Showing papers on "Charge density published in 2012"
TL;DR: This work prepared metal–insulator–semiconductor field-effect transistors based on vanadium dioxide and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state.
Abstract: In the classic transistor, the number of electric charge carriers--and thus the electrical conductivity--is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system's state, as successfully demonstrated in electric-field-induced ferromagnetism and superconductivity. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas-Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal-insulator-semiconductor field-effect transistors based on vanadium dioxide--a strongly correlated material with a thermally driven, first-order metal-insulator transition well above room temperature--and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal-insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.
668 citations
TL;DR: It is shown that both classical as well as linear quantum mechanical descriptions of the system fail even for moderate incident light intensities and the coupling between the two nanoparticles and the field enhancement is reduced as compared to linear theory.
Abstract: A fully quantum mechanical investigation using time-dependent density functional theory reveals that the field enhancement in a coupled nanoparticle dimer can be strongly affected by nonlinear effects. We show that both classical as well as linear quantum mechanical descriptions of the system fail even for moderate incident light intensities. An interparticle current resulting from the strong field photoemission tends to neutralize the plasmon-induced surface charge densities on the opposite sides of the nanoparticle junction. Thus, the coupling between the two nanoparticles and the field enhancement is reduced as compared to linear theory. A substantial nonlinear effect is revealed already at incident powers of 109 W/cm2 for interparticle separation distances as large as 1 nm and down to the touching limit.
458 citations
TL;DR: In this paper, the static and dynamic properties of spontaneous superstructures formed by electrons are reviewed, and a special attention is paid to the collective effects in pinning and sliding of these structures, and the glassy properties at low temperature.
Abstract: This article reviews the static and dynamic properties of spontaneous superstructures formed by electrons. Representations of such electronic crystals are charge density waves (CDW) and spin density waves in inorganic as well as organic low-dimensional materials. A special attention is paid to the collective effects in pinning and sliding of these superstructures, and the glassy properties at low temperature. Charge order and charge disproportionation which occur in organic materials resulting from correlation effects are analysed. Experiments under magnetic field, and more specifically field-induced CDWs are discussed. Properties of meso-and nanostructures of CDWs are also reviewed.
397 citations
IBM1
TL;DR: It is shown that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and density functional theory calculations are used to verify that these maps reflect the intramolecular distribution of charge.
Abstract: Kelvin probe force microscopy can reveal the distribution of charge within a single naphthalocyanine molecule.
305 citations
TL;DR: In this article, the authors review the process that led to the development of one of the most widely used force fields in the area of ionic liquids modeling, analyze its subsequent expansions and alternative models, and consider future routes of improvement to overcome present limitations.
Abstract: In this account, we review the process that led to the development of one of the most widely used force fields in the area of ionic liquids modeling, analyze its subsequent expansions and alternative models, and consider future routes of improvement to overcome present limitations. This includes the description and discussion of (1) the rationale behind the generic and systematic character of the Canongia Lopes & Padua (CLP (2) the families of ionic liquids that have been (and continue to be) parameterized over the years and those that are the most challenging both in theoretical and applied terms; (3) the steps that lead to a correct parameterization of each type of ion and its homologous family, with special emphasis on the correct modeling of their flexibility and charge distribution; (4) the validation processes of the CLP and finally (5) the compromises that have to be attained when choosing between generic or specific force fields, coarse-grain or atomistic models, and polarizable or non-polarizable methods. The application of the CL&P and other force fields to the study of ionic liquids using quantum- and statistical-mechanics methods has led to the discovery and analysis of the unique nature of their liquid phases, that is, the notion that ionic liquids are nano-segregated fluids with structural and dynamic heterogeneities at the nanoscopic scale. This successful contribution of theoretical chemistry to the field of ionic liquids will also serve as a guide throughout the ensuing discussion.
290 citations
01 Jan 2012
TL;DR: A guided tour of modern charge density analysis can be found in this article, where the authors provide an overview of charge density and its applications in materials and energy science. But the authors do not discuss the application of experimental charge density in bio-molecular reactions.
