bluuebunny/crossref_metadata_2025 huggingface dataset:
Water mediated attraction between repulsive ions: A cluster-based simulation approach
Authors: Keasler Samuel J. , Nellas Ricky B. , Chen Bin
Published: October 2006

Could two like ions be attractive to each other in the presence of water? To address this question and to further interrogate the intriguing solvent effects at a molecular level on multiply charged species, a “bottom-up” simulation approach was formulated, from which the inter-ionic potential of mean force and other properties were monitored closely with the gradual addition of the water molecules. This approach was first tested on a commonly studied ion pair (namely, Na+ and Cl−), where excellent agreement with the published bulk-phase data was found. Further application of this approach to the like-ion pair indicated that an attractive interaction between two anions or two cations can be induced by the addition of an appropriate number of water molecules. This result corroborates a recent experimental report of an intriguing folding of a dianionic polymer into a more compact structure with the addition of water molecules in gas phase as well as previous theoretical findings of possible attraction between like-ion pairs in bulk aqueous phases.
Accurate circuit analysis of resonant-type left handed transmission lines with inter-resonator coupling
Authors: Gil I. , Bonache J. , Gil M. , García-García J. , Martín F. , Marqués R.
Published: October 2006

In this paper, a circuit model for the description of left handed transmission lines based on complementary split rings resonators (CSRRs) is proposed. As compared to previous models, coupling between adjacent resonators is included in the present work. The conditions that make this coupling significant are discussed. Specifically, it will be shown that it is barely present when circular CSRRs are used to implement the left handed transmission line. However, if the line is loaded with rectangular CSRRs separated by a small distance, inter-resonator coupling is important and it significantly influences the electromagnetic behavior of the structures. It will be also shown that under low or moderate coupling, it is possible to describe the structures by means of a simplified model with modified parameters. Several prototype devices with different CSRR topologies and coupling levels have been fabricated to illustrate the phenomenology associated with these structures and the accuracy of their model descriptions. The results of this work can be of interest for the design of planar microwave circuits based on CSRR left handed lines.
Comment on “Contact mechanisms and design principles for alloyed Ohmic contacts to n-GaN” [J. Appl. Phys. 95, 7940 (2004)]
Authors: Lin Yow-Jon
Published: October 2006

The contact mechanism and design principles for alloyed Ohmic contacts to n-GaN were investigated in Mohammad’s paper [J. Appl. Phys. 95, 7940 (2004)]. Mohammad’s study demonstrated that both tunneling and thermionic emission were equally important for low resistivity at the metals/n-GaN interfaces. As regards this Comment, we point out the fundamental errors of this interpretation. In addition, we find that the results shown in Figs. 3–6 of Mohammad’s paper are incorrect because the lower effective barrier height (BH) of less than 0eV (due to the induced BH reduction by image force lowering or band gap narrowing) and a variable (c1) were neglected by the author.
Dielectric study on the flow alignment in 4-n-pentyl-4′-cyanobiphenyl
Authors: Negita K. , Nishikawa T. , Inamasu Y.
Published: October 2006

Dielectric permittivity and loss are measured under steady shear flow as functions of temperature, shear rate, electric field frequency, and electric field strength in the nematic (N) and the isotropic (I) phases of 4-n-pentyl-4′-cyanobiphenyl. In the N phase, the dielectric permittivity in the quiescent state is largely modified if the steady shear flow is applied. These behaviors are discussed based on the Leslie-Ericksen theory [Q. J. Mech. Appl. Math. 19, 357 (1966); Arch. Ration. Mech. Anal. 4, 231 (1960)], showing that the dielectric properties under the shear flow are consistently interpreted in terms of the flow alignment of the director, a unit vector specifying the orientation of the liquid crystals. It is also suggested that the behaviors of dielectric permittivities are similar to those of the viscosities.
Functional perfluoropolyether spreading on a solid substrate
Authors: Li Xin , Hu Yuanzhong , Wang Hui
Published: October 2006

Molecular dynamics simulations based on a coarse-grained, bead-spring model are adopted to investigate the spreading of functional perfluoropolyether on a solid substrate. The spreading phenomenon has been described by the thickness profiles and molecule movements which evolve with time. The spreading mechanism has been analyzed in terms of both the driving term from the gradient of disjoining pressure and the resistance term from the bead-substrate friction and the film viscosity by dividing the spreading film into three regions.
n
-type emitter surface passivation in c-Si solar cells by means of antireflective amorphous silicon carbide layers
Authors: Ferre R. , Martín I. , Ortega P. , Vetter M. , Torres I. , Alcubilla R.
Published: October 2006

Emitter saturation current densities (JOe) of phosphorus-diffused planar c-Si solar cell emitters passivated by silicon carbide (SiCx) layers have been determined in a wide sheet resistance range (20–500Ω∕sq). Phosphorus diffusions were performed using solid planar diffusion sources without employing any drive-in step. Stacks of two SiCx layers were deposited by plasma enhanced chemical vapor deposition: first a thin silicon rich layer with excellent passivating properties and then an antireflective carbon rich layer. The thickness of the passivating layer was optimized, reaching a trade-off between the better passivation achieved for thicker layers and the increased light absorption within the layer, which reduced the photocurrent. The surface recombination velocity and the optical losses were determined for each configuration and used to calculate photovoltaic conversion efficiency limits for 50 and 90Ω∕sq emitters. In both cases, optimum configuration is for the stacks with passivating layers that are about 8nm thick.
Rb
87
NMR relaxation of RbCdCl3 and Rb4CdCl6 single crystals with the electric-quadrupole-type interactions
Authors: Lim Ae Ran , Han Tae Jong , Noiret I.
Published: October 2006

