bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure prediction of high-pressure phases for alkali metal sulfides
Authors: Schön J. C. , Čančarević Ž. , Jansen M.
Published: July 2004

For a given chemical system we present a systematic approach to predict structures, which may exist at high pressure, by investigating the global enthalpy landscape. We combine global optimizations, based on empirical potential energy functions, and local optimizations (volume, cell shape, and atomic positions) on both Hartree-Fock and density functional theory level. We predict the existence of high-pressure phases for the alkali metal sulfides of the composition M2S (M=Li, Na, K, Rb, Cs), together with the transition pressures among these phases.
Biomolecular free energy profiles by a shooting/umbrella sampling protocol, “BOLAS”
Authors: Radhakrishnan Ravi , Schlick Tamar
Published: July 2004

We develop an efficient technique for computing free energies corresponding to conformational transitions in complex systems by combining a Monte Carlo ensemble of trajectories generated by the shooting algorithm with umbrella sampling. Motivated by the transition path sampling method, our scheme “BOLAS” (named after a cowboy’s lasso) preserves microscopic reversibility and leads to the correct equilibrium distribution. This makes possible computation of free energy profiles along complex reaction coordinates for biomolecular systems with a lower systematic error compared to traditional, force-biased umbrella sampling protocols. We demonstrate the validity of BOLAS for a bistable potential, and illustrate the method’s scope with an application to the sugar repuckering transition in a solvated deoxyadenosine molecule.
Theoretical calculations of effective exchange integrals by spin projected and unprojected broken-symmetry methods. III. Cluster models of three-dimensional KNiF3 solid
Authors: Onishi Taku , Yamaguchi Kizashi
Published: July 2004

Previously, we have performed the spin-polarized hybrid-density functional theory (HUDFT) calculations for elucidating magnetic properties of the two-dimensional (2D) K2NiF4 and K2CuF4 solids. In Part I, it has been concluded that the half-and-half-(HH-) type HUDFT method is one of the best calculation methods for these species. On the other hand, in Part II, we have demonstrated that potential curves for cluster models of K2CuF4 and KCuF3 are reasonably calculated by the HH-type HUDFT method under the approximate spin projection, and the lattice distortion resulted by Jahn-Teller effect is expressed as the second-order polynomial. In this study, we pay attention to the three-dimensional (3D) magnetic interactions in KNiF3. Our effective exchange integral Jab schemes for 3D cluster models such as KNi8F12 (4) with and without point charges have provided the reasonable Jab values (−30.24–34.48 cm−1), in comparison with the experimental one (−30.58 cm−1). The 3D magnetic interactions have been investigated from viewpoints of the Mulliken spin density and charge density populations, the natural orbital analysis, and chemical indices. Point charges located in positions of fluorine anion adjacent to cluster model have reproduced Jab values well. Roles of potassium as counter cation in KNiF3 solid were also investigated. It was concluded that potassium has a role of stabilizing the 3D magnetic structures. Finally, the mutual relationships between broken-symmetry and symmetry-adapted approaches are discussed on the basis of chemical indices.
Vibrational and rotational dynamics of cyanoferrates in solution
Authors: Sando Gerald M. , Zhong Q. , Owrutsky J. C.
Published: July 2004

Ultrafast infrared spectroscopy has been used to measure vibrational energy relaxation (VER) and reorientation (Tr) times for the high frequency vibrational bands of potassium ferrocyanide and ferricyanide (CN stretches), and sodium nitroprusside (SNP, CN, and NO stretches) in water and several other solvents. Relatively short VER times (4–43 ps) are determined for the hexacyano species and for the NO band of SNP, but the CN band of SNP relaxes much more slowly (55–365 ps). The solvent dependence of the VER times is similar for all the solutes and resembles what has been previously observed for triatomic molecular ions [Li et al., J. Chem. Phys. 98, 5499 (1993)]. Anisotropy decay times are also measured from the polarization dependence of the transient absorptions. The Tr times determined for SNP are different for the different vibrational bands; for the nondegenerate NO mode of nitroprusside (SNP) they are much longer (>15 ps), correlate with solvent viscosity, and are attributed to overall molecular rotation. The short Tr (<10 ps) times for the CN band in SNP and for the hexacyanoferrates are due to dipole orientational relaxation in which the transition moment rapidly redistributes among the degenerate modes. There is no evidence of intramolecular vibrational relaxation (IVR) to other high frequency modes. VER times measured for hexacarbonyls and SNP in methanol are similar, which suggests that the generally faster VER for the latter is in part because they are soluble in more strongly interacting polar solvents. The results are compared to those for small ions and metal carbonyls and are discussed in terms of the importance of solute charge and symmetry on VER.
Growth kinetics, structure, and morphology of para-quaterphenyl thin films on gold(111)
Authors: Müllegger S. , Salzmann I. , Resel R. , Hlawacek G. , Teichert C. , Winkler A.
Published: July 2004