Abstract: A guided tour through modern charge density analysis.- Electron densities and related properties from the ab-initio simulation of crystalline solids.- Modeling and analysing thermal motion in experimental charge density studies.- Spin and the Complementary Worlds of Electron Position and Momentum Densities.- Past, present and future of charge density and density matrix refinements.- Using wavefunctions to get more information out of diffraction experiments.- Local Models for Joint Position and Momentum Density Studies.- Magnetization densities in material science.- Beyond Standard Charge Density Topological Analyses.- On the Interplay Between Real and Reciprocal Space Properties.- Intermolecular interaction energies from experimental charge density studies.- Chemical Information from Charge Density Studies.- Charge density in materials and energy science.- A generic force field based on Quantum Chemical Topology.- Frontier Applications of Experimental Charge Density and Electrostatics to Bio-Macromolecules.- Charge densities and crystal engineering.- Electron Density Topology of Crystalline Solids at High Pressure.- Bonding changes along solid-solid phase transitions using the Electron Localization Function approach.- Multi-temperature electron density studies.- Transient Charge Density Maps from Femtosecond X-Ray Diffraction.- Charge density and chemical reactions: a unified view from Conceptual DFT.
246 citations
TL;DR: Investigation of the influence of electrostatic surface potential distribution of monoclonal antibodies (MAbs) on intermolecular interactions and viscosity finds replacement of charge residues in the sequence of MAb-2, M-10, did not invoke charge distribution to the same extent as M Ab-1 and hence exhibited a similar viscolysis and self-association profile as Mab-2.
Abstract: The present work investigates the influence of electrostatic surface potential distribution of monoclonal antibodies (MAbs) on intermolecular interactions and viscosity. Electrostatic models suggest MAb-1 has a less uniform surface charge distribution than MAb-2. The patches of positive and negative potential on MAb-1 are predicted to favor intermolecular attraction, even in the presence of a small net positive charge. Consistent with this expectation, MAb-1 exhibits a negative second virial coefficient (B₂₂), an increase in static structure factor, S((q→0)), and a decrease in hydrodynamic interaction parameter, H((q→0)), with increase in MAb-1 concentration. Conversely, MAb-2 did not show such heterogeneous charge distribution as MAb-1 and hence favors intermolecular repulsion (positive B₂₂), lower static structure factor, S((q→0)), and repulsion induced increase in momentum transfer, H((q→0)), to result in lower viscosity of MAb-2. Charge swap mutants of MAb-1, M-5 and M-7, showed a decrease in charge asymmetry and concomitantly a loss in self-associating behavior and lower viscosity than MAb-1. However, replacement of charge residues in the sequence of MAb-2, M-10, did not invoke charge distribution to the same extent as MAb-1 and hence exhibited a similar viscosity and self-association profile as MAb-2.
242 citations
TL;DR: In this paper, experimentally determined reaction orders depend not only on the shape of the density of states but also on the spatial distribution of carriers in solar cells with small depletion regions due to small active layer thicknesses or due to large unintentional background doping of polymers.
Abstract: Nongeminate recombination in polymer:fullerene solar cells is frequently characterized using transient optoelectronic measurements that allow the determination of recombination rates, charge carrier lifetimes, and average charge carrier concentrations as a function of voltage. These data are often interpreted in terms of an empirical reaction order defining how recombination depends on measured charge density. In polymer:fullerene solar cells, the empirical reaction orders are often considerably larger than 2, which had previously been explained in terms of the nonlinear relationship between mobile and trapped charge carriers in the presence of an exponential tail of localized states. Here, we show that experimentally determined reaction orders depend not only on the shape of the density of states but also on the spatial distribution of carriers. In particular, in solar cells with small depletion regions due to small active layer thicknesses or due to large unintentional background doping of the polymers, the reaction order can assume values that are much larger than the value expected from the shape of the density of states alone.
198 citations
TL;DR: A number of the charge-density-wave materials reveal a transition to the macroscopic quantum state around 200 K, and graphene-like mechanical exfoliation of TiSe(2) crystals is used to prepare a set of films with different thicknesses.
