bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimal large eddy simulation of turbulent channel flow based on direct numerical simulation statistical data
Authors: Völker S. , Moser R. D. , Venugopal P.
Published: September 2002

It has been shown that there is a large eddy simulation (LES) evolution, the ideal LES, that guarantees accurate single-time statistics and at the same time produces the most accurate short-time dynamics of the simulated turbulence. In optimal LES, models are constructed by formally approximating ideal LES using stochastic estimation. In this paper, optimal LES modeling is applied to the turbulent flow in a channel, using statistical data from a direct numerical simulation to form the stochastic estimates. Due to the data requirements, the modeling process pursued here does not directly yield generally applicable LES models; instead, the current study provides information on the required characteristics of subgrid models for wall-bounded turbulence. In the channel flow, and other wall-bounded flows, the strong inhomogeneity near the wall introduces several complications. There is a mean subgrid stress that must be represented, and there are subgrid contributions to turbulent transport. It is found that formulating the optimal LES models to reproduce a priori the important terms in the Reynolds stress transport equations was necessary to produce accurate LES. Models formulated this way were found to produce good predictions of mean velocity, wall shear stress and turbulent intensities. The results of this study indicate that for inhomogeneous flows, the subgrid model must represent processes other than transfer of energy to small scales. Specifically, it must represent subgrid transport, mean subgrid Reynolds stress and subgrid intercomponent transfer due to pressure strain.
Instability of time-dependent wind-driven ocean gyres
Authors: van der Vaart Paul C. F. , Schuttelaars Henk M. , Calvete Daniel , Dijkstra Henk A.
Published: September 2002

The wind-driven ocean circulation at midlatitudes is susceptible to several types of instabilities. One of the simplest models of these flows is the quasigeostrophic barotropic potential vorticity equation in an idealized ocean basin. In this model, the route to complex spatio/temporal flows is through successive bifurcations. The aim of this study is to describe the physics of the destabilization process of a periodic wind-driven flow associated with a secondary bifurcation. Although bifurcation theory has proven to be a valuable tool to determine the physical mechanisms of destabilization of fluid flows, the analysis of the stability of time-dependent (for example, periodic) flows, using this methodology, is computationally unpractical, due to the large number of degrees-of-freedom involved. The approach followed here is to construct a low-order model using numerical Galerkin projection of the full model equations onto the dynamically active eigenmodes. The resulting reduced model is shown to capture the local dynamics of the full model. The physical mechanism of the destabilization of the periodic wind-driven flow is deduced from the reduced model. While there are several stabilizing processes, notably rectification, the destabilization occurs due to time-dependent increase of the background horizontal shear in the flow.
Force detection using a fiber-optic cantilever
Authors: Budakian Raffi , Putterman Seth J.
Published: September 2002

A force measurement technique has been developed that utilizes a clamped fiber-optic element both as a cantilever and as a highly sensitive probe of the static and dynamic displacement of a sample that is mounted near its free end. Light from a 1.5 mW superluminescent diode coupled into the fiber is used to detect displacement with 6×10−13 m/Hz sensitivity for frequencies above 40 kHz. This technique has been used to study the interaction between two gold surfaces by simultaneously measuring the interaction force and contact stiffness as a function of the conductance and elongation of the contact. Excellent correspondence between steps in contact stiffness and electrical conductivity was observed.
Streaming flows in a channel with elastic walls
Authors: Shugan I. V. , Smirnov N. N. , Legros J. C.
Published: September 2002

The two-way coupling model for wave motion in a two-phase medium “fluid–elastic walls” accounting for interaction between those phases is considered within the frames of the boundary layer type approximation. An asymptotic linearized model including Navier–Stokes equations for the viscous incompressible fluid and the deformable plate motion equations for elastic walls is analyzed within a wide range of governing parameters variations. The dispersion relationship for different Reynolds numbers shows different oscillation regimes. The structure of the fluid flow is studied in detail for the two asymptotic limit cases of high and low Reynolds numbers. The results show that the intensity of fluid mass transfer induced in a fluid-filled channel with vibrating walls increases when increasing the vibrational Reynolds number. Contrary to the existing opinion, which considers the traveling wave regime of wall oscillations to be the basic mechanism generating the fluid flow, it is proved that the mean flux induced by standing waves could surpass the flux induced by the traveling waves in a range of one order of Reynolds number.
Ferroelectric (116) SrBi2Nb2O9 thin films epitaxially grown by pulsed laser deposition on epitaxial (110) Pt/(110) SrTiO3 electrode
Authors: Duclère J. R. , Guilloux-Viry M. , Perrin A. , Cattan E. , Soyer C. , Rèmiens D.
Published: September 2002

Epitaxial SrBi2Nb2O9 thin films have been grown by pulsed-laser deposition on (110)Pt bottom electrode epitaxially grown on (110) SrTiO3 by dc sputtering. X-ray φ-scans and electron channeling patterns reveal the epitaxial growth of the (116) SrBi2Nb2O9/(110)Pt bilayers. The hysteresis loop, dielectric response, and capacitance–voltage characteristics of this bilayer are presented. The remnant polarization was measured to be close to 5 μC/cm2, and the coercive field was calculated to be ∼120 kV/cm. The zero field permittivity was about 150 and the dielectric loss was ∼2%.
Experimental study of gravity-driven dense suspension jets
Authors: Nicolas Maxime
Published: September 2002