The adsorption, desorption, and growth kinetics as well as the thin film morphology and crystal structure of p-quaterphenyl (4P) grown under ultrahigh vacuum conditions on single crystalline Au(111) have been investigated. Thermal desorption spectroscopy (TDS) reveals two distinct first-order peaks attributed to monolayer desorption followed by a zero-order multilayer desorption. The saturation coverage of the full 4P monolayer has been quantitatively measured with a quartz microbalance to be 8×1013 molecules/cm2. Using low energy electron diffraction the structures of the 0.5 and 1 ML (monolayer) adsorbates have been studied, showing highly regular arrangements of the 4P molecules, which are affected by the (111) surface structure. At the transition from 0.5 to 1 ML a structural compression of the overlayer has been observed. The behavior of thicker 4P films has been investigated by combined TDS-XPS (XPS—x-ray photoelectron spectroscopy). A temperature-induced recrystallization process at about 270 K has been observed for a 7 nm thick 4P film grown at 93 K, corresponding to a transition from a disordered layerlike growth to a crystalline island growth. Ex situ optical microscopy and atomic-force microscopy investigations have revealed needle-shaped 4P islands. Applying x-ray diffraction the crystalline order and epitaxial relationship of the 4P films with 30 nm and 200 nm mean thicknesses have been determined.
Preparation and resolution of molecular states by coherent sequences of phase-locked ultrashort laser pulses
Authors: Ramos-Sánchez Saúl , Romero-Rochı́n Vı́ctor
Published: July 2004

We study the application of nonlinear wave packet interferometry to the preparation and resolution of the overlaps of nonstationary nuclear wave functions evolving in an excited electronic state of a diatomic molecule. It is shown that possible experiments with two phase-locked ultrashort pulsepairs can be used to determine a specific vibrational wave packet state in terms of coherent states of the ground electronic state. We apply this scheme to an idealized molecule with harmonic potential energy surfaces and to the X←B transition states of the iodine molecule. Our results indicate that this scheme is very promising as a potential tool to quantum control.
High frequency acoustic excitations in ordered diblock copolymer studied by inelastic x-ray scattering
Authors: Kriegs H. , Steffen W. , Fytas G. , Monaco G. , Dreyfus C. , Fragouli P. , Pitsikalis M. , Hadjichristidis N.
Published: July 2004

The phonon propagation in lamellar nanostructures formed via self-assembling of short styrene-b-isoprene (SI) as well as of its more incompatible styrene-b-(ethylene-alt-propylene) (SEP) counterpart was studied by inelastic x-ray scattering. Irrespective of the physical state of the block copolymers, a single acoustic phonon was observed in SI (ordered and disordered) and SEP (ordered). At GHz frequencies, inelastic light scattering from the same samples revealed very small dispersion in the sound phase velocity but a short phonon lifetime.
High-quality HfSixOy gate dielectrics fabricated by solid phase interface reaction between physical-vapor-deposited metal–Hf and SiO2 underlayer
Authors: Watanabe Heiji , Saitoh Motofumi , Ikarashi Nobuyuki , Tatsumi Toru
Published: July 2004

We fabricated high-quality Hf–silicate (HfSixOy) gate dielectrics by utilizing the solid phase interface reaction between physical-vapor-deposited metal–Hf (typically 0.5nm thick) and SiO2 underlayers. Metal diffusion to the SiO2 layer increases the permittivity of the underlayer, while preservation of the initial SiO2∕Si bottom interface ensures good electrical properties of the gate dielectrics. The Hf–silicate layer remains amorphous and the poly-Si∕HfSixOy gate stack endures activation annealing at 1000°C. The interface trap density was comparable to that of conventional SiO2 dielectrics and the hysteresis of capacitance–voltage curves was as low as 4mV for a bias swing between −2 and +2.5V. Moreover, high electron mobility, equal to 89% of the universal mobility, was obtained for the high-k gate transistor.
Classical trajectory calculations of intramolecular vibrational energy redistribution. I. Methanol-water complex
Authors: Yamamoto Norifumi , Nishino Yumi , Miyoshi Eisaku
Published: July 2004

Intramolecular vibrational energy redistributions of the O-H stretching (νOH) vibration for the methanol monomer and its water complex, the methanol-water dimer, are investigated by using ab initio full-dimensional classical trajectory calculations. For the methanol monomer, in the high-energy regime of the 5νOH overtone, the time dependence of the normal-mode energies indicates that energy flowed from the initial excited O-H stretching mode to the C-H stretching mode. This result confirms the experimental observation of energy redistribution between the O-H and C-H stretching vibrations [L. Lubich et al., Faraday Discuss. 102, 167 (1995)]. Furthermore, a lot of dynamical information in the time domain is contained in the power spectra, whose density is given by the Fourier transformation of the total momentum obtained from trajectory calculations. For the methanol-water hydrogen-bonded complex, at the high-energy level of the 5νOH overtone, the calculated power spectrum shows considerable splitting and broadening, indicating significant energy redistribution through strong coupling between the O-H stretching vibration and other vibrations. It is thus clear that the A-H⋯B hydrogen-bond formation facilitates energy redistribution subsequent to the vibrational excitation of the hydrogen-bonded A-H stretching mode.
Classical trajectory calculations of intramolecular vibrational energy redistribution. II. Phenol-water complex
Authors: Yamamoto Norifumi , Miyoshi Eisaku
Published: July 2004

Ab initio classical trajectory calculations have been applied to the intramolecular vibrational energy redistribution process of an O-H stretching vibration for phenol cation, [phenol]+, and its hydrogen-bonded water complex, [phenol-water]+. In phenol cation, a single narrow peak in the power spectrum, obtained by Fourier transformation of the autocorrelation function of its total momentum, indicates that the initial energy given to the O-H stretching oscillator of the phenol moiety is conserved and no energy flow occurs. On the other hand, for phenol-water cation, the calculated broadened power spectrum implies that the initial energy is not conserved and the energy flow causes an energy redistribution among various vibrational modes.
Li intercalation in TiO2 anatase: Raman spectroscopy and lattice dynamic studies
Authors: Smirnov M. , Baddour-Hadjean R.
Published: July 2004

