Increasing Rod Length Research Articles

Lateral charge migration in 1D semiconductor-metal hybrid photocatalytic systems.

Colloidal nanorods based on CdS or CdSe, functionalized with metal particles, have proven to be efficient catalysts for light-driven hydrogen evolution. Seeded CdSe@CdS nanorods have shown increasing performance with increasing rod length. This observation was rationalized by the increasing lifetime of the separated charges, as a large distance between holes localized in the CdSe seed and electrons localized at the metal tip decreases their recombination rate. However, the impact of nanorod length on the electron-to-tip localization efficiency or pathway remained an open question. Therefore, we investigated the photo-induced electron transfer to the metal in a series of Ni-tipped CdSe@CdS nanorods with varying length. We find that the transfer processes occurring from the region close to the semiconductor-metal interface, the rod region, and the CdSe seed region depend in different ways on the rods' length. The rate of the fastest process from excitonic states generated directly at the interface is independent of the rod length, but the relative amplitude decreases with increasing rod length, as the weight of the interface region is decreasing. The transfer of electrons to the metal tip from excitons generated in the CdS rod region depends strongly on the length of the nanorods, which indicates an electron transport-limited process, i.e., electron diffusion toward the interface region, followed by fast interface crossing. The transfer originating from the CdSe excitonic states again shows no significant length dependence in its time constant, as it is probably limited by the rate of overcoming the shallow confinement in the CdSe seed.

Lightning Current Performance of Conventional and Enhanced Rod Ground Electrodes

The correct operation of surge arresters under lightning and switching transient conditions requires that grounding be effective at high frequencies as well as at power system frequency. Effective high frequency grounding also limits the electrocution risk to substation personnel due to transient ground potential rise. This article assesses the performance of high frequency electrodes at typical lightning impulse current magnitudes. The enhancement of such electrodes by addition of bonded horizontal arms in cross and star arrangement is also investigated. The effective impulse resistances for electrodes having lengths up to 4.8 m are calculated over a range of impulse current magnitudes at the same location. The results obtained indicate that, for low impulse current magnitudes, the preionization electrode's resistance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> ) falls with increasing rod length, a behavior reflected in the measured dc resistance. The presence of horizontal enhancement was found to reduce <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> in all cases. The occurrence of soil ionization in the immediate vicinity of the electrode resulted in a reduced postionization resistance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) at higher currents, tending toward a common asymptotic value independent of both the length of the rod and the presence of electrode enhancements. The observed behavior is supported by numerical simulation of electric field and current density distributions, indicating that the high current performance of a grounding rod is heavily influenced by soil ionization and breakdown in high-field regions at the electrode extremities.

Diffusion of Thin Nanorods in Polymer Melts.

The diffusion of monomerically thin nanorods in polymer melts is studied by molecular dynamics simulations. We focus on the systems where chains are long enough to screen the hydrodynamic interactions such that the diffusion coefficient D ∥ for the direction parallel to the rod decreases linearly with increasing rod length l. In unentangled polymers, the diffusion coefficient for the direction normal to the rod exhibits a crossover from D ⊥ ~ l -2 to D ⊥ ~ l -1 with increasing l, corresponding to a progressive coupling of nanorod motion to the polymers. Accordingly, the rotational diffusion coefficient D R ≈ D ⊥ l -2 ~ l -4 and then D R ~ l -3 as l increases. In entangled polymers, D ⊥ and D R are suppressed for l larger than the entanglement mesh size a. D ⊥ ~ l -3 and D R ~ l -5 for l sufficiently above a in agreement with de Gennes' rod reptation model.

Light Adaptation in Phycobilisome Antennas: Influence on the Rod Length and Structural Arrangement

Phycobilisomes, the light-harvesting antennas of cyanobacteria, can adapt to a wide range of environments thanks to a composition and function response to stress conditions. We study how structural changes influence excitation transfer in these supercomplexes. Specifically, we show the influence of the rod length on the photon absorption and subsequent excitation transport to the core. Despite the fact that the efficiency of individual disks on the rod decreases with increasing rod length, we find an optimal length for which the average rod efficiency is maximal. Combining this study with experimental structural measurements, we propose models for the arrangement of the phycobiliproteins inside the thylakoid membranes, evaluate the importance of rod length, and predict the corresponding transport properties for different cyanobacterial species. This analysis, which links the functional and structural properties of full phycobilisome complexes, thus provides further rationales to help resolve their exact structure.

