bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of metal-oxide-based charge generation layers used in stacked organic light-emitting diodes
Authors: Qi Xiangfei , Li Ning , Forrest Stephen R.
Published: January 2010

We study electron and hole injection in MoO3 charge generation layers (CGLs) commonly used for establishing balanced injection in multilayer stacked organic light-emitting diodes (SOLEDs). A compound CGL consisting of 100-Å-thick MoO3 and Li-doped 4,7-diphenyl-1,10-phenanthroline in a 1:1 molar ratio is demonstrated to have a high electron generation efficiency. Charge injection from the compound CGL is modeled based on a two-step process consisting of tunneling-assisted thermionic emission over an injection barrier of (1.2±0.2) eV and a trap level due to oxygen vacancies at (0.06±0.01) eV above the MoO3 valence band edge. Peak external quantum efficiencies (EQEs) of (10.5±0.2)%, (10.1±0.2)%, (8.6±0.2)%, and (8.9±0.2)% are obtained for tris-(phenylpyridine)iridium-based electrophosphorescent OLEDs with indium tin oxide (ITO) anode/CGL cathode, CGL anode/CGL cathode, CGL anode/Al cathode, and ITO anode/Al cathode contacts, respectively. Based on our analysis, a three-element green emitting electrophosphorescent SOLED is demonstrated with a peak forward-viewing EQE=(24.3±1.0)% and a power efficiency of (19±1) lm/W.
Highly efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes
Authors: Kondakova Marina E. , Deaton Joseph C. , Pawlik Thomas D. , Giesen David J. , Kondakov Denis Y. , Young Ralph H. , Royster Tommie L. , Comfort Dustin L. , Shore Joel D.
Published: January 2010

We demonstrate highly efficient white and nonwhite hybrid organic light-emitting diodes (OLEDs) in which singlet and triplet excited states, generated in the recombination zone, are utilized by fluorescence and phosphorescence, respectively. The excited states are formed at a blue fluorescent light-emitting layer (LEL), and the triplets diffuse through a spacer layer to one or more phosphorescent LEL(s). A key feature enabling the triplet diffusion in such OLEDs is the use of a blue fluorescent emitter with triplet energy above, or not much below, that of the fluorescent host. Additional material properties required for triplet harvesting are outlined. At 1000 cd/m2 a blue and yellow harvesting OLED shows 13.6% external quantum efficiency, 3.8 V, 30.1 lm/W, and color characteristics suitable for display application. High-efficiency harvesting R+G+B white, and B+G and B+R nonwhite OLEDs are also demonstrated. The triplet-harvesting mechanism was verified in all devices by physical methods including spectral analysis, time-resolved electroluminescence, magnetic field effect, and electron paramagnetic resonance.
Effect of film thickness on the thermal resistance of confined semiconductor thin films
Authors: Landry E. S. , McGaughey A. J. H.
Published: January 2010

The thermal resistance of semiconductor thin films is predicted using lattice dynamics (LD) calculations and molecular dynamics (MD) simulations. We consider Si and Ge films with thicknesses, LF, between 0.2 and 30 nm that are confined between larger extents of the other species (i.e., Ge/Si/Ge and Si/Ge/Si structures). The LD predictions are made in the classical limit for comparison to the classical MD simulations, which are performed at a temperature of 500 K. For structures with LF<2 nm, the thin film thermal resistance increases rapidly with increasing film thickness, a trend we attribute to changes in the allowed vibrational states in the film. These changes are found to affect the dependence of the phonon transmission coefficient on incidence angle for the Ge/Si/Ge structures and on frequency for the Si/Ge/Si structures. When LF>2 nm, the MD-predicted thermal resistances are independent of the film thickness for the Ge/Si/Ge structures and increase with increasing film thickness for the Si/Ge/Si structures. We attribute these results to phonon transport that is ballistic in the Ge/Si/Ge structures and more diffusive in the Si/Ge/Si structures based on comparisons to the LD predictions, which assume ballistic phonon transport. We find that this difference between the structures cannot be predicted by comparing the mode-averaged phonon mean free path to the film thickness. It can be predicted, however, by considering the frequency dependence of the phonon mean free paths.
Homogenization theory for the cumulative effect of Te inclusions in CdZnTe radiation detectors
Authors: Bale Derek S.
Published: January 2010

In this paper, homogenization theory based on a multiple scale perturbation of the charge-transport equation is used to derive a mathematical framework for modeling the cumulative effect of Te inclusions in radiation detectors based on semi-insulating cadmium zinc telluride (CdZnTe). The derived framework naturally incorporates a wide range of physical models that may posit either a reduced electron lifetime due to enhanced trapping at inclusions, or an altered carrier speed due to a distorted electric field at inclusions, or both. The new framework is applied to a simplified version of the geometric model introduced by Bolotnikov et al. [Nucl. Instrum. Methods Phys. Res. A 571, 687 (2007)], and it is shown that this results in a closed-form approximation to the reduced electron trapping time that depends in a rather simple way on fundamental inclusion parameters such as their mean size and number density. It is also demonstrated that this effective trapping time compares well with previously published simulation data for the geometric model. Further, the electron mobility-lifetime product that results from the reduced carrier lifetime is easily incorporated into Monte Carlo device simulation. Examples of simulated induction maps and pulse-height spectra for pixelated detectors that contain inclusions of various mean sizes and number densities are presented.
Pressure induced phase transformation of Ba8Ga16Ge30 clathrate studied by x-ray diffraction and Raman spectroscopy
Authors: Kume Tetsuji , Ohno Satoshi , Sasaki Shigeo , Shimizu Hiroyasu , Ohishi Yasuo , Okamoto Norihiko L. , Kishida Kyosuke , Tanaka Katsushi , Inui Haruyuki
Published: January 2010

