bluuebunny/crossref_metadata_2025 huggingface dataset:
Negative thermodiffusion of polymers and colloids in solvent mixtures
Authors: de Gans Berend-Jan , Kita Rio , Müller Beate , Wiegand Simone
Published: April 2003

Results on thermodiffusion of poly(ethylene oxide) and colloidal boehmite (γ-AlOOH) rods in ethanol/water mixtures are presented. Data were obtained using thermal diffusion forced Rayleigh scattering. The sign of the Soret coefficient of the boehmite rods changes from positive to negative with increasing water content, i.e., at sufficiently high water content the colloidal particles move to higher temperatures. The sign of the Soret coefficient of the poly(ethylene oxide) in ethanol/water mixtures is negative, i.e., the poly(ethylene oxide) molecules move to higher temperatures, whereas in pure water the sign is positive. To our knowledge this is the first time that a sign change has been observed for polymers in solution. The analysis of the static light scattering on poly(ethylene oxide) allows the determination of the preferentially solvating solvent. In the investigated concentration range the preferentially solvating solvent is ethanol, in spite of being the poorer solvent for poly(ethylene oxide).
Computer modeling of synthesis of proteinlike copolymer via copolymerization with simultaneous globule formation
Authors: Berezkin Anatolii V. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: April 2003

Using a Monte Carlo simulation technique, we have modeled the process of copolymerization of hydrophobic and hydrophilic monomers in a selective (polar) solvent. The composition of an emerging polymer chain is such that macromolecule adopts a globular conformation. The preferential sorption of hydrophobic monomers in the core of the globule is explicitly taken into account. It is shown that such the copolymerization process automatically leads to the formation of the core–shell microstructure in the resulting globule and to the well-pronounced long-range correlations of the Levy-flight type in obtained sequences of monomer units. Thus, this type of synthesis provides a robust one-step method of producing of “proteinlike” copolymers, i.e., copolymers that exhibit in the globular state a microstructure with a hydrophobic core wrapped in a hydrophilic envelope.
Comment on “Transport equations for concentrated electrolyte solutions: Reference frame, mutual diffusion” [J. Chem. Phys. 116, 2085 (2002)]
Authors: Felderhof B. U.
Published: April 2003

The definition of the mutual diffusion coefficient of a binary electrolyte solution is discussed on the basis of irreversible thermodynamics. An expression for the mutual diffusion coefficient in terms of Onsager transport coefficients and thermodynamic derivatives is derived. It is shown how the mutual diffusion coefficient is related to another coefficient defined recently [J.-F. Dufrêche, O. Bernard, and P. Turq J. Chem. Phys. 116, 2085 (2002)].
Incidence angle dependence of scattering and dissociation of O2 on Al(111): Possible weakly bound molecular precursors
Authors: Weiße Olaf , Wesenberg Claudia , Binetti Marcello , Hasselbrink Eckart , Corriol Cécile , Darling George R. , Holloway Stephen
Published: April 2003

Molecular beams have been used to search for evidence for a weakly bound molecular precursor in the interaction of O2 with Al(111). The experiments are consistent with a precursor whose binding energy is smaller than 0.1 eV. The total reflectivity as a function of incidence angle shows a pronounced dip at 25° for Etrans between 90 and 300 meV. This feature corroborates an earlier observation by Österlund et al. in sticking measurements. Modeling using a reduced dimensionality potential energy surface shows a similar behavior which is caused by steering into a shallow molecular adsorption well located at the same site in the unit cell as the maximum in the barrier towards dissociative adsorption. This effect is not observed if the molecular adsorption well is located at the same site as the minimum energy pathway to dissociative adsorption.
Budding and fission dynamics of two-component vesicles
Authors: Yamamoto Satoru , Hyodo Shi-aki
Published: April 2003

We studied the shape deformation induced by the phase separation of two-component vesicles using a dissipative particle dynamics simulation. Two types of amphiphiles, which have the same architecture but segregate from each other, are modeled by connecting particles representing the hydrophilic head and hydrophobic tail groups. After vesicle formation using a single component system, some of the amphiphiles are replaced by a second component, and then phase separation on the vesicle is simulated. Under appropriate conditions, typical shape deformations of a vesicle, such as crenated and invaginated shapes, are observed. We demonstrate that the budding and the fission are facilitated by lateral phase separation upon vesicle coupling to an asymmetric transversal distribution of amphiphiles in the bilayer. Moreover, simulations show a possible new pathway for the fission process triggered by cleavage along the domain boundary. Mesoscopic simulation based on a simple coarse-graining model reveals the dynamics of amphiphiles in vesicles that is essentially unpredictable using a conventional continuum model or full atomistic simulation.
High level theoretical study of the structure and rotational barriers of trans-stilbene
Authors: Kwasniewski S. P. , Claes L. , François J.-P. , Deleuze M. S.
Published: April 2003

The relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree–Fock, second and third-order Møller–Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T)], in conjunction with basis sets of increasing size, containing up to 1130 basis functions. A focal point analysis has been carried out in order to determine how the energy differences and rotational barriers approach convergence, enabling extrapolation of the CCSD(T) results to a near-complete basis set. The investigated saddle points pertain to independent rotations of the phenyl rings about the single C–C bond, and to pedalling motions described by a twofold rotation of the central ethylene bond about the longitudinal axis of the molecule. The benchmark calculations presented in this study lead to the conclusion that, in the nonrelativistic limit and within the frozen core approximation, trans-stilbene in vacuum is a strictly planar molecule in its absolute energy minimum form, in sharp contrast with many previous theoretical studies. This point has been ultimately confirmed by an MP2 geometry optimization using the aug-cc-pVDZ basis set. At last, the energy of cis-stilbene relative to the trans-isomer is accurately evaluated.
Nd
0.8
Y
0.2
NiO
3
thin films with room-temperature metal–insulator transition deposited by pulsed laser ablation
Authors: Hamet Jean-François , Ambrosini Andrea , Retoux Richard
Published: April 2003

A solid solution between neodymium and yttrium nickelates has been stabilized that displays a room-temperature metal–insulator transition and lattice parameters predicted by Vegard’s law. Nd0.8Y0.2NiO3 thin films were grown on (110) NdGaO3 by pulsed laser deposition, and structural investigations were carried out by x-ray diffraction and transmission electron microscopy, which revealed the presence of domains with the c axis either parallel or perpendicular to the substrate plane. The growth conditions leading to a metal–insulator transition at room temperature (311 K) were defined. The strain effect induced by the substrate was explained, and a critical film thickness close to 100 nm was established.
Luminescence, radiation damage, and color center creation in Eu3+-doped Bi4Ge3O12 fiber single crystals
Authors: Shim J. B. , Yoshikawa A. , Bensalah A. , Fukuda T. , Solovieva N. , Nikl M. , Rosetta E. , Vedda A. , Yoon D. H.
Published: April 2003

Doped Bi4Ge3O12 (BGO) fiber single crystals were grown by the micro-pulling-down method with the resistance heating system. Optical absorption measurements before and after x-ray irradiation (induced absorption) were performed in order to investigate the radiation damage of the crystals. The Eu3+-doped BGO has shown higher radiation resistance with respect to the equivalently grown undoped BGO fibers. Radio- and photoluminescence characteristics were obtained to clarify the role of Eu3+ in luminescence and scintillation mechanism. Radioluminescence spectra show the intense and slow (decay time of 1.61 ms) D0,15−7Fx(x=0−6) radiative transitions of Eu3+, while the intrinsic BGO emission becomes weaker with respect to the undoped BGO. Photoluminescence decay of the intrinsic BGO emission becomes nonexponential and noticeably faster in Eu-doped samples, which evidences the nonradiative energy transfer from intrinsic emission centers to Eu3+ ions. Thermally stimulated luminescence (TSL) measurements above room temperature prove that Eu3+ doping strongly influences the concentration of TSL active traps.
(
FeCo/Co–M
)
n
soft magnetic multilayers with uniaxial anisotropy and very high saturation magnetization
Authors: Pan G. , Du H.
Published: April 2003

Nanocrystalline (FeCo/CoNbZr)n and (FeCo/CoZr)n uniaxial-anisotropy soft magnetic multilayers with saturation flux density up to 22.5 kG were prepared by rf sputtering. Magnetic and microstructural properties of these films were studied before and after magnetic annealing. Cross-sectional transmission electron microscopy study showed that the soft magnetic Co–M (M=Zr, Nb, etc.) amorphous layers act as grain refiners as well as soft magnetic grain-boundary materials for the FeCo layers. The resultant multilayer films exhibit excellent uniaxial-anisotropy soft magnetic properties with small coercivity, high anisotropy field Hk, and excellent thermal stability after magnetic field annealing, which are very attractive for applications in thin-film heads for ultrahigh-density magnetic recording. The origin of the soft magnetism was discussed in the light of the random anisotropy model. The improvement of soft magnetic properties of the laminated films is closely associated with the reduction in grain size of the FeCo layers due to lamination. Qualitative agreement between the experimental data and theoretical calculations can be established. Magnetic field annealing was very effective in improving the soft magnetic properties of the films. However, further study is required in order to understand the mechanism. The saturation magnetization of the film follows a simple summation function of the saturation magnetization of the two sublayer materials, with a maximum achievable value of 22.5 kG within the uniaxial-anisotropy regime.
Mixed electromechanical fatigue in lead zirconate titanate
Authors: Lupascu Doru C. , Aulbach Emil , Rödel Jürgen
Published: April 2003

The effects of mixed fatigue by phase-shifted electrical and mechanical loading on the material properties of a commercial lead–zirconate–titanate ceramic are presented. Both the loss in switchable polarization and strain are more pronounced than in purely unipolar electric, but much less than in bipolar electric fatigue. Fatigue-induced offsets are described. Point defect motion and point defect agglomeration are discussed as sources of fatigue in ferroelectrics. Fatigue microcracking occurs at the interface to the load pistons irrespective of Poisson’s ratio of the piston.
Pulsed discharge nozzle cavity ringdown spectroscopy of cold polycyclic aromatic hydrocarbon ions
Authors: Biennier Ludovic , Salama Farid , Allamandola Louis J. , Scherer James J.
Published: April 2003

