bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhancement of photoluminescence in Ge nanoparticles by neighboring amorphous C in composite Ge/C thin films
Authors: Zhu Y. , Yuan C. L. , Ong P. P.
Published: May 2003

Composite thin films of Ge and C prepared with the pulsed laser deposition method were found to exhibit enhancement of photoluminescence (PL). X-ray diffractometry and atomic force microscopy established that the structure of the as-prepared sample consisted of Ge nanocrystals and amorphous C. A blue PL from the films was found in the wavelength region of 400–550 nm with two distinct humps in its peak. For the as-prepared sample, the PL intensity was found to peak at a certain Ge:C composition ratio. For films with the same Ge:C ratio, the PL intensity was found to increase monotonically with intensity of annealing in the open atmosphere. The origin of the PL is attributed to localized Ge/O-related defects and that of the humps to the two nonbonding electrons of Ge(O). It is argued that the two nonbonding electrons are related to the intermediate Ge oxidation states (GOx, 0<x<2), one of them being related to x near 0, and the other to x near 2, that is, near the saturated GeO2 state. The observed enhancement in the PL of the Ge nanoparticles is attributed to the presence of amorphous C because it enhances the formation of Ge nanoparticles and the localized Ge/O-related defects at the Ge/C interfacial regions. This phenomenon offers a possible useful way to control the PL intensity of the film by adjusting its Ge:C ratio or its intensity of annealing.
CO adsorption on the multiple-site Ru(112̄1) surface: The role of bonding competition
Authors: Fan C. Y. , Bonzel H. P. , Jacobi K.
Published: May 2003

The chemisorption and dissociation of CO on Ru(112̄1) were investigated by using high-resolution electron energy-loss spectroscopy and thermal desorption spectroscopy. Three different adsorption states of CO can be distinguished. The most strongly bound β-state, characterized by a C–O stretch frequency of 166 meV, is attributed to CO adsorbed in a fourfold hollow site of the Ru(112̄1) unit cell. This state occurs only at low total coverage and dissociates at T>300 K. A more weakly bound state is α1-CO with a stretch frequency of 240–255 meV, attributed to CO on-top bonded to first and second layer Ru atoms. This species converts to β-CO at moderate total coverage and T>360 K, increasing the amount of dissociated CO. The α1-CO species dominates the coverage regime up to 1.5 ML. The α2-CO species is most weakly bound in the coverage range up to 2 ML and is characterized by a stretch frequency of 220 meV. It is proposed to be located in twofold bridge sites. The ratio of on-top to bridge bonded CO is equal to three at saturation. The finite existence range for β-CO is rationalized by a bond competition effect, due to neighboring α1-CO species destabilizing the β-state at increasing coverage. Consequently β-CO converts to α1-CO under these conditions. A decrease of the β-state coverage via dissociation of CO may initiate the reverse process of α1- to β-CO conversion.
Assessing a microcanonical theory of gas-surface reactivity: Applicability to thermal equilibrium, nonequilibrium, and eigenstate-resolved dissociation of methane on Ni(100)
Authors: Bukoski A. , Harrison I.
Published: May 2003

A simple, three-parameter microcanonical theory of gas-surface reactivity is shown to predict experimental dissociative sticking probabilities for methane dissociative chemisorption on the Ni(100) surface over roughly ten orders of magnitude variation in both pressure and sticking—even at quantum state resolved levels of detail. Facile energy randomization within the transiently formed gas-surface collision complexes is postulated to make the pooled energy from 15 local degrees of freedom statistically available to surmount the barrier to dissociation. The apparent threshold energy for C–H bond cleavage of CH4 incident on Ni(100) is 67 kJ/mol, down from 432 kJ/mol in the gas phase.
Complete description of linear molecule photoionization achieved by vector correlations using the light of a single circular polarization
Authors: Lebech M. , Houver J. C. , Lafosse A. , Dowek D. , Alcaraz C. , Nahon L. , Lucchese R. R.
Published: May 2003

In this paper we demonstrate that the vector correlation approach for the study of dissociative photoionization (DPI) of linear molecules enables us to achieve a complete description of molecular photoionization by performing a single experiment using only one state of circularly, or elliptically, polarized light. This is illustrated by the derivation of the complex dipole matrix elements for the benchmark DPI reaction of the NO molecule, where (4σ)−1 inner-valence ionization is induced by left-handed circularly polarized synchrotron radiation at hν=23.65 eV. The importance of electronic correlation for this process is emphasized by comparing the experimental results with multichannel Schwinger configuration interaction calculations. The energy dependence of the transition matrix elements and that of the electronic correlation in the 25–40 eV energy range are illustrated by the calculations and compared with the present results and recent experimental studies at 40.8 eV.
Dynamical corrections to quantum transition state theory
Authors: Cheney Brian G. , Andersen Hans C.
Published: May 2003

Various versions of quantum transition state theory (QTST) for the calculation of chemical reaction rate coefficients have been developed. The Hansen–Andersen formulation of QTST approximates the reactive flux autocorrelation function of Miller, Schwartz, and Tromp for the problem of interest by the correlation function for a one-dimensional system with a suitably chosen parabolic reference potential. In this paper, we present a systematic method for correcting this approximation in order to obtain more accurate rate constants at low temperatures. Values of the correlation function for real or imaginary times, such as might be obtained from path integral Monte Carlo calculations, are used to construct an improved approximation for the flux autocorrelation function. Our method uses a Bayesian analysis that incorporates a modified form of the maximum entropy method. In contrast to other applications of the maximum entropy method to this problem, the improved approximation is consistent with the known analyticity properties of the correlation function. The method is applied to several test problems, including one based on the H2+Cl reaction. Our method yields significant corrections using a remarkably small amount of information.
Molecular simulation of a hydrated electron at different thermodynamic state points
Authors: Nicolas Cédric , Boutin Anne , Lévy Bernard , Borgis Daniel
Published: May 2003

