bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic structures at the interfaces between copper phthalocyanine and layered materials
Authors: Shimada T. , Hamaguchi K. , Koma A. , Ohuchi F. S.
Published: July 2002

Copper phtalocyanine (CuPc) films with the thickness controlled in molecular scales have been grown epitaxially on (0001) surfaces of layered materials, and electronic interaction at the interfaces have been studied by photoelectron spectroscopy. Materials with different electronic properties having different work functions (Evac) were chosen as the substrates; semiconducting MoTe2 (Evac=4.0 eV), semi-metallic highly oriented pyrolytic graphite (Evac=4.5 eV) and metallic TaSe2 (Evac=5.5 eV). Formation of interface dipole layers was found at CuPc/TaSe2 interfaces and molecular orbitals involved were identified.
Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices
Authors: Vispute R. D. , Talyansky V. , Choopun S. , Sharma R. P. , Venkatesan T. , He M. , Tang X. , Halpern J. B. , Spencer M. G. , Li Y. X. , Salamanca-Riba L. G. , Iliadis A. A. , Jones K. A.
Published: July 2002

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.
Centimeter scale atomic force microscope imaging and lithography
Authors: Minne S. C. , Adams J. D. , Yaralioglu G. , Manalis S. R. , Atalar A. , Quate C. F.
Published: July 2002

We present a 4 mm2 image taken with a parallel array of 10 cantilevers, an image spanning 6.4 mm taken with 32 cantilevers, and lithography over a 100 mm2 area using an array of 50 cantilevers. All of these results represent scan areas that are orders of magnitude larger than that of a typical atomic force microscope (0.01 mm2). Previously, the serial nature and limited scan size of the atomic force microscope prevented large scale imaging. Our design addresses these issues by using a modular micromachined parallel atomic force microscope array in conjunction with large displacement scanners. High-resolution microscopy and lithography over large areas are important for many applications, but especially in microelectronics, where integrated circuit chips typically have nanometer scale features distributed over square centimeter areas.
Hot electron relaxation time in GaN
Authors: Ye Hong , Wicks G. W. , Fauchet P. M.
Published: July 2002

The hot electron relaxation time is studied in an n-type GaN film grown by molecular beam epitaxy on sapphire. A femtosecond pump–probe technique is used in which the electrons are excited by an infrared pump and the carrier dynamics are monitored by a tunable near ultraviolet probe. Complex transients, showing bleaching or induced absorption, are observed. The data are fitted by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The LO-phonon emission time is measured to be 0.2 ps.
Blue luminescence from amorphous GaN nanoparticles synthesized in situ in a polymer
Authors: Yang Yi , Leppert Valerie J. , Risbud Subhash H. , Twamley Brendan , Power Phillip P. , Lee Howard W. H.
Published: July 2002

Amorphous GaN nanoparticles were synthesized by the in situ thermal decomposition of cyclotrigallazane incorporated into a polystyrene–poly(N,N-dimethyl-4-vinylaniline) copolymer. Transmission electron microscopy, energy dispersive x-ray spectrometry, and x-ray photoelectron spectroscopy show that the composite material consists of amorphous GaN nanoparticles (average diameter ∼40 nm) well dispersed in the copolymer. The photoluminescence spectra show blue light emission from the amorphous GaN nanoparticles, peaked at ∼426 nm.
Strain and oxygenation effects on superconductivity of La1.85Sr0.15CuO4 thin films
Authors: Si Weidong , Li Hong-Cheng , Xi X. X.
Published: July 2002

We have measured normal-state and superconducting properties of La1.85Sr0.15CuO4 thin films as a function of lattice strain and oxygen content. Strain was controlled by depositing SrLaAlO4 buffer layers of different thickness on SrTiO3 substrates, and an ozone/molecular oxygen mixture was used during cooling to achieve better oxygen uptake. We found that both full oxygenation and compressive in-plane strain are critical for the properties of the La1.85Sr0.15CuO4 thin films, and they are closely correlated.
Electrical properties of hafnium silicate gate dielectrics deposited directly on silicon
Authors: Wilk G. D. , Wallace R. M.
Published: July 2002

Hafnium silicate (HfSixOy) gate dielectric films with ∼6 at. % Hf exhibit significantly improved leakage properties over SiO2 in the ultrathin regime while remaining thermally stable in direct contact with Si. Capacitance–voltage measurements show an equivalent oxide thickness (tox) of less than 18 Å for a 50 Å HfSixOy film deposited directly on a Si substrate, with no significant dispersion of the capacitance for frequencies ranging from 10 kHz to 1 MHz. Current–voltage measurements show for the same film a leakage current of 1.2×10−6 A/cm2 at 1 V bias. Hysteresis in these films is measured to be less than 20 mV, the breakdown field is measured to be EBD∼10 MV/cm, and the midgap interface state density is Dit∼1011 cm−2 eV−1. Cross-sectional transmission electron microscopy shows no signs of reaction or crystallization in HfSixOy films on Si after being annealed at 800 °C for 30 min.
Reduction of the interfacial Si displacement of ultrathin SiO2 on Si(100) formed by atmospheric-pressure ozone
Authors: Kurokawa Akira , Nakamura Ken , Ichimura Shingo , Moon Dae Won
Published: July 2002