This article presents experimental results from a study of a jet of dense suspension falling under gravity in a quiescent liquid bath of miscible liquid. The initial jet velocity v0 scales with the square of the initial jet diameter. Four different flow behaviors are observed. The jet remains cylindrical and stable when viscous forces are dominant. A capillary-like instability with formation of blobs occurs when a Reynolds number based on the particle diameter and a free-falling velocity is over unity. The blobs are stable and settle without changing shape only for a blob Reynolds number below a critical number. Dispersion of the jet particles is observed when the particle Reynolds number is over 1, and an atomization behavior occurs when particle inertia is large compared to viscous forces, i.e., when the Stokes number of the particles is large compared to unity.
Potential model of a two-dimensional Bunsen flame
Authors: Denet Bruno
Published: September 2002

The Michelson–Sivashinsky equation, which models the nonlinear dynamics of premixed flames, has been recently extended to describe oblique flames. This approach was extremely successful to describe the behavior on one side of the flame, but some qualitative effects involving the interaction of both sides of the front were left unexplained. We use here a potential flow model, first introduced by Frankel, to study numerically this configuration. Furthermore, this approach allows us to provide a physical explanation of the phenomena occuring in this geometry by means of an electrostatic analogy.
Structure functions and energy dissipation dependence on Reynolds number
Authors: Boffetta G. , Romano G. P.
Published: September 2002

The dependence of the statistics of energy dissipation on the Reynolds number is investigated in an experimental jet flow. In a range of about one decade of Reλ (from about 200 to 2000) the adimensional mean energy dissipation is found to be independent on Reλ, while the higher moments of dissipation show a power-law dependence. The scaling exponents are found to be consistent with a simple prediction based on the multifractal model for inertial range structure functions. This is an experimental confirmation of the connection between inertial range quantities and dissipation statistics predicted by the multifractal approach.
Vector level identity for dynamic subgrid scale modeling in large eddy simulation
Authors: Morinishi Youhei , Vasilyev Oleg V.
Published: September 2002

The most commonly used dynamic subgrid scale model is based on the Smagorinsky eddy viscosity model with the model coefficient computed dynamically through the tensor level identity by Germano et al. However, the tensor level identity does not explicitly account for the effect of the discretization of the governing equations, and thus the computational results strongly depend on grid resolution, especially in a simulation with poor resolution. In this paper, we propose a new dynamic procedure with the vector level identity, which takes the effect of grid resolution into consideration. The new procedure is tested for the dynamic Smagorinsky eddy viscosity model with the vector level identity. All computational tests were done on turbulent channel flow. The numerical results confirm that the mean velocity profile computed using the new subgrid scale model does not depend on the grid resolution.
A new variational approach to gas flow in a rotating system
Authors: Babarsky Richard J. , Herbst Ira W. , Wood Houston G.
Published: September 2002

Onsager’s classic treatment of axisymmetric, stationary flow in a rapidly rotating centrifuge is based on the theorem of minimum entropy production. For the case of end-driven flows, this approach yields a sixth-order, self-adjoint equation (the “pancake” equation) for a master potential, χ, whose second-order radial derivative is equivalent to axial mass flow. This formulation allows for a straightforward application of mass flow conditions on the axial boundaries, and χ provides a theoretical foundation for describing axial mass-driven flow in a centrifuge. Alternatively, several authors have described axisymmetric flow in a centrifuge by a sixth-order partial differential equation for temperature. So far, however, there has been no established theoretical connection between Onsager’s minimum principle and the thermal problem. This topic is considered in this paper and the corresponding derivation also results in a self-adjoint problem, in this case in terms of a temperature potential, Φ. Moreover, it may be shown that the corresponding variational form of the thermal problem is equivalent to the original Onsager entropy integral. The resulting Euler equation in the temperature potential Φ gives rise to a radial operator, M6, which, in terms of the composition of fundamental third-order operators (i.e., M6=L3L3* where L3 is the radial shear operator and L3* is the radial heat flux operator), is the commutation of the original “pancake” operator, L6 (i.e., L6=L3*L3). Furthermore, based on this relationship, M6 and L6 are shown to be isospectral except for the point 0, which is an eigenvalue of M6 but not of L6.
Three-dimensional simulation of a Richtmyer–Meshkov instability with a two-scale initial perturbation
Authors: Cohen Ronald H. , Dannevik William P. , Dimits Andris M. , Eliason Donald E. , Mirin Arthur A. , Zhou Ye , Porter David H. , Woodward Paul R.
Published: September 2002