The properties of the hydrated electron are studied by quantum-classical molecular-dynamics simulation in a wide range of temperature and pressure, from ambient to supercritical conditions. The calculations are based on a newly developed electron-water pseudo-potential based on rigorous quantum-mechanical calculations in the static exchange limit, as well as a novel methodological approach in which the electron wave function is expanded in a basis set of spherical Gaussians, distributed on a regular cubic lattice. Although the agreement with experiment is not completely quantitative, the strong experimental red shift of the absorption spectrum found experimentally with increasing temperature is recovered and a microscopic interpretation is proposed. It is also demonstrated that the observed shift is a density rather than a temperature effect. Finally, a striking, nonmonotonic evolution of the band width with increasing temperature, or decreasing density, is pointed out.
Adsorption of HF and HCl molecules on ice at 190 and 235 K from molecular dynamics simulations: Free energy profiles and residence times
Authors: Toubin C. , Picaud S. , Hoang P. N. M. , Girardet C. , Lynden-Bell R. M. , Hynes James T.
Published: May 2003

Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl.
Absolute and convective instability of a radially expanding liquid sheet
Authors: Lin S. P. , Jiang W. Y.
Published: May 2003

A radially expanding liquid sheet of a finite radius can be formed by impacting a liquid jet on a circular disk [Savart, Ann. Chim. Phys. 54, 55 (1833); Ann. Chim. 54, 113 (1833)] or by impinging two jet heads on against each other [Savart, Ann. Chim. Phys. 55, 257 (1833)]. The breakup of the liquid sheet first observed by Savart at the outer rim of the sheet is explained from the point of view of absolute and convective instability. Whether the sheet is convectively or absolutely unstable depends on the local Weber number We=ρU2H/S, where ρ and S are, respectively, the liquid density and the surface tension, and H and U are, respectively, the local half-sheet thickness and the local average liquid velocity. It is shown that absolute instability occurs at We=1, and convective instability occurs in the region where We>1. In the radially expanding liquid sheet, H decreases inversely with the radial distance from the center of the sheet, but U remains constant. Thus, the local Weber number that is greater than one near the center must decrease radially, and eventually reaches one where the sheet encounters absolute instability. In absolute instability, the disturbance propagates upstream to terminate the liquid sheet. This is one of the two regimes of breakup. The onset of absolute instability in this regime always occurs at the broken free edge, where the local Weber number is 1, regardless of the sheet radius. Taylor [Proc. R. Soc. London, Ser. A 253, 296 (1959)] obtained the same criterion from a balance of the surface tension force with a time rate of change of momentum experienced by the fluid at the broken edge. In regime II, it is shown that the sheet may break up at local Weber numbers greater than 1, due to convective instability. The global implications of the local absolute instability are discussed in connection with the related phenomena of breakups of planar sheets, water bells and annular jets.
Excited states of the 3d transition metal monoxides
Authors: Dai Bing , Deng Kaiming , Yang Jinlong , Zhu Qingshi
Published: May 2003

The electron affinities and low-lying excited states of all the 3d transition metal monoxide molecules are studied using the density functional theory (DFT) and time-dependent (TD) DFT method. The calculated results are compared with the available theoretical ones and used to assign the features of these monoxides in photoelectron spectroscopies. It shows that TDDFT, by and large, can be used to get good results for the excited states of the open-shell transition metal oxides. The effect of basis sets on the calculated results is also discussed.
Theoretical absorption spectrum of the Ar–CO van der Waals complex
Authors: López Cacheiro Javier , Fernández Berta , Pedersen Thomas Bondo , Koch Henrik
Published: May 2003

The three-dimensional intermolecular electric dipole moment surface of Ar–CO is calculated at the coupled cluster singles and doubles level of theory with the aug-cc-pVTZ basis set extended with a 3s3p2d1f1g set of midbond functions. Using the rovibrational energies and wave functions of our recent study [J. Chem. Phys. 117, 6562 (2002)], temperature-dependent spectral intensities are evaluated and compared to available experimental data. Based on the theoretical spectrum, alternative assignments of the experimentally observed lines in the fundamental band of CO around 2160 and 2166 cm−1 are suggested.
Accurate atomization energies and dipole moments from Ornstein–Uhlenbeck diffusion quantum Monte Carlo calculations for small first-row polyatomic molecules
Authors: Lu Shih-I
Published: May 2003

The total atomization energies and equilibrium dipole moments of some small first-row polyatomic molecules are calculated at the Ornstein–Uhlenbeck diffusion quantum Monte Carlo with the floating spherical Gaussians. By using explicitly correlated wave functions built from the spherical Gaussian-type orbitals and geminals to be the trial wave function, this work gives accuracy that is comparable to or better than previously theoretical estimates. The accuracies are less than 0.6 kcal mol−1 and 0.007 D for total atomization energies and equilibrium dipole moments, respectively. We also compare the performance of three different approximations in estimating expectation values of equilibrium dipole moment.
Chain model of a magnetorheological suspension in a rotating field
Authors: Melle Sonia , Martin James E.
Published: May 2003

