Coulombic Screening Research Articles

Elucidating the Impact of the Dielectric Environment on Excitons and Trions in 2D Semiconductor Electrodes

Transition metal dichalcogenides (TMDs) (MX2; M = Mo, W; X = S, Se) are a promising class of materials for photoelectrochemical solar energy conversion applications due to their optimal bandgaps and natural abundance. In this configuration, TMDs are placed onto a conductive substrate and employed as a photoelectrode. A key step in the solar energy conversion process is to photo-excite the semiconductor and dissociate the photogenerated excitons to drive a reaction of interest (e.g., H2 evolution). Because these materials are so physically thin, the excitons are sensitive to the local environment. In this work, we characterize the exciton and trion formation behavior as a function of solvent dielectric, ionic strength, and chemistry. As the dielectric constant of the solvent decreases, trion formation increases. The lower dielectric solvent causes lower Coulombic screening, increasing the probability that an exciton will encounter another electron to generate a trion. I will discuss how these findings relate to a working photoelectrochemical cell.

N‐Involved Optimization of Out‐of‐Plane Optoelectronic Performances in Nanoscale Ruddlesden–Popper Perovskite

AbstractRuddlesden–Popper (RP) metal‐halide perovskites are attracting increasing attention due to their improved stability. The intercalated hydrophobic organic layer accounts for this improvement, however, it is detrimental to the out‐of‐plane optoelectronics. Here, the n‐involved performance optimization in RP (PEA)2(Cs)n−1PbnI3n+1 nanomaterials is reported. The exciton is confirmed as the majority of photo‐induced species, but it is more free carrier‐like in n‐larger samples due to the weakened quantum confinement effects. After 90‐day treatment in 65% humidity, it is found that only RP nanomaterials originally designed as n = 4 ((PEA)2(Cs)3Pb4I13) possess the best residual optoelectronic performances (photo‐responsivity and specific detectivity) at minimal drop proportions (≈20%). Particularly, the Coulombic screening effect triggers saturated optoelectronic performances at low driving voltage, due to the almost total depletion of responsive photo‐current even under enhanced external excitations. This work balances the optoelectronic performances and moisture resistance of PEA‐based RP nanomaterials, and is mainly aiming to alleviate the deterioration of optoelectronic performance in 90‐day moisture‐treated (65%) conditions.

Metal-Ion Oligomerization Inside Electrified Carbon Micropores and its Effect on Capacitive Charge Storage.

Ion adsorption inside electrified carbon micropores is pivotal for the operation of supercapacitors. Depending on the electrolyte, two main mechanisms have been identified so far, the desolvation of ions in solvents and the formation of superionic states in ionic liquids. Here, it is shown that upon confinement inside negatively charged micropores, transition-metal cations dissolved in water associate to form oligomer species. They are identified using in situ X-ray absorption spectroscopy. The cations associate one with each other via hydroxo bridging, forming ionic oligomers under the synergic effect of spatial confinement and Coulombic screening. The oligomers display sluggish dissociation kinetics and accumulate upon cycling, which leads to supercapacitor capacitance fading. They may be dissolved by applying a positive potential, so an intermittent reverse cycling strategy is proposed to periodically evacuate micropores and revivify the capacitance. These results reveal new insights into ion adsorption and structural evolution with their effects on the electrochemical performance, providing guidelines for designing advanced supercapacitors.

Influence of charged impurity screening and surface optical phonons on the n=0 Landau level splitting of graphene with polar substrate

The effect of charged Coulombic impurity screening on the n = 0 Landau level splitting of graphene with a polar substrate under a static magnetic field is investigated theoretically considering the electron-phonon Fröhlich interaction from surface optical phonons of the substrate. The screening is improved using a static dielectric function under the random-phase approximation. A method of linear combination operators is adopted to deal with the influence from a magnetic field and a weak electron-phonon coupling theory is used to discuss the effect from the surface optical phonons. It is found that the splitting due to phonons can be broadened by the charged impurity screening and the broadening increases as increasing of the magnetic field. Our results may provide a possible way to enlarge the energy gap of monolayer graphene. • The n = 0 Landau level splitting of graphene with a substrate under a magnetic field affected by Coulombic impurity screening is obtained. • The screening uses a static dielectric function under the random-phase approximation. • The splitting due to phonons can be broadened by the charged impurity screening. • The broadening increases as increasing of the magnetic field.

