Electrostatic Potential Profile Research Articles

Transport Measurements of Strongly Correlated Electrons on Helium in a Classical Point-Contact Device

We present transport measurements of electrons on the surface of liquid helium in a microchannel device in which a constriction may be formed by a split-gate electrode. The surface electron current passing through the microchannel first decreases and is then completely suppressed as the split-gate voltage is swept negative. The current decreases in a steplike manner, due to changes in the number of electrons able to pass simultaneously through the constriction. We investigate the dependence of the electron transport on the AC driving voltage and the DC potentials applied to the sample electrodes, in order to understand the electrostatic potential profile of the constriction region. Our results are in good agreement with a finite element modeling analysis of the device. We demonstrate that the threshold of current flow depends not only on the applied potentials but also on the surface electron density. The detailed understanding of the characteristics of such a device is an important step in the development of mesoscopic experiments with surface electrons on liquid helium.

Structure of Colloidal Solution in Presence of Mixed Electrolytes: A Solvent Restricted Primitive Model Study

The structure of colloidal solution in presence of mixed electrolytes is studied using Monte Carlo simulation and density functional theory, based on a four-component model of the spherical double layer. In this model the ions and solvent molecules are treated as charged and neutral hard spheres, respectively, having equal diameter, and in addition the mixture of mono- and multivalent co-ions are also considered. The macroion is considered as a uniformly charged hard sphere surrounded by the electrolyte and the solvent. The density functional theory is based on a partially perturbative scheme, where the electrical part is calculated through perturbation with respect to uniform density and the hard sphere contribution is approximated using a weighted density approach. The theory is found to be in quantitative agreement with the Monte Carlo simulation results, for singlet density as well as the mean electrostatic potential profiles. The system is studied over a wide range of parametric conditions, viz. with different ionic valences as well as size, at varying electrolyte concentration ratio of mono- and multivalent co-ions of mixed electrolytes, at different surface charge densities, and radius of the macroion. The present work reflects that even a simple primitive model for the solvent is able to manipulate the hard-sphere and electrostatic correlations of the diffuse double layer in the ionic density as well as mean electrostatic potential profiles.

Chiral states in bilayer graphene: Magnetic field dependence and gap opening

At the interface of electrostatic potential kink profiles one dimensional chiral states are found in bilayer graphene (BLG). Such structures can be created by applying an asymmetric potential to the upper and the lower layer of BLG. We found that: i) due to the strong confinement by the single kink profile the uni-directional states are only weakly affected by a magnetic field, ii) increasing the smoothness of the kink potential results in additional bound states which are topologically different from those chiral states, and iii) in the presence of a kink-antikink potential the overlap between the oppositely moving chiral states results in the appearance of crossing and anti-crossing points in the energy spectrum. This leads to the opening of tunable minigaps in the spectrum of the uni-directional topological states.

Measurement of electric field across individual wurtzite GaN quantum dots using electron holography

Electrostatic potential profiles across wurtzite AlN/GaN quantum dot (QD) superlattices grown by molecular beam epitaxy have been measured using off-axis electron holography. The profiles for individual GaN QDs show large phase shifts which can be understood in terms of spontaneous polarization and piezoelectric fields and the accumulation of positive and negative charge at the GaN/AlN interfaces. An electric field with magnitude of ∼7.8 ± 2 MV/cm was measured across the center of a GaN QD, in reasonable agreement with reported simulations.

Poisson–Boltzmann Theory of pH-Sensitive (Annealing) Polyelectrolyte Brush

We present a self-consistent field analytical theory of a polymer brush formed by weakly charged pH-sensitive (annealing) polyelectrolytes tethered to a solid-liquid interface and immersed in buffer solution of low molecular weight salt. We use the Poisson-Boltzmann framework, applied by us previously to polyelectrolyte (PE) brushes with quenched charge (Zhulina, E. B.; Borisov, O. V. J. Chem. Phys. 1997, 107, 5952). This approach allows for detailed analysis of the internal structure of annealing PE brush in terms of polymer density distribution, profiles of electrostatic potential and of local degree of chain ionization as a function of buffer ionic strength and pH without any assumptions on mobile ion distribution imposed in earlier scaling-type models. The presented analytical theory recovers all major asymptotic dependences for average brush properties predicted earlier. In particular, a nonmonotonic dependence of brush thickness on ionic strength and grafting density is confirmed and specified with accuracy of numerical coefficients including crossover regions. Moreover, the theory predicts qualitatively new effects, such as, e.g., disproportionation of tethered polyions into weakly charged concentrated proximal and strongly charged sparse distal brush domains at low salt and moderate grating densities. The presented results allow us to quantify responsive features of annealing PE brushes whose large-scale and local conformational properties can be manipulated by external stimuli.