Three-dimensional high-resolution simulations (up to 8 billion zones) have been performed for a Richtmyer–Meshkov instability produced by passing a shock through a contact discontinuity with a two-scale initial perturbation. The setup approximates shock-tube experiments with a membrane pushed through a wire mesh. The simulation produces mixing-layer widths similar to those observed experimentally. Comparison of runs at various resolutions suggests a mixing transition from unstable to turbulent flow as the numerical Reynolds number is increased. At the highest resolutions, the spectrum exhibits a region of power-law decay, in which the spectral flux is approximately constant, suggestive of an inertial range, but with weaker wave number dependence than Kolmogorov scaling, about k−6/5. Analysis of structure functions at the end of the simulation indicates the persistence of structures with velocities largest in the stream-wise direction. Comparison of three-dimensional and two-dimensional runs illustrates the tendency toward forward cascade in three dimensions, versus inverse cascade in two dimensions. Comparison of the full simulation with a simulation of a single-scale perturbation indicates that the coupling of the disparate scales leads to destruction of the small-scale bubbles and spikes except near the spike growing from the large-scale perturbation. Finally, an analysis of the sub-grid-scale stresses in filtered data indicates significant correlation of the resultant forward and back transfer of energy with the determinant of the rate-of-strain tensor of the resolved scale flow. A possible relation between this trend and alignment of vorticity on small scales with the principal directions of strain on large scales is discussed. The observed correlation lends support to the use of sub-grid-scale models proportional to the determinant of the rate-of-strain tensor for large-eddy simulation.
Turbulent flow through a plane sudden expansion of modest aspect ratio
Authors: Escudier M. P. , Oliveira P. J. , Poole R. J.
Published: September 2002

The results are reported of an experimental investigation of turbulent flow through a plane sudden expansion of expansion ratio R=D/d=4 and aspect ratio A=w/h=5.33. It is well known that plane sudden expansions of high aspect ratio (A>10) with R greater than 1.5 produce asymmetric flows and this was again seen in this study. The literature for the asymmetric flow situation is surprisingly limited and only axial velocity and axial turbulence intensity results have been reported previously. A laser Doppler anemometer was used here to measure mean and rms axial velocities, U and u′, as well as the transverse mean and rms velocities, V and v′, and the Reynolds shear stress, uv¯. Not only was the mean flow found to be strongly asymmetric, but integration of the mean axial velocity profiles revealed significant departures from two dimensionality along the centerplane of the expansion duct. Results are reported at three spanwise locations to highlight this three dimensionality and qualitative arguments are made to relate this to the influence on corner vortices of the modest aspect ratio.
Direct numerical simulations of two-phase laminar jet flows with different cross-section injection geometries
Authors: Abdel-Hameed H. , Bellan J.
Published: September 2002

Direct numerical simulations are performed of spatial, three-dimensional, laminar jets of different inlet geometric configurations for the purpose of quantifying the characteristics of the flows; both single-phase (SP) and two-phase (TP) free jets are considered. The TP jets consist of gas laden with liquid drops randomly injected at the inlet. Drop evaporation ensues both due to the gaseous flow being initially unvitiated by the vapor species corresponding to the liquid drops, and to drop heating as the initial drop temperature is lower than that of the carrier gas. The conservation equations for the TP flow include complete couplings of mass, momentum, and energy based on thermodynamically self-consistent specification of the vapor enthalpy, internal energy, and latent heat of vaporization. Inlet geometries investigated are circular, elliptic, rectangular, square, and triangular. The results focus both on the different spreading achieved according to the inlet geometry, as well as on the considerable change in the flow field due to the presence of the drops. The most important consequence of the drop interaction with the flow is the production of streamwise vorticity that alters entrainment and species mixing according to the inlet geometry. Similar to their SP equivalent, TP jets are shown to reach steady-state entrainment; examination of the flows at this time station shows that the potential cores of TP jets are shorter by an order of magnitude than their SP counterpart. Moreover, whereas the TP circular jet exhibits a symmetric entrainment pattern well past the streamwise location of the potential core, noncircular jets display at the same location strong departures from symmetry. Furthermore, the SP-jet phenomenon of axis switching is no longer present in TP jets. The distributions of drop-number density, liquid mass, and evaporated species are compared for different inlet cross sections and recommendations are made regarding the optimal choice for different applications.
A stirrer for magnetohydrodynamically controlled minute fluidic networks
Authors: Qian Shizhi , Zhu Jianzhong , Bau Haim H.
Published: September 2002

Magnetohydrodynamics may potentially provide a convenient means for controlling fluid flow and stirring fluids in minute fluidic networks. The branches of such fluidic networks consist of conduits with rectangular cross sections. Each conduit has two individually controlled electrodes positioned along opposing walls and additional disk-shaped electrodes deposited in the conduit’s interior away from its sidewalls. The network is positioned in a uniform magnetic field. When one applies a potential difference between a disk-shaped electrode and two wall electrodes acting in tandem, circulatory motion is induced in the conduit. When the potential difference alternates periodically across two or more such configurations, complicated (chaotic) motions evolve. As the period of alternation increases, so does the complexity of the flow. We derive a two-dimensional, time-independent expression for the magnetohydrodynamic creeping flow around a centrally positioned disk-shaped electrode in the limit of zero radius. With the aid of this expression, the trajectories of passive tracers are computed as functions of the alternations protocol and the electrodes’ locations. The theoretical results are qualitatively compared with flow visualization experiments.
Visible luminescence from silicon surfaces microstructured in air
Authors: Wu C. , Crouch C. H. , Zhao L. , Mazur E.
Published: September 2002

