bluuebunny/crossref_metadata_2025 huggingface dataset:
Grain-size effect on the deformation mechanisms of nanostructured copper processed by high-pressure torsion
Authors: Liao X. Z. , Zhao Y. H. , Zhu Y. T. , Valiev R. Z. , Gunderov D. V.
Published: June 2004

The unique nonuniform deformation characteristic of high-pressure torsion was used to produce nanostructures with systematically varying grain sizes in a copper disk, which allows us to study the grain-size effect on the deformation mechanisms in nanostructured copper using a single sample. The as-processed copper disk has 100–200 nm grains near its center and 10–20 nm grains at its periphery. High densities of full dislocations (2×1016/m2) were distributed nonuniformly in large grains, implying that dislocation slip is the dominant deformation mechanism. With increasing dislocation density, the dislocations accumulated and rearranged, forming elongated nanodomains. The originally formed nanodomains remain almost the same crystalline orientation as their parent large grains. Further deformation occurred mainly through partial dislocation emissions from nanodomain boundaries, resulting in high density of nanotwins and stacking faults in the nanodomains. The elongated nanodomains finally transformed into equiaxed nanocrystalline grains with large-angle grain boundaries. The results suggest that grain boundary rotation and grain boundary sliding might play a significant role in the formation of large-angle grain boundaries in nanocrystalline grains. These experimental results show that different deformation mechanisms operate at different length scales and confirm unambiguously the deformation mechanisms of nanocrystalline grains predicted by molecular dynamic simulations.
Reaction path potential for complex systems derived from combined ab initio quantum mechanical and molecular mechanical calculations
Authors: Lu Zhenyu , Yang Weitao
Published: June 2004

Combined ab initio quantum mechanical and molecular mechanical calculations have been widely used for modeling chemical reactions in complex systems such as enzymes, with most applications being based on the determination of a minimum energy path connecting the reactant through the transition state to the product in the enzyme environment. However, statistical mechanics sampling and reaction dynamics calculations with a combined ab initio quantum mechanical (QM) and molecular mechanical (MM) potential are still not feasible because of the computational costs associated mainly with the ab initio quantum mechanical calculations for the QM subsystem. To address this issue, a reaction path potential energy surface is developed here for statistical mechanics and dynamics simulation of chemical reactions in enzymes and other complex systems. The reaction path potential follows the ideas from the reaction path Hamiltonian of Miller, Handy and Adams for gas phase chemical reactions but is designed specifically for large systems that are described with combined ab initio quantum mechanical and molecular mechanical methods. The reaction path potential is an analytical energy expression of the combined quantum mechanical and molecular mechanical potential energy along the minimum energy path. An expansion around the minimum energy path is made in both the nuclear and the electronic degrees of freedom for the QM subsystem internal energy, while the energy of the subsystem described with MM remains unchanged from that in the combined quantum mechanical and molecular mechanical expression and the electrostatic interaction between the QM and MM subsystems is described as the interaction of the MM charges with the QM charges. The QM charges are polarizable in response to the changes in both the MM and the QM degrees of freedom through a new response kernel developed in the present work. The input data for constructing the reaction path potential are energies, vibrational frequencies, and electron density response properties of the QM subsystem along the minimum energy path, all of which can be obtained from the combined quantum mechanical and molecular mechanical calculations. Once constructed, it costs much less for its evaluation. Thus, the reaction path potential provides a potential energy surface for rigorous statistical mechanics and reaction dynamics calculations of complex systems. As an example, the method is applied to the statistical mechanical calculations for the potential of mean force of the chemical reaction in triosephosphate isomerase.
Aqua dissociation nature of cesium hydroxide
Authors: Odde Srinivas , Pak Chaeho , Lee Han Myoung , Kim Kwang S. , Mhin Byung Jin
Published: June 2004

To understand the mechanism of aqueous base dissociation chemistry, the ionic dissociation of cesium–hydroxide in water clusters is examined using density functional theory and ab initio calculations. In this study, we report hydrated structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of CsOH⋅(H2O)n=0–4. With the addition of water molecules, the Cs–OH bond lengthened significantly from 2.46 Å for n=1 to 3.08 Å for n=4, which causes redshift in Cs–O stretching frequency. It is found that three water molecules are needed for the dissociation of Cs–OH, in contrast to the case of strong acid dissociation which requires at least four water molecules. However, the dissociation for n=3 could be considered as incomplete because a very weak CS…OH stretch mode is still present, while that for n=4 is complete since the Cs…OH mode no longer exists. This study can be related with hydration chemistry of cations and anions, and extended into the intra- and intercharge-transfer phenomena.
Explicitly correlated second order perturbation theory: Introduction of a rational generator and numerical quadratures
Authors: Ten-no Seiichiro
Published: June 2004

A rational generator, which fulfills the cusp conditions for singlet and triplet electron pairs, is proposed and applied to explicitly correlated second order Møller–Plesset perturbation theory calculations. It is shown that the generator in conjunction with frozen geminals improves the convergence of correlation energy without introducing any variational parameters in explicitly correlated functions. A new scheme for three-electron integrals based on numerical quadratures is also illustrated. The method is tested for the convergence of reaction enthalpies with various basis sets.
Interplay between shear flow and elastic deformations in liquid crystals
Authors: Marenduzzo D. , Orlandini E. , Yeomans J. M.
Published: June 2004

