Strong Electrostatic Field Research Articles

Field-effect control of metallic superconducting systems

Static electric fields have a negligible influence on the electric and transport properties of a metal because of the screening effect. This belief was extended to conventional metallic superconductors. However, recent experiments have shown that the superconductor properties can be controlled and manipulated by the application of strong electrostatic fields. Here, the authors review the experimental results obtained in the realization of field-effect metallic superconducting devices exploiting this phenomenon. The authors start by presenting the pioneering results on superconducting Bardeen–Cooper–Schrieffer wires and nanoconstriction Josephson junctions (Dayem bridges) made of different materials, such as titanium, aluminum, and vanadium. Then, the authors show the mastering of the Josephson supercurrent in superconductor-normal metal-superconductor proximity transistors, suggesting that the presence of induced superconducting correlations is enough to see this unconventional field-effect. Later, the authors present the control of the interference pattern in a superconducting quantum interference device, indicating the coupling of the electric field with the superconducting phase. The authors conclude this review by discussing some devices that may represent a breakthrough in superconducting quantum and classical computation.

Efficiency Enhancement of GaN Based Light-emitting Diodes with a Heterojunction Type Last Quantum Barrier

A GaN/Al0.15Ga0.85N/GaN heterojunctionis designed to enhance the electron confinement and improve the hole injection efficiency of GaN-based MQW LEDs. The physical and optical properties of GaN-based MQW LEDs with and without the heterojunction structure are investigated. The simulation results show that the LEDs with the heterojunction structure quantum barrier exhibit much higher output power and lower threshold voltage as compared to those with the traditional structure due to the enhancement of the electron confinement and improvement of the hole injection from p-type region, which are induced by the strong reverse electrostatic fields in the heterojunction structure

Si quantum dots enhanced hydrogen bonds networks of liquid water in a stimulated Raman scattering process.

Stimulated Raman scattering (SRS) of silicon quantum dots (Si QD) water solutions of different sizes (2 and 5nm) are investigated using Nd:YAG laser. Since strong and weak hydrogen bonds are formed by the charge transfer between water molecules and Si QDs, two SRS peaks of OH stretching vibrations of Si QDs solutions are observed in the forward direction. Simultaneously, characteristic feature peaks related to the interaction between OH groups and excess electrons are obtained in the backward SRS of 2nm Si QDs solutions. The excess electrons induce a strong electrostatic field, leading to the transformation from water to an ice-VIII structure.

Ion counting demonstrates a high electrostatic field generated by the nucleosome.

In eukaryotes, a first step towards the nuclear DNA compaction process is the formation of a nucleosome, which is comprised of negatively charged DNA wrapped around a positively charged histone protein octamer. Often, it is assumed that the complexation of the DNA into the nucleosome completely attenuates the DNA charge and hence the electrostatic field generated by the molecule. In contrast, theoretical and computational studies suggest that the nucleosome retains a strong, negative electrostatic field. Despite their fundamental implications for chromatin organization and function, these opposing views of nucleosome electrostatics have not been experimentally tested. Herein, we directly measure nucleosome electrostatics and find that while nucleosome formation reduces the complex charge by half, the nucleosome nevertheless maintains a strong negative electrostatic field. Our studies highlight the importance of considering the polyelectrolyte nature of the nucleosome and its impact on processes ranging from factor binding to DNA compaction.

Structure Distribution of Gaseous Ions in Strong Electrostatic Fields.