We examined the structure around the interface of SiO2 and Si using medium-energy ion scattering spectroscopy (MEIS) to investigate the interfacial Si displacement of an ultrathin silicon dioxide formed by oxidation of a Si(100) substrate with atmospheric-pressure ozone at a substrate temperature of 375 °C. A thermally grown oxide with the same thickness as an ozone-formed oxide was also measured with MEIS for comparison. The ozone-formed oxide exhibited considerably less Si displacement in the oxide layers near the interface than a thermally grown oxide, which indicates that an ozone oxide is homogenous. These results explain well our previous findings that an ozone oxide exhibits a constant HF etching rate of silicon dioxide while a thermally grown oxide slows the etching rate near the interface.
Synthesis and optical properties of gallium arsenide nanowires
Authors: Duan Xiangfeng , Wang Jianfang , Lieber Charles M.
Published: July 2002

Gallium arsenide (GaAs) nanowires have been synthesized in bulk quantities and high purity by laser-assisted catalytic growth. Field-emission scanning electron microscopy and transmission electron microscopy investigations show that the GaAs nanowires are produced in >90% yield, are single crystals with 〈111〉 growth axes, and have diameters varying from three to tens of nanometers, and lengths extending to tens of micrometers. Photoluminescence (PL) measurements made on individual GaAs nanowires show large blueshifts in the PL peak position compared to bulk GaAs, and are consistent with strong quantum confinement. The implications of these results are discussed.
Addition energies and quasiparticle gap of CdSe nanocrystals
Authors: Franceschetti Alberto , Zunger Alex
Published: July 2002

Using atomistic pseudopotential wave functions we calculate the quasiparticle gap, the optical gap and the electron and hole addition energies of CdSe nanocrystals. We find that the quasiparticle gap and the addition energies depend strongly on the dielectric constant of the surrounding material, while the optical gap is rather insensitive to the environment. We provide scaling lows for these quantities as a function of the quantum dot size, and compare our results with recent scanning tunneling spectroscopy experiments.
Low-temperature scanning laser microscopy of individual filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x tapes
Authors: Sivakov A. G. , Lukashenko A. V. , Abraimov D. , Müller P. , Ustinov A. V. , Leghissa M.
Published: July 2002

The method of low-temperature scanning laser microscopy is applied to visualize the resistive state in individual superconducting filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x/Ag tapes. This technique is capable of imaging the distributions of the critical currents over a sample. Using the nonbolometric response, a spatial resolution of about 1 μm is demonstrated for 10-μm-thick filaments. Some of the resistively visualized grain boundaries between crystallites show Josephson behavior.
Terahertz demultiplexing by a single-shot time-to-space conversion using a film of squarylium dye J aggregates
Authors: Furuki Makoto , Tian Minquan , Sato Yasuhiro , Pu Lyong Sun , Tatsuura Satoshi , Wada Osamu
Published: July 2002

We applied time-to-space conversion using femtosecond nonlinear-optical response of squarylium-dye (SQ) J-aggregates film. A pump pulse and a train of four probe pulses were illuminated on the same area (10 mm φ) of the film in direction of oblique and normal to the film plane, respectively. Due to the oblique illumination, the pump pulse met probe pulses (interval time: 1 ps) at separate places. The film picked out part of each probe pulse by its transmittance change, which was observed for a transmitted image of spatially separated four lines. Response time of the SQ J aggregates is enough for the single-shot 1 THz demultiplexing.
Cryogenic variable temperature ultrahigh vacuum scanning tunneling microscope
Authors: Foley E. T. , Kam A. F. , Lyding J. W.
Published: July 2002

A cryogenic variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been developed. This design utilizes a novel vibration isolation that provides an active thermal link to the cooling source without the standard tradeoff of compromising mechanical isolation. A welded bellows serves as the basis for the vibration isolation system. This bellows houses a heat exchanger which uses helium gas to form a thermal link between the STM and the cryogenic. This STM is fully integrated into a multichamber UHV STM system now in place in our laboratory and atomic resolution operation is demonstrated from 11 to 300 K.
Reactivity and electronic states of O4 along minimum energy paths
Authors: Hernández-Lamoneda R. , Ramı́rez-Solı́s A.
Published: July 2002

Ab initio calculations, of the ground and low-lying electronic states of O4 along the minimum energy paths (MEP) for the reactions O2(X 3Σg−)+O2(X 3Σg−)→O4(X 1A1) and O2(X 3Σg−)+O2(X 3Σg−)→O3(X 1A1)+O(3P) have been performed. Our CASSCF(16,12)+CASMP2 calculations using the 5s4p3d2 f ANO basis set provide a solid basis to establish the stability of the O4 chemically bound molecule. Surface crossings between singlet and triplet states have been found and further characterized by evaluating their spin–orbit coupling matrix elements. Our calculations show that for the ozone formation reaction there should be spin–orbit coupling transitions allowing vibrational to electronic energy transfer around the saddle point region. The formation of vibrationally excited O2(a 1Δg,v) is predicted to contribute to the dark channel reported in experiments.
Glass transition in chlorobenzene–decalin under pressure
Authors: Köplinger J. , Kasper G. , Hunklinger S.
Published: July 2002