Molecular dynamics simulations of ion transport through carbon nanotubes. II. Structural effects of the nanotube radius, solute concentration, and applied electric fields

The reported work extends previously published research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the solution structuring are investigated in terms of spatial density distributions, pair distribution functions, and electrostatic potential profiles. Several simulated structural features are consistent with general theoretical results of nanofluidics and can be interpreted fairly well with respect to these (such as the Donnan-type voltages established at the channel apertures depending on the logarithm of the maximum ion concentration). The simulated properties are based on averages over the largest data collection times reported in the literature (0.8 μs), providing accurate estimates of the measured quantities.

Oxygen-deficient metabolism and corneal edema

Wear of low-oxygen-transmissible soft contact lenses swells the cornea significantly, even during open eye. Although oxygen-deficient corneal edema is well-documented, a self-consistent quantitative prediction based on the underlying metabolic reactions is not available. We present a biochemical description of the human cornea that quantifies hypoxic swelling through the coupled transport of water, salt, and respiratory metabolites. Aerobic and anaerobic consumption of glucose, as well as acidosis and pH buffering, are incorporated in a seven-layer corneal model (anterior chamber, endothelium, stroma, epithelium, postlens tear film, contact lens, and prelens tear film). Corneal swelling is predicted from coupled transport of water, dissolved salts, and especially metabolites, along with membrane-transport resistances at the endothelium and epithelium. At the endothelium, the Na+/K+ - ATPase electrogenic channel actively transports bicarbonate ion from the stroma into the anterior chamber. As captured by the Kedem-Katchalsky membrane-transport formalism, the active bicarbonate-ion flux provides the driving force for corneal fluid pump-out needed to match the leak-in tendency of the stroma. Increased lactate-ion production during hypoxia osmotically lowers the pump-out rate requiring the stroma to swell to higher water content. Concentration profiles are predicted for glucose, water, oxygen, carbon dioxide, and hydronium, lactate, bicarbonate, sodium, and chloride ions, along with electrostatic potential and pressure profiles. Although the active bicarbonate-ion pump at the endothelium drives bicarbonate into the aqueous humor, we find a net flux of bicarbonate ion into the cornea that safeguards against acidosis. For the first time, we predict corneal swelling upon soft-contact-lens wear from fundamental biophysico-chemical principles. We also successfully predict that hypertonic tear alleviates contact-lens-induced edema.

Electroosmotic Velocity and Electric Conductivity in a Fibrous Porous Medium in the Transverse Direction

The steady electroosmosis and electric conduction in a fibrous medium constructed by a homogeneous array of parallel, identical, charged, circular cylinders filled with an electrolyte solution is analytically examined. The imposed electric field is constant and normal to the axes of the cylinders. The electric double layer surrounding each dielectric cylinder may have an arbitrary thickness relative to the radius of the cylinder. A unit cell model that allows for the overlap of the double layers of adjacent cylinders is employed. The electrokinetic equations that govern the ionic concentration distributions, the electrostatic potential profile, and the fluid flow field in the electrolyte solution surrounding the charged cylinder in a cylindrical cell are linearized assuming that the system is only slightly distorted from equilibrium. Through the use of a regular perturbation method, these linearized equations are solved with the surface charge density (or zeta potential) of the cylinder as the small perturbation parameter. Analytical expressions for the electroosmotic velocity of the fluid solution and the effective electric conductivity in the array of cylinders are obtained in closed forms as functions of the porosity of the fiber matrix and other characteristics of the porous system. Comparisons of the results of the cell model with different conditions at the outer boundary of the cell are made. The cell model predicts that, under otherwise identical conditions, the electric conductivity in a porous medium composed of an array of parallel cylinders in the transverse direction in general is smaller than that of a suspension of spheres, but there are some exceptions. The effect of interactions among the cylinders or spheres on the effective conductivity can be significant under appropriate conditions.

