bluuebunny/crossref_metadata_2025 huggingface dataset:
Cohesive failure of thin layers of soft model adhesives under tension
Authors: Derks Didi , Lindner Anke , Creton Costantino , Bonn Daniel
Published: February 2003

The cohesive failure of soft adhesives is studied using a yield stress fluid as a model adhesive, which allow to relate the viscoelastic properties of the fluid directly to its adhesive performance. We derive a theoretical expression for the force—distance curve as a function of the yield stress, which describes our experimental results very well. The theoretical prediction is obtained by assuming a circular air—adhesive interface; surprisingly, good agreement between theory and experiment is also obtained when strong fingering instabilities are observed in the experiment. This result is confirmed by the fact that we do not find a significant reduction in the work of adhesion when fingering instabilities are present. In addition, we discuss the morphology of the fingering instabilities.
Structure and secondary electron emission properties of MgO films deposited by pulsed mid-frequency magnetron sputtering
Authors: Cheng Y. H. , Kupfer H. , Richter F. , Giegengack H. , Hoyer W.
Published: February 2003

A pulsed mid-frequency magnetron sputtering technique was used to deposit MgO films. Atomic force microscopy, Rutherford backscattering, x-ray diffraction, and a diode discharge device were used to characterize surface morphology, oxygen content, crystalline structure, and the secondary electron emission (γ) coefficient of the films, respectively. The oxygen content (56 at. %) in all the films remained constant. However, surface morphology, crystalline structure, and the secondary electron emission properties of the films are strongly dependent on the O2 flow rate. As the O2 flow rate is increased from 3 to 10 sccm, a sudden decrease in the grain size and the γ coefficient of the films can be observed, and the crystalline orientation evolves from a strong (200) preferred orientation continuously to a fully (220) preferred orientation. Further increase of O2 flow rate results in a slight change in the grain size and the crystalline orientation, but a considerable increase in the full width at half maximum of the x-ray diffraction peaks and a significant decrease in the γ coefficient of the films. We found that preferred orientation and crystalline quality are the main factors that influence the secondary electron emission properties of the MgO films.
Formation of light-emitting FeSi2 in Fe thin films on ion-implanted (111)Si
Authors: Lu H. T. , Chen L. J. , Chueh Y. L. , Chou L. J.
Published: February 2003

The formation of iron silicides on (111)Si and effects of ion implantation on phase transformation have been investigated by sheet resistance measurements, grazing-incidence x-ray diffractometry, transmission electron microscopy, energy-dispersive x-ray analysis, and secondary ion mass spectroscopy. Ion implantation was found to enhance the growth of light-emitting β-FeSi2. Phase transformation from FeSi to β-FeSi2 begins at 600 °C and completes at 700 °C. P+ implantation was found to lower the transformation temperature from 700 to 600 °C. Wider than 20 nm As-decorated grain boundaries were observed in the As+-implanted samples annealed at 600–700 °C. The As-rich grain boundaries disappeared after 800 °C annealing, leading to a decrease in resistivity.
Preparation of Pb(Zr,Ti)O3 thin films by metalorganic chemical vapor deposition for low voltage ferroelectric memory
Authors: Gilbert S. R. , Hunter S. , Ritchey D. , Chi C. , Taylor D. V. , Amano J. , Aggarwal S. , Moise T. S. , Sakoda T. , Summerfelt S. R. , Singh K. K. , Kazemi C. , Carl D. , Bierman B.
Published: February 2003

To realize the full potential of high density embedded ferroelectric memory, ferroelectric film thickness must be scaled below 100 nm to ensure that the capacitor operating voltage is compatible with advanced, low voltage logic transistors. In this article, we describe recent progress in the preparation of sub-100 nm thick Pb(Zr,Ti)O3 (PZT) thin films by metalorganic chemical vapor deposition on 200 mm wafers using an industry-standard processing platform. Within the full range of thicknesses investigated, 134–52 nm, capacitor operating voltage scales linearly with film thickness, yielding 71 nm thick films with a switched polarization (Psw) of ∼40 μC/cm2 at 1.2 V. Below ∼50 nm, PZT surface roughness makes further thickness scaling difficult. With improved surface morphology, however, even lower operating voltages should be feasible.
Effect of compressed CO2 on the size and stability of reverse micelles: Small-angle x-ray scattering and phase behavior study
Authors: Zhang Jianling , Han Buxing , Liu Juncheng , Zhang Xiaogang , Yang Guanying , He Jun , Liu Zhimin , Jiang Tao , Wang Jun , Dong Baozhong
Published: February 2003

Synchrotron radiation small-angle x-ray scattering (SAXS) and phase behavior investigations were combined to study the effect of compressed CO2 on the stability and structure of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles in isooctane at different temperatures, CO2 pressures, AOT concentrations, and the molar ratios of water to surfactant w ([H2O]/[AOT]). SAXS measurements indicate that the size of the reverse micelles depends markedly on the pressure of CO2 and w. Increasing pressure of CO2 results in reduction of the micellar size. The effect of CO2 pressure on the size and structure of the reverse micelles is nearly independent of the concentration of the surfactant. The results of SAXS also give the information about the stability of the reverse micelles, which agrees with that obtained from the phase behavior investigation.
Nondestructive evaluation of dislocation structure in cyclically deformed Ni3Fe single crystals using magnetic techniques
Authors: Yasuda Hiroyuki Y. , Sasaki Akiko , Umakoshi Yukichi
Published: February 2003

