Local Charge Separation Research Articles

Structural and Dielectric Properties of CdS Nanoparticle Embedded Polyurethane Nanocomposite Samples

AbstractThe polymer nanocomposite samples have been prepared by solution embedding of CdS nanoparticles in polyurethane (PU). The structural properties have been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM and AFM images represent the dispersion of CdS nanoparticles in a polymer matrix. The permittivity of nanocomposite samples increases with temperature due to formation of new dipoles or accumulation of charge carriers in a nanoparticle‐‐polymer interface. The increase in permittivity with temperature can be attributed to the heat‐assisted dipole moment. Furthermore, it has been observed that permittivity is the function of nanoparticle concentration, temperature, and frequency. The high concentration of nanoparticles lowers permittivity with frequency. The positive and negative tangential loss has been observed in PU‐‐CdS nanocomposite samples. The negative loss phenomenon could be understood in terms of a local space charge separation. The phase transition of PU and PU‐‐CdS nanocomposites has been studied by using a thermally stimulated discharge current technique. © 2012 Wiley Periodicals, Inc. Adv Polym Techn 32: E274–E286, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21273

Dynamic subauroral ionospheric electric fields observed by the Falkland Islands radar during the course of a geomagnetic storm

[1] We present an analysis of ionospheric electric field data observed during a geomagnetic storm by the recently deployed HF radar located on the Falkland Islands. On 3 August 2010 at ∼1800 UT evidence of the onset of a geomagnetic storm was observed in ground magnetometer data in the form of a decrease in the Sym-H index of ∼100 nT. The main phase of the storm was observed to last ∼24 hours before a gradual recovery lasting ∼3 days. On 4 August, during the peak magnetic disturbance of the storm, a high velocity (>1000 m s−1) channel of ionospheric plasma flow, which we interpret as a subauroral ion drift (SAID), located between 53° and 58° magnetic south and lasting ∼6.5 hours, was observed by the Falkland Islands radar in the pre-midnight sector. Coincident flow data from the DMSP satellites and the magnetically near-conjugate northern hemisphere Blackstone HF radar reveal that the SAID was embedded within the broader subauroral polarization streams (SAPS). DMSP particle data indicate that the SAID location closely followed the equatorward edge of the auroral electron precipitation boundary, while remaining generally poleward of the equatorward boundary of the ion precipitation. The latitude of the SAID varied throughout the interval on similar timescales to variations in the interplanetary magnetic field and auroral activity, while variations in its velocity were more closely related to ring current dynamics. These results are consistent with SAID electric fields being generated by localized charge separation in the partial ring current, but suggest that their location is more strongly governed by solar wind driving and associated large-scale magnetospheric dynamics.

Effects of ion solvation on phase equilibrium and interfacial tension of liquid mixtures

We study the bulk thermodynamics and interfacial properties of electrolyte solution mixtures by accounting for electrostatic interaction, ion solvation, and inhomogeneity in the dielectric medium in the mean-field framework. Difference in the solvation energy between the cations and anions is shown to give rise to local charge separation near the interface, and a finite Galvani potential between two coexisting solutions. The ion solvation affects the phase equilibrium of the solvent mixture, depending on the dielectric constants of the solvents, reflecting the competition between the solvation energy and translation entropy of the ions. Miscibility is decreased if both solvents have low dielectric constants and is enhanced if both solvents have high dielectric constant. At the mean-field level, the ion distribution near the interface is determined by two competing effects: accumulation in the electrostatic double layer and depletion in a diffuse interface. The interfacial tension shows a nonmonotonic dependence on the salt concentration: it increases linearly with the salt concentration at higher concentrations and decreases approximately as the square root of the salt concentration for dilute solutions, reaching a minimum near 1 mM. We also find that, for a fixed cation type, the interfacial tension decreases as the size of anion increases. These results offer qualitative explanations within one unified framework for the long-known concentration and ion size effects on the interfacial tension of electrolyte solutions.

Nanoimaging of localized plasmon-induced charge separation

Distribution of the plasmon-induced charge separation sites at the Ag nanorod-TiO(2) interface is visualized by AFM. The charge separation sites are localized in almost the same way as electric fields around the nanorods, indicating that the charge separation is induced or promoted by the localized electric fields.

Fluctuation in electrolyte solutions: The self energy

We address the issue of the self energy of the mobile ions in electrolyte solutions within a general Gaussian renormalized fluctuation theory using a field-theoretic approach. We introduce the Born radii of the ions in the form of a charge distribution allowing for different Born radii between the cations and anions. The model thus automatically yields a theory free of divergences and accounts for the solvation of the ions at the level of continuous dielectric media. In an inhomogeneous dielectric medium, the self energy is in general position dependent and differences in the self energy between cations and anions can give rise to local charge separation in a macroscopically neutral system. Treating the Born radius a as a smallness parameter, we show that the self energy can be split into an O(a(-1)) nonuniversal contribution and an O(a0) universal contribution that depends only on the ion concentration, valency, and the spatially varying dielectric constant. For a weakly inhomogeneous dielectric medium, the nonuniversal part of the self energy is shown to have the form of the Born energy with the local dielectric constant. This self energy is incorporated into the Poisson-Boltzmann equation as a simple means of including this local fluctuation effect in a mean-field theory. We illustrate the phenomenon of charge separation by considering cations and anions of difference sizes and valencies in a periodic dielectric medium.