Hybrid Polariton Devices: Ultrafast Response of a Hybrid Device Based on Strongly Coupled Monolayer WS2 and Photonic Crystals: The Effect of Photoinduced Coulombic Screening (Laser Photonics Rev. 14(4)/2020)

In article number 1900419, Lei Shi, Tian Jiang, and co-workers fabricate a hybrid polariton device in which strong coupling interactions occur between a photonic crystal (PhC) slab and monolayer tungsten disulfide (WS2). Moreover, transient dynamics studies show that the nonequilibrium coulombic screening effect in WS2 can explicitly influence the ultrafast response of this polariton device. These results are expected to deepen the understanding of photophysics in polariton and permit the active tunability of light-matter interactions.

How Ethylene Glycol Chains Enhance the Dielectric Constant of Organic Semiconductors: Molecular Origin and Frequency Dependence.

Incorporating ethylene glycols (EGs) into organic semiconductors has become the prominent strategy to increase their dielectric constant. However, EG’s contribution to the dielectric constant is due to nuclear relaxations, and therefore, its relevance for various organic electronic applications depends on the time scale of these relaxations, which remains unknown. In this work, by means of a new computational protocol based on polarizable molecular dynamics simulations, the time- and frequency-dependent dielectric constant of a representative fullerene derivative with EG side chains is predicted, the origin of its unusually high dielectric constant is explained, and design suggestions are made to further increase it. Finally, a dielectric relaxation time of ∼1 ns is extracted which suggests that EGs may be too slow to reduce the Coulombic screening in organic photovoltaics but are definitely fast enough for organic thermoelectrics with much lower charge carrier velocities.

Ultrafast Response of a Hybrid Device Based on Strongly Coupled Monolayer WS2and Photonic Crystals: The Effect of Photoinduced Coulombic Screening

AbstractQuantum interactions between transition metal dichalcogenides (TMDs) and optical cavities are rapidly becoming an appealing research topic since these interactions underly a broad spectrum of optical phenomena. Here, we fabricate a simple device in which coherent strong coupling interactions occur between a photonic crystal (PhC) slab and monolayer tungsten disulfide (WS2). Both steady‐state angle‐resolved spectroscopy and transient absorption microscopy (TAM) are employed to explore the coupling behavior of this device. Specifically, anticrossing dispersions are observed in the hybrid device, indicating a Rabi splitting of 40.2 meV. A newly formed spectral feature emerges in the transient absorption (TA) spectrum of this polariton device under near‐resonant excitation, which is subsequently evidenced to be a signature of the upper hybrid exciton–polariton state. Moreover, by carefully analyzing the ultrafast responses of both bare WS2and the WS2‐PhC polariton device excited both off resonance and near resonance, it is found that nonequilibrium thermal decay induces Coulombic screening in the monolayer WS2, which has a major impact on the formation of exciton–polariton. The results of this work could not only improve the current understanding of photophysics in the strong light–matter coupling regime but also lay the foundation for tailoring the development of TMD‐based coherent devices.

Influence of Mg2+ and Ca2+ on nanodisc formation by diisobutylene/maleic acid (DIBMA) copolymer.

Most membrane-solubilising amphiphilic copolymers such as diisobutylene/maleic acid (DIBMA) and styrene/maleic acid (SMA) carry high negative charge densities. Their polyanionic character results in strong Coulombic repulsion, both between polymer chains and lipid membranes during the solubilisation process as well as among polymer-encapsulated nanodiscs after solubilisation. Coulombic repulsion is attenuated by charge screening and, more efficiently, by counterion association, which is particularly strong for multivalent cations binding to polyanionic copolymers. Here, we investigated the effects of the two common alkaline earth metal ions Mg2+ and Ca2+ on the solubilisation properties of and the nanodiscs formed by DIBMA. By quantifying the kinetics and the equilibrium efficiency of lipid solubilisation by static and dynamic light scattering, respectively, we found that millimolar concentrations of Mg2+ or Ca2+ accelerated DIBMA-mediated lipid solubilisation several-fold and resulted in considerably smaller nanodiscs than without divalent cations. Time-resolved Förster resonance energy transfer spectroscopy revealed that collisional transfer of phospholipids among nanodiscs was up to ∼20 and ∼25 times faster in the presence of 10 mM Mg2+ or 7.5 mM Ca2+ than in the absence of divalent cations. These major effects of Mg2+ and Ca2+ contrasted with a moderate influence on the morphology and the thermotropic phase behaviour of the nanodiscs. Finally, we compared the yields of membrane-protein extraction from Escherichia coli membranes, which increased by up to two-fold upon addition of Mg2+ or Ca2+. None of these effects could be explained by Coulombic screening alone, as the change in ionic strength resulting from low millimolar concentrations of divalent cations was minor. Thus, we conclude that Mg2+ and Ca2+ specifically associated with DIBMA to neutralise part of the polymer's carboxylate groups.