The deformation substructure in Ni3Fe single crystals cyclically deformed at constant plastic shear strain amplitudes (γpl) consisted of two phases: the matrix with planar dislocation arrangement and the persistent slip band (PSB) having a three dimensional cell structure. The saturation stress remained almost constant regardless of γpl by adjusting the volume fraction of PSB. Anisotropy of high-field susceptibility in cyclically deformed Ni3Fe single crystals was measured in the (111) discs. Two types of magnetic anisotropy caused by atom rearrangement near the antiphase boundary (APB) and internal stress around dislocations were observed in fatigued Ni3Fe single crystals. The anisotropy data were subjected to the Fourier-type transformation and were split into separate anisotropy functions depending on the type of lattice defects. The magnitude ratio of dislocation- to APB-dependent anisotropy increases linearly with increasing γpl. This strongly suggests that Winter’s two-phase model can be applied to both mechanical and magnetic properties. Thus, nondestructive evaluation of dislocation structure in cyclically deformed Ni3Fe single crystals could be done by analyzing the magnetic anisotropy induced by cyclic deformation.
Plasma enhanced chemical vapor deposited SiCOH dielectrics: from low-k to extreme low-k interconnect materials
Authors: Grill Alfred
Published: February 2003

Carbon doped oxide dielectrics comprised of Si, C, O, and H (SiCOH) have been prepared by plasma enhanced chemical vapor deposition. Low-k films with a dielectric constant (k) of about 2.8 have been deposited from tetramethylcyclotetrasiloxane (TMCTS). The dielectric constant has been further reduced to extreme low-k values of k<2.1 by admixing an organic precursor to TMCTS and annealing the films to remove the organic fragments and create porosity in the films. The entire range of SiCOH films is characterized by relatively low coefficients of thermal expansion of about 12×10−6 K and mechanical properties that make them suitable for integration as the interconnect dielectric in ultralarge scale integration (ULSI) devices. The range of dielectric constants makes the films potentially useful for several generations of ULSI chips.
Indium segregation to dislocation loops induced by ion implantation damage in silicon
Authors: Noda Taiji
Published: February 2003

Indium segregation to dislocation loops and indium co-diffusion were investigated using secondary ion mass spectrometry (SIMS) and a physically-based diffusion model. High doses of As (30 keV, 1×1015 cm2) and B (5 keV, 1×1015 cm2) were implanted in indium-doped Si wafers which were then annealed at 750 °C and 1000 °C in a nitrogen ambient. SIMS profiles showed indium segregation around the implantation damage in both As- and B-implanted Si samples. Indium segregation around the B implantation damage confirms that indium segregation is induced primarily by implantation damage. We present a diffusion model that describes the interaction of indium and dislocation loops and the electric field/chemical ion pairing effect. A simulation that includes this model can predict indium segregation to dislocation loops produced by As/B implantation damage. As/B implants also induce the transient enhanced diffusion of indium. A difference in diffusivity enhancement of indium is shown between As implants and B implants. It is proposed that the difference can be attributed to the difference in the amount of the net excess interstitials, primarily due to ion mass effects.
Micro-Raman and infrared properties of SnO2 nanobelts synthesized from Sn and SiO2 powders
Authors: Peng X. S. , Zhang L. D. , Meng G. W. , Tian Y. T. , Lin Y. , Geng B. Y. , Sun S. H.
Published: February 2003

Rutile structured SnO2 nanobelts have been synthesized from the mixture of Sn powders and SiO2 nanoparticle powders. Each nanobelt has a uniform width of about several hundred nanometers and a thickness of about tens of nanometers along its entire length. Micro-Raman spectrum measurement on the SnO2 nanobelts shows that the first-order Raman A1g mode (632.9 cm−1) is very strong, and two weak Raman bands 498 and 694 cm−1 seem to correspond to infrared (IR)-active longitudinal optical (LO) and transverse optical (TO) of A2u modes. In addition, the IR spectrum of the SnO2 nanobelts shows the A2u (LO) (701.9 cm−1) and Eu (1) (TO) (634.5 cm−1) modes and one surface mode (565.2 cm−1). The IR-active bands in the Raman spectrum and the surface mode in IR spectrum, which may be due to the nanoscale morphology of the nanobelts.
Two-color optical technique for characterization of x-ray radiation-enhanced electron transport in SiO2
Authors: Marka Z. , Pasternak R. , Albridge R. G. , Rashkeev S. N. , Pantelides S. T. , Tolk N. H. , Choi B. K. , Fleetwood D. M. , Schrimpf R. D.
Published: February 2003

Damage enhanced electron transport, across thin oxides in x-ray irradiated Si/SiO2 samples, was measured via a contactless two-color laser technique. This method involves two steps: (1) optically stimulated electron injection into the oxide and (2) detection of transport, trapping, and recombination rates using time-dependent electric-field-induced second-harmonic generation arising from charge separation at the interface. Measured electron transport rates across an irradiated oxide are found to be substantially higher in comparison to unirradiated oxides. This effect is attributed to the presence of x-ray irradiation-induced defects that act as intermediate trapping sites facilitating enhanced electron tunneling through the oxide. The possible nature of the radiation-induced trapping sites is discussed.
Role of halogens in the mechanism of sensitization of uncooled PbSe infrared photodetectors
Authors: Torquemada M. C. , Rodrigo M. T. , Vergara G. , Sánchez F. J. , Almazán R. , Verdú M. , Rodrı́guez P. , Villamayor V. , Gómez L. J. , Montojo M. T. , Muñoz A.
Published: February 2003