We study shear flow in liquid crystal cells with elastic deformations using a lattice Boltzmann scheme that solves the full, three-dimensional Beris–Edwards equations of hydrodynamics. We consider first twisted and hybrid aligned nematic cells, in which the deformation is imposed by conflicting anchoring at the boundaries. We find that backflow renders the velocity profile non Newtonian, and that the director profile divides into two regions characterized by different director orientations. We next consider a cholesteric liquid crystal, in which a twist deformation is naturally present. We confirm the presence of secondary flow for small shear rates, and are able to follow the dynamical pathway of shear-induced unwinding, for higher shear rates. Finally, we analyze how the coupling between shear and elastic deformation can affect shear banding in an initially isotropic phase. We find that for a nematic liquid crystal, elastic distortions may cause an asymmetry in the dynamics of band formation, whereas for a cholesteric, shear can induce twist in an initially isotropic sample.
Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy
Authors: Knoesel Ernst , Bonn Mischa , Shan Jie , Wang Feng , Heinz Tony F.
Published: June 2004

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.
Influence of platinum loading on photoluminescence of TiO2 powder
Authors: Nakajima Hiromitsu , Mori Toshiyuki , Watanabe Mamoru
Published: June 2004

Photoluminescence (PL) and PL excitation spectra of pure and Pt-loaded TiO2 powders were measured at room temperature, and the influence of the Pt loading on PL was considered. We found a blueshift in the PL excitation band of the Pt loaded TiO2 powders and our results suggested that the enhancement of the charge separation efficiency in TiO2 by the Pt loading can be evaluated by PL spectroscopy.
Magnetic behavior of (Al,Ge)65CuMn quaternary alloy quasicrystals
Authors: Chatterjee Ratnamala , Kanjilal Aloke
Published: June 2004

We report the results of field and temperature dependent magnetization measurements on (Al,Ge)65CuMn quasicrystalline samples with ∼22 and 24 at. % Mn. The Ge content is also kept ∼25 at. %. The data for these samples are then compared with the data taken on samples with low Ge content (∼10 and 0 at. %). Al40Ge25Cu10Mn25, a composition very close to one of the alloys studied here, is reported [Tsai et al., Jpn. J. Appl. Phys., 27, L2252 (1988)] in literature as a ferromagnetic, icosahedral QC with large TC. The detailed magnetic studies described here prove that the magnetism observed in these quaternary alloys is highly disordered. The two important observations made in our studies are (i) observation of short-range ferromagnetism in samples with high Ge and high Mn content, up to very high fields (H=5 T) and (ii) oscillatory magnetization (or existence of multiple minima in the M-T curves) observed for the same samples for H⩾1 kOe.
Magnetic order in Co-doped and (Mn, Co) codoped ZnO thin films by pulsed laser deposition
Authors: Yan L. , Ong C. K. , Rao X. S.
Published: June 2004

Co-doped Zn1−xCoxAl0.01O (x=0.15,0.3) and (Mn, Co) codoped Zn0.7(Mn0.15Co0.15)O thin films were fabricated on Al2O3 (0001) by pulsed laser deposition. The doped ZnO thin films with well wurtzite structures could be deposited at a low temperature of 400 °C and low oxygen pressure of 5×10−5 Pa. The Co-doped ZnO showed metallic conductivity with low resistance, while the (Mn, Co) codoped ZnO was semiconductor with high resistance, which was confirmed by the resistance vs temperature measurements. All the three doped ZnO films showed room temperature (290 K) ferromagnetism, with the saturated magnetic moments of 0.08μB/Co, 0.17μB/Co, and 0.19μB/(0.5Co+0.5Mn) for Zn0.85Co0.15Al0.01O, Zn0.7Co0.3Al0.01O, and Zn0.7(Mn0.15Co0.15)O, respectively.
Electrical properties of superfilled sub-micrometer silver metallizations
Authors: Josell D. , Burkhard C. , Li Y. , Cheng Y.-W. , Keller R. R. , Witt C. A. , Kelley D. R. , Bonevich J. E. , Baker B. C. , Moffat T. P.
Published: June 2004

Electrical properties of damascene silver wires with widths between ≈60 and 840 nm and heights between ≈100 nm and 300 nm are presented. The superconformal electrodeposition process by which the seam-free and void-free metallizations were fabricated is summarized. The chemical-mechanical polishing plus oblique ion sputtering process by which metal overburden was removed from the field adjacent to the wires is detailed. The size-dependent resistivity of the wires is obtained and interpreted in terms of intrinsic resistivity, grain boundary reflection, and surface scattering. Quantitative analysis of the last is accomplished using a different implementation of the Fuchs-Sondheimer formalism for wires of rectangular geometry and nonzero surface specularity that is derived herein.
Comparison of the passivation effects on self- and non-self-aligned InP/InGaAs/InP double heterostructure bipolar transistors by low-temperature deposited SiNx
Authors: Jin Z. , Prost W. , Neumann S. , Tegude F.-J.
Published: June 2004

