Local Charge Density Research Articles

Refractive index sensitivity of gold double concentric nanoshells: Tuning the intensity discrepancy of two-band plasmonic absorption

Refractive index sensitivity of gold double concentric nanoshells based on plasmonic absorption intensity has been investigated by using the quasistatic calculation. Increasing the environmental dielectric constant leads to the absorption intensity of | ω − − 〉 mode gets intense, whereas the absorption intensity of | ω − + 〉 mode fades down. Thus the intensity discrepancy between the two-band plasmonic absorption is greatly dependent on the environmental dielectric constant, which could be used in the refractive index sensing. Furthermore, the sensing ability can be improved by decreasing the thickness of the inner gold shell or increasing the thickness of the outer gold shell. A mechanism based on inter-shell plasmonic coupling and local electric field dependent surface charge density was investigated to illuminate the different effects from inner and outer gold shell on the refractive index sensitivity.

Lateral dynamics of charged lipids and peripheral proteins in spatially heterogeneous membranes: Comparison of continuous and Monte Carlo approaches

Biological membranes are complex environments whose physico-chemical properties are of utmost importance for the understanding of many crucial biological processes. Much attention has been given in the literature to the description of membranes along the z-axis perpendicular to the membrane. Here, we instead consider the lateral dynamics of lipids and peripheral proteins due to their electrostatic interaction. Previously, we constructed a Monte Carlo automaton capable of simulating mutual diffusive dynamics of charged lipids and associated positively charged peptides. Here, we derive and numerically analyze a system of Poisson-Boltzmann-Nernst-Planck (PBNP) equations that provide a mean-field approximation compatible with our Monte Carlo model. The thorough comparison between the mean-field PBNP equations and Monte Carlo simulations demonstrates that both the approaches are in a good qualitative agreement in all tested scenarios. We find that the two methods quantitatively deviate when the local charge density is high, presumably because the Poisson-Boltzmann formalism is applicable in the so-called weak coupling limit, whose validity is restricted to low charge densities. Nevertheless, we conclude that the mean-field PBNP approach provides a good approximation for the considerably more detailed Monte Carlo model at only a fraction of the associated computational cost and allows simulation of the membrane lateral dynamics on the space and time scales relevant for the realistic biological problems.

Spin and charge transport in U-shaped one-dimensional channels with spin-orbit couplings

A general form of the Hamiltonian for electrons confined to a curved one-dimensional (1D) channel with spin-orbit coupling (SOC) linear in momentum is rederived and is applied to a U-shaped channel. Discretizing the derived continuous 1D Hamiltonian to a tight-binding version, the Landauer--Keldysh formalism (LKF) for nonequilibrium transport can be applied. Spin transport through the U-channel based on the LKF is compared with previous quantum mechanical approaches. The role of a curvature-induced geometric potential which was previously neglected in the literature of the ring issue is also revisited. Transport regimes between nonadiabatic, corresponding to weak SOC or sharp turn, and adiabatic, corresponding to strong SOC or smooth turn, is discussed. Based on the LKF, interesting charge and spin transport properties are further revealed. For the charge transport, the interplay between the Rashba and the linear Dresselhaus (001) SOCs leads to an additional modulation to the local charge density in the half-ring part of the U-channel, which is shown to originate from the angle-dependent spin-orbit potential. For the spin transport, theoretically predicted eigenstates of the Rashba rings, Dresselhaus rings, and the persistent spin-helix state are numerically tested by the present quantum transport calculation.

Imaging charge density fluctuations in graphene using Coulomb blockade spectroscopy

Using scanning tunneling microscopy, we have imaged local charge density fluctuations in monolayer graphene. By placing a small gold nanoparticle on the end of the STM tip, a charge sensor is created. By raster scanning the tip over the surface and using Coulomb blockade spectroscopy, we map the local charge on the graphene. We observe a series of electron and hole doped puddles with a characteristic length scale of about 20 nm. Theoretical calculations for the correlation length of the puddles based on the number of impurities are in agreement with our measurements.

Simultaneous low extinction and high local field enhancement in Ag nanocubes**Project supported by the National Natural Science Foundation of China (Grant Nos. 60736041 and 10874238), and the National Key Basic Research Special Foundation of China (Grant No. 2007CB613205).