Lead selenide layers were obtained by thermal evaporation in vacuum on thermally oxidized silicon. As-deposited layers present no response to infrared radiation and an activation procedure is mandatory. The sensitization process described in this work consists of a thermal treatment developed in two different stages: The first step in an iodine and oxygen rich atmosphere at 220 °C, and the second one at a higher temperature (450 °C) on air. After this treatment, the developed photodetectors show high sensitivity to medium wavelength infrared radiation at room temperature. We have analyzed the structural, compositional, electrical, and morphological changes of the films occurred during the activation process. After processing many films, we conclude that iodine plays a key role in the PbSe sensitization. This halogen behaves as a transport agent during the PbSe recrystallization process, and promotes a fast growth of PbSe microcrystals. Oxygen is trapped into the PbSe lattice during the recrystallization process, as it happens in chemically deposited PbSe films. The introduction of halogens in the PbSe sensitization procedure is a highly efficient technique for the incorporation of oxygen to the semiconductor lattice in electrically active positions.
Anisotropic refractive-index change in silica glass induced by self-trapped filament of linearly polarized femtosecond laser pulses
Authors: Yamada Kazuhiro , Watanabe Wataru , Nishii Junji , Itoh Kazuyoshi
Published: February 2003

Permanent refractive-index change can be induced by a self-trapped filament of intense ultrashort laser pulses in silica glass. We investigated the dependence of refractive-index change on polarization of incident laser pulses. As the region of refractive-index change can be selectively etched by hydrofluoric acid (HF), we first polished the sample and etched the cross section of the regions. The HF-etched cross section of refractive-index change that was induced by linearly polarized pulses was seen to be elliptical when observed with a scanning-electron microscope. By polarimetric analysis of diffraction from gratings fabricated by polarized ultrashort laser pulses, the index ellipsoid of refractive-index change was confirmed to be uniaxial and negative.
Crystallization behavior of amorphous Fe–P strengthened with embedded carbon nanotubes
Authors: Wei Bingqing , Li Yubao , Kohler-Redlich Philipp , Lück Reinhard , Xie SiShen
Published: February 2003

Fe 80 P 20 amorphous alloy, amorphous Fe–P–C, and amorphous Fe–P strengthened with embedded carbon nanotubes were fabricated by the rapid solidification process. Magnetothermal analysis, high-resolution transmission electron microscopy, and x-ray diffraction were employed to investigate the crystallization behavior of these amorphous alloys. Carbon nanotubes embedded in the amorphous matrix increases the apparent crystallization temperature by about 100 K and modifies the crystallization process compared to those of amorphous Fe–P and amorphous Fe–P–C. The role of the added carbon nanotubes will be discussed.
Laser shadowgraph measurements of electromagnetically-driven cylindrical shock-wave implosions in water
Authors: Rodriguez G. , Roberts J. P. , Echave J. A. , Taylor A. J.
Published: February 2003

Experiments performed at the Los Alamos National Laboratory’s Atlas 23-MJ z-pinch capacitor bank facility allows for experimental characterization of electromagnetically-driven cylindrical shock-wave implosions. Time-resolved laser shadowgraphy is used to dynamically image an Atlas-generated liner–target radial shock implosion in water with two-dimensional imaging detail that provides benchmark results for numerical hydrocode validation efforts at Los Alamos. Our laser shadowgraph measurements capture eight separate image frames of shock-wave motion in the water, with overall spatial and temporal resolutions of ±0.25 mm and 10 ns, respectively. Imaging of the shock-wave radial shape and position inside the water volume allows for quantitative comparison with numerical simulations. A linear fit to the average radius one-dimensional shock-wave trajectory plot yields a shock speed in the water of 6.65 mm/μs, corresponding to a Mach-4 shock. Comparison of the measured shock-wave trajectory with the numerical hydrodynamic simulation codes is performed.
Oxidation of nitrogen-implanted silicon: Energy dependence of oxide growth and defect characterization of the silicon substrate
Authors: Skarlatos D. , Tsamis C. , Tsoukalas D.
Published: February 2003

It is well known that nitrogen implantation in silicon leads in reduction of the SiO2 thickness in implanted wafers, which makes this a very promising technique for present and future device processing. In this work we investigate the influence of the implantation energy on oxidation kinetics in nitrogen-implanted silicon. Nitrogen was implanted in silicon wafers at low (3 keV), and medium (25, 80, and 150 keV) energies and oxidations were performed at various temperatures (800–900 °C) and times. The experiment shows that the decrease in the oxide thickness is smaller when nitrogen is implanted closer to the silicon surface. We attribute this to nitrogen out-diffusion during the ramping and the initial oxidation steps, which is more effective when nitrogen is placed closer to the surface. Additional experiments varying the ramping time support this explanation. An additional study of the growth of extended defects under the different implantation conditions has been performed. This study enabled the estimation of an energy-dose “window,” in which we can observe very small oxide thickness (25−40 Å) without extended defects formation.
Characteristics and stress-induced degradation of laser-activated low temperature polycrystalline silicon thin-film transistors
Authors: Peng Du-Zen , Chang Ting-Chang , Chang Chun-Yen , Tsai Ming-Liang , Tu Chun-Hao , Liu Po-Tsun
Published: February 2003