In drift tube experiments, where ions move in gases under the action of an electrostatic field, collision excitation is implemented for the study of the energy partitioning in the molecular degrees of freedom and the corresponding relaxation rates when the excitation is turned off. In the case of flexible ions, the vibration modes related to metastable molecular structures have been activated in ion mobility spectrometry and their population has been probed with respect to the field strength and the gas temperature. Here, we study the angular vibrational excitation and relaxation of such systems by examining the motion of molecular ions with one bending mode at strong fields using a nonequilibrium molecular dynamics simulation method. The relatively stable structures are introduced through the use of an intramolecular angular potential with minima at the position of the most stable conformations. We calculate the first few moments of the velocity and angular velocity distribution functions as well as the distribution of the conformers, and find that they follow unified curves when plotted with respect to the relative ion-atom collision energy. At high field strengths, the angular vibration is excited and a portion of the ions interchanges conformations continuously in time with the populations of the molecular structures to attain limiting values. In addition, orientational alignment, with the perpendicular angular momentum being greater than the one parallel to the field, is observed. Our observations, although based on a specific system, must be rather general for the case of large flexible molecular ions.

Electrospinning fiberization of carbon nanotube hybrid sulfonated poly (ether ether ketone) ion conductive membranes for a vanadium redox flow battery

It is challenging for ion conductive membranes to achieve both high proton conduction and low vanadium ion permeation because both ions transport mainly through the hydrophilic domains in membranes. A novel strategy of electrospinning fiberization of the carbon nanotube hybrid sulfonated poly (ether ether ketone) is proposed. The improved conductivity is achieved through interconnective proton pathways induced by the electrospun nanofiber. Multiwall carbon nanotubes contain carboxyl groups, align and disperse well in the nanofibers under a strong electrostatic field of 1.3 kV cm−1 as evidenced by TEM and SAXS, therefore result in enhanced hydrogen bond networks for proton hopping. In contrast, vanadium ions cannot transport through hopping and, thus, are blocked by the carbon nanotubes in hydrophilic domains. Consequently, with the carbon nanotube content increasing from 0 to 0.5 wt %, the area resistance of the electrospun membrane remains unchanged, but vanadium ion permeability dramatically decreases by approximately 67.6%. A vanadium redox flow battery assembled with an electrospun membrane exhibits high chemical and capacity stability as well as an energy efficiency of 83.4%, even after 100 cycles at 100 mA cm−2. These parameters are substantially superior to those of a battery assembled with Nafion 211 with a similar membrane thickness.

Enhanced laser-driven ion acceleration from a low-density-PMMA coated metal-foil

Strong enhancement in proton energy was investigated from a two-dimensional particle-in-cell simulation where an ultraintense laser pulse irradiates a 2-μm thick metal foil coated with a low density, 1-μm thick PMMA (polymethylmathacrylate - C5H8O2) on the rear surface. The reduction of PMMA density effectively increases resistivity of hot electrons, which results in the generation of a strong electrostatic field at the metal-PMMA interface in addition to the sheath electrostatic field at the PMMA-vacuum boundary. The interaction of each proton beam accelerated by the two electrostatic fields leads to the enhancement of energy for the protons originated from the PMMA-vacuum side. With a laser intensity of 1×1020 W/cm2, maximum proton energy of 80 MeV was investigated with a modulation in energy spectrum, which is 2.2 times higher than those from a metal-contamination layer target or a metal-high density PMMA target. It is also interesting that there is an energy peak around 18 MeV, which is caused by an interaction with heavier ions.

A bipolar electret-based micro-vibration-driven energy harvester with multi-air-gap structure