We have investigated the dielectric properties of a 17.2 mol % mixture of chlorobenzene and decalin in the supercooled and in the glassy state under hydrostatic pressure up to 400 MPa. The dielectric loss shows the well-known primary (α) relaxation and a secondary (β) relaxation the origin of which is still being debated. The glass transition temperature Tg increases linearly with pressure. The fragility is pressure independent. With increasing pressure the relaxation strength of the β process rises at the expense of the α process. Above around 160 K, where devitrification occurs at atmospheric pressure, the activation energy of the secondary relaxation process is roughly twice that at lower temperatures. Both energies are proportional to Tg, indicating a strong correlation between α and β processes. A possible origin of the secondary relaxation is rationalized by critical clusters in the melt.
Influence of microwave energy on semiconductors during ion implantation process
Authors: Gay H-C. , Gibert P. , Lin-Kwang J.
Published: July 2002

Microwave energy is used during the implantation-annealing process of semiconductors where it is shown that the implant ion damage anneal takes place at the same time as the implantation step. The microwave energy is observed to be focused within the plasma volume created by the electron–hole pairs, where the conductivity is increased considerably. It is shown that the implantation takes place perpendicularly with respect to the wafer surface and that there is very little lateral redistribution of ions. The transmission electron microscopy and measurements of the implanted zone confirm the predicted good results of this process.
Molecular dynamics investigation of thickness effect on liquid films
Authors: Weng Jian-Gang , Park Seungho , Lukes Jennifer R. , Tien Chang-Lin
Published: July 2002

This work applies the molecular dynamics simulation method to study a Lennard-Jones liquid thin film suspended in the vapor and to explore the film thickness effect on its stability. For the accurate estimation of local pressure distributions in the film, an improved method is proposed and used. Simulation results indicate that profiles of the local surface tension distribution vary widely with film thickness, while surface tension values and density profiles show little variation. As the film gets thinner, the two liquid–vapor interfacial regions begin to overlap and liquid-phase molecules in the center region of the film experience larger tension in the direction parallel to the film surface. Such interface overlapping is believed to destabilize the film and the occurrence of film rupture depends on the system temperature and the cross-sectional area of the computational domain.
Optical fiber interconnect with the ends offset and axial loading: What could be done to reduce the tensile stress in the fiber?
Authors: Suhir E.
Published: July 2002

We examine an optical fiber interconnect subjected to the ends offset and subsequent axial loading in tension or compression. Such loading typically occurs because of the thermal expansion or contraction mismatch of the glass fiber with the material of the enclosure. The objective of the study is to find out what could possibly be done to reduce the tensile stress in the fiber. We show that, if the ends offset is not very small, the reactive tension and the subsequent external (“active”) tension not only lead to elevated tensile stresses in the fiber, but also, more importantly, can result in a considerable increase in the bending stress. In order to minimize the tensile stress in the fiber, the ends offset and the active tension should be reduced. In the case of the compressive loading, however, there is an opportunity to reduce the tensile stress substantially and, if the offset is not very large, even remove it completely. This could be done by a proper selection of the coefficient of thermal expansion (contraction) of the material of the enclosure. Based on the analysis, using the developed analytical stress models, we suggest that this coefficient be chosen in such a way that the induced compressive force in the fiber is about 44% of the critical value. We found that a 34% relief in the bending stress can be achieved in such a situation (compared to the case of zero compression). Larger compressive force is not advisable, because of the rapid increase in the fiber curvatures (bending stress) and, as a result of that, in the total maximum tensile stress as well.
X-ray induced modification of electronic properties of boron nitride thin films
Authors: Ageev V. , Ugarov M. , Frolov V. , Karabutov A. , Loubnin E. , Badi N. , Bensaoula A.
Published: July 2002

X-ray induced modification of the electronic properties of nanocrystalline boron nitride (BN) films with different compositions and carbon impurity contents is reported. The related changes of the surface composition and valence band structures of the irradiated films are discussed. X-ray irradiation of nanocrystalline BN films is shown to widen areas with intermediate values of electroconductivity (associated with areas of high emission density) and to increase the average value of electroconductivity by redistributing surface potentials and electron emission sites. Improvement of the field emission is observed with both increasing electron current and diminishing thresholds. Longer x-ray irradiation times yield greater improvement.
Room-temperature stimulated emission of excitons in ZnO/(Mg, Zn)O superlattices
Authors: Ohtomo A. , Tamura K. , Kawasaki M. , Makino T. , Segawa Y. , Tang Z. K. , Wong G. K. L. , Matsumoto Y. , Koinuma H.
Published: July 2002

