Image Charge Effects Research Articles

Solvent-Solvent Correlations across Graphene: The Effect of Image Charges.

Wetting experiments show pure graphene to be weakly hydrophilic, but its contact angle (CA) also reflects the character of the supporting material. Measurements and molecular dynamics simulations on suspended and supported graphene often reveal a CA reduction due to the presence of the supporting substrate. A similar reduction is consistently observed when graphene is wetted from both sides. The effect has been attributed to transparency to molecular interactions across the graphene sheet; however, the possibility of substrate-induced graphene polarization has also been considered. Computer simulations of CA on graphene have so far been determined by ignoring the material's conducting properties. We improve the graphene model by incorporating its conductivity according to the constant applied potential molecular dynamics. Using this method, we compare the wettabilities of suspended graphene and graphene supported by water by measuring the CA of cylindrical water drops on the sheets. The inclusion of graphene conductivity and concomitant polarization effects leads to a lower CA on suspended graphene, but the CA reduction is significantly bigger when the sheets are also wetted from the opposite side. The stronger adhesion is accompanied by a profound change in the correlations among water molecules across the sheet. While partial charges on water molecules interacting across an insulator sheet attract charges of the opposite sign, apparent attraction among like charges is manifested across the conducting graphene. The change is associated with graphene polarization, as the image charges inside the conductor attract equally signed partial charges of water molecules on both sides of the sheet. Additionally, using a nonpolar liquid (diiodomethane), we affirm a detectable wetting translucency when liquid-liquid forces are dominated by dispersive interactions. Our findings are important for predictive modeling toward a variety of applications including sensors, fuel cell membranes, water filtration, and graphene-based electrode materials in high-performance supercapacitors.

Understanding attenuated solvent reorganization energies near electrode interfaces.

In this manuscript, we examine the role of image charge effects on the electrostatic potential fluctuations experienced by ionic species in the vicinity of an electrode surface. We combine simulation and theory to quantify these fluctuations and how they vary with distance from the electrode surface. We observe that the potential distribution narrows significantly for species within a few electrolyte screening lengths of the electrode. We attribute this narrowing to the effects of image charge fluctuations originating from the polarization response of the electrode. We show that the physical consequences of these image charge effects can be captured in the context of a simple analytical field theory with anti-symmetric boundary conditions. We contextualize these results by discussing their implications for rates of Marcus-like outer-sphere interfacial electron transfer.

Image-charge effects on an electrostatic pendulum in mechanical equilibrium

A charged pendulum suspended in mechanical equilibrium by an insulated string and deflected by a uniform electric field was investigated theoretically and experimentally. When the uniform electric field is generated by a pair of large parallel conducting plates, there is an additional force of attraction between the pendulum and its corresponding image charges, which can dramatically affect the pendulum's equilibrium deflection. Consequently, the deflection depends non-linearly on the pendulum's net charge and the plate potentials, which results in additional unstable-equilibrium solutions, saddle-point solutions, or conditions for which no equilibrium occurs. Inclusion of this experiment within introductory and/or advanced undergraduate physics courses is discussed.

Study on ultra-high sensitivity piezoelectric effect of GaN micro/nano columns

High-quality GaN micro/nano columns were prepared with self-organized catalytic-free method. Young’s modulus of GaN nanocolumns were measured under both compressive stress and tensile stress. It was found that the Young’s modulus decreases with the increasing of nanocolumn diameter due to the increase of face defect density. Furthermore, we measured the piezoelectric properties and found that there was a 1000-fold current increase under a strain of 1% with a fixed bias voltage of 10 mV. Based on the Schottky Barrier Diode model, we modified it with the effect of polarization charge, image charge and interface state to analyze the experiment results which reveals that the strong piezopolarization effect plays an important role in this phenomenon. Therefore, the GaN nanocolumns has a great prospect to be applied in high-efficiency nanogenerators and high-sensitivity nanosensors.

Effect of Coating of Dielectric in a 3-Phase GIB with Particle Movement

The dielectric coating effect on the motion of the Particle inside a Gas Insulated Bus duct (GIB) of three phase using epoxy resin coating of dielectric inside f GIB outer enclosure of inner surface without image charge is presented in this paper. Simulation is performed by considering various parameters like drag, gravitational and the electrostatic forces acting on the particle and a mathematical model was derived. A 2nd order differential equation for the particle motion is solved and restitution coefficient was considered at each impact of particle with the GIB enclosure. At the particle locations, estimation of electric fields instantaneously was made by CSM. The particle motion in the absence of effect of image charge is considered in a 3-phase GIB with dielectric coating. For voltage levels like 220kV, 400kV, 600kV and 800kV class, the movement of radial in nature is found for particles like aluminum and copper inside GIB. Analysis of all the results were done and presented in this paper.