Micro mechanical energy harvester as a kind of micro power source for micro electro mechanical systems (MEMS) has been paid considerable attention. Electret-based vibration energy harvester (E-MVEH) has been a research highlight due to its high sensitivity, simplicity and compatibility to the fabrication process of MEMS. In this manuscript, a novel E-MVEH with multiple-air-gap structure based on bipolar electrets is comparatively investigated. The E-MVEH with single-air-gap structure consists of an electret membrane (EM) with an electrode on one surface and a counter-electrode supported by means of micro-springs. The E-MVEH with double air gaps is made of a bipolar electret membrane put in the middle of two counter-electrodes that are supported by means of micro-springs. It is essentially equivalent to two variable capacitors in series. The E-MVEH with four air gaps consists of two double-air-gaps E-MVEH in parallel connection. When an external vibration causes a relative displacement between the electret membrane (EM) and the counter-electrode, the capacitance varies, leading to re-organization of the charges on the counter-electrode. Thus, an alternating current is generated and the vibration energy is converted into electric energy. This alternating current can be changed into direct current through a full bridge rectifier. The bipolar EM used in this study is a sandwich FEP/THV/FEP perfluoropolymer EM reported previously by our group. It can display persistent and strong electrostatic field with both polarities on the top and bottom surface, respectively. The experimental results of the output characteristic for the proposed E-MVEH show that there exists a resonant frequency in the vibration frequency range of 30−180 Hz, at which the open-circuit voltage and short-circuit current and hence the output power reach their respective maximum. The resonant frequency moves towards the low frequency with increasing vibration intensity, whereas the effect of air-gap number on the resonant frequency is rather small. On the basis of experimental results and theoretical analysis, the maximum output power is positively correlated to the vibration driving force, the surface potential and the area of the electret membrane, and the air-gap number. When the surface potential of the electret membrane is 1200±50 V, the maximum output power is only 0.19 μW for single-air-gap structure and reaches 27.02 μW for the four-air-gap structures under a vibration driving force of 0.5 N and a load resistance of 55 MΩ. For the four-air-gap structures, the maximum output power increases to 59.12 μW when the surface potential of the electret membrane increases to 2150 V. The multi-air-gap energy harvester fabricated with bipolar electret membrane can produce larger variable capacitance; therefore, the output power is significantly enhanced. This novel E-MVEH provides an up-to-date strategy for the design of self-powered sensor systems.

Dielectric response in the vicinity of an ion: A nonlocal and nonlinear model of the dielectric properties of water.

The goal of this work is to propose a simple continuous model that captures the dielectric properties of water at the nanometric scale. We write an electrostatic energy as a functional of the polarisation field containing a term in P4 and non-local Gaussian terms. Such a hamiltonian can reproduce two key properties of water: the saturation of the polarisation response of water in the presence of a strong electrostatic field and the nanometric dipolar correlations of the solvent molecules modifying the long range van der waals interaction. This model explores thus two fundamental aspects that have to be included in implicit models of electrolytes for a relevant description of electrostatic interactions at nanometric scales.

Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator

A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating, accelerating and guiding protons, using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism. In addition, the problems involved are studied by using two-dimensional particle-in-cell simulations. The results show that the proton beam can be collimated, accelerated and guided effectively through this type of target. Theoretically, a formula is derived for the combined electric field of accelerating protons. Compared with a proton beam without a beam collimator, the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10% respectively. Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field, and that the beam collimator plays a role in focusing energy. In addition, the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I. The more prominent point is that the proton number of type II is 2.2 times higher than that of type I. This kind of target has important applications in many fields, such as fast ion ignition in inertial fusion, high energy physics and proton therapy.

Electrostatic field-induced tip-electrospray ionization mass spectrometry for direct analysis of raw food materials.

Rapid characterization of metabolites and risk compounds such as chemical residues and natural toxins in raw food materials such as vegetables, meats, and edible living plants and animals plays an important part in ensuing food quality and safety. To rapidly characterize the analytes in raw food materials, it is essential to develop in situ method for directly analyzing raw food materials. In this work, raw food materials including biological tissues and living samples were placed between an electrode and mass spectrometric (MS) inlet under a strong electrostatic field; analytes were rapidly induced to generate electrospray ionization (ESI) from the sample tip by adding a drop of solvent onto the sample. Therefore, the electrostatic field-induced tip-ESI-MS allows raw samples to avoid contacting high voltage, and thus this method has the advantage for in vivo analysis of food living plants and animals. Metabolite profiling, residues of pesticides and veterinary drugs, and natural toxins from raw food materials have been successfully detected. The analytical performances, including the linear ranges, sensitivity, and reproducibility, were investigated for direct sample analysis. The ionization mechanism of electrostatic field-induced tip-ESI was also discussed in this work.