The characteristics and reliability of laser-activated polycrystalline silicon thin-film transistors (poly-Si TFTs) under the stress condition of drain voltage (Vd)=12 V and gate voltage (Vg)=15 V have been investigated. In spite of reducing the source/drain resistivity by using laser activation method, the leakage current (Vg<0) is larger for laser-activated poly-Si TFTs in comparison with traditional furnace-activated counterparts. It is also found that the post-stress leakage and on/off current ratio degrade much faster for laser-activated poly-Si TFTs, while the degradation rates of threshold voltage and subthreshold swing are comparable to those of traditional furnace-activated TFTs. The laser activation modifies the grain structure between drain and channel region and causes grain discontinuity extending from the drain side to the channel region. The grain discontinuity near drain side in the polysilicon film has been investigated by transmission electron microscopy analysis. The effective trap state density calculated from typical I−V curve has been compared for laser-activated and furnace-activated TFTs. As a result, an inferior reliability with extra trap state density and larger leakage current was observed in the laser-activated poly-Si TFTs.
Surfactant effect of In for AlGaN growth by plasma-assisted molecular beam epitaxy
Authors: Monroy E. , Daudin B. , Bellet-Amalric E. , Gogneau N. , Jalabert D. , Enjalbert F. , Brault J. , Barjon J. , Dang Le Si
Published: February 2003

In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.
Optical spectroscopy and frequency upconversion properties of Tm3+ doped tungstate fluorophosphate glasses
Authors: Poirier Gaël , Jerez Vladimir A. , de Araújo Cid B. , Messaddeq Younes , Ribeiro Sidney J. L. , Poulain Marcel
Published: February 2003

Tungstate fluorophosphate glasses of good optical quality were synthesized by fusion of the components and casting under air atmosphere. The absorption spectra from near-infrared to visible were obtained and the Judd–Ofelt parameters determined from the absorption bands. Transition probabilities, excited state lifetimes and transition branching ratios were determined from the measurements. Pumping with a 354.7 nm beam from a pulsed laser resulted in emission at 450 nm due to transition D21→F43 in Tm3+ ions and a broadband emission centered at ≈550 nm attributed to the glass matrix. When pumping at 650 nm, two emission bands at 450 nm (1D2→3F4) and at 790 nm (3H4→3H6) were observed. Excitation spectra were also obtained in order to understand the origin of both emissions. Theoretical and experimental lifetimes were determined and the results were explained in terms of multiphonon relaxation.
Blue, green, red, and white electroluminescence from multichromophore polymer blends
Authors: Hu Bin , Karasz Frank E.
Published: February 2003

We extend our previous investigations of electro-optically active polymer blends to report enhanced blue, green, red, or white electroluminescence (EL) from a multicomponent multiphase polymer blend containing blue, green, and red light emitting chromophores in different proportions as required together with both hole and electron transport materials. The general architecture of the EL devices employed was indium–tin–oxide (ITO)/poly p-phenylene vinylene (PPV)/blend/Ca or Al, in which a PPV layer was interspersed between the ITO and the blend layers to provide more efficient hole injection, enhance overall emission, and increase device longevity. The relevant EL mechanisms are discussed in terms of color related and efficiency related processes. The color related EL processes involve blueshifted emissions controlled by chromophore dilution, Förster energy transfer controlled mainly by spectral overlap between absorption and emission in chromophores, and an interfacial emission from the PPV layer. The efficiency related processes involve double charge injection controlled by the nature and concentration of the hole and electron transport materials present, the confinement of injected charge carriers controlled by the blend morphology, and potential barriers within the polymer blend layer. The design of devices which emit the desired color, efficiently balance the double charge injection, and optimize the confinement of injected charge carriers to provide high quantum yields by using polymer blends, is discussed.
Ethylene glycol dimers in the liquid phase: A study by x-ray and neutron diffraction
Authors: Bakó Imre , Grósz Tamas , Pálinkás Gábor , Bellissent-Funel Marie Claire
Published: February 2003

Molecular conformations and intermolecular H bonding in liquid ethylene glycol have been studied by neutron diffraction with hydrogen/deuterium isotopic substitution, and by x-ray diffraction measurements at room temperature. For comparison, conformations of molecules and their dimers in the gas phase have also been calculated, based on the density-functional theory. Energies, geometries, and vibrational frequencies of dimers were analyzed. Diffraction data analyzed by the Monte Carlo method resulted in a molecular structure in agreement with the findings from gas-phase electron diffraction experiments and density-functional calculations. Conformers of ethylene glycol molecules in the liquid phase were identified as a gauche OCCO arrangement, stabilized by intramolecular hydrogen bonding, which is in agreement with the results of gas-phase density-functional calculations. Data analysis resulted in an average of three hydrogen-bonded nearest neighbors of molecules in liquid ethylene glycol. In addition, experimental composite pair correlation functions RHO and RR have been extracted from the diffraction data for future comparison with computer simulation results.
No “delayed” muonium-formation in organic liquids
Authors: Walker David C. , Karolczak Stefan , Porter Gerald B. , Gillis Hugh A.
Published: February 2003