We report on the observation of stimulated emission in ZnO/MgxZn1−xO superlattices well above room temperature. Two kinds of superlattices grown by laser molecular-beam epitaxy showed clear systematics on the quantum subband levels in absorption and spontaneous emission spectra. Stimulated emission with excitonic origin could be observed at very low optical pumping levels. The threshold excitation intensity changed from 11 to 40 kW/cm2, and the emission energy could be tuned between 3.2 and 3.4 eV, depending on the well thickness and/or the Mg content in the barrier layers. The excitonic stimulated emission could be observed up to 373 K and the characteristic temperature was as high as 87 K.
Phase diagrams of polydisperse van der Waals fluids
Authors: Bellier-Castella L. , Xu H. , Baus M.
Published: July 2002

The phase behavior of a system composed of spherical particles with a monomodal size distribution is investigated theoretically within the context of the van der Waals approximation for polydisperse fluids. It is shown how the binodals, spinodals, cloud-point and shadow curves as well as all the (polydispersity induced) critical points can be obtained for a variety of interaction potentials. The polydispersity induced modifications of the phase diagram (even for a polydispersity index I as small as I≈1.01) should be observable in some colloidal dispersions.
Unimodular relativity and cosmological constant
Authors: Finkelstein David R. , Galiautdinov Andrei A. , Baugh James E.
Published: July 2002

Unimodular relativity is a theory of gravity and space–time with a fixed absolute space–time volume element, the modulus, which we suppose is proportional to the number of microscopic modules in that volume element. In general relativity an arbitrary fixed measure can be imposed as a gauge condition, while in unimodular relativity it is determined by the events in the volume. Since this seems to break general covariance, some have suggested that it permits a nonzero covariant divergence of the material stress-energy tensor and a variable cosmological “constant.” In Lagrangian unimodular relativity, however, even with higher derivatives of the gravitational field in the dynamics, the usual covariant continuity holds and the cosmological constant is still a constant of integration of the gravitational field equations.
Light scattering from red pigment particles: Multiple scattering in a strongly absorbing system
Authors: McNeil L. E. , French R. H.
Published: July 2002

In the optical analysis of collections of particles embedded in a matrix, the ultimate goal is to predict the optical properties of the final film, given the optical characteristics of the materials of which it is made, the particle size, and the fraction of the volume occupied by the particles. The first task in the pursuit of this goal is to make connections between the optical properties of the bulk materials of which the particles and medium are made and the spatial distribution of the scattered intensity from the individual particles. The second is to relate the calculated far-field scattering and absorption cross sections for a single particle to the measured optical characteristics of real films. Here we accomplish these tasks for a regime that is rarely considered in the literature but is quite important in applications. This regime is characterized by three conditions: (a) the extinction coefficient of the particle is significant; (b) the particle size is comparable to the wavelength in the medium; and (c) the optical density of the films is large enough that multiple scattering is significant, and yet direct transmission of collimated radiation through the film is not negligible. We have measured the visible diffuse reflectance and transmission of films of quinacridone particles in a transparent resin at various particle volume concentrations, and used the Kubelka–Munk formalism as a parametrization method to extract scattering and absorption parameters for this strongly absorbing, multiple-scattering system in the resonant regime. We have modeled the scattering parameter S* as a convolution of the angular distribution of the scattering from a single particle (derived from Mie theory) and a multiple-scattering function that characterizes the effects of the concentration and arrangement of the particles in the film. We find that the multiple-scattering function has an angular distribution that is strongly peaked in the backscattering direction. This accounts for the transformation of the collimated and strongly forward-scattered light in the top layers of the film into diffusely reflected light as the film is traversed. The multiple-scattering function also accounts for the differences in diffuse reflection observed for films of identical volume fraction occupied by particles (particle volume concentration) but different grinding time. This method of analysis begins to allow one to make an explicit connection between the fundamental optical properties of the particles and the experimentally accessible parameters. We have thus developed a way to begin to bridge the gap between the fundamental physics of the scattering of light from individual particles and the practical characterization of a film using the simple Kubelka–Munk analysis.
Two-dimensional vortex shedding of a circular cylinder
Authors: Wen C.-Y. , Lin C.-Y.
Published: July 2002

The Strouhal–Reynolds number relationship for the two-dimensional (2-D) vortex shedding of a circular cylinder at low to medium Reynolds numbers (Re, ranging from 45 to 560) is investigated experimentally. Both horizontal and vertical soap film tunnels are used to set up a truly 2-D experiment. It is found that two separate 3-D instabilities of the natural wake at Re≅180 and 260 disappear. The Strouhal–Reynolds number curve is in good agreement with the 2-D computations of Barkley and Henderson [J. Fluid Mech. 322, 215 (1996)]. The 2-D asymptote of 0.2417 of Strouhal number is also approached.
Epitaxial-strain-induced insulator-superconductor transition in undoped and lightly doped La2CuO4
Authors: Si Weidong , Xi X. X.
Published: July 2002