Both the self- and non-self-aligned graded-base InP/InGaAs/InP double heterostructure bipolar transistors (DHBTs) were passivated by room-temperature deposited SiNx. Current gains were found to increase significantly after SiNx passivation. The passivation resulted in an increase in the leakage current of base-collector junction. The larger leakage current in the non-self-aligned DHBT made the sudden increase of the collector current in the common-emitter I–V curve occur at smaller collector-emitter bias than that in the self-aligned DHBT, which has smaller leakage current. Further increase in the current gain was found after the DHBTs were exposed to air for 200 days. The leakage current in the base-collector diode was reduced. The sudden increase of the collector current in the non-self-aligned DHBT in the common-emitter I–V characteristics was also suppressed. Investigation of reverse Gummel plots showed that the surface recombination in the base-collector diode was suppressed by the passivation. The leakage current in the base-collector diode was attributed to the increase of the generation rate.
Perpendicular magnetic anisotropy and structural properties of NiCu/Cu multilayers
Authors: Ruotolo A. , Bell C. , Leung C. W. , Blamire M. G.
Published: June 2004

Perpendicular magnetic anisotropy (PMA) was studied at low temperature (T=30 K) in dc-magnetron sputtered Ni60Cu40/Cu multilayers. PMA has been observed in many multilayer structures for ferromagnetic layer thicknesses less than a certain thickness t⊥. In general cases t⊥ is less than a few nanometers, making such structures unsuitable for low-cost fabrication techniques. Our results show a strong perpendicular easy direction of magnetization for NiCu layer thickness between 4.2 nm and 34 nm. The thickness t⊥ at which the multilayers change the preferential orientation from perpendicular to in-plane is estimated to be 55 nm. Structural studies show that the low magnetostatic energy density is likely to be the main reason for the large t⊥ value obtained in this system.
Structural phase transition in indium tungstate
Authors: Sivasubramanian V. , Ravindran T. R. , Nithya R. , Arora A. K.
Published: June 2004

Structural phase transition in In2W3O12 has been studied using differential scanning calorimetry, dilatometry, and Raman spectroscopy. The phase transition at 523 K has been found to be of first order with a heat of transition of 1.22 kJ/mole. In contrast to the earlier reports, the present dilatometric studies suggest that high temperature phase could have a small positive thermal expansion. Phonon frequencies in the monoclinic phase of this network structure were obtained from the Raman spectrum at 10 K. Large discontinuous changes in phonon frequencies are not found across the phase transition; however, temperature coefficients of phonon frequencies exhibit changes. Present results indicate that the local environment of tungstate ions in the high temperature phase is not significantly different from that in the monoclinic phase. Furthermore, weak temperature dependence of the phonon frequencies suggests that their anharmonicities are small in the high temperature phase and this could be responsible for the low value of thermal expansion.
Temperature dependence of intensity and peak position from photocurrent response in p-CdIn2Te4 crystal
Authors: Jeong T. S. , Hong K. J.
Published: June 2004

A p-CdIn2Te4 crystal has been investigated using photocurrent (PC) spectroscopy as a function of temperature. The three peaks are associated with band-to-band transitions. In addition, the PC intensities were observed to decrease with decreasing temperature. This is attributed to the trapping centers due to native defects and impurities in the p-CdIn2Te4 on the basis of Simmons and Taylor’s theory [J. Phys. C 7, 3051 (1974)]. From the relation of log Jph vs 1/T, where Jph is the PC density, the exponential variation of the PC with varying temperature showed that there are two dominant levels in the high- and moderate-temperature region. However, the PC signal in the low-temperature region below 50 K remains constant. Finally, the temperature dependence of the optical band gap of the p-CdIn2Te4 obtained from the PC measurement has been precisely estimated.
Role of oxygen pressure during pulsed laser deposition on the electrical and dielectric properties of antiferroelectric lanthanum-doped lead zirconate stannate titanate thin films
Authors: Yao Yingbang , Lu S. G. , Chen Haydn , Zhai Jiwei , Wong K. H.
Published: June 2004

Lanthanum-doped lead zirconate titanate stannate antiferroelectric thin films of ∼420 nm with compositions in the antiferroelectric tetragonal region have been prepared on Pt-buffered Si substrates by pulsed laser deposition. Effects of oxygen pressure during deposition were studied, with emphasis placed on the electrical and dielectric properties of the films. The dielectric constant and the maximum polarization increased with the oxygen pressure during deposition, from 75 to 125 mTorr. So did the dielectric strength. This property enhancement with deposition oxygen pressure was believed to be due to the reduction of pyrochlore phase in the films. However, increasing the oxygen pressure beyond 150 mTorr during deposition had led to the increase of surface roughness, which eventually resulted in film cracking. It was also found that increasing the oxygen pressure did not benefit the fatigue performance in any appreciable way.
Passivation of double-positioning twin boundaries in CdTe
Authors: Yan Yanfa , Al-Jassim M. M. , Jones K. M.
Published: June 2004