The phase transition and the vibrational properties of Ba8Ga16Ge30 have been investigated at high pressures up to 40 GPa at room temperature. The combined study of the high-pressure Raman and synchrotron powder x-ray diffraction (XRD) experiments revealed the occurrence of a first-order phase transition at 33 GPa, on which a volume decrease of about 3% was found. Rietveld refinements of the XRD data demonstrated the atomic displacements precursory to the phase transition, allowing us to discuss the mechanism of the phase transition. In the Raman experimental data, anomalies were observed in the spectral feature and the guest vibration around 17 GPa. By combining the Raman results with the XRD ones, the vibrational frequency of the guest Ba was investigated as a function of the host cage size. As a result, a linear relation between guest vibrational frequency and the guest-host distance was identified.
Nanocrystalline structure of nanobump generated by localized photoexcitation of metal film
Authors: Ivanov Dmitriy S. , Lin Zhibin , Rethfeld Baerbel , O’Connor Gerard M. , Glynn Thomas J. , Zhigilei Leonid V.
Published: January 2010

The extreme cooling rates in material processing can be achieved in a number of current and emerging femtosecond laser techniques capable of highly localized energy deposition. The mechanisms of rapid solidification of a nanoscale region of a metal film transiently melted by a localized photoexcitation are investigated in a large-scale atomistic simulation. The small size of the melted region, steep temperature gradients, and fast two-dimensional electron heat conduction result in the cooling rate exceeding 1013 K/s and create conditions for deep undercooling of the melt. The velocity of the liquid/crystal interface rises up to the maximum value of ∼80 m/s during the initial stage of the cooling process and stays approximately constant as the temperature of the melted region continues to decrease. When the temperature drops down to the level of ∼0.6Tm, a massive homogeneous nucleation of the crystal phase inside the undercooled liquid region takes place and prevents the undercooled liquid from reaching the glass transition temperature. The prediction of the nanocrystalline structure of the surface features generated in laser nanoprocessing has implications for practical applications of nanostructured surfaces and calls for experimental verification of the computational results.
An interpretation of the anomalous Π1 vibronic structure in the far-UV spectrum of CO
Authors: Lefebvre-Brion H. , Liebermann H. P. , Vázquez G. J.
Published: January 2010

The far-UV spectrum of carbon monoxide presents numerous abnormal Π1 rovibronic levels in the region 92 000–105 000 cm−1 which have been observed by several experimentalists. Yet, and in spite of various attribution attempts carried out over the past two decades, the nature of these levels is poorly understood and they still lack a definitive assignment. The absorption lines in this energy region are characterized by irregular energy level positions and spacings, and odd, smaller than expected, rotational constants. In the current contribution we address this puzzle by relying on recent ab initio calculations of several Rydberg and valence states of CO [G. J. Vázquez, J. M. Amero, H. P. Liebermann, and H. Lefebvre-Brion, J. Phys. Chem. A 113, 13395 (2009)], and on further new calculations in which we compute electronic transition moments between the ground state and several excited Π1 states. We focus on the perturbations between the adiabatic Π1 states, specifically on the interaction between the second and third potential energy curves, reported in our previous paper. The second adiabatic potential energy curve, which we refer to as E-E′, displays a distorted shape with two minima as a result of an avoided crossing with the third one. We report here the computation of the lowest vibronic levels of a system of two electronic states which undergo a strong Rydberg-valence interaction. Our vibronic calculations proceed as follows: from the second and third computed adiabatic curves we first obtain approximate diabatic curves for the (X Σ2+)3pπ E Π1 Rydberg state and for the valence E′ Π1 state. Then we solve a system of 2×2 coupled equations in order to obtain the perturbed vibronic energy levels and wave functions for the interacting E and E′ states. The computed vibronic levels obtained from the coupled equation treatment are compared to the first six observed Π1 levels. A good agreement is found with experiment for the four lowest vibronic levels and a reasonable accord for two higher levels.
Structure and giant magnetoresistance of granular Co–Cu nanolayers prepared by cross-beam pulsed laser deposition
Authors: Jesche A. , Gorbunoff A. , Mensch A. , Stöcker H. , Levin A. A. , Meyer D. C.
Published: January 2010