We have studied the epitaxial-strain effects on transport properties of pulsed-laser-deposited undoped and lightly doped La2CuO4 thin films. The epitaxial strain is controlled by using SrLaAlO4 buffer layers of different thicknesses on SrTiO3 substrates. The La2CuO4 films are insulating under tensile epitaxial strain, but when the strain is sufficiently compressive they become superconducting. Since the undoped and lightly doped La2CuO4 are known to be superconducting when interstitial oxygen is inserted, the epitaxial-strain-induced insulator–superconductor transition suggests a significant interplay between strain and oxygenation in the films.
High-speed, high-sensitivity aerosol fluorescence spectrum detection using a 32-anode photomultiplier tube detector
Authors: Pan Yong-le , Cobler Patrick , Rhodes Scott , Potter Alexander , Chou Tim , Holler Stephen , Chang Richard K. , Pinnick Ronald G. , Wolf Jean-Pierre
Published: July 2002

We employ a 32-anode photomultiplier tube (PMT) in a fluorescence detection system and demonstrate its ability to record broad fluorescence spectra at frame rates in excess of 1.4×103 Hz, which is 56× faster than the frame rate of an intensified charge coupled device detector. The multi-anode PMT has single-photon detectable sensitivity. A new data acquisition and processing system for the multi-anode PMT, together with the system-controlling software, has been developed. The performance characteristics of the fluorescence detection system, including the data rate capability, dynamic range, signal-to-noise ratio, and crosstalk among the different anodes, are reported. The 32-anode PMT and acquisition system are suitable for a real-time, field-portable, multichannel optical analyzer.
Cyclization of a polymer with charged reactive end groups
Authors: Stampe J. , Sokolov I. M.
Published: July 2002

We calculate the cyclization rate k(t) of a single polymer chain with charged end groups, where cyclization takes place with probability of one when the ends approach each other at a distance smaller than the reaction radius R. Using Monte Carlo simulations in a kink-jump model we show that the long-time asymptotic value of k for longer chains and attractive Coulomb interaction does not depend on R, but depends strongly on the Onsager radius, which characterizes the strength of electrostatic interaction. This result relies only on the large-scale behavior of the chain and is reproduced within a simple harmonic-spring approximation. The harmonic-spring model leads to a diffusion equation which is solved numerically. The solution for long chains is confirmed with analytical approximations.
Fluctuation field and time dependence of magnetization in TbFeCo amorphous rare earth-transition metal thin films for perpendicular magnetic recording
Authors: Chekanov A. , Matsumoto K. , Ozaki K.
Published: July 2002

In this work we report on the experimental analysis of the time decay of magnetization in the films of amorphous TbFeCo alloys for perpendicular magnetic recording. It is shown that the experimental results can be explained on the basis of the thermally activated incoherent subnetwork magnetization reversal model. Thermally activated reversal of the transition metal subnetwork and associated reversal of the antiferromagnetically coupled rare-earth subnetwork may result in effective stabilization of the net film magnetization. Experimentally obtained activation diameter value Da∼5–6 nm corresponds to the exchange length or the domain wall width in the films under study. The small activation diameter value combined with high stability factor k∼250 mean that the TbFeCo amorphous film should be capable of supporting a thermally stable perpendicular recording at over 100 Gbit/in2 recording density.
Electronic structure and magnetic properties of Ni clusters
Authors: Duan H. M. , Gong X. G. , Zheng Q. Q. , Lin H. Q.
Published: July 2002

A systematic study on the ground state electronic structure and magnetic properties of Nin (n=2–39 and 55) clusters are performed using the density functional calculation with the local spin-density approximation. The binding energy per atom increases monotonically. The occupation numbers do not change significantly for n>8, which helps us to understand the experimental observation of the photoelectron spectroscopy. The obtained average magnetic moment per atom is larger than the bulk value, and the evolvement of these moments towards the bulk is in good agreement with the experimental trend. However, the average moments of “core” atoms are comparable to or even larger than that of surface atoms, which indicates that the usual conclusion of surface enhancement on magnetism in transition metal clusters may be improper for these Nin (n=14–39) clusters.
Alkali metal cation-ligand affinities: Basis set superposition correction for the Gaussian protocols
Authors: Siu F. M. , Ma N. L. , Tsang C. W.
Published: July 2002

The effect of basis set superposition error (BSSE) on Gaussian-2 and Gaussian-3 calculated alkali metal cation-ligand affinities has been studied. For these systems, we found that the standard Boys–Bernadi full counterpoise (CP) method often leads to correction terms that are physically incorrect. This problem may be rectified by using the geometry corrected counterpoise (GCP) method. The relationship between CP, GCP corrections, and deformation energy is discussed. In order to yield good agreement with existing experimental Li+ and Na+ ligand affinities, we recommend the adoption of either the G3 (with GCP correction) or the G2(MP2,SVP)-FC (without GCP correction) protocols. In the case of K+, the GCP correction is of negligible magnitude, and hence GCP corrections may be omitted in the G2(MP2,SVP)-ASC affinity calculations for these complexes.
Nanoscale intrinsic localized modes in an antiferromagnetic lattice
Authors: English L. Q. , Sato M. , Sievers A. J.
Published: July 2002