Cutoff radius effect of the isotropic periodic sum and Wolf method in liquid–vapor interfaces of water

As a more economical but similarly accurate computation method than the Ewald sum, the isotropic periodic sum (IPS) method for nonpolar molecules (IPSn) and polar molecules (IPSp), along with the Wolf method are of interest, but the cutoff radius dependence is an important issue. To evaluate the cutoff radius effect of the three methods, a water-vapor interfacial system has been studied by molecular dynamics. The Wolf method can produce adequate results for surface tension compared to that of the Ewald sum (within 2.9%) at a long enough cutoff radius, r(c). However, the estimation of the electrostatic potential profile and dipole orientational function is poor. The Wolf method cannot estimate electrostatic configuration at r(c) ≤ L(z)∕2 (L(z) is the longest lattice of the system). We have found that the convergence of the surface tension and the electrostatic configuration of the IPSn method is faster than that of the IPSp method. Moreover, the IPSn method is most accurate among the three methods for the same cutoff radius. Furthermore, the behavior of the surface tension against the cutoff radius shows a greater difference for the IPSn and IPSp method. The surface tension of the IPSp method fluctuates and presents a similar result to that of the Ewald sum, but the surface tension for the IPSn method greatly deviates near r(c) = L(z)∕3. The cause of this deviation is the difference between the interfacial configuration of the water surface and the cutoff treatment of the IPS method. The deviation becomes insignificant far from r(c) = L(z)∕3. In spite of this shortcoming, the IPSn method gives the most accurate result in estimating the surface tension at r(c) = L(z)∕2. From all the results in this work, the IPSn and IPSp method have been found to be more accurate than the Wolf method. In conclusion, the surface tension and structure of water-vapor interface can be calculated by the IPSn method when r(c) is greater than or equal to the longest lattice of the system. The IPSp method and the Wolf method require a longer cutoff radius than the longest lattice of the system to estimate interfacial properties.

Electric Conductivity and Electrophoretic Mobility in Suspensions of Charged Porous Spheres

The electric conduction and electrophoresis of a suspension of charged porous spheres in an electrolyte solution with an arbitrary thickness of the electric double layers are analytically studied. The porous particle can be a solvent-permeable and ion-penetrable polyelectrolyte molecule or charged floc with uniformly distributed frictional segments and fixed charges. The effect of particle interactions is taken into account by employing a unit cell model, and the overlap of the electric double layers of adjacent particles is allowed. The electrokinetic equations, which govern the electrostatic potential profile, the ionic concentration (or electrochemical potential energy) distributions, and the fluid velocity field inside and outside the porous particle in a unit cell, are linearized by assuming that the system is only slightly distorted from equilibrium. Through the use of a regular perturbation method, these linearized equations are solved with the dimensionless density of the fixed charges as the small perturbation parameter. Analytical expressions for the electrophoretic mobility of each charged porous sphere and for the effective electric conductivity of the suspension correct to the first and second orders, respectively, of the fixed charge density are obtained in closed forms. The effect of particle interactions on the electrophoresis and electric conduction of the suspension can be significant in typical situations. Comparisons of the results of the cell model with different conditions at the outer boundary of the cell are made. The dependence of the electrophoretic mobility and the electric conductivity on the particle volume fraction and other properties of the particle−solution system is found to be quite complicated.

Experimental study of radial electric field and electrostatic potential fluctuation in the Large Helical Device

A heavy ion beam probe was installed on the Large Helical Device (LHD) to investigate the roles of radial electric fields (Er) in magnetically confined high-temperature plasmas. Two new observations are presented. One is the observation of electrostatic potential profiles during the formation of extremely hollow density profiles of impurities, called the impurity hole (Ida K et al 2009 Phys. Plasmas 16 056111), in the LHD plasmas. The measured Er is negative, and the Er determined by the ambipolarity condition of neoclassical particle fluxes is consistent with this observation. However, the transport analysis indicates that the formation of the extremely hollow profile is not attributable to the impurity fluxes driven by Er and the density and temperature gradients of the impurity. The other new observation is on the geodesic acoustic mode (GAM). The electrostatic potential fluctuation associated with the GAM, which is probably induced by energetic particles, in plasmas with the reversed or weak magnetic shear is identified. The GAM is localized in the core region of the plasma.