A series of CoxCu100−x (x=0, 40–75, 100) layers with thicknesses between 13 and 55 nm were prepared on silicon substrates using cross-beam pulsed laser deposition. Wide-angle x-ray diffraction (WAXRD), transmission electron microscopy (TEM), and electrical transport measurements revealed a structure consisting of decomposed cobalt and copper grains with grain sizes of about 10 nm. The influence of cobalt content and layer thickness on the grain size is discussed. Electron diffraction indicates the presence of an intermetallic Co–Cu phase of Cu3Au structure type. Thermal treatment at temperatures between 525 and 750 K results in the progressive decomposition of Co and Cu, with an increase of the grain sizes up to about 100 nm. This is tunable by controlling the temperature and duration of the anneal, and is directly observable in WAXRD patterns and TEM images. A careful analysis of grain size and the coherence length of the radiation used allows for an accurate interpretation of the x-ray diffraction patterns, by taking into account coherent and noncoherent scattering. The alloy films show a giant magnetoresistance of 1%–2.3% with the maximum obtained after annealing at around 725 K.
Modeling nonlinear electromechanical behavior of shocked silicon carbide
Authors: Clayton J. D.
Published: January 2010

A model is developed for anisotropic ceramic crystals undergoing potentially large deformations that can occur under significant pressures or high temperatures. The model is applied to describe silicon carbide (SiC), with a focus on α-SiC, specifically hexagonal polytype 6H. Incorporated in the description are nonlinear anisotropic thermoelasticity, electrostriction, and piezoelectricity. The response of single crystals of α-SiC of various orientations subjected to one-dimensional shock loading is modeled for open- and short-circuit boundary conditions. The influences of elastic and electromechanical nonlinearity and anisotropy on the response to impact are quantified. For elastic axial compressive strains less than 0.1, piezoelectricity, electrostriction, and thermal expansion have a negligible influence on the mechanical (stress) response, but the influences of nonlinear elasticity (third-order elastic constants) and anisotropy are not insignificant. The model is extended to incorporate inelastic deformation and lattice defects. Addressed are Shockley partial dislocations on the basal plane and edge dislocation loops on the prism plane, dilatation from point defects and elastic fields of dislocation lines, and cleavage fracture. The results suggest that electric current generated in shock-loaded α-SiC crystals of certain orientations could affect the dislocation mobility and hence the yield strength at high pressure.
Electric double layer transistor with a (Ga,Mn)As channel
Authors: Endo M. , Chiba D. , Shimotani H. , Matsukura F. , Iwasa Y. , Ohno H.
Published: January 2010

The authors have fabricated electric double layer transistors with a (Ga,Mn)As channel to investigate the possibility of larger modulation of magnetism by the application of electric fields. The sheet conductance as well as the Curie temperature can be modulated up to a few tens of percents by application of gate voltage of a few volts, which is almost one-order of magnitude smaller than that required in conventional metal-insulator-semiconductor structures. The 14 K modulation of the Curie temperature by applying gate voltage ranging from −1 to 3 V is the highest modulation ratio reported so far in ferromagnetic semiconductors.
Effect of strain rate on the yielding mechanism of amorphous metal foam
Authors: Schramm Joseph P. , Demetriou Marios D. , Johnson William L. , Poon Benny , Ravichandran Guruswami , Rittel Daniel
Published: January 2010

Stochastic amorphous Pd43Ni10Cu27P20 foams were tested in quasistatic and dynamic loading. The strength/porosity relations show distinct slopes for the two loading conditions, suggesting a strain-rate-induced change in the foam yielding mechanism. The strength/porosity correlation of the dynamic test data along with microscopy assessments support that dynamic foam yielding is dominated by plasticity rather than elastic buckling, the mechanism previously identified to control quasistatic yielding. The strain-rate-induced shift in the foam yielding mechanism is attributed to the rate of loading approaching the rate of sound wave propagation across intracellular membranes, thereby suppressing elastic buckling and promoting plastic yielding.
A universal semiempirical model for the Fowler–Nordheim programming of charge trapping devices
Authors: Sundararaman Ravishankar , Tiwari Sandip
Published: January 2010

An approximate analytical solution to the dynamics of charge trapping and detrapping by Fowler–Nordheim tunneling is constructed within a simplified model that captures the essential features of the programming characteristics of tunneling devices with the fewest possible parameters. This solution is demonstrated to be an excellent fit for experimental programming characteristics of various tunneling-based nonvolatile memory devices from different laboratories including devices based on traps in silicon nitride, interface traps in silicon dioxide, and silicon nanocrystals.
Band structure and optical gain of tensile-strained germanium based on a 30 band k⋅p formalism
Authors: El Kurdi Moustafa , Fishman Guy , Sauvage Sébastien , Boucaud Philippe
Published: January 2010

We have investigated the band structure of tensile-strained germanium using a 30 band k⋅p formalism. This multiband formalism allows to simultaneously describe the valence and conduction bands, including the L, Δ, and Γ valleys. We calculate the energy band variation as a function of strain and obtain that the crossover from indirect to direct band gap occurs for a tensile in-plane strain of 1.9%. The effective masses of density of states are deduced from the calculated conduction and valence band density of states. Significant deviations are observed as compared to the effective masses of density of states values of unstrained bulk germanium. We finally calculate the optical gain that can be achieved with tensile-strained bulk germanium. An optical gain larger than 3000 cm−1 is predicted for a carrier density of 1×1018 cm−3 and a 3% in-plane biaxial strain. This optical gain is larger than the one of GaAs calculated with the same formalism and is much larger than the experimental free-carrier absorption losses. This gain should be sufficient to achieve lasing in these structures.
Theory of temperature dependence of the conductivity in carbon nanotubes
Authors: Fujita S. , Suzuki A.
Published: January 2010

