bluuebunny/crossref_metadata_2025 huggingface dataset:
Vacuum ultraviolet photodissociation and surface morphology change of water ice films dosed with hydrogen chloride
Authors: Yabushita Akihiro , Kanda Daichi , Kawanaka Noboru , Kawasaki Masahiro
Published: October 2007

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100–145K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100–140K, while the microstructure of the ice surface stayed of polycrystalline at 145K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157nm photodissociation of neutral HCl adsorbed on water ice films at 100–140K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100–140K.
Magneto-optic properties of electron cyclotron resonance ion beam sputtered and magnetron sputtered Co/Pt multilayers
Authors: Fiedler Steffen , Stillrich Holger , Oepen Hans Peter
Published: October 2007

An investigation of the magneto-optic properties of single Co films sandwiched by Pt films is presented. The films are grown via electron cyclotron resonance ion beam sputtering and magnetron sputtering. Kerr rotation and ellipticity are measured for the multilayers, varying Pt cap, Co, as well as Pt buffer layer thickness. The set of data allows the separation of optic and magneto-optic contributions to the Kerr signals. The magneto-optic constants (Voigt constants) are determined and compared with literature values. The striking result is the strong difference of the magneto-optic constants for ultrathin and thick Co films. The analysis of the data reveals that for ultrathin films, the Kerr signals have contributions that come presumably from the interfaces, indicative of polarized Pt.
Diffusion of nanoparticles in dense fluids
Authors: Ould-Kaddour F. , Levesque D.
Published: October 2007

The diffusion process of a single spherical nanoparticle immersed in a fluid solvent is studied by molecular dynamics simulations. When the nanoparticle mass stays constant, it is shown that, at short times, the decay of the nanoparticle velocity autocorrelation function is strongly modified when the particle diameter increases. It is also shown that, at large times, the characteristic algebraic decay induced by the hydrodynamic correlations between the solvated particle and the solvent presents a scaling behavior depending on the particle diameter.
Cleaning properties of atomic oxygen excited to metastable state
2
s
2
2
p
4
(
S
1
)
Authors: Shun’ko E. V. , Belkin V. S.
Published: October 2007

A possibility to treat surfaces of various materials by bombardment with neutral atomic oxygen excited to the metastable state 2s22p4(S10), (4.1891 V from the ground) is described. The oxygen dissociation and excitation was developed in plasmas generated in reactors of capacitively coupled dielectric barrier discharge configurations comprised of a quartz or ceramic tube passing throughout two annular electrodes and containing a gas mixture of ∼98% Ar with ∼2% O2 ionized at the atmospheric pressure by the radio frequency discharge at 13.56 MHz and flowing at about 6 m/s. As shown in the experiments, the transition 2s22p4(S10)−2s22p4(D12) with a high degree of probability is responsible for the yellow color of afterglow products, and the lifetime of the metastable state can be as long as 5.4 ms in certain cases, allowing one to separate the excited atomic medium from plasma that it produced. It has been shown that the cleaning process included significant Van der Waals bonds liberation to the depth of several hundred angstroms of surfaces treated, which drastically reduced the percentage of carbon-containing contaminants and changed, to some degree, the chemical structure of surfaces containing chemically bonded oxygen.
Nonlinear excitations in electron-positron-ion plasmas in accretion disks of active galactic nuclei
Authors: Moslem W. M. , Kourakis I. , Shukla P. K. , Schlickeiser R.
Published: October 2007

The propagation of acoustic nonlinear excitations in an electron-positron-ion (e-p-i) plasma composed of warm electrons and positrons, as well as hot ions, has been investigated by adopting a two-dimensional cylindrical geometry. The electrons and positrons are modeled by hydrodynamic fluid equations, while the ions are assumed to follow a temperature-parametrized Boltzmann distribution (the fixed ion model is recovered in the appropriate limit). This situation applies in the accretion disk near a black hole in active galactic nuclei, where the ion temperature may be as high as 3 to 300 times that of the electrons. Using a reductive perturbation technique, a cylindrical Kadomtsev-Petviashvili equation is derived and its exact soliton solutions are presented. Furthermore, real situations in which the strength of the nonlinearity may be weak are considered, so that higher-order nonlinearity plays an important role. Accordingly, an extended cylindrical Kadomtsev-Petviashvili equation is derived, which admits both soliton and double-layer solutions. The characteristics of the nonlinear excitations obtained are investigated in detail.
Visualizing two qubits
Authors: Avron J. E. , Bisker G. , Kenneth O.
Published: October 2007

The notions of entanglement witnesses and separable and entangled states for a two qubit system can be visualized in three dimensions using the SLOCC equivalence classes. This visualization preserves the duality relations between the various sets and allows us to give “proof by inspection” of a nonelementary result of Horodecki et al. [Phys. Lett. A 223, 1–8 (1996)] that for two qubits, Peres PPT (positive partial transpose) test for separability is iff. We then show that the CHSH Bell inequalities can be visualized as circles and cylinders in the same diagram. This allows us to give a geometric proof of yet another result of Horodecki et al. [Phys. Lett. A 200, 340–344 (1995)], which optimizes the violation of the CHSH Bell inequality. Finally, we give numerical evidence that, remarkably, allowing Alice and Bob to use three rather than two measurements each does not help them to distinguish any new entangled SLOCC equivalence class beyond the CHSH class.
Confinement, entropy, and single-particle dynamics of equilibrium hard-sphere mixtures
Authors: Mittal Jeetain , Shen Vincent K. , Errington Jeffrey R. , Truskett Thomas M.
Published: October 2007