We theoretically investigate surface plasmon resonance properties in Au and Ag cubic nanoparticles and find a novel plasmonic mode that exhibits simultaneous low extinction and high local field enhancement properties. We analyse this mode from different aspects by looking at the distribution patterns of local field intensity, energy flux, absorption and charge density. We find that in the mode the polarized charge is highly densified in a very limited volume around the corner of the nanocube and results in very strong local field enhancement. Perturbations of the incident energy flux and light absorption are also strongly localized in this small volume of the corner region, leading to both low absorption and low scattering cross section. As a result, the extinction is low for the mode. Metal nanoparticles involving such peculiar modes may be useful for constructing nonlinear compound materials with low linear absorption and high nonlinearity.

Multiple wavelength reflectance microscopy to study the multiphysical behavior of microelectromechanical systems

In order to characterize surface chemomechanical phenomena driving microelectromechanical systems behavior, we propose herein a method to simultaneously obtain a full kinematic field describing the surface displacement and a map of its chemical modification from optical measurements. Using a microscope, reflected intensity fields are recorded for two different illumination wavelengths. Decoupling the wavelength-independent and -dependent contributions to the measured relative intensity changes then yields the sought fields. This method is applied to the investigation of the electroelastic coupling, providing images of both the local surface electrical charge density and the device deformation field.

Numerical simulation of positive corona discharge in air

Electrical discharges play a key role in technologies, the corona discharge commonly occurs in many engineering devices and processes. Diverse applications demand a solid physical and chemical understanding of the operational principals of such discharges. All of this reflects on the great importance of the evaluation of the corona performance characteristics. Numerical simulation of the corona discharge is necessary for predicting the performance of a device. The present work is devoted to the numerical determination of the properties of corona discharge in a DC positive point-to-plane gap in air. The inception and propagation of the avalanche is simulated by Monte Carlo method. This method provides the spatial-temporal local field and charge density variations as well as the avalanche front velocity.

Superconductivity in the presence of a pseudogap induced by local charge density waves

Superconductivity in the presence of a pseudogap induced by local charge density waves

Solid Solution Softening Mechanisms in Mg-Ca Alloy

In the present work, electronic structures of pure Mg, Mg-Ca and Mg-Al alloys were investigated by first-principle calculations. The charge density between the solute and solvent atoms in the Mg-Ca alloy was lower than that in the Mg-Al alloy. The local charge density of states (LDOS) of Mg atom in the Mg-Ca alloy was almost the same as the LDOS of the Mg atom in the pure Mg, suggesting that the solute Ca atoms had little effect on the Mg-Mg bonding in the Mg-Ca alloy. On the other hand, there is more and deeper valley in the LDOS of the Mg atom in the Mg-Al alloy, compared with that in the Mg-Ca alloy, suggesting that the solute Al atom may increase the covalency of the Mg-Mg bonds. A series of the calculations indicates that Ca atoms enhance the formation of double kinks rather than hinder the movement of dislocations, resulting in solid solution softening of Mg-Ca alloy. [doi:10.2320/matertrans.M2011109]

Overall Charge and Local Charge Density of Pectin Determines the Enthalpic and Entropic Contributions to Complexation with β-Lactoglobulin

The complex formation between β-lactoglobulin and pectins of varying overall charge and local charge density were investigated. Isothermal titration calorimetry experiments were carried out to determine the enthalpic contribution to the complex formation at pH 4.25 and various ionic strengths. Complex formation was found to be an exothermic process for all conditions. Combination with previously published binding constants by Sperber et al. (Sperber, B. L. H. M.; Cohen Stuart, M. A.; Schols, H. A.; Voragen, A. G. J.; Norde, W. Biomacromolecules 2009, 10, 3246-3252) allows for the determination of the changes in the Gibbs energy and the change in entropy of the system upon complex formation between β-lactoglobulin and pectin. The local charge density of pectin is found to determine the balance between enthalpic and entropic contributions. For a high local charge density pectin, the main contribution to the Gibbs energy is of an enthalpic nature, supported by a favorable entropy effect due to the release of small counterions. A pectin with a low local charge density has a more even distribution of the enthalpic and entropic part to the change of the Gibbs energy. The enthalpic part is reduced due to the lower charge density, while the relative increase of the entropic contribution is thought to be caused by a change in the location of the binding place for pectin on the β-lactoglobulin molecule. The association of the hydrophobic methyl esters on pectin with an exposed hydrophobic region on β-lg results in the release of water molecules from the hydrophobic region and surrounding the methyl esters of the pectin molecule. An increase in the ionic strength decreases the enthalpic contribution due to the shielding of electrostatic attraction in favor of the entropic contribution, supporting the idea that the release of water molecules from hydrophobic areas plays a part in the complex formation.