Electrical surface charge patterns induced by droplets sliding over polymer and photoresist surfaces

We have studied the generation of surface charge distributions on polymer surfaces due to the deposition, motion and evaporation of liquid droplets. Using liquids with different static dielectric constants, we found that the magnitude of the surface charge conforms well to the Boltzmann relation for the dissociation probability of ions in solution, indicating that the phenomenon is electrokinetic in nature. The origin of the sensitive dependence of the surface charge on the substrate thickness was identified to be an image charge effect. Experiments on polycarbonate and chemically-amplified photoresist yield surface charges of opposite polarity. By varying the photoresist composition, we identified the leaching of negative quencher ions as the reason for the charge inversion.

Polarizable potentials for metals: The density readjusting embedded atom method (DR-EAM)

In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely used classical molecular dynamics force fields. We present a modification of the embedded atom method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This ``density readjusting EAM'' (DR-EAM) preserves nearly all of the useful qualities of traditional EAM, including bulk elastic properties and surface energies. However, it also allows valence electron density to migrate through the metal in response to external perturbations. We show that DR-EAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DR-EAM also captures some of the behavior of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. We further show that it predicts charge transfer between the constituent atoms in alloys, leading to novel predictions about unit cell geometries in layered L$1_0$ structures.

Determination of Energy-Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Oligophenylene Thiols and Dithiols on Ag, Au, and Pt Electrodes.

We report detailed measurements of transport and electronic properties of molecular tunnel junctions based on self-assembled monolayers (SAMs) of oligophenylene monothiols (OPT n, n = 1-3) and dithiols (OPD n, n = 1-3) on Ag, Au, and Pt electrodes. The junctions were fabricated with the conducting probe atomic force microscope (CP-AFM) platform. Fitting of the current-voltage ( I-V) characteristics for OPT n and OPD n junctions to the analytical single-level tunneling model allows extraction of both the HOMO-to-Fermi-level offset (εh) and the average molecule-electrode coupling (Γ) as a function of molecular length ( n) and electrode work function (Φ). Significantly, direct measurements of εhUPS by ultraviolet photoelectron spectroscopy (UPS) for OPT n and OPD n SAMs on Ag, Au, and Pt agree remarkably well with the transport estimates εhtrans, providing strong support-beyond the high quality I-V simulations-for the relevance of the analytical single-level model to simple molecular tunnel junctions. Because the UPS measurements involve SAMs bonded to only one metal contact, the correspondence of εhUPS and εhtrans also indicates that the top contact has a weak effect on the HOMO energy. Corroborating ab initio calculations definitively rule out a dominant contribution of image charge effects to the magnitude of εh. Thus, the effective molecular tunnel barrier εh is determined, and essentially pinned, by the formation of a single metal-S covalent bond per OPT n or OPD n molecule.

Controlling the stability of Pickering emulsions by pH-responsive nanoparticles.

Electrostatic dissipative particle dynamics simulations were conducted to model the interactions between emulsion droplets stabilized by pH-sensitive polyelectrolyte-grafted nanoparticles. Using a steered molecular dynamics approach, a mechanistic study of forced coalescence was performed to probe the resistance between two particle-covered droplets. The degree of ionization of the grafted polyelectrolytes was adjusted to capture the pH responsiveness. The maximal resistance forces were measured to quantitatively discriminate the efficacy of particles in stabilizing emulsions at different degrees of ionization. Through analyzing droplet dynamics, resistance force variation, and electric field, we discovered that the resistance is attributed to direct electrostatic repulsion, the image charge effect near the water-oil interface, and steric hindrance among extended polymers. When the particle density on the droplet surface is relatively low, the increasing resistance forces at higher degrees of ionization can effectively prevent droplet coalescence. Oppositely, the ionization compromises emulsion stability when the particle surface coverage is high. Substantial desorption of particles from the interface was triggered as the degree of ionization increases. This in turn reduces resistance force and facilitates coalescence. Moreover, the nanoparticles prevent coalescence at high surface coverages by forming dense layers at individual interfaces, while the particle bridges straddling two interfaces were found at low surface coverages, which can also keep the droplets apart.

Molecular Dynamics Simulation of Polarizable Gold Nanoparticles Interacting with Sodium Citrate.

