Dependence Of Charge Density Research Articles

Enhancement of Charge Transport in Polythiophene Semiconducting Polymer by Blending with Graphene Nanoparticles.

This paper describes a study on the charge transport in a composite of liquid-exfoliated graphene nanoparticles (GNPs) and a polythiophene semiconducting polymer. While the former component is highly conducting, although it consists of isolated nanostructures, the latter offers an efficient charge transport path between the individual GNPs within the film, overall yielding enhanced charge transport properties of the resulting bi-component system. The electrical characteristics of the composite layers were investigated by means of measurements of time-of-flight photoconductivity and transconductance in field-effect transistors. In order to analyze both phenomena separately, charge density and charge mobility contributions to the conductivity were singled out. With the increasing GNP concentration, the charge mobility was found to increase, thereby reducing the time spent by the carriers on the polymer chains. In addition, for GNP loading above 0.2 % (wt.), an increase of free charge density was observed that highlights an additional key role played by doping. Variable-range hopping model of a mixed two- and three-dimensional transport is explained using temperature dependence of mobility and free charge density. The temperature variation of free charge density was related to the electron transfer from polythiophene to GNP, with an energy barrier of 24 meV.

Viscosity effects for ion transport in electrochemical systems

Ion transport in a fluid medium is often described by the Poisson-Nernst-Planck (PNP) equation, a time-dependent mean-field theory that ignores the ion excluded-volume effects and electrostatic correlations, and a number of theoretical procedures have been recently proposed to alleviate the mean-field approximation. In this work, we investigate the charging kinetics of an electric double layer (EDL) of organic electrolytes using the dynamic density functional theory (DDFT). Unlike the PNP equation, DDFT predicts a nonlinear dependence of the surface charge density on the charging voltage. For systems with low solvent viscosity, the collective behaviors caused by inertial effects lead to an oscillatory variation of the surface charge density, occurrences in the charging process of the electrochemical system. Both the strength and frequency of charge oscillation are affected by the solvent viscosity and the degree of EDL overlap.

Structure of Liquid Coacervates formed by Oppositely Charged Polyelectrolytes.

We develop a scaling theory and perform molecular dynamic simulations of weakly interacting coacervates with electrostatic interaction energy per charge less than thermal energy kT. Such liquid coacervates formed by oppositely charged polyelectrolytes can be asymmetric in charge density and number of charges per chain. We predict that these coacervates form interpenetrating solutions with two correlation lengths and two qualitatively different types of conformations of polyelectrolytes with lower and higher charge densities, which are analogous to chain conformations in quasi-neutral and in polyelectrolyte solutions, respectively. Weaker charged chains are attracted to and adsorbed on stronger charged chains forming a screening "coat" around the stronger charged polyelectrolytes. Salt added at lower concentrations screens the repulsion between stronger charged chains, thereby reducing the thickness of the screening coat and resulting in the non-zero net polymer charge in the coacervate. At higher salt concentrations salt screens the attraction between oppositely charged chains, decreasing the coacervate concentration and its polymeric charge density. Thus, we predict a non-monotonic salt concentration dependence of polymeric charge density for asymmetric coacervates. Phase diagram for a mixture of oppositely charged polyelectrolytes at various compositions is proposed for different salt concentrations.

Control of polarization reversal temperature behavior by surface screening in thin ferroelectric films

Ferroelectric surfaces and interfaces are unique physical objects for fundamental studies of various screening mechanisms of spontaneous polarization by free carriers and possible ion exchange between the polar surface and ambient media. The theory of the polarization charge compensation at ferroelectric surface by ambient screening charges requires a detailed comparison of different screening models. In the article, we study the free energy of a thin ferroelectric film covered by a screening charge layer of different nature and calculate hysteresis loops of polarization and screening charge in the system at different temperatures. The dependence of the screening charge density on electric potential was considered for three basic models, namely for the linear Bardeen-type surface states (BS) and nonlinear the Fermi-Dirac (FD) density of states describing two-dimensional electron gas at the film surface, and the strongly nonlinear electrochemical Stephenson-Highland (SH) model describing the surface charge density of absorbed ions. Among considered surface screening models, the most various behavior of polarization and screening charge hysteresis loops is inherent to SH model. Obtained results give new insight to the understanding of bound charge compensation at ferroelectric surfaces at different temperatures, as well as they open the way to control the polarization reversal in thin ferroelectric films by appropriate choice of the surface charge nature and screening mechanism.