Raman spectra of the electrochemically lithiated TiO2 anatase are reported. They evidence spectral features induced by Li intercalation in the high-frequency region. Emergence of these lines supports recent hypothesis [M. Wagemaker et al., J. Am. Chem. Sec. 125, 840 (2003)] for formation of short Li–O valence bonds. This suggestion is verified by the lattice dynamics simulation based on the potential model with Li–O force constants extrapolated from known studies of the lithium oxide compounds. The obtained results confirm assumption of multiple Li positions and provide assignment of all the observed spectral features.
Oppositely charged colloidal binary mixtures: A colloidal analog of the restricted primitive model
Authors: Caballero José B. , Puertas Antonio M. , Fernández-Barbero Antonio , de las Nieves F. Javier
Published: July 2004

The equilibrium phase diagram of a colloidal system composed of 1:1 mixture of positive and negative particles with equal charge is studied by means of Monte Carlo simulations. The system is the colloidal analog of the restricted primitive model (RPM) for ionic fluids. A liquid-gas transition is found in the low-temperature–low-density region, similar to the liquid-gas transition in the RPM. The fluid-crystal transition is also studied, and the liquid phase is shown to be stable in a narrow range of temperatures. In the liquid, the pair distribution function shows alternating layers of particles with opposite sign of charge surrounding every particle. In the vapor phase, clusters of particles are observed, again in agreement with the RPM. However, a decreasing distribution of clusters is obtained, instead of the discrimination between charged and neutral clusters found in the RPM.
Interplay of secondary structures and side-chain contacts in the denatured state of BBA1
Authors: Wen Edward Z. , Luo Ray
Published: July 2004

The denatured state of a miniprotein BBA1 is studied under the native condition with the AMBER/Poisson-Boltzmann energy model and with the self-guided enhanced sampling technique. Forty independent trajectories are collected to sample the highly diversified denatured structures. Our simulation data show that the denatured BBA1 contains high percentage of native helix and native turn, but low percentage of native hairpin. Conditional population analysis indicates that the native helix formation and the native hairpin formation are not cooperative in the denatured state. Side-chain analysis shows that the native hydrophobic contacts are more preferred than the non-native hydrophobic contacts in the denatured BBA1. In contrast, the salt-bridge contacts are more or less nonspecific even if their populations are higher than those of hydrophobic contacts. Analysis of the trajectories shows that the native helix mostly initiates near the N terminus and propagates to the C terminus, and mostly forms from 310-helix/turn to α helix. The same analysis shows that the native turn is important but not necessary in its formation in the denatured BBA1. In addition, the formations of the two strands in the native hairpin are rather asymmetric, demonstrating the likely influence of the protein environment. Energetic analysis shows that the native helix formation is largely driven by electrostatic interactions in denatured BBA1. Further, the native helix formation is associated with the breakup of non-native salt-bridge contacts and the accumulation of native salt-bridge contacts. However, the native hydrophobic contacts only show a small increase upon the native helix formation while the non-native hydrophobic contacts stay essentially the same, different from the evolution of hydrophobic contacts observed in an isolated helix folding.
Vibrational effects in laser-driven molecular wires
Authors: Lehmann Jörg , Kohler Sigmund , May Volkhard , Hänggi Peter
Published: July 2004

The influence of an electron-vibrational coupling on the laser control of electron transport through a molecular wire that is attached to several electronic leads is investigated. These molecular vibrational modes induce an effective electron-electron interaction. In the regime where the wire electrons couple weakly to both the external leads and the vibrational modes, we derive within a Hartree-Fock approximation a nonlinear set of quantum kinetic equations. The quantum kinetic theory is then used to evaluate the laser driven, time-averaged electron current through the wire-leads contacts. This formalism is applied to two archetypical situations in the presence of electron-vibrational effects, namely, (i) the generation of a ratchet or pump current in a symmetrical molecule by a harmonic mixing field and (ii) the laser switching of the current through the molecule.
A SPR and AFM study of the effect of surface heterogeneity on adsorption of proteins
Authors: Huang Yu-Wen , Gupta Vinay K.
Published: July 2004

The effect of chemical heterogeneity of surfaces on the adsorption of proteins was investigated using model surfaces prepared by self-assembly of ω-functionalized alkanethiols on gold substrates. Surface plasmon resonance was used to monitor the adsorption kinetics of bovine serum albumin (BSA) and the morphology of the adsorbed BSA was imaged with tapping mode atomic force microscopy. The experiments show that the morphology of the adsorbed protein layer was altered significantly only when the surface heterogeneity was distributed in a patchwise manner on a nanometer length scale, which is commensurate with the dimension of the protein. In contrast to linear flexible polymers where the initial adsorption rate remained unchanged upon introduction of the chemical heterogeneity, the initial rate for the globular protein changed from the value observed on homogeneous surfaces and was dependent on the heterogeneous distribution of the chemical sites.
Nuclear and electron spin relaxation in paramagnetic complexes in solution: Effects of the quantum nature of molecular vibrations
Authors: Kruk Danuta , Kowalewski Jozef , Westlund Per-Olof
Published: July 2004