We develop an athermal chain model for magnetorheological suspensions in rotating magnetic fields. This model is based on a balance of hydrodynamic and magnetostatic forces and focuses on the mechanical stability of chains. Using a linear approximation of the chain shape, we compute the orientation and size of a critical chain in a rotating magnetic field as a function of the Mason number Mn, which is the ratio of dipolar to hydrodynamic forces between two particles in contact. The critical chain length is found to scale with the inverse square root of Mn, and its orientation relative to the instantaneous field is independent of Mn. The actual nonlinear shape of a chain in a rotating field is then computed self-consistently. Finally, the effect of local fields on the dipolar interaction force is considered, leading to predictions for the chain shape and orientation that depend rather strongly on the magnetic permeability of the particles. A principal finding is the possibility of brittle or ductile chain fracture, depending on the permeability of the particles. Single-chain simulations confirm this prediction, as do experimental measurements.
Compressibility effects on turbulence structures of axisymmetric mixing layers
Authors: Thurow Brian , Samimy Mo , Lempert Walter
Published: May 2003

A real-time flow visualization system that produces 17 images over a time span of 150 μs is used to visualize the mixing layers of Mach 1.3 (Mc=0.59) and Mach 2.0 (Mc=0.87) ideally expanded high Reynolds number axisymmetric jets. These image sequences reveal details about the influence of compressibility on the dynamics of turbulence structures. In general, the behavior observed in axisymmetric jets is similar to that observed in planar shear layers at similar convective Mach numbers. In addition, large streamwise vortices are apparent in cross-stream images of the flow. Large-scale structures become more three-dimensional and less organized with increasing compressibility and more difficult to identify and track. Planar space–time correlations are used to track structures as they convect downstream. The histogram of the convective velocity for the Mach 1.3 jet revealed a broad distribution of convective velocities with an ensemble average of 266 m/s, which is much higher than the theoretical prediction of 206 m/s. The Mach 2.0 jet, however, exhibited a bimodal convective velocity distribution with an ensemble average of 402 m/s for the “fast” and 190 m/s for the “slow” mode. These modes are equally spaced from the theoretical convective velocity of 303 m/s. Approximately 2/3 of the measured velocities were in the slow mode.
Molecular dynamics study of shock-induced chemistry in small condensed-phase hydrocarbons
Authors: Elert M. L. , Zybin S. V. , White C. T.
Published: May 2003

Molecular dynamics simulations using an empirical bond order potential have been performed to investigate shock-induced chemistry in solid acetylene, ethylene, and methane. Acetylene was found to undergo significant polymerization reactions for flyer plate impact speeds above 10 km/s. These conditions are similar to those which would be experienced upon planetary impact of comets, which are known to contain condensed-phase acetylene. Ethylene exhibits similar reactivity above 15 km/s. Methane undergoes hydrogen abstraction reactions at flyer plate impact speeds of 16–20 km/s and produces hydrocarbon chains at higher impact speeds. The latter results are significant for elucidating the fate of atmospheric methane upon cometary or meteor impact, and for predicting the initial reaction steps in the reactivity of methane ices in the high-pressure, high-temperature interiors of Neptune and Uranus.
Spectroscopy of mass-selected VCo and VFe in argon matrices
Authors: Zhao Bing , Lu Haiyan , Likhtina Ija , Jules Joseph , Lombardi John R.
Published: May 2003

We examine the resonance Raman spectroscopy of the heteronuclear dimers VCo and VFe, using a mass-selected cluster source. Cluster cations are produced in a sputtering source and mass filtered with a Wien filter, then neutralized and deposited in an Ar matrix at low temperatures. For VCo we obtain the resonance Raman spectra in Ar matrix excited with 457.9, 488.0, 496.5, 501.7, and 514.5 nm laser lines. We observe Raman shifts of 459 and 916 cm−1, and we assign these lines to the fundamental and first overtone of the ground state. The resulting force constant is ke=3.42 mdyne/Å. Resonance Raman spectra of VFe, obtained at the same five excitation wavelengths, display Raman shifts at 423 and 836 cm−1, and we assign these to the fundamental and first overtone of the ground-state vibration. The resulting force constant is ke=2.94 mdyne/Å. We compare these observed force constants and configurations with those of other V–X dimers (X=Ti–Ni), as well as related isoelectronic species. The value for VFe, with 13 valence electrons, is considerably lower than its neighbors. This molecule most likely has a Σ+2 ground state, stemming from a (3dσ)2(3dπ)4(3dδ)4(4sσ)2(4sσ*)1 configuration. The higher force constant for VCo with 14 valence electrons indicates either a (3dσ)2(3dπ)4(3dδ)4(3dδ*)2(4sσ)2 configuration with a Σ3 state, or a (3dσ)2(3dπ)4(3dδ)4(3dδ*)1(4sσ)2(4sσ*)1 configuration with a Δ3 state.
Influence of pore roughness on high-frequency permeability
Authors: Cortis Andrea , Smeulders David M. J. , Guermond Jean Luc , Lafarge Denis
Published: May 2003