The gas-phase electronic absorption spectra of the naphthalene (C10H8+) and acenaphthene (C12H10+) cations have been measured in the visible range in a free jet planar expansion in an attempt to collect data in an astrophysically relevant environment. The direct absorption spectra of two out of four bands measured of the gas-phase cold naphthalene cation along with the gas-phase vibronic absorption spectrum of the cold acenaphthene cation are reported for the first time. Direct absorption spectra of their van der Waals complexes with argon are also reported for the first time. The study has been carried out using the ultrasensitive and versatile technique of cavity ringdown spectroscopy (CRDS) coupled to a pulsed discharge slit nozzle (PDN). The new PDN-CRDS set up is described and its characteristics are evaluated. The direct-absorption spectra of the polycyclic aromatic hydrocarbon (PAH) ions are discussed and compared to the gas-phase and solid-phase data available in the literature. The analysis of the results show that cold, free flying PAH ions are generated in the argon discharge primarily through soft Penning ionization. This enables the intrinsic band profiles to be measured, a key requirement for astrophysical applications.
Translational diffusion of polymer chains with excluded volume and hydrodynamic interactions by Brownian dynamics simulation
Authors: Liu Bo , Dünweg Burkhard
Published: April 2003

Within Kirkwood theory, we study the translational diffusion coefficient of a single polymer chain in dilute solution, and focus on the small difference between the short-time Kirkwood value D(K) and the asymptotic long-time value D. We calculate this correction term by highly accurate large-scale Brownian dynamics simulations, and show that it is in perfect agreement with the rigorous variational result D<D(K), and with Fixman’s Green–Kubo formula, which is re-derived. This resolves the puzzle posed by earlier numerical results [Rey et al., Macromolecules 24, 4666 (1991)], which rather seemed to indicate D>D(K); the older data are shown to have insufficient statistical accuracy to resolve this question. We then discuss the Green–Kubo integrand in some detail. This function behaves very differently for pre-averaged versus fluctuating hydrodynamics, as shown for the initial value by analytical considerations corroborated by numerical results. We also present further numerical data on the chain’s statics and dynamics.
Effects of confinement and crowding on the thermodynamics and kinetics of folding of a minimalist β-barrel protein
Authors: Friedel Miriam , Sheeler Daniel J. , Shea Joan-Emma
Published: April 2003

We report the results of a molecular dynamics (MD) study of the effects of confinement and molecular crowding on the thermodynamics and kinetics of folding of a 46 residue off-lattice minimalist β-barrel protein. Crowding was mimicked by restricting the protein to a sphere with a soft well repulsive potential. MD simulations were performed on the protein in an unconfined environment, as well as confined to spheres of two different radii, 5.88σ and 11.76σ. Here, σ is the bond length between two adjacent residues and the radius of gyration of the protein in its native state is 2.87σ. We find that for the larger sphere (11.76σ), the folding and collapse temperatures are virtually unchanged from their bulk values, but the average folding time is decreased by 35%. By contrast, the smaller sphere has a much more significant thermodynamic effect (the folding temperature is raised by 28%) but the average folding time is only decreased by 58%. Confinement is also seen to restrict the conformational space accessible to the protein in its denatured state. In addition, confinement appears to change the folding mechanism for this protein, as long-lived intermediates present in the bulk are both modified and have shorter lifetimes when the protein is confined.
Freezing in one-dimensional liquids
Authors: Koga Kenichiro
Published: April 2003

Freezing of liquids in one dimension is studied by a lattice model that is an extension of the model solvent of the hydrophobic attraction. The model in one dimension, which is exactly solvable, exhibits a continuous phase change between a high-temperature disordered “liquid” state and a low-temperature ordered “solid” state but also does exhibit a first-order freezing transition at some finite temperature with either one of the two model parameters taken to be infinite. In this theoretical framework the sharpness of the freezing in one dimension is expressed by a simple function of the microscopic model parameters and thus is related with other macroscopic properties of the substance. These results may account for continuity and discontinuity of the liquid and solid reported for different one-dimensional substances.
Thermophysical properties of liquid refractory metals: Comparison between hard sphere model calculation and electrostatic levitation measurements
Authors: Ishikawa Takehiko , Paradis Paul-François , Itami Toshio , Yoda Shinichi
Published: April 2003

Thermophysical properties of molten refractory metals (titanium, zirconium, hafnium, and niobium) have been measured using a containerless method. Using an in-house developed electrostatic levitator, the density, the heat capacity, the entropy, the surface tension, and the viscosity of liquid phases have been measured over a wide temperature range. The measured data showed good agreement with theoretical calculations based on the hard sphere model.
Aqueous Na+Cl− pair association from liquidlike to steamlike densities along near-critical isotherms
Authors: Chialvo A. A. , Simonson J. M.
Published: April 2003

An extensive molecular-based study of ion-pair formation in near-critical dilute aqueous NaCl solutions is performed along three near- (super- and sub-) critical isotherms and from liquidlike to steamlike densities. The study encompasses the determination of the ion-pair association constant via potential of mean force calculations. The main goal is to find answers to some relevant questions regarding the thermodynamic and corresponding microscopic behavior of the ion-pair formation at steamlike densities, where experimental data are extremely difficult to obtain accurately. A direct comparison is made between simulation, theoretical developments, and experiment to aid the interpretation of experimental data and their macroscopic modeling.
Interaction of a spherical particle with linear chains. II. Chains end-grafted at the particle surface
Authors: Klos J. , Pakula T.
Published: April 2003

