Image Charge Effects Research Articles

Coarse-grained polarizable soft solvent models, with applications in dissipative particle dynamics.

We critically examine a broad class of explicitly polarizable soft solvent models aimed at applications in dissipative particle dynamics. We obtain the dielectric permittivity using the fluctuating box dipole method in linear response theory and verify the models in relation to several test cases, including demonstrating ion desorption from an oil-water interface due to image charge effects. We additionally compute the Kirkwood factor and find that it uniformly lies in the range gK≃0.7-0.8, indicating that dipole-dipole correlations are not negligible in these models. This is supported by the measurements of dipole-dipole correlation functions. As a consequence, Onsager theory over-predicts the dielectric permittivity by 20%-30%. The mean square molecular dipole moment can be accurately estimated with a first-order Wertheim perturbation theory.

Image charge effects under metal and dielectric boundary conditions

Image charge effects under metal and dielectric boundary conditions

CP-AFM Molecular Tunnel Junctions with Alkyl Backbones Anchored Using Alkynyl and Thiol Groups: Microscopically Different Despite Phenomenological Similarity.

In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of n-alkyl chains anchored with acetylene groups (CnA; n = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (I-V) characteristics of CnA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance GAg < GAu < GPt is found to follow the same ordering as the metal work function ΦAu < ΦAu < ΦPt, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CnT) to which we refer in view of some similarities. Furthermore, unlike in CnT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CnA an additional MO shift, whose independence of size (n) rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CnA and CnT junctions different. What ultimately makes CnA unique at the microscopic level is a salient difference never reported previously, namely, that CnA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CnA from the CnT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.

Coalescence-Induced Droplet Jumping for Electro-Thermal Sensing.

Jumping droplet condensation, whereby microdroplets (ca. 1-100 μm) coalescing on suitably designed superhydrophobic surfaces jump away from the surface, has recently been shown to have a 10× heat transfer enhancement compared to filmwise condensing surfaces. However, accurate measurements of the condensation heat flux remain a challenge due to the need for low supersaturations (<1.1) to avoid flooding. The low corresponding heat fluxes (<5 W/cm2) can result in temperature noise that exceeds the resolution of the measurement devices. Furthermore, difficulties in electro-thermal measurements such as droplet and surface electrostatic charge arise in applications where direct access to the condensing surface, such as in isolated chambers and small integrated devices, is not possible. Here, we present an optical technique that can determine the experimental electro-thermal parameters of the jumping droplet condensation process with high fidelity through the analysis of jumping droplet trajectories. To measure the heat flux, we observed the experimental trajectories of condensate droplets on superhydrophobic nanostructures and simultaneously matched them in space and time with simulated trajectories using the droplet dynamic equations of motion. Two independent approaches yielded mean heat fluxes of approximately 0.13 W/cm2 with standard deviations ranging from 0.047 to 0.095 W/cm2, a 79% reduction in error when compared with classical energy balance-based heat flux measurements. In addition, we analyzed the trajectories of electrostatically interacting droplets during flight and fitted the simulated and experimental results to achieve spatial and temporal agreement. The effect of image charges on a jumping droplet as it approaches the surface was analyzed, and the observed acceleration has been numerically quantified. Our work presents a sensing methodology of electro-thermal parameters governing jumping droplet condensation.

INITIATION OF A STREAMER IN A LIQUID FROM THE SURFACE OF A BUBBLE IN WHICH A PARTIAL DISCHARGE OCCURRED

The analysis of experimental data on the initiation of streamers in a liquid di- electric from the surface of a free-floating up gas bubble after partial discharge is performed. Calculations of electric fields in a dielectric after partial discharge in bubbles corresponding to experimental conditions, with different degrees of deformation of the bubbles and their dif- ferent position relative to flat electrodes, have been carried out. The effect of image charges on the value of the maximum electric field on the surface of a spherical bubble is estimated. The probability of initiation of branching streamers from the bubble surface into the liquid is analyzed.

Image charge effect in layered materials: Implications for the interlayer coupling in MoS2

Interlayer coupling often has profound effects on the electronic structure of layered materials. Its strength depends on the ratio between interlayer hopping and band offset. In multilayer materials, the image charge effect causes a layer-dependent variation of the band offset. Here, the authors study the effect of this variation on the interlayer coupling by combining the tight-binding method with an image self-energy. In MoS${}_{2}$, the image charge effect leads to the formation of a surface-layer band gap.