The high-frequency behavior of the fluid velocity patterns for smooth and corrugated pore channels is studied. The classical approach of Johnson et al. [J. Fluid Mech. 176, 379 (1987)] for smooth geometries is obtained in different manners, thus clarifying differences with Sheng and Zhou [Phys. Rev. Lett. 61, 1591 (1988)] and Avellaneda and Torquato [Phys. Fluids A 3, 2529 (1991)]. For wedge-shaped pore geometries, the classical approach is modified by a nonanalytic extension proposed by Achdou and Avellaneda [Phys. Fluids A 4, 2561 (1992)]. The dependency of the nonanalytic extension on the apex angle of the wedge was derived. Precise numerical computations for various apex angles in two-dimensional channels confirmed this theoretical dependency, which is somewhat different from the original Achdou and Avellaneda predictions. Moreover, it was found that the contribution of the singularities does not alter the parameters of the classical theory by Johnson et al..
Dynamics of particle sedimentation in a vertical channel: Period-doubling bifurcation and chaotic state
Authors: Aidun Cyrus K. , Ding E-Jiang
Published: May 2003

The dynamics and interaction of two circular cylinders settling in an infinitely long narrow channel (width equal to four times the cylinder diameter) is explained by direct computational analysis. The results show that at relatively low Reynolds numbers (based on the average particle velocity and diameter), the particles undergo complex transitions to reach a low-dimensional chaotic state represented by a strange attractor. As the Reynolds number increases, the initial periodic state goes through a turning point and a subcritical transition to another periodic branch. Further increase in the Reynolds number results in a cascade of period-doubling bifurcations to a chaotic state represented by a low dimensional chaotic attractor. The entire sequence of transitions takes place in a relatively narrow range of Reynolds number between 2 and 6. The physical reason for the period-doubling transitions is explained based on the interaction of the particles with each other and the channel walls. The particles undergo near contact interactions as settling in the channel. To accurately capture the dynamics, the computational method requires accurate resolution of the particle interactions. The computational results are obtained with our lattice-Boltzmann method developed for suspended particles near contact. The results show that even in the most simple and ideal multiparticle sedimentation, the system undergoes transition to complex dynamics at relatively low Reynolds number.
Coulomb blockade devices of Co dot arrays on tungsten-nanowire templates fabricated by using only a thin film technique
Authors: Lee Yun-Hi , Kim Dong-Ho , Shin Kyung-Sik , Choi Chang-Hoon , Jang Yoon-Taek , Ju Byeong-Kwon
Published: May 2003

A simple fabrication of gate-controlled nanodevices made of the Co nanodots on tungsten (W)-nanowire templates by using conventional photolithography and a thin film technique is reported. The combined multiple grain nanobridge allows the observation of the Coulomb gap up to a temperature as high as 200 K and shows nonlinear current–voltage characteristics up to 250 K. The combination of Co dots with straight W-nanowire templates opens possibilities of reproducible blockade devices.
Aspects of planar, oblique and interacting shock waves in an ideal dissociating gas
Authors: Sanderson S. R. , Hornung H. G. , Sturtevant B.
Published: May 2003

We develop a compact dimensionless framework for the analysis of canonical thermo-chemical nonequilibrium flow fields involving normal, oblique and interacting shock waves. Discontinuous solutions of the conservation equations are coupled with thermodynamic and kinetic models for an ideal dissociating gas. Convenient forms are provided for the variation of the relevant dimensionless parameters across shock waves in dissociating gases. The treatment is carried through in a consistent manner for the pressure–flow deflection angle plane representation of shock wave interaction problems. The contribution of the current paper is a careful nondimensionalization of the problem that yields a tractable formulation and allows results with considerable generality to be obtained.
On the optimization of mixing protocol in a certain class of three-dimensional Stokes flows
Authors: Rodrigo A. J. S. , Mota J. P. B. , Lefèvre A. , Saatdjian E.
Published: May 2003

Mixing in a special class of three-dimensional, non-inertial periodic flows is studied numerically. In the type of flow considered here, the cross-sectional velocity components are independent of the axial flow and the axial flow is independent of the axial coordinate. Using the eccentric helical annular mixer as a prototype, we consider the counter-rotating case with steady rotation of the outer cylinder and sinusoidal modulation of the inner one. Apart from the mixer geometry, the behavior of the system is governed by two dimensionless parameters obtained by scaling the cross-sectional stirring protocol with respect to the characteristic residence time of the fluid in the mixer. The first parameter is related to the average number of turns of the outer cylinder and the second one is related to the average number of modulation periods of the inner cylinder. The convection-diffusion equation is solved numerically, with temperature as a passive scalar, at high Péclet number. For a given three-dimensional mixer geometry and axial flow rate we show that there is an optimum modulation frequency for which the exit standard deviation of the temperature field is a minimum. Lagrangian simulations at infinite Péclet number and the use of other tools to study mixing, such as stretching calculations and tracer tracking methods, confirm that the optimized protocol does result in very effective mixing.
Spatial ordering in InP/InGaP nanostructures
Authors: Bortoleto J. R. R. , Gutiérrez H. R. , Cotta M. A. , Bettini J. , Cardoso L. P. , de Carvalho M. M. G.
Published: May 2003