Preparation and Characterization of Biocompatible Electrospun Nanofiber Scaffolds

Nanoscale fibers were prepared for the fabrication of scaffolds by using a strong electrostatic field on the polymer solution. Electrospinning is widely applied for production of drug delivery, tissue engineering, and regenerative medicine systems as well as biosensors and enzyme immobilization. Nanofibers, thanks to their high surface area to volume ratio, can also mimic the extracellular matrix, thus it has been recognized as a suitable technique for the fast fabrication of scaffolds. This article demonstrates the fabrication of several nanofibrous scaffolds from biopolymers such as polycaprolactone, poly(lactic acid), poly(lactide-co-glycolide), poly(lactide-co-caprolactone) and poly(hydroxybutyrate-co-hydroxy valerate). The characterization and comparison of the scaffolds were achieved based on the morphology and surface characteristic of the nanofibers. The samples showed hydrophobic characteristic, thus a plasma surface treatment was applied successfully to increase hydrophilicity and the effect of the treatment was evaluated based on the wettability and the change in elemental composition of the surface based on X-ray photoelectron spectroscopy.

(Invited) Multiscale Modeling of Transport Phenomena in Ion-Conducting Membranes and Aqueous CO2 Reduction Cells

Transport phenomena is a critical component of electrochemical engineering of energy-conversion devices including fuel cells and solar-fuel generators. In fuel cells, ion-conducting polymers, such as perflurosulfonic-acid (PFSA) membranes, typically nanophase separate into conducting and nonconducting or structural domains that arrange together on the mesoscale and result in macroscopically observable properties. To examine these interactions, a multiscale model has been developed that includes a mean-field local-density theory at the nanoscale, a physically derived resistor-network for the mesoscale that leads up to macroscale. The nanoscale includes cation and water chemical potentials to model transport in the aqueous domains including cation-polymer electrostatic interactions, solvation energy, and a concentrated-solution transport theory to model transport and account for dielectric friction induced by the strong electrostatic fields of charged polymer groups. For the mesoscale, the model accounts for transport along the segments of the mesoscale network of the aqueous domains where mass and charge are conserved at each node (i.e. branching point) of the network. The difference in ionic potential and species chemical potentials between connected network nodes are driving forces for transport. The resistance of each segment of the network is determined from the nanoscale model. The results reveal nonlinear and nonintuitive transport pathways, where the overall properties agree with experimental property measurements. In solar-fuel generators, including CO2 reduction cells, transport phenomena play a critical, yet not fully realized, role in controlling the overall reaction rate. To understand this interplay, multiscale modeling is used to examine the reaction mechanism of CO2 reduction to CO over a planar silver catalyst. Traditional Nernst-Planck expressions for ion and gas transport are coupled to a microkinetics model, where the reaction rates and barriers for the various reaction steps are taken from DFT calculations. The modeling approach helps to resolve the reaction mechanism and demonstrates good agreement with experimental data and trends without fitting parameters. Acknowledgements The fuel-cell work was funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract DE-AC02-05CH11231. The CO2 reduction was funded under the Joint Center for Artificial Photosynthesis (JCAP), a BES Energy Innovation Hub.

Interplay between morphological and shielding effects in field emission via Schwarz-Christoffel transformation

It is well known that sufficiently strong electrostatic fields are able to change the morphology of Large Area Field Emitters (LAFEs). This phenomenon affects the electrostatic interactions between adjacent sites on a LAFE during field emission and may lead to several consequences, such as: the emitter's degradation, diffusion of absorbed particles on the emitter's surface, deflection due to electrostatic forces, and mechanical stress. These consequences are undesirable for technological applications, since they may significantly affect the macroscopic current density on the LAFE. Despite the technological importance, these processes are not completely understood yet. Moreover, the electrostatic effects due to the proximity between emitters on a LAFE may compete with the morphological ones. The balance between these effects may lead to a non trivial behavior in the apex-Field Enhancement Factor (FEF). The present work intends to study the interplay between proximity and morphological effects by studying a model amenable for an analytical treatment. In order to do that, a conducting system under an external electrostatic field, with a profile limited by two mirror-reflected triangular protrusions on an infinite line, is considered. The FEF near the apex of each emitter is obtained as a function of their shape and the distance between them via a Schwarz-Christoffel transformation. Our results suggest that a tradeoff between morphological and proximity effects on a LAFE may provide an explanation for the observed reduction of the local FEF and its variation at small distances between the emitter sites.