Formation of Open Framework Uranium Germanates: The Influence of Mixed Molten Flux and Charge Density Dependence in U-Silicate and U-Germanate Families.

Seven novel open-framework uranyl germanates, K2(UO2)GeO4, K6(UO2)3Ge8O22, α-Cs2(UO2)Ge2O6, β-Cs2(UO2)Ge2O6, Cs2(UO2)GeO4, and A(UO2)3(Ge2O7)2 (A = [NaK6Cl]6+, [Na2Cs6Cl2]6+), were grown from different mixed molten fluxes. The three-dimensional (3D) structure of K2(UO2)GeO4 with 8-ring channels can be built upon [UGe4] pentamer secondary building units (SBUs). The 3D framework of K6(UO2)3Ge8O22 with trapezoid (Ge8O22)12- clusters consists of two types of [UGe4] pentamers. The 3D framework of α-Cs2(UO2)Ge2O6 with 10-ring channels, crystallizing in the P21/ n space group, is constructed by [UGe4] pentamers. The structure of β-Cs2(UO2)Ge2O6 contains achter (eight) single germanate chains and is composed of [UGe6] heptamers and [UGe4] pentamers. The structure of Cs2(UO2)GeO4 with hexagonal 10-ring channels is composed of [U3Ge4] heptamers and twisting five-fold GeO4 tetrahedra in four-membered Ge4O12 rings occur. 3D frameworks of NaK6Cl(UO2)3(Ge2O7)2 (space group Pnnm) and Na2Cs6Cl2(UO2)3(Ge2O7)2 ( P21/ c) can be constructed from the same SBUs [UGe4] pentamers. Thermal stability of salt-inclusions was studied by TG and PXRD analysis. Analysis of charge density for the U-Si-O system indicates that the polymerization of silicate units reduces the cross-links of the 3D frameworks. The concept of SBUs combined with the cutting and gluing strategy was applied to understand and analyze the distinct 8-, 10-, 12-, and 14- membered channels for the uranyl germanate family. The charge density of all known 3D U-Si/Ge-O frameworks has been investigated, which shows strong correlations with chemical composition of corresponding phases. The increase of Si/O (Ge/O) ratios in silicate units results in the decrease of negative charge density. Moreover, the charge density increases with decreasing countercation size within the same Si/O ratio. The correlations can be used to predict inclusion phase formation within U-Si/Ge-O families. Raman spectra of the studied uranyl germanates were measured, and bands were assigned on the basis of structural features.

Computational Modeling and Validation of Thermally Induced Electrical Capacitance Changes for Lipid Bilayer Membranes Irradiated by Pulsed Lasers.