If Squeezed, a Camel Passes Through the Eye of a Needle: Voltage-Mediated Stretching of Dendrimers Facilitates Passage Through a Nanopore.

Herein, we report uni-molecular observations of electric potential- and electrolyte-dependent elasticity of poly(amidoamine) (PAMAM)-G1.5 dendrimers containing sodium carboxylate surface groups, using the electric field-assisted migration through the α-hemolysin nanopore (α-HL). Although at moderate transmembrane potentials the dendrimer (~ 2.5nm in diameter) is sterically excluded from translocation across the constriction region of the nanopore (~ 1.5nm in diameter), we found a threshold for its translocation that depends on both the electrolyte pH and ionic strength. We posit that the decreased repulsive intramolecular interactions among dendrimer's branches at low when compared to neutral pH, caused mainly by the protonation of surface groups on the dendrimer, determine a larger propensity of the dendrimer to collapse and deform. This in turns enables the dendrimer to adopt more favorably conformations that facilitate its optimal squeezing through the α-HL's constriction region at low pH, despite the fact that the estimated net force acting on it becomes approximately one order of magnitude lower than at neutral pH. Experiments performed in a low ionic strength buffer, which decreases Coulombic screening, enhance the intramolecular forces on the dendrimer and renders the dendrimer stiffer than in high ionic strength buffer, confirming the dendrimer elastic properties-dependent threshold for deformation inside the nanopore.

Photoinduced Bulk Polarization and Its Effects on Photovoltaic Actions in Perovskite Solar Cells.

This article reports an experimental demonstration of photoinduced bulk polarization in hysteresis-free methylammonium (MA) lead-halide perovskite solar cells [ITO/PEDOT:PSS/perovskite/PCBM/PEI/Ag]. An anomalous capacitance-voltage (CV) signal is observed as a broad "shoulder" in the depletion region from -0.5 to +0.5 V under photoexcitation based on CV measurements where a dc bias is gradually scanned to continuously drift mobile ions in order to detect local polarization under a low alternating bias (50 mV, 5 kHz). Essentially, gradually scanning the dc bias and applying a low alternating bias can separately generate continuously drifting ions and a bulk CV signal from local polarization under photoexcitation. Particularly, when the device efficiency is improved from 12.41% to 18.19% upon chlorine incorporation, this anomalous CV signal can be enhanced by a factor of 3. This anomalous CV signal can be assigned as the signature of photoinduced bulk polarization by distinguishing from surface polarization associated with interfacial charge accumulation. Meanwhile, replacing easy-rotational MA+ with difficult-rotational formamidinium (FA+) cations largely minimizes such anomalous CV signal, suggesting that photoinduced bulk polarization relies on the orientational freedom of dipolar organic cations. Furthermore, a Kelvin probe force microscopy study shows that chlorine incorporation can suppress the density of charged defects and thus enhances photoinduced bulk polarization due to the reduced screening effect from charged defects. A bias-dependent photoluminescence study indicates that increasing bulk polarization can suppress carrier recombination by decreasing charge capture probability through the Coulombic screening effect. Clearly, our studies provide an insightful understanding of photoinduced bulk polarization and its effects on photovoltaic actions in perovskite solar cells.

Phonon linewidth due to electron-phonon interactions with strong forward scattering in FeSe thin films on oxide substrates

The discovery of an enhanced superconducting transition temperature $T_c$ in monolayers of FeSe grown on several oxide substrates has opened a new route to high-$T_c$ superconductivity through interface engineering. One proposal for the origin of the observed enhancement is an electron-phonon (e-ph) interaction across the interface that peaked at small momentum transfers. In this paper, we examine the implications of such a coupling on the phononic properties of the system. We show that a strong forward scattering leads to a sizable broadening of phonon lineshape, which may result in charge instabilities at long-wavelengths. However, we further find that the inclusion of Coulombic screening significantly reduces the phonon broadening. Our results show that one might not expect anomalously broad phonon linewidths in the FeSe interface systems, despite the fact that the e-ph interaction has a strong peak in the forward scattering (small $q$) direction.