Dissipative Particle Dynamics Simulation on Self‐Assembly Behavior of Rod–Coil–Rod Triblock Copolymer in Solutions

Self‐assembly behaviors of rod–coil–rod triblock copolymers in the selective solvent are systematically investigated by dissipative particle dynamic simulations. Three selective solvents are considered: the pure coil‐selective solvent, the pure rod‐selective solvent, and the mixed solvent. The concentration‐induced morphologies and morphological transition affected by the rod and coil length are examined. The micelle adopts the overall shape of sphere, nematic bundle, worm, cylinder, lamella, coil‐, and rod‐aggregated hollow cylinders, and network. In the coil‐selective solvent, increasing coil length can defer the phase transition from sphere to other morphologies while increasing rod length can advance the transition. In the rod‐selective solvent, an opposite influence rule is found.

Length tunable penta-twinned palladium nanorods: seedless synthesis and electrooxidation of formic acid.

Palladium nanorods with controlled lengths from 100 to 500 nm and a fixed width of 20 nm were synthesized for the first time by a seedless approach. These rods show higher peak current densities than Pd cubes for formic acid oxidation and the catalytic activity decreases with increasing rod length.

The phase transition of ɛ-InxFe2−xO3 nanomagnets with a large thermal hysteresis loop (invited)

A large thermal hysteresis loop was observed in the phase transition on rod-shaped ɛ-InxFe2−xO3 (x ∼ 0.04) nanomagnets. The width of the thermal hysteresis loop, ΔT, increased with increasing rod length (l), i.e., ΔT = 6 K (l = 25 nm), 14 K (40 nm), 25 K (80 nm), and 47 K (170 nm). The observed ΔT value of 47 K is one of the largest values among insulating ferromagnetic materials. The thermal hysteresis loops were analyzed by the Slichter and Drickamer model, and the results showed that the transition enthalpy and entropy do not change. However, the elastic interaction parameter between the transition sites increases with an increasing l value. Maybe the correlation length of a propagating phonon due to elastic interaction competes with the rod length of the samples, causing the rod-length dependence of the thermal hysteresis loop.

Cavitation and Crazing in Rod-Containing Nanocomposites

Addition of nanoparticles to a polymer can drastically affect the mechanical properties and structure of the nanocomposite. Here we investigate the nucleation and growth of voids that precede craze formation and the early crazing itself by use of coarse-grained Monte Carlo and molecular dynamics simulations. We investigate the role of deformation rate, local density, local rod orientation, nonaffine displacements, and local elastic moduli. We observe that for both pure polymers and nanocomposites, regions of low local elastic modulus are more prone to failure. Additionally, it is found that Voronoi volume can anticipate void formation and that it is also a predictor of failure, particularly in composites. After the onset of crazing, it is found that nanorods incorporated into the crazes rapidly orient themselves to match the direction of the polymer fibrils, but nanorods in bulk regions remain randomly oriented. We further find that attractive rods resist incorporation into the developing crazes and that this effect is stronger with increasing rod length.

Morphology and internal structure control of rod–coil copolymer aggregates by mixed selective solvents

The molecular packing of rod–coil copolymers, which relates closely to their aggregate morphology, significantly influences their optoelectronic properties. The morphological phase diagram of rod–coil block copolymers in mixed selective solvents and the internal structures of the resulting aggregates are studied by using dissipative particle dynamics. The coil-selective solvent is mixed well with the rod-selective solvent and the effective solvent quality can be tuned by their relative amount. The diverse morphologies and morphological transitions of structural aggregates induced by mixed solvents depend on the mixed solvent composition, rod-block length, and copolymer concentration. Our simulation results show that while the overall morphology of the aggregate is controlled primarily by the rod length and polymer concentration, the domain morphology within the aggregate, as well as the degree of local rod alignment, are primarily controlled by the rod length and solvent quality. Specifically, we observe that spheres, cylinders, perforated sheets, and three-dimensional networks are sequentially formed with increasing rod length or copolymer concentration irrespective of the solvent quality. On the other hand, changing the solvent quality influences the morphology within an aggregate both by determining any preference for the exposed surface of the aggregate, and also because the rod-selective solvent tends to disrupt the orderly packing of the rod blocks. The range of morphologies within the aggregate is even richer, and interesting structures, such as an inverted vesicle, a structural sphere, an inverted cylindrical vesicle, a segmented cylinder, and a segmented network are observed. Unique segmented-like structures exhibiting the most ordered conformation in the rod domains appear when roughly equal amounts of rod- and coil-selective solvents are used.

Energy Efficiency for Standard Penetration Tests

The need for standardization of the measured blow count number Nspt into a normalized reference energy value is now fully recognized. The present paper extends the existing theoretical approach using the wave propagation theory as framework and introduces an analysis for large displacements enabling the influence of rod length in the measured Nspt values to be quantified. The study is based on both calibration chamber and field tests. Energy measurements are monitored in two different positions: below the anvil and above the sampler. Both experimental and numerical results demonstrate that whereas the energy delivered into the rod stem is expressed as a ratio of the theoretical free-fall energy of the hammer, the effective sampler energy is a function of the hammer height of fall, sampler permanent penetration, and weight of both hammer and rods. Influence of rod length is twofold and produces opposite effects: wave energy losses increase with increasing rod length and in a long rod composition the gain in potential energy from rod weight is significant and may partially compensate measured energy losses. Based on this revised approach, an analytical solution is proposed to calculate the energy delivered to the sampler and efficiency coefficients are suggested to account for energy losses during the energy transference process.