We report the observation of a spatially-ordered bidimensional array of self-assembled InP quantum dots grown on slightly In-rich InGaP layers. The alignment of InP dots is observed along [100] and [010] directions. This effect is enhanced when 2° off vicinal substrates are used; it is also strongly dependent on growth temperature. Our results suggest that the density and size of CuPt-type atomically ordered regions as well as compositional modulation of InGaP layers play an important role on the spatial alignment of InP/InGaP quantum dots.
Experiments on shock wave reflection transition and hysteresis in low-noise wind tunnel
Authors: Ivanov M. S. , Kudryavtsev A. N. , Nikiforov S. B. , Khotyanovsky D. V. , Pavlov A. A.
Published: May 2003

Experiments on the transition between steady regular and Mach reflections of shock waves have been performed in a low-noise wind tunnel. Owing to a very low level of free-stream disturbances inherent in this facility, the hysteresis phenomenon has been observed in close agreement with theoretical predictions and results of the previous numerical simulations. When increasing the incident shock wave angle, regular reflection persists in most of the dual solution domain. Thus, the detachment criterion can be considered as a true one for the transition from regular to Mach reflection, and an earlier transition observed in many wind tunnel experiments is certainly caused by free-stream disturbances, which were present in those facilities.
Monolithic low-transition-temperature superconducting magnetometers for high resolution imaging magnetic fields of room temperature samples
Authors: Baudenbacher F. , Fong L. E. , Holzer J. R. , Radparvar M.
Published: May 2003

We have developed a monolithic low-temperature superconducting quantum interference device (SQUID) magnetometer and incorporated the device in a scanning microscope for imaging magnetic fields of room temperature samples. The instrument has a ∼100 μm spatial resolution and a 1.4 pT/Hz1/2 field sensitivity above a few hertz. We discuss design constraints on and potential applications of the SQUID microscope.
Er
3+
photoluminescence from Er-doped amorphous SiOx films prepared by pulsed laser deposition at room temperature: The effects of oxygen concentration
Authors: Ha Jeong Sook , Bae Chang Hyun , Nam Sang Hwan , Park Seung Min , Jang Young Rae , Yoo Keon Ho , Park Kyoungwan
Published: May 2003

We have fabricated Er-doped amorphous SiOx films by laser ablation of a Si:Er2O3 target in He atmosphere. The photoluminescence intensity at 1.54 μm was highly dependent on the oxygen content in the film, which turned out to be changed significantly by the ambient He pressure. Also, we have adopted time-of-flight quadrupole mass spectroscopy to obtain kinetic energies of ionic species in a plume produced by laser ablation. Si and Er ions do not overlap spatially as they expand toward the Si substrate and Er ions impinge on the preformed SiOx layer.
Very high average power at room temperature from λ≈5.9-μm quantum-cascade lasers
Authors: Yu J. S. , Slivken S. , Evans A. , David J. , Razeghi M.
Published: May 2003

We report a very high average output power at room temperature for quantum-cascade lasers emitting at λ≈5.9 μm. For high-reflectivity-coated 2-mm-long cavities, a low threshold current density of 1.7 kA/cm2 was obtained at room temperature. From 300 to 400 K, the characteristic temperature (T0) was 198 K. A maximum average output power of 0.67 W was achieved. In addition, 0.56 W average output power was observed at a duty cycle of 56%.
Electric-field effects of two-level systems observed with single-molecule spectroscopy
Authors: Bauer M. , Kador L.
Published: May 2003

The fluorescence excitation spectra of single chromophores in a solid often consist of several components at low temperature, which is due to the interaction with matrix two-level systems (TLSs). For a number of terrylene molecules in n-hexadecane we observed electric-field-induced changes of the intensity and splitting of the spectral components. With some basic assumptions the data allow us to calculate all the characteristic parameters of a single TLS, viz., the (zero field) asymmetry, the tunnel parameter, and the permanent electric dipole moment. In one case we found an exceptionally large dipole moment of about 8 Debye.
Velocity slip and temperature jump coefficients for gaseous mixtures. I. Viscous slip coefficient
Authors: Sharipov Felix , Kalempa Denize
Published: May 2003

The viscous slip coefficient was calculated for binary gaseous mixtures on the basis of the McCormack kinetic model of the Boltzmann equation, which was solved by the discrete velocity method. The calculations were carried out for the three mixtures of noble gases: neon–argon, helium–argon, and helium–xenon. It was showed that for the mixture of helium and xenon, which has a large ration of the molecular masses, the slip coefficient significantly differs from that for a single gas. A comparison of the present results with those obtained by the moment method applied to the Boltzmann equation showed that the McCormack model equation provides reliable results with modest computational efforts. An example of application of the viscous slip coefficient was given.
Microstructuring by printing and laser curing of nanoparticle solutions
Authors: Bieri N. R. , Chung J. , Haferl S. E. , Poulikakos D. , Grigoropoulos C. P.
Published: May 2003

In this letter, the process of printing and laser curing of nanoparticle solutions is presented. A liquid solvent is employed as the carrier of gold nanoparticles (the material of interest in this study) possessing a low melting temperature compared to that of bulk gold. Using a specifically designed printing system, the gold nanoparticle solution is deposited on a substrate and cured with laser radiation. In this manner, the potential of writing gold structures on temperature sensitive substrates is demonstrated. The interaction between the laser radiation and nanoparticles drives the solvent evaporation and controls the quality of the microstructures printing process. The latter is also affected by thermocapillary flow at the free surface, developing during the curing process. An optical method for estimating the curing times is also developed and discussed.
Impact of stress on oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films
Authors: Klenov Dmitri O. , Donner Wolfgang , Foran Brendan , Stemmer Susanne
Published: May 2003