To study the structure of a citrate-capped gold nanoparticle and forces involved in citrate-gold interactions, we performed a molecular dynamics simulation of a truncated-octahedron nanoparticle containing Au(111) and Au(100) surfaces with sodium citrate. In this paper, we employed an approach to the modeling of interactions of a gold nanoparticle with citrate molecules taking into account the image charge effect in the metal. First, we built models of 6 and 14 nm nanoparticles, which can reproduce the polarization effects, based on a rigid-rod gold model and the GolP-CHARMM force field. To verify the simulation results, we analyzed density plots, radial distributions, distributions perpendicular to Au(111) and Au(100) surfaces, the electric potential of the system, and the dynamics of citrate crown formation. We observed formation of a stable citrate crown around the nanoparticle and detected nonuniform surface distribution of citrate ions with the preference for Au(111) facets over Au(100) ones. Testing of the model of the citrate-capped gold nanoparticle in a simulation at high concentrations of Na+ and Cl- ions (0.8 M) showed incorporation of chloride anions into the citrate crown. We compared the results of citrate crown formation between polarizable and nonpolarizable gold models and noticed a difference in the citrate distribution on the surface of the gold nanoparticle. We found that polarization effects in the metal are involved in the mechanism of interaction of the gold nanoparticle and citrate ions. The obtained results are in good agreement with experimental data and computer simulations from a number of other studies, which prove the validity of the proposed model.

Influence of image charge effect on the binding energy of hydrogen-like donor impurity in a near-surface quantum well under transverse electric field

We present a systematic theoretical study of the influence of image charges on the spectrum of a shallow donor impurity located anywhere in a near-surface quantum well under the electric field applied along the growth direction of the system. We calculate the binding energies of the ground and excited donor impurity states in the effective-mass approximation using the variational method for various values of the quantum well width, electric field strength, and donor positions within the well. Theoretical results indicate that the role of image charges is especially important for small widths of a quantum well. In contrast to the ground state, the influence of image charges on the binding energy of excited states is significant only at small values of the electric field. Therefore such fields can serve as a means to control the transition energies between ground and first excited states.

Image charge effect on the light emission of rutile TiO2(110) induced by a scanning tunneling microscope**Project supported by the National Key R&D Program of China (Grant Nos. 2016YFA0300901 and 2017YFA0205003), the National Natural Science Foundation of China (Grant Nos. 11634001 and 21725302), and the Key Research Program of the Chinese Academy of Sciences (Grant No. XDPB08-1).

The plasmon-enhanced light emission of rutile TiO2(110) surface has been investigated by a low-temperature scanning tunneling microscope (STM). We found that the photon emission arises from the inelastic electron tunneling between the STM tip and the conduction band or defect states of TiO2(110). In contrast to the Au(111) surface, the maximum photon energy as a function of the bias voltage clearly deviates from the linear scaling behavior, suggesting the non-negligible effect of the STM tip on the band structure of TiO2. By performing differential conductance (dI/dV) measurements, it was revealed that such a deviation is not related to the tip-induced band bending, but is attributed to the image charge effect of the metal tip, which significantly shifts the band edges of the TiO2(110) towards the Femi level (EF) during the tunneling process. This work not only sheds new lights onto the understanding of plasmon-enhanced light emission of semiconductor surfaces, but also opens up a new avenue for engineering the plasmon-mediated interfacial charge transfer in molecular and semiconducting materials.

Impurity-Bound Polaronic States in ZnSe Quantum Dots Under Electric Field: Image Charge Effect

A Landau–Pekar variational approach is used to study the ground and first excited states of an impurity-bound polaron in a ZnSe quantum dot under an external electric field, by taking into account the image charge effect (ICE). The binding energy (BE) of the donor impurity is calculated considering the interaction of an electron with both the bulk-type longitudinal optical phonons and the interface optical phonons. Both electric field and parabolic confinement effects on the BE and its polaronic shift for 1s and 2s hydrogen-like states with and without ICE are investigated in detail. We have shown that the ICE strongly reduces the donor BEs in both ignoring and considering electron–optical phonon interaction.

Solution processed alkali-metal and alkaline-earth-metal compounds as the efficient electron injection layer in organic light-emitting diodes

To enable the use of high work-function metals as the cathode in organic light emitting diodes (OLEDs), different types of alkali-metal and alkaline-earth-metal based compounds are examined as the electron injection layer (EIL). All the studied compounds can improve the device performance for aluminum, gold, and silver cathodes. Potassium hydroxide (KOH) and potassium carbonate (K2CO3) are employed as EIL for the first time in OLEDs. The performance of the device with sodium hydroxide (NaOH)/silver cathode almost doubles the conventional barium/aluminum device’s efficiency. By examining the work-function of EIL-modified metals through Kelvin Probe, a general correlation between the device performance and cathode work-function is revealed. The image-charge effect is found to be a contributor to the reduction of the metal work-function.