We report visible luminescence from SiOx formed by microstructuring silicon surfaces with femtosecond laser pulses in air. Incorporation of oxygen into the silicon lattice occurs only where the laser beam strikes the surface. Laser microstructuring therefore offers the possibility of writing submicrometer luminescent features without lithographic masks. The amount of oxygen incorporated into the silicon surface depends on the laser fluence; the peak wavelength of the primary luminescence band varies between 540 and 630 nm and depends on the number of laser shots. Upon annealing, the intensity of the primary luminescence band increases significantly without any change in the luminescence peak wavelength, suggesting that the luminescence comes from defects rather than quantum confinement.
Exchange-coupled Sm–Co/Nd–Co nanomagnets: correlation between soft phase anisotropy and exchange field
Authors: Guo Z. J. , Jiang J. S. , Pearson J. E. , Bader S. D. , Liu J. P.
Published: September 2002

The effect of the magnetic anisotropy of the Nd–Co soft phase on its exchange field (Hex) is reported for epitaxial Sm–Co/Nd–Co bilayers. It is found that Hex gradually increases with anisotropy K of the soft phase. The experimental values of Hex as well as its variation with K are quantitatively interpreted using an analytical model based on the formation of a partial domain wall on the soft phase side of the interface. The results suggest that one can enhance Hex, and hence, the volume fraction of the soft phase for effective exchange spring coupling between the hard and soft phases, by tailoring the anisotropy of the soft phase.
Effect of 3 MeV electron irradiation on the photoluminescence properties of Eu-doped GaN
Authors: Nakanishi Y. , Wakahara A. , Okada H. , Yoshida A. , Ohshima T. , Itoh H.
Published: September 2002

The effect of 3 MeV electron irradiation on the photoluminescence (PL) properties of Eu-doped GaN was investigated. Eu was introduced into GaN epitaxial layers grown on sapphire substrates by ion implantation. The peak concentration of implanted Eu was found to be a few atomic percent. The electron dose was in the range of 1016–3×1017 cm−2. PL was measured in the temperature range of 13–295 K by using a He–Cd laser as the excitation source. PL intensity corresponding to the transition of D05−F27 in Eu3+ was hardly dependent on the electron fluence. In contrast, the PL intensity of the near-band-edge emission from undoped GaN decreased when increasing the electron fluence.
Polarity determination for GaN thin films by electron energy-loss spectroscopy
Authors: Kong X. , Hu G. Q. , Duan X. F. , Lu Y. , Liu X. L.
Published: September 2002

The intensity of the N K edge in electron energy-loss spectra from a GaN thin film shows a pronounced difference when the orientation of the film approaches the (0002) and (000-2) Bragg reflections, along the polar direction. This experimental result can be interpreted by the effect associated with interference between the Bloch waves of the incident electron in the GaN crystal. The theoretical calculations indicate that, at the Bragg condition of g=0002 along the Ga–N bond direction, the thickness-averaged electron current density on the N atom plane is much higher than that at g=0002̄, with a maximum as the specimen thickness is about 0.4ξ0002 (the two-beam extinction distance). The delocalization effect on the experimental spectra is also discussed.
Zinc oxide nanowires on carbon nanotubes
Authors: Kim Hansoo , Sigmund Wolfgang
Published: September 2002

Multiwalled nanotubes (MWNTs) were heat-treated with zinc (Zn) at various temperatures. Hereby, zinc oxide (ZnO) was produced on the outermost shells of MWNTs in the forms of ultrathin films, quantum dots, or nanowires/nanorods. ZnO nanowires were grown on the surface of MWNTs without the presence of a catalyst, suggesting the vapor–liquid–solid mechanism is not applicable for the current growth process. The average diameter of nanowires on MWNTs prepared at 600 °C was about 30 nm and the length was up to 1.5 μm, yielding aspect ratios of 7 to 50. ZnO nanorods grown on MWNTs at 800 °C had diameters from 80 to 150 nm with lengths from 0.5 to 1 μm (aspect ratios of 6 to 10).
On a possible direct effect of the eddy viscosity gradient in turbulence modeling
Authors: Germano Massimo
Published: September 2002

In this paper a possible direct effect of the eddy viscosity gradient in modeling the subgrid scale turbulent stress is argued on the grounds of simple heuristic arguments based on a multilevel decomposition of the turbulent field and on the gradient approximation.
Using ultrathin elastomeric stamps to reduce pattern distortion in microcontact printing
Authors: Tormen Massimo , Borzenko Tatiana , Steffen Bernd , Schmidt Georg , Molenkamp Laurens W.
Published: September 2002

We demonstrate that ultrathin (thickness below 1 μm) elastomeric stamps, directly fabricated on rigid substrates can be reliably used in microcontact printing (μCP), showing enhanced mechanical properties with respect to the thicker ones commonly utilized. Macroscopic distorsion (due to thermal expansion and external stress) and microscopic effects such as the membrane relaxation in wide unsupported areas are drastically reduced. A mix and match process (optical lithography followed by μCP) performed with submicron overlay accuracy proves the reliability of the present approach and its applicability to multilevel production processes.
Infrared dielectric response function of strained In1−xGaxAs/InP epilayers
Authors: Yu G. , Rowell N. L. , Lockwood D. J. , Poole P. J.
Published: September 2002