The Rb87 spin-lattice relaxation time T1 and spin-spin relaxation time T2 of RbCdCl3 and Rb4CdCl6 single crystals grown by the slow evaporation method were measured over the temperature range of 160–410K. In the case of the RbCdCl3 crystal, changes were observed in the Rb87 T1−1 near 340, 363, and 395K, which correspond to phase transitions. The abrupt decrease in T1−1 observed at 395K is due to a shortening in the c direction as a result of a phase transition from a tetragonal to a cubic structure. We suggest that the cubic Rb environment is favored above 395K, which causes relaxation and averages out the quadrupolar splittings. On the other hand, the Rb87 T1 in Rb4CdCl6 decreased with increasing temperature, and no phase transitions were observed in the temperature range considered. For both RbCdCl3 and Rb4CdCl6, the relaxation rate of Rb was found to be proportional to the square of the temperature, indicating that the relaxation proceeds via a Raman process. This behavior for the electric-quadrupolar-type RbCdCl3 and Rb4CdCl6 crystals stands in contrast to a previous report that the relaxations in Cs2CuCl4 and Cs2CoCl4 crystals with the magnetic relaxation type proceed via single-phonon processes. Therefore, the relaxation mechanism in crystals with an electric quadrupolar NMR interaction mechanism is completely different from that in crystals with magnetic-type NMR behavior.
Surface characteristics of epitaxially grown Ni layers on Al surfaces: Molecular dynamics simulation
Authors: Lee Soon-Gun , Chung Yong-Chae
Published: October 2006

The deposition behavior for Ni thin film growth on Al substrates of various orientations according to the incident energy of adatoms was investigated by molecular dynamics simulation. In spite of the low adatom incident energy of 0.1eV, Ni–Al intermixing occurred actively at the surfaces of Al(001), Al(011), and Al(111) at 80K and Ni atoms apparently favored the island growth mode irrespective of the Al surface orientation. The highest surface roughness was shown for the case of Al(111) surface. The steering effect, which results in rougher surface, was significantly observed at low incident energy. The steering effect was quantitatively investigated through the extensive measurement of the trajectory and deposition flux of atoms with the variation of incident energy near the artificially structured Ni step positioned on Al surfaces.
Effective multipoles and Yukawa electrostatics in dressed molecule theory
Authors: Ramirez Rosa , Kjellander Roland
Published: October 2006

In this paper we derive the multipolar expansion of the screened Coulomb potential in electrolyte solutions with molecular solvent. The solute and solvent molecules can have arbitrary sizes, shapes, and internal charge distributions. We use the exact statistical mechanical definition of renormalized charge distributions coming from “dressed molecule theory” to determine the effective multipoles of a molecule immersed in an electrolyte. The effects of many-body correlations are fully included in our formally exact theory. We restrict ourselves to sufficiently dilute solutions so the screened Coulomb potential decays for large distances like a Yukawa function, exp(−κr)∕r, where r is the distance and 1∕κ is the decay length (it is normally different from the Debye length). The resulting “Yukawa electrostatics” differ in many respects from ordinary, unscreened electrostatics. The “Yukawa charge” of a molecule (the lowest order moment in the multipolar expansion) is in general not equal to its Coulombic charge and it is not the integral of the renormalized charge distribution of the molecule. Moreover, as shown in this paper, the multipolar expansion of the Yukawa potential does not correspond, contrary to the case of the Coulomb potential, to its asymptotic expansion for large r. As a consequence, the charge term in the multipolar expansion is not the leading term in the asymptotic expansion. Instead, for large r values, multipoles of all orders contribute to the leading asymptotic term. Thus, the electrostatic potential from, for example, an electroneutral solvent molecule in an electrolyte solution has generally the same range as that from an ion. The proper asymptotic expansion for electrostatic interactions in electrolytes is derived. It is briefly shown how the multipole expansion formalism can also be applied in the Poisson-Boltzmann approximation for primitive model electrolytes.
Large remnant polarization and excellent fatigue property of vanadium-doped SrBi4Ti4O15 thin films
Authors: Sun Hui , Zhu Jun , Fang Hong , Chen Xiao-bing
Published: October 2006

We report the enhancement of electric and fatigue-endurance properties of vanadium-doped SrBi4Ti4O15 (SBTi) thin films deposited by a refined sol-gel method. The structures and surface morphologies of the films were characterized by x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Their electric properties were investigated systematically. Compared to the undoped SBTi, V-doped SBTi [(SrBi3.99Ti3.97V0.03O15) (SBTV)] showed a larger remnant polarization (2Pr), lower coercive electric field, lower leakage current density, and a better fatigue resistance. The 2Pr of SBTV was 35.9μC∕cm2, which was much higher than that of SBTi (25.3μC∕cm2). Importantly, at a low frequency of 50kHz, the SBTV film showed no variation of Pnv and −Pnv after 2.2×109 switching cycles, suggesting excellent fatigue-endurance characteristics.
Noise considerations in field-effect biosensors
Authors: Deen M. J. , Shinwari M. W. , Ranuárez J. C. , Landheer D.
Published: October 2006