Linear flexible polymers end-grafted onto the spherical surface (“hairy sphere”) are simulated using the cooperative motion algorithm. The simulations are performed in two extremes of the surrounding matrix: (1) The hairy sphere immersed in a pure solvent of single beads and (2) the hairy sphere in a melt of linear chains. The static properties of the grafted chains are analyzed by calculating the polymer concentration profiles, the distributions of polymer centers-of-mass and of the free chain ends for various values of the sphere radius, various chain lengths, and variable coverage of the sphere surface by the grafting-chain ends (surface coverage). Ordering phenomena of the polymers and their intramolecular structure are taken into account by considering the orientation correlation parameter, the mean-squared radius of gyration and the mean-squared end-to-end distance as functions of the position of the polymer centers-of-mass with respect to the sphere surface. In case (2) some properties of the melt chains have also been analyzed. The simulations indicate that (1) the concentration profiles of the end-grafted chains under good solvent conditions are noticeably different from those in a melt, (2) in both cases, they are strongly affected by the values of the surface coverage, (3) as the sphere radius increases, the monomer concentration profile changes from concave to convex, (4) both the free ends and the centers of mass of the grafted chains reveal a tendency to concentrate at some distance from the surface, i.e., the profiles have noticeable maxima, (5) for small surface coverage the chain centers of mass penetrate into the sphere, especially when anchored to the smaller sphere in the melt, (6) depending on the position of the center-of-mass of the end-grafted chains, both tangential and radial ordering of the polymers relative to the sphere exists, (7) the end-to-end vectors and radii of gyration show a paraboliclike shape with minima at a finite distance from the surface.
Waiting time distribution and nonexponential relaxation in single molecule spectroscopic studies: Realization of entropic bottleneck in a simple model
Authors: Chakrabarti Dwaipayan , Bagchi Biman
Published: April 2003

We study a dynamical disorder model for environmental modulation of rate processes where a need of dynamical cooperativity presents an entropy barrier, rather than an energy barrier. The rate depends on a control variable, Q, that describes the collective instantaneous state of the environment and is itself a random walker in finite discrete space with continuous time. We obtain the waiting time distribution for the relaxation by simulating the model. The time dependence of the average survival probability is derived there from and also by a numerical solution through the Liouville-master equation approach to the theoretical problem. We present an analytical treatment of the first passage time problem that is posed by a limiting case of our model. As the rate of the environmental fluctuation, τenv−1, slows down, the decay of the average survival probability is found to be more and more nonexponential in short times, but to change to exponential at longer times. The average survival time, τ, exhibits a fractional power law dependence on κ(=τenvk0), where time is scaled in terms of k0−1, k0 being the intrinsic rate coefficient for the relaxation. The mean first passage time in the limiting case of the model exhibits an exponential dependence on the total number of the environmental subsystems N and a non-Arrhenious temperature dependence over the temperature range studied. We note the likely relevance of a part of this result to single molecule spectroscopic studies that reveal a tail in the waiting time distribution at long times.
GaN evaporation and enhanced diffusion of Ar during high-temperature ion implantation
Authors: Usov I. , Parikh N. , Kudriavtsev Y. , Asomoza R. , Reitmeier Z. , Davis R.
Published: April 2003

GaN films were implanted with 150 keV Ar+ at temperatures up to 1100 °C to a dose of 3×1015 cm−2. Concentration profiles of Ar were measured by secondary ion mass spectroscopy and depth distributions of ion-induced damage were estimated from Rutherford backscattering/channeling spectra. No redistribution of Ar atoms was detected up to 700 °C. At 1000 °C a deep penetrating diffusion tail and a shift of the Ar peak to the surface were observed. At temperatures higher than 800 °C shift of the damage peak to the surface was also observed. We attributed the shift of the Ar peak and the damage peaks to evaporation of thin layer of GaN during high-temperature implantation and estimated its temperature dependence.
Accessing the optical limiting properties of metallo-dielectric photonic band gap structures
Authors: Larciprete M. C. , Sibilia C. , Paoloni S. , Bertolotti M. , Sarto F. , Scalora M.
Published: April 2003

The optical limiting properties of a one-dimensional, transparent metallodielectric photonic band gap structure are studied. Due to light confinement in the structure that enhances the nonlinear response of the layers, a nonlinear transmission dependence on the incident light intensity is found. Experimental results are reported for a four period sample where the single period consists of ZnO and Ag layers 109 and 17 nm thick, respectively. The structure was designed to exhibit a transmission resonance at 532 nm. Under the action of a Q-switched frequency doubled Nd:yttrium–aluminum–garnet laser, a decrease in transmission of approximately 50% is obtained for a maximum incident light intensity of 2 GW/cm2. These results are explained in terms of a dynamic change of the absorption coefficient due to the enhancement of the two-photon absorption process. These results suggest that the structure is suitable for optical limiting applications in the visible range.
Formation of quasi-single-domain 3C-SiC on nominally on-axis Si(001) substrate using organosilane buffer layer
Authors: Nakazawa Hideki , Suemitsu Maki
Published: April 2003