Neural stimulation has widespread applications in investigating brain functions, restoring impaired neural functions, and treating numerous neurological/psychiatric diseases. Use of infrared pulses to stimulate neurons (infrared neural stimulation) offers a direct and non-invasive technique. Recent research has demonstrated that transient heating associated with the absorption of infrared light by the local aqueous medium around the cell membrane can stimulate nerves. One mechanism for this stimulation is due to a thermally induced increase in the membrane electrical capacitance, which causes cell depolarization as well as action potential production under certain physiological conditions. A theoretical and computational model helps better understand the mechanism of thermally induced electrical capacitance changes and optimize the stimulus parameters. In this article, we develop the existing theoretical models for membrane electrical capacitance and its thermally induced changes. We improve the formulation of Gouy-Chapman-Stern theory by Genet et al. and Shapiro et al. with the addition of a diffuse layer to the electrical double layer and by modifying the relation of Stern layer capacitance, to calculate the membrane capacitive charge and capacitive current. We also present a new method to calculate the membrane electrical capacitance and the rate of its thermally induced changes. In our calculations, two new factors are considered including the temperature dependence of the surface charge density and the hydrophobic core dielectric constant of the lipid bilayer. Our developed model predicts rates of 0.3 and 0.26%/°C for the thermally induced capacitance changes of the artificial lipid bilayer under two different sets of conditions previously reported by Shapiro et al. and Carvalho-de-Souza et al., respectively. Our model is in very good agreement with the corresponding experimental values given by these groups. The presented model is also able to calculate the membrane capacitive currents and investigate the voltage dependence of this current.

Binding of trivalent metal ions (Al3+, In3+, La3+) with phosphatidylcholine liposomal membranes investigated by microelectrophoresis

.Interactions between trivalent metal ions (Al3+, In3+, La3+) and phosphatidylcholine (PC) liposomes are studied by microelectrophoresis. The dependence of the PC membrane surface charge density and zeta potential on p{rm H} (p{rm H} range from 2 to 10) of the aqueous metal chloride solutions is determined. The obtained results indicate the adsorption of Al3+, In3+ and La3+ ions on phosphatidylcholine model membranes, leading to changes in the electrical properties of the membranes. The theoretical considerations on equilibria occurring between phosphatidylcholine liposomal membrane and trivalent metal ions are presented. A mathematical model describing the interactions in a quantitative way is proposed.Graphical abstract

Charge Density and Band Offsets at Heterovalent Semiconductor Interfaces

The well‐known insensitivity of the band offset (BO) of isovalent heterojunctions with the zincblende structure to the orientation, abruptness, and atomic structure of the interface was recently shown to be attributable to a localness in the dependence of charge density on the atomic structure. In contrast, a sharp dependence of the BO on interface specifics has been observed at heterovalent heterojunctions. Here, detailed analyses of the relationship between the BO, interface structure, and charge distribution have been carried out for many lattice‐matched heterovalent interfaces between zincblende and diamond structure semiconductors. From thermodynamic considerations, three types of neutral interfaces were investigated, each with equal densities of donor‐ and acceptor‐like heterovalent bonds, constructible in all orientations. Distinctively different, yet approximately orientation‐independent, valence BOs were found. The equilibrium charge density of the heterovalent interface could be recreated with the charge densities of bulk semiconductors and oligo‐cells. Because charge transfer between heterovalent bonds is identifiable with that for dopants in semiconductor and its effect accountable by linear response, a combination of neutral polyhedra theory, previously developed for isovalent heterojunctions, and dielectric screening theory was found to explain BO trends throughout, allowing a strategy that facilitates adjustment in the BO of all isovalent heterojunctions.

Finite extensibility and deviation from Gaussian elasticity of dimethylacrylamide-based gels with different charge density: Insight into pH/solvent-dependent swelling and surfactant interactions