Polarized infrared reflectivity measurements have been performed on strained In1−xGaxAs epilayers grown by chemical beam epitaxy on semi-insulating InP(100) with x varied from 0.314 to 0.556. Fitting of the reflectance spectra yielded excellent agreement with three longitudinal and transverse optical phonon pairs, corresponding to GaAs-like, disorder-induced, and InAs-like modes. The dielectric response function and phonon frequency dependences for all modes were obtained versus Ga fraction.
Chemical bonding of hydrogen in MgH2
Authors: Noritake T. , Aoki M. , Towata S. , Seno Y. , Hirose Y. , Nishibori E. , Takata M. , Sakata M.
Published: September 2002

MgH 2 is one of the promising base materials for hydrogen storage, which is a key technology of clean energy source. In this study, the bonding nature of hydrogen in MgH2 was fully uncovered by examining the charge density distribution of this substance obtained by the maximum entropy method from the synchrotron radiation powder data. MgH2 can be expressed as Mg1.91+ H0.26−, which is much weaker ionicity than the theoretical expectations. It also shows weak covalence between Mg and H. Though the bonding nature of hydrogen in MgH2 is rather complex, i.e., the mixture of ionic and covalent bonding, it is certain that hydrogen is weakly bonded to Mg, which must be a big advantage of hydrogenation–dehydrogenation of this substance.
Submicron imaging of buried integrated circuit structures using scanning confocal electron microscopy
Authors: Frigo Sean P. , Levine Zachary H. , Zaluzec Nestor J.
Published: September 2002

Two-dimensional images of model integrated circuit components were collected using the technique of scanning confocal electron microscopy. For structures embedded about 5 μm below the surface of a silicon oxide dielectric, a lateral resolution of 76±9 nm was measured. Elemental mapping via x-ray emission spectrometry is demonstrated. A parallax analysis of images taken for various tilt angles to the electron beam allowed determination of the spacing between two wiring planes. The results show that scanning confocal electron microscopy is capable of probing buried structures at resolutions that will be necessary for the inspection of next-generation integrated circuit technology.
Contamination due to memory effects in filtered vacuum arc plasma deposition systems
Authors: Martins D. R. , Salvadori M. C. , Verdonck P. , Brown I. G.
Published: September 2002

Thin film synthesis by filtered vacuum arc plasma deposition is a widely used technique with a number of important emerging technological applications. A characteristic feature of the method is that during the deposition process not only is the substrate coated by the plasma, but the plasma gun itself and the magnetic field coil and/or vacuum vessel section constituting the macroparticle filter are also coated to some extent. If then the plasma gun cathode is changed to a new element, there can be a contamination of the subsequent film deposition by sputtering from various parts of the system of the previous coating species. We have experimentally explored this effect and compared our results with theoretical estimates of sputtering from the stopping and range of ions in matter code. We find film contamination of the order of 10−4–10−3, and the memory of the prior history of the deposition hardware can be relatively long lasting.
Radiography of nonaxisymmetric objects: An onion-peeling inversion method
Authors: Schwierz-Iosefzon T. , Notea A. , Deutsch M.
Published: September 2002

An onion-peeling method for obtaining the linear attenuation coefficient distribution within a body from a single radiographic projection is presented. Unlike previous methods, which are applicable only to axi- or centrosymmetric objects, ours requires only mirror symmetry relative to the plane of the radiograph. An example of the use of the method is presented and discussed.
High critical current density and improved irreversibility field in bulk MgB2 made by a scaleable, nanoparticle addition route
Authors: Wang J. , Bugoslavsky Y. , Berenov A. , Cowey L. , Caplin A. D. , Cohen L. F. , MacManus Driscoll J. L. , Cooley L. D. , Song X. , Larbalestier D. C.
Published: September 2002

Bulk samples of MgB2 were prepared with 5, 10, and 15 wt % Y2O3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ∼3–5 nm YB4 nanoparticles embedded within MgB2 grains of ∼400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density JC was observed, most notably for 10% doping. At 4.2 K, the lower bound JC value was ∼2×105 A cm−2 at 2 T. At 20 K, the corresponding value was ∼8×104 A cm−2. Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K.
High-energy electrons near the dielectric-plasma boundary in a large-area surface-wave plasma excited at 915 MHz
Authors: Nagatsu Masaaki , Niwa Takahiko , Sugai Hideo
Published: September 2002

High-energy electrons produced in a planar ultrahigh frequency surface-wave plasma excited at 915 MHz have been studied using the Druyvesteyn technique of Langmuir probe measurements. Near the dielectric-plasma boundary, the hump of energetic electrons was clearly observed in the electron energy probability functions. The hump steeply shifted its center energy from about 20 eV at z=0.2 cm to almost zero at z∼1.4 cm apart from the quartz window, while the bulk electrons were strongly heated there. Peak energy and width of the hump seemed to have a strong spatial correlation with the electric field intensities.
Combinatorial study of the long-term stability of organic thin-film solar cells
Authors: Hänsel Helmut , Zettl Heiko , Krausch Georg , Schmitz Christoph , Kisselev Roman , Thelakkat Mukundan , Schmidt Hans-Werner
Published: September 2002

We present a setup that allows us to combinatorially study organic multilayer devices for durability and degradation mechanisms. Arrays of 8×8 thin-film organic devices are prepared by vacuum deposition and investigated under vacuum conditions. Measurements of nine types of organic solar cells are presented. Continuous solar-like illumination at 80 mW/cm2 and a temperature of 80 °C provide realistic testing conditions. The degradation of solar cells has been investigated as a function of layer combination and layer thickness over a period of 800 h. Different time scales of degradation are identified and attributed to the layer structure of the devices.
CuGaSe
2
solar cell cross section studied by Kelvin probe force microscopy in ultrahigh vacuum
Authors: Glatzel Th. , Marrón D. Fuertes , Schedel-Niedrig Th. , Sadewasser S. , Lux-Steiner M. Ch.
Published: September 2002