Visualization of charge transport through Landau levels in graphene

Monitoring the photocurrent generated as a laser scans across a graphene field-effect device subjected to low temperature and high magnetic fields enables the spatial distribution of Landau levels across a graphene sheet to be mapped. This in turn allows the relative contribution of bulk and edge states to the macroscopic electrical characteristics of these devices to be determined. Band bending and the associated spatially inhomogeneous population of Landau levels play a central role in the physics of the quantum Hall effect (QHE) by constraining the pathways for charge-carrier transport and scattering1. Recent progress in understanding such effects in low-dimensional carrier gases in conventional semiconductors has been achieved by real-space mapping using local probes2,3. Here, we use spatially resolved photocurrent measurements in the QHE regime to study the correlation between the distribution of Landau levels and the macroscopic transport characteristics in graphene. Spatial maps show that the net photocurrent is determined by hot carriers transported to the periphery of the graphene channel, where QHE edge states provide efficient pathways for their extraction to the contacts. The photocurrent is sensitive to the local filling factor, which allows us to reconstruct the local charge density in the entire conducting channel of a graphene device.

Electrostatic origins of polyelectrolyte adsorption: Theory and Monte Carlo simulations

Electrostatic interactions may promote or abate polyelectrolyte adsorption onto a charged surface depending on a number of interrelated factors including the surface and polymer charge densities, the salt concentration, and nonelectrostatic interactions such as van der Waals and hydrophobic forces. Even without the nonelectrostatic interactions, the electrostatic behavior of polyelectrolyte systems is often counterintuitive and cannot be explained with conventional theories of polymers or simple electrolytes. In this work, a nonlocal density functional theory (NLDFT) and Monte Carlo simulations are used together to investigate polyelectrolyte adsorption at both oppositely charged and like-charged surfaces (one due to the direct electrostatic attraction and the other due to counterion correlations). The simulation results provide a stringent test of the numerical performance of the NLDFT, in particular for systems containing multivalent counterions where electrostatic correlations are important. A systematic study of the effects of ion valence, salt concentration, and polyion chain length reveals that polyelectrolyte attraction to an oppositely charged surface is nearly a neutralization effect, little influenced by the polyion chain length and counterion valence. Neither the surface mean electrostatic potential nor the integrated local charge density shows no significant sign of charge inversion. Both theory and simulation predict polyelectrolyte adsorption onto a like-charged surface when the system contains multivalent counterions. In that case, the surface excess is sensitive to the surface charge density, the counterion valence, and the salt concentration. The surface mean electrostatic potential shows a clear evidence of charge inversion when two layers of like charges are mediated by multivalent counterions. The theoretical investigations indicate that most likely, the electrostatic correlation mediated by multivalent counterions is responsible for the layer-by-layer assembly of oppositely charged polyelectrolyte films.

The anatomy of the double layer and capacitance in ionic liquids with anisotropic ions: Electrostriction vs. lattice saturation

We investigate the basic mechanisms of the electrical double layer formation in ionic liquids near a flat charged wall, by performing Monte Carlo simulations of several model liquids with spherical and elongated ions that contain charged ‘heads’ and neutral ‘tails’. We analyze the structural transitions in response to the variation of the voltage across the double layer and their effects on the formation of the characteristic ‘camel shape’ capacitance recently observed in experiments [1,2]. The camel shape was predicted earlier by the mean-field theory [3] for the case of an ionic liquid with large percentage of voids which allowed for substantial ‘electrostriction’ and thereby increase of the local charge density in the double layer. However, we show that in contrast to a liquid composed of spherical ions, in a liquid with anisotropic ions the double-hump ‘camel shape’ of the capacitance curve does not require high compressibility of the liquid. Many ionic liquids have elongated shape of ions, with charged ‘heads’ and neutral ‘tails’. We show that these neutral tails play a role of space fillers, or ‘latent voids’ that can be replaced by charged groups, following rotations and translations of ions. This feature allows an electric field-induced increase of the counter charge density without a substantial compression of the liquid. It causes rising branches of capacitance at small positive and negative electrode polarizations. However, we show that at large positive and negative voltages it has to be inevitably replaced by the ‘lattice saturation’, a universal effect, which does not depend on the model, in which the capacitance decreases inversely proportional to the square root of the voltage. This conclusion is supported by our analysis of the experimental results of Lockett et al. (2008) [1] that appear to follow this law staring from the potentials ∼±0.7–0.9 V. The competition between the two trends, one prevailing at low voltages and the other one at high voltages, results in the camel shape of capacitance, seen in our simulations.

