Interfacial Electrostatic Potential Research Articles

Plasmonic Local Electric Field-Enhanced Interface toward High-Efficiency Cu2ZnSn(S,Se)4 Thin-Film Solar Cells.

The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have shown a continuous rise in power conversion efficiencies in the past years. However, the encountered interfacial problems with respect to charge recombination and extraction losses at the CdS/CZTSSe heterojunction still hinder their further development. In this work, an additional plasmonic local electric field is imposed into the CdS/CZTSSe interface through the electrostatic assembly of a two-dimensional (2D) ordered Au@SiO2 NP array onto an aminosilane-modified surface absorber. The interfacial electric properties are tuned by controlling the coverage particle distance, and the finite-difference time domain (FDTD) simulation demonstrates that the strong near-field enhancement mainly occurs near the p-n junction interface. It is shown that the imposed local electric field leads to interfacial electrostatic potential (Velec) augmentation and improves the charge extraction and recombination processes. These electric benefits enable remarkable improvements in open-circuit voltage (Voc) and short-circuit current (Jsc), leading to the cell efficiency being increased from 10.19 to 11.50%. This work highlights the dramatic role of the plasmonic local electric field and the use of the 2D Au@SiO2 NP array to modify a surface absorber instead of the extensively used ion passivation, providing a new strategy for p-n junction engineering in kesterite photovoltaics.

In-plane quasi-single-domain BaTiO3 via interfacial symmetry engineering

The control of the in-plane domain evolution in ferroelectric thin films is not only critical to understanding ferroelectric phenomena but also to enabling functional device fabrication. However, in-plane polarized ferroelectric thin films typically exhibit complicated multi-domain states, not desirable for optoelectronic device performance. Here we report a strategy combining interfacial symmetry engineering and anisotropic strain to design single-domain, in-plane polarized ferroelectric BaTiO3 thin films. Theoretical calculations predict the key role of the BaTiO3/PrScO3{({{{{{boldsymbol{110}}}}}})}_{{{{{{bf{O}}}}}}} substrate interfacial environment, where anisotropic strain, monoclinic distortions, and interfacial electrostatic potential stabilize a single-variant spontaneous polarization. A combination of scanning transmission electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis loop measurements, and second harmonic generation measurements directly reveals the stabilization of the in-plane quasi-single-domain polarization state. This work offers design principles for engineering in-plane domains of ferroelectric oxide thin films, which is a prerequisite for high performance optoelectronic devices.

Probing Heterogeneous Charge Distributions at the α-Al2O3(0001)/H2O Interface.

Unlike metal or semiconductor electrodes, the surface charge resulting from the protonation or deprotonation of insulating mineral oxides is highly localized and heterogeneous in nature. In this work the Stark active C≡N stretch of potassium thiocyanate is used as a molecular probe of the heterogeneity of the interfacial electrostatic potential at the α-Al2O3(0001)/H2O interface. Vibrational sum frequency generation (vSFG) measurements performed in the OH stretching region suggest that thiocyanate species organize interfacial water similarly to halide ions. Changes in the electrostatic potential are then tracked via Stark shifts of the vibrational frequency of the thiocyanate stretch. Our vSFG measurements show that we can simultaneously measure the vSFG response of SCN- ions experiencing charged and neutral surface sites. We assign local potentials of +308 and -154 mV to positively and negatively charged aluminol groups that are present at pH = 4 and pH = 10, respectively. Thiocyanate anions at positively charged surface sites and negatively charged surface sites and those participating in contact ion pairing adopt similar orientations and are oppositely oriented relative to thiocyanate ions near neutral surface sites. All four species followed Langmuir adsorption isotherms. Density functional theory-molecular dynamics (DFT-MD) simulations of SCN- near the neutral α-Al2O3(0001)/H2O interface show that the vSFG response in the C≡N stretch region originates from a SCN-H-O-Al complex, suggesting the surface site specificity of these experiments. To our knowledge this is the first spectroscopic measurement of local potentials associated with a heterogeneously charged surface. The ability to probe the evolution of local charges in situ could provide vital insight into many industrial, electrochemical, and geochemically relevant interfaces.