Quasi-single-domain 3C-SiC films have been successfully grown on nominally on-axis Si(001) substrate. The starting surface is either of 2×1 quasi-single-domain or of 2×1+1×2 double-domain. The point here is to use dc-resistive heating of the substrate and to form a low-temperature (650 °C) interfacial buffer layer using monomethylsilane (H3 C-SiH3). The dc resistive heating serves to form a single-domain Si(001)-2×1 or 1×2 starting surface or to develop a single-domain 3C-SiC(001)-2×3 or 3×2 surface on a 2×1+1×2 double-domain Si(001) substrate. When a single-domain Si(001) starting surface is utilized, it is not the dc polarity during growth but the surface reconstruction of the starting surface that determines the dominant domain in the 3C-SiC film. The thickness of the single-domain 3C-SiC film is as thin as ∼45–200 nm, which is about three orders of magnitude smaller than that required in a previous study (>5 μm).
Effects of spacer thickness on optical properties of stacked Ge/Si quantum dots grown by chemical vapor deposition
Authors: Chang Wen-Hao , Chen Wen-Yen , Chou An-Tai , Hsu Tzu-Min , Chen Pan-Shiu , Pei Zingway , Lai Li-Shyue
Published: April 2003

Photoluminescence investigations on stacked Ge/Si dots with different spacer thicknesses are presented. According to the emission energy shift in the Ge dots, we found that a thinner spacer layer will lead to remarkable Ge–Si intermixing during the stacking of the Ge/Si dots. Such material intermixing not only shallows the dot potential depth, but also softens the sharpness of the dot/spacer interface. In addition, the temperature of photoluminescence quenching also varies with the spacer thickness. Finally, we point out some important factors that are relevant to the room-temperature luminescence efficiency of stacked Ge/Si quantum dots.
Density profiles of a hard disk mixture inside a small circular cavity: Effect of the conservation of the total angular momentum
Authors: Román F. L. , González A. , White J. A. , Velasco S.
Published: April 2003

We investigate the finite-size effect due to the conservation of the total angular momentum L in a hard-disk mixture confined to a hard circular cavity. The study is made by means of molecular dynamics simulations in the microcanonical ensemble. Given the geometry of the cavity and the nature of interparticle interactions, L is conserved if the cavity hard wall is smooth, that is, if one considers particle–wall specular collisions. Conversely, L fluctuates about its mean value for a rough hard wall for which the (energy-conserving) collisions are not specular. The size effect due to the conservation of L becomes apparent in several situations where the two types of wall lead to density profiles that are significantly different. The rough-wall results are related to the smooth-wall ones by means of a series expansion in terms of the fluctuations in the total angular momentum.
Quantum chemical investigation of biexcitons in conjugated polymers
Authors: Pasquinelli Melissa A. , Yaron David
Published: April 2003

The formation of biexcitons in conjugated polymers is examined within the Pariser–Parr–Pople (PPP) model of polyacetylene. The calculations are done using a scattering formalism that provides a size-consistent description of excited states containing both single and double electron–hole pair excitations. The excited-state absorption spectrum from the 1 1Bu exciton state was calculated and examined for signatures of biexciton formation. Calculations were performed on polyenes with up to 9 unit cells, and on long chains using periodic boundary conditions. While polyenes with 7 to 31 unit cells exhibit states with some of the features expected for biexcitons, such states are not seen in the limit of long chains. This suggests that, within the two-band model considered here, exciton–exciton interactions are not of sufficient strength to lead to biexciton formation in the limit of long chains, and that their presence in shorter chains is due to confinement effects. In the long-chain limit, transitions are found to states consisting of overlapping electron–hole pairs, but these states have energies above that required to make two free excitons. These states may result from scattering resonances between excitons.
Thermal oxidation of silicon nanocrystals in O2 and NO ambient
Authors: Scheer K. C. , Rao R. A. , Muralidhar R. , Bagchi S. , Conner J. , Lozano L. , Perez C. , Sadd M. , White B. E.
Published: April 2003

We have studied the oxidation of Si nanocrystals as a function of oxidizing ambient, temperature, time, and initial nanocrystal size using x-ray photoelectron spectroscopy, transmission electron microscopy, and energy-filtered transmission electron microscopy. Thicker oxide shells are obtained by oxidation in O2 ambient compared with NO ambient. Oxidation in O2 is observed to be self-limiting at temperatures below the viscoelastic temperature of SiO2 because of compressive stress normal to the SiO2/Si interface, which retards the surface oxidation rate. Oxidation in NO also results in self-limiting oxidation due to the incorporation of N at the Si/SiOx interface. This N-rich interfacial layer acts as an effective barrier against oxidant diffusion and also blocks the reaction sites on the Si surface. Therefore, NO oxidation is successful in slowing further oxidation of Si cores, even in a severe oxidizing ambient such as O2 at 1050 °C.
On the nanoscale measurement of friction using atomic-force microscope cantilever torsional resonances
Authors: Reinstädtler M. , Rabe U. , Scherer V. , Hartmann U. , Goldade A. , Bhushan B. , Arnold W.
Published: April 2003