We investigate oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films grown by sputter deposition on (001) LaAlO3 and (001) SrTiO3. After annealing at 500 °C under oxygen partial pressures greater than those used during deposition, films transform to a long-range oxygen vacancy ordered structure with orthorhombic symmetry. Observed orientation variants of the oxygen vacancy ordered structures are different for the two substrates. We discuss the relationship between film stress due to lattice and thermal mismatch with the substrate, and vacancy ordering.
Eu
3+
ion as fluorescent probe for detecting the surface effect in nanocrystals
Authors: Yan Chun-Hua , Sun Ling-Dong , Liao Chun-Sheng , Zhang Ying-Xin , Lu Yi-Qiang , Huang Shi-Hua , Lü Shao-Zhe
Published: May 2003

Nanocrystalline yttrium vanadate doped with trivalent europium ions, YVO4:Eu3+, was synthesized via the microemulsion mediated hydrothermal process. Laser selective excitation spectra were employed to probe the local environments of Eu3+ ions in YVO4 nanocrystals. The local environments of the dopant ions were diversified because of the surface effect in nanocrystals, which led to an increase in linewidth in excitation and emission spectra. Furthermore, the change of site symmetry from the interior to the surface of nanoparticles was clearly exhibited in the emission spectra of Eu3+ ions under different excitation wavelengths. The results show that the fluorescent behavior of doped Eu3+ ions may be a useful probe for detecting the surface effect in nanosize materials.
High-dielectric-constant all-polymer percolative composites
Authors: Huang Cheng , Zhang Q. M. , Su Ji
Published: May 2003

We report here an all-polymer high-dielectric (dielectric constant K>1000 at 1 kHz) percolative composite material, fabricated by a combination of conductive polyaniline particles (K>105) within a poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer matrix (K>50). These high-K polymer hybrid materials also exhibit high electromechanical responses. For example, 1.5% strain, which is proportional to the square of the field applied, can be induced by a field of 9.5 MV/m, an eightfold reduction in field applied compared with that in a fluoroterpolymer matrix.
Direct fabrication of nanowires in an electron microscope
Authors: Silvis-Cividjian N. , Hagen C. W. , Kruit P. , v.d. Stam M. A. J. , Groen H. B.
Published: May 2003

Electron-beam-induced deposition (EBID) is a potentially fast and resistless deposition technique which might overcome the fundamental resolution limits of conventional electron-beam lithography. We advance the understanding of the EBID process by simulating the structure growth. The merit of our model is that it explains the shapes of structures grown by EBID quantitatively. It also predicts the possibility to directly fabricate structures with lateral sizes smaller than 10 nm and points out the ideal conditions to achieve this goal. We verify these predictions by fabricating sub-10-nm lines and dots in a state-of-the-art scanning transmission electron microscope.
A method to extract quantitative information in analyzer-based x-ray phase contrast imaging
Authors: Pagot E. , Cloetens P. , Fiedler S. , Bravin A. , Coan P. , Baruchel J. , Härtwig J. , Thomlinson W.
Published: May 2003

Analyzer-based imaging is a powerful phase-sensitive technique that generates improved contrast compared to standard absorption radiography. Combining numerically two images taken on either side at ±1/2 of the full width at half-maximum (FWHM) of the rocking curve provides images of “pure refraction” and of “apparent absorption.” In this study, a similar approach is made by combining symmetrical images with respect to the peak of the analyzer rocking curve but at general positions, ±α⋅FWHM. These two approaches do not consider the ultrasmall angle scattering produced by the object independently, which can lead to inconsistent results. An accurate way to separately retrieve the quantitative information intrinsic to the object is proposed. It is based on a statistical analysis of the local rocking curve, and allows one to overcome the problems encountered using the previous approaches.
In situ photoemission characterization of terminating-layer-controlled La0.6Sr0.4MnO3 thin films
Authors: Kumigashira H. , Horiba K. , Ohguchi H. , Ono K. , Oshima M. , Nakagawa N. , Lippmaa M. , Kawasaki M. , Koinuma H.
Published: May 2003

We report on the fabrication of terminating-layer-controlled La0.6Sr0.4MnO3 (LSMO) thin films and in situ characterization of the films using photoemission spectroscopy. The terminating layer of the LSMO films was changed from B to A site by inserting one atomic layer of SrO between the LSMO film and a TiO2-terminated SrTiO3(001) substrate. The successful control of the terminating layer was confirmed by measuring the angular dependence of core-level spectra. Detailed analysis of the Sr 3d core levels revealed considerable differences in chemical bonding states of Sr atoms in the surface regions of films with different terminating layers.
Electrical properties of HfO2 deposited via atomic layer deposition using Hf(NO3)4 and H2O
Authors: Conley J. F. , Ono Y. , Solanki R. , Stecker G. , Zhuang W.
Published: May 2003

We report on the electrical properties of HfO2 deposited via atomic layer deposition using Hf(NO3)4 precursor for metal/oxide/semiconductor gate dielectric applications. Thin films, with less than 1% variation in accumulation capacitance over a 150 mm wafer, have been deposited directly on hydrogen-terminated Si wafers. The effective dielectric constant of thin (<10 nm) films was in the range of κeff=10–12, the breakdown voltage was about 6–9 MV/cm, and the leakage current was between 3–6 orders of magnitude lower than that of SiO2. The relative benefit of lower leakage current of HfO2 over SiO2 decreased with decreasing effective thickness. Electron trapping was observed under constant voltage stressing.
Structural and optical properties of epitaxial BaTiO3 thin films grown on GdScO3(110)
Authors: Schubert J. , Trithaveesak O. , Petraru A. , Jia C. L. , Uecker R. , Reiche P. , Schlom D. G.
Published: May 2003