We use discontinuous molecular dynamics and grand-canonical transition-matrix Monte Carlo simulations to explore how confinement between parallel hard walls modifies the relationships between packing fraction, self-diffusivity, partial molar excess entropy, and total excess entropy for binary hard-sphere mixtures. To accomplish this, we introduce an efficient algorithm to calculate partial molar excess entropies from the transition-matrix Monte Carlo simulation data. We find that the species-dependent self-diffusivities of confined fluids are very similar to those of the bulk mixture if compared at the same, appropriately defined, packing fraction up to intermediate values, but then deviate negatively from the bulk behavior at higher packing fractions. On the other hand, the relationships between self-diffusivity and partial molar excess entropy (or total excess entropy) observed in the bulk fluid are preserved under confinement even at relatively high packing fractions and for different mixture compositions. This suggests that the excess entropy, calculable from classical density functional theories of inhomogeneous fluids, can be used to predict some of the nontrivial dynamical behaviors of fluid mixtures in confined environments.
Exchange-hole dipole moment and the dispersion interaction revisited
Authors: Becke Axel D. , Johnson Erin R.
Published: October 2007

We have recently introduced a model of the dispersion interaction based on the position-dependent dipole moment of the exchange hole [J. Chem. Phys. 122, 154104 (2005)]. The original derivation, involving simple dipole-induced-dipole electrostatics, was somewhat heuristic, however, and lacking in rigor. Here we present a much more satisfying derivation founded on second-order perturbation theory in the closure approximation and a semiclassical evaluation of the relevant interaction integrals. Expressions for C6, C8, and C10 dispersion coefficients are obtained in a remarkably straightforward manner. Their values agree very well with ab initio reference data on dispersion coefficients between the atoms H, He, Ne, Ar, Kr, and Xe. We also highlight the importance of the exchange-hole contribution to the dispersion coefficients, especially to C6.
Integral equation theory study on the phase separation in star polymer nanocomposite melts
Authors: Zhao Lei , Li Yi-Gui , Zhong Chongli
Published: October 2007

The polymer reference interaction site model theory is used to investigate phase separation in star polymer nanocomposite melts. Two kinds of spinodal curves were obtained: classic fluid phase boundary for relatively low nanoparticle-monomer attraction strength and network phase boundary for relatively high nanoparticle-monomer attraction strength. The network phase boundaries are much more sensitive with nanoparticle-monomer attraction strength than the fluid phase boundaries. The interference among the arm number, arm length, and nanoparticle-monomer attraction strength was systematically investigated. When the arm lengths are short, the network phase boundary shows a marked shift toward less miscibility with increasing arm number. When the arm lengths are long enough, the network phase boundaries show opposite trends. There exists a crossover arm number value for star polymer nanocomposite melts, below which the network phase separation is consistent with that of chain polymer nanocomposite melts. However, the network phase separation shows qualitatively different behaviors when the arm number is larger than this value.
Slow electron velocity-map imaging spectroscopy of the C4H− and C4D− anions
Authors: Zhou Jia , Garand Etienne , Neumark Daniel M.
Published: October 2007

High resolution photodetachment spectra of C4H− and C4D− obtained via slow electron velocity-map imaging (SEVI) are presented. The spectra reveal closely spaced transitions to the neutral Σ+2 and Π2 states which can be distinguished based on the corresponding photoelectron angular distributions. The C4H ground state is confirmed as the X̃Σ+2 state, with the excited ÃΠ2 state lying only 213cm−1 higher (201cm−1 for C4D). The electron affinities (EAs) are slightly revised to EA (C4H)=28497±8cm−1 and EA (C4D)=28478±10cm−1. Progressions in low frequency bending vibrations are observed in both states, yielding experimental frequencies of ν7=179(169)cm−1 and ν6=408(392)cm−1 for the X̃Σ+2 state of C4H (C4D), and ν7=220(215)cm−1 and ν6=446(437)cm−1 for the ÃΠ2 state.
Proteins in a shear flow
Authors: Szymczak P. , Cieplak Marek
Published: October 2007

The conformational dynamics of a single protein molecule in a shear flow is investigated using Brownian dynamics simulations. A structure-based coarse grained model of a protein is used. We consider two proteins, ubiquitin and integrin, and find that at moderate shear rates they unfold through a sequence of metastable states—a pattern which is distinct from a smooth unraveling found in homopolymers. Full unfolding occurs only at very large shear rates. Furthermore, the hydrodynamic interactions between the amino acids are shown to hinder the shear flow unfolding. The characteristics of the unfolding process depend on whether a protein is anchored or not, and if it is, on the choice of an anchoring point.
Photoluminescence and electromodulation spectroscopy characterization of a Zn0.81Be0.04Mg0.15Se mixed crystal
Authors: Huang P. J. , Hsu H. P. , Huang Y. S. , Firszt F. , Łęgowski S. , Męczyńska H. , Marasek A. , Tiong K. K.
Published: October 2007