The weak dissipation of spin waves in magnetic materials make antiferromagnets realistic condensed matter systems for the experimental observation of intrinsic localized modes of nanoscale size. By employing a chirped high power microwave pulse, a few microseconds long, the lowest frequency antiferromagnetic uniform mode of the quasi-one-dimensional biaxial antiferromagnet (C2H5NH3)2CuCl4 has been efficiently driven into an unstable dynamical region. Intrinsic localized spin waves form below the gap. Our experimental findings demonstrate that a simple optimal control scheme can be used to investigate this kind of nonlinear excitation.
Self-assembled near-zero-thickness molecular layers as diffusion barriers for Cu metallization
Authors: Krishnamoorthy A. , Chanda K. , Murarka S. P. , Ramanath G. , Ryan J. G.
Published: July 2002

Devising ultrathin barrier layers to prevent Cu diffusion into SiO2-based dielectrics is a major challenge that must be met to increase the speed, number density, and performance of microelectronics devices. Here, we demonstrate the use of near-zero-thickness (<2-nm-thick) self-assembled molecular monolayers (SAMs) as candidates for this application. Cu/SiO2/Si(001) metal–oxide–semiconductor capacitors, with and without SAMs at the Cu/SiO2 interface, were annealed at 200 °C in a 2 MV cm−1 electrical field. Capacitance–voltage and current–voltage measurements of SAM-coated capacitors indicate that SAMs with aromatic terminal groups inhibit Cu diffusion into SiO2. They consistently show more than four-orders-of-magnitude lower leakage currents and a factor-of-4 higher time to failure when compared with the corresponding values from samples without SAMs at the interface. SAMs with short tail lengths or aliphatic terminal groups are ineffective in hindering Cu diffusion, indicating that the molecular length and chemical configuration are key factors determining the efficacy of SAMs as barriers. We propose that the steric hindrance offered by the terminal groups in the SAMs are responsible for the barrier properties.
The segmental dynamics of a polymer electrolyte investigated by coherent quasielastic neutron scattering
Authors: Carlsson P. , Zorn R. , Andersson D. , Farago B. , Howells W. S. , Börjesson L.
Published: July 2002

The structural dynamics of a polymer electrolyte model material, poly(prolyene oxide) (PPO)–LiClO4 (and PPO for reference), has for the first time been studied using coherent quasielastic neutron scattering. By a combination of neutron spin echo and inverse time-of-flight techniques we investigate the relaxation function in an experimental time window 10−12≲t≲10−8 s at a momentum transfer corresponding to the distance between neighboring interchain segments. We find that the relaxation of the correlation between neighboring chains is slower and more stretched in the polymer salt complex compared to the pure polymer. The data can, for both PPO and PPO–LiClO4, be described by a stretched exponential function with temperature independent stretching parameters. While the relaxation times follow the macroscopic viscosity for the former, they do not for the latter. The slower relaxation in PPO–LiClO4 compared to PPO and the failure of the viscosity scaling in PPO–LiClO4 may be explained in terms of a temperature dependent effective molecular weight induced by cations acting as cross links between chains. We discuss the origin of the extra stretching of the relaxation in the polymer salt complex under the aspect of heterogeneity, comparing it with data in the literature. We find that the stretching to the major part is intrinsic or at most due to heterogeneities on an atomic length scale. The molecular length scale of the experiment allows for the first time a direct connection to the renewal time in the dynamic disordered hopping model for ion transport in polymer electrolytes.
Nonlinear neoclassical transport in toroidal edge plasmas
Authors: Fülöp T. , Helander P.
Published: July 2002

In conventional neoclassical theory, the density and temperature gradients are not allowed to be as steep as frequently observed in the tokamak edge. In this paper the theory of neoclassical transport in a collisional, impure plasma is extended to allow for steeper profiles than normally assumed. The dynamics of highly charged impurity ions then becomes nonlinear, which affects the transport of all species. As earlier found in the banana regime, when the bulk plasma gradients are large the impurity ions undergo a poloidal redistribution, which reduces their parallel friction with the bulk ions and suppresses the neoclassical ion particle flux. The neoclassical confinement is thus improved in regions with large radial gradients. When the plasma is collisional and the gradients are large, the impurities accumulate on the inboard side of the torus.
Band alignment at a ZnO/GaN (0001) heterointerface
Authors: Hong Soon-Ku , Hanada Takashi , Makino Hisao , Chen Yefan , Ko Hang-Ju , Yao Takafumi , Tanaka Akinori , Sasaki Hiroyuki , Sato Shigeru
Published: July 2002

We report the experimental results of the valence band offset at a ZnO/GaN (0001) heterointerface. The ZnO/GaN (0001) heterointerface is prepared by growing a ZnO layer on (0001) GaN/Al2O3, in which the ZnO layer is epitaxially deposited by plasma-assisted molecular-beam epitaxy, while the GaN template is prepared by metalorganic chemical-vapor deposition. Ex situ ultraviolet and x-ray photoelectron spectroscopy have been used to measure the valence band offset ΔEV. The photoelectron spectroscopy measurements are done before and after Ar+ ion cleaning of the surfaces. Type-II band alignments with band offsets of ΔEV=1.0 eV (before cleaning) and 0.8 eV (after cleaning) with the valence band maximum of GaN being placed above that of ZnO are obtained.
Calibrating an ellipsometer using x-ray reflectivity
Authors: Richter Andrew , Guico Rodney , Wang Jin
Published: July 2002