Kelvin probe force microscopy under ultrahigh vacuum conditions has been used to image the electronic structure of a Mo/CuGaSe2/CdS/ZnO thin film solar cell. Due to the high energy sensitivity together with a lateral resolution in the nanometer range we obtained detailed information about the various interfaces within the heterostructure. The absolute work function of the different materials was measured on a polished cross section. To obtain a flat and clean surface we optimized the sputtering process with Ar ions. The presence of an additional MoSe2 layer between the Mo backcontact and the CuGaSe2 absorber layer was observed.
Properties of AlxGa1−xN layers grown by plasma-assisted molecular-beam epitaxy under Ga-rich conditions
Authors: He L. , Reshchikov M. A. , Yun F. , Huang D. , King T. , Morkoç H.
Published: September 2002

Al x Ga 1−x N films were grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates under Ga-rich conditions. To control the AlxGa1−xN composition over the entire range, the Al and Ga arrival rates were fixed while the nitrogen arrival rate was varied. We have found that the Al fraction increased with decreasing N flow due to preferentially favorable bonding of Al and N over Ga and N. Consequently, the growth rate decreased as the Al mole fraction increased. A photoluminescence quantum efficiency at 15 K was markedly higher for the AlxGa1−xN layers grown under Ga-rich conditions (3%–48%) compared to the layers grown under N-rich conditions (1%–10%), indicating much reduced nonradiative recombination in samples grown under Ga-rich conditions.
Dielectric property and thermal stability of HfO2 on silicon
Authors: Lin Y.-S. , Puthenkovilakam R. , Chang J. P.
Published: September 2002

A stoichiometric, uniform, and amorphous hafnium oxide thin film is deposited by an atomic layer deposition process. The as-deposited hafnium oxide films showed superior electrical properties compared to zirconium oxides, including a dielectric constant of 23, a flatband voltage shift of +0.3 V, a hysteresis of 25 mV, an interfacial trap density of 1.8×1011 cm−2 eV−1, and a leakage current density several orders of magnitude lower than SiO2 at an equivalent oxide thickness of 9.3 Å, suitable for metal–oxide–semiconductor device applications. The thermal stability of hafnium oxide on silicon was determined to be better than that of zirconium oxide. Post-deposition annealing in oxygen and ammonia further improved the thermal stability of HfO2 to 1000 and 1100 °C, respectively.
Spherulitic growth kinetics of protein crystals
Authors: Chow P. S. , Liu X. Y. , Zhang J. , Tan R. B. H.
Published: September 2002

The spherulitic growth mechanism of lysozyme crystals from aqueous solutions was examined quantitatively by taking advantage of a “buffer zone” of liquid–liquid phase separation occurring in lysozyme aqueous solution. This buffer zone technique allows us to obtain constant concentrations in both liquid phases during the whole process of crystallization from solution. Under such a constant concentration (or supersaturation) condition, a constant growth rate along the radial direction of lysozyme spherulites was obtained for the spherulitic growth from solution. This evidently indicates that the growth of lysozyme spherulites is controlled by surface kinetics rather than by volume diffusion. In addition, it was found that microscopic droplets resulted from phase separation, and heterogeneous impurities normally serve as cores causing the formation of the radial pattern of spherulites.
Effect of structure ordering on charge carrier mobilities in green-emitting poly(phenylene vinylene)s
Authors: Yu Liang-Sheng , Tseng Hao-En , Lu Hsin-Hung , Chen Show-An
Published: September 2002

Charge transport behaviors and performances of electroluminescent (EL) devices of the two greenemitting poly(phenylene vinylene)s, the homopolymer of 2-[3′-(3,7-dimethyloctyloxy)phenylene]p-phenylene-vinylene and the copolymer of 50% by mole 2-[4′-(3,7-dimethyloctyloxy)phenylene]p-phenylene-vinylene and 50% 2-[3′-(3,7-dimethyloctyloxy)phenylene]-p-phenylene-vinylene are investigated. The former is found to have more ordered chain alignment than the latter. Such ordered alignment leads to significant increases of the mobilities of charge carriers, especially for that of an electron, which is promoted to a level equivalent to that of a hole. The balance of electron and hole mobility could be the reason for its high brightness (100 000 cd/m2) and efficiency (12 cd/A) of the EL device.
An experimental study of the breakage of liquid bridges at stability limit of minimum volume
Authors: Espino J. L. , Meseguer J. , Laverón-Simavilla A.
Published: September 2002

An experimental apparatus to study the breaking process of axisymmetric liquid bridges has been developed, and the breaking sequences of a large number of liquid bridge configurations at minimum-volume stability limit have been analyzed. Experimental results show that very close to the breaking moment the neck radius of the liquid bridge varies as t1/3, where t is the time to breakage, irrespective of the value of the distance between the solid disks that support the liquid column.
Probing the step structure of buried metal/semiconductor interfaces using quantized electron states: The case of Pb on Si(111) 6×6-Au
Authors: Yu Hongbin , Jiang C.-S. , Ebert Ph. , Shih C.-K.
Published: September 2002