We present density-functional total-energy calculations to investigate the passivation effects of impurity elements such as Br, Cl, S, O, H, and Na on double-positioning twin boundaries in CdTe. We find that Br, Cl, S, and O atoms present passivation effects on the boundaries with different degrees, whereas H and Na do not passivate the boundaries. Of all these impurities, Cl atoms present the best passivation effects on the double-positioning twin boundaries. The superior passivation effects are realized by either terminating the Cd atoms with dangling bonds, or substituting the Te atoms with dangling bonds in the double-positioning twin boundaries in CdTe by Cl atoms.
Secondary electron emission yield on poled silica based thick films
Authors: Braga D. , Poumellec B. , Cannas V. , Blaise G. , Ren Y. , Kristensen M.
Published: June 2004

Studies on the distribution of the electric field produced by a thermal poling process in a layer of Ge-doped silica on silicon substrate, by using secondary electron emission yield (SEEY) measurements (δ) are presented. Comparing δ0 between poled and unpoled areas, the SEEY at the origin of electron injection, we pointed out an electric field 0.5 μm below the surface for our poling conditions and directed in the same direction as the external field applied during the poling process. Then, the dependence of δ on the injected dose of electrons allows us to deduce that the poling process disturbs the glass structure strongly enough for leading to a weak conductivity. It is then easy to display the poled areas. We have also pointed out an effect of the electric properties of the glass on the measurements obtained with the Electron Probe for MicroAnalysis.
Modeling and measurement of surface displacements in BaTiO3 bulk material in piezoresponse force microscopy
Authors: Felten F. , Schneider G. A. , Saldaña J. Muñoz , Kalinin S. V.
Published: June 2004

Piezoresponse force microscopy (PFM) is applied to image ferroelastic formed c domains in a single crystal ferroelectric barium titanate bulk material. A simple model and an analytical approach are presented, which provides a basis to understand the complex tip-surface interactions responsible for the image contrast in PFM. In particular, the measured amplitude of the piezoresponse out-of-plane surface displacements of a C− domain is compared with theoretical results based upon a three-dimensional Green’s function solution. The electric field distribution in the tip-surface contact is determined using image-charge calculations for a spherical tip separated by a thin water layer from a mechanically isotropic and electrically anisotropic dielectric half plane.
Recrystallization, redistribution, and electrical activation of strained-silicon/Si0.7Ge0.3 heterostructures with implanted arsenic
Authors: Sugii Nobuyuki , Irieda Shigefumi , Morioka Jun , Inada Taroh
Published: June 2004

For strained-silicon metal-oxide-semiconductor field-effect-transistors (MOSFETs), the formation of a low-resistance source and drain region is as important in terms of enhanced performance as optimizing the strain-induced increase in inversion-carrier mobility. The crystallographic, compositional, and electrical properties of an arsenic-ion implanted strained silicon/Si0.7Ge0.3 heterostructure were investigated in order to acquire basic information for fabricating low-resistance source and drain region of the n-channel strained-silicon MOSFET. The strained-silicon layer is completely recrystallized by rapid thermal annealing after arsenic implantation, but misfit dislocations were formed by annealing (at 1000 °C) in the case thickness of the strained-silicon layer exceeding the critical thickness. Arsenic diffusivity in strained silicon was identical to that in silicon, whereas that in SiGe was higher than that in silicon. Germanium recoil and enhanced diffusion caused by arsenic implantation to the SiGe layer was observed. This phenomenon can degrade the abruptness of the strained-silicon/SiGe interface during the formation of a shallow source and drain region. Solubility limit of arsenic in strained silicon was 2×1020 cm−3 and identical to that in silicon, on the other hand, the solubility limit in Si0.7Ge0.3 reduced by half. Electron mobility in strained silicon was by about 20%–30% higher than that in silicon, and that in Si0.7Ge0.3 was by about 20%–30% lower than those in silicon. In order to reduce parasitic resistance in the shallow source and drain region, thicker strained silicon layer is thus desirable, however, it should be very careful to control the strained-silicon thickness below the critical value.
Phase diagram of the adhesive hard sphere fluid
Authors: Miller Mark A. , Frenkel Daan
Published: June 2004

The phase behavior of the Baxter adhesive hard sphere fluid has been determined using specialized Monte Carlo simulations. We give a detailed account of the techniques used and present data for the fluid–fluid coexistence curve as well as parametrized fits for the supercritical equation of state and the percolation threshold. These properties are compared with the existing results of Percus–Yevick theory for this system.
Relaxation dynamics of a linear molecule in a random static medium: A scaling analysis
Authors: Moreno Angel J. , Kob Walter
Published: June 2004

We present extensive molecular dynamics simulations of the motion of a single linear rigid molecule in a two-dimensional random array of fixed overlapping disklike obstacles. The diffusion constants for the center of mass translation, DCM, and for rotation, DR, are calculated for a wide range of the molecular length, L, and the density of obstacles, ρ. The obtained results follow a master curve Dρμ∼(L2ρ)−ν with an exponent μ=−34 and 14 for DR and DCM, respectively, that can be deduced from simple scaling and kinematic arguments. The nontrivial positive exponent ν shows an abrupt crossover at L2ρ=ζ1. For DCM we find a second crossover at L2ρ=ζ2. The values of ζ1 and ζ2 correspond to the average minor and major axis of the elliptic holes that characterize the random configuration of the obstacles. A violation of the Stokes–Einstein–Debye relation is observed for L2ρ>ζ1, in analogy with the phenomenon of enhanced translational diffusion observed in supercooled liquids close to the glass transition temperature.
Probes of spin conservation in heavy metal reactions: Experimental and theoretical studies of the reactions of Re+ with H2, D2, and HD
Authors: Armentrout P. B. , Li Feng-Xia
Published: June 2004