We present an optical characterization of a Bridgman-grown zinc-blende-type Zn0.81Be0.04Mg0.15Se mixed crystal in the near-band-edge interband transitions using temperature-dependent photoluminescence (PL), contactless electroreflectance (CER), and photoreflectance (PR) in the temperature range of 15–400K. Typical PL spectrum at low temperature consists of an exciton line, an edge emission due to recombination of shallow donor-acceptor pairs, and a broad band related to recombination through deep level defects. Interband transitions, originating from the band edge and spin-orbital splitting critical points of the sample, have been observed in the CER/PR spectra. The peak positions of the excitonic emission lines in the PL spectra correspond quite well to the energies of the fundamental transitions determined from electromodulation data. The parameters that describe the temperature dependence of the fundamental and spin split-off band gaps, and the broadening function of the band edge exciton are evaluated and discussed.
Growth and morphological study of zinc oxide nanoneedles grown on the annealed titanate nanotubes using hydrothermal method
Authors: Kim G. S. , Ansari S. G. , Seo H. K. , Kim Y. S. , Shin H. S.
Published: October 2007

Hydrothermal growth of ZnO on the annealed titanate nanotube films results in the oriented hexagonal-needlelike structures. The size, shape, density, and alignment of ZnO film are significantly affected by annealing temperature and orientation of the beneath titanate layer. It is believed that oxygen and hydrogen vacancies, generated due to dehydration of interlayered OH groups while annealing of the titanate, are responsible for the changes in the morphology of the ZnO. Microscopic observations clearly resolved nanoneedles with the base diameter of ∼150nm and length of ∼5μm with lattice spacing of 0.52nm, indicating single crystalline ZnO and grown along the (0001) direction. A growth model is presented based on the layer-by-layer growth (three-step growth) as a function of growth time (2–6h). Thicknesses of these three steps were found increasing with growth time. The second step (II) of growth exhibits the same feature as that of the first step (I), i.e., bounded with six crystallographic, lower surface energy facets of {011¯0} surfaces and capped with {011¯1} planes, but with higher step thickness and sharper tip. Finally, the obtained ZnO nanoneedles exhibit {011¯1} facets, the third step (III), with sharp tip in the [0001] direction.
Influence of impurities on the x-ray photoelectron spectroscopy and Raman spectra of single-wall carbon nanotubes
Authors: Li Zhongrui , Biris Alexandru S. , Dervishi Enkeleda , Saini Viney , Xu Yang , Biris Alexandru R. , Lupu Dan
Published: October 2007

The effect of impurities on the properties of single-wall carbon nanotubes (SWNTs) was investigated with multiple analytical techniques. Charge transfer is believed to occur between the impurities and the SWNTs as observed by combining the Raman scattering and x-ray photoelectron measurements. The impurity condition (type and level) was found to strongly affect the electronic and vibrational properties of the SWNT. The metal catalysts in the impurity usually behave as electron donors, which can downshift the graphitic (G) band as well as the radial breathing mode frequencies. The low temperature air oxidation of as-prepared SWNT material usually upshifts the radial breathing mode Raman peaks to higher frequencies.
Comment on “Influence of dielectric core and embedding medium on the local field enhancement for gold nanoshells” [J. Appl. Phys. 100, 026104 (2006)]
Authors: Wu DaJian , Xu XiaoDong , Liu XiaoJun
Published: October 2007

The weak peak at ultraviolet region observed by Zhu and Zhang in gold nanoshells should not be ascribed to the coupling of the “quadruple resonance,” which may be interpreted in terms of the contributions of the interband transitions in the Au layer. In addition, the quasistatic theory used by Zhu and Zhang is not suitable for investigation of the optical properties of gold nanoshells when the total radius of the shell is larger than the quasistatic limit. Furthermore, it is obvious that the strength and position of the local field factor peak should vary with the radial distance and the polar angle.
Observation of sheath modification in laboratory dusty plasma
Authors: Adhikary Nirab Chandra , Bailung Heremba , Pal Arup Ratan , Chutia Joyanti , Nakamura Yoshiharu
Published: October 2007

The nature of the electrostatic sheath near a plasma boundary is experimentally investigated in the presence of charged dust grains. The experiment is performed in a dusty double plasma device. The dusty plasma parameters are measured and the influence of dust density on the dust charging process has been presented. The average charge on a dust grain is found to decrease when dust density in the plasma volume is increased. The negatively charged dust particles are found to modify the sheath parameter and bulk plasma parameters. The influence of different dust density as well as of the negative bias to the metallic plate on the ion sheath structure is presented.
Conformation-specific infrared and ultraviolet spectroscopy of tyrosine-based protonated dipeptides
Authors: Stearns Jaime A. , Guidi Monia , Boyarkin Oleg V. , Rizzo Thomas R.
Published: October 2007

We present the spectroscopy and photofragmentation dynamics of two isomeric protonated dipeptides, H+AlaTyr and H+TyrAla, in a cold ion trap. By a combination of infrared-ultraviolet double resonance experiments and density functional theory calculations, we establish the conformations present at low temperature. Interaction of the charge at the N-terminus with the carbonyl group and the tyrosine π-cloud seems to be critical in stabilizing the low-energy conformations. H+AlaTyr has the flexibility to allow a stronger interaction between the charge and the aromatic ring than in H+TyrAla, and this interaction may be responsible for many of the differences we observe in the former: a significant redshift in the ultraviolet spectrum, a much larger photofragmentation yield, fewer stable conformations, and the absence of fragmentation in excited electronic states.
Growth mode control of the free carrier density in SrTiO3−δ films
Authors: Ohtomo A. , Hwang H. Y.
Published: October 2007