Statistics of charge carriers of quantum semiconductor film in the presence of strong lateral electrostatic field

The influence of an external strong electrostatic field on the statistical characteristics of equilibrium free charge carriers in a quasi-two-dimensional structure is considered using an example of an InAs semiconductor quantum film. The analysis is carried out for the case when a strong quantization regime for charge carriers is realized in the film and only the first and second film sub-bands of their quantized motion are occupied. The general analytic expressions for the energy of the quantized motion of charge carriers in a film in the presence of a strong electrostatic field are given. An explicit dependence of the width of the band gap of the sample on the value of the external field is obtained. It is shown that with the increase in the external field, the width of band gap of the sample increases. The calculations and corresponding analytical expressions for the chemical potential, concentration, energy and heat capacity of free charge carriers of an InAs quantum film in the presence of a strong electrostatic field are also presented. It is shown that with an increase in the external electrostatic field, the chemical potential of the system retains its negative sign, but increases by the magnitude. For a given value of the external field, the chemical potential of the system increases linearly with the increase in temperature. It is shown that at a given temperature the carriers’ density decreases with the increase in the external field. At a fixed value of the field, the concentration increases with the increase in temperature. The behavior of the energy and heat capacity of the charge carriers is also determined by the same dependence on the temperature of the system and the magnitude of the external field. It is also shown that the main contribution to the indicated characteristics of the sample is made namely by the first filled sub-band of quantized motion of charge carriers. If the second sub-band is also filled, its contribution to the determination of the statistical characteristics of the sample is exponentially smaller than the contribution of the first filled sub-band.

Nano- and Mesoscale Ion and Water Transport in Perfluorosulfonic-Acid Membranes

High performance of polymer-electrolyte fuel cells requires rapid cation conduction through perflurosulfonic-acid (PFSA) membranes. Consequently, understanding water and aqueous cation transport in aqueous nanoscale domains (nano length scale) and connectivity and tortuosity of the microscale domains (meso length scales) is critical to optimizing PFSA membrane design. Due to transport coupling, membrane ionic conduction depends not only on electrostatic potential gradients but also on water content gradients (i.e. electro-osmosis). In this work, we model coupled cation and water transport in PFSA membranes at the nano- and mesoscale and predict macroscopic properties. We adopt a mean-field local-density theory for cation and water chemical potentials to predict transport in the nanoscale PFSA aqueous domains. Cation-polymer electrostatic interactions and solvation energy are included. Concentrated solution transport theory (Stefan-Maxwell formulation) models diffusion and specifically accounts for dielectric friction induced by the strong electrostatic fields of charged polymer groups. To model the mesoscale, a resistor network that includes Stefan-Maxwell coupling quantifies transport along the segments of the collection of aqueous domains in PFSA membranes. The nanoscale model parameterizes the properties of each segment of the resistor network. With this methodology, we decouple the influences of nano- and mesoscale resistances on macroscopic water and cation transport and electro-osmosis. At the nanoscale there is coupling between water and cation transport from both diffusional drag and electrostatic-driven convection. Electrostatic and pressure forces on the membrane are countered by immobilization of the polymer matrix. The models show the nature of electro-osmosis depends on water content and the vapor and electrostatic potential boundary conditions of the membrane. This work provides a compelling approach to elucidate the multiscale resistances to water and cation transport in PFSA membranes.