Temperature-Dependent Transport Properties of Poly[2-(methacryloyloxy)ethyl]trimethylammonium Chloride Brushes Resulting from Ion Specific Effects

Combined use of electrochemical techniques (electrochemical impedance spectroscopy and cyclic voltammetry) and quartz crystal microbalance with dissipation allowed to resolve separately the thermal effects on diffusion and electron-transfer steps of the electrochemical reaction of the [Fe(CN)6]3–/4– redox couple at a Au electrode modified with poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PMETAC) brushes. Arrhenius-type dependences of the kinetic constant and the diffusion coefficient with temperature were observed in different electrolytes. Ion-paired collapsed polyelectrolyte brushes in NaClO4 result in compact stiff structures with less amount of entrapped water and markedly different from the same brushes with a collapse driven by pure Coulombic screening in NaCl. A remarkable difference related to the type of counterion is the occurrence of a thermal transition for the polyelectrolyte brush in the presence of ClO4– ions at near-ambient temperature (∼17 °C). Activation energies for electro...

Low-Energy Models for Correlated Materials: Bandwidth Renormalization from Coulombic Screening

We provide a prescription for constructing Hamiltonians representing the low-energy physics of correlated electron materials with dynamically screened Coulomb interactions. The key feature is a renormalization of the hopping and hybridization parameters by the processes that lead to the dynamical screening. The renormalization is shown to be non-negligible for various classes of correlated electron materials. The bandwidth reduction effect is necessary for connecting models to materials behavior and for making quantitative predictions for low-energy properties of solids.

Kinetic Salt Effects on an Ionic Reaction in Ionic Liquid/Methanol Mixtures —Viscosity and Coulombic Screening Effects—

Abstract The kinetic salt effect on the disproportionation reaction between diiodide anions in methanol has been examined using two ionic liquids and one inorganic salt. The ionic reaction was accelerated significantly, depending on the cation of the salt, and the ionic liquids enhanced the reaction more effectively. At higher concentrations the reaction rates decrease owing to increasing viscosities.

Second order coalescence conditions of molecular wave functions

Kato's cusp condition gives the exact first order dependence of molecular wave functions on interparticle separation near the coalescence of two charged particles. We derive conditions correct to second order in interparticle separation, which concern second order derivatives of the wave function at the coalescence point. For identical particle coalescence, we give equations correct to third order. In addition to a universal, particle dependent term, a system and state dependent term arises in the higher order conditions, which we interpret as an effect of Coulombic screening. We apply our analysis to the standard orbital-based methods of quantum chemistry and discuss the implications for Jastrow- and R12-type correlation factors.

The Ion Sensitivity of Surface Conductive Single Crystalline Diamond

Charge build-up at the solid/aqueous interface is a ubiquitous phenomenon that determines the properties of interfacial electrical double layers. Due to its unique properties, the surface of diamond offers an attractive platform to investigate charging mechanisms in aqueous solutions. We investigate the surface charge by studying the ion sensitivity of H-terminated single crystalline diamond surface conductive layers. The effect of monovalent and divalent salts has been probed at different pH values. For a pH above 3.5, increasing the ionic strength results in a decrease of the surface conductivity, in contrast to the results obtained for pH below 3.5. Electrokinetic experiments are in good agreement with the surface conductivity measurements, showing an isoelectric point at pH 3.5 for the H-terminated diamond surface. We discuss the results in terms of the Coulombic screening by electrolyte ions of the surface potential, which is induced by a pH-dependent surface charge. The origin of this surface charge is discussed in terms of charge regulation by amphoteric hydroxyl surface groups and unsymmetrical adsorption of hydroxide and hydronium ions induced by the hydrophobic nature of the H-terminated diamond surface. This surface charge can have important consequences for processes governed by the diamond/aqueous interface, such as electron transfer to charged redox molecules, adsorption of charged molecules and proteins, and ion sensitivity.

Switching the Properties of Polyelectrolyte Brushes via “Hydrophobic Collapse”

We have studied the changes in physical and chemical properties of cationic poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) brushes after collapse driven by ion-pairing interactions in the presence of ClO4- anions. Results derived from the quartz crystal microbalance technique, atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle goniometry indicate that ion-paired collapsed polyelectrolyte brushes suffer a dramatic loss of water accompanied by conformational changes leading to markedly different mechanical properties. This scenario is completely different from polyelectrolyte brushes whose collapse is simply driven by pure Coulombic screening, for example, in the presence of Cl- anions. In addition, wetting measurements indicated that ion-pairing interactions can be used to switch surface characteristics from hydrophilic to hydrophobic in a reversible manner. The immediate implications of these experimental results are related to the promising use of polyelectrolyt...