A model of the paramagnetic relaxation enhancement is developed in terms of electron-spin relaxation caused by the zero-field splitting (ZFS) fluctuating in time due to a coupling between the electron-spin variables and quantum vibrations. The ZFS interaction provides a coupling between the electron-spin variables and vibrational degrees of freedom, and is represented as a Taylor series expansion in a set of vibrational modes (normal coordinates). A two-level harmonic oscillator subsystem is assumed, and the electron-spin relaxation associated with T2V and T1V vibrational relaxation is considered. The description of vibrationally induced electron-spin dynamics is incorporated into the calculations of the paramagnetic relaxation enhancement by the Solomon–Bloembergen–Morgan approach as well as in the framework of the general slow-motion theory. The theoretical predictions are compared with the experimental paramagnetic relaxation enhancement values for the Ni(H2O)62+ complex in aqueous solution. The parameters required by the model are obtained from quantum chemical and molecular dynamics studies. Comparison is made between the current model and its recently published classical counterpart.
High-resolution kinetic energy release distributions and dissociation energies for fullerene ions Cn+, 42⩽n⩽90
Authors: Głuch K. , Matt-Leubner S. , Echt O. , Concina B. , Scheier P. , Märk T. D.
Published: July 2004

We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, Cn+→Cn−2++C2, for sizes 42⩽n⩽90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values.
Density functional investigations of the properties and thermochemistry of UF6 and UF5 using valence-electron and all-electron approaches
Authors: Batista Enrique R. , Martin Richard L. , Hay P. Jeffrey , Peralta Juan E. , Scuseria Gustavo E.
Published: July 2004

The structural properties and thermochemistry of UF6 and UF5 have been investigated using both Hartree-Fock and density functional theory (DFT) approximations. Within the latter approach, the local spin-density approximation, the generalized gradient approximation, and hybrid density functionals were considered. To describe the uranium atom we employed small-core (60 electrons) and large-core (78 electrons) relativistic effective core potentials (RECPs), as well as the all-electron approximation based on the two-component third-order Douglas-Kroll-Hess Hamiltonian. For structural properties, we obtained very good agreement with experiment with DFT and both large and small-core RECPs. The best match with experiment is given by the hybrid functionals with the small-core RECP. The bond dissociation energy (BDE) was obtained from the relative energies of the fragments [UF6→UF5+F], corrected for zero-point energy and spin-orbit interaction. Very good agreement was found between the BDE obtained from all-electron calculations and those calculated with the small-core RECP, while those from the large-core RECP are off by more than 50%. In order to obtain good agreement with experiment in the BDE it is imperative to work with hybrid density functionals and a small-core RECP.
Transport properties of single-crystal tetracene field-effect transistors with silicon dioxide gate dielectric
Authors: Newman Christopher R. , Chesterfield Reid J. , Merlo Jeffrey A. , Frisbie C. Daniel
Published: July 2004

Single-crystal organic field-effect transistors (SX-OFETs) with channel lengths of 1 and 100μm have been fabricated by adhering thin crystals of tetracene to freshly ashed SiO2 substrates containing countersunk gold contacts. The intrinsic transport properties of the tetracene single crystals, corrected for potential contact effects by using a standard four-probe configuration, have been measured from room temperature down to 4.2K. These OFETs exhibit mobilities as high as 0.1cm2V−1s−1, subthreshold swings of <500mV∕decade, and Ion∕Ioff ratios in excess of 109. The larger devices (L=100μm,W=1000μm) show thermally activated mobilities over the temperature range 200K<T<300K, but thermally induced cracks in the crystal prevent this analysis from being extended to lower temperatures. The smaller devices have a greater probability of surviving to low temperatures without a crack permeating the active channel, and representative devices have been investigated over the full range 4.2K<T<300K. The transport mechanism in these smaller devices can be varied from thermally activated to nearly temperature invariant by the application of drain fields >105V∕cm.
On the nonlinear evolution of wind-driven gravity waves
Authors: Alexakis A. , Calder A. C. , Dursi L. J. , Rosner R. , Truran J. W. , Fryxell B. , Zingale M. , Timmes F. X. , Olson K. , Ricker P.
Published: July 2004

We present a study of wind-driven nonlinear interfacial gravity waves using numerical simulations in two dimensions. We consider a case relevant to mixing phenomenon in astrophysical events such as novae in which the density ratio is approximately 1:10. Our physical setup follows the proposed mechanism of Miles [J. Fluid Mech. 3, 185 (1957)] for the amplification of such waves. Our results show good agreement with linear predictions for the growth of the waves. We explore how the wind strength affects the wave dynamics and the resulting mixing in the nonlinear stage. We identify two regimes of mixing, namely, the overturning and the cusp-breaking regimes. The former occurs when the wind is strong enough to overcome the gravitational potential barrier and overturn the wave. This result is in agreement with the common notion of turbulent mixing in which density gradients are increased to diffusion scales by the stretching of a series of vortices. In the latter case, mixing is the result of cusp instabilities. Although the wind is not strong enough to overturn the wave in this case, it can drive the wave up to a maximum amplitude where a singular structure at the cusp of the wave forms. Such structures are subject to various instabilities near the cusp that result in breaking the cusp. Mixing then results from these secondary instabilities and the spray-like structures that appear as a consequence of the breaking.
Single-crystalline gallium-doped indium oxide nanowires
Authors: Chun Hye Jin , Choi Young Sang , Bae Seung Yong , Choi Hyun Chul , Park Jeunghee
Published: July 2004