The electrical conductivity of carbon nanotubes varies, depending on the temperature and the radius and pitch of the sample. In majority cases, the resistance decreases with increasing temperature, suggesting a thermally activated process. The standard band theory based on the Wigner–Seitz cell model predicts a gapless semiconductor, which does not account for the thermal activation. A new band model in which an “electron” (“hole”) has a carbon hexagon size for graphene is proposed. The normal charge carriers in graphene transport are electrons and holes. The electrons (holes) wavepackets extend over the carbon hexagon and carry the charges −e(+e). Electrons or holes thermally activated are shown to generate the observed temperature behavior of the conductivity in the nanotubes.
Terahertz waveguide emitter with subwavelength confinement
Authors: Martl Michael , Darmo Juraj , Dietze Daniel , Unterrainer Karl , Gornik Erich
Published: January 2010

The generation of broadband terahertz pulses on the facet of waveguides is presented as an alternative to widely used coupling techniques. Dielectric loaded subwavelength waveguide structures with lateral confinement are investigated with respect to propagating modes and waveguide losses. The results show the terahertz waveguide emitter to be a promising tool for terahertz spectroscopy in the near field and for the probing of microstructured devices such as quantum cascade lasers.
Exciton-exciton annihilation in organic lanthanide complexes
Authors: Mezyk J. , Di Nuzzo D. , Mech A. , Tubino R. , Meinardi F.
Published: January 2010

We have investigated the mechanism of exciton-exciton annihilation in organic lanthanide complexes by measuring the excitation density dependence of photoluminescence yield in neat films of a model europium chelate. The observed luminescence efficiency reduction at high excitation intensities has been attributed to the operation of the mutual annihilation of the ligand singlet excitons. The second order interaction rate constant, the exciton diffusivity, and the singlet exciton diffusion length in this material have been determined. These results were used to evaluate the role of exciton annihilation in organic light emitting diodes, lanthanide-based organic lasers, and light up-converters. Indications concerning the optimization of the performances of such devices have also been given.
Flight behavior of charged droplets in electrohydrodynamic inkjet printing
Authors: Yudistira Hadi Teguh , Nguyen Vu Dat , Dutta Prashanta , Byun Doyoung
Published: January 2010

Flight behaviors of charged droplets are presented for electrohydrodynamic (EHD) inkjet printing. Three different kinds of EHD spraying techniques, pulsed dc, ac, and single potential (SP) ac, have been investigated and both conductive and dielectric target surfaces were considered. Experimental results show that the flight paths of charged droplets may deviate from their regular straight route, i.e., directly from the nozzle to the substrate. Depending on the droplet charge and applied electric field, droplets may deflect, reflect, or retreat to the meniscus. We can solve these drawbacks by SP EHD printing.
Stuffed fullerenelike boron carbide nanoclusters
Authors: Prasad Dasari L. V. K. , Jemmis Eluvathingal D.
Published: January 2010

Viable stuffed fullerenelike boron carbide nanoclusters, C50B34, C48B36−2, and their isomers based on an icosahedral B84 fragment of elemental β-rhombohedral boron have been investigated using density functional theory calculations. The structure and the stability of these clusters are rationalized using the polyhedral skeletal electron counting and ring-cap orbital overlap compatibility rules. The curvature of the fullerene was found to play a vital role in achieving the most stable isomer C50B34(3B). The large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, three dimensional aromaticity, and electron detachment energies support their high stability. Further, the IR and Raman active modes were recognized.
The influence of a single thiol group on the electronic and optical properties of the smallest diamondoid adamantane
Authors: Landt Lasse , Staiger Matthias , Wolter David , Klünder Kathrin , Zimmermann Peter , Willey Trevor M. , van Buuren Tony , Brehmer Daniel , Schreiner Peter R. , Tkachenko Boryslav A. , Fokin Andrey A. , Möller Thomas , Bostedt Christoph
Published: January 2010

At the nanoscale, the surface becomes pivotal for the properties of semiconductors due to an increased surface-to-bulk ratio. Surface functionalization is a means to include semiconductor nanocrystals into devices. In this comprehensive experimental study we determine in detail the effect of a single thiol functional group on the electronic and optical properties of the hydrogen-passivated nanodiamond adamantane. We find that the optical properties of the diamondoid are strongly affected due to a drastic change in the occupied states. Compared to adamantane, the optical gap in adamantane-1-thiol is lowered by ∼0.6 eV and UV luminescence is quenched. The lowest unoccupied states remain delocalized at the cluster surface leaving the diamondoid’s negative electron affinity intact.
Raman spectroscopy and field emission characterization of delafossite CuFeO2
Authors: Pavunny Shojan P. , Kumar Ashok , Katiyar R. S.
Published: January 2010