Impact of Nano- and Mesoscales on Macroscopic Cation Conductivity in Perfluorinated-Sulfonic-Acid Membranes

A mean-field local-density theory is outlined for ion transport in perfluorinated-sulfonic-acid (PFSA) membranes. A theory of molecular-level interactions predict nanodomain and macroscale conductivity. The effects of solvation, dielectric saturation, dispersion forces, image charge, finite size, and confinement are included in a physically consistent 3D-model domain geometry. Probability-distribution profiles of aqueous cation concentration at the domain-scale are in agreement with atomistic simulations using no explicit fitting parameters. Measured conductivities of lithium-, sodium-, and proton-form membranes with equivalent weights of 1100, 1000, and 825 g/mol(SO3) validate the macroscale predictions using a single-value mesoscopic fitting parameter. Cation electrostatic interactions with pendant sulfonate groups are the largest source of migration resistance at the domain-scale. Tortuosity of ionically conductive domains is the largest source of migration resistance at the macroscale. Our proposed tr...

Modeling quantum yield, emittance, and surface roughness effects from metallic photocathodes

Detailed measurements of momentum distributions of emitted electrons have allowed the investigation of the thermal limit of the transverse emittance from metal photocathodes. Furthermore, recent developments in material design and growth have resulted in photocathodes that can deliver high quantum efficiency and are sufficiently robust to use in high electric field gradient photoinjectors and free electron lasers. The growth process usually produces photoemissive material layers with rough surface profiles that lead to transverse accelerating fields and possible work function variations, resulting in emittance growth. To better understand the effects of temperature, density of states, and surface roughness on the properties of emitted electrons, we have developed realistic three-dimensional models for photocathode materials with grated surface structures. They include general modeling of electron excitation due to photon absorption, charge transport, and emission from flat and rough metallic surfaces. The models also include image charge and field enhancement effects. We report results from simulations with flat and rough surfaces to investigate how electron scattering, controlled roughness, work function variation, and field enhancement affect emission properties. Comparison of simulation results with measurements of the quantum yield and transverse emittance from flat Sb emission surfaces shows the importance of including efficient modeling of photon absorption, temperature effects, and the material density of states to achieve agreement with the experimental data.

Comparative study of the exciton binding energies of thin and ultrathin organic-inorganic perovskites due to dielectric mismatch effects

Multi-quantum well organic-inorganic perovskites offer an approach to tunable exciton binding energies based on well-barrier dielectric mismatches, that is, the image-charge effect. As with other two-dimensional materials, ultrathin exfoliated perovskites are highly sensitive to their dielectric environments. We investigate ultrathin crystalline two-dimensional van der Waals layers of organic-inorganic perovskite crystals adjacent to the surface of a substrate. The exciton binding energies are strongly influenced by the surrounding dielectric environment. The Keldysh model describes this dependence. We compare our binding-energy results with data in the (C6H13NH3)2PbI4 perovskite and estimate binding-energy values for (C4H9NH3)2PbBr4.

Molecular mechanics models for the image charge, a comment on “including image charge effects in the molecular dynamics simulations of molecules on metal surfaces”

We re-investigate the image charge model of Iori and Corni (Iori and Corni, J. Comput. Chem. 2008, 29, 1656). We find that a simple symmetrization of their model allows to obtain quantitatively correct results for the electrostatic interaction of a water molecule with a metallic surface. This symmetrization reduces the magnitude of the electrostatic interaction to less than 10% of the total interaction energy. © 2017 Wiley Periodicals, Inc.

Polaronic effects on the off-center donor impurity in AlAs/GaAs/SiO2 spherical core/shell quantum dots

The ground state of a conduction electron coupled to an off-center impurity donor in a AlAS/GaAs spherical core/shell quantum dot is investigated theoretically. The image-charge effect and the influence of the electron-polar-LO-phonon interaction are considered. The electron-impurity binding energy is calculated via a variational procedure and is reported both as a function of the shell width and of the radial position of the donor atom. The polaronic effects on this quantity are particularly discussed.

EFFECTS OF ELECTRIC FIELD AND IMAGE CHARGES ON THE IMPURITY-RELATED OPTICAL ABSORPTION COEFFICIENTS IN A SPHERICAL QUANTUM DOT

The influence of an image charge effect on the impurity-related energies, the oscillator strengths of transitions from the 1s-like state to 2px and 2pz states as well as the optical absorption coefficients in a spherical quantum dot in the presence of an external electric field have been studied. The results show that the image charges lead to significant changes in the optical coefficients of the system.