Dynamics of Hydration Water Plays a Key Role in Determining the Binding Thermodynamics of Protein Complexes

Interfacial waters are considered to play a crucial role in protein–protein interactions, but in what sense and why are they important? Here, using molecular dynamics simulations and statistical thermodynamic analyses, we demonstrate distinctive dynamic characteristics of the interfacial water and investigate their implications for the binding thermodynamics. We identify the presence of extraordinarily slow (~1,000 times slower than in bulk water) hydrogen-bond rearrangements in interfacial water. We rationalize the slow rearrangements by introducing the “trapping” free energies, characterizing how strongly individual hydration waters are captured by the biomolecular surface, whose magnitude is then traced back to the number of water–protein hydrogen bonds and the strong electrostatic field produced at the binding interface. We also discuss the impact of the slow interfacial waters on the binding thermodynamics. We find that, as expected from their slow dynamics, the conventional approach to the water-mediated interaction, which assumes rapid equilibration of the waters’ degrees of freedom, is inadequate. We show instead that an explicit treatment of the extremely slow interfacial waters is critical. Our results shed new light on the role of water in protein–protein interactions, highlighting the need to consider its dynamics to improve our understanding of biomolecular bindings.

Linear and nonlinear optical response of one-dimensional semiconductors: finite-size and Franz–Keldysh effects

The dipole moment formalism for the optical response of finite electronic structures breaks down in infinite ones, for which a momentum-based method is better suited. Focusing on simple chain structures, we compare the linear and nonlinear optical response of finite and infinite one-dimensional semiconductors. This comparison is then extended to cases including strong electro-static fields breaking translational invariance. For large electro-static fields, highly non-perturbative Franz–Keldysh (FK) features are observed in both linear and nonlinear spectra. It is demonstrated that dipole and momentum formalisms agree in the limit of large structures provided the intraband momentum contributions are carefully treated. This convergence is established even in the presence of non-perturbative electro-static fields.

New scheme for enhancement of maximum proton energy with a cone-hole target irradiated by a short intense laser pulse

Improvement of proton energy from short intense laser interaction with a new proposal of a cone-hole target is investigated via two-dimensional particle-in-cell simulations. The configuration of the target is a cone structure with a hole of changeable diameter through the center of the tip, with proton layers contaminated both on the target rear surface and at the rear part of the hole. In the interacting process, the cone-hole geometry enables the focus of the laser pulse by the cone structure and the consequent penetration of the intensified laser through the tip along the hole instead of reflection, which can increase the energy coupling from laser field to plasmas. The heated electrons, following the target normal sheath acceleration scheme, induce a much stronger electrostatic field in the longitudinal direction at the rear surface of the target than that in the traditional foil case. The simulation results indicate that the accelerated proton beam from the cone-hole target has a cutoff energy about 5.7 and 2.1 times larger than the foil case and the hollow cone case, respectively. Furthermore, the case of the cone-hole target without the proton layer in the hole is also analyzed to demonstrate the effect of the proton layer position and the results show that not only can the existence of the central proton layer improve the proton energy but also lead to a better collimation. The dependence of proton energy on the hole diameter and the scaling law of the maximum proton energy relative to laser intensity are also presented.

Formation and evolution of a pair of collisionless shocks in counter-streaming flows

A pair of collisionless shocks that propagate in the opposite directions are firstly observed in the interactions of laser-produced counter-streaming flows. The flows are generated by irradiating a pair of opposing copper foils with eight laser beams at the Shenguang-II (SG-II) laser facility. The experimental results indicate that the excited shocks are collisionless and electrostatic, in good agreement with the theoretical model of electrostatic shock. The particle-in-cell (PIC) simulations verify that a strong electrostatic field growing from the interaction region contributes to the shocks formation. The evolution is driven by the thermal pressure gradient between the upstream and the downstream. Theoretical analysis indicates that the strength of the shocks is enhanced with the decreasing density ratio during both flows interpenetration. The positive feedback can offset the shock decay process. This is probable the main reason why the electrostatic shocks can keep stable for a longer time in our experiment.