Studies of the yields of muonium atoms and diamagnetic-species in hexane in the presence of added scavengers have shown, unequivocally, that muonium is not formed by the “delayed” (microsecond) mechanism proposed recently. Instead, the lifetimes evaluated show that if the combination reaction of μ++e−→Mu contributes to muonium formation in “chemically active” liquids, then it is complete within <10−10 s. In fact the results are consistent only with the “hot” model, in which the initial distribution of muons between muonium atoms and diamagnetic molecules is determined at the epithermal level, as the last step of the muon’s charge-exchangecycles and thermalization.
Chain length dependence of the state diagram of a single stiff-chain macromolecule: Theory and Monte Carlo simulation
Authors: Stukan M. R. , Ivanov V. A. , Grosberg A. Yu. , Paul W. , Binder K.
Published: February 2003

We present a Monte Carlo computer simulation and theoretical results for the dependence of the state diagram of a single semiflexible chain on the chain length. The calculated transition lines between different structures in the state diagrams for both studied chain lengths N=40 and N=80 can be described by theoretical predictions which include chain length dependence explicitly. The stability criteria of different structures are discussed. The theoretically predicted exponent in the dependence of the toroid size on the chain length is compatible with computer simulation results.
Large area multiturn superfluid phase slip gyroscope
Authors: Bruckner Niels , Packard Richard
Published: February 2003

We have built and tested a large area multiturn superfluid He4 phase slip gyroscope. This device demonstrates quantum-mechanical phase coherence of superfluid He4 over a macroscopic length scale (1.4 m). The sensing loop area of this device is two orders of magnitude larger than our proof-of-principle model, with an improvement in sensitivity of ∼20 over any other superfluid He4 gyroscope. We find that this rotation sensor has excellent long-term stability. In addition, there are no new noise sources preventing further enhancements in sensitivity.
Energy landscape and thermally activated switching of submicron-sized ferromagnetic elements
Authors: E Weinan , Ren Weiqing , Vanden-Eijnden Eric
Published: February 2003

Thermally activated switching and the energy landscape of submicron-sized magnetic elements are studied using the string method. For thin films, we found that switching proceeds by two generic scenarios: Domain-wall propagation and reconnection followed by edge domain switching, or vortex nucleation at the boundary followed by vortex propagation through the sample. The second scenario is preferred for thicker films whereas the first is preferred for thin (less than 20 nm) films. The energy landscape of such a system is nicely summarized on the plane spanned by the average magnetization in the in-plane directions. For three-dimensional samples, we found that switching proceeds by vortex propagation through the sample. The implication of the Landau–Lifshitz dynamics is also discussed.
Kinetic-energy release in Coulomb explosion of metastable C3H52+
Authors: Głuch K. , Fedor J. , Matt-Leubner S. , Echt O. , Stamatovic A. , Probst M. , Scheier P. , Märk T. D.
Published: February 2003

C 3 H 5 2+ , formed by electron impact ionization of propane, undergoes metastable decay into C2H2++CH3+. We have monitored this reaction in a magnetic mass spectrometer of reversed geometry that is equipped with two electric sectors (BEE geometry). Three different techniques were applied to identify the fragment ions and determine the kinetic-energy release (KER) of spontaneous Coulomb explosion of C3H52+ in the second and third field free regions of the mass spectrometer. The KER distribution is very narrow, with a width of about 3% [root-mean square standard deviation]. An average KER of 4.58±0.15 eV is derived from the distribution. High level ab initio quantum-chemical calculations of the structure and energetics of C3H52+ are reported. The activation barrier of the reverse reaction, CH3++C2H2+ (vinylidene), is computed. The value closely agrees with the experimental average KER, thus indicating that essentially all energy available in the reaction is partitioned into kinetic energy.
Correlation between electronic and molecular structure distortions and vibrational properties. II. Amide I modes of NMA–nD2O complexes
Authors: Ham Sihyun , Kim Joo-Hee , Lee Hochan , Cho Minhaeng
Published: February 2003

Hydration effects on the molecular structure and amide I mode frequency of a prototype peptide molecule, N-methylacetamide (NMA), when it is solvated by a few water molecules, were investigated by carrying out ab initio calculations for a number of NMA–water complexes. The harmonic frequency shift of the amide I mode in NMA–nD2O (n=1–5) complex was found to originate from the combination of the molecular cubic anharmonicity and displacement of the amide I coordinate when the NMA is hydrated. Using a multivariate least-square fitting method, the effective transition charges of six NMA sites were determined. A brief discussion on how this empirical model can be used to quantitatively describe solvatochromic frequency shift of the NMA amide I mode in solution is presented.
Energy relaxation and quenching processes of doped rare-gas clusters with a shell-like geometric structure
Authors: Laarmann T. , von Haeften K. , Wabnitz H. , Möller T.
Published: February 2003