Field-effect sensors used to detect and identify biological species have been proposed as alternatives to other methods such as fluorescence deoxyribonucleic acid (DNA) microarrays. Sensors fabricated using commercial complementary metal-oxide-semiconductor technology would enable low-cost and highly integrated biological detection systems. In this paper, the small-signal and noise modeling of biosensors implemented with electrolyte-insulator-semiconductor structures is studied, with emphasis on design guidelines for low-noise performance. In doing so, a modified form of the general charge sheet metal-oxide-semiconductor field-effect transistor model that better fits the electrolyte-insulator-semiconductor structure is used. It is discussed how if the reference electrode and the insulator-electrolyte generate no noise associated with charge transport, then the main noise mechanisms are the resistive losses of the electrolyte and the low-frequency noise of the field-effect transistor. It is also found that for realistic sensor geometries and high electrolyte concentrations, the noise from the field-effect transistor (FET) dominates the thermal noise from the electrolyte resistance, and the optimal biasing point for the FET for minimum noise is found to be around moderate inversion.
Fluorescence shifts of rare-earth ions in nonoxide glasses
Authors: Yang Zhiyong , Luo Lan , Chen Wei
Published: October 2006

1.3 μ m fluorescence shifts of Pr3+ or Dy3+ ion doped nonoxide glasses are investigated in terms of the peak fluorescence wavelength (PFW) shift. Related influencing factors are analyzed and discussed. The PFW shift with glass matrix is controlled by the scalar crystal field strength Nv. A larger Nv can lead to a redshift of the 1.3μm fluorescence. The quenching of luminescence dominates the redshift of PFW with increasing doping concentration. The PFW shift in codoped glass is controlled by the energy transfer and quenching of luminescence.
Effect of texture on dielectric properties and thermal depoling of Bi4Ti3O12 ferroelectric ceramics
Authors: Yan Haixue , Reece Michael J. , Liu Jing , Shen Zhijian , Kan Yanmei , Wang Peiling
Published: October 2006

Ordinary fired Bi4Ti3O12 ceramics show a gradual reduction in their room temperature d33 after annealing at temperatures from room temperature to 450°C. Textured ceramics show a greater resistance to thermal depoling. At about 450°C there is a rapid drop of d33 for the textured materials, which may be a consequence of a phase transition. Between 500 and 650°C the d33 is stable. The depolarization is assisted by internal mechanical stresses. These stresses are smaller in textured materials, which explains the increasing resistance to thermal depoling with increasing texture.
In
Mn
O
3
: A biferroic
Authors: Serrao Claudy Rayan , Krupanidhi S. B. , Bhattacharjee Joydeep , Waghmare Umesh V. , Kundu Asish K. , Rao C. N. R.
Published: October 2006

In Mn O 3 which has a hexagonal structure similar to that of YMnO3 is found to show a canted antiferromagnetic behavior below 50K (TN) and a ferroelectric (FE) transition at 500K accompanied by hystersis. We have determined the structure, polarization, and energetics of the FE and paraelectric (PE) phases of InMnO3 using first-principles density functional theory calculations based on pseudopotentials and a plane-wave basis, and find the polarization of the PE phase to be a half-integer quantum. The difference in polarization of the FE and PE phases calculated along a simple path is different from the absolute value of polarization of the FE phase. A weak piezoelectric response is exhibited by InMnO3 to uniaxial strain.
The contribution of dipoles and space charge to low temperature relaxation in polyethylene terephthalate
Authors: Neagu Eugen R. , Neagu Rodica M.
Published: October 2006

The fine structure of the complex low temperature dielectric relaxation process in biaxially stretched semicrystalline polyethylene terephthalate was investigated by the final thermally stimulated discharge current technique. By this technique, the apparent peaks that are mainly related to space charge detrapping from shallow traps are discriminated from the actual peaks related to the movement of dipoles attached to polymer chain or the peaks related to the detrapped charge from traps associated with specific polymer chain segments. Five relaxations around 100, 150, 210, 240, and 290K have been clearly identified. While the first three relaxations are of dipolar origin and are assigned to the motions of end hydroxyl groups and carbonyl groups, the last two relaxations are related to space charge detrapping and are assigned to phenyl group motions in the amorphous and crystalline phases. The very good agreement between the data determined by the final thermally stimulated discharge current technique and by other electrical and mechanical methods confirms the high potentiality of the final thermally stimulated discharge current technique to study the fine structure of the complex relaxation processes such as the low temperature relaxation of polyethylene terephthalate.
Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area
Authors: Li Chang-Peng , Roshchin Igor V. , Batlle Xavier , Viret Michel , Ott Frédéric , Schuller Ivan K.
Published: October 2006