We have prepared 1-μm-thick epitaxial BaTiO3 thin films on GdScO3(110) using pulsed laser deposition. The structural perfection of the films was revealed by a rocking curve width of Δω=0.13° for the 002 BaTiO3 reflection and a Rutherford backscattering spectrometry/channeling minimum yield, χmin, of 0.5% measured for the Ba signal behind the surface peak. High-resolution transmission electron microscopy revealed an epitaxial relationship between BaTiO3 and GdScO3 and a sharp interface between the substrate and the film. The refractive index of the BaTiO3 film was no=2.329±0.002 and ne=2.307±0.002 at a wavelength of 632.8 nm and no=2.248±0.002 and ne=2.228±0.002 at a wavelength of 1523 nm. The optical losses were less than 2 dB/cm at a wavelength of 632.8 nm.
Surface modes for near field thermophotovoltaics
Authors: Narayanaswamy Arvind , Chen Gang
Published: May 2003

Thermal radiative energy transfer between closely spaced surfaces has been analyzed in the past and shown not to obey the laws of classical radiation heat transfer owing to evanescent waves and, more recently, electromagnetic surface modes. We have analyzed the energy transfer between layered media, one of the layers being the thermal source, using a Green’s functions method and the fluctuation-dissipation theorem. Based on the analysis, we propose a structure that can utilize the surface modes to increase the power density and efficiency of low temperature thermophotovoltaic generators.
A field-emission display with a self-focus cathode electrode
Authors: Choi Y. S. , Cho Y. S. , Kang J. H. , Kim Y. J. , Kim I. H. , Park S. H. , Lee H. W. , Hwang S. Y. , Lee S. J. , Lee C. G. , Oh T. S. , Choi J. S. , Kang S. K. , Kim J. M.
Published: May 2003

A field-emission display (5-in.) using thick-film carbon nanotube emitters is fabricated. A thick-film insulating layer and an emitter layer are employed for low-cost manufacturing and scalability to a large panel. A self-focus cathode for this display is proposed. An auxiliary electrode in contact with the cathode electrode surrounds the emitter layer at the center of gate aperture. The structure has several advantages in manufacturing. According to simulation results, this self-focus cathode structure shows excellent focusing effects in spite of its simple manufacturing process and structure.
Determining the curvature dependence of surface tension
Authors: Kashchiev Dimo
Published: May 2003

The curvature dependence of the surface tension of one-component droplets and bubbles is determined approximately. In the scope of the approximation used, it is found that for both droplets and bubbles of any size the surface tension diminishes monotonically with decreasing the radius of the surface of tension.
Effect of Al2O3 capping layer on suppression of interfacial SiO2 growth in HfO2/ultrathin SiO2/Si(001) structure
Authors: Kundu Manisha , Miyata Noriyuki , Nabatame Toshihide , Horikawa Tsuyoshi , Ichikawa Masakazu , Toriumi Akira
Published: May 2003

We investigated the effect a 1.2-nm-thick Al2O3 capping layer had on suppressing interfacial Si oxidation in a 2.6-nm-HfO2/0.35-nm-SiO2/Si(001) structure during postdeposition annealing in an oxygen ambient. An incubation period (IP) was initially observed during which the HfO2/Si interface exhibited remarkable stability without any interfacial SiO2 growth. This was then followed by very slow interface oxidation. Our detailed study suggested that low oxidant diffusion through the capping layer determined the effective IP. Furthermore, HfO2/Si interface oxidation, which proceeded through a two-step process that was similar to an uncapped structure, was severely constrained by the limited availability of oxygen at the Al2O3/HfO2 interface.
Ferroelectric behavior of orientation-controlled PbBi4Ti4O15 thin films
Authors: Xianyu Wen Xu , Lee Wan In , Ko Taegyung , Lee June Key
Published: May 2003

Ferroelectric lead bismuth titanate (PbBi4Ti4O15) thin films, selectively controlled in c-axis and off-c-axis orientation, were fabricated on a Pt layer by a chemical solution deposition method. The off-c-axis oriented PbBi4Ti4O15 films demonstrated much higher remanent polarization (8.7 μC/cm2) than those of c-axis oriented films (3.7 μC/cm2). Regardless of grain orientation, PbBi4Ti4O15 films were not fatigued up to 1010 cycles under 9-V application. It is deduced that the role of Bi2O22+ layer in inducing fatigue-free property for this Bi-layered perovskite structure is the self-regulation of space charge.
Domain structure study of SrBi2Ta2O9 ferroelectric thin films by scanning capacitance microscopy
Authors: Leu Ching-Chich , Chen Chih-Yuan , Chien Chao-Hsin , Chang Mao-Nan , Hsu Fan-Yi , Hu Chen-Ti
Published: May 2003