Energy relaxation processes of photo-excited Kr50¯ clusters covered with a shell of Ar atoms (up to 40), which are embedded inside large Ne7500¯ clusters are investigated with energy resolved fluorescence spectroscopy. In the energy range of the characteristic Ne cluster absorption (16.5–18 eV) a strong energy transfer to the embedded Kr cluster is observed, which results in the desorption of electronically excited Kr* atoms. Kr* atoms move through the Ne cluster, desorb and emit visible and near-infrared light in the vacuum (5p→5s). By coating the Kr clusters with Ar atoms, the Kr lines disappear and 4p→4s transitions of Ar* become dominant. Additionally, new emission bands occur, which are assigned to transitions of perturbed atomic Kr 5p-states inside Ne clusters. Due to the interaction of electronically excited Kr* atoms with neutral Ar atoms in the surrounding shell, several excited Kr states namely 5p [1/2]0 and 5p [3/2]2 decay nonradiatively. This is in agreement with the well-known “energy-gap law.” The results give experimental evidence that clusters with a multishell structure can be prepared by a sequential pick-up technique. This allows the preparation of Kr clusters embedded inside Ne clusters and coated with a shell of Ar atoms. Such clusters cannot be prepared with conventional coexpansion techniques.
Direct Monte Carlo simulation of chemical reaction systems: Prediction of ultrafast detonations
Authors: Anderson James B. , Long Lyle N.
Published: February 2003

Ultrafast detonations having steady-state velocities greater than predicted by the Chapman–Jouguet (C–J) and the Zeldovich–von Neumann–Döring (ZND) theories are predicted by direct simulations of detonation waves in gaseous reaction systems. The simulations are made using Bird’s direct simulation Monte Carlo method which produces the full details of the coupled gas-dynamic and reaction effects as well as temperature, velocity, density, pressure, and species profiles for the detonation waves. The systems are simplified and clarified by restriction to one-dimensional flow with the reaction A+M→B+M having variable energy release and rate characteristics. For a slow reaction the reaction and shock regions are separated and the detonation wave proceeds at the forward speed of sound in the burned gas as predicted by the C–J and ZND theories. For a very fast reaction the reaction and shock waves overlap, the assumptions required for the C–J and ZND theories are no longer valid, the von Neumann spike is reduced, and the detonation velocity exceeds that predicted by the C–J and ZND theories to produce an ultrafast detonation.
Continuum predictions from molecular dynamics simulations: Shock waves
Authors: Root Seth , Hardy Robert J. , Swanson David R.
Published: February 2003

Techniques are investigated for obtaining continuously distributed local properties from the positions and velocities of constituent atoms. A localization function is used to calculate the local density, temperature, and velocity from the results of molecular dynamics simulations of shock waves in a two-dimensional model system. The two-dimensional spatial variations of the local properties are found, and the width of the localization function is varied to optimize the presentation. A vector plot of the local velocity shows turbulence behind the shock with a resolution on the order of a nanometer. Contour plots show the mass density and local temperature at a hot spot caused by a collapsed void.
Structure of nonuniform three-component fluid mixtures: A density-functional approach
Authors: Patra Chandra N. , Ghosh Swapan K.
Published: February 2003

The structure of nonuniform three-component fluid mixtures is studied using a self-consistent density-functional approach as well as a simple weighted density approximation and also by computer simulation. The second-order direct correlation function and the bridge function of the corresponding uniform fluid mixture required as input in the self-consistent theory are obtained from integral equation theory using an accurate closure relation, whereas the available analytic expressions of second-order correlation function within the Percus–Yevick approximation is used in the simple weighted density approximation. This new approach is employed to predict the structure of ternary hard sphere fluid mixtures near a hard wall as well as the structure of ternary uniform hard sphere mixtures. The calculated density and concentration profiles of the nonuniform fluid mixtures as well as the radial distribution function profiles of the uniform fluid mixtures are shown to compare well with the computer simulation results performed in the present work.
Electrostatic detachment of an adhering particle from a micromanipulated probe
Authors: Saito Shigeki , Himeno Hideo , Takahashi Kunio
Published: February 2003

During micromanipulation, the influence of gravitational force becomes extremely small. The adhesional force is more significant for smaller objects. An adhered object can be detached by electrostatic interaction. In our earlier study, the electrostatic force generated by an applied voltage and the voltage required for detachment have been theoretically analyzed using the boundary element method (BEM). The system consists of a manipulation probe, a spherical microparticle, and a substrate plate. These objects are all conductive. In this study, the voltage for detachment of a microparticle with a 30-μm diameter is experimentally clarified, and is revealed to be in good agreement with the voltage predicted by BEM analysis. In addition, phenomena other than detachment are discussed based on observations through an optical video microscope. These results provide us with knowledge about the strategy for reliable electrostatic micromanipulation.
Photovoltage detection of x-ray absorption and magnetic circular dichroism spectra of magnetic films grown on semiconductors
Authors: Mallett J. J. , Dudzik E. , Schwarzacher W. , Thompson S. M. , Tsang H. K. , van der Laan G.
Published: February 2003

We describe a measurement technique for x-ray absorption spectroscopy, in which the sample is used as the detector. Co L2,3 absorption spectra were taken from cobalt films grown on n-GaAs by measuring the photovoltage generated by the x rays transmitted across the rectifying metal–semiconductor contact. After correcting the effects of signal saturation good agreement was obtained with spectra acquired using the conventional total electron yield method. X-ray magnetic circular dichroism measurements using the method gave values for the spin moment of cobalt between 1.48 and 1.89 μB/atom, compared to the value in the literature of 1.55 μB/atom for the bulk. An anomalously high average value of 0.39 μB/atom was found for the orbital moment and attributed to the presence of oxide in the films.
Analysis of single- and two-photon-excited green emission spectra of thin-film cadmium sulfide
Authors: Ullrich B. , Yano S. , Schroeder R. , Sakai H.
Published: February 2003