We have studied the growth dynamics and electronic properties of SrTiO3−δ homoepitaxial films by pulsed laser deposition. We find that the two dominant factors determining the growth mode are the kinetics of surface crystallization and of oxidation. When matched, persistent two-dimensional layer-by-layer growth can be obtained for hundreds of unit cells. By tuning these kinetic factors, oxygen vacancies can be frozen in the film, allowing controlled, systematic doping across a metal-insulator transition. Metallic films can be grown, exhibiting Hall mobilities as high as 25000cm2∕Vs.
Fabrication and electrical, photosensitive properties of p-poly(9,9-diethylfluorene)/n-silicon nanowire heterojunction
Authors: Cheng Yanzhao , Fang Guojia , Li Chun , Yuan Longyan , Ai Lei , Chen Bingruo , Zhao Xingzhong , Chen Zhiyuan , Bai Weibin , Zhan Caimao
Published: October 2007

P -poly(9, 9-diethylfluorene)/n-silicon nanowire (SiNW) heterojunctions were prepared by spin coating p-type conductive organic polymer on as-prepared n-SiNWs, which were fabricated by electroless metal deposition method. This hybrid nanowire p-n heterojunction shows good rectifying behavior with turn-on voltage of about 1V at room temperature. Temperature-dependent current-voltage properties in the range of 253–413K were investigated and they follow the standard p-n heterojunction electron transport mechanism. The hybrid nanowire heterojunction shows good photovoltaic properties and good sensitivity toward visible light, and a responsivity of 3.23mA∕W under a reverse bias of 10V was obtained. These results present potential applications in the future nanoelectronic and photonic devices based on organic and inorganic p-n heterojunction arrays.
Current shot noise characteristics in resonant tunneling step-barrier structures
Authors: Zhu Rui , Guo Yong
Published: October 2007

The current shot noise behavior in resonant tunneling step-barrier structures is investigated based on the standard scattering approach. The relations between the shot noise and the applied bias, the Fermi energies, as well as the structural parameters are revealed. By making a comparison of resonant tunneling among single square-barrier structures, step-barrier structures, and double-barrier structures, the distinct curve shape and extremely large suppression of the shot noise are revealed in the step-barrier structures. It is shown that in the step-barrier structures, a valley of the shot noise occurs at the bias where the current peak locates and the Fano factor drops down to minimum at the valley bottom. It is also found that the shot noise suppression is larger for larger chemical potentials and maximal suppression with the Fano factor close to 0.05 appears at a particular structure configuration. These results are helpful to improve the signal-to-noise ratio of quantum devices based on resonant-tunneling structures.
Photoluminescence of 6H–SiC nanostructures fabricated by electrochemical etching
Authors: Botsoa J. , Bluet J. M. , Lysenko V. , Marty O. , Barbier D. , Guillot G.
Published: October 2007

Photoluminescence (PL) spectra of 6H–SiC nanostructures (nanoporous layers and nanopowder) fabricated by electrochemical etching of bulk wafers consist of broad subgap emission bands as well as above-gap tails. These features are explained, respectively, in terms of radiative recombinations via N–Al donor-acceptor electronic levels and surface states as well as quantum confinement phenomenon in small nanocrystallites. An excitation power dependent PL study allowed differentiation of the radiative channels mentioned above. The particular role of surface states on the room temperature PL of the SiC nanopowder is highlighted. A concentration dependent PL study on aqueous suspensions of the nanopowder points out the quenching of PL emission coming from recombination of quantum-confined excitons in small SiC nanoparticles interacting with the larger ones.
Influence of swift ions and proton implantation on the formation of optical waveguides in lithium niobate
Authors: Kumar P. , Moorthy Babu S. , Ganesamoorthy S. , Karnal A. K. , Kanjilal D.
Published: October 2007

Optically polished titanium doped congruent lithium niobate single crystals were implanted with protons of energy of 120keV (at fluences of 1×1015, 1×1016, and 1×1017ions∕cm2). Some loss of lithium from the surface upon ion implantation was recovered by irradiation with 50MeV lithium ions (at fluences varying from 1×1011to1×1013ions∕cm2). The near surface region defects created in the crystal were analyzed using high resolution x-ray diffraction technique, atomic force microscopy, fourier transform infrared, and optical transmittance (UV-visible) studies. The marked lattice strain induced by the energetic ions was characterized by high resolution x-ray diffraction measurements. Three-dimensional defect clusters were observed from atomic force microscopy with nanoscale resolution. Variations of O–H bond stretching vibrations as a function of fluences were observed. Optical transparency of these samples was found to depend on implantation, irradiation, and combined processes. Correlating the structural information with surface morphology experiments, the existence of a waveguide layer on lithium niobate surface has been ascertained.
Synthesis and in situ characterization of low-resistivity TaNx films by remote plasma atomic layer deposition
Authors: Langereis E. , Knoops H. C. M. , Mackus A. J. M. , Roozeboom F. , van de Sanden M. C. M. , Kessels W. M. M.
Published: October 2007