Porous alumina masks are fabricated by anodization of aluminum films grown on both semiconducting and insulating substrates. For these self-assembled alumina masks, pore diameters and periodicities within the ranges of 10–130 and 20–200nm, respectively, can be controlled by varying anodization conditions. 20nm periodicities correspond to pore densities in excess of 1012 per square inch, close to the holy grail of media with 1Tbit∕in.2 density. With these alumina masks, ordered sub-100-nm planar ferromagnetic nanodot arrays covering over 1cm2 were fabricated by electron beam evaporation and subsequent mask lift-off. Moreover, exchange-biased bilayer nanodots were fabricated using argon-ion milling. The average dot diameter and periodicity are tuned between 25 and 130nm and between 45 and 200nm, respectively. Quantitative analyses of scanning electron microscopy (SEM) images of pore and dot arrays show a high degree of hexagonal ordering and narrow size distributions. The dot periodicity obtained from grazing incidence small angle neutron scattering on nanodot arrays covering ∼2.5cm2 is in good agreement with SEM image characterization.
Highly efficient organic-inorganic poly(3,4-ethylenedioxythiophene)-molybdenum trioxide nanocomposite electrodes for electrochemical supercapacitor
Authors: Murugan A. Vadivel , Viswanath Annamraju Kasi , Gopinath Chinnakonda S. , Vijayamohanan K.
Published: October 2006

In this paper, we report a highly efficient organic-inorganic nanocomposite electrode with enhanced double layer capacitance, which has been synthesized using 3,4-ethylenedioxythiophene and crystalline molybdenum trioxide (MoO3) in the presence of an external oxidizing agent. The interlayer spacing of MoO3 upon intercalation expands from 6.93to13.46Å and is followed by an exfoliation and restacking process. The resulting nanocomposite is characterized by powder x-ray diffraction, scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, and four probe conductivity measurements. The application potential of this nanocomposite as an electrode material for electrochemical supercapacitors has been investigated, highlighting the unusual enhancement of double layer capacitance of poly(3,4-ethylenedioxythiphene) (PEDOT-MoO3) nanocomposites (∼300Fg−1) compared to that of pristine MoO3 (∼40mFg−1). The improved electrochemical performance is attributed to the intercalation of electronically conducting PEDOT between MoO3 layers with enhanced bidimensionality and an increase in the surface area.
A consistent extension of the local spin density approximation to account for quantum dot mass and dielectric mismatches
Authors: Pi M. , Royo M. , Planelles J.
Published: October 2006

A consistent extension of local spin density approximation (LSDA) to account for mass and dielectric mismatches in nanocrystals is presented. The extension accounting for variable effective mass is exact. Illustrative comparisons with available configuration interaction calculations show that the approach is also very reliable when it comes to account for dielectric mismatches. The modified LSDA is as fast and computationally low demanding as LSDA. Therefore, it is a tool suitable to study large particle systems in inhomogeneous media without much effort.
Study of a nonlinear mechanical response of reentrant spin glass antiferromagnet Mn–Cu alloys
Authors: Tanji Toru , Kawanami Toru , Moriwaki Shigenori , Mio Norikatsu , Yin Fuxing , Matsushita Akiyuki
Published: October 2006

We have studied the relation between the mechanical response and the magnetic susceptibility of γ-phase Mn–Cu binary alloys, which are reentrant spin glass (RSG) antiferromagnets. A close correlation between the temperature dependency of the hysteretic magnetization due to magnetic freezing and internal friction was observed. When the Cu concentration reached up to 30at.%, where the antiferromagnetic long-range order disappeared, the magnetically correlated internal friction also disappeared. This composition effect and a remarkable vibration-amplitude dependency of the internal friction suggested that antiferromagnetic domain walls were responsible for these phenomena. Thus, we concluded that RSG freezing influences the dynamics of antiferromagnetic domain walls in a similar way as that of ferromagnets, and leads to internal friction.
Organic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography
Authors: Sun Yiru , Forrest Stephen R.
Published: October 2006

High efficiency white organic light emitting devices (WOLEDs) with optical outcoupling enhanced by hexagonal polymethylmethacrylate microlens arrays fabricated by imprint lithography on a glass substrate are demonstrated. Monte Carlo and finite difference time domain simulations of the emitted light are used to optimize the microlens design. The measured enhancement of light outcoupling and the angular dependence of the extracted light intensity are in agreement with the simulation. Using microlens arrays, we demonstrate a fluorescent/phosphorescent WOLED with a maximum external quantum efficiency of (14.3±0.3)% at 900cd∕m2 and power efficiency of 21.6±0.5lm∕W at 220cd∕m2. The electroluminescent spectra at viewing angles from normal to the substrate plane, to 60° off normal, remain almost unchanged, giving a color rendering index of 87.
Photoluminescence lifetimes of Si quantum dots
Authors: Zianni X. , Nassiopoulou A. G.
Published: October 2006

We present a continuum model for the calculation of the electron states in Si dots that accounts for the effects of size, shape, and crystallographic orientation of the dots. This formalism has been used to study the behavior of the photoluminescence (PL) lifetime in Si quantum dots. This is due to the anisotropy of the silicon band structure and the confinement in quantum dots, which result in a cluster of energy levels from the different valleys of Si. Although these levels are very close in energy, they have very different recombination rates. Hence, there are (relatively) fast and slow levels at approximately the same energy. This feature causes a temperature dependence of PL in Si nanostructures, hence it is suggested that dispersion in the magnitude of the PL lifetimes in Si dots is at the origin of the observed stretched exponential behavior of PL lifetime in porous Si. Both zero phonon and phonon-assisted recombinations have been included in the calculations. Zero phonon recombination dominates in small dots (∼2nm) and the lifetime is ∼10μs. In larger dots, of a size of ∼4nm and above, phonon-assisted transitions become dominant and PL lifetimes are of the order of 1–10ms.
Microstructure and magnetostriction of Tb0.3Dy0.7Fe1.95 prepared under different solidification conditions by zoning and modified Bridgman techniques
Authors: Palit Mithun , Pandian S. , Balamuralikrishnan R. , Singh A. K. , Das Niranjan , Chandrasekharan V. , Markandeyulu G.
Published: October 2006