Employing nanosecond and femtosecond laser pulses at 355 nm (3.49 eV) and 768 nm (1.61 eV), green single- and two-photon-excited interband emission at 2.412 and 2.381 eV of thin-film cadmium sulfide (CdS) on glass has been measured at room temperature. The spectra are fitted and analyzed by a theory based on the detailed balance principle. The results demonstrate that the interband emission is independent of the mode of excitation and that the energy shift of the spectra is solely due to self-absorption of the emission evoked by two-photon excitation. The work also addresses energy dissipation in thin-film CdS excited beyond the Mott transition.
In situ growth of superconducting MgB2 thin films by molecular-beam epitaxy
Authors: Ueda K. , Naito M.
Published: February 2003

The in situ growth of superconducting MgB2 thin films was examined from various perspectives. The paper discusses (1) growth temperature, (2) the effect of excess Mg, (3) the effect of residual gas during growth, (4) the effect of in situ annealing, (5) thickness dependence, and (6) the effect of substrates. Our results provide a guide to the preparation of high-quality superconducting MgB2 films for potential electronics applications.
Elastic, mechanical, and thermal properties of nanocrystalline diamond films
Authors: Philip J. , Hess P. , Feygelson T. , Butler J. E. , Chattopadhyay S. , Chen K. H. , Chen L. C.
Published: February 2003

Nanocrystalline columnar-structured diamond films with column diameters less than 100 nm and thicknesses in the range of 1–5 μm were grown on silicon substrates by chemical vapor deposition (CVD) in a microwave plasma reactor with purified methane and hydrogen used as the reactants. Uniform conformal nucleation densities in excess of 1012 cm−2 were accomplished prior to growth by seeding with explosively formed nanodiamonds, which resulted in good optical quality films. The film thickness was measured in situ by the laser reflectometry method. The grain size and optical quality of the films were characterized by scanning electron microscopy and Raman measurements. Broadband surface acoustic wave pulses were used to measure the anomalous dispersion in the layered systems. The experimental dispersion curves were fitted by theory, assuming the diamond film as an isotropic layer on an anisotropic silicon substrate, to determine mean values of the density and Young’s modulus of the diamond films. The density was close to the density of single crystal diamond or polycrystalline diamond plates grown by the CVD technique, whereas the Young’s modulus varied strongly with the nucleation density between 517 and 1120 GPa. Young’s moduli close to the single crystal values were obtained for films grown with a nucleation density ⩾1012 cm−2. The thermal diffusivity in these films was measured by the traveling wave technique. The value for ∼3.5-μm-thick nanocrystalline diamond films with nucleation densities ⩾1012 cm−2 was ∼7.2 cm2/s, whereas those with lower nucleation densities showed a value of ∼5.5 cm2/s.
Finite collection time effects in autocovariance function measurements
Authors: Menzel A. , Conrad E. H. , Tringides M. C. , Kammler M. , Horn von Högen M.
Published: February 2003

In this paper we discuss how finite data sets influence experimental measurements of the autocovariance function. Autocovariance estimators are biased, meaning that the expectation value for any measured autocovariance function is not identical to the actual autocovariance function. In this work we show that the measured autocovariance function for a finite length time series must become negative for some lag times. We derive analytic corrections to these finite time errors for different types of correlated random sequences. Our results explain the apparent anticorrelated noise observed in experimental observations.
The dynamics of the dissociative adsorption of methane on Pt(533)
Authors: Gee Adam T. , Hayden Brian E. , Mormiche Claire , Kleyn Aart W. , Riedmüller Bernd
Published: February 2003

The influence of well-defined steps on the dynamics of the dissociative chemisorption of methane on Pt(533) has been investigated using molecular beam techniques. The initial dissociative chemisorption probability S0 has been determined as a function of incident energy Ei, angle of incidence θi, and surface temperature TS. For incident kinetic energies in the range 26<Ei(meV)<1450, the initial dissociation probability of CH4 on the Pt(533) surface is higher than on Pt(111), for all surface temperatures investigated. This enhancement in dissociation is associated with the additional direct sticking mediated by the step sites, with no evidence for any additional indirect dynamical channel to dissociation induced by the step sites in the range of energies studied. The Ei dependence can be separated into the contributions of the (111) terraces and the (100) steps. The latter exhibits an effective activation barrier for dissociation ≈300 meV lower than the (111) terraces. The angular dependence can also be interpreted as having two contributions, one associated with the (111) terraces, and the second associated with the steps. The angular dependence associated with the step sites is broader than the dependence expected for the (111) terraces, and has a maximum for incident trajectories with an angle between the angles corresponding to the normal directions of the (111) and (100) facets. An enhanced TS dependence is also observed on the Pt(533) surface over Pt(111). This is also associated with the influence of the step sites, and results either from the lower barrier to dissociation, or more likely a more effective coupling of the energy from the surface into the reaction coordinate.
Liquid-phase adaptive femtosecond quantum control: Removing intrinsic intensity dependencies
Authors: Brixner T. , Damrauer N. H. , Kiefer B. , Gerber G.
Published: February 2003