The Importance of the Water Molecular Quadrupole for Estimating Interfacial Potential Shifts Acting on Ions Near the Liquid-Vapor Interface.

Interfacial electrostatic potential gradients arise from nonuniform charge distributions encountered crossing the interface. The charges involved can include the molecular charges predominantly bound to each neutral solvent molecule and distributions of ions (or electrons) free to move in the interfacial region. This paper focuses on the solvent contribution to the interfacial potential. Quasichemical theory (QCT) provides a physical framework for the analysis of near-local (chemical) and far-field contributions to ion solvation free energies. Here, we utilize QCT to analyze cavity net potentials that contribute to the single-ion real solvation free energy. In particular, we discuss the results of molecular dynamics simulations of water droplets large enough to exhibit bulklike behavior in the droplet interior. A multipolar analysis of the cavity potential illustrates the importance of the solvent molecular quadrupole due to the near-cancellation of the dipolar contributions from the cavity-liquid and liquid-vapor interfaces. The results reveal the physical origin of the previously observed strong classical model dependence of the cavity potential.

The interfacial electrostatic potential modulates the insertion of cell-penetrating peptides into lipid bilayers.

Cell-penetrating peptides (CPP) are short sequences of cationic amino-acids that show a surprising ability to traverse lipid bilayers. CPP are considered to be some of the most effective vectors to introduce membrane-impermeable cargos into cells, but the molecular basis of the membrane translocation mechanisms and its dependence on relevant membrane physicochemical properties have yet to be fully determined. In this paper we resort to Molecular Dynamics simulations and experiments to investigate how the electrostatic potential across the lipid/water interface affects the insertion of hydrophilic and amphipathic CPP into two-dimensional lipid structures. Simulations are used to quantify the effect of the transmembrane potential on the free-energy profile associated with the transfer of the CPP across a neutral lipid bilayer. It is found that the electrostatic bias has a relatively small effect on the binding of the peptides to the membrane surface, but that it significantly lowers the permeation barrier. A charge compensation mechanism, arising from the segregation of counter-ions while the peptide traverses the membrane, determines the shape and symmetry of the free-energy curves and underlines relevant mechanistic considerations. Langmuir monolayer experiments performed with a variety of amphiphiles model the incorporation of the CPP into the external membrane leaflet. It is shown that the dipole potential of the monolayer controls the extent of penetration of the CPP into the lipid aggregate, to a greater degree than its surface charge.

Inorganic Surface Engineering to Enhance Perovskite Solar Cell Efficiency

The photoconversion efficiency of perovskite solar cells (PSCs) is enhanced by the deposition of inorganic nanoparticles (NPs) at the interface between the compact TiO2 electron-selective contact and the mesoporous TiO2 film. The NPs used are core/shell Au@SiO2, where a thin SiO2 coating protects the Au core from the direct chemical interaction with CH3NH3PbI3 halide perovskite used as light-harvesting material. The samples prepared with Au@SiO2 NPs exhibit a higher external quantum efficiency in the complete wavelength range at which perovskite presents light absorption and not just at the wavelengths at which Au@SiO2 NPs present their absorption peak. This fact rules out a direct plasmonic process as responsible for the enhancement of cell performance. A detailed characterization by photoluminescence, impedance spectroscopy, and open-circuit voltage decay unveils a modification of the interfacial properties with an augmentation of the interfacial electrostatic potential that increases both photovoltage and photocurrent. This article highlights the dramatic role of interfaces in the performance of PSCs. The use of reduced quantities of highly stable inorganic compounds to modify the PSC interface instead of the extensively used organic compounds opens the door to a new surface engineering based on inorganic compounds.

Molecular-level insight into the binding of arginine to a zwitterionic Langmuir monolayer

Arginine molecules bind to a DPPC monolayer, altering the interfacial electrostatic potential and the lateral mobility of the lipids, while having little effect on the compression isotherm of the monolayer.