X-ray reflectivity has been used to find the optical refractive index of polymer thin film in order to calibrate a Stokes ellipsometer for film thickness measurements during the deposition procedure. A thin, spun-cast film of poly(tert-butyl acrylate) (PtBA) was made with a film thickness of ∼500 Å. An x-ray reflectivity measurement was taken and the data were fit to determine the thickness of the PtBA film and the underlying silicon–oxide layer. This measurement was then used to calculate the optical refractive index for PtBA at the ellipsometer wavelength. Using this value for the refractive index subsequently allowed us to determine the film thickness for a series of PtBA films made by using a number of polymer solution concentrations resulting in film thickness ranging from 100 to 1300 Å. These film thicknesses were found to be generally the same as those found using x-ray reflectivity. The success of this procedure suggests a useful method for calibrating an ellipsometer for fast in-lab measurements, especially on ultrathin films when simultaneous determination of the film thickness and the refractive index is less reliable.
Temporal parameters of the X-pinch x-ray source
Authors: Sinars D. B. , Pikuz S. A. , Shelkovenko T. A. , Chandler K. M. , Hammer D. A.
Published: July 2002

The X pinch has proved to be an excellent source of 2.5–10 keV radiation for point-projection radiography with spatial resolution of 2 μm or even better. The pulse duration of the x-ray bursts has been investigated for a wide variety of wire materials in the 1.5–10 keV energy range using a set of fast diamond photoconducting detectors with different filters, and using an x-ray streak camera to observe the source through four different filters on each pulse. All wires tested have intense continuum up to at least 6 keV, and the duration of the pulse is shorter for the harder radiation component for all materials. However, there are substantial differences between materials. For example, the pulse duration for Al with filtering for energy ⩾1.5 and ⩾5 keV are about 1 and 0.5 ns, respectively. By contrast, for Mo with filtering for energy ⩾2.5 and ⩾5 keV, the pulse durations are about 200 ps and ⩽80 ps, respectively.
The vibrational spectrum of crystalline benzoic acid: Inelastic neutron scattering and density functional theory calculations
Authors: Plazanet M. , Fukushima N. , Johnson M. R. , Horsewill A. J. , Trommsdorff H. P.
Published: July 2002

Vibrational spectra of several isotopomers of benzoic acid (BA) crystals have been recorded by inelastic neutron scattering and are compared with spectra calculated for different potential energy surfaces (PES). These PES were obtained within the harmonic approximation from quantum chemical density functional theory (DFT) calculations made for the monomer, the isolated dimer, and the crystal using different codes and different levels of basis functions. Without refinement of the force constants, agreement between calculated and observed spectra is already sufficient for an unambiguous assignment of all vibrational modes. The best agreement was obtained with periodic DFT calculations. The most prominent discrepancy between calculated and observed frequencies was found for the out-of-plane O–H bending modes. For these modes (as well as for the in-plane bending and the O–H stretching modes) the anharmonicity of the potential was calculated, and the anharmonic correction was shown to account for about one-third of the discrepancy. The origin of this difference is attributed to the slight compression of the hydrogen bonds in the calculated structure of the dimer, which also leads to a significant lowering of the frequency of the O–H stretch mode.
New statistical mechanical treatment of systems near surfaces. V. Incompressible blend of interacting polydisperse linear polymers
Authors: Batman Richard , Chhajer Mukesh , Gujrati P. D.
Published: July 2002

We consider a lattice model of an incompressible blend of interacting (repulsive, attractive, or neutral) polydisperse polymers of two species, A and B. The blend is next to an infinite plane surface whose interaction with A can be attractive, repulsive, or neutral. This is the only parameter required to completely specify the effect of the surface on both components of the blend. We numerically study various density profiles and surface functions, as we move away from the surface, by using the method of Chhajer and Gujrati that has already been successfully applied to study a polymer solution next to a surface. The resulting density profiles show the oscillations that are seen in Monte Carlo simulations (but with magnitude enhanced and range diminished due to the presence of free volume in simulations), and the enrichment of the smaller species at a neutral surface. The method is computationally ultrafast and can be carried out on a PC, even in the incompressible case, when Monte Carlo simulations are not feasible. The calculations usually take a few seconds to a minute.
Optical recording media using laser-induced size reduction of Au nanoparticles
Authors: Sugiyama M. , Inasawa S. , Koda S. , Hirose T. , Yonekawa T. , Omatsu T. , Takami A.
Published: July 2002

Silica-gel films containing Au nanoparticles were fabricated by a photoreduction method, which functioned as a medium for high-density optical recording. On irradiation of 532 nm laser light, which corresponds to the surface plasmon resonance absorption of Au particles, the absorption spectrum of the film showed a blueshift. Using this property, a hologram was recorded on the film. Transmitting electron microscopy supported that the change in absorption spectrum was due to size reduction of embedded particles caused by laser irradiation. For recording the hologram, a threshold of laser fluence existed which corresponded to the heat of vaporization of Au particles.
Fabrication of gold nanowires on insulating substrates by field-induced mass transport
Authors: Calleja M. , Tello M. , Anguita J. , Garcı́a F. , Garcı́a R.
Published: July 2002