A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cation, Re+, with molecular hydrogen and its isotopologues. A flow tube ion source produces Re+ in its S37 electronic ground state. Reaction with H2, D2, and HD forms Re H+(Re D+) in endothermic processes. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energy of D0(Re+–H)=2.29±0.07 eV (221±6 kJ/mol). The experimental thermochemistry is consistent with ab initio calculations, performed here and in the literature. Theory also provides the electronic structures of these species and is used to examine the reactive potential energy surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that the late metal ion, Re+, reacts largely via a statistical mechanism. This is consistent with the potential energy surfaces which locate a stable Re H2+(5B2) complex. Results for this third-row transition metal system are compared with the first-row congener (Mn+) and found to have much higher reactivity towards dihydrogen and stronger M+–H bonds. These differences can be attributed to efficient coupling among surfaces of different spin along with lanthanide contraction and relativistic effects.
The role of angular momentum in collision-induced vibration–rotation relaxation in polyatomics
Authors: McCaffery Anthony J. , Osborne Mark A. , Marsh Richard J. , Lawrance Warren D. , Waclawik Eric R.
Published: June 2004

Vibrational relaxation of the 61 level of S1(1B2u) benzene is analyzed using the angular momentum model of inelastic processes. Momentum–(rotational) angular momentum diagrams illustrate energetic and angular momentum constraints on the disposal of released energy and the effect of collision partner on resultant benzene rotational excitation. A kinematic “equivalent rotor” model is introduced that allows quantitative prediction of rotational distributions from inelastic collisions in polyatomic molecules. The method was tested by predicting K-state distributions in glyoxal–Ne as well as J-state distributions in rotationally inelastic acetylene–He collisions before being used to predict J and K distributions from vibrational relaxation of 61 benzene by H2, D2, and CH4. Diagrammatic methods and calculations illustrate changes resulting from simultaneous collision partner excitation, a particularly effective mechanism in p-H2 where some 70% of the available 61→00 energy may be disposed into 0→2 rotation. These results support the explanation for branching ratios in 61→00 relaxation given by Waclawik and Lawrance and the absence of this pathway for monatomic partners. Collision-induced vibrational relaxation in molecules represents competition between the magnitude of the energy gap of a potential transition and the ability of the colliding species to generate the angular momentum (rotational and orbital) needed for the transition to proceed. Transition probability falls rapidly as ΔJ increases and for a given molecule–collision partner pair will provide a limit to the gap that may be bridged. Energy constraints increase as collision partner mass increases, an effect that is amplified when Ji>0. Large energy gaps are most effectively bridged using light collision partners. For efficient vibrational relaxation in polyatomics an additional requirement is that the molecular motion of the mode must be capable of generating molecular rotation on contact with the collision partner in order to meet the angular momentum requirements. We postulate that this may account for some of the striking propensities that characterize polyatomic energy transfer.
A stimulated emission pumping study of the first excited singlet state of germylidene (H2C=Ge)
Authors: He Sheng-Gui , Tackett Brandon S. , Clouthier Dennis J.
Published: June 2004

The Ã 1A2 states of H2CGe and D2CGe have been explored for the first time by Ã–X̃ laser-induced fluorescence (LIF) spectroscopy of the orbitally forbidden S1–S0 transition and stimulated emission pumping (SEP) and wavelength resolved fluorescence studies of the allowed B̃–Ã electronic transition. Medium-resolution SEP studies gave the excited Ã state ν2, ν3, ν4, and ν6 vibrational frequencies for H2C74Ge and D2C74Ge. The 41 and 61 levels and higher combination and overtone states are strongly Coriolis coupled, which perturbs the rotational subband structure, limiting the accuracy of the determination of the vibrational frequencies. High-resolution SEP studies of the B̃–Ã 000 band have allowed us to determine the rotational constants of the Ã state of H2C74Ge, from which we were able to calculate an approximate r0 structure with the CH bond length constrained to the ground state value. The zero-point level of D2C74Ge is substantially perturbed, most plausibly by interaction with an excited vibrational level of the nearby triplet (ã 3A2) state.
Phase transitions of a polymer threading a membrane coupled to coil-globule transitions
Authors: Matsuyama Akihiko
Published: June 2004

We theoretically study phase transitions of a polymer threading through a pore imbedded in a membrane. We focus on the coupling between a partition of the polymer segments through the membrane and a coil-globule transition of the single polymer chain. Based on the Flory model for collapse transitions of a polymer chain, we calculate the fraction of polymer segments and the expansion factor of a polymer coil on each side of the membrane. We predict a first-order phase transition of a polymer threading a membrane; polymer segments in one side are discontinuously translocated into the other side, depending on solvent conditions and molecular weight of the polymer. We also discuss the equilibrium conformation of the polymer chain on each side of the membrane.
Combined effect of preferential orientation and Zr/Ti atomic ratio on electrical properties of Pb(ZrxTi1−x)O3 thin films
Authors: Gong Wen , Li Jing-Feng , Chu Xiangcheng , Gui Zhilun , Li Longtu
Published: June 2004