Comprehensive model for ideal reverse leakage current components in Schottky barrier diodes tested in GaN-on-SiC samples

A model to predict the ideal reverse leakage currents in Schottky barrier diodes, namely, thermionic emission and tunneling components, has been developed and tested by means of current–voltage–temperature measurements in GaN-on-SiC devices. The model addresses both current components and both forward and reverse polarities in a unified way and with the same set of parameters. The values of the main parameters (barrier height, series resistance, and ideality factor) are extracted from the fitting of the forward-bias I–V curves and then used to predict the reverse-bias behavior without any further adjustment. An excellent agreement with the I–V curves measured in the forward bias in the GaN diode under analysis has been achieved in a wide range of temperatures (275–475 K). In reverse bias, at temperatures higher than 425 K, a quasi-ideal behavior is found, but additional mechanisms (most likely trap-assisted tunneling) lead to an excess of leakage current at lower temperatures. We demonstrate the importance of the inclusion of image-charge effects in the model in order to correctly predict the values of the reverse leakage current. Relevant physical information, like the energy range at which most of the tunnel injection takes place or the distance from the interface at which tunneled electrons emerge, is also provided by the model.

Effective theory for matter in non-perturbative cavity QED

Starting from a general material system of NN particles coupled to a cavity, we use a coherent-state path integral formulation to produce a effective theory for the material degrees of freedom. We tackle the effects of image charges, the A^2A2 term and a multimode arbitrary-geometry cavity. The resulting (non-local) action has the photonic degrees of freedom replaced by an effective position-dependent interaction between the particles. In the large-NN limit, we discuss how the theory can be cast into an effective Hamiltonian where the cavity induced interactions are made explicit. The theory is applicable, beyond cavity QED, to any system where bulk material is linearly coupled to a diagonalizable bosonic bath. We highlight the differences of the theory with other well-known methods and numerically study its finite-size scaling on the Dicke model. Finally, we showcase its descriptive power with three examples: photon condensation, the 2D free electron gas in a cavity and the modification of magnetic interactions between molecular spins; recovering, condensing and extending some recent results in the literature.

Numerical study of effects of electrode parameters and image charge on the electric field configuration of RPCs

The working of a resistive plate chamber depends on the electric field applied inside the gas gap. The strength of this electric field inside the gas gap depends on parameters like electrode thickness, permittivity, and gas gap, among others. The applied electric field can get significantly modified by the space charge effect. This can be a major concern while working in high rate particle physics experiments. Accumulation of space charge generated due to consecutive avalanches can distort the applied field. During this investigation, we have observed that the strength of this perturbation in the electric field is a function of the amount of charge, electrode thickness, permittivity, and gas gap. In this paper, we have studied the dependence of the applied field on parameters linked to RPC fabrication mentioned above. In addition, we have used the method of image to calculate the additional polarisation field due to space charge for a basic five layers of the geometry of an RPC. The variation of the perturbation in the electric field with the same three parameters electrode thickness, permittivity, gas gap has also been discussed.

Image Charge effect on Metallic Particle Movement in a single phase Gas Insulated Bus Duct with Dielectric Coated Enclosure

Image Charge effect on Metallic Particle Movement in a single phase Gas Insulated Bus Duct with Dielectric Coated Enclosure

Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols.

A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, εh, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, εhtrans and εhUPS, respectively. We take the difference in these quantities to be the image potential energy eVimage. In the theoretical approach, we use density functional theory (DFT) to calculate directly eVimage between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eVimage ∼ -0.1 eV per metal contact, meaning that the total image potential energy is ∼-0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25-30% of the total offset εh, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.

Unearthing of a new science from nanostructured carbons

Graphene, single-wall carbon nanotubes, and porous carbons have zero-dimensional nanowindows, one-dimensional (1D) pores, and three-dimensional pores consisting of two-dimensional (2D) unit pores, respectively. A molecular dynamics simulation study on nanowindows shows that they enable the efficient separation of O2 from N2 and Ar at an extremely high permeance. Intense confinement of sulfur atoms in 1D-carbon nanotube pore spaces in vacuo leads to the formation of metallic 1D-sulfur chains. Evidence of the partial breaking of the Coulombic law for ionic liquids confined as monolayers in 2D pores, which is caused by an evident image charge effect of conductive carbon-pore walls, was provided by hybrid-reverse Monte Carlo simulation-aided X-ray scattering. The solid-like structural formation of oxygen molecules in 2D carbon pores induces highly efficient and rapid adsorption separation of 18O2 from 16O2 near 112 K, which is the boiling point of methane. Abundant water vapor can be adsorbed on microporous carbon through cluster-associated hydrophobic-hydrophilic transformations. Water molecules form aggregated clusters on the adsorbed branch, which transforms into a continuous structure at a relative pressure of 1, giving rise to obvious adsorption hysteresis. Logical and challenging studies to improve our fundamental understanding, along with new approaches, produce completely new findings, even for carbon materials that have been thoroughly studied.