An intermetallic compound of nominal composition Tb0.3Dy0.7Fe1.95 was conventionally cast in the form of cylindrical rods of 8mm diameter and directionally solidified by zoning at three different growth rates. Compounds of the same nominal composition were also directionally solidified in the form of cylindrical rods of 20mm diameter by modified Bridgman technique, at two different growth rates. The microstructure of the directionally solidified compounds has been investigated as a function of solidification rate and compared with that of conventionally cast compound. The observed microstructural features of these samples have been correlated with the magnetostriction measured on the corresponding samples. Further, by examining the longitudinal sections cut along its cylindrical axis, a correlation of microstructure with magnetostriction has been brought out for each directionally solidified sample as a function of distance from the initially solidified end to the other. It has been observed that a large magnetostriction (at 4.5kOe) and a high initial slope of magnetostriction versus applied magnetic field are realized in the directionally solidified samples as compared to the conventionally cast sample. An improvement in the property has been observed among the directionally solidified samples when solidified with higher growth rates. It is surmised on the basis of the observed microstructural features in this study that a large magnetostriction is realized if formation of (Tb,Dy)Fe2 occurs by congruent solidification rather than by the peritectic reaction L+(Tb,Dy)Fe3→(Tb,Dy)Fe2.
A measurement of electron-wall interactions using transmission diffraction from nanofabricated gratings
Authors: Barwick Brett , Gronniger Glen , Yuan Lu , Liou Sy-Hwang , Batelaan Herman
Published: October 2006

Electron diffraction from metal coated freestanding nanofabricated gratings is presented, with a quantitative path integral analysis of the electron-grating interactions. Electron diffraction out to the 20th order was observed indicating the high quality of our nanofabricated gratings. The electron beam is collimated to its diffraction limit with ion-milled material slits. Our path integral analysis is first tested against single slit electron diffraction, and then further expanded with the same theoretical approach to describe grating diffraction. Rotation of the grating with respect to the incident electron beam varies the effective distance between the electron and grating bars. This allows the measurement of the image charge potential between the electron and the grating bars. Image charge potentials that were about 15% of the value for that of a pure electron-metal wall interaction were found. We varied the electron energy from 50to900eV. The interaction time is of the order of typical metal image charge response times and in principle allows the investigation of image charge formation. In addition to the image charge interaction there is a dephasing process reducing the transverse coherence length of the electron wave. The dephasing process causes broadening of the diffraction peaks and is consistent with a model that ascribes the dephasing process to microscopic contact potentials. Surface structures with length scales of about 200nm observed with a scanning tunneling microscope, and dephasing interaction strength typical of contact potentials of 0.35eV support this claim. Such a dephasing model motivated the investigation of different metallic coatings, in particular Ni, Ti, Al, and different thickness Au–Pd coatings. Improved quality of diffraction patterns was found for Ni. This coating made electron diffraction possible at energies as low as 50eV. This energy was limited by our electron gun design. These results are particularly relevant for the use of these gratings as coherent beam splitters in low energy electron interferometry.
Microscopic simulations of molecular cluster decay: Does the carrier gas affect evaporation?
Authors: Tang Hoi Yu , Ford Ian J.
Published: October 2006

We develop a kinetic theory of cluster decay by considering the stochastic motion of molecules within an effective potential of mean force (PMF) due to the cluster. We perform molecular dynamics simulations on a 50-atom argon cluster to determine the mean radial force on a component atom and hence the confining potential of mean force. Comparisons between isolated clusters and clusters thermostatted through the presence of a 100-atom helium carrier gas show that the heat bath has only a slight effect upon the PMF. This confirms the validity of calculations of cluster properties using isolated cluster simulations. The PMF is used to calculate the atomic evaporation rate from these clusters, and results are compared with the predictions of the capillarity approximation together with detailed balance, both components of the classical theory of aerosol nucleation.
Simulating thermal explosion of octahydrotetranitrotetrazine-based explosives: Model comparison with experiment
Authors: Yoh Jack J. , McClelland Matthew A. , Maienschein Jon L. , Nichols Albert L. , Tarver Craig M.
Published: October 2006