Weakening of the Cu/Cu3Sn(100) Interface by Bi Impurities

We have performed density functional theory calculations to examine the role of Bi impurities in modifying the structure and properties of the Cu/Cu(3)Sn(100) interface. Analysis of the heat of supercell formation reveals that Bi atoms preferentially replace Cu atom in the Cu slab. Substitution of Cu by Bi reduces the adhesion energy of the interface from 1.5 J/m(2) to 1.1 J/m(2), primarily due to the atomic size effect. The size effect leads to expansion of the interface, because surrounding Cu and Sn atoms are pushed away from the misfit Bi atoms. Analysis of electronic density indicates that the local charge density is dispersed around Bi atoms. The presence of a Bi atom makes surrounding atoms less active, which is attributed to the reduction of hybridizations.

Many-body van der Waals forces involving chains

Van der Waals (VDW) forces arise from quantum mechanical fluctuations of local charge densities. Whereas computing VDW interactions between two atoms requires only 2-body interactions, by definition, computing interactions between macroscopic bodies required multi-body interactions, where the presence of a third atom affects how any two atoms interact, and a fourth atom affects the first three, and so on. Often, 3-body interactions have been found to be small, and 4-body and higher interactions are almost always neglected in calculating interactions between atoms. But is it possible that 4-body and higher order interactions can actually be more important than 3-body interactions, and indeed comparable to 2-body interactions? We explored the following question: how important is the sum of 4-body and higher order interactions? A set of problems involving finite and infinite chains was explored numerically and analytically, including a single chain, a pair of parallel, long chains and a two-dimensional array of long, parallel chains. The coupled dipole method was used, providing a type of Lifshitz theory for calculations of nanoscale VDW interactions. Calculations were made for the static polarizability, as well as the VDW interaction between chains. This latter energy was compared with that found by summing 2-body interactions for the chains. It was found that when a coupling constant ν, which is the polarizability per unit volume of the material, is large, then the sum of 4-body and higher order interactions dominates both the 2-body and the 3-body interactions. In addition, it was found that for all geometries examined, a divergence of the polarizability and a dynamical energy instability occur simultaneously when ν reaches a limiting value ν max , giving a well-known polarization catastrophe.

Electro-elastic coupling modelling from multiple wavelengths imaging microscopy

Due to their ability to interact with their environment, the interest in micro- cantilever sensors in biological or chemical applications is increasing. Moreover measurement of the sensor bending induced by molecular adsorption is improved by electrochemical actuation of the cantilever. This is why modelling the electro-elastic coupling is a key issue. Different methods are used to measure the bending induced by this electrochemical actuation. Among them, multiple wavelengths imaging microscopy provides full-field measurements. The local surface electrical charge density and the deformation field are obtained by decoupling wavelength-dependent and -independent contributions to the collected intensity. Based on a modelling of the electro-elastic coupling and on these full-field measurements, an identification method of the coupling parameters of the system is proposed herein.

Equilibrium charge density on a thin curved wire

We address the electrostatic problem of a thin, curved, and cylindrical conductor (a conducting filament) and show that the corresponding linear charge density slowly tends to uniformity as the inverse of the logarithm of a characteristic parameter, which is the ratio of the diameter to the smaller of the length and minimum radius of curvature of the filament. An alternative derivation of this result based on energy minimization is given. These results follow from a general asymptotic analysis of the electric field components and potential near a charged filament in the limit of vanishing diameter. It is found that the divergent parts of the radial and azimuthal electric field components are determined by the local charge density, while the axial component is determined by the local dipole density. For a straight filament our results reduce to those known for conducting needles. For curved filaments, the configuration of charges and fields is no longer azimuthally symmetric, and there is an additional length scale arising from the finite radius of curvature of the filament. The basic uniformity result survives the added complications, which include an azimuthal variation in the surface charge density of the filament. As with the variations in linear charge density along the filament, the azimuthal variations vanish with the characteristic parameter, only more rapidly. These findings allow us to derive an asymptotic formula for the capacitance of a curved filament that generalizes a result first obtained by Maxwell. The examples of a straight filament with uniform and linearly varying charge densities and a circular filament with a uniform charge distribution are treated analytically and found to agree with the general analysis. Numerical calculations illustrating the slow convergence of linear charge distribution to uniformity for an elliptical filament are presented. An interactive computer program implementing and animating the numerical calculations is available.