Delafossite p-type CuFeO2 (CFO) semiconductors were synthesized by a modified solid state reaction technique and investigated by x-ray diffraction, x-ray photoemission spectroscopy (XPS), energy dispersive x-ray spectroscopy, and scanning electron microscopy, revealing the single-phase nature of CFO with 1:1 Cu/Fe atomic ratio. The valance states of CFO were examined by XPS and suggest Cu and Fe ions are in +1 and +3 valance states with high spin S=5/2. The “turn-on field” which is the macroscopic field needed to get an emission current of 9 nA, was calculated as 5.72 V/μm. Room temperature Raman spectra of CFO displayed two main Raman active modes at Eg∼351 cm−1 and Ag∼692 cm−1 in accord with other delafossite structures. Temperature dependent Raman spectra showed that both the modes shifted to lower frequency with significant decrease in intensity with increase in temperature. Frequency shift and linewidth of both phonon lines matched well with the theoretical damped harmonic oscillator model based on thermal expansion of the lattice and their anharmonicity coupling with other phonons.
Ultrafast spin-transfer switching in spin valve nanopillars with perpendicular anisotropy
Authors: Bedau D. , Liu H. , Bouzaglou J.-J. , Kent A. D. , Sun J. Z. , Katine J. A. , Fullerton E. E. , Mangin S.
Published: January 2010

Spin-transfer switching with short current pulses has been studied in spin-valve nanopillars with perpendicularly magnetized free and reference layers. Magnetization switching with current pulses as short as 300 ps is demonstrated. The pulse amplitude needed to reverse the magnetization is shown to be inversely proportional to the pulse duration, consistent with a macrospin spin-transfer model. However, the pulse amplitude duration switching boundary depends on the applied field much more strongly than predicted by the zero temperature macrospin model. The results also demonstrate that there is an optimal pulse length that minimizes the energy required to reverse the magnetization.
Microwave modulation of terahertz quantum cascade lasers: a transmission-line approach
Authors: Maineult W. , Ding L. , Gellie P. , Filloux P. , Sirtori C. , Barbieri S. , Akalin T. , Lampin J.-F. , Sagnes I. , Beere H. E. , Ritchie D. A.
Published: January 2010

We report on microwave impedance measurements of metal-metal ridge-waveguide terahertz quantum cascade lasers. Experimental data, recorded at 4 K in the 100 MHz–55 GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3 dB modulation bandwidth of ∼70 GHz for a 1 mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3 THz QCL up to 24 GHz.
Communications: Nanomagnetic shielding: High-resolution NMR in carbon allotropes
Authors: Kim Y. , Abou-Hamad E. , Rubio A. , Wågberg T. , Talyzin A. V. , Boesch D. , Aloni S. , Zettl A. , Luzzi D. E. , Goze-Bac C.
Published: January 2010

The understanding and control of the magnetic properties of carbon-based materials is of fundamental relevance in applications in nano- and biosciences. Ring currents do play a basic role in those systems. In particular the inner cavities of nanotubes offer an ideal environment to investigate the magnetism of synthetic materials at the nanoscale. Here, by means of C13 high resolution NMR of encapsulated molecules in peapod hybrid materials, we report the largest diamagnetic shifts (down to −68.3 ppm) ever observed in carbon allotropes, which is connected to the enhancement of the aromaticity of the nanotube envelope upon doping. This diamagnetic shift can be externally controlled by in situ modifications such as doping or electrostatic charging. Moreover, defects such as C-vacancies, pentagons, and chemical functionalization of the outer nanotube quench this diamagnetic effect and restore NMR signatures to slightly paramagnetic shifts compared to nonencapsulated molecules. The magnetic interactions reported here are robust phenomena independent of temperature and proportional to the applied magnetic field. The magnitude, tunability, and stability of the magnetic effects make the peapod nanomaterials potentially valuable for nanomagnetic shielding in nanoelectronics and nanobiomedical engineering.
Disentangling mechanical and mass effects on nanomechanical resonators
Authors: Grüter R. R. , Khan Z. , Paxman R. , Ndieyira J. W. , Dueck B. , Bircher B. A. , Yang J. L. , Drechsler U. , Despont M. , McKendry R. A. , Hoogenboom B. W.
Published: January 2010

Micro and nanomechanical resonators are powerful and label-free sensors of analytes in various environments. Their response, however, is a convolution of mass, rigidity, and nanoscale heterogeneity of adsorbates. Here we demonstrate a procedure to disentangle this complex sensor response, to simultaneously measure both mass and elastic properties of nanometer thick samples. This turns an apparent disadvantage of these resonators into a striking and unique asset, enabling them to measure more than mass alone.
Off-state and turn-on characteristics of solid electrolyte switch
Authors: Tsuji Y. , Sakamoto T. , Banno N. , Hada H. , Aono M.
Published: January 2010

We have investigated off-state and turn-on characteristics of a Ta2O5-based solid-electrolyte switch, the resistance of which changes when the metallic current path is formed in the solid-electrolyte. The turn-on voltages are found to vary widely even when the switches are in an off-state with similar resistance. The variation is induced by the residual metal with different shapes that remains in the solid-electrolyte after a switch is turned off. The residual metal with a sharp point enhances the electrochemical reaction, resulting in the turn-on voltage lowering. We also developed a screening scheme to reduce the variation of the turn-on voltages.
Free energy landscapes and volumes of coexisting phases for a colloidal dispersion
Authors: Lang Trinh Hoa , Wang G. F. , Lai S. K.
Published: January 2010