The three-dimensional step structure at the buried Pb on Si(111) 6×6-Au interface is determined by utilizing the presence of quantum well states. We demonstrate that the spatial step positions as well as the step heights can be extracted nondestructively and with atomic layer precision by scanning tunneling microscopy and spectroscopy.
Characterization of surface layers in Zn-diffused LiNbO3 waveguides by heavy ion elastic recoil detection
Authors: Espeso-Gil O. , Garcı́a G. , Agulló-López F. , Climent-Font A. , Sajavaara T. , Domenech M. , Cantelar E. , Lifante G.
Published: September 2002

The surface layers formed in LiNbO3 waveguides, fabricated by Zn diffusion from the vapor phase, have been investigated by time of flight elastic recoil detection analysis using I127 ions. The key features of this technique, simultaneous profiling of all ions and a depth of analysis <1 μm, have allowed a detailed and quantitative characterization of the surface layers. The Zn diffusion into LiNbO3 can be understood as a 2Li↔Zn exchange process. As a consequence, an outermost layer of several hundreds of nanometers is formed, consisting of LiNbO3 and ZnNb2O6 phases showing complementary profiles. A good correlation has been found between the composition profiles and the optical waveguiding behavior. After thermal annealing of the waveguides, a thinner layer containing a uniform mixture of ZnO and LiNbO3 is generated followed by a transition to a graded solid solution of Zn into LiNbO3.
Observation of enhanced transport in carbon-doped InGaAsN after in situ anneal and its impact on performance of NpN InGaP/InGaAsN heterojunction bipolar transistors
Authors: Monier C. , Baca A. G. , Sun S. Z. , Armour E. , Newman F. , Hou H. Q.
Published: September 2002

The incorporation of a low band gap carbon-doped InGaAsN material into a standard GaAs heterojunction bipolar transistor (HBT) has great potential to achieve higher operating efficiency at lower bias conditions. In order to improve the performance of the initial as-grown InGaAsN-based HBTs (with 1% N and 3% In for an energy band gap EG of 1.2 eV), the effects of different thermal treatments on material quality and their impact on dc and rf characteristics on small-area devices are examined in this letter. A degradation of the base transport is observed after a postgrowth anneal associated with lower current gain β and degraded microwave properties. An in situ anneal approach under inert ambient conducted following the emitter growth, increases the active doping level (with a base sheet resistance RSH three times lower than the as-grown structure and demonstrates suitable high frequency performance for a device with such a large amount of nitrogen in the base.
Analysis of heavily tailed size distributions of ZnTe/ZnSe quantum dot structures by using the bootstrap methodology
Authors: Lee Johnson , Chou W. C. , Jan G. J.
Published: September 2002

We have used the bootstrap methodology to analyze dot size distributions of ZnTe quantum dot (QD) structures. The photoluminescence (PL) spectrum indicates that the ZnTe QD structure belongs to a type-II band alignment. The broadness with small fluctuations in the PL represents the spatial inhomogeneity of the QD sizes. The Schrödinger equation together with the first-order perturbation correction was numerically solved to correlate the dot size and the photon energy. Using the bootstrap “loess” curve fitting method, the PL spectrum was determined to be a normal distribution with a high significance level of 46% tested by a null hypothesis H0. By examining the slope of the complementary cumulative distribution function, we found that the size distribution is heavy tailed.
Fabrication and characterization of gated field emitter arrays with self-aligned carbon nanotubes grown by chemical vapor deposition
Authors: Han In Taek , Kim Ha Jin , Park Young-Jun , Lee Naesung , Jang Jae Eun , Kim Jung Woo , Jung Jae Eun , Kim Jong Min
Published: September 2002

Field emitter arrays with multiwall carbon nanotubes (CNTs) grown inside their gated holes were fabricated on glass substrates. The Fe–Ni–Co alloy catalyst dots on which the CNTs would be grown were deposited into the gated holes by a self-aligned method to maintain a constant distance between CNT emitters and gate electrodes. The CNTs were synthesized by thermal chemical vapor deposition using a gas mixture of CO and H2 at 500 °C. The CNT lengths were controlled by changing ratios of CO to H2. Field emission currents and images were monitored as a function of gate and anode voltages. It was shown that the CNT emitters grown just up to the gate electrode height operated best in a triode mode.
Characterization of a diamond p-n junction using electron-beam-induced current and cathodoluminescence
Authors: Sekiguchi Takashi , Koizumi Satoshi
Published: September 2002

We have characterized a diamond p-n junction by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL). The diamond p-n junction was fabricated by growing a B-doped p-type layer and a P-doped n-type layer on the {111} diamond substrate by microwave plasma enhanced chemical vapor deposition. The cross section of p-n junction was revealed by the mesa etching. The substrate, B- and P-doped layers were distinguished by CL spectra. The EBIC profiles across the p-n junction were recorded under various reverse bias conditions. These data confirmed that the p-n junction was actually formed at the interface between B- and P-doped layers. The energy-band profile suggests that the carrier concentration of P-doped layer is more than ten times higher than that of B-doped layer. The observed results strongly support the fact that the UV luminescence is emitted from the p-n junction region in diamond light-emitting diode.
Holographic data storage on nonphotosensitive glass with a single femtosecond laser pulse
Authors: Li Yan , Watanabe Wataru , Itoh Kazuyoshi , Sun Xiudong
Published: September 2002