Femtosecond adaptive pulse shaping of 800-nm laser pulses is applied to control the multiphoton molecular excitation of the charge-transfer coordination complex [Ru(dpb)3](PF6)2 (where dpb=4,4′-diphenyl-2,2′-bipyridine) dissolved in methanol. A phase-only femtosecond pulse shaper provides a mechanism for multiparameter (128) variation of the incident field, and a closed-loop evolutionary algorithm optimizes pulse shapes within the vast search space. Molecular emission at 620 nm is used as experimental feedback which is proportional to the excited-state population in the long-lived MLCT3 (metal-to-ligand charge-transfer) state. The dominant intensity dependence of the multiphoton excitation process is removed by using second-harmonic generation (SHG) in a thin optical crystal as a general “reference” signal. Successful control of the emission/SHG ratio demands that the field adapt to the electronic structure or dynamic needs of the molecule in solution. This suggests that adaptive femtosecond pulse shaping can provide a general means of finding field shapes capable of selectively exciting molecules based on their unique optical properties.
Short H-bonds and spontaneous self-dissociation in (H2O)20: Effects of H-bond topology
Authors: Kuo Jer-Lai , Ciobanu Cristian V. , Ojamäe Lars , Shavitt Isaiah , Singer Sherwin J.
Published: February 2003

There are 30026 symmetry-distinct ways to arrange 20 water molecules in a dodecahedral cage with nearly optimum hydrogen bond lengths and angles, analogous to the arrangements that give rise to the zero-point entropy in ice-Ih. The energy of hydrogen bond isomers in (H2O)20, assumed to be similar in the past, differs by up to 70 kcal/mol. The isomers differ widely in their hydrogen bond lengths, some exhibiting bond lengths as short as ∼2.4 Å. The differences among the isomers extends to their chemical properties: In some arrangements one or more water molecules spontaneously self-dissociate, giving rise to spatially separated excess proton and hydroxyl ion units in the cluster. Isomers that exhibit these unusual properties can be identified by features of their hydrogen bond topology.
Linear and nonlinear infrared signatures of local α- and 310-helical structures in alanine polypeptides
Authors: Moran Andrew M. , Park Seung-Min , Dreyer Jens , Mukamel Shaul
Published: February 2003

Vibrational exciton Hamiltonians for the amide I and amide A modes of both the α- and 310-helical conformations of a fifteen unit polyalanine oligomer CH3–CO(Ala)15–NHCH3 are constructed using density-functional calculations for smaller model peptides. Energy levels as well as the transition dipoles of all singly and doubly excited-state manifolds are calculated. A variety of C13-substituted isotopic derivatives are examined with respect to their ability to reveal differences in local secondary structures in two-dimensional infrared spectra in the amide I region. Amide mode anharmonicities are predicted to be valid indicators of secondary helical structures.
Spin counting experiments in the dipolar-ordered state
Authors: Cho H. , Cory D. G. , Ramanathan C.
Published: February 2003

The dipolar-ordered state (ρ∝Hd) can be prepared from a state of Zeeman equilibrium at high field by adiabatically removing the Zeeman field. It is also possible to produce dipolar-ordered states in the rotating frame using two well-known methods—adiabatic demagnetization in the rotating frame and the Jeener–Broekaert (JB) pulse pair. Using multiple quantum nuclear magnetic resonance techniques, we illustrate the transient dynamics of the spin system following the JB pulse pair and its evolution to a dipolar-ordered state over a period of approximately 60 μs in a single crystal of CaF2 oriented along the [100] direction, and approximately 100 μs when the crystal is oriented along the [110] direction. By encoding the coherence numbers in an orthogonal basis (the x basis) to the Zeeman basis, we show that the dipolar-ordered state is a two spin correlated state. The observed ratio between double quantum and zero quantum coherences in the x basis confirms the presence of the Ii+Ij−+Ii−Ij+ (flip–flop) terms in the experimentally prepared dipolar-ordered state.
Influence of aluminum additions on phase formation in boron nitride films deposited by magnetron sputtering
Authors: Pfeifer Th. , Richter F. , Welzel Th. , Kupfer H. , Willich P.
Published: February 2003

Boron nitride (BN) thin films have been deposited by magnetron sputtering of an hexagonal BN (h-BN) target with an additional flux of aluminum provided by a special Al sputtering target. Up to an aluminum percentage of about 1.0 at. % the formation of the cubic BN (c-BN) phase was not influenced by the aluminum addition. An Al content of 1.3 at. % was connected with a remarkable delay in the transition from h-BN to c-BN growth as well as a certain deterioration in the crystal quality. An aluminum flux causing a concentration of 1.5 at. % did prevent the growth of the cubic BN phase completely. The dynamics of impurity incorporation were investigated using secondary ion mass spectrometry measurements. We found that the aluminum electrode plays an important role in the incorporation of hydrogen in the films. In connection with other results it can be concluded that for both h-BN and c-BN strong evidence for substitutional incorporation of aluminum on boron sites exists. Cross-section transmission electron microscopy revealed that the formation of the c-BN phase starts at particular sites (c-BN nuclei) which broaden via a wedge shaped growth up to the point where the c-BN nucleation is completed and a nanocrystalline, pure c-BN layer grows. Hence, the interface between h-BN and c-BN has a jagged shape. This structure of the films h-BN/c-BN was found to be essentially independent of the Al incorporation.