Treating the repulsive part of a pairwise potential by the hard-sphere form and its attractive part by the effective depletion potential form, we calculate using this model potential the colloidal domains of phase separation. Differing from the usual recipe of applying the thermodynamic conditions of equal pressure and equal chemical potential where the branches of coexisting phases are the ultimate target, we employ the free energy density minimization approach [G. F. Wang and S. K. Lai, Phys. Rev. E 70, 051402 (2004)] to crosshatch the domains of equilibrium phases, which consist of the gas, liquid, and solid homogeneous phases as well as the coexistence of these phases. This numerical procedure is attractive since it yields naturally the colloidal volume of space occupied by each of the coexisting phases. In this work, we first examine the change in structures of the fluid and solid free energy density landscapes with the effective polymer concentration. We show by explicit illustration the link between the free energy density landscapes and the development of both the metastable and stable coexisting phases. Then, attention is paid to the spatial volumes predicted at the triple point. It is found here that the volumes of spaces of the three coexisting phases at the triple point vary one dimensionally, whereas for the two coexisting phases, they are uniquely determined.
Optical phase and the ionization-dissociation dynamics of excited H2
Authors: Kirrander A. , Fielding H. H. , Jungen Ch.
Published: January 2010

We investigate the influence of optical phase on the dynamics of hydrogen molecules excited to a spectral region with competition between predominantly rotational ionization, and dissociation. We show that an appropriate choice of optical phase changes the relative timing of the ionization and dissociation. Furthermore, the temporal width of the ionization and dissociation fluxes can also be controlled, in a matter-wave analogy of transform-limited optical pulses. The close link between the optical phase and the photoinduced electronic and molecular dynamics has important implications for femtochemistry.
An exchange-Coulomb model potential energy surface for the Ne–CO interaction. II. Molecular beam scattering and bulk gas phenomena in Ne–CO mixtures
Authors: Dham Ashok K. , McBane George C. , McCourt Frederick R. W. , Meath William J.
Published: January 2010

Four potential energy surfaces are of current interest for the Ne–CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler–London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the N20e–C12O16 van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne–CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne–CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne–CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne–CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne–CO interaction.
Numerical coarse-graining of fluid field theories
Authors: Villet Michael C. , Fredrickson Glenn H.
Published: January 2010

We present a formalism for the systematic numerical coarse-graining of field-theoretic models of fluids that draws upon techniques from both the Monte Carlo renormalization group and particle-based coarse-graining literature. A force-matching technique initially developed for coarse-graining particle-based interaction potentials is adapted to calculate renormalized field-theoretic coupling coefficients in a complex-valued field theory, and a related method is introduced for coarse-graining field-theoretic operators. The viability of this methodology is demonstrated by coarse-graining a field-theoretic model of a Gaussian-core fluid and thereby reducing lattice discretization errors.
Paraboloid electronic eye cameras using deformable arrays of photodetectors in hexagonal mesh layouts
Authors: Jung Inhwa , Shin Gunchul , Malyarchuk Viktor , Ha Jeong Sook , Rogers John A.
Published: January 2010

We report on a type of digital camera that uses a hexagonal array of silicon photodetectors on a substrate whose surface has parabolic curvature. This elliptical paraboloid shape closely matches the image surface formed by a simple, planoconvex lens. The hexagonal arrangement provides high area coverage with an approximately circular peripheral view. Details of the design strategies and underlying features of the mechanics and optics are described. Full imaging with these parabolic cameras and comparison to planar layouts reveals improved uniformity of illumination and focus across a wide field of view.
Thickness dependence of the critical current density in superconducting films: A geometrical approach
Authors: Hengstberger F. , Eisterer M. , Weber H. W.
Published: January 2010

We analyze the influence of the magnetic field generated by the supercurrents (self-field) on the current density distribution by numerical simulations. The thickness of the superconducting film determines the self-field and consequently the critical current density at zero applied field. We find an equation, which derives the thickness dependence of the critical current density from its dependence on the magnetic induction. Solutions of the equation reproduce numerical simulations to great accuracy, thus enabling a quantification of the dependence of the self-field critical current density with increasing film thickness. This result is technologically relevant for the development of coated conductors with thicker superconducting layers.
Carbon in palladium catalysts: A metastable carbide
Authors: Seriani Nicola , Mittendorfer Florian , Kresse Georg
Published: January 2010

The catalytic activity of palladium toward selective hydrogenation of hydrocarbons depends on the partial pressure of hydrogen. It has been suggested that the reaction proceeds selectively toward partial hydrogenation only when a carbon-rich film is present at the metal surface. On the basis of first-principles simulations, we show that carbon can dissolve into the metal because graphite formation is delayed by the large critical nucleus necessary for graphite nucleation. A bulk carbide Pd6C with a hexagonal six-layer fcc-like supercell forms. The structure is characterized by core level shifts of 0.66–0.70 eV in the core states of Pd, in agreement with experimental x-ray photoemission spectra. Moreover, this phase traps bulk-dissolved hydrogen, suppressing the total hydrogenation reaction channel and fostering partial hydrogenation.
An NMR study of macromolecular aggregation in a model polymer-surfactant solution
Authors: Barhoum Suliman , Yethiraj Anand
Published: January 2010