A model comparison with measurements for the thermal, chemical, and mechanical behaviors in a thermal explosion experiment is presented. Confined high explosives (HEs) are heated at a rate of 1°C∕h until an explosion is observed. The heating, ignition, and deflagration phases are modeled using an arbitrarily Lagrangian-Eulerian (ALE3D) code that can handle a wide range of time scales that vary from a structural to a hydrodynamic time scale. During the preignition phase, quasistatic mechanics and diffusive thermal transfer from a heat source to the HE are coupled with the finite chemical reactions that include both endothermic and exothermic processes. Once the HE ignites, a hydrodynamic calculation is performed as a burn front propagates through the HE. Two octahydrotetranitrotetrazine (HMX)-based explosives, LX-04 and LX-10, are considered, whose chemical-thermal-mechanical models are constructed based on measurements of thermal and mechanical properties along with small-scale thermal explosion measurements. The present HMX modeling work shows the violence calculations with thermal predictions associated with a confined thermal explosion test. The simulated dynamic response of HE confinement during the explosive phase is compared to measurements in larger scale thermal explosion tests. The explosion temperatures for both HEs are predicted to within 1°C. Calculated and measured wall strains provide an indication of vessel pressurization during the heating phase and violence during the explosive phase.
Void formation and surface energies in Cu(InGa)Se2
Authors: Lei C. , Rockett A. , Robertson I. M. , Shafarman W. N. , Beck M.
Published: October 2006

The spontaneous formation of voids has been observed in Cu(In,Ga)Se2 films deposited by multistage evaporation processes but was not found commonly in single-stage evaporated material. These voids may be isolated in the grains or may occur at the grain boundaries. The voids exhibit strongly faceted geometries with interior surfaces being close-packed {112} planes of the chalcopyrite structure. A model based on the Kirkendal effect [Trans. AIME 171, 130 (1947)] and the interdiffusion and phase reaction in the bilayer process is proposed to explain the observation. The void shape is analyzed and an approximate Wulff construction [Z. Kristallogr. Mineral. 34, 449 (1901)] is used to estimate the relative surface energies of the predominant facets. Certain dislocation cores are also found to be unstable and convert to nanovoids in the presence of sufficient atomic vacancies moving in the lattice.
Evolution of magnetization reversal mechanism in Fe-Cr alloy films
Authors: Gao T. R. , Hao S. P. , Zhou S. M. , Sun L.
Published: October 2006

A large thin film sample of FexCr1−x alloy with a composition gradient (x=0.38−0.52) has been prepared by co-sputtering to investigate magnetic anisotropy and magnetization effects on reversal mechanisms. The single-phased Fe-Cr films have a bcc structure with [110] preferred orientation. Since the magnetization decreases as the Fe content is decreased and the uniaxial anisotropy energy keeps at almost constant, the uniaxial anisotropic field HK is increased. At the same time, the pinning field Hp(0) changes little since the lattice constant of Fe-Cr alloy film does not change much with the composition. Consequently, the HK can be much larger than Hp(0) for low Fe concentrations and becomes comparable with increasing Fe concentration. As a result, the magnetization reversal mechanism evolves from a mode based on the pinning and motion of domain wall to another mode based on modified Kondorsky model as the Fe content is increased.
Deep levels in silicon Schottky junctions with embedded arrays of β-FeSi2 nanocrystallites
Authors: Tsormpatzoglou A. , Tassis D. H. , Dimitriadis C. A. , Dózsa L. , Galkin N. G. , Goroshko D. L. , Polyarnyi V. O. , Chusovitin E. A.
Published: October 2006

Schottky contacts on p-type silicon, with embedded arrays of β-FeSi2 nanocrystallites, were studied by current-voltage (I-V), deep level transient spectroscopy (DLTS), and low-frequency noise measurements. Forward I-V characteristics on logarithmic scale indicate that space-charge limited current (SCLC) dominates the carrier transport in these diodes. From an analysis of the SCLC characteristics, we found that two arrays of β-FeSi2 nanocrystallites induce a trap level at 320meV above the valence band edge with concentration of 5×1014cm−3, which is in relatively good agreement with the trap detected by DLTS. By inserting ten arrays of β-FeSi2 nanocrystallites, two trap levels at 300 and 340meV with concentrations of 2.6×1014 and 1.1×1015cm−3, respectively, were found from the SCLC analysis. By increasing the number of the inserted arrays of β-FeSi2 nanocrystallites from 2 to 10, a trap at 402meV with concentration of 6.5×1015cm−3 has been detected by DLTS, failing to detect the second trap observed by SCLC measurements. The normalized power spectral density SI∕I2 vs V exhibits a peak at the transition voltage from the trap-filling regime to the SCLC regime, corresponding to a deep trap level. It is demonstrated that the noise technique is more sensitive than the SCLC and DLTS techniques in extracting information about the deep trap parameters.
Transmission electron microscopy investigation of the structural damage formed in GaN by medium range energy rare earth ion implantation
Authors: Gloux F. , Wojtowicz T. , Ruterana P. , Lorenz K. , Alves E.
Published: October 2006

The crystallographic nature of the damage created in GaN by 300keV rare earth ions has been investigated following implantation at room temperature by varying the fluence of Er, Eu, or Tm from 7×1013to2×1016at.∕cm2. There is a build up of point defects clusters, which increases in density and depth versus the ion fluence. When a threshold around 3×1015at.∕cm2 is reached, a nanocrystalline surface layer is observed. From the lowest fluence, we point out the formation of basal stacking faults, with a majority of I1. Their density also increases with the fluence, but it is seen to saturate at the onset of the observation of the surface nanocrystalline layer. Extrinsic E faults bounded by the c∕2 partials have also been identified; however, most of the E stacking faults transform to I1 which are noticed to fold easily from basal to prismatic planes by switching or not to the Drum atomic configuration.
S
(
D
2
1
)
atomic orbital polarization in the photodissociation of OCS at 193nm: Construction of the complete density matrix
Authors: Lee Suk Kyoung , Silva Ruchira , Thamanna Shakera , Vasyutinskii Oleg S. , Suits Arthur G.
Published: October 2006