The quasi-binary ionic gels composed of non-ionic N,N-dimethylacrylamide (DMAAm) and ionic comonomer sodium acrylate (NaA) were prepared to investigate the charge density dependence of the equilibrium swelling and elasticity under various environmental conditions. By altering the chemical composition, highly ionic poly(N,N-dimethylacrylamide-co-sodium acrylate) P(DMAAm-co-NaA) hydrogels and cryogels were obtained which were highly sensitive to pH- and ion-concentration of the external solution containing surfactant or halide ions. The mechanical properties and the corresponding degree of swelling in water/solvent mixtures both for hydrogels and cryogels were followed by the pulsatile swelling/deswelling experiments. The swelling behaviors of the prepared hydrogels and cryogels with specific network structures were dependent on the pH of external solution, NaA content, associative interactions between polymer and surfactant species and the concentration of the monovalent-salt solutions. By the results of dynamic swelling test, the influence of NaA content on the swelling rate, swelling coefficients and diffusional behavior of water in the prepared gels were determined. The variation in the charge density in P(DMAAm-co-NaA) hydrogel networks was theoretically explained in the framework of model which treats to explain the dependence of the elastic free energy on their swelling ratio. Although the introduction of electrostatic interactions decreased the reduced elastic modulus at low swelling degree, the deviation from Gaussian elasticity due to finite chain extensibility was clearly detected at higher ionic comonomer concentration in which the reduced modulus increases with increasing swelling. The elasticity data showed that Gaussian statistics fails to describe the statistical properties of PDMAAm chains at higher swelling degree and the equilibrium swollen P(DMAAm-co-NaA) hydrogels are in the non-Gaussian regime. Developed theory containing adjustable parameters allows to obtain semiquantitative matching of experimental results on swollen state properties of highly charged gels and non-Gaussian elasticity.

Charge density dependence of elasticity of anionically modified N,N-dimethylacrylamide-based gels with (meth)acrylic acid segments: An insight by quantitative analysis of electrostatic contributions

Swollen state properties of modified polyelectrolyte gels with anionic species is highly dependent on the effective charge density distributions as well as the interactions between the mobile and immobile groups. A more systematic and comparative study of the role of structure and content of the anionic comonomers has been made on poly(N,N-dimethylacrylamide) PDMAAm-based hydrogels attention being focused on their Gaussian elasticity and corresponding swollen state properties. A thermodynamic framework for interpreting gel-swelling model was presented to describe the aqueous equilibrium swelling properties of a series of weakly ionic polyelectrolyte hydrogels based on acidic comonomers; acrylic acid (AAc), methacrylic acid (MAAc), sodium acrylate (NaA) and a base monomer DMAAm as well as the effect of ionizable groups on their resulting mechanical properties. The introduction of the electrostatic interactions decreased the elastic modulus of ionic PDMAAm hydrogels and cryogels. The model predicts well the effect of the gel charge density on the elasticity; however, the correlation of the obtained results with the equilibrium swelling data suggests that a change of the gel microstructure with electrostatic interactions is the responsible of the observed background effect. Although the ionizable functional groups of AAc and MAAc display weak anionic polyelectrolyte characteristics, the swelling ratio is enhanced with respect to homopolymeric PDMAAm, increasing in the order DMAAm<MAAc<AAc<NaA.

Effect of interlamellar interactions on shear induced multilamellar vesicle formation.

Shear-induced multilamellar vesicle (MLV) formation has been studied by coupling the small-angle neutron scattering (SANS) technique with neutron spin echo (NSE) spectroscopy. A 10% mass fraction of the nonionic surfactant pentaethylene glycol dodecyl ether (C12E5) in water was selected as a model system for studying weak inter-lamellar interactions. These interactions are controlled either by adding an anionic surfactant, sodium dodecyl sulfate, or an antagonistic salt, rubidium tetraphenylborate. Increasing the charge density in the bilayer induces an enhanced ordering of the lamellar structure. The charge density dependence of the membrane bending modulus was determined by NSE and showed an increasing trend with charge. This behavior is well explained by a classical theoretical model. By considering the Caillé parameters calculated from the SANS data, the layer compressibility modulus B¯ is estimated and the nature of the dominant inter-lamellar interaction is determined. Shear flow induces MLV formation around a shear rate of 10 s-1, when a small amount of charge is included in the membrane. The flow-induced layer undulations are in-phase between neighboring layers when the inter-lamellar interaction is sufficiently strong. Under these conditions, MLV formation can occur without significantly changing the inter-lamellar spacing. On the other hand, in the case of weak inter-lamellar interactions, the flow-induced undulations are not in-phase, and greater steric repulsion leads to an increase in the inter-lamellar spacing with shear rate. In this case, MLV formation occurs as the amplitude of the undulations gets larger and the steric interaction leads to in-phase undulations between neighboring membranes.

