion temperatures >250eV, which are an order of magnitude higher than Thomson measurements, indicating ion heating through magnetic relaxation. The CHI discharges that exhibit current and poloidal flux buildup after bubble burst show sustained ion heating during current drive.
Viscosity and slip velocity in gas flow in microchannels
Authors: Fichman M. , Hetsroni G.
Published: December 2005

It was shown that as a result of the interaction of part of the molecules with a wall, the viscosity in the Knudsen layer is lower in comparison with the viscosity in the bulk fluid. The slip condition of Maxwell depends on the normal gradient of the tangential velocity on the wall. The reduction of the viscosity increases the tangential velocity and its normal gradient, and, therefore, increases the slip. The correction factors for the Maxwell expression for one-dimensional plane and tubular isothermal and incompressible flows as a function of the Knudsen number and of the kind of interaction of molecules with the wall were derived.
Mixed donor-acceptor molecular heterojunctions for photovoltaic applications. I. Material properties
Authors: Rand Barry P. , Xue Jiangeng , Uchida Soichi , Forrest Stephen R.
Published: December 2005

In this and the following paper (Parts I and II, respectively), we discuss the properties of mixed donor-acceptor organic thin films and their application to organic solar cells. In Part I, we present a study of the material properties of mixed donor-acceptor thin films. Through optical absorption, x-ray diffraction, microscopy, and charge transport measurements, we determine the relationships among film microstructure, mixing ratio, and charge conduction in mixtures of two organic molecular species. We find that mixed layers of the molecular pair of 1:1 (by weight) copper phthalocyanine in C60 have electron and hole mobilities reduced by more than one order of magnitude compared to corresponding films of pure composition. In Part II, we demonstrate that the performance of organic hybrid planar-mixed heterojunction photovoltaic cells based on a mixed donor-acceptor molecular layer sandwiched between the donor and acceptor layers of homogeneous composition can have improved performance over conventional planar heterojunction cells containing no mixed composition layers.
Efficacy of single and double SiNx interlayers on defect reduction in GaN overlayers grown by organometallic vapor-phase epitaxy
Authors: Yun F. , Moon Y. -T. , Fu Y. , Zhu K. , Ozgür Ü. , Morkoç H. , Inoki C. K. , Kuan T. S. , Sagar Ashutosh , Feenstra R. M.
Published: December 2005

We report on the growth of and evolution of defects in GaN epilayers having single- and double-layer SiNx nanoporous insertion layers. The SiNx was formed in situ in the growth chamber of an organometallic vapor-phase epitaxy system by simultaneous flow of diluted silane and ammonia. The GaN epilayers and SiNx interlayers were grown on 6H-SiC substrates using three different nucleation layers, namely, low-temperature GaN, high-temperature GaN, and high-temperature AlN nucleation layers. X-ray-diffraction rocking curves and cross-sectional and plan-view transmission electron microscope analyses indicated that a nanoporous SiNx layer can reduce the dislocations density in the GaN overgrown layer to ∼3×108cm−2 range; below this level the defect blocking effect of SiNx would saturate. Therefore the insertion of a second SiNx layer becomes much less effective in reducing dislocations, although it continues to reduce the point defects, as suggested by time-resolved photoluminescence measurements. The insertion of SiNx interlayers was found to improve significantly the mechanical strength of the GaN epilayers resulting in a much lower crack line density.
Micro-Raman spectroscopy on a-C:H nanoparticles
Authors: Hong Suk-Ho , Winter Jörg
Published: December 2005

We present the Raman spectra of amorphous hydrogenated carbon nanoparticles prepared in an Ar–CH4 hydrocarbon plasma. The measured Raman spectra are compared with that of both soft polymerlike and hard diamondlike a-C:H films deposited from pure CH4 plasma. The film properties were studied by Rutherford backscattering and spectroscopic ellipsometry. It is shown that the plasma characteristics are continuously changed from a “precursor-rich” state to a “precursor” state during the particle formation process. As a result, the net flux of the carbon atoms to the surface of the nanoparticles (growth rate) decreases linearly as a function of time as the nanoparticles are growing up. Also, the hydrogen content in nanoparticles increases as a function of nanoparticle size. The particle growth process is then examined by a simple ballistic model. It is found that the volume polymerization process, i.e., the growth of the nanoparticles at a large particle size (⩾100nm), is essentially the same as the surface polymerization process occurring at a surface of a substrate.
Hydrogen passivation of germanium (100) surface using wet chemical preparation
Authors: Rivillon Sandrine , Chabal Yves J. , Amy Fabrice , Kahn Antoine
Published: December 2005

A wet chemical preparation involving de-ionized water, hydrogen peroxide, and hydrofluoric acid is used to passivate germanium (Ge) (100) surfaces. Infrared absorption spectroscopy and x-ray photoemission spectroscopy studies show that oxide free and hydrogen-terminated Ge (100) surfaces can be obtained. As in the case for silicon (100) surfaces etched in hydrofluoric acid, hydrogen-terminated Ge (100) surfaces are atomically rough, with primarily mono- and dihydride terminations.
Effect of electronic structure on carrier multiplication efficiency: Comparative study of PbSe and CdSe nanocrystals
Authors: Schaller Richard D. , Petruska Melissa A. , Klimov Victor I.
Published: December 2005

Recently, we demonstrated that PbSe nanocrystal quantum dots can efficiently produce multiple electron-hole pairs (excitons) in response to a single absorbed photon. To address the generality of this carrier-multiplication phenomenon to other materials, we perform a comparative study of multiexciton generation in PbSe and CdSe nanocrystals that have distinctly different electronic structures. We find that both materials exhibit high-efficiency carrier multiplication and the activation threshold is lower in CdSe nanocrystals than in PbSe nanocrystals (∼2.5 vs ∼2.9 energy gaps). Furthermore, the efficiencies of multiexciton generation are nearly identical for both materials despite a vast difference in both energy structures and carrier relaxation behaviors, strongly suggesting that this phenomenon is general to quantum-confined semiconductor nanocrystals.
Magnesium diboride nanobridges fabricated by electron-beam lithography
Authors: Malisa A. , Charlebois S. , Lindström T.
Published: December 2005

MgB 2 nanobridges were fabricated by e-beam lithography and Ar-ion beam milling. Nanobridges of widths ranging from 60 nm to 1μm and 3μm in length were realized by Ar-ion beam milling using amorphous carbon as etching mask. The processing did not harm the superconducting properties appreciably. High values of the critical current density, more than 10MA∕cm2, were measured for bridges with widths down to 60 nm. Current-voltage (I-V) characteristics showed a behavior typical of a bridge going normal, after the critical current is exceeded, and remaining normal as the current is decreased to a lower switch back value due to Joule heating. We could also observe switching behavior in some bridges indicating formation of normal hotspots in the bridges before they returned to their superconducting state. Alternative explanations may include natural grain boundaries in the film or the movement of Abrikosov vortices. The current-voltage (I-V) characteristics showing critical current densities up to 5×107⁢A∕cm2 indicates excellent film properties in the nanobridges.