Binding of β-Lactolobulin to Pectins Varying in their Overall and Local Charge Density

The formation of complexes between proteins and polysaccharides is of great importance for many food systems like foams, emulsions, acidified milk drinks, and so on. The complex formation between beta-lactoglobulin (beta-lg) and pectins with a well-defined physicochemical fine structure has been studied to elucidate the influence of overall charge and local charge density of pectin on the complex formation. Binding isotherms of beta-lg to pectin are constructed using fluorescence anisotropy, which is shown to be an excellent technique for this purpose, as it is fast and requires low sample volumes. From the binding isotherms the maximal adsorbed amount, binding constant (k(obs)) and the cooperativity of binding are obtained at different ionic strengths. The Hill model is used to fit the binding isotherms and is shown to be preferable over a Langmuir fit. At pH 4.25, k(obs) shows a maximum at an ionic strength of 10 mM when using a low methyl esterified pectin (LMP) due to the balance of attractive and repulsive electrostatic forces between beta-lg and pectin and beta-lg neighbors. For two high methyl esterified pectins, one with a blockwise distribution of methyl esters (HMP(B)) and one with a random distribution (HMP(R)), this ionic strength maximum is absent and k(obs) decreases with increasing ionic strength. k(obs) is found to be largest for LMP and HMP(B) and considerably lower for HMP(R). A positive cooperativity is observed for both LMP (above an ionic strength of 45 mM) and HMP(R) (above an ionic strength of 15 mM) but not for HMP(B). Positive cooperativity is thought to be caused by a rearrangement of the pectin helix structure caused by binding of beta-lg, thus creating new or binding sites with a higher affinity. To attain strong binding of beta-lg to pectin it is preferable to use a pectin with a blockwise distribution of methyl esters. When complex formation takes place in high ionic strength media an LMP gives the best results, while at low ionic strength a high methyl esterified pectin with blockwise distribution may give better results, due to reduced electrostatic repulsion between both pectin and beta-lg and beta-lg neighbors.

Chiral magnetic effect in SU(2) lattice gluodynamics at zero temperature

The chiral magnetic effect is the appearance of a quark electric current along a magnetic-field direction in topologically nontrivial gauge fields. There is evidence that this effect is observed in collisions between heavy ions at the RHIC collider. The features of the chiral magnetic effect in SU(2) lattice gluodynamics at zero temperature have been investigated. It has been found that the electric current increases in the magnetic-field direction owing to quantum fluctuations of gluon fields. Fluctuations of the local charge density and chirality also increase with the magnetic field strength, which is a signature of the chiral magnetic effect.

Imaging of Surface Charge and the Mechanism of Desorption Electrospray Ionization Mass Spectrometry

Distributions of charge deposited on surfaces in desorption electrospray ionization mass spectrometry (DESI-MS) were investigated using a static charge measurement apparatus, which gives an output voltage proportional to the local surface charge density. By scanning the probe along the surface and taking measurements at fixed intervals, a contour image of relative charge density reflecting the charge distribution on the surface can be plotted. Through the measured charge distribution and the derived charge density gradient, the motion of charged droplets in the DESI experiment can be inferred. Measurements taken under various DESI operating conditions, including spray pressure, angle, flow rate, and sprayer tip-to-surface distance, show that charge is spread over an area of a few square centimeters under typical conditions; effective desorption occurs from a much smaller area (∼1 mm2) of highest charge density. Higher sheath gas pressures and smaller sprayer tip-to-surface distances lead to concentration of charge distribution into a smaller area, whereas smaller spray angles favor charge distribution over a larger area. The appearance of the highest charge density in front of the DESI sprayer tip and near the MS inlet suggests that charged droplets are moved toward the MS inlet by pneumatic forces and by the vacuum suction, in agreement with results of earlier simulations. The present observations are consistent with previous studies using other techniques and support the accepted droplet splashing DESI mechanism.