Abstract: A number of the charge-density-wave materials reveal a transition to the macroscopic quantum state around 200 K. We used graphene-like mechanical exfoliation of TiSe2 crystals to prepare a set of films with different thicknesses. The transition temperature to the charge-density-wave state was determined via modification of Raman spectra of TiSe2 films. It was established that the transition temperature can increase from its bulk value to ∼240 K as the thickness of the van der Waals films reduces to the nanometer range. The obtained results are important for the proposed applications of such materials in the collective-state information processing, which require room-temperature operation.
157 citations
TL;DR: Observations confirm key aspects of a predicted electric double layer structure from an analytical Landau-Ginzburg-type continuum theory incorporating ion correlation effects, and provide a new baseline for understanding the fundamental nanoscale response of RTILs at charged interfaces.
Abstract: The nanoscale interactions of room temperature ionic liquids (RTILs) at uncharged (graphene) and charged (muscovite mica) solid surfaces were evaluated with high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations. At uncharged graphene surfaces, the imidazolium-based RTIL ([bmim+][Tf2N–]) exhibits a mixed cation/anion layering with a strong interfacial densification of the first RTIL layer. The first layer density observed via experiment is larger than that predicted by simulation and the apparent discrepancy can be understood with the inclusion of, dominantly, image charge and π-stacking interactions between the RTIL and the graphene sheet. In contrast, the RTIL structure adjacent to the charged mica surface exhibits an alternating cation–anion layering extending 3.5 nm into the bulk fluid. The associated charge density profile demonstrates a pronounced charge overscreening (i.e., excess first-layer counterions with respect to the adjacent surface charge), highlighti...
152 citations
TL;DR: In this article, the simple substitution of carbon with nitrogen atoms has been identified as the most common doping configuration, indicating a reduction of local charge density on top of the nitrogen atoms and a charge transfer to the neighboring carbon atoms.
Abstract: Nitrogen-doped epitaxial graphene grown on SiC(0001) was prepared by exposing the surface to an atomic nitrogen flux Using scanning tunneling microscopy and scanning tunneling spectroscopy (STS), supported by density functional theory (DFT) calculations, the simple substitution of carbon with nitrogen atoms has been identified as the most common doping configuration High-resolution images reveal a reduction of local charge density on top of the nitrogen atoms, indicating a charge transfer to the neighboring carbon atoms Local STS spectra clearly evidenced the energy levels associated with the chemical doping by nitrogen, localized in the conduction band Various other nitrogen-related defects have been observed The bias dependence of their topographic signatures demonstrates the presence of structural configurations more complex than substitution as well as hole doping © 2012 American Physical Society
TL;DR: It is found that water molecules hydrogen-bonded to the surface have different orientations depending on the strength of the hydrogen bonds and this observation is used to explain the features in the surface vibrational spectra measured by sum frequency generation spectroscopy.
Abstract: The organization of water at the interface with silica and alumina oxides is analysed using density functional theory-based molecular dynamics simulation (DFT-MD). The interfacial hydrogen bonding is investigated in detail and related to the chemistry of the oxide surfaces by computing the surface charge density and acidity. We find that water molecules hydrogen-bonded to the surface have different orientations depending on the strength of the hydrogen bonds and use this observation to explain the features in the surface vibrational spectra measured by sum frequency generation spectroscopy. In particular, 'ice-like' and 'liquid-like' features in these spectra are interpreted as the result of hydrogen bonds of different strengths between surface silanols/aluminols and water.
TL;DR: A fourth-order modified Poisson equation is developed that captures the essential features in a simple continuum framework for nonlocal electrostatics of interacting effective charges and allows for simple calculations of electrokinetic flows in correlated ionic fluids.
Abstract: The classical theory of electrokinetic phenomena is based on the mean-field approximation that the electric field acting on an individual ion is self-consistently determined by the local mean charge density. This paper considers situations, such as concentrated electrolytes, multivalent electrolytes, or solvent-free ionic liquids, where the mean-field approximation breaks down. A fourth-order modified Poisson equation is developed that captures the essential features in a simple continuum framework. The model is derived as a gradient approximation for nonlocal electrostatics of interacting effective charges, where the permittivity becomes a differential operator, scaled by a correlation length. The theory is able to capture subtle aspects of molecular simulations and allows for simple calculations of electrokinetic flows in correlated ionic fluids. Charge-density oscillations tend to reduce electro-osmotic flow and streaming current, and overscreening of surface charge can lead to flow reversal. These effects also help to explain the suppression of induced-charge electrokinetic phenomena at high salt concentrations.