Redefining electrical double layer thickness in narrow confinements: Effect of solvent polarization

In this paper we delineate the consequences of field-dependent solvent polarization in the electric double layer (EDL) electrostatic potential distribution, and the effective EDL thickness in narrow nanofluidic confinements with thick (or overlapping) EDLs. The EDL, formed at the interface between a charged substrate and an electrolyte solution, induces a large electric field spanning across few nanometer distances from the interface. As a result, a polar solvent like water gets polarized, making its relative permittivity a function of the EDL electric field. This affects the overall EDL electrostatic potential distribution and most importantly, leads to a significant reduction of the effective EDL thickness, with the extent of the reduction being dictated by the value of field independent EDL thickness, strength of the solvent polarization, and the substrate-liquid interfacial electrostatic potential. Such a finding will necessitate redefining the classical EDL thickness, which will be of overwhelming significance in nanofluidic transport.

Absolute Hydration Free Energy Scale for Alkali and Halide Ions Established from Simulations with a Polarizable Force Field

A polarizable potential function for the hydration of alkali and halide ions is developed on the basis of the recent SWM4-DP water model [Lamoureux, G.; MacKerell, A. D., Jr.; Roux, B. J. Chem. Phys. 2003, 119, 5185]. Induced polarization is incorporated using classical Drude oscillators that are treated as auxiliary dynamical degrees of freedom. The ions are represented as polarizable Lennard-Jones centers, whose parameters are optimized to reproduce the binding energies of gas-phase monohydrates and the hydration free energies in the bulk liquid. Systematic exploration of the parameters shows that the monohydrate binding energies can be consistent with a unique hydration free energy scale if the computed hydration free energies incorporate the contribution from the air/water interfacial electrostatic potential (-540 mV for SWM4-DP). The final model, which can satisfyingly reproduce both gas and bulk-phase properties, corresponds to an absolute scale in which the intrinsic hydration free energy of the proton is -247 kcal/mol.

First-principles study of oxygen-deficientSrTiO3∕MgO(100)interfaces

The electronic structure and the energetics of oxygen deficient interfaces, constituted by MgO(100) substrates that are capped by epitaxial $\mathrm{Ti}{\mathrm{O}}_{2}$ or $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ layers, are studied within first principles. Various configurations with one O vacancy per $(1\ifmmode\times\else\texttimes\fi{}1)$ surface unit cell, which is located either in the MgO or in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ interfacial layers, are compared. The excess electrons either fill Ti states at the bottom of the conduction band or form an $F$-center as in MgO(100), depending not only upon the side of the interface in which the vacancy is created, but also on the actual vacancy environment. In most cases that we consider, the interface has a metallic character. Our main result regards the order of stability of the various configurations; the oxygen vacancies can be formed more easily in the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ overlayer than in the MgO substrate. Moreover, we show that the formation energies of O vacancies can be related to the nature and energy level of the excess electron state. Such a behavior is primarily determined by the interfacial electrostatic potential, which plays a prominent role as in other oxide/oxide interfaces.

The effect of surface probe density on DNA hybridization.

The hybridization of complementary strands of DNA is the underlying principle of all microarray-based techniques for the analysis of DNA variation. In this paper, we study how probe immobilization at surfaces, specifically probe density, influences the kinetics of target capture using surface plasmon resonance (SPR) spectroscopy, an in situ label-free optical method. Probe density is controlled by varying immobilization conditions, including solution ionic strength, interfacial electrostatic potential and whether duplex or single stranded oligonucleotides are used. Independent of which probe immobilization strategy is used, we find that DNA films of equal probe density exhibit reproducible efficiencies and reproducible kinetics for probe/target hybridization. However, hybridization depends strongly on probe density in both the efficiency of duplex formation and the kinetics of target capture. We propose that probe density effects may account for the observed variation in target-capture rates, which have previously been attributed to thermodynamic effects.