A method for the fabrication of nanometer size gold wires on insulating surfaces is presented. An oscillating gold-coated atomic force microscope tip is brought into close proximity of a silicon dioxide surface. The application of a negative sample voltage produces the transport of gold atoms from the tip to the surface. The voltage is applied when there is a tip–surface separation of ∼3 nm. The finite tip–surface separation enhances the tip lifetime. It also allows the application of sequences of multiple voltage pulses. Those sequences allow the fabrication of continuous nanowires. The atomic force microscope gold deposition is performed at room temperature and in ambient conditions which makes the method fully compatible with standard lithographic techniques. Electron transport measurements of the wires show a clear metallic behavior. Electrical resistivities of ∼3×10−7 Ω m and current densities of up to 5×1011 A m−2 are reported.
Near-field scanning optical microscope studies of the anisotropic stress variations in patterned SiN membranes
Authors: Campillo A. L. , Hsu J. W. P.
Published: July 2002

A near-field scanning optical microscope (NSOM) is used in transmission to study the anisotropic stress variations in SiN membranes, nominally 50 nm thick, containing arrays of submicron size holes. A polarization modulation technique is employed in combination with the NSOM to obtain quantitative measurements of the stress induced optical anisotropy on a ∼100 nm scale. A method is developed to remove the dichroic contribution from the measured retardance in order to determine stress variation. Stress patterns observed in the images provide insight into how the membrane relaxes in response to the patterning process.
Ab initio study of the absorption spectra of Agn (n=5–8) clusters
Authors: Bonačić-Koutecky Vlasta , Veyret Vincent , Mitrić Roland
Published: July 2002

The absorption spectra of Ag5–8 have been determined in the framework of the linear response equation-of-motion coupled cluster method and related techniques employing 11-electron relativistic effective core potential. In these treatments electron correlation effects for 11 electrons per atom are included, providing an accurate description of excited states of silver clusters. The calculations of transition energies and oscillator strengths have been carried out in a large energy interval for the stable structures and for the isomeric forms higher in energy. This allowed us to investigate the influence of structural properties on the spectroscopic patterns and to determine the role of d-electrons. Inclusion of d-electrons in the correlation treatment is mandatory to obtain accurate values for transition energies, but the excitations of s-electrons are primarily responsible for the spectroscopic patterns. They are characterized by the interference phenomena known in molecular spectroscopy which lead to a small number of intense and a large number of weak resonances. The calculated absorption spectra for the stable structures provide accurate predictions of the optical response properties in the gas phase and at the zero temperature. Since for neutral silver clusters the experimental data in the gas phase are not yet available, we also calculated spectra for deformed structures which model the influence of the environment such as rare-gas atoms, solid Ar-matrix or He-droplet. Comparison of our results with available experimental data permits us to identify structural properties responsible for the recorded spectral features.
Monolithic intracavity laser-modulator device fabrication using postgrowth processing of 1.55 μm heterostructures
Authors: Aimez V. , Beauvais J. , Beerens J. , Ng S. L. , Ooi B. S.
Published: July 2002

In this letter, we present the attractive characteristics of a fabrication method based on quantum-well intermixing induced by low energy ion implantation for the realization of photonic integrated circuits on GaInAsP–InP heterostructures. Intracavity electro-absorption modulators monolithically integrated with laser devices were fabricated, using this postgrowth technique. The modulator section of the integrated devices was blueshifted by 75 nm while keeping the laser section unshifted and preserving very low values of the lasing threshold current density. Modulation depths in excess of 10 dB/V at 1.55 μm were obtained on these integrated devices which incorporate both a modulator and a laser.
Analysis of mechanisms which lead to electrical breakdown in argon using the time delay method
Authors: Pejović Momčilo M. , Ristić Goran S.
Published: July 2002

The mechanisms leading to initiation of electrical breakdown in geometrically identical argon-filled tubes at different pressures were investigated by the memory curves. It was shown that the positive ions and metastable states, remaining from the previous discharge, have the dominant role in the initiation of breakdown in early and late afterglow periods, respectively. Contributions to the breakdown of gamma photons from radioactive sources (60Co) and cosmic rays were also analyzed.
Spring constant and damping constant tuning of nanomechanical resonators using a single-electron transistor
Authors: Schwab K.
Published: July 2002

By fabricating a single-electron transistor onto a mechanical system in a high magnetic field, it is shown that one can manipulate both the mechanical spring constant and damping constant by adjusting a potential of a nearby gate electrode. The spring constant effect is shown to be usable to control the resonant frequency of silicon-based nanomechanical resonators, while an additional damping constant effect is relevant for the resonators built upon carbon nanotube or similar molecular-sized materials. This could prove to be a very convenient scheme to actively control the response of nanomechanical systems for a variety of applications including radio-frequency signal processing, ultrasensitive force detection, and fundamental physics explorations.