TL;DR: Using state-of-the-art, aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy with atomic-scale spatial resolution, experimental evidence for an intrinsic electronic reconstruction at the LAO/STO interface is shown.
Abstract: We present direct, atomic-column-resolved scanning transmission electron microscopy and electron energy loss measurements of atomic displacements and Ti valence in abrupt, conductive LAO/STO interfaces. We find that two distinct but interrelated mechanisms are responsible for screening the diverging electric potential in the LAO film: 1) charge injection in the interfacial Ti planes, and 2) dielectric relaxation in both LAO and STO through ionic displacements. The injected charge density decays over a length of nearly 3 unit cells within the STO substrate. The total injected charge is lower than predicted by pure electronic reconstruction. The origin of this discrepancy is attributed to cation and oxygen displacements, which we observe in both LAO and STO, and generate a polarization opposite to the intrinsic polarization of the LAO film. Our data attribute a minor role to oxygen vacancies and cation intermixing.
Journal Article•
TL;DR: The AIMD-c charge was found to predict experimental results better than the other four sets of charges, indicating that fitting charges from crystal phase DFT calculations, instead of extensive sampling of the liquid phase configurations, is a simple and reliable way to derive atomic charges for condensed phase ionic liquid simulations.
Abstract: The atomic charges for two ionic liquids (ILs), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) and 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF6]), were derived from periodic crystal phase calculations with density functional theory (DFT) and plane wave basis sets (denoted as “AIMD-c charge”). For both ILs, the total charge was found to be ±0.8 e for the cation and anion, respectively, due to the charge transfer between ions and polarization caused by the environment. These atomic charges were used in a force field developed within the general Amber force field framework. Using this force field, static, dynamic, and thermodynamic properties were computed for the two ILs using molecular dynamics simulation. The results were compared against results obtained using the same Amber force field but four different sets of partial charges, denoted as full charge, scaled charge, AIMD-l charge, and AIMD-b charge, respectively. The full charge was derived from quantum chemistry calculation of isolated ions in a vacuum and resulted in a total charge of unity on each ion. The scaled charge was obtained by uniformly scaling the full charge by 0.8. AIMD-l and AIMD-b charges were derived from liquid phase ab initio molecular dynamics simulations. The scaled charges have the same total charge on the ions as the AIMD-c charge but different distributions. It was found that simulation results not only depend on the total charge of each ion, but they are also sensitive to the charge distribution within an ion, especially for dynamic and thermodynamic properties. Overall, for the two ILs under study, the AIMD-c charge was found to predict experimental results better than the other four sets of charges, indicating that fitting charges from crystal phase DFT calculations, instead of extensive sampling of the liquid phase configurations, is a simple and reliable way to derive atomic charges for condensed phase ionic liquid simulations.
TL;DR: In this article, the effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations, and it was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the voltage level.
Abstract: Effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations The considered insulator consisted of a glass fiber reinforced epoxy core covered with a layer of silicone rubber and terminated by metallic electrodes with rounded smooth edges In the experiments, the insulator surface was charged by external corona while keeping the electrodes grounded and different charging levels were realized by varying its intensity A series of disruptive discharge tests were carried out on the charged insulator under negative dc voltages It was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the flashover voltage level A theoretical model has been developed and utilized for analyzing the experimental results In the model, surface charge density profiles deduced from measured surface potential distributions were used as boundary conditions for calculations of electric fields The measured and calculated flashover voltages were found to be in agreement indicating that the observed variations in the flashover characteristics could be attributed to the modifications of the electric field produced by the surface charges
TL;DR: Molecular dynamics simulations were performed for a room-temperature ionic liquid near idealized spherical OLCs and reveal that the surface charge density increases almost linearly with the potential applied on electric double layers (EDLs) near O LCs.