Confinement of electron plasma by levitating dipole magnet

A recent experiment on the Ring Trap 1 device has demonstrated long-term (exceeding 300 s) confinement of non-neutral (pure electron) plasma in a dipole magnetic field; particles diffuse inward, steepening the density gradient and self-organizing into a stable vortex structure [Z. Yoshida et al., Phys. Rev. Lett. 104, 235004 (2010)]. In this study, the internal structures of the plasma are experimentally investigated, and it is shown that the observations are consistent with rigidly rotating charged particle clump. The radial profiles of electrostatic potential and electron density consistently show that the drift velocity has homogeneous angular frequency in the confinement region. The electrostatic fluctuations also rotate rigidly with a phase velocity that agrees with the drift velocity. The magnetospheric system should have a wide application in confining single-species and even multiple-species charged particles.

Performance study of high operating temperature HgCdTe mid wave infrared detector through numerical modeling

The design of present generation uncooled Hg1−xCdxTe infrared photon detectors relies on complex heterostructures with a basic unit cell of type ṉ+/π/p̱+. We present an analysis of double barrier ṉ+/π/p̱+ mid wave infrared (x=0.3) HgCdTe detector for near room temperature operation using numerical computations. The present work proposes an accurate and generalized methodology in terms of the device design, material properties, and operation temperature to study the effects of position dependence of carrier concentration, electrostatic potential, and generation-recombination (g-r) rates on detector performance. Position dependent profiles of electrostatic potential, carrier concentration, and g-r rates were simulated numerically. Performance of detector was studied as function of doping concentration of absorber and contact layers, width of both layers and minority carrier lifetime. Responsivity ∼0.38 A W−1, noise current ∼6×10−14 A/Hz1/2 and D∗ ∼3.1×1010cm Hz1/2 W−1 at 0.1 V reverse bias have been calculated using optimized values of doping concentration, absorber width and carrier lifetime. The suitability of the method has been illustrated by demonstrating the feasibility of achieving the optimum device performance by carefully selecting the device design and other parameters.

Development of 6-MeV Heavy Ion Beam Probe on LHD

A heavy-ion beam probe (HIBP) using a 3-MV tandem accelerator was installed in Large Helical Device (LHD). It is designed to measure the electrostatic potential in the core region directly. The electrostatic potential profiles can be measured successfully using the HIBP, and the radial electric field predicted by the neoclassical theory is consistent with that measured using the HIBP as long as the ambipolarity condition of the neoclassical particle flux has a single solution. Although the turbulent fluctuation is not detected because of low signal-to-noise ratio, several coherent fluctuations, which are inferred to be reversed-shear-induced Alfvén eigenmode and the geodesic acoustic mode, are observed directly in core plasmas, and the spatial distribution is revealed.

Structure of Spherical Electric Double Layers Containing Mixed Electrolytes: A Systematic Study by Monte Carlo Simulations and Density Functional Theory

The structure of spherical electric double layers in the presence of mixed electrolytes is studied using Monte Carlo simulation and density functional theory within the restricted primitive model. The macroion is modeled as an impenetrable charged hard sphere carrying a uniform surface charge density, surrounded by the small ions represented as charged hard spheres, and the solvent is taken as a dielectric continuum. The density functional theory uses a partially perturbative scheme, where the hard-sphere contribution to the one-particle correlation function is evaluated using weighted density approximation and the ionic interactions are calculated using a second-order functional Taylor expansion with respect to a bulk electrolyte. The Monte Carlo simulations have been performed in canonical ensemble. The system is studied at varying ionic concentrations, at different concentration ratios of mono- and multivalent counterions of mixed electrolytes, at different diameters of hard spheres, at different macroion radius, and at varying polyion surface charge densities. The theoretical predictions in terms of the density profiles and the mean electrostatic potential profiles are found to be in good agreement with the simulation results. This model study shows clear manipulations of ionic size and charge correlations in dictating a number of interesting phenomena relating to width of the diffuse layer and charge inversion under different parametric conditions.