High-density gallium (Ga)-doped indium oxide (In2O3) nanowires whose Ga content [Ga∕(In+Ga) atomic ratio] is 0%, 7%, and 45%, were synthesized by thermal evaporation. They have an average diameter of 50nm and consist of nearly single-crystalline cubic In2O3 structure with the [010] growth direction. High-resolution x-ray diffraction analysis reveals that as the Ga content increases the position of In2O3 peaks shifts to the higher angle and a crystalline form (Ga,In)2O3 is produced. We estimated a 0.4% reduction of the lattice constant for 45% Ga-doped nanowires.
Replacement equivalence of H− and argon in small (Ar)nH− clusters from optimized structure calculations
Authors: Sebastianelli F. , Baccarelli I. , Di Paola C. , Gianturco F. A.
Published: July 2004

The structural properties of some of the smaller ionic clusters of argon atoms containing the atomic impurity H−, ArnH− with n from 2 up to 7, are examined using different modeling for the interactions within each cluster and by employing different theoretical treatments, both classical and quantum, for the energetics. The same calculations are also carried out for the corresponding neutral homogeneous clusters Arn+1. The results of the calculations, the physical reliability of the interactions modeling, and the similarities and the difference between the anionic and the neutral complexes are discussed in some detail. The emerging picture shows that, due to specific features of the employed atom-atom potentials, the ArnH− and Arn+1 clusters present very similar structures, where the H− dopant substitutes for one of the outer Ar atoms but does not undergo as yet solvation within such small clusters.
Exploring spin valve magnetization reversal dynamics with temporal, spatial and layer resolution: Influence of domain-wall energy
Authors: Kuch W. , Vogel J. , Camarero J. , Fukumoto K. , Pennec Y. , Pizzini S. , Bonfim M. , Kirschner J.
Published: July 2004

The magnetization reversal dynamics of an Fe20Ni80∕Cu∕Co spin valve is investigated on the nanosecond time scale by magnetic microscopy with time and layer resolution. It is found that the speed by which micron-sized magnetic domains in the magnetically soft Fe20Ni80 layer are expanded by external field pulses exhibits a dependence on the change in domain-wall length, and on the coupling to the local magnetization direction of the Co layer.
Visible-wavelength super-refraction in photonic crystal superprisms
Authors: Baumberg J. J. , Perney N. M. B. , Netti M. C. , Charlton M. D. C. , Zoorob M. , Parker G. J.
Published: July 2004

We demonstrate the fabrication of superprism devices in photonic crystal waveguides with excellent transmission through 600 rows of 160nm diameter holes. Broadband spectral and angular measurements allow mapping of the chromatic refractivity. This shows the ability of such devices to super-refract by more than 1°∕nm close to the principal band gaps, 10× more than equivalent gratings, and 100× more than equivalent prisms. Simple theories based on plane-wave models give excellent agreement with these results.
Cooling of high-power-density microdevices using liquid metal coolants
Authors: Miner A. , Ghoshal U.
Published: July 2004

High electrically conducting fluids such as liquid metals offer a unique solution to the current and future cooling needs of high power density heat sources. The two principal advantages of developing single phase cooling systems based on liquid metals lie in their superior thermophysical properties and in the ability to pump these liquids efficiently with silent, nonmoving pumps. Closed loop systems based on liquid metals and liquid metal pumps enable gravity independent high performance cooling systems. Analytical and experimental work is presented showing heat transfer coefficients on the order of 10W∕cm2∕K and miniature pumps operating at greater than 8kPa maximum pressure rise and 1% maximum efficiency.
Full-field coherence-gated holographic imaging through scattering media using a photorefractive polymer composite device
Authors: Dean P. , Dickinson M. R. , West D. P.
Published: July 2004

We report full-field, retroreflective holographic imaging through turbid media using a photorefractive polymer composite as a coherence gate. A four-wave mixing geometry was used to record and reconstruct two-dimensional images of test objects through 6.5 scattering mean-free-paths in real-time. Images with a transverse spatial resolution better than 42μm were acquired in a few seconds using a 6.3mW helium neon laser at 633nm. The photorefractive devices used are based on a poly (N-vinylcarbazole) (PVK):2,4,7-trinitro-9-fluorenone dimalenitrile (TNFDM) charge transport network, doped with the electro-optic chromophore 1-(2′-ethylhexyloxy)-2,5-dimethyl-4-(4″-nitrophenylazo)benzene (EHDNPB).
Laser processing of nanocrystalline TiO2 films for dye-sensitized solar cells
Authors: Kim H. , Kushto G. P. , Arnold C. B. , Kafafi Z. H. , Piqué A.
Published: July 2004

Pulsed-laser deposition and laser direct-write have been applied to deposit dense (30nm thick) and porous nanocrystalline TiO2 (nc-TiO2, 5–20μm thick) layers incorporated in dye-sensitized solar cells. Laser direct-write is a laser-induced forward transfer technique that enables the fabrication of conformal structures containing metals, ceramics, polymers, and composites on rigid and flexible substrates without the use of masks or additional patterning steps. A pulsed UV laser (355nm) was used to forward transfer a suspension of TiO2(P25) nanopowder onto a F-doped SnO2 coated glass substrate. In this letter we demonstrate the use of laser transfer techniques to produce porous nc-TiO2 films required for dye-sensitized solar cells. The dye solar cells fabricated with the laser processed TiO2 layers on glass showed a power conversion efficiency of ∼4.3% under an illumination of 10mW∕cm2.
Resonance-like electrical control of electron spin for microwave measurement
Authors: Glavin B. A. , Kim K. W.
Published: July 2004