Lead zirconate titanate [Pb(ZrxTi1−x)O3, PZT] thin films with various compositions, whose Zr/Ti ratio were varied as 40/60, 48/52, 47/53, and 60/40, were deposited on Pt(111)/Ti/SiO2/Si substrates by sol-gel method. A seeding layer was introduced between the PZT layer and the bottom electrode to control the texture of overlaid PZT thin films. A single perovskite PZT thin film with absolute (100) texture was obtained, when lead oxide was used as the seeding crystal, whereas titanium dioxide resulted in highly [111]-oriented PZT films. The dielectric and ferroelectric properties of PZT films with different preferential orientations were evaluated systemically as a function of composition. The maximums of relative dielectric constant were obtained in the morphotropic phase boundary region for both (100)- and (111)-textured PZT films. The ferroelectric properties also greatly depend on films’ texture and composition. The intrinsic and extrinsic contributions to dielectric and ferroelectric properties were discussed.
Magnetic properties of metallic ferromagnetic nanoparticle composites
Authors: Ramprasad R. , Zurcher P. , Petras M. , Miller M. , Renaud P.
Published: June 2004

Magnetic properties of nanoparticle composites, consisting of aligned ferromagnetic nanoparticles embedded in a nonmagnetic matrix, have been determined using a model based on phenomenological approaches. Input materials parameters for this model include the saturation magnetization (Ms), the crystal anisotropy field (Hk), a damping parameter (α) that describes the magnetic losses in the particles, and the conductivity (σ) of the particles; all particles are assumed to have identical properties. Control of the physical characteristics of the composite system—such as the particle size, shape, volume fraction, and orientation—is necessary in order to achieve optimal magnetic properties (e.g., the magnetic permeability) at GHz frequencies. The degree to which the physical attributes need to be controlled has been determined by analysis of the ferromagnetic resonance (FMR) and eddy current losses at varying particle volume fractions. Composites with approximately spherical particles with radii smaller than 100 nm (for the materials parameters chosen here), packed to achieve a thin film geometry (with the easy magnetization axes of all particles aligned parallel to each other and to the surface of the thin film) are expected to have low eddy current losses, and optimal magnetic permeability and FMR behavior.
Structural characteristics of epitaxial BaTiO3/LaNiO3 superlattice
Authors: Liang Yuan-Chang , Wu Tai-Bor , Lee Hsin-Yi , Hsieh Yung-Wei
Published: June 2004

Artificial superlattices consisting of ferroelectric BaTiO3 (BTO) and conductive LaNiO3 (LNO) sublayers were epitaxially grown on Nb-doped SrTiO3(001) single crystal substrates by a dual-gun rf magnetron sputtering system. A symmetric sublayer structure with the designed thickness varying in the range from 3 nm to 70 nm was adopted. The formation of superlattice structure was confirmed from the (00L) Bragg reflection of x ray and the depth profile of secondary ion mass spectrometry. The in-plane diffraction shows that the BTO and LNO sublayers have the same in-plane lattice spacing for the superlattices with stacking periodicity below 16 nm. The lattice parameter obtained from in-plane diffraction also exhibits a partial but nearly constant relaxation of in-plane strain in the superlattices, even though the sublayer thickness is below the critical value for generation of misfit dislocations. X-ray reflectivity measurement reveals that all the above superlattices have about the same interface roughness of BTO/LNO. Consequently, nearly the same extent of dielectric enhancement results from the strained BTO layer, along with a highly conductive interface zone in the superlattiecs.
Photoluminescence dynamics in ensembles of wide-band-gap nanocrystallites and powders
Authors: Bergman Leah , Chen Xiang-Bai , Morrison John L. , Huso Jesse , Purdy Andrew P.
Published: June 2004

We present photoluminescence (PL) studies of GaN and ZnO nanocrystallites and powders. Our studies show that in addition to the intrinsic photoluminescence characteristics, the photoluminescence properties of the porous media are also a strong function of conditions such as ensemble size and powder density, ultraviolet-laser excitation power, and vacuum state. PL redshifts up to 120 meV were observed for GaN and ZnO crystallites and were attributed to laser heating and heat trapping in the ensemble. The electron-phonon interaction model for GaN indicated ensemble temperature ∼550 K, which is consistent with the finding obtained via high-temperature PL and Raman experiments. The PL in the vacuum state exhibited a significant redshift, ∼80 meV relative to that in air, and the PL of a dense ZnO pellet was found to resemble that of the bulk more than does a loose powder. The PL analyses indicated an excitonic emission at room temperature for both GaN and ZnO crystallites with intensity saturation occurring for large ensembles at high laser power.
Resonant microphone based on laser beam deflection
Authors: Roark Kevin , Diebold Gerald J.
Published: June 2004

A microphone consisting of a flexible membrane coupled to a Helmholtz resonator can be constructed to have a resonance at a specific frequency making it, unlike conventional broadband microphones, a frequency selective detector of sound. The present device uses a laser beam reflected from the membrane and directed onto a split photodiode to record the motion of the membrane. Since the microphone has a lightly damped resonance, both the thermal noise fluctuations in the displacement of the membrane from its equilibrium position and the response of the microphone to sound at the resonance frequency are large. The large amplitude of both the signal and the noise fluctuations means that effect of amplifier noise on the microphone’s sensitivity is diminished relative to that in broadband microphones. Applications of the microphone include photoacoustic detection of gases employing low power lasers.
Raman spectroscopy of ion irradiated diamond
Authors: Brunetto R. , Baratta G. A. , Strazzulla G.
Published: June 2004