A model complex-forming nonionic polymer–anionic surfactant system in aqueous solution has been studied at different surfactant concentrations. Using pulsed-field-gradient diffusion NMR spectroscopy, we obtain the self-diffusion coefficients of poly(ethylene glycol) (PEO) and sodium dodecyl sulfate (SDS) simultaneously and as a function of SDS concentration. In addition, we obtain NMR relaxation rates and chemical shifts as a function of SDS concentration. Within the context of a simple model, our experimental results yield the onset of aggregation of SDS on PEO chains (CAC=3.5 mM), a crossover concentration (C2=60 mM) which signals a sharp change in relaxation behavior, as well as an increase in free surfactant concentration and a critical concentration (Cm=145 mM) which signals a distinct change in diffusion behavior and a crossover to a solution containing free micelles. Cm also marks the concentration above which obstruction effects are definitely important. In addition, we obtain the concentration of SDS in monomeric form and in the form of free micelles, as well as the average number of SDS molecules in a PEO-SDS aggregate (NAggr). Taken together, our results suggests continuous changes in the aggregation phenomenon over much of the concentration but with three distinct concentrations that signal changes in the nature of the aggregates.
Excited-state polarizabilities of solvated molecules using cubic response theory and the polarizable continuum model
Authors: Ferrighi Lara , Frediani Luca , Ruud Kenneth
Published: January 2010

The theory and an implementation of the solvent contribution to the cubic response function for the polarizable continuum model for multiconfigurational self-consistent field wave functions is presented. The excited-state polarizability of benzene, para-nitroaniline, and nitrobenzene has been obtained from the double residue of the cubic response function calculated in the presence of an acetonitrile and dioxane solvent. The calculated excited-state polarizabilities are compared to results obtained from the linear response function of the explicitly optimized excited states.
Communications: Explicitly correlated equation-of-motion coupled cluster method for ionized states
Authors: Bokhan Denis , Ten-no Seiichiro
Published: January 2010

The ionization potential equation-of-motion (IP-EOM) method, based on linear explicitly correlated coupled cluster singles and doubles theory [CCSD(F12)] is reported. Numerical tests have shown that the present IP-EOM-CCSD(F12) method provides vertical IPs accurate to 0.05 eV compared to those in the complete basis set limit. The corresponding error in vertical IP of 2s shells does not exceed 0.1 eV.
Determination of the minority carrier diffusion length in compositionally graded Cu(In,Ga)Se2 solar cells using electron beam induced current
Authors: Brown Gregory , Faifer Vladimir , Pudov Alex , Anikeev Sergey , Bykov Eugene , Contreras Miguel , Wu Junqiao
Published: January 2010

A method is proposed and tested which allows for the accurate determination of the carrier collection efficiency and minority carrier diffusion length in Cu(In,Ga)Se2 solar cells using energy dependent electron beam induced current. Gallium composition gradients across the film thickness introduce quasielectric fields that are found to improve collection efficiency when they are located toward the rear of the sample. The quasielectric fields are also shown to reduce the influence of back surface recombination. The strengths and limitations of this technique are discussed and compared with external quantum efficiency measurements.
Surface enhanced spin-flip scattering in lateral spin valves
Authors: Erekhinsky Mikhail , Sharoni Amos , Casanova Fèlix , Schuller Ivan K.
Published: January 2010

Nonlocal transport in Py/Cu lateral spin valves shows that the Cu spin diffusion length and the apparent Py spin polarization increase with Cu thickness. A proper quantitative analysis shows that the Cu spin diffusion length is dominated by surface spin-flip scattering and that the Py and Cu thickness dependence of spin polarization is due to strong spin-flip back-scattering at the Py/Cu interface. This solves a long-standing puzzle regarding the discrepancy in Py spin polarizations obtained from different measurements. Interestingly, the Cu surface oxidation causes enhanced spin diffusion, contrary to expectations. These surface effects substantially affect the performance of lateral spin valves.
Impact of interfacial roughness on spin filter tunneling
Authors: Miller Casey W. , Belyea Dustin D.
Published: January 2010

The impact of interface roughness on spin filter tunneling is considered at low biases as functions of temperature and barrier parameters. Roughness reduces the maximum achievable spin polarization, which results from tunneling “hot spots” (thin regions of the barrier) having intrinsically reduced spin filtering efficiency. Surveying a range of experimentally reasonable roughness and mean barrier thickness values allows us to conclude that roughness values greater than 10% of the mean barrier thickness have an adverse impact on the spin polarization. Atomic-scale roughness may thus be critical for achieving 100% spin polarization in spin filter tunnel junctions at low biases.
Phase stabilization of δ-Bi2O3 nanostructures by epitaxial growth onto single crystal SrTiO3 or DyScO3 substrates
Authors: Proffit D. L. , Bai G.-R. , Fong D. D. , Fister T. T. , Hruszkewycz S. O. , Highland M. J. , Baldo P. M. , Fuoss P. H. , Mason T. O. , Eastman J. A.
Published: January 2010