We demonstrate that the spin-polarized electron current can interact with a microwave electric field in a resonant manner. The spin–orbit interaction gives rise to an effective magnetic field proportional to the electric current. In the presence of both dc and ac electric field components, electron spin resonance occurs if the ac frequency matches the spin precession frequency that is controlled by the dc field. In a device consisting of two spin-polarized contacts connected by a two-dimensional channel, this mechanism allows electrically tuned detection of the ac signal frequency and amplitude. For GaAs, such detection is effective in the frequency domain around tens of gigahertz.
Lateral n–p junction for acoustoelectric nanocircuits
Authors: Hosey T. , Talyanskii V. , Vijendran S. , Jones G. A. C. , Ward M. B. , Unitt D. C. , Norman C. E. , Shields A. J.
Published: July 2004

We report the experimental realization of a device comprising a lateral n–p junction grown by focused-ion molecular-beam epitaxy and a transducer to generate a surface acoustic wave. Acoustic charge transport across the junction and the accompanying photon emission are demonstrated. This type of light-emitting diode is suitable for integration into acoustoelectric nanocircuits in which quasione-dimensional semiconductor channels serve as “wires” through which packets of charge are transported by surface acoustic waves. The diode provides a means by which to extend the functionality of acoustoelectric nanocircuits into the optical domain.
Interlayer composition of HfO2∕Si(001) films
Authors: Copel M. , Reuter M. C. , Jamison P.
Published: July 2004

We report medium energy ion scattering results that determine the extent of Hf incorporation in the interfacial region of HfO2∕Si(001) films. The lack of change in the Hf backscatter peak after interlayer growth by in situ oxidation indicates extremely low levels of Hf incorporation. We conclude that silicate formation is not a significant factor in determining capacitances of HfO2∕Si(001) structures, provided that the deposition technique does not involve creation of a silicide as an intermediate step.
Demonstration of all in situ magnesium diboride superconductor thin-film tunnel junctions
Authors: Kim Tae Hee , Moodera Jagadeesh S.
Published: July 2004

High-quality planar tunnel junctions of MgB2 superconductor have been fabricated, all in situ, on Si single crystal substrates with Al2O3 as well as MgO barriers. Two superconducting energy gaps due to the σ and π bands of MgB2 have been unambiguously determined from superconductor-insulator-superconductor tunneling conductance measurements, showing good agreement with the theoretical predictions. Phonon structures have also been observed in tunneling. Our junction fabrication technique and well defined tunneling characteristics show a distinct possibility for the development of superconductive electronics based on MgB2 junctions.
Improved fill factors in amorphous silicon solar cells on zinc oxide by insertion of a germanium layer to block impurity incorporation
Authors: Ganguly G. , Carlson D. E. , Hegedus S. S. , Ryan D. , Gordon R. G. , Pang D. , Reedy R. C.
Published: July 2004

Amorphous silicon based solar cells deposited on zinc oxide (ZnO) exhibit reduced fill factor and open circuit voltage in comparison with tin oxide (SnO2). One approach has been to use higher conductivity nanocrystalline layers to overcome the “higher contact resistance.” Recent measurements have found the ZnO–p-layer contact resistance to be unchanged relative to SnO2, while instead, the diode ideality factor is poorer on ZnO. We show that the insertion of a thin, amorphous germanium layer at the ZnO–p-layer interface improves the cell performance and diode ideality factor by suppression of oxygen and zinc incorporation in the silicon layers.
Microstructure and properties of single crystal BaTiO3 thin films synthesized by ion implantation-induced layer transfer
Authors: Park Young-Bae , Ruglovsky Jennifer L. , Atwater Harry A.
Published: July 2004

Single crystal BaTiO3 thin films have been transferred onto Pt-coated and Si3N4-coated substrates by the ion implantation-induced layer transfer method using H+ and He+ ion coimplantation and subsequent annealing. The transferred BaTiO3 films are single crystalline with root mean square roughness of 17nm. Polarized optical and piezoresponse force microscopy (PFM) indicate that the BaTiO3 film domain structure closely resembles that of bulk tetragonal BaTiO3 and atomic force microscopy shows a 90° a–c domain structure with a tetragonal angle of 0.5°–0.6°. Micro-Raman spectroscopy indicates that the local mode intensity is degraded in implanted BaTiO3 but recovers during anneals above the Curie temperature. The piezoelectric coefficient, d33, is estimated from PFM to be 80–100pm∕V and the coercive electric field (Ec) is 12–20kV∕cm, comparable to those in single crystal BaTiO3.
Evidence of strong nanotube alignment and for iron preferential growth axis in multiwalled carbon nanotube carpets
Authors: Pichot Vincent , Launois Pascale , Pinault Mathieu , Mayne-L’Hermite Martine , Reynaud Cécile
Published: July 2004