Non-Gaussian elasticity and charge density-dependent swelling of strong polyelectrolyte poly(N-isopropylacrylamide-co-sodium acrylate) hydrogels.

The mechanical properties and charge density-dependent swelling of strong polyelectrolyte poly(N-isopropylacrylamide-co-sodium acrylate) P(NIPA-co-NaA) hydrogels prepared at a fixed total monomer concentration and crosslinker ratio, but at various charge densities, i.e. NaA content in the feed between 0 and 90 mol%, were investigated. The elasticity results were discussed to explain the relationship between the elastic free energy ΔGel and the swelling ratio α as well as to fit the existing theories to the swelling data. The implications of the obtained results for the deviation from the Gaussian chain statistics were considered. Given the swollen elastic modulus and the dependence of charge density on the equilibrium gel volume, it would seem that the latter factor is an important determinant of non-Gaussian elasticity of polyelectrolyte P(NIPA-co-NaA) hydrogels containing strongly dissociated groups. The dependence of the reduced modulus on the equilibrium gel volume was found to be Gr ≈ (Veq)-0.47 at low swelling degree and Gr ≈ (Veq)0.64 at high swelling degree and the deviation was interpreted as the non-Gaussian elasticity of equilibrium swollen P(NIPA-co-NaA) hydrogels. The detailed theoretical treatments of non-Gaussian elasticity of P(NIPA-co-NaA) hydrogels and, in particular, the influence of the charge density on the elasticity showed that the knowledge of several swollen state parameters and the effective charge density distribution of hydrogels were strongly required to explain the variation of the elastic properties depending on the ionic group content. Within this framework, the dominant mechanism responsible for the deviation from Gaussian elasticity and the finite chain extensibility of ionic P(NIPA-co-NaA) hydrogels was described and the results were used to explain the dependence of the elastic modulus on the equilibrium gel volume.

Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors

Polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. The recent development of semicrystalline polymers with strongly improved charge transport properties forces a review of the current understanding of the charge transport mechanisms and how they relate to the polymer's chemical and structural properties. Here, the charge density dependence of field effect mobility in semicrystalline polymer semiconductors is studied. A simultaneous increase in mobility and its charge density dependence, directly correlated to the increase in average crystallite size of the polymer film, is observed. Further evidence from charge accumulation spectroscopy shows that charges accumulate in the crystalline regions of the polymer film and that the increase in crystallite size affects the average electronic orbitals delocalization. These results clearly point to an effect that is not caused by energetic disorder. It is instead shown that the inclusion of short range coulomb repulsion between charge carriers on nanoscale crystalline domains allows describing the observed mobility dependence in agreement with the structural and optical characterization. The conclusions that are extracted extend beyond pure transistor characterization and can provide new insights into charge carrier transport for regimes and timescales that are relevant to other optoelectronic devices.

On the dependence of charge density on surface curvature of an isolated conductor

A study of the relation between the electrostatic charge density at a point on a conducting surface and the curvature of the surface (at that point) is presented. Two major papers in the scientific literature on this topic are reviewed and the apparent discrepancy between them is resolved. Hence, a step is taken towards obtaining a general analytic formula for relating the charge density with surface curvature of conductors. The merit of this formula and its limitations are discussed.