Remote plasma atomic layer deposition (ALD) of TaNx films from Ta[N(CH3)2]5 and H2, H2-N2, and NH3 plasmas is reported. From film analysis by in situ spectroscopic ellipsometry and various ex situ techniques, data on growth rate, atomic composition, mass density, TaNx microstructure, and resistivity are presented for films deposited at substrate temperatures between 150 and 250°C. It is established that cubic TaNx films with a high mass density (12.1gcm−3) and low electrical resistivity (380μΩcm) can be deposited using a H2 plasma with the density and resistivity of the films improving with plasma exposure time. H2-N2 and NH3 plasmas resulted in N-rich Ta3N5 films with a high resistivity. It is demonstrated that the different TaNx phases can be distinguished in situ by spectroscopic ellipsometry on the basis of their dielectric function with the magnitude of the Drude absorption yielding information on the resistivity of the films. In addition, the saturation of the ALD surface reactions can be determined by monitoring the plasma emission, as revealed by optical emission spectroscopy.
Dynamics of photodeformations and space charge field in photorefractive Fe:LiNbO3 studied with synchrotron area diffractometry
Authors: Calamiotou M. , Chrysanthakopoulos N. , Papaioannou G. , Baruchel J.
Published: October 2007

The method of synchrotron area diffractometry has been used to map with a high spatial resolution in real time the distribution over the crystal surface of the lattice deformation tensor components induced by a macroscopic visible light beam on a z-cut iron-doped photorefractive LiNbO3 crystal. The xy planes exhibit a tensile strain, on the order of 10−4, in the center and long range, extending up to 780μm, shear deformations at the borders of the illuminated region, respectively. Photodeformations evolve with illumination time with relaxation type, time dependence, and time constants of the order of minutes. The observed lateral distribution of deformation tensor components, as well as their temporal evolution, has been examined, considering the coupling of the converse piezoelectric effect with the strong space charge field generated by the damage inducing beam along the z axis due to the bulk photovoltaic effect. The observed strain in the center can be attributed to the bulk photovoltaic field of the order of 107V∕m, while the long range shear deformations are mainly associated with the lateral components of the electric field which are present at the borders of the space charge regions. Both photodeformations and space charge field evolve at the same time scale, the dependence of time constants on the incident light intensity following the predictions of the one-center model charge redistribution due to the bulk photovoltaic effect. This work demonstrates that the method of synchrotron area diffractometry is a very powerful tool to study in situ the dynamics and spatial variation of microstuctural changes (deformations) induced by an external field (electric, magnetic, or temperature).
Criteria for the existence of helical instabilities in subsonic impinging jets
Authors: Panickar Praveen , Raman Ganesh
Published: October 2007

Understanding instability modes in impinging jets is important for the design of advanced vertical takeoff and landing aircraft. Our initial experimental observations of helical modes in subsonic impinging jets were contrary to expectations based on published stability theory results. This paper focuses on the curious phenomenon of the occurrence of these, hitherto unexpected, helical modes in subsonic impinging jets and provides, both experimental and analytical proof for their existence. Past results, based on linear stability analysis, that call for an absence of helical modes in cold, subsonic impinging jets have been re-examined to reconcile new experimental data. Based on this re-examination, a new threshold Mach number has been proposed, above which helical modes are permissible. The revised theory put forth in this paper indicates that the threshold Mach number for the existence of the helical mode depends on the jet temperature and that this threshold decreases as the jet temperature increases. This threshold Mach number has been experimentally verified for unheated jets. Additionally, the experimental results reveal that the threshold Mach number also depends on the stand-off distance. Finally, it has been shown that the experimentally obtained mean Strouhal numbers for the helical mode show excellent agreement with the Strouhal number of the least dispersive wave of the same mode calculated at various Mach numbers. The results presented here offer hope for better understanding of impinging jet instability modes.
Dicarbon defects in as-grown and annealed carbon-doped InAs
Authors: Najmi S. , Chen X. K. , Thewalt M. L. W. , Watkins S. P.
Published: October 2007

Annealing measurements performed on heavily carbon-doped InAs samples grown by organometallic vapor phase epitaxy confirm that the n-type behavior observed in this material is due to the formation of dicarbon donor defects. The structural, electrical, and optical properties of n-type carbon-doped InAs annealed at temperatures of 400 °C and higher were studied. Raman and infrared spectroscopy showed a reduction in the intensity of carbon acceptor and carbon-hydrogen lines along with an increase in the strength of the dicarbon local vibrational mode after annealing. X-ray diffraction revealed that the lattice constant of annealed samples increased toward that of the undoped samples. These changes were accompanied by increases in the electron concentration and mobility. Based on the expected energy level of dicarbon donors in GaAs, we predict that the dicarbon defects in InAs should introduce a resonant level close to or slightly above the conduction band minimum.
Effects of substrate on the structure and orientation of ZnO thin film grown by rf-magnetron sputtering
Authors: Liu H. F. , Chua S. J. , Hu G. X. , Gong H. , Xiang N.
Published: October 2007