The absolute velocity-dependent alignment and orientation for S(D21) atoms from the photodissociation of OCS at 193nm were measured using the dc slice imaging method. Three main peaks ascribed to specific groups of high rotational levels of CO in the vibrational ground state were found, with rotationally resolved rings in a fourth slow region ascribed to weak signals associated with excited vibrational states of CO. The observed speed-dependent β and polarization parameters support the interpretation that there are two main dissociation processes: a simultaneous two-surface (A′ and A″) excitation and the initial single-surface (A′) excitation followed by the nonadiabatic crossing to ground state. At 193nm photodissociation, the nonadiabatic dissociation process is strongly enhanced relative to longer wavelengths. The angle- and speed-dependent S(D21) density matrix can be constructed including the higher order (K=3,4) contributions for the circularly polarized dissociation light. This was explicitly done for selected energies and angles. It was found in one case that the density matrix is sensitively affected by the rank 4 terms, suggesting that the higher order contributions should not be overlooked for an accurate picture of the dissociation dynamics in this system.
Molecular gap and energy level diagram for pentacene adsorbed on filled d-band metal surfaces
Authors: Baldacchini Chiara , Mariani Carlo , Betti Maria Grazia , Gavioli L. , Fanetti M. , Sancrotti M.
Published: October 2006

The authors present a combined photoemission and scanning-tunneling spectroscopy study of the filled electronic states, the molecular energy gap, and the energy level diagram of highly ordered arrays of pentacene deposited on the Cu(119) vicinal surface. The states localized at the interface are clearly singled out, comparing the results at different pentacene thicknesses and with gas-phase photoemission data. The molecular gap of 2.35eV, the hole injection barrier of 1.05eV, and the electron injection barrier of 1.30eV determine the energy level diagram of the states localized at the pentacene molecules.
High breakdown voltage with low on-state resistance of p-InGaN∕n-GaN vertical conducting diodes on n-GaN substrates
Authors: Nishikawa Atsushi , Kumakura Kazuhide , Makimoto Toshiki
Published: October 2006

p - In Ga N ∕ n - Ga N vertical conducting diodes were grown on freestanding n-GaN substrates by metal organic vapor phase epitaxy. The homoepitaxial growth produced a high-quality GaN layer, as evidenced by the full width at half maximum of the (0002) x-ray rocking curve is as low as 34arcsec. For a diode with a 3.6-μm-thick n-GaN layer, a high breakdown voltage (VB) of 571V is obtained with a low on-state resistance (Ron) of 1.23mΩcm2, leading to the figure of merit, (VB)2∕Ron, of 265MW∕cm2. This is the highest value among those previously reported for GaN-based vertical conducting Schottky and p-n junction diodes.
In situ transmission electron microscopy observations of 1.8μm and 180nm Cu interconnects under thermal stresses
Authors: An J. H. , Ferreira P. J.
Published: October 2006

In situ heating transmission electron microscopy was used to observe the stress relaxation behavior in 1.8μm and 180nm wide Cu interconnects in real time. 1.8μm lines exhibit dislocation nucleation at the grain boundaries, while void nucleation/growth was observed in the 180nm lines. The difference in the stress relaxation mechanism is due to distinct stress states among the two lines, namely, biaxial for the 1.8μm lines and quasihydrostatic stress for the 180nm lines. Quasihydrostatic stresses in the 180nm lines are likely to lead to an absence of dislocation motion.
Millimeter-wave magnetoelectric effects in bilayers of barium hexaferrite and lead zirconate titanate
Authors: Srinivasan G. , Zavislyak I. V. , Tatarenko A. S.
Published: October 2006

Millimeter-wave magnetoelectric interactions have been studied through electric field effects on magnetic excitations in bilayers of single crystal barium ferrite and lead zirconate titanate (PZT). An electric field E produces a mechanical deformation in PZT, resulting in a shift δf in the frequency for electromagnetic modes in the ferrite. Reflected power versus frequency profiles at 40–55GHz for a series of bias magnetic field and E=0–10kV∕cm along the c axis of the ferrite showed an increase in δf to a maximum of 8MHz. Theoretical estimates of δf are in agreement with the data.
Three-dimensional imaging of nonspherical silicon nanoparticles embedded in silicon oxide by plasmon tomography
Authors: Yurtsever Aycan , Weyland Matthew , Muller David A.
Published: October 2006

Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies.
Electrical characterization of defects introduced in n-type Ge during indium implantation
Authors: Auret F. D. , Janse van Rensburg P. J. , Hayes M. , Nel J. M. , Meyer W. E. , Decoster S. , Matias V. , Vantomme A.
Published: October 2006

The authors have employed deep level transient spectroscopy to investigate the defects introduced in n-type Ge during 160keV indium (In) ion implantation. Our results show that In implantation introduces three prominent electron traps with energy levels at EC−0.09eV, EC−0.15eV, and EC−0.30eV, respectively. The authors have found that these defects are different from the point defects introduced by electron irradiation but that they do not involve In. Annealing at 600°C removed all the defects introduced during In implantation but results in a single prominent defect with a level at EC−0.35eV.
Ion cutting and transfer of sub-100-nm silicon layers using low-keV H, D, and He ions
Authors: Desrosiers Nicholas , Terreault Bernard
Published: October 2006