Ion irradiation experiments of diamond samples at room temperature have been performed by using in situ Raman spectroscopy as diagnostic technique. Different ions are used with energies of 200 or 400 keV. The area of virgin diamond Raman band (at 1332 cm−1) decreases exponentially as the ion fluence increases. This is due to changes in the optical properties of the damaged samples in the visible spectral range. Results from different ions demonstrate that this effect is correlated with the number of displacements/cm2, i.e., with the energy lost by ions through elastic collisions with target nuclei. Amorphous carbon (sp2) is formed after a threshold of about 2×1022 vacancies/cm3, or about 16 eV/C-atom deposited by elastic collisions. The peak position and full width at half maximum of the D line and G line of the synthesized amorphous carbon are studied. In particular, the G-line peak position shifts from the initial 1545 cm−1 to about 1515 cm−1 at the higher doses. The results are also discussed in view of their relevance in astrophysical environments.
Relaxation processes in the coloration of amorphous WO3 thin films studied by combined impedance and electro-optical measurements
Authors: Garcia-Belmonte Germà , Bueno Paulo R. , Fabregat-Santiago Francisco , Bisquert Juan
Published: June 2004

We introduce in this paper a phenomenological model (Havriliak-Negami function) to describe the relaxation of the intercalated Li ions into amorphous WO3 electrochromic films. Kinetic behavior of intercalation was analyzed by a combined measurement of electrochemical impedance and modulated transmittance. The frequency-dependent capacitance and modulated transmittance show the processes of ion diffusion and rearrangement towards equilibrium in the stationary sites of the structure. In case of Li-doped samples, capacitance and transmittance spectra overlap along a wide interval of frequencies, thus indicating that ion diffusion does not play a central role within the measuring frequency range employed, so that the kinetics of coloring mechanism can be wholly described by HN relaxation. The rates of the coloring process are located near 100 mHz, although their probability decays slowly towards very low frequencies.
Gallium adsorption onto (112̄0) gallium nitride surfaces
Authors: McLaurin M. , Haskell B. , Nakamura S. , Speck J. S.
Published: June 2004

We report on a systematic study of transients in reflection high energy electron diffraction specular intensities due to adsorption and subsequent desorption of gallium onto (112̄0) GaN surfaces both during the gallium adsorption by itself or during the growth of GaN by plasma-assisted molecular beam epitaxy. We determined the boundaries between N-rich growth, Ga-rich growth with only a two-dimensional phase of adsorbed Ga, and Ga-rich growth with droplets of liquid Ga. The thermal dependence of the boundary between the Ga-rich regimes with and without droplets was found to be approximately 3.2 eV for both the cases of gallium adsorption and GaN growth. This temperature dependence is explained in terms of a quasiequilibrium model for the interaction between the surface phases. Evidence is presented indicating that the two-dimensional phase saturates at submonolayer coverage for temperatures below 700 °C.
Electronic and bonding structures of B-C-N thin films investigated by x-ray absorption and photoemission spectroscopy
Authors: Ray S. C. , Tsai H. M. , Bao C. W. , Chiou J. W. , Jan J. C. , Kumar K. P. Krishna , Pong W. F. , Tsai M.-H. , Chattopadhyay S. , Chen L. C. , Chien S. C. , Lee M. T. , Lin S. T. , Chen K. H.
Published: June 2004

X-ray absorption near-edge structure (XANES) and valence-band photoelectron spectroscopy (PES) were used to investigate the electronic and bonding structures of B-C-N thin films. The intensities of the sp2-bonded features in the C K-edge XANES spectra are found to generally decrease as the C concentration increases, whereas the intensities of the sp2-bonded features in the spectra of N K-edge XANES increase with the N concentration. The decrease of the intensities of the sp2-bonded features in the C and N K-edges XANES spectra correlates with the increase of the C/B and N/B concentration ratios and the increase of Young’s modulus. Valence-band PES spectra are found to be insensitive to the variations of the B and C concentrations in B-C-N compounds
Photoionization cross section of hydrogenic impurities in cylindrical quantum wires: Infinite well model
Authors: Ham Heon , Lee C. J. , Spector Harold N.
Published: June 2004

We have used a variational wave function for hydrogenic impurities in cylindrical quantum wires to calculate the dependence of the photoionization cross section of such impurities on the photon energy. The calculation has been performed using the infinite confining potential well model. The results are presented as a function of photon frequency for various wire radii and for different locations of the impurity in the wire. We find that the photoionization cross section depends upon the polarization of the photons which is an effect not noted in previous work by Sali, Fliyou and Loumrhari [Physica B, 233, 196 (1997)]. For the polarization of the radiation parallel to the axis of the wire, the cross section initially increases from zero at the threshold frequency for photoionization to occur while for the polarization of the radiation perpendicular to the axis of the wire, the cross section rises steeply at the threshold frequency and then decreases with the photon frequency.
Investigations of transient phase formation in Ti/Si thin film reaction
Authors: Chaix-Pluchery O. , Chenevier B. , Matko I. , Sénateur J. P. , La Via F.
Published: June 2004