X-ray diffractions, Nomarski microscopy, scanning electron microscopy, and photoluminescence have been used to study the effects of substrate on the structure and orientation of ZnO thin films grown by rf-magnetron sputtering. GaAs(001), GaAs(111), Al2O3(0002) (c-plane), and Al2O3(11¯02) (r-plane) wafers have been selected as substrates in this study. X-ray diffractions reveal that the ZnO film grown on GaAs(001) substrate is purely textured with a high c-axis orientation while that grown on GaAs(111) substrate is a single ZnO(0002) crystal; a polycrystalline structure with a large-single-crystal area of ZnO(0002) is obtained on a c-plane Al2O3 substrate while a ZnO(112¯0) single crystal is formed on an r-plane Al2O3 substrate. There is absence of significant difference between the photoluminescence spectra collected from ZnO∕GaAs(001), ZnO∕GaAs(111), and ZnO∕Al2O3(0002), while the photoluminescence from ZnO∕Al2O3(11¯02) shows a reduced intensity together with an increased linewidth, which is, likely, due to the increased incorporation of native defects during the growth of ZnO(112¯0).
Low temperature piezoelectric and dielectric properties of lead magnesium niobate titanate single crystals
Authors: Guo Wei , Jin Dafei , Wei Wanchun , Maris Humphrey J. , Tian Jian , Huang Xingling , Han Pengdi
Published: October 2007

Lead magnesium niobate-lead titanate (PMN-xPT) single crystal plates were prepared close to the morphotropic phase boundary with PT compositions 27%−31%. The piezoelectric and dielectric properties of these plates at liquid-helium temperature (4.2 K) were obtained by fitting measured impedance curves. In particular, the piezoelectric strain constant e33 is found to be in the range of 5.1−5.7 C m−2 at 4.2 K, which indicates an extraordinarily large piezoelectric effect compared with other materials even at such a low temperature. This result shows that PMN-xPT single crystals are promising candidates for ultrasonic transducers at low temperatures.
Enhanced gas sensing of Au nanocluster-doped or -coated zinc oxide thin films
Authors: Socol G. , Axente E. , Ristoscu C. , Sima F. , Popescu A. , Stefan N. , Mihailescu I. N. , Escoubas L. , Ferreira J. , Bakalova S. , Szekeres A.
Published: October 2007

We demonstrated that doping or covering with Au nanoclusters boosts gas sensing effectiveness of optical metal oxide sensors. The sensing response of pulsed laser deposited ZnO films as sensing element was tested by m-line technique for low concentration (1000ppm) of butane in environmental N2. The optical interrogation was performed for three types of coatings: undoped ZnO, undoped ZnO structures partially covered with Au nanoclusters, or obtained from Au (0.5wt%) doped ZnO targets. Nanocluster coating tripled the sensitivity, while doping resulted in an increase of up to 45% as compared with simple structures.
Optical characterization of niobium-doped rhenium disulphide single crystals
Authors: Dumcenco D. O. , Huang Y. S. , Liang C. H. , Tiong K. K.
Published: October 2007

In this study, the optical properties of niobium-doped rhenium disulphide (ReS2:Nb) single crystals have been reported. The doping effects of the material were characterized by polarization-dependent transmittance, photoluminescence (PL), and piezoreflectance (PzR) measurements in the temperature range of 10–300K. The indirect energy gap of ReS2:Nb shows a slight redshift with respect to the undoped sample. The low temperature PL spectra reveal two near direct band-edge excitonic peaks as well as two additional prominent features at higher energy side. The results agree well with that of the PzR investigation of the sample. In comparison with the undoped ReS2, the excitonic transition energies remain practically unchanged, while the broadening parameter of the excitonic transition features slightly increases due to impurity scattering. In addition, the parameters that describe the temperature variations of the band-edge excitonic transitions were evaluated and discussed.
Temperature dependence of the elastic and vibronic behavior of Si, Ge, and diamond crystals
Authors: Gu Mingxia , Zhou Yichun , Pan Likun , Sun Zhuo , Wang Shanzhong , Sun Chang Q.
Published: October 2007

The thermally induced softening of the elastic and vibronic identities in crystals and their correlations have long been a puzzle. Analytical solutions have been developed, showing that the detectable elastic and vibronic properties could be related directly to the bonding parameters, such as bond length and strength, and their response to the temperature change. Reproduction of measured T-dependent Young’s modulus and Raman shift of Si, Ge, and diamond reveals that the thermally driven softening of the elasticity and the optical Raman frequency arises from bond expansion and vibration, with derived information about the atomic cohesive energy and clarification of their interdependence.
Emission and shock visualization in nonequilibrium nitrogen afterglow plasma
Authors: Adamovich Igor V. , Rich J. William
Published: October 2007

Kinetic modeling of propagating and stationary normal shocks in nonequilibrium nitrogen afterglow plasma is used to simulate the results of shock emission measurements in nitrogen afterglow. Emission intensity overshoot behind the shock predicted by the model is in satisfactory agreement with the experimental results and is consistent with previous analytic estimates. The model demonstrates that the first and the second positive band emission overshoot behind the shock are produced by energy transfer processes among the triplet electronic states of nitrogen generated in the electric discharge. On the other hand, charge separation and ambipolar electric field produced across the shock layer do not result in electron heating and additional electron impact excitation of electronic states. The calculations show that emission overshoot makes possible accurate detection of a stationary shock layer in supersonic flowing afterglow experiments.
Effect of intentional bulk contamination with iron on multicrystalline silicon solar cell properties
Authors: Dubois S. , Palais O. , Ribeyron P. J. , Enjalbert N. , Pasquinelli M. , Martinuzzi S.
Published: October 2007