Friedel oscillation near a van Hove singularity in two-dimensional Dirac materials

We consider Friedel oscillation in the two-dimensional Dirac materials when the Fermi level is near the van Hove singularity. Twisted graphene bilayer and the surface state of topological crystalline insulator are the representative materials which show low-energy saddle points that are feasible to probe by gating. We approximate the Fermi surface near saddle point with a hyperbola and calculate the static Lindhard response function. Employing a theorem of Lighthill, the induced charge density due to an impurity is obtained and the algebraic decay of is determined by the singularity of the static response function. Although a hyperbolic Fermi surface is rather different from a circular one, the static Lindhard response function in the present case shows a singularity similar with the response function associated with circular Fermi surface, which leads to the at large distance R. The dependences of charge density on the Fermi energy are different. Consequently, it is possible to observe in twisted graphene bilayer the evolution that near Dirac point changes to above the saddle point. Measurements using scanning tunnelling microscopy around the impurity sites could verify the prediction.

Imaging Local Electric Field Distribution by Plasmonic Impedance Microscopy

We report on imaging of local electric field on an electrode surface with plasmonic electrochemical impedance microscopy (P-EIM). The local electric field is created by putting an electrode inside a micropipet positioned over the electrode and applying a voltage between the two electrodes. We show that the distribution of the surface charge as well as the local electric field at the electrode surface can be imaged with P-EIM. The spatial distribution and the dependence of the local charge density and electric field on the distance between the micropipet and the surface are measured, and the results are compared with the finite element calculations. The work also demonstrates the possibility of integrating plasmonic imaging with scanning ion conductance microscopy (SICM) and other scanning probe microscopies.

Understanding Open‐Circuit Voltage Loss through the Density of States in Organic Bulk Heterojunction Solar Cells

The field of organic photovoltaics has recently produced highly efficient single‐junction cells with power conversion efficiency &gt;10%, yet the open‐circuit voltage (VOC) remains relatively low in many high performing systems. An accurate picture of the density of states (DOS) in working solar cells is crucial to understanding the sources of voltage loss, but remains difficult to obtain experimentally. Here, the tail of the DOS is characterized in a number of small molecule bulk heterojunction solar cells from the charge density dependence of VOC, and is directly compared to the disorder present within donor and acceptor components as measured by Kelvin probe. Using these DOS distributions, the total energy loss relative to the charge transfer state energy (ECT)—ranging from ≈0.5 to 0.7 eV—is divided into contributions from energetic disorder and from charge recombination, and the extent to which these factors limit the VOC is assessed.

Density-dependent electron transport and precise modeling of GaN high electron mobility transistors

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors (HEMTs). Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 × 107 cm/s at a low sheet charge density of 7.8 × 1011 cm−2. An optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated longitudinal optical (LO) phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

Antenna Enhanced Graphene THz Emitter and Detector.

Recent intense electrical and optical studies of graphene have pushed the material to the forefront of optoelectronic research. Of particular interest is the few terahertz (THz) frequency regime where efficient light sources and highly sensitive detectors are very challenging to make. Here we present THz sources and detectors made with graphene field effect transistors (GFETs) enhanced by a double-patch antenna and an on-chip silicon lens. We report the first experimental observation of 1-3 THz radiation from graphene, as well as more than 3 orders of magnitude performance improvements in a half-edge-contacted GFET thermoelectric detector operating at ∼2 THz. The quantitative analysis of the emitting power and its unusual charge density dependence indicate significant nonthermal noise contribution from the GFET. The polarization resolved detection measurements with different illumination geometries allow for detailed and quantitative analysis of various factors that contribute to the overall detector performance. Our experimental results represent a significant advance toward practically useful graphene THz devices.

Adsorption of Monovalent Ions to Phosphatidylcholine-Cardiolipin Membranes

We examined the adsorption of Na+, Cl−, H+, and OH− ions on phosphatidylcholine-cardiolipin liposomal membrane surfaces using both experimental and theoretical approaches. The dependence of the membrane surface charge density on the pH of the electrolyte solution was determined using microelectrophoresis. A mathematical model comprising eight equilibria was formulated to describe the adsorption of the electrolyte ions. Combination of this model with experimental surface charge density values enabled the determination of association constants. From the good agreement between the experimental and theoretical data, we can conclude that the assumed model is compatible with the experimental data.