This letter reports structural results obtained from x-ray scattering measurements performed on individual carbon nanotube carpets. Carpets of aligned multiwalled nanotubes are synthesized by the aerosol pyrolysis method using liquid hydrocarbon∕metallocene aerosols. Nanotube alignment of ±5.5° with respect to the average growth axis is obtained. In addition, nanotubes are partially filled with iron, and we demonstrate that the body of the carpet contains mainly one phase (γ-Fe) encapsulated inside nanotubes which exhibits a preferential crystallographic ⟨110⟩ axis along the nanotubes.
The structural and optical behaviors of K-doped ZnO∕Al2O3(0001) films
Authors: Kim Sung-Kyu , Kim Shin Ae , Lee Chang-Hee , Lee Hyeon-Jun , Jeong Se-Young , Cho Chae Ryong
Published: July 2004

K-doped ZnO films were prepared on Al2O3(0001) substrates by solution deposition. For the prepared thin films of Zn1−xKxO (x=0.002, 0.01, 0.02, 0.05, 0.1), we carried out x-ray diffraction, transmittance spectroscopy, photoluminescence, Hall measurement and x-ray photoemission spectroscopy study, and found that properties like the crystallinity, optical band gap, carrier concentrations and chemical binding states changed at a K doping concentration of 2mol%. From these results, we suggest that the doped K in ZnO films processed by the sol–gel method plays a different role at K doping concentrations below 2mol% and above 2mol%.
Remarkable differences in photoluminescent properties between LaPO4:Eu one-dimensional nanowires and zero-dimensional nanoparticles
Authors: Song Hongwei , Yu Lixin , Lu Shaozhe , Wang Tie , Liu Zhongxin , Yang Linmei
Published: July 2004

Photoluminescent properties of zero-dimensional LaPO4:Eu nanoparticles (NPs) and one-dimensional nanowires (NWs) prepared by the same wet-chemical synthesis technique were studied and compared. The results indicate that in NP Eu3+ occupied only one site, A, while in NW Eu3+ occupied the same site, A and an additional site B due to crystal anisotropy. Furthermore, the electronic transition rate of D15–∑JFJ7 in the NW increased from 14.9to28.9ms−1 compared to the NP, while the nonradiative transition rate of D15–D05 decreased from 24.1to19.7ms−1. The luminescent quantum efficiency thus improved from 30% to 59%. This work demonstrates that a one-dimensional NW may be a more favorable device than a zero-dimensional NP for photoluminescence.
Reconciliation of ab initio theory and experimental elastic properties of Al2O3
Authors: Gladden J. R. , So Jin H. , Maynard J. D. , Saxe P. W. , Le Page Y.
Published: July 2004

Ab initio calculations of the basic properties of solids have advanced significantly, and it is now possible to simply access a crystal structure database, and from the given constituent atoms and their positions, calculate reasonably accurate values for elastic constants and thermomechanical properties that may be derived from them. However, progress has been impeded by a unique discrepancy involving the sign of one of the elastic constants of an important material: Al2O3. In this letter, this longstanding discrepancy is resolved with experimental measurements.
Superluminal time advance of a complex audio signal
Authors: Munday J. N. , Henderson R. H.
Published: July 2004

There has been much recent interest in the study of superluminal pulse propagation; however, all previous findings have been extremely limited, only observing superluminal group velocities for simple Gaussian-like pulses. We now report the observation of a superluminal time advance for a complex, broadband signal. Such an advance causes all frequency components and envelopes to appear to exit a circuit 0.078±0.004ms before entering it. Although this may seem counterintuitive, the results are not in conflict with relativistic causality and could lead to many useful devices.
Stress map for ion irradiation: Depth-resolved dynamic competition between radiation-induced viscoelastic phenomena in SiO2
Authors: van Dillen T. , Siem M. Y. S. , Polman A.
Published: July 2004

The dynamic competition between structural transformation, Newtonian viscous flow, and anisotropic strain generation during ion irradiation of SiO2, leads to strongly depth-dependent evolution of the mechanical stress, ranging between compressive and tensile. From independent in situ stress measurements during irradiation, generic expressions are derived of the nuclear stopping dependence of both the structural transformation rate and the radiation-induced viscosity. Using these data we introduce and demonstrate the concept of a “stress map” that predicts the depth-resolved saturation stress in SiO2 for any irradiation up to several MeV.
Reversible hydriding and dehydriding properties of CaSi: Potential of metal silicides for hydrogen storage
Authors: Aoki M. , Ohba N. , Noritake T. , Towata S.
Published: July 2004

We found that CaSi reversibly absorbs and desorbs hydrogen. First-principles calculations theoretically indicated that CaSi hydride is thermodynamically stable. The hydriding and dehydriding properties of CaSi were experimentally determined using pressure-composition (p-c) isotherms and x-ray diffraction analysis. The p-c isotherms clearly demonstrated plateau pressures in a temperature range of 473–573K. The maximum hydrogen content was 1.9wt% under a hydrogen pressure of 9MPa at 473K. The reversible hydriding and dehydriding properties of CaSi suggest the potential of metal silicides for hydrogen storage.
Die-upset hybrid Pr–Fe–B nanocomposite magnets
Authors: Gabay A. M. , Zhang Y. , Hadjipanayis G. C.
Published: July 2004

Anisotropic nanocomposite R–Fe–B/Fe magnets (R=Pr, Tb) were synthesized by hot pressing and subsequent die upsetting blends of R-rich and R-lean melt-spun ribbons. The magnets have a layered structure, in which alternating layers of the two starting alloys lay perpendicularly to the pressing direction. A crystallographic alignment of the R2Fe14B grains is observed in the R-rich layers, whose microstructure is identical to that of the conventional die-upset magnets. The R-lean layers consisting of exchange-coupled R2Fe14B and α-Fe grains retain the random crystallographic orientation. The obtained bulk R-lean magnets show better properties than magnets of the same overall composition prepared from a single alloy.
Wide-range (0.33%–100%) 3C–SiC resistive hydrogen gas sensor development
Authors: Fawcett Timothy J. , Wolan John T. , Myers Rachael L. , Walker Jeremy , Saddow Stephen E.
Published: July 2004