We studied friction and stick-slip phenomena on bare and lubricated silicon samples by measuring the torsional contact resonances of atomic force microscope cantilevers. A piezoelectric transducer placed below the sample generates in-plane sample surface vibrations which excite torsional vibrations of the cantilever. The resonance frequencies of the vibrating beam depend on the tip-sample forces. At low lateral surface amplitudes the cantilever behaves like a linear oscillator with viscous damping. Above a critical surface amplitude, typically 0.2 nm, the amplitude maximum of the resonance curves does not increase any more and the shape of the resonance curves changes, indicating the onset of sliding friction. The critical amplitude increases with increasing static cantilever load. For a bare silicon sample it is higher than for the lubricated silicon. Microslip known from macroscopic contacts causes energy dissipation in the atomic force microscope tip-contact before sliding friction sets in.
Scanning transmission electron microscopy study of Au/Zn/Au/Cr/Au and Au/Ti/Pt/Au/Cr/Au contacts to p-type InGaAs/InP
Authors: Huang J. S. , Vartuli C. B.
Published: April 2003

We studied the interfacial reaction of Au/Zn/Au/Cr/Au and Au/Ti/Pt/Au/Cr/Au contacts to p-InGaAs/p-InP using scanning transmission electron microscopy. We found that the alloying morphology was distinctly different in the two contact systems. For Au/Zn/Au/Cr/Au, significant interdiffusion between the metal and InGaAs contact layer occurred. Two types of compound were formed: one was rich in Au and the other was rich in Ga and As. Another interesting observation was that a significant amount of As has outdiffused into the Cr layer after alloying. For the Au/Ti/Pt/Au/Cr/Au, only interfacial layers were involved in the reaction. Compounds of Au-Ga-In, Ti-As, and Au-Ga were formed, and the Cr layer remained intact. The mechanisms of compound formation are discussed.
Ohmic loss in frequency-selective surfaces
Authors: Raynolds James E. , Munk Ben A. , Pryor Jonathan B. , Marhefka Ronald J.
Published: April 2003

The present study was undertaken in order to quantify absorption effects due to ohmic loss in frequency-selective surfaces (FSS) at infrared frequencies. The structures considered in this work act as electromagnetic filters, and as such, are of interest for use as thermophotovoltaic spectral control devices. For this application, absorption is of primary concern since it leads to reduced filter efficiency. This work focuses on the behavior of single-layer, free-standing FSS arrays comprised of circular apertures (holes) and circular loop apertures (rings). Numerical calculations of the transmission, reflection, and absorption characteristics of various arrays were carried out for wavelengths between 1 and 15 μm using a commercial finite-element software package. Absorption effects were included using measured optical properties as input parameters to a surface impedance boundary condition. Analytical techniques were then employed to determine the absorption behavior in the static limit. An interesting result of this analysis is the fact that absorption at 15 μm is predicted by the finite-element calculations to increase versus aperture radius (for fixed aperture separation), while in the static limit absorption was found (through the use of analytical techniques) to decrease. The results of the finite-element calculations were confirmed using another numerical method based on the periodic moment method. We are thus led to the conclusion that the qualitative relationship for absorption versus aperture radius must reverse for some frequency regime yet to be investigated.
Temperature dependence of phonon sidebands in line-narrowed fluorescence spectra of chromophores in glasses
Authors: Verdal Nina , Kelley Anne Myers
Published: April 2003

Site-selective line-narrowed fluorescence spectra have been measured from 1.5 to 60–90 K for the S1→S0 electronic transitions of phenanthrene and of perylene doped into four amorphous or nearly amorphous matrices: polyethylene, polystyrene, poly(methyl methacrylate), and ethanol. The band shapes (zero-phonon line plus phonon wing) are examined for the best-isolated strong vibronic band in the emission spectrum of each chromophore and compared with simulations using two different models that assume harmonic phonons linearly coupled to the electronic transition. One model extracts the spectral density of coupled phonons from the lowest-temperature experimental data, while the other employs an empirically determined set of harmonic phonon frequencies and coupling strengths. Good fits between simulation and experiment are obtained over the full temperature range for all chromophore–matrix combinations examined, which vary considerably in electron–phonon coupling strength. The assumption of linearly coupled harmonic phonons therefore appears to be reasonably good for these systems. Some broadening of the zero-phonon lines with increasing temperature in polystyrene and in ethanol suggests a contribution from quadratically coupled phonons as well in these matrices.
The measurement of the solid–liquid surface free energy of xenon
Authors: Stalder I. , Bilgram J. H.
Published: April 2003

The solid–liquid surface free energy γSL is an important parameter in the study of thermodynamics of the solid–liquid interface. It is a crucial parameter in the description of dendritic solidification. Xenon is a model substance for thermodynamic studies and it is a transparent model substance for metals. Xenon is used to study structure formation far from equilibrium and to model dendritic solidification of metals. γSL of xenon has been determined from equilibrium shapes of grain boundary grooves. The solid–liquid interface has been stabilized by an imposed temperature gradient. Experimental groove shapes have been fitted to a model function. Grains with various crystallographic orientations and interfaces stabilized with different gradients have been studied. γSL is found to be 9.95±0.68 mJ/m2.
Carrier-density-wave transport property depth profilometry using spectroscopic photothermal radiometry of silicon wafers II: Experimental and computational aspects
Authors: Shaughnessy Derrick , Mandelis Andreas
Published: April 2003