Hysteresis Behavior and Photoresponse Enhancement in Au Nanoparticle-Decorated Ge Photodetectors

Au nanoparticles (NPs) were drop-casted onto germanium (Ge) devices to investigate the influence of Au NPs on the electronic transport and photodetection properties of Ge metal–semiconductor–metal photodetectors. A significant hysteresis behavior was observed in the current–voltage curves of the Au NP-decorated Ge photodetector, which was ascribed to the energy band bending below the Au NPs and the electrostatic effect of image charges in the Au NPs. More interestingly, at the optical communication wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.55~\mu \text{m}$ </tex-math></inline-formula> , the introduction of Au NPs effectively improved the responsivity of the device from 0.18 to 0.92 A/W. Rather than the localized surface plasmon resonance effect, the increase of responsivity in the Au NP-decorated Ge photodetector was caused by a type-II liked energy band alignment, which enhanced the spatial electron–hole separation effect. These results provide inspiration for the development of novel optoelectronic devices.

Point-to-point Coulomb effects in high brightness photoelectron beam lines for ultrafast electron diffraction

In an effort to increase spatial and temporal resolution of ultrafast electron diffraction and microscopy, ultra-high brightness photocathodes are actively sought to improve electron beam quality. Beam dynamics codes often approximate the Coulomb interaction with mean-field space charge, which is a good approximation in traditional beams. However, point-to-point Coulomb effects, such as disorder induced heating and the Boersch effect, can not be neglected in cold, dense beams produced by such photocathodes. In this paper, we introduce two new numerical methods to calculate the important effects of the photocathode image charge when using a point-to-point interaction model. Equipped with an accurate model of the image charge, we calculate the effects of point-to-point interactions on two high brightness photoemission beamlines for ultrafast diffraction. The first beamline uses a 200 keV gun, whereas the second uses a 5 MeV gun, each operating in the single-shot diffraction regime with $10^5$ electrons/pulse. Assuming a zero photoemission temperature, it is shown that including stochastic Coulomb effects increases the final emittance of these beamlines by over a factor of 2 and decreases the peak transverse phase space density by over a factor of 3 as compared to mean-field simulations. We then introduce a method to compute the energy released by disorder induced heating using the pair correlation function. This disorder induced heating energy was found to scale very near the theoretical result for stationary ultracold plasmas, and it accounts for over half of the emittance growth above mean-field simulations.

Optimization of Electrostatic Quadrupole Profile to Reduce Image Charge Effects

Vertical electric fields need to remain orders of magnitude smaller than the horizontal electric field in storage ring electric dipole moment experiments. Otherwise, the coupling with the magnetic dipole moment dominates the spin precession, eventually leading to a false signal. This work presents first quantitative studies regarding electrostatic quadrupole profile optimization, which helps suppressing image beam induced vertical electric fields. Besides the demonstration of the profile optimization, its suppression performance is also studied for various beam offsets and beam size configurations. It is found that with the optimum profile, the vertical electric field can be reduced by two orders of magnitude.

Comprehensive theoretical framework for the analysis of microgap thermionic energy conversion under constant heat inputs

We present a comprehensive model to fully characterize the behavior of thermionic energy conversion devices subject to constant heat flux inputs. Thermionic energy converters (TECs) have recently gained enormous attention mainly due to technological advances enabling the fabrication of converters with micro/nanometer interelectrode gaps. The reduced gap appeared to solve the long-lasting issue of negative space-charge build-up within TECs. However, the decrease in gap distance leads to the appearance of new physics in the energy interactions between the two electrodes, such as the image-charge effect and near-field thermal radiation. The intertwined energy exchange mechanisms require a deeper look and a systematic approach for accurately modeling the performance of TECs in the micro/nanoscales, especially in practical applications where the device is exposed to external energy sources such as concentrated solar power. We conducted a steady-state energy balance analysis to calculate the operating temperatures of the electrodes by considering constant heat flux inputs (such as concentrated solar energy) to the emitter and various cooling rates of the collector. The potential barrier profile within the interelectrode gap, the net current density, and the power output are subsequently calculated for a TEC with electrodes made of tungsten with and without electron reflection. The optimal values of the gap distance are also reported for various input scenarios revealing considerable deviation from the previously reported ones. An overall efficiency of 38.3% with a power output of 19.2 kW/m2 can be reached for a TEC with perfectly absorbing electrodes at the gap distance of 6μm operating under a heat flux input of 50 kW/m2 and convective heat transfer coefficient of 500 W/m2K. We believe that the present work provides an unabridged framework for the rigorous analysis of thermionic power generation with (sub) micrometer interelectrode distances and offers substantial insight into the design and characterization of TECs.