Silicon carbide (SiC) resistive hydrogen gas sensors have been fabricated and tested. NiCr planar ohmic contacts were deposited on both a 4μm 3C–SiC epitaxial film grown on n-type Si(001) and directly on Si to form the resistive sensor structures. Detection at concentrations as low as 0.33% and as high as 100% (H2 in Ar) was observed with the 3C–SiC sensor while the Si sensor saturated at 40%. The 3C–SiC sensors show a remarkable range of sensitivity without any saturation effects typically seen in other solid-state hydrogen gas sensors. Under a constant 2V bias, these sensors demonstrated an increase in current up to 17mA upon exposure to pure H2. Preliminary experiments aimed at determining the gas sensing mechanism of these devices have been conducted and are also reported.
Nanoindentation-induced amorphization in silicon carbide
Authors: Szlufarska Izabela , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya
Published: July 2004

The nanoindentation-induced amorphization in SiC is studied using molecular dynamics simulations. The load-displacement response shows an elastic shoulder followed by a plastic regime consisting of a series of load drops. Analyses of bond angles, local pressure, and shear stress, and shortest-path rings show that these drops are related to dislocation activities under the indenter. We show that amorphization is driven by coalescence of dislocation loops and that there is a strong correlation between load-displacement response and ring distribution.
Small angle scattering signals for (neutron) computerized tomography
Authors: Strobl M. , Treimer W. , Hilger A.
Published: July 2004

Small angle neutron scattering is a well-established tool for the determination of microscopic structures in various materials. With the ultrasmall angle neutron scattering technique (USANS), structures with sizes of approximately 50nm to 50μm can be resolved by a double crystal diffractometer (DCD). USANS signals recorded with a special DCD were used for tomographic purposes investigating the macroscopic structure of a sample with a maximum resolution of 200μm. Thereby, macroscopic regions within the sample with different ultrasmall angle scattering properties, i.e., with different microscopic structures, could be imaged by the means of tomographic reconstruction from projections (on a macroscopic scale).
Structural dependence of carrier capture time in semiconductor quantum-well lasers
Authors: Hader J. , Moloney J. V. , Koch S. W.
Published: July 2004

A fully microscopic model based on generalized quantum Bolzman equations for electron–electron and electron–phonon scattering is used to calculate the carrier capture dynamics in quantum-well lasers. The capture time and dynamics are governed by transitions between quantum states that are delocalized throughout the whole structure. Good agreement with experimental results is demonstrated for InGaPAs- and InGaAlAs-based multi-quantum-well systems.
Cavity enhancement of the magneto-optic Kerr effect for optical studies of magnetic nanostructures
Authors: Qureshi N. , Schmidt H. , Hawkins A. R.
Published: July 2004

We present a study of cavity enhancement of the magneto-optic Kerr effect using dielectric multilayers in order to facilitate optical studies of individual single-domain nanomagnets. We develop a transfer matrix theory to analyze Kerr rotation from an arbitrary number of possibly lossy dielectric layers. The combination of one lossless and one thin metallic layer is found to be most favorable for studying individual nanomagnets, providing the best tradeoff between signal enhancement and spatial resolution. Accounting for the microscopic surface structure, we find good agreement between theory and experiment. Using this technique, we demonstrate Kerr enhancements by a factor of more than 16.
Importance of dewetting in organic molecular-beam deposition: Pentacene on gold
Authors: Beernink G. , Strunskus T. , Witte G. , Wöll Ch.
Published: July 2004

Organic molecular-beam deposition of pentacene on gold substrates has been investigated using a multitechnique approach. The morphology of the organic thin films depends strongly on the substrate temperature. Pronounced dewetting and island formation are observed at room temperature. Whereas pentacene molecules adopt a planar monolayer structure, they continue to grow in an upright orientation in multilayer films as inferred from x-ray absorption spectroscopy and atomic force microscopy. These results are in pronounced contrast to a recent scanning tunneling microscopy (STM) study by Kang and Zhu [Appl. Phys. Lett. 82, 3248 (2003)] and indicate fundamental problems in the interpretation of STM measurements for organic thin films.
Nickel silicidation on n and p-type junctions at 300°C
Authors: Jiang Yu-Long , Agarwal Aditya , Ru Guo-Ping , Qu Xin-Ping , Poate John , Li Bing-Zong , Holland Wayne
Published: July 2004

The electrical and materials properties of ∼20nm nickel silicide films, formed at 300°C, on n+∕p and p+∕n junctions are investigated. The sheet resistance of the silicide on p+∕n junctions is found to be more than twice as high as that of the silicide on n+∕p junctions. Cross section transmission electron microscopy, Rutherford backscattering spectroscopy, and x-ray photoelectron energy spectroscopy reveal that a pure Ni2Si layer forms on n+∕p junctions while a thicker Ni2Si∕NiSi double layer (∼60% Ni2Si) forms on p+∕n junctions. But the electrical differences are found to correlate only with differences in grain size and dopant concentration in the silicide.