The authors have investigated the ion cutting and transfer of silicon layers thinner than 100nm using 5–8keV H, D, or He ions. The thickness and morphology of the transferred layers have been characterized as a function of ion fluence. Ion cutting of bonded layers occurs in a much wider fluence range than blistering of free surfaces. The optimal surface roughness (2nm) is obtained with rather high H or D doses of (7–8)×1016at.∕cm2, although the threshold dose is 1.5×1016H∕cm2 only. These results demonstrate the importance of the boundary conditions imposed on the splitting process by the bonded wafer.
Optimizing the performance of a vertical-cavity surface-emitting laser
Authors: Chatterjee S. , Wohlleben W. , Lange C. , Stolz W. , Motzkus M. , Rühle W.
Published: October 2006

The pulse response of an optically excited vertical-cavity surface-emitting laser is optimized by shaping the pump pulse. The temporal phase and amplitude of the excitation are adjusted by a learning loop with live spectrotemporal monitoring of excitation and emission pulses using a streak camera. The genetic algorithm of the loop is inspired by the optimization occurring in evolution in biology. The pulse response is such decreased from 4.8to2ps. Emission intensity shows strong correlation with the pulse delay and also improves by 63%.
Thermionic cooling in cylindrical semiconductor nanostructures
Authors: Lyu Pin , Zhang Chao
Published: October 2006

The authors analyzed the thermionic cooling efficiency of the cylindrical semiconductor nanostructures. It is shown that due to the reduced emission current from the inner electrode, the cooling efficiency can be enhanced if the outer cylinder is the cold electrode. The threshold voltage for thermionic cooling is lower in cylindrical devices as compared to that in planar devices. The competition between the heat transport by electrons and the heat conduction by phonons is responsible for the efficiency enhancement and the voltage reduction.
Variable core model and the Peierls stress for the mixed (screw-edge) dislocation
Authors: Lubarda Vlado A. , Markenscoff Xanthippi
Published: October 2006

A variable core model of a moving crystal dislocation is proposed and used to derive an expression for the Peierls stress. The dislocation width varies periodically as a dislocation moves through the lattice, which leads to an expression for the Peierls stress in terms of the difference of the total energies in the crystal corresponding to stable and unstable equilibrium configurations of the dislocation, rather than the difference in the misfit energies alone. Results for both edge and mixed dislocations are given and proposed to be used in conjunction with ab initio calculations.
On the deposition rate in a high power pulsed magnetron sputtering discharge
Authors: Alami J. , Sarakinos K. , Mark G. , Wuttig M.
Published: October 2006

The effect of the high pulse current and the duty cycle on the deposition rate in high power pulsed magnetron sputtering (HPPMS) is investigated. Using a Cr target and the same average target current, deposition rates are compared to dc magnetron sputtering (dcMS) rates. It is found that for a peak target current density ITpd of up to 570mAcm−2, HPPMS and dcMS deposition rates are equal. For ITpd>570mAcm−2, optical emission spectroscopy shows a pronounced increase of the Cr+∕Cr0 signal ratio. In addition, a loss of deposition rate, which is attributed to self-sputtering, is observed.
Correlation between optical and electrical properties of Mg-doped AlN epilayers
Authors: Nakarmi M. L. , Nepal N. , Ugolini C. , Altahtamouni T. M. , Lin J. Y. , Jiang H. X.
Published: October 2006

Deep UV photoluminescence and Hall-effect measurements were employed to characterize Mg-doped AlN grown by metal organic chemical vapor deposition. A strong correlation between the optical and electrical properties was identified and utilized for material and p-type conductivity optimization. An impurity emission peak at 4.7eV, attributed to the transition of electrons bound to triply charged nitrogen vacancies to neutral magnesium impurities, was observed in highly resistive epilayers. Improved conductivity was obtained by suppressing the intensity of the 4.7eV emission line. Mg-doped AlN epilayers with improved conductivities predominantly emit the acceptor-bound exciton transition at 5.94eV. From the Hall-effect measurements performed at elevated temperatures, the activation energy of Mg in AlN was measured to be about 0.5eV, which is consistent with the value obtained from previous optical measurements. Energy levels of nitrogen vacancies and Mg acceptors in Mg-doped AlN have been constructed.
Ultraviolet free-exciton light emission in Er-passivated SnO2 nanocrystals in silica
Authors: Brovelli S. , Chiodini N. , Meinardi F. , Lauria A. , Paleari A.
Published: October 2006

Sn O 2 nanocrystals are grown in silica starting from a sol-gel method and using Er doping to passivate the cluster boundaries. As a result, emission at 3.8eV from the decay of SnO2 free excitons is observed in nanostructured SnO2:SiO2, besides the extrinsic 2eV luminescence of defects in SnO2 and ascribable to substoichiometric nanocluster boundaries. The analysis of the extrinsic emission competitive with the ultraviolet (UV) luminescence evidences the involvement of a phonon mode at 210cm−1 from a SnO-like phase. The feasibility of passivated wide-band-gap nanocrystals in silica gives interesting perspectives for UV-emitting optical devices.