Abstract: Recent experiments have revealed that onion-like carbons (OLCs) offer high energy density and charging/discharging rates when used as the electrodes in supercapacitors. To understand the physical origin of this phenomenon, molecular dynamics simulations were performed for a room-temperature ionic liquid near idealized spherical OLCs with radii ranging from 0.356 to 1.223 nm. We find that the surface charge density increases almost linearly with the potential applied on electric double layers (EDLs) near OLCs. This leads to a nearly flat shape of the differential capacitance versus the potential, unlike the bell or camel shape observed on planar electrodes. Moreover, our simulations reveal that the capacitance of EDLs on OLCs increases with the curvature or as the OLC size decreases, in agreement with experimental observations. The curvature effect is explained by dominance of charge overscreening over a wide potential range and increased ion density per unit area of electrode surface as the OLC becomes smaller.
TL;DR: It is seen thereby that the same building principle that is used by nature to create an excitation energy funnel in the FMO protein also allows for efficient dissipation of the excitons’ excess energy.
Abstract: We report a method for the structure-based calculation of the spectral density of the pigment–protein coupling in light-harvesting complexes that combines normal-mode analysis with the charge density coupling (CDC) and transition charge from electrostatic potential (TrEsp) methods for the computation of site energies and excitonic couplings, respectively The method is applied to the Fenna–Matthews–Olson (FMO) protein in order to investigate the influence of the different parts of the spectral density as well as correlations among these contributions on the energy transfer dynamics and on the temperature-dependent decay of coherences The fluctuations and correlations in excitonic couplings as well as the correlations between coupling and site energy fluctuations are found to be 1 order of magnitude smaller in amplitude than the site energy fluctuations Despite considerable amplitudes of that part of the spectral density which contains correlations in site energy fluctuations, the effect of these correla
TL;DR: In this article, the dominant temperature and illumination-dependent charge extraction measurements were performed under open circuit and short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC61BM) and PTB7:PC71BM (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4, 5-b′]dithiophene]-dith
Abstract: Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility and protection of trapped charge carriers from recombination by a donor–acceptor phase separation until re-emission from these deep states. In order to clarify which mechanism is dominant temperature- and illumination-dependent charge extraction measurements are performed under open circuit and short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC61BM) and PTB7:PC71BM (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) solar cells in combination with current–voltage characteristics. It is shown that the charge carrier density n dependence of the mobility μ and the recombination prefactor are different for P3HT:PC61BM at temperatures below 300 K and PTB7:PC71BM at room temperature. Therefore, in addition to μ(n), a detrapping limited recombination in systems with at least partial donor–acceptor phase separation is required to explain the high recombination orders.
TL;DR: In this article, temperature and illumination dependent charge extraction measurements under open circuit as well as short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C$_{61}$butyric acid methyl ester (P3HT:PC$_{ 61}$BM) and PTB7:PC $_{71} $BM (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4, 5-b']
Abstract: Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility, and protection of trapped charge carriers from recombination by a donor--acceptor phase separation until reemission from these deep states. In order to clarify which mechanism is dominant we performed temperature and illumination dependent charge extraction measurements under open circuit as well as short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C$_{61}$butyric acid methyl ester (P3HT:PC$_{61}$BM) and PTB7:PC$_{71}$BM (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) solar cells in combination with current--voltage characteristics. We show that the charge carrier density $n$ dependence of the mobility $\mu$ and the recombination prefactor are different for PC$_{61}$BM at temperatures below 300K and PTB7:PC$_{71}$BM at room temperature. Therefore, in addition to $\mu(n)$ a detrapping limited recombination in systems with at least partial donor--acceptor phase separation is required to explain the high recombination orders.
TL;DR: In this article, a density functional theory (DFT) study of an oxygen reduction reaction (ORR) involving graphyne and demonstrate that graphyne is a good, metal-free electrocatalyst for ORRs in acidic fuel cells.
Abstract: Graphyne, a new two-dimensional periodic carbon allotrope with a one-atom-thick sheet of carbon built from triple- and double-bonded units of two sp- and sp2-hybridized carbon atoms, has been shown in recent studies to have the potential for high-density hydrogen and lithium storage. We report here a density functional theory (DFT) study of an oxygen reduction reaction (ORR) involving graphyne and demonstrate that graphyne is a good, metal-free electrocatalyst for ORRs in acidic fuel cells. We optimized the geometrical structure, calculated the charge densities on each carbon atom in the graphyne, and simulated each step of the ORR reaction involving graphyne. The simulation results indicate that the distribution of the charge density at each carbon atom on the graphyne plane is not uniform and that a large number of positively charged carbon atoms, which are beneficial to the adsorption of O2 and OOH+ molecules, can behave as catalytic sites to facilitate ORRs. When H+ is introduced into the system, a se...
