Showing papers on "Electric potential published in 2012"

The Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors

Abstract: We present low temperature electrical transport experiments in five field effect transistor devices consisting of monolayer, bilayer and trilayer MoS2 films, mechanically exfoliated onto Si/SiO2 substrate. Our experiments reveal that the electronic states in all films are localized well up to the room temperature over the experimentally accessible range of gate voltage. This manifests in two dimensional (2D) variable range hopping (VRH) at high temperatures, while below \sim 30 K the conductivity displays oscillatory structures in gate voltage arising from resonant tunneling at the localized sites. From the correlation energy (T0) of VRH and gate voltage dependence of conductivity, we suggest that Coulomb potential from trapped charges in the substrate are the dominant source of disorder in MoS2 field effect devices, which leads to carrier localization as well.

Intracellular recordings of action potentials by an extracellular nanoscale field-effect transistor

Abstract: A silicon