A kinetic model for dynamic interfacial tension variation in an acidic oil/alkali/surfactant system

A chemical kinetic model is developed to describe the dynamic variation of oil–water interfacial tension (IFT) in an acidic oil/alkaline solution system where the aqueous phase contains a surfactant. The effect of mass transfer resistances on the transport of surface active compounds in the bulk is examined. The effect of the variation of interfacial electrostatic potential on adsorption, desorption and ionization is accounted for in the model. The model is solved numerically to predict a wide range of experimentally observed phenomena. Parameters such as the energy barriers for the various processes occurring in the system are estimated from the experimental data. The effects of bulk concentrations and mass transfer resistances are discussed. The significance of the kinetics of the interfacial dissociation reaction is explored.

Single Cation Adsorption Equation for the Solution–Metal Oxide Interface

Simultaneous solution of the Gibbs–Lewis thermodynamic equations for equilibrium proton and cation complexation at the solution–metal oxide interface results in a single cation adsorption equation in closed mathematical form. The simple form of the cation adsorption equation allows direct assessment of cation adsorption dependence on: (1) solution pH, (2) protonation differences of metal oxide surfaces, (3) differences in adsorption affinity for different cations, (4) concentration of solid versus cations in the dispersion, etc. Experimental and calculated cation adsorption results are compared. From a knowledge of the cation adsorption behavior combined with electrostatic potential equations, the electrostatic behavior in an interfacial system can also be determined. The resulting solution and interfacial electrostatic potential equations are single equations in closed mathematical form expressed as explicit functions of solution and interfacial variables. The dependence of these variables on solution and interfacial electrostatic potential is also examined.

Monovalent cations differentially affect membrane surface properties and membrane curvature, as revealed by fluorescent probes and dynamic light scattering

The effects of monovalent cations on the interfacial electrostatic potential ( ψ d), hydrodynamic shear boundary distance ( d s), and membrane curvature were studied in large unilamellar phospholipid and galacto/sulfolipid liposomes containing different fractions of negatively charged lipids. The differential effects of alkali metal ions on ψ d could be accurately determined at physiological surface charge densities with a surface-anchored fluorescent probe. Li + and Na + more effectively decrease ψ d and exhibit higher association constants ( K as) than K + and Cs +. These two groups of cations display qualitatively different perturbations of the interfacial structure. Combining K as values with the electrokinetic (ζ) potentials yielded the respective d s values. At low ionic strength d s more substantially increases with Li + or Na + than with K + or Cs +. Increasing surface charge density causes increased membrane curvature in the presence of K + or Cs +, but this is largely prevented by Li + or Na +. Membrane binding of the amphiphilic cation acridine orange decreases surface charge and membrane curvature more extensively than H 3O +, Li +, and Na +. The differential interface-perturbing behavior of monovalent cations is discussed with regard to their different hydration tendencies that will modulate the extent and stability of the hydrogen-bond network along the charged membrane surface.

Effects of average molecular charge on amino acid interfacial partitioning in reversed micelles

The effect of pH on amino acid interfacial partitioning in reversed micelles is reported. A simple thermodynamic analysis of the data allows estimates of the interfacial electrostatic potential in these reversed micelles to be made, from which the effects of external electrolyte type and concentration may be inferred. Good agreement with the predictions of the Poisson-Boltzmann equation is obtained. Inferences can be drawn as to the orientation of the amino acids within the structured interfaces, and it is argued that the amino acid headgroup dipole must be aligned almost normal to the direction of the electric field. Changes in amino acid speciation in reversed micellar systems, coupled with an apparent shift in pH of the external aqueous phase, can be attributed to a concentration of the hydroxonium ion in the reversed micellar water pools, and not to changes in intrinsic dissociation constants. It is demonstrated that species distribution in the AOT Winsor II system closely resembles various chromatographic processes, there being a direct analogy between the chromatographic retention coefficient and the reversed micellar interfacial partition coefficient.