The influence of intentional iron bulk contamination on the performances of boron doped p-type multicrystalline silicon solar cells was investigated. Solar cells were made from iron contaminated wafers, with an initial dissolved iron concentration 100 times higher than that of standard wafers. Nevertheless, the conversion efficiency of these cells was not impacted by this intentional contamination. We showed that this tolerance toward iron was due to the efficiency of the gettering and hydrogenation effects, complementary in this material. While phosphorus diffusion (extracting more than 99% of the iron from the bulk) is slightly limited in regions of high dislocation density, hydrogen diffuses through the whole thickness of the wafer and passivates defects and remaining impurities, with its diffusion being faster along extended defects
Bound magnetic polarons in p-type Cu2Cd0.25Fe0.75GeSe4 and Cu2FeGeTe4
Authors: Quintero E. , Quintero M. , Morocoima M. , Bocaranda P.
Published: October 2007

Measurements of magnetic susceptibility χ as a function of temperature T and of magnetization M as a function of applied magnetic field B at a number of fixed temperatures were made on polycrystalline samples of Cu2Cd0.25Fe0.75GeSe4 and Cu2FeGeTe4. The χ vs T data show that the Cu2Cd0.25Fe0.75GeSe4 alloy is antiferromagnetic with a Néel temperature TN of about 12K, while the Cu2FeGeTe4 has a magnetic transition at about 160K. The M vs B results obtained on Cu2Cd0.25Fe0.75GeSe4 indicated that bound magnetic polarons (BMPs), associated with holes bound to acceptors, occur in the paramagnetic and antiferromagnetic phases, and that the effects from BMPs disappear at approximately 50K. In the case of the Cu2FeGeTe4 compound, effects due to the presence of BMPs were not observed in the material. For the Cu2Cd0.25Fe0.75GeSe4, the M vs B curves were well fitted by a Langevin-type of equation and the variation of the fitting parameters determined as a function of temperature. It was found that, in the temperature range between 15 and 50K, the number of BMPs remained practically constant having a mean value of 3.8×1018∕cm3. The analysis gave a value of 305μB for the average magnetic moment of a BMP, corresponding to 61 Fe atoms. Using a simple spherical model, this gives the BMP radius of about 14Å; this value is close to the expected radius of the hole orbit, of the order of 10Å.
Realization of dot- and antidot-type two-dimensional photonic crystals by double holographic method
Authors: Yeo Jong-Bin , Yun Sang-Don , Lee Hyun-Yong
Published: October 2007

Two-dimensional photonic crystals (2D PCs) were fabricated on a 1.18-μm-thick polymer resist (DMI-150) by a double-exposure holographic method using a HeCd laser with a wavelength of 442nm. The 2D PC arrays exhibited various primitive lattice structures from triangle to square according to the change of the rotation angle (γ) of the double-exposure beams. In addition, the period, filling factor, and form (dot or antidot) of the PCs could be controlled by the experimental conditions such as the incident interference angle (θ), the exposure power, and duration. The following exposure procedure was applied: first holographic exposure, followed by sample rotation, and finally, second holographic exposure. The first and second holographic exposures were carried out at a fixed angle (θ=11°) and the rotation was performed in the range of γ=45°–90°. The variation of diffraction efficiency (η) during the exposure process and after the development was monitored using a HeNe laser in real time. The images of the prepared 2D PCs were observed by scanning electron microscopy and atomic force microscopy, and the data on their reciprocal lattices were obtained by the diffracted patterns. We believe that the double holographic method is a suitable tool to realize the dot- and antidot-type 2D PCs with a periodic array of a large area.
Switching of nonfunctionalized spiropyran thin films on single crystalline MgO(100)
Authors: Karcher M. , Rüdt C. , Elsäßer C. , Fumagalli P.
Published: October 2007

Thin films of spiropyran with thicknesses in the range of 5–40 nm were prepared on nonfunctionalized single crystalline MgO(100) substrates in situ in high vacuum. The films were studied in detail for wavelengths between 400 and 800 nm by means of optical absorption spectroscopy. The absorption maximum of the spiropyran state is found at 390 nm, comparable to microcrystalline powder. In order to investigate the switching kinetics between the closed-ring spiropyran and the ring-opened merocyanine form, time-resolved measurements were performed. The experimental data could be fit for both switching directions using a biexponential model indicating optically and thermally induced processes. A quantum efficiency for the switching toward the merocyanine state of about 2% was calculated. The reversibility of the switching process was found to be better than 90%.
Integrable Rosochatius deformations of the restricted soliton flows
Authors: Zhou Ruguang
Published: October 2007

A method to construct integrable Rosochatius deformations of the restricted soliton flows in the setup of Lax formulation is presented. The integrable Rosochatius deformations of the restricted soliton flows such as the restricted Ablowitz-Kaup-Newell-Segur flow, the restricted Tu-Meng flow, the restricted Tu flow with Neumann-type constraints, and the restricted modified Korteweg–de Vries flow, together with their Lax representations, are presented. In addition, a Lax representation of the Jacobi-Rosochatius system is obtained.
Dye-sensitized solar cells using network structure of electrospun ZnO nanofiber mats
Authors: Kim Il-Doo , Hong Jae-Min , Lee Byong Hong , Kim Dong Young , Jeon Eun-Kyung , Choi Duck-Kyun , Yang Dae-Jin
Published: October 2007