We observe that the high-temperature δ-phase of Bi2O3 is stabilized to room temperature by the epitaxial growth of nanostructures onto either (001)-oriented SrTiO3 or (001)p-oriented DyScO3 single crystal substrates. In addition, the morphology can be controlled by the miscut of the substrate. Synchrotron x-ray scattering observations at controlled temperatures and oxygen partial pressures reveal that the δ-Bi2O3 nanostructures are coherently strained to the substrates at room temperature. Annealing the nanostructures at 600 °C causes gradual conversion of the (001)-oriented δ-phase to an unidentified strain-relaxed phase.
Photoluminescence characteristics of high quality ZnO nanowires and its enhancement by polymer covering
Authors: Liu K. W. , Chen R. , Xing G. Z. , Wu T. , Sun H. D.
Published: January 2010

We investigated the photoluminescence (PL) properties of ZnO nanowires with and without covering with polymethyl methacrylate (PMMA). Low temperature PL spectra of as-grown ZnO nanowires are dominated by near band edge (NBE) emission due to donor bound excitons and free-to-bound recombination (FB). FB emission persists till 300 K and together with free exciton emission governs the lineshape of the PL spectra. After covering with PMMA, the integral intensity of NBE emission increases about three times, indicating significantly improved excitonic emission efficiency. A model based on surface states and energy bands theory was proposed to interpret this emission enhancement.
A rolling locomotion method for untethered magnetic microrobots
Authors: Hou Max T. , Shen Hui-Mei , Jiang Guan-Lin , Lu Chiang-Ni , Hsu I-Jen , Yeh J. Andrew
Published: January 2010

It is a challenge to achieve free and efficient motion of microrobots on arbitrary surfaces. We report a rolling locomotion method for a magnetic microrobot with a rectangular body (300×200×50 μm3); this method is based on an external rotating magnetic field. The magnetic force, accompanied by normal and friction forces, enables the successive rotations of the microrobot. A magnetic field with a rotational speed of 2 rps rolls the microrobot, giving it a translation speed of 1.4 mm/s. With this locomotion ability, microrobots can move along a line or curve and can climb slopes or stairs.
Variational solution of the Schrödinger equation using plane waves in adaptive coordinates: The radial case
Authors: Pérez-Jordá José M.
Published: January 2010

A new method for solving the Schrödinger equation is proposed, based on the following details. First, a map u=u(r) from Cartesian coordinates r to a new coordinate system u is chosen. Second, the solution (orbital) ψ(r) is written in terms of a function U depending on u so that ψ(r)=|Ju|−1/2U(u), where |Ju| is the Jacobian determinant of the map. Third, U is expressed as a linear combination of plane waves in the u coordinate, U(u)=∑kckeik⋅u. Finally, the coefficients ck are variationally optimized to obtain the best energy, using a generalization of an algorithm originally developed for the Coulomb potential [J. M. Pérez-Jordá, Phys. Rev. B 58, 1230 (1998)]. The method is tested for the radial Schrödinger equation in the hydrogen atom, resulting in micro-Hartree accuracy or better for the energy of ns and np orbitals (with n up to 5) using expansions of moderate length.
Image dipoles approach to the local field enhancement in nanostructured Ag–Au hybrid devices
Authors: David Christin , Richter Marten , Knorr Andreas , Weidinger Inez M. , Hildebrandt Peter
Published: January 2010

We have investigated the plasmonic enhancement in the radiation field at various nanostructured multilayer devices that may be applied in surface enhanced Raman spectroscopy. We apply an image dipole method to describe the effect of surface morphology on the field enhancement in a quasistatic limit. In particular, we compare the performance of a nanostructured silver surface and a layered silver-gold hybrid device. It is found that localized surface plasmon states provide a high field enhancement in silver-gold hybrid devices, where symmetry breaking due to surface defects is a supporting factor. These results are compared to those obtained for multishell nanoparticles of spherical symmetry. Calculated enhancement factors are discussed on the background of recent experimental data.
The complete iodine and nitrogen nuclear electric quadrupole coupling tensors for fluoroiodoacetonitrile determined by chirped pulse Fourier transform microwave spectroscopy
Authors: Grubbs G. S. , Kadiwar G. , Bailey W. C. , Cooke S. A.
Published: January 2010

Molecular pulsed jet, chirped-pulse Fourier transform microwave spectroscopy has been used to record 499 transitions for the title molecule. Measurements have been made in the 8–16 GHz regions. Vibrational and electronic ground state rotational constants A, B, and C have been obtained, together with centrifugal distortion terms. The complete iodine and nitrogen nuclear quadrupole coupling tensors have been determined for the first time. Quantum chemical calculations have been performed to aid with analyses and, in particular, to aid in determining the signs of the off-diagonal components of the nuclear quadrupole coupling tensors. An experimentally determined relative electronegativity scale for several polyhalomethyl groups is proposed.