The analysis of temperature and time dependence of the Ti film microstructure evolution in Ti/Si thin films indicates Si diffusion occurs before any crystal phase is formed. Diffusion occurs first in Ti grain boundaries and then, within the grains. Ti5Si3 has been identified as a transient phase forming before C49 TiSi2 grows. Ti5Si3 only grows in isothermal runs in a narrow range of annealing temperatures (412–434 °C in our study). It has not been observed in temperature runs in which C49 TiSi2 forms. The mechanisms of Ti5Si3 formation and transformation into C49 have been found to be diffusion limited. Phases of alloyed Ti, amorphous silicide, Ti5Si3, and C49 TiSi2 have been simultaneously observed. The lifetime and the volume fraction of the initial metal layer transformed into Ti5Si3 are naturally tightly temperature dependent.
Nanoscale observation of failures in organic light-emitting diodes
Authors: Czerw R. , Carroll D. L. , Woo H. S. , Kim Y. B. , Park J. W.
Published: June 2004

We introduce a simple and low-cost method for examining interface morphologies of nanoscale organic layers to demonstrate failure modes in organic light-emitting diodes. Cross-sectional scanning electron microscopy of nonencapsulated organic light-emitting diodes, fabricated on silicon wafers using poly[1-methoxy-4-(2-ethylhexyloxy-2,5-phenylvinylene)] as an emitting layer, were examined after failure. Two primary modes of failure are easily identified in the micrographs. The first process involves the formation of “black spots” caused by the pyrolitic carbonization of the emitting layer’s polymer, and the second is the delamination and concurrent swelling of the cathode.
Modeling bilayer polymer/fullerene photovoltaic devices
Authors: Koehler M. , Roman L. S. , Inganäs O. , da Luz M. G. E.
Published: June 2004

We investigate the transport properties of organic photovoltaic devices formed by a heterojunction of a semiconducting polymer poly {3-[4′-(1″,4″,7″-trioxaoctyl) phenyl] thiophene} and the fullerene (C60). Under monochromatic light of different wavelengths we measure the current-voltage (I–V) characteristic of diodes with variable thickness of the C60 layer. We propose an analytical model assuming that; (i) holes are created in the polymer by charge carrier generation at the heterojunction; and (ii) the C60 layer behaves like a photoconductor under illumination. By using the electrical conductivity of the C60 layer as fitting parameter we reproduce quite well the experimental data, including the I–V curves and the changes of the open-circuit voltage with the variation of the C60 layer thickness. We show that the values of the conductivity are closely related to the fullerene optical absorption coefficient, implying a large contribution of the C60 film to the diode photocurrent.
Synthesis of sheath voltage drops in asymmetric radio-frequency discharges
Authors: Yonemura Shigeru , Nanbu Kenichi , Iwata Naoaki
Published: June 2004

A sheath voltage drop in asymmetric discharges is one of the most important parameters of radio-frequency capacitively coupled plasmas because it determines the kinetic energy of the ions incident on the target or substrate. In this study, we developed a numerical simulation code to estimate the sheath voltage drops and, consequently, the self-bias voltage. We roughly approximated general asymmetric rf discharges to one-dimensional spherical ones. The results obtained by using our simulation code are consistent with measurements and Lieberman’s theory.
Ripening and annealing effects in InAs/GaAs(001) quantum dot formation
Authors: Krzyzewski T. J. , Jones T. S.
Published: June 2004

Scanning tunneling microscopy has been used to investigate the effects of annealing and quenching on InAs/GaAs(001) quantum dots (QD) grown at a low growth rate by molecular beam epitaxy (MBE). Significant changes in QD number density, average volume (of more than one order of magnitude), wetting layer morphology, and QD volume density have been observed after in situ annealing for up to 90 s at a growth temperature of 485 °C providing direct evidence for a QD ripening process resembling that observed in other heteroepitaxial growth systems. Inefficient sample removal from the growth chamber and cooling (quenching) leads to unintentional annealing and ripening of the QDs and/or the appearance of clusters on the sample surface. The appearance of these clusters can have a significant impact on any statistical analysis of QD samples. Our results show that MBE-grown InAs/GaAs(001) QDs are kinetically controlled structures even at low growth rates, and underline the importance of the quench method in minimizing postgrowth changes in the characteristics of the uncapped QD array and preserving the in situ surface morphology for ex situ surface studies.
Probing composition and interfacial interaction in oxide passivated core-shell iron nanoparticles by combining x-ray absorption and magnetic circular dichroism
Authors: Fauth K. , Goering E. , Schütz G. , Kuhn L. Theil
Published: June 2004

We present x-ray absorption and x-ray magnetic circular dichroism studies of iron oxide passivated iron nanoparticles. The combination of measurements with different probing depths allows us to determine the oxide composition while the magnetic data reveal the magnetic interactions across the interface between the α-Fe core and the spin canted ferrimagnetic iron oxide shell. These studies shed light on the mechanisms that govern the magnetic properties of core-shell nanoparticles.