Nanostructured semiconducting metal oxides and particularly nanofiber-based photoelectrodes can provide enhanced energy conversion efficiencies in dye-sensitized solar cells (DSSCs). In this study ZnO/poly(vinyl acetate) composite nanofiber mats were directly electrospun onto a glass substrate coated with F:SnO2, then hot pressed at 120°C and calcined at 450°C. This resulted in multiple nanofiber networks composed of a twisted structure of 200–500nm diameter cores with ∼30nm single grains. The DSSCs using ZnO nanofiber mats exhibited a conversion efficiency of 1.34% under 100mW∕cm2 (AM-1.5G) illumination.
Conductivity fluctuation within a crystalline domain and its origin in pentacene thin-film transistors
Authors: Ohashi Noboru , Tomii Hiroshi , Matsubara Ryousuke , Sakai Masatoshi , Kudo Kazuhiro , Nakamura Masakazu
Published: October 2007

Surface topography and high-resolution potential images in a thin-film transistor with a polycrystalline pentacene active layer have been measured by atomic-force-microscope potentiometry. A potential fluctuation independent of topographic features was found in large flat molecular terraces. The origin of the potential fluctuation was concluded to be the fluctuation of the top level of the highest-occupied-molecular-orbital band, which results in the variation of local carrier concentration. The full width at half maximum of the band fluctuation was estimated to be 12meV, which might reduce the mean carrier velocity in crystalline domains.
Optimized stray-field-induced enhancement of the electron spin precession by buried Fe gates
Authors: Meier L. , Salis G. , Moll N. , Ellenberger C. , Shorubalko I. , Wahlen U. , Ensslin K. , Gini E.
Published: October 2007

The magnetic stray field from Fe gates is used to modify the spin precession frequency of InGaAs∕GaAs quantum-well electrons in an external magnetic field. By using an etching process to position the gates directly in the plane of the quantum well, the stray-field influence on the spin precession increases significantly compared with results from previous studies with top-gated structures. In line with numerical simulations, the stray-field-induced precession frequency increases as the gap between the ferromagnetic gates is reduced. The inhomogeneous stray field leads to additional spin dephasing.
Anomalous absorption of surface plasma wave by particles adsorbed on metal surface
Authors: Kumar Gagan , Tripathi V. K.
Published: October 2007

A surface plasma wave (SPW) propagating on a metal surface, covered with thin layer of metallic nanoparticles, excites resonant plasma oscillations in the particles. The particles absorb energy from the wave via electrons, incurring attenuation of the surface plasma wave. For spherical metallic particles with plasma frequency ωPe, the resonant plasma oscillations occur at ω=ωPe∕3, where ω is the frequency of the SPW. In the vicinity of this frequency, a sharp increase in the absorption of surface plasma wave by the metallic particles, depending upon its size, occurs.
Sample size effect and microcompression of Mg65Cu25Gd10 metallic glass
Authors: Lee C. J. , Huang J. C. , Nieh T. G.
Published: October 2007

Micropillars with diameters of 1 and 3.8μm were fabricated from Mg-based metallic glasses using focus ion beam, and then tested in compression at strain rates ranging from 6×10−5to6×10−1s−1. The apparent yield strength of the micropillars is 1342–1580MPa, or 60%–100% increment over the bulk specimens. This strength increase can be rationalized using the Weibull statistics for brittle materials, and the Weibull modulus of the Mg-based metallic glasses is estimated to be about 35. Preliminary results indicated that the number of shear bands increased with the sample size and strain rates.
Velocity dependence of nanoscale wear in atomic force microscopy
Authors: Bhushan Bharat , Kwak Kwang Joo
Published: October 2007

The velocity dependence of nanoscale wear is studied over a wide range of velocities up to 100mm∕s. High sliding velocities are achieved by modifying an existing setup with a custom calibrated piezostage. Pt-coated atomic force microscope probes are scanned against uncoated diamondlike carbon-coated Si wafers and with an overcoat of a lubricant film. The wear rate increases with logarithm of velocity up to a certain value and then levels off. A wear model is developed to explain the velocity dependence of nanoscale wear based on thermally activated atomic-scale stick slip and tribochemical wear.
Passband filters for terahertz radiation based on dual metallic photonic structures
Authors: Gallant A. J. , Kaliteevski M. A. , Wood D. , Petty M. C. , Abram R. A. , Brand S. , Swift G. P. , Zeze D. A. , Chamberlain J. M.
Published: October 2007

This paper reports on the development of two-dimensional metallic microstructures for the filtering of terahertz radiation. These structures are fabricated using ultraviolet-based processing of thick SU8 resist. This micromachining technique enables the array patterns, dimensions, and consequently the filter characteristics to be readily defined. In particular, we demonstrate that a filter with an isolated near-square-shaped passband can be realized on the basis of a combination of two different metallic photonic arrays of optimized design.
High-density energy conversion using compact magnetohydrodynamic electrical power generator
Authors: Murakami Tomoyuki , Okuno Yoshihiro
Published: October 2007

We describe high-density energy conversion using a compact closed-cycle magnetohydrodynamic electrical power generator. The slightly divergent disk-shaped generator exhibits improved energy-conversion performance, in particular, a high power density of 0.76GW∕m3. The application of high- and uniform-density magnetic flux to the entire generator, the high electrical conductivity, the symmetric plasma structure and stable plasma behavior, and the sufficient pressure gradient used to drive the fluid contribute to the outstanding performance of the generator.