Holographic data storage on fused silica, soda-lime, and lead glasses with a single 130 fs laser pulse at a wavelength of 800 nm is presented. After the sample is exposed to the interference fringe pattern of the object beam and the reference beam, a relief microhologram is recorded through surface ablation. The recorded information can be reconstructed without destruction of the hologram when the power of the reference beam is reduced below the ablation threshold. The preliminary experimental results demonstrate the possibility of holographic data storage on nonphotosensitive glasses.
Design and fabrication of impurity band-based photonic crystal waveguides for optical delay lines
Authors: Sugimoto Y. , Lan S. , Nishikawa S. , Ikeda N. , Ishikawa H. , Asakawa K.
Published: September 2002

We have designed, fabricated, and characterized efficient optical delay lines for ultrashort pulses based on photonic crystal waveguides formed by coupled defects. The basic structure consists of one-dimensional cylindrical air holes formed in silicon on silicon dioxide ridge waveguides. The defects are created by intentionally increasing the separation of two neighboring air holes. Through proper design, quasiflat impurity bands located at ∼1.55 μm can be successfully obtained. In addition, the wavelength tunability of the impurity bands was clearly demonstrated. Good agreement was found between the experimental observations and numerical simulations.
Femtosecond nonlinear optical properties of carbon nanoparticles
Authors: Li Dan , Liu Ye , Yang Heqing , Qian Shixiong
Published: September 2002

The nonlinear optical properties and ultrafast electron-relaxation dynamics of carbon nanoparticles were investigated by using the femtosecond optical Kerr effect and pump-probe techniques. The blueshift of the absorption edge with the decrease of the size of the nanoparticles reveals the opening of the gap. The magnitude of χ(3) for carbon nanoparticles is calculated to be 8.3×10−13 esu, which arises from the contribution of delocalized feature of the π electrons. The decay of photobleaching includes a fast and a slow component, which are assigned to the relaxation of the free carriers and trapped carriers, respectively. It is found that the lifetimes of two components of bleaching decrease as temperature of heat treatment is increased.
Ultrafast frequency-selective optical switching based on thin self-assembled organic chromophoric films with a large second-order nonlinear response
Authors: Wang Gang , Zhu Peiwang , Marks Tobin J. , Ketterson J. B.
Published: September 2002

Thin films consisting of self-assembled chromophoric superlattices exhibit very large second-order nonlinear responses [χ(2)]. Using such films, a “static” diffraction grating is created by the interference of two coherent infrared beams from a pulsed yttritium–aluminum–garnet laser. This grating is used to switch the second-harmonic and third-harmonic “signal” beams (generated from the fundamental “pump” beam or mixed within the chromophoric superlattice) into different channels (directions). Ultrafast switching response as a function of the time overlap of the pumping beams is demonstrated. It is suggested that such devices can be used to spatially and temporally separate signal trains consisting of pulses having different frequencies and arrival times.
Alternative mechanism for misfit dislocation generation during high-temperature Ge(Si)/Si (001) island growth
Authors: Zou J. , Liao X. Z. , Cockayne D. J. H. , Jiang Z. M.
Published: September 2002

The misfit dislocations in [001] Ge(Si)/Si islands grown at 700 °C were investigated using transmission electron microscopy. 30° partial misfit dislocations are found both in the island/substrate interface and near the island surface. Since the 30° partial leads the movement of the 60° dissociated misfit dislocation in a (001) compressively strained system such as (001) GeSi/Si, a generation mechanism of misfit dislocations through partial misfit dislocations half loops is proposed.
Electron temperature in a sodium–iodide lighting plasma
Authors: Karabourniotis D. , Drakakis E.
Published: September 2002

The time-dependent electron temperature at the axis of an ac mercury–sodium–iodide arc discharge is determined experimentally. It modulates in phase with the discharge voltage and differs in modulation and magnitude from the temperature obtained assuming thermodynamic equilibrium. The method used is independent of plasma equilibrium assumptions as well as of line-transition probabilities, and since it is based on relative emission measurements, it can, in principle, be applied to discharges contained within translucent envelopes.
Laser-induced surface potential transients observed in III-nitride heterostructures
Authors: Koley G. , Cha Ho-Young , Thomas C. I. , Spencer M. G.
Published: September 2002

We report on very long surface potential transients induced by ultraviolet laser illumination, which have been observed in nitride heterostructures. These surface potential transients correlate with current transients when measured simultaneously. Under illumination, electron–hole pairs are generated, which screen the electric field in the AlGaN barrier layer causing the surface potential to decrease. The holes move toward the surface assisted by the electric field in the AlGaN while the electrons increase the sheet charge concentration at the AlGaN/GaN interface, decreasing the net charge dipole across the barrier layer. This reduction can cause the surface potential to vary as much as 1 V between the unilluminated and illuminated states. The long transient response observed after the laser is turned off is explained by the slow recombination of the holes with the electrons thermionically emitted from the potential well at the interface. The thermionic emission is modeled by a continuously varying barrier potential, which results in a stretched exponential-type response.