The experimental verification of a previously presented theoretical model for the photothermal radiometric (PTR) signal from an Si wafer excited by a laser of arbitrary wavelength is presented. A multiparameter fitting algorithm is developed and is used to fit experimental frequency scans to the theoretical model. The recombination lifetime and surface recombination velocity values extracted from the fits are consistent for all of the experiments performed. The diffusion coefficients for the more strongly absorbed excitation wavelengths are greater than those measured when using deeper penetrating excitation wavelengths. This discrepancy is discussed in terms of the dependence of the PTR signal on injected carrier densities and the nonlinearity of the PTR signal with temperature. The sensitivity of the PTR signal to a localized defect is shown to increase with the proximity of the defect to the centroid of the injected carrier density. The method amounts to carrier-density-wave depth profilometry of the relevant electronic transport parameters.
Soft x-ray excited optical luminescence from poly(N-vinylcarbazole)
Authors: Naftel S. J. , Kim P.-S. G. , Sham T. K. , Sammynaiken R. , Yates B. W. , Hu Y.-F.
Published: April 2003

X-ray excited optical luminescence (XEOL) using tunable soft x rays from a synchrotron light source, together with x-ray absorption fine structure spectroscopy has been used to study the electronic structure and optical properties of thin films of poly(N-vinylcarbazole). It is found that carbon core level excitation enhances the formation of excimers emitting at 380 and 410 nm. A third excimer at 310 nm is also noted. In addition, excitations across the C K edge and the N K edge show noticeably different optical response. These results are interpreted in terms of the site specificity of the XEOL technique.
Ballistic and quasiballistic tunnel transit time oscillators for the terahertz range: Linear admittance
Authors: Gribnikov Z. S. , Vagidov N. Z. , Mitin V. V. , Haddad G. I.
Published: April 2003

We have considered interactions between ballistic (or quasiballistic) electrons accelerated by a dc electric field in an undoped transit space (T space) and a small ultrahigh frequency ac electric field and have calculated the linear admittance of the T space. Electrons in the T space have a conventional, nonparabolic dispersion relation. After consideration of the simplest specific case when the current is limited by the space charge of the emitted electrons, we turned to an actual case when the current is limited by a heterostructural tunnel barrier (B barrier) separating the heavily doped cathode contact and the T space. We assumed that the B barrier is much thinner than the T space and both dc and ac voltages drop mainly across the T space. The emission tunnel current through the B barrier is determined by the electric field E(0) in the T space at the boundary B barrier/T space. The more substantial is, the tunnel current limitation the higher the electric field E(0) becomes. We have shown that for a space-charge limited current the change from parabolic dispersion to the nonparabolic branch induces narrowing and closing of the frequency windows of transit-time negative conductance starting with the lowest-frequency windows. These narrowing and closing frequency windows become effective only for very high voltages U across the T space: U≫mVS2/2e, where m is the effective mass for the parabolic branch and VS is the saturated velocity for the nonparabolic branch. For moderate voltages U, the effects of nonparabolicity are not very substantial. The tunnel current limitation decreases the space-charge effects in the T space and diminishes the role of the detailed electron dispersion relation. As a result, restoration of the frequency windows of transit-time negative conductance and an increase in the value of this negative conductance occur. The implementation of the considered tunnel injection transit time oscillator diode promises to lead to efficient and powerful sources of terahertz range radiation.
Carrier-density-wave transport property depth profilometry using spectroscopic photothermal radiometry of silicon wafers I: Theoretical aspects
Authors: Shaughnessy Derrick , Mandelis Andreas
Published: April 2003

A theoretical model for the photothermal radiometric (PTR) signal from an indirect band-gap semiconductor excited by a laser of arbitrary wavelength is presented. The model has been used to investigate the spectral dependence of the sensitivity of the PTR signal to variations in the electronic transport parameters of the sample. Simulations show slight variations of the sensitivity to carrier lifetime and carrier diffusivity with excitation wavelength due to changes in the strength of the thermal contribution to the signal that are a result of changes in the difference between the photon energy and the band gap. The sensitivity of the PTR signal to changes in the front surface recombination velocity is shown to have a strong dependence on the excitation wavelength with the sensitivity decreasing as the absorption depth of the excitation source increases, allowing spectroscopic carrier-density-wave depth profilometric measurements.
Effect of growth rate and gallium source on GaAsN
Authors: Kurtz Sarah , Geisz J. F. , Keyes B. M. , Metzger W. K. , Friedman D. J. , Olson J. M. , Ptak A. J. , King R. R. , Karam N. H.
Published: April 2003