A Coupled 3-D PNP/ECP Model for Ion Transport in Biological Ion Channels

Ion channels are proteins embedded in the membrane of all biological cells, folded in a manner that creates nanoscopic pores that control the flow of ions in and out of the cell. All ion channels carry a highly localized distribution of permanent charge and possess specific properties (e.g., selectivity and switching) that are interesting to the device engineering community. The Poisson-Nernst-Planck (PNP), or drift-diffusion theory, can be used to compute macroscopic current in ion channels quickly. However, PNP theory can be problematic when applied to regions of constricted volume, such as the interior of ion channels, because, firstly, PNP theory replaces discrete charges with a continuous distribution function, grossly underestimating the dielectric boundary force. Additionally, traditional PNP theory ignores the finite volume occupied by the ions and water molecules, as well as the nonsingular distribution of charge on the ion. As a result, Coulombic screening can be overestimated, particularly in highly charged regions, leading to unphysically high ion densities. PNP theory also fails to explain the specific selectivity observed in certain ion channels. The entropic effects of finite-sized ions and water molecules, and the nonsingular charge distribution on the ion can be introduced into the PNP formulism by including an additional component to the electrochemical potential. The so-called excess chemical potential (ECP) represents the difference between the electrochemical potential of a "real" ionic solution and that of an idealized solution. Our goal was to incorporate in a 3D PNP solver (realized with the computational platform PROPHET) an available model for ECP correction (Gillespie, Nonner and Eisenberg, 2002). The ECP has a hard-sphere component, arising from the fact that ions are not point particles so there is an upper limit to the ion density in any given confined volume, and an electrostatic component, due to the fact that ions are not point charges. Both components of the ECP are functions of local charge carrier and water density, and are obtained using density functional theory (DFT). The ECP terms are added to the electrostatic potential in the flux equations, yielding a modified set of PNP equations. To solve the 3D system of PNP/ECP equations self-consistently we use a decoupled feedback method similar to Gummel's iteration: the discretized PNP model is solved with Newton's method. The resulting charge carrier densities are used to update the ECP equations. The new values for the ECP are fed back into the PNP equations and the entire process is iterated until convergence.

Ultramicroelectrode Voltammetric Investigation of Intermicellar Interaction and Micellar Growth of Sodium Dodecyl Sulfate in Aqueous NaCl Solutions

Ultramicroelectrode voltammetric measurements have been carried out to calculate the micellar self-diffusion coefficient (Ds) and intermicellar interaction parameter (Kd) of sodium dodecyl sulfate (SDS) in aqueous NaCl solutions at 298.15 K. The hydrodynamic radius ( ), the aggregation number, and the molecular weight of SDS micelles at various NaCl concentrations are also estimated. It is found that the Ds value is a function of SDS concentrations and the electrolyte concentration dependence on Ds exhibits two regions of behavior. With increasing NaCl concentration, Ds initially increases because of increasing Coulombic screening and, then, decreases linearly because of a linear spherical expansion of the micelles resulting from the increasing aggregation number. Kd is found to be a function of electrolyte concentrations. The stepwise decrease of Kd reflects that, due to Coulombic repulsion, the electrostatic interaction is gradually screened with increasing NaCl concentration. As the linear interaction theory applies rigorously over the electrolyte concentration range from 0.20 to 1.00 mol dm-3, it suggests that the micellar structure remains effectively spherical in the region, the linear spherical expansion of the micelle is occurring, and no dramatic structural changes have taken place in the micellar solutions. In addition, the micellar interaction energy is calculated, which implies that it is the Coulombic nature of the interaction between micellar particles that controls the micellar self-diffusion behavior in aqueous solutions.

Voltammetric Measurement of Intermicellar Interaction Parameters; Correlation with Predicted Interaction Energies

We have used the rotating disk electrode (RDE) to measure micellar long-time self-diffusion coefficients (Ds) and intermicellar interaction parameters (kd) for cetyltrimethylammonium chloride (CTAC) micelles as a function of KCl concentration. Ds values were found to be a function of surfactant concentration and the observed behavior conformed to the linear interaction theory up to 1.00 mol dm-3 KCl. The electrolyte concentration dependence on Ds exhibited three regions of behavior. With increasing KCl concentration, Ds initially increased because of increasing Coulombic screening and then decreased linearly because of a linear spherical expansion of the micelles resulting from the increasing aggregation number, Nagg. Finally, Ds decreased precipitously because of the micellar structural transition from spherical to rod-shaped particles and associated electrolyte-dependent micellar elongation. Intermicellar interaction parameters were also found to be a function of electrolyte concentration and exhibited ...