TL;DR: In this article, current trends in crystal engineering are critically discussed based on experimental charge density analysis for quantifying various interatomic interactions, from weak van der Waals to coordinate bonds, as applied to the design of crystalline materials.
TL;DR: In this article, a drain current model for triple-gate n-type junctionless nanowire transistors is proposed based on the solution of the Poisson equation, which is validated using 3-D TCAD simulations where the drain current and its derivatives, the potential, and the charge density have been compared, showing a good agreement for all parameters.
Abstract: This paper proposes a drain current model for triple-gate n-type junctionless nanowire transistors. The model is based on the solution of the Poisson equation. First, the 2-D Poisson equation is used to obtain the effective surface potential for long-channel devices, which is used to calculate the charge density along the channel and the drain current. The solution of the 3-D Laplace equation is added to the 2-D model in order to account for the short-channel effects. The proposed model is validated using 3-D TCAD simulations where the drain current and its derivatives, the potential, and the charge density have been compared, showing a good agreement for all parameters. Experimental data of short-channel devices down to 30 nm at different temperatures have been also used to validate the model.
TL;DR: The experimentally well-established apparent excess surface conductivity follows from the model for all hydrodynamic boundary conditions without additional assumptions, and fits multiple published sets of experimental data on hydrophilic and hydrophobic surfaces with striking accuracy.
Abstract: We calculate the electro-osmotic mobility and surface conductivity at a solid–liquid interface from a modified Poisson–Boltzmann equation, including spatial variations of the dielectric function and the viscosity that where extracted previously from molecular dynamics simulations of aqueous interfaces. The low-dielectric region directly at the interface leads to a substantially reduced surface capacitance. At the same time, ions accumulate into a highly condensed interfacial layer, leading to the well-known saturation of the electro-osmotic mobility at large surface charge density regardless of the hydrodynamic boundary conditions. The experimentally well-established apparent excess surface conductivity follows from our model for all hydrodynamic boundary conditions without additional assumptions. Our theory fits multiple published sets of experimental data on hydrophilic and hydrophobic surfaces with striking accuracy, using the nonelectrostatic ion–surface interaction as the only fitting parameter.
TL;DR: In this paper, the second-order optical nonlinearity is derived from the hydrodynamic description of electron plasma and originates from the presence of material interfaces in the case of small metal particles.
Abstract: The efficient resonant nonlinear coupling between localized surface plasmon modes is demonstrated in a simple and intuitive way using boundary integral formulation and utilizing second-order optical nonlinearity. The nonlinearity is derived from the hydrodynamic description of electron plasma and originates from the presence of material interfaces in the case of small metal particles. The coupling between fundamental and second-harmonic modes is shown to be symmetry selective and proportional to the spatial overlap between polarization dipole density of the second-harmonic mode and the square of the polarization charge density of the fundamental mode. Particles with high geometrical symmetry will convert a far-field illumination into dark nonradiating second-harmonic modes, such as quadrupoles. Effective second-harmonic susceptibilities are proportional to the surface-to-volume ratio of a particle, emphasizing the nanoscale enhancement of the effect.
TL;DR: The study rules out the hypervalent description of the sulfur atom in the sulfate group and suggests a bonding situation where the S-O interactions can be characterized as highly polarized, covalent bonds, with the "single bond" description significantly prevailing over the "double bond" picture.
Abstract: One of the most basic concepts in chemical bonding theory is the octet rule, which was introduced by Lewis in 1916, but later challenged by Pauling to explain the bonding of third-row elements. In the third row, the central atom was assumed to exceed the octet by employing d orbitals in double bonding leading to hypervalency. Ever since, polyoxoanions such as SO42–, PO43–, and ClO4– have been paradigmatic examples for the concept of hypervalency in which the double bonds resonate among the oxygen atoms. Here, we examine S–O bonding by investigating the charge density of the sulfate group, SO42–, within a crystalline environment based both on experimental and theoretical methods. K2SO4 is a high symmetry inorganic solid, where the crystals are strongly affected by extinction effects. Therefore, high quality, very low temperature single crystal X-ray diffraction data were collected using a small crystal (∼30 μm) and a high-energy (30 keV) synchrotron beam. The experimental charge density was determined by m...