GaAs 1−x N x with x=0.2% is grown by metal–organic chemical vapor deposition with growth rates between 2 and 7 μm/h and with two gallium sources. The GaAsN grown with trimethylgallium at high growth rates shows increased carbon contamination (>1017 cm−3), low photoluminescent lifetimes (∼0.2 ns), and high background acceptor concentrations (>1017 cm−3). The GaAsN is improved if it is grown with a lower growth rate or if triethylgallium is used, resulting in lower carbon contamination (∼1016 cm−3), longer photoluminescent lifetimes (2–9 ns), and slightly lower background acceptor concentrations (<1017 cm−3). The lifetime decreases with carbon concentration, implying that the low lifetimes in this sample set may be caused by nonradiative recombination at a center containing both N and C.
Unambiguous x-ray phase retrieval from Fraunhofer diffraction data
Authors: Siu Karen K-W , Nikulin Andrei Y. , Wells Peter , Harvey Erol , Bigault Thierry , Freund Andreas K. , Ishikawa Tetsuya
Published: April 2003

An unambiguous inverse solution from Fraunhofer diffraction data has been achieved for an amorphous sample of low-molecular weight. The complex scattering amplitude has been reconstructed with submicron spatial resolution using the phase retrieval x-ray diffractometry technique. The technique relies on a logarithmic dispersion relation to determine the x-ray wave phase from the scattered intensity profile. Successful experimental localization of the zeros of the complex scattering amplitude was achieved by utilizing two data sets taken at different incident x-ray energies, permitting a unique solution.
Oriented ferromagnetic Fe-Pt alloy nanoparticles produced in Al2O3 by ion-beam synthesis
Authors: White C. W. , Withrow S. P. , Sorge K. D. , Meldrum A. , Budai J. D. , Thompson J. R. , Boatner L. A.
Published: April 2003

Oriented Fe1−xPtx nanoparticles have been formed in single-crystal Al2O3 host matrices by the sequential implantation of Fe and Pt ions followed by thermal annealing. For x in the range of ∼35–55 at. % Pt, these nanoparticles are in the chemically ordered tetragonal L10 structure of FePt and appear to be fully ordered. The nanoparticles are ferromagnetic, and the magnetic coercivity is a strong function of the alloy composition, reaching values in excess of 20 kOe for x∼45%. The crystallographic orientation and morphology of the nanoparticles are strongly dependent on the implantation conditions. Under certain implantation conditions, a buried amorphous layer can be formed in the Al2O3 matrix which crystallizes during annealing giving rise to the formation of an interconnected network of large FePt particles with a single orientation. Oriented nanoparticles of Fe3Pt and FePt3 were also synthesized. The Fe3Pt and FePt3 particles have the ordered, cubic L12 structure with an order parameter of 0.5–0.8; and these particles also exhibit ferromagnetic behavior. The magnetic and structural properties of the Fe1−xPtx nanoparticles are compared to and contrasted with those of nanoparticles of Fe and Pt in Al2O3 that were also formed by ion-beam synthesis. Evidence for Al incorporation into the nanoparticles is also presented.
Alignment of liquid crystals with periodic submicron structures ablated in polymeric and indium tin oxide surfaces
Authors: Behdani M. , Keshmiri S. H. , Soria S. , Bader M. A. , Ihlemann J. , Marowsky G. , Rasing Th.
Published: April 2003

We show that a periodic nanostructure patterned into a polymeric or indium tin oxide (ITO) surface is capable of aligning liquid crystal (LC) molecules. Gratings of different depths were created on thin polymeric or ITO surfaces with submicron and micron periods by superposition of ultraviolet plane waves. The depth of the gratings was varied by changing the fluence of the laser. This method allows to pattern orientations over small areas and does not suffer from the disadvantages of rubbing based alignment methods. LC alignment was tested by forming twisted nematic cells. Anchoring energies were calculated from measurements of the twist angles.
One- and two-photon photocurrents from tunable organic microcavity photodiodes
Authors: Koeppe R. , Müller J. G. , Lupton J. M. , Feldmann J. , Scherf U. , Lemmer U.
Published: April 2003

We have constructed multilayer thin-film organic microcavity photodiodes with the photoactive layer comprised of a spin-coated conjugated polymer and an evaporated C60 layer. The electrodes are designed as semitransparent mirrors which form a resonant cavity structure. The photocurrent spectra show distinct maxima at the optical resonances of the cavities, which are located up to 200 nm below the fundamental optical transition of the polymer. The design allows a simple tuning of the spectral response by varying the layer thickness. Microcavity photodiodes are also shown to be highly sensitive two-photon detectors, which exhibit a factor 500 improvement in the two-photon response compared to devices without photonic confinement.
Evolution of the band structure of β-In2S3−3xO3x buffer layer with its oxygen content
Authors: Barreau N. , Marsillac S. , Bernède J. C. , Assmann L.
Published: April 2003

The evolution of the band structure of β-In2S3−3xO3x (BISO) thin films grown by physical vapor deposition, with composition x, is investigated using x-ray photoelectron spectroscopy. It is shown that the energy difference between the valence-band level and the Fermi level remains nearly constant as the optical band gap of the films increases. As a consequence, the difference between the conduction band level and the Fermi level increases as much as the optical band gap of the films. The calculation of the electronic affinity χ of the BISO thin films shows that it decreases linearly from 4.65 to 3.85 eV when x varies from 0 to 0.14. This will facilitate fabrication of efficient Cu(InGa)Se2-based solar cells having different absorber layer band gap.