Scanning capacitance microscopy was used to image the polarization-induced microstructural patterns of sol-gel derivative SrBi2Ta2O9 (SBT) thin films. A sharp image contrast was induced between the nanosized domains owing to the various polarities, so that the domain structure in the SBT thin film was clearly revealed. As a result, the switched and unswitched regions could be unequivocally identified. This investigation also confirms that the reversal polarization process of a ferroelectric domain is much easier inside a large grain than in a small grain.
Compositionally-tuned epitaxial cubic MgxZn1−xO on Si(100) for deep ultraviolet photodetectors
Authors: Yang W. , Hullavarad S. S. , Nagaraj B. , Takeuchi I. , Sharma R. P. , Venkatesan T. , Vispute R. D. , Shen H.
Published: May 2003

We report on the epitaxial growth of wide-band-gap cubic-phase MgxZn1−xO thin films on Si(100) by pulsed-laser deposition and fabrication of oxide-semiconductor-based ultraviolet photodetectors. The challenges of large lattice and thermal expansion mismatch between Si and MgxZn1−xO have been overcome by using a thin SrTiO3 buffer layer. The heteroepitaxy of cubic-phase MgxZn1−xO on Si was established with epitaxial relationship of MgxZn1−xO(100)//SrTiO3(100)//Si(100) and MgxZn1−xO[100]//SrTiO3[100]//Si[110]. The minimum yield of the Rutherford backscattering ion channeling in MgxZn1−xO layer was only 4%, indicating good crystalline quality of the film. Smooth surface morphology with rms roughness of 0.6 nm was observed using atomic force microscopy. Photodetectors fabricated on Mg0.68Zn0.32O/SrTiO3/Si show peak photoresponse at 225 nm, which is in the deep UV region.
Multiple liquid–liquid transitions in supercooled water
Authors: Brovchenko Ivan , Geiger Alfons , Oleinikova Alla
Published: May 2003

Three distinct liquid–liquid coexistence regions were observed for ST2 model water by restricted ensemble Monte Carlo simulations of the isotherms of homogenized systems and by phase equilibria simulations in the Gibbs ensemble. The lowest density liquid–liquid transition meets the liquid–vapor phase transition at a triple point and ends in a metastable critical point. A percolation analysis evidences, that the phase separations at the lowest and highest densities can be attributed to the separation of differently coordinated water molecules. The densities of the obtained four phases of supercooled water correlate with experimentally observed densities of amorphous ice.
Photoelimination of C2H2 and H2 from styrene at 193 nm
Authors: Lee Ya-Rong , Chen Cheng-Chu , Lin Shen-Maw
Published: May 2003

In this letter, we report the photodissociation of styrene at 193 nm by product translational spectroscopy. The results show that the molecule undergoes predominantly molecular elimination of C2H2 and H2 with a branching ratio C2H2/H2≈7. In addition, a secondary dissociation channel from the internally excited C6H6 product was identified. Product translational energy distributions were measured for all reaction channels. The lack of the product anisotropy is consistent with previous investigations that fragment separation occurred after structural rearrangement.
Dependence of the pyroelectric response on internal stresses in ferroelectric thin films
Authors: Ban Z.-G. , Alpay S. P.
Published: May 2003

The role of internal stresses on the pyroelectric properties of ferroelectric thin films is analyzed theoretically via a thermodynamic model. The pyroelectric coefficient as a function of the misfit strain is calculated for (001) Ba0.6Sr0.4TiO3 epitaxial thin films. It is shown that this property is highly dependent on the misfit strain. A very large pyroelectric response (0.65 μC/cm2 K) is theoretically predicted at a critical misfit strain (∼−0.05%) corresponding to the ferroelectric to paraelectric phase transformation. The analysis shows that internal tensile stresses are particularly not desirable with significant degradation close to an order of magnitude in the pyroelectric response.
Effect of nitrogen fraction on the temperature dependence of GaNAs/GaAs quantum-well emission
Authors: Potter R. J. , Balkan N. , Carrère H. , Arnoult A. , Bedel E. , Marie X.
Published: May 2003

The effects of nitrogen fraction on the temperature dependence of GaNxAs1−x/GaAs (x<2.8%) quantum-well emission was investigated using steady-state photoluminescence between 2 and 300 K. At low temperatures, a characteristic S-shape behavior indicative of carrier localization was observed for each of the samples. This is believed to result from the large miscibility gap induced by the nitrogen, which results in structural/compositional fluctuations in the well. In the high temperature regime (T>150 K) where the emission has a linear dependence, a strong reduction in emission temperature dependence was observed with increasing nitrogen. The temperature dependence was modeled using the band anticrossing approach, with the interaction matrix element parameter CNM (VMN=−CMNx) and the nitrogen level parameter γ (EN=EN0−γx) used as fitting parameters.
Room-temperature operation of an InAs–GaAs–AlAs quantum-cascade laser
Authors: Carder D. A. , Wilson L. R. , Green R. P. , Cockburn J. W. , Hopkinson M. , Steer M. J. , Airey R. , Hill G.
Published: May 2003

We report the shortest-wavelength (λ∼8.5 μm) room-temperature laser operation so far achieved for GaAs-based quantum-cascade (QC) lasers. By depositing InAs monolayers in the device active regions during growth, we are able to both reduce the emission wavelength and minimize thermally activated carrier leakage into quasicontinuum states. This approach results in laser operation up to 305 K, with a peak optical power of ∼10 mW at room temperature. A reduced temperature sensitivity of the threshold current compared with similar GaAs–Al0.45Ga0.55As QC lasers is also observed.