Microstructure and field mapping of AlInN‐based heterostructures and devices

AbstractThe microstructure, composition and electrostatic fields of near‐lattice‐matched Al1‐xInxN/AlN/GaN heterostructures and devices have been characterized using a variety of electron microscopy techniques. A thin parastic Ga‐rich layer was often observed immediately above the AlN spacer and was attributed to GaN nucleation during specimen cooling. Mapping of electrostatic potential profiles across a GaN/Al0.85In0.15N/AlN/ GaN heterostructure using off‐axis electron holography showed polarization‐induced fields of 7.5MV/cm within the AlN layer, and 2.2MV/cm within the Ga‐rich layer. A two‐dimensional electron gas with a density of ∼9.8×1012 cm–2 was observed in the underlying GaN layer located very close to the AlN/GaN interface. Contact inclusions were observed extending into the AlInN and GaN layers along mixed‐type threading dislocations under the source and drain regions of HFET devices. The density and size of the contact inclusions was determined by the annealing temperature (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization effects in 2-DEG and 2-DHG AlGaN/AlN/GaN multi-heterostructures measured by electron holography

The electrostatic potential profiles and charge distributions in modulation-doped n-type and p-type AlGaN/AlN/GaN heterostructures have been measured by electron holography with high spatial resolution. For n-type two-dimensional electron gas structure a negative curvature and for p-type two-dimensional hole gas structure a positive curvature in the potential profile at the AlN/GaN interface were observed, which demonstrated the accumulation of two-dimensional carriers. The measured electrostatic potential profiles were also compared with the calculated band diagram in the heterostructures.

DEPPDB — DNA ELECTROSTATIC POTENTIAL PROPERTIES DATABASE: ELECTROSTATIC PROPERTIES OF GENOME DNA

The electrostatic properties of genome DNA influence its interactions with different proteins, in particular, the regulation of transcription by RNA-polymerases. DEPPDB--DNA Electrostatic Potential Properties Database--was developed to hold and provide all available information on the electrostatic properties of genome DNA combined with its sequence and annotation of biological and structural properties of genome elements and whole genomes. Genomes in DEPPDB are organized on a taxonomical basis. Currently, the database contains all the completely sequenced bacterial and viral genomes according to NCBI RefSeq. General properties of the genome DNA electrostatic potential profile and principles of its formation are revealed. This potential correlates with the GC content but does not correspond to it exactly and strongly depends on both the sequence arrangement and its context (flanking regions). Analysis of the promoter regions for bacterial and viral RNA polymerases revealed a correspondence between the scale of these proteins' physical properties and electrostatic profile patterns. We also discovered a direct correlation between the potential value and the binding frequency of RNA polymerase to DNA, supporting the idea of the role of electrostatics in these interactions. This matches a pronounced tendency of the promoter regions to possess higher values of the electrostatic potential.

Three component model of cylindrical electric double layers containing mixed electrolytes: A systematic study by Monte Carlo simulations and density functional theory

The structure of electric double layer around a hard rigid impenetrable cylindrical polyion is studied using density functional theory as well as Monte Carlo simulations. The three component model, presented here, is an extension of solvent primitive model where the solvent molecules are treated as the neutral hard spheres, counterions and coions as the charged hard spheres, all of equal diameters, and in addition the mixture of mono- and multivalent counterions are also considered. The theory is partially perturbative where the hard sphere interactions are treated within the weighted density approach and the corresponding ionic interactions have been evaluated through second-order functional Taylor expansion with respect to the bulk electrolyte. The theoretical predictions in terms of the density profiles and the mean electrostatic potential profiles are found to be in good agreement with the simulation results. The presence of neutral hard spheres incorporate the effects of exclude volume interactions (ionic size correlations) while the mixture of mono- and multivalent counterions enhance the ionic charge correlation effects. Thus, this model study shows clear manipulations of ionic size and charge correlations in dictating the ionic density profiles as well as mean electrostatic potential profiles of the diffuse layer. The behavior of diffused double layer has been characterized at varying ionic concentrations, at different concentration ratios of mono- and multivalent counterions of mixed electrolytes, at different diameters of hard spheres, and at varying polyion surface charge density.

ZnO–ZnTe nanocone heterojunctions

We report heterojunctions made of vertically aligned ZnO–ZnTe nanocones synthesized using a combination of thermal vapor deposition and pulsed-laser deposition. ZnO nanocones and nanorods were controlled by utilizing the growth rate difference between central and boundary sites of precursor domains. The p–n heterojunctions were subsequently formed by growing ZnTe as shells on the nanocone surface. Structural and electric characteristics indicate that nanocones are more feasible than nanorods for forming heterojunction. Furthermore, theoretical modeling demonstrates that the nanocone-based junction exhibits an electrostatic potential profile that is much more effective for carrier transport than the electrostatic potential for the nanorod-based junction.