Image-charge effects on ion adsorption near aqueous interfaces

Electrostatic interactions near surfaces and interfaces are ubiquitous in many fields of science. Continuum electrostatics predicts that ions will be attracted to conducting electrodes but repelled by surfaces with lower dielectric constant than the solvent. However, several recent studies found that certain "chaotropic" ions have similar adsorption behavior at air/water and graphene/water interfaces. Here we systematically study the effect of polarization of the surface, the solvent, and solutes on the adsorption of ions onto the electrode surfaces using molecular dynamics simulation. An efficient method is developed to treat an electrolyte system between two parallel conducting surfaces by exploiting the mirror-expanded symmetry of the exact image-charge solution. With neutral surfaces, the image interactions induced by the solvent dipoles and ions largely cancel each other, resulting in no significant net differences in the ion adsorption profile regardless of the surface polarity. Under an external electric field, the adsorption of ions is strongly affected by the surface polarization, such that the charge separation across the electrolyte and the capacitance of the cell is greatly enhanced with a conducting surface over a low-dielectric-constant surface. While the extent of ion adsorption is highly dependent on the electrolyte model (the polarizability of solvent and solutes, as well as the van der Waals radii), we find the effect of surface polarization on ion adsorption is consistent throughout different electrolyte models.

A gap opening in graphene covered by a conducting polymer: The role of many-body effects

We study the adsorption of the conducting polymer poly-para-phenylene (PPP) on graphene using first-principles electronic structure calculations. In this article, we decided to investigate the role of polarization of the polymer on the band gap renormalization of the graphene sheet. For this purpose, we supposed that the polymer is lying flat on the sheet of graphene. The polymer adsorption has caused the polarization of graphene and vice versa. We demonstrate the anisotropic polarization of the polymer significantly affects the energy band structure of graphene by opening a gap at the Dirac point. Upon polymer adsorption, the conical shape of the graphene at the Dirac point is partially transformed and leads to opening the band gap about 260 meV at this point. This finding can change the metallic property of the graphene and created semiconducting property inside the compound. This result can only be explained by many-body effects which are treated within the G0W0 quasi-particle approximation while the Kohn-Sham single-particle band structure of graphene is hardly modified by the presence of the polymer. Our findings can be understood as a renormalization due to image charge effects on the anisotropic polymer chain.

Study on the effect of singlewalled carbon nanotubes on junction properties of Safranin –T dye-based organic device

In this work, junction properties of metal-organic dye contact have been studied and alteration of these properties in presence of single walled carbon nanotubes has been observed. Junction properties of metal-organic interface significantly influence the device performance. Present work studies barrier potential and trap energy under the influence of single walled carbon nanotubes. Barrier inhomogeneity and the effect of image charge on lowering of barrier potential have also been studied. Formation of the organic device has been done by sandwiching Safranin - T dye in between two electrodes which are Indium Tin Oxide coated glass substrate and Aluminium respectively. Charge flow at the metal–organic layer interface has been analyzed by Richardson –Schottky thermionic emission theory. Both barrier potential and trap energy have been estimated from the steady-state current-voltage characteristics of the device. The incorporation of single walled carbon nanotubes lowers both of these parameters results in improvement of current flow at the metal-organic contact.

Vibrational Strong Coupling in Subwavelength Nanogap Patch Antenna at the Single Resonator Level.

Vibrational strong coupling (VSC) between a vacuum field and molecules in a cavity offers promising applications in cavity-modified chemical reactions and ultrasensitive vibrational spectroscopy. At present, in order to realize VSC, bulky microcavities with large mode volume are utilized, which limits their potential applications at the nanoscale. Here, we report on the experimental realization of strong coupling between molecular vibrations and infrared photons confined within a deeply subwavelength nanogap patch antenna cavity. Our system exhibits a characteristic anticrossing dispersion, indicating a Rabi splitting of 108 cm-1 at the single resonator level with excellent angular insensitivity. The numerical simulations and theoretical analyses quantitatively reveal that the strength of coupling depends on the cavity field-molecule overlap integral and the image charge effect. VSC at the single nanogap patch antenna level paves the way for molecular-scale chemistry, ultrasensitive biosensors, and the development of ultralow-power all-optical devices in the mid-infrared spectral range.