TL;DR: In this paper, the authors analyzed the chemoelectrical behavior of ionic polymer metal composites (IPMCs) in the small voltage range with a novel hypothesis on the charge dynamics in proximity of the electrodes.
Abstract: In this paper, we analyze the chemoelectrical behavior of ionic polymer metal composites (IPMCs) in the small voltage range with a novel hypothesis on the charge dynamics in proximity of the electrodes. In particular, we homogenize the microscopic properties of the interfacial region through a so-called composite layer which extends between the polymer membrane and the metal electrode. This layer accounts for the dissimilar properties of its constituents by describing the charge distribution via two species of charge carriers, that is, electrons and mobile counterions. We model the charge dynamics in the IPMC by adapting the multiphysics formulation based on the Poisson-Nernst-Planck (PNP) framework, which is enriched through an additional term to capture the electron transport in the composite layer. Under the hypothesis of small voltage input, we use the linearized PNP model to derive an equivalent IPMC circuit model with lumped elements. The equivalent model comprises a resistor connected in series with the parallel of a capacitor and a Warburg impedance element. These elements idealize the phenomena of charge build up in the double layer region and the faradaic impedance related to mass transfer, respectively. We validate the equivalent model through measurements on in-house fabricated samples addressing both IPMC step response and impedance, while assessing the influence of repeated plating cycles on the electrical properties of IPMCs. Experimental results are compared with theoretical findings to identify the equivalent circuit parameters. Findings from this study are compared with alternative impedance models proposed in the literature.
TL;DR: An interesting trend is observed in steady-state current-potential measurements using single conical nanopores, where a threshold low-conductivity state is observed upon the dilution of electrolyte concentration and the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification.
Abstract: Current rectification is well known in ion transport through nanoscale pores and channel devices. The measured current is affected by both the geometry and fixed interfacial charges of the nanodevices. In this article, an interesting trend is observed in steady-state current–potential measurements using single conical nanopores. A threshold low-conductivity state is observed upon the dilution of electrolyte concentration. Correspondingly, the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification. This novel trend at opposite bias polarities is employed to differentiate the ion flux affected by the fixed charges at the substrate–solution interface (surface effect), with respect to the constant asymmetric geometry (volume effect). The surface charge density (SCD) of individual nanopores, an important physical parameter that is challenging to measure experimentally and is known to vary from one nanopore to another, is directly quantified by...
TL;DR: In this article, the intrinsic limitations of electrochemical impedance spectroscopy (EIS) in measuring electric double layer (EDL) capacitance have been clarified, and a characteristic time for ion diffusion τm was identified.
École Centrale Paris1, University of Paris-Sud2, Charles University in Prague3, Helmholtz-Zentrum Berlin4, University of Zaragoza5, Spanish National Research Council6, Jožef Stefan Institute7, Stony Brook University8, Brookhaven National Laboratory9, KAERI10, Max Planck Society11, University of Cambridge12, Autonomous University of Barcelona13
TL;DR: In this paper, a wide variety of experimental techniques were combined to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature, and the transition at $T$ $=$ 140.3 K was shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface.
Abstract: We combine a wide variety of experimental techniques to analyze two heretofore mysterious phase transitions in multiferroic bismuth ferrite at low temperature. Raman spectroscopy, resonant ultrasound spectroscopy, electron paraelectric resonance, x-ray lattice constant measurements, conductivity and dielectric response, and specific heat and pyroelectric data have been collected for two different types of samples: single crystals and, in order to maximize surface/volume ratio to enhance surface phase transition effects, BiFeO${}_{3}$ nanotubes were also studied. The transition at $T$ $=$ 140.3 K is shown to be a surface phase transition, with an associated sharp change in lattice parameter and charge density at the surface. Meanwhile, the 201 K anomaly appears to signal the onset of glassy behavior.