The interfacial calcium ion concentration as modulator of the latency phase in the hydrolysis of dimyristoylphosphatidylcholine liposomes by phospholipase A2

Analysis of the time course of hydrolysis of dimyristoylphosphatidylcholine liposomes catalyzed by porcine pancreatic phospholipase A2 at 18 degrees C shows that, in the presence of 10 mM NaCl, the length of the latency period in the presteady-state phase increases from 3 to 10.5 min when the CaCl2 concentration is reduced from 15 to 1 mM. This inverse dependence of the lag period on calcium ion concentration is seen more readily at 1 M NaCl, where the induction time changes from 13.5 to 42 min by decreasing the concentration of CaCl2 from 15 to 1 mM. To interpret these results, we took into account the small amount of fatty acid that is produced during the latency phases. The fatty acid generates a negative surface electrostatic potential and makes the interfacial concentration of calcium ions different from the concentration in the bulk solvent. Variations in the analytical concentrations of NaCl and CaCl2 affect both the interfacial calcium ion concentration and electrostatic potential, as estimated theoretically from Grahame and Boltzmann equations. According to these estimates, the length of the latency period diminishes with the increase of the interfacial calcium concentration, but does not show any logical dependence on the change in surface electrostatic potential.

Electrostatic potentials and protein partitioning in aqueous two‐phase systems

AbstractA thermodynamic analysis unambiguously relates interfacial‐electrostatic‐potential differences measured with Ag/AgCl capillary electrodes to the equilibrium properties of an aqueous two‐phase system. Interfacial electrostatic potentials were measured as a function of total α‐cyclodextrin concentration in an aqueous two‐phase system containing 9.1 wt. % poly (ethylene glycol), 6.1 wt. % Dextran T‐70, and 1‐mM potassium iodide. An order‐of‐magnitude increase in the interfacial electrostatic potential was observed as the total concentration of α‐cyclodextrin increased from 0 to 1 mM. Measured partition coefficients for α‐chymotrypsin, lysozyme, and bovine serum albumin depend strongly on α‐cyclodextrin concentration. For example, as the concentration of α‐cyclodextrin rises from 0 to 1 mM, the partition coefficient of lysozyme decreases from 1.7 to 0.55. These measurements are in good agreement with theoretical expectations.

Interfacial electrostatic potential of polyions as probed by acid‐base indicator dyes covalently attached to the main chain

AbstractPolyanions with various charge densities with a solvatochromic acid‐base indicator dye attached to the main chain were prepared by the terpolymerization of acrylic acid, acrylamide, and a small mole fraction of 1‐(2‐methacryloyloxyethyl)‐4‐(2‐(4‐hydroxyphenyl)ethenyl)pyridinium bromide. The apparent pK value for the polymer‐bound indicator dye (pKabs) was determined by the spectroscopic pH titration. The interfacial electrostatic potential (ψ) was calculated from pKobs. Dielectric contribution to the acid‐base equilibrium was taken into account for the calculation. The electrostatic potentials thus obtained were compared with those calculated from the nonlinearized Poisson‐Boltzmann equation for a cylinder model. The indicator dye moieties are suggested to be reporting the mean field potential at a distance of about 10 Å from the axis of the polyion cylinder. The observed values as a function of the charge density were also discussed in connection with Manning's theory.

A single spectroscopic probe for the determination of both the interfacial solvent properties and electrostatic surface potential of model lipid membranes

2,6-Diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenoxide, ET(30), has been investigated in order to ascertain its suitability as a probe for both the effective interfacial dielectric constant (Iµeff) and the electrostatic surface potential (ψ0) of model lipid membranes in aqueous solution. This work establishes that the solvatochromic visible absorption band for ET(30) can be used to provide a good estimate of the Iµeff for cationic micelles. It is also shown that the acid–base dissociation of ET(30) can be utilized to obtain a quantitative measure of the ψ0 in the case of cationic micelles. There are problems and uncertainties associated with the use of ET(30) in aqueous solutions of other types of charged self-assembled surfactant aggregates, and these are discussed.