Effect of Radiant Energy on Near-Surface Water

While recent research on interfacial water has focused mainly on the few interfacial layers adjacent to the solid boundary, century-old studies have extensively shown that macroscopic domains of liquids near interfaces acquire features different from the bulk. Interest in these long-range effects has been rekindled by recent observations showing that colloidal and molecular solutes are excluded from extensive regions next to many hydrophilic surfaces [Zheng and Pollack Phys. Rev. E 2003, 68, 031408]. Studies of these aqueous "exclusion zones" reveal a more ordered phase than bulk water, with local charge separation between the exclusion zones and the regions beyond [Zheng et al. Colloid Interface Sci. 2006, 127, 19; Zheng and Pollack Water and the Cell: Solute exclusion and potential distribution near hydrophilic surfaces; Springer: Netherlands, 2006; pp 165-174], here confirmed using pH measurements. The main question, however, is where the energy for building these charged, low-entropy zones might come from. It is shown that radiant energy profoundly expands these zones in a reversible, wavelength-dependent manner. It appears that incident radiant energy may be stored in the water as entropy loss and charge separation.

Repulsive exciton-exciton interaction in quantum dots

Biexcitons localized in single InAs/GaAs quantum dots (QD's) are investigated by cathodoluminescence, demonstrating an anticorrelation of the biexciton binding energy and the exciton transition energy. The binding energy decreases with increasing transition energy changing its sign at about 1.24 eV. The ``binding'' to ``antibinding'' transition is attributed to three-dimensional confinement, quenching correlation, and exchange and causing local charge separation. Model calculations of the biexciton in truncated InAs/GaAs QD's demonstrate the observed trend to result from the decreasing number of localized excited states with decreasing QD size. The interaction with resonant states in the wetting layer is found to be negligible.

Cosmological magnetogenesis driven by radiation pressure

The origin of large scale cosmological magnetic fields remains a mystery, despite the continuous efforts devoted to that problem. We present a new model of magnetic field generation, based on local charge separation provided by an anisotropic and inhomogeneous radiation pressure. In the cosmological context, the processes we explore take place at the epoch of the reionization of the Universe. Under simple assumptions, we obtain results (i) in terms of the order of magnitude of the field generated at large scales and (ii) in terms of its power spectrum. The amplitudes obtained $(B\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremath{\mu}\mathrm{G})$ are considerably higher than those obtained in usual magnetogenesis models and provide suitable seeds for amplification by adiabatic collapse and/or dynamo during structure formation.

Shape-Dependent Exciton Dynamics in InGaAs/GaAs Quantum Dots

The influence of local charge separation in self-organized In(Ga)As/GaAs quantum dots (QDs) on the exciton dynamics under resonant excitation of the confined exciton in the first excited state is investigated by time-resolved photoluminescence spectroscopy. The oscillator strength and the relaxation rate are shown to depend oppositely on the charge separation, which again depends on the shape and composition profile. A pronounced phonon bottleneck effect is demonstrated for excitons in flat, truncated QDs, whereas recombination-limited dynamics are observed for QDs favouring local charge separation. The results point out possible pathways for optimizing such QDs for device applications.

Ion Confinement in a Linear Plasma Column

It is shown by means of PIC (particle-in-cell) simulations that in magnetized plasmas with pressure gradients local charge separation occurs due to different ion and electron gyration radii. The dimensionless ratio χ = nplm/(ϵ0B2) being the dielectric susceptibility (with the plasma density npl, the mass m and the magnetic field strength B) is the main parameter in the analysis.

Ion Confinement in a Linear Plasma Column

It is shown by means of PIC (particle-in-cell) simulations that in magnetized plasmas with pressure gradients local charge separation occurs due to different ion and electron gyration radii. The dimensionless ratio χ = nplm/(ϵ0B2) being the dielectric susceptibility (with the plasma density npl, the mass m and the magnetic field strength B) is the main parameter in the analysis.

Boundary structure of low‐energy ions associated with the nightside convection reversal

The low‐energy H+ data with 6‐s resolution from the retarding ion mass spectrometer instrument on DE 1 have been used to reveal an ion boundary structure at midaltitudes that is associated with the nightside convection reversal. The convection reversal provides a free energy source for the perpendicular ion heating that generates ion conics. A detailed analysis of a Y shape distribution observed in the spin‐spectrogram (Plates 1‐4) shows that the altitude of the ion heating decreases monotonically as latitude decreases along the satellite orbit. This suggests that the convection boundary makes an angle of approximately 3° to 10° with the magnetic field line. Near the convection reversal, the nonlinear transverse heating leads to a local charge separation, which forms a double layer along the boundary. The observations of dc electric field are fitted by a model with a Gaussian charge distribution.

Parity selection rule for electron tunneling matrix element. Quantum oscillations in delayed fluorescence of hydrated chlorophyll

The submicrosecond oscillatory delayed fluorescence observed in non-polar solutions of hydrated chlorophyll a is attributed to H 2O·Chl-a·OH 2 in which a water molecule is bound to the Mg atom on each side of the Chl-a macrocycle. The observed quantum oscillations are described in terms of symmetry selection rules for electron tunneling matrix elements and electron shuttling between bonding and antibonding states in a photoexcited system possessing nearly degenerate localized charge separation states. The transfer matrix element, J, between these states is estimated to be 0.67 × 10 −8 eV.

On the Dynamics of Langmuir Wave Soliton

Various descriptions for the one-dimensional Langmuir wave soliton are classified according to the relations among various smallness parameters. Four different regimes are found to emerge depending on the relative importance of various forces acting on the electrons and are graphically represented in the parameter space. When the field energy becomes as large as the electron kinetic energy, second harmonic generation and local charge separation (in slow time scale) are found to occur, but their effect on the description of the Langmuir wave soliton is found insignificant. Dynamic behavior of the Langmuir wave soliton is discussed in terms of the trajectory in the parameter space and is compared with recent experimental and numerical analyses.