Optical properties of coupled metallic nanorods for field-enhanced spectroscopy

The optical properties of coupled metallic nanorods are studied to investigate the use of coupled plasmonic structures in field-enhanced spectroscopies. Light scattering by coupled nanorods is calculated with the boundary element method, including retardation. The modes of coupled nanorod systems are calculated by the boundary charge method and discussed in terms of their symmetry. Similar scattering behavior for isolated nanorods and pairs of nanorods can mask the very different local responses that produce near-field enhancement. The response of isolated rods redshifts with increasing rod length because intrarod restoring forces are reduced. The near- and far-field responses increase monotonically with increasing rod length increasing polarization along the rod. For coupled nanorods, coupling localizes charge at the gap between the rod ends and splits degenerate modes. The localized charge depolarizes the intrarod response and provides an additional redshift. Moreover, the near-field enhancement in the gap between the nanorods is dramatically increased by coupling-induced charge localization at the gap. For short nanorods, the near-field response in coupled systems is determined by the geometry of the rod ends that define the gap. For longer nanorods, the response in coupled systems is determined by the rod length. Changing the dimensions and geometry of the nanorods to modify the interrod coupling has a major effect on the local-field enhancement. The effects of the environment and the actual metallic material do not have as big an influence on the field enhancement.

Electromechanical stiffening of rods and tubes

Electrostatic interactions are shown to exert a significant effect on the buckling instability of a rod. In particular, the threshold value of the compressional force needed to induce buckling is found to be independent of rod length for long charged rods. In the case of rods of intermediate length, the critical buckling force crosses over from the classic inverse-square length dependence to asymptotic length-independent form with increasing rod length. It is suggested that this effect leads to the possibility of electromechanical stiffening of nanotubes, which would allow relatively long segments of them to be used as atomic force probes.

Shape control of III-V semiconductor nanocrystals: synthesis and properties of InAs quantum rods.

A novel approach for synthesis of soluble semiconductor quantum rods using metal nanoparticles to direct and catalyze one-dimensional growth is developed. The method is useful in particular for III-V semiconductors with cubic lattice, where the utilization of surfactant-controlled rod-growth is not easily realized. The growth takes place via the solution-liquid-solid (SLS) mechanism where proper precursors are injected into a coordinating solvent. Centrifugation is used for separation of rod-fractions with different lengths. The reaction is demonstrated for InAs, InP and GaAs. Focusing on InAs rods as a model system, we examined the effects of the type of metal catalyst, and the tuning of reaction conditions with respect to temperature, concentration, catalyst content and reaction time. Within the three types of metal catalysts used--Au, Ag and In, Au was found to provide the best control for achieving rod-growth even though the melting point of bulk gold is significantly higher then the reaction temperature. The structural properties of the rods were characterized by transmission electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. Rods have a cubic lattice and grow mainly along the [111] direction. The relative gold content decreases in shorter rods suggesting Au depletion as a cause for limiting the growth. Room and low temperature absorption and photoluminescence measurements show that the band-gap shifts to the red upon increasing rod length revealing strong quantum confinement along the long axis in InAs rods, providing spectral coverage of the near-IR range relevant for telecommunication applications. Emission intensity also decreases with increased rod-length. These length dependent properties manifest the transition from 0D to 1D quantum confined systems.

Enhanced stability of layered phases in parallel hard spherocylinders due to addition of hard spheres

There is increasing evidence that entropy can induce microphase separation in binary fluid mixtures interacting through hard particle potentials. One such phase consists of alternating two-dimensional liquidlike layers of rods and spheres. We study the transition from a uniform miscible state to this ordered state using computer simulations, and compare results to experiments and theory. We conclude the following: (1) There is stable entropy driven microphase separation in mixtures of parallel rods and spheres. (2) Adding spheres smaller than the rod length decreases the total volume fraction needed for the formation of a layered phase, and therefore small spheres effectively stabilize the layered phase; the opposite is true for large spheres. (3) The degree of this stabilization increases with increasing rod length.

Mesophasen in gemischten Tensidsystemen / Mesophases in mixed surfactant systems

Mixed surfactant solutions of the zwitterionic surfactant C 14 DMAO, the anionic surfactant SDS and the co-surfactant hexanol were studied in the whole molar mixing ratio. The single phase region of the dilute L α -phases was established. The lowest surfactant concentration for which the L α -phase is formed depends on the mole fraction of the anionic surfactant X a and shifts with increasing X a to higher surfactant concentrations. For a 100 mM surfactant solution, the L α -phase disappears for X a values above 0.3. At the overall surfactant concentration 100 mM the phase diagram of the quaternary surfactant system C 14 DMAO/SDS/hexanol/water is given. The phase boundaries of the L 1 -phase were determined by conductivity measurements. Conductivity decreases markedly in the two-phase L 1 /L α -region which contains vesicles. The strongly synergistic behaviour between C 14 DMAO and SDS is also observed in the presence of hexanol and is reflected in a minimum of the surface and interfacial tensions with increasing X a . The synergism is still marked at the phase boundary of the L 1 -phase in the presence of hexanol. The surface and interfacial tension values decrease continuously with the addition of hexanol and the interfacial tension reaches a value of 0.15 mN/m for X a ≤ 0.6 at the phase boundary. The phase boundary of the L 1 -phase is shifted to higher hexanol concentrations with increasing mole fractions of SDS. The addition of hexanol to slightly charged solutions leads to increasing rod length and zero shear viscosities (η°). The structural relaxation times and the η° values reach their highest values at the L 1 -phase boundary for X a = 0.15. Four different relaxation processes with different signs were observed with electric birefringence investigations as a function of the hexanol concentration and of the mixing ratio of C 14 DMAO and SDS. The first, third and fourth processes have a negative sign, the second process a positive sign. The 100 mM surfactant mixtures show the four electric birefringence signals between 0 < X a < 0.6 as a function of the hexanol concentration. In the L 1 /L α -two-phase region we observe for a constant surfactant concentration c° and constant X a micellar rods in the coexisting L 1 -phase and the anomaly of the electric birefringence. With increasing hexanol concentrations in the two phase solutions the length of the rod-like micelles decreases and for higher hexanol concentrations only globular micelles are found.

Orientational ordering in lipid monolayers: A two-dimensional model of rigid rods grafted onto a lattice

A simple model for lipid monolayers on water surfaces at high spreading pressure is investigated in this work. In this model, the hydrophilic head group of the lipid molecules form a rigid regular triangular lattice, and the hydrophobic alkane chains (assumed to be in an all-trans state) are represented by rigid rods with two angular degrees of freedom (θ, φ). The rods consist of ‘‘effective monomers,’’ and between the effective monomers on neighboring rods a Lennard-Jones interaction is assumed. The model is studied by exact ground-state calculations, mean-field theory, and Monte Carlo simulations. Basic parameters are rod length a and lattice constant b. The ground-state phase diagram shows the following phases: for small b, the rods are oriented perpendicularly to the monolayer plane (no-tilt phase, 〈θ〉=0); for somewhat larger b, a sixfold degenerate uniform-tilt state occurs with all rods tilted towards one of their next-nearest neighbors. For still larger b, the rods are tilted nonuniformly and form a ‘‘striped’’ structure. These unexpected phases do not occur if we allow a rectangular distortion of the lattice. For T&amp;gt;0, the simplest mean-field theory predicts a gradual disordering of the uniform-tilt state via a second-order phase transition. For the transition region, the Monte Carlo results disagree with this picture. Instead they show a strong asymmetric first-order phase transition with pronounced hysteresis. The transition temperature increases with increasing rod length a, qualitatively similar to experiment.

Magneto-optical effect in anodized Al2O3 film with micropores electrodeposited with Co

We have electrodeposited Co into the micropores of an anodic Al2O3 coating on Al, making Co rods of about 120 Å in diameter and 0.3–1 μm in length. The Al2O3 is amorphous, while the Co is crystalline (hexagonal form) with no preferred orientation exhibiting saturation magnetization 80% of that of bulk Co. The composite Al2O3/Co film shows prominent magnetic anisotropy perpendicular to the surface due to the shape anisotropy of the Co rods, having coercive force of ∼1.2 kOe. The Kerr rotation and the reflectivity of the films at λ=0.63 μm, Θ̂k =±15′ to ±50′, and R=5 to 15% varied from sample to sample due to interference effect of light in the films. The values of θk and R change irreversibly when heated above ∼150 °C. Stripping the Al2O3-Co film from the Al substrate by dissolving the Al, we have measured the Faraday effect and light transmittance. They increase linearly with increasing rod length, giving Θ̂f(rotation) =−0.41 deg/μm, Ψ̂f(ellipticity) =1.14 deg/μm, and α̂(absorption coefficient)=1.1 μm−1. These values conform with what is expected from the effective dielectric tensor, including off-diagonal elements, which we [Jpn. J. Appl. Phys. 23, 1580 (1984)] have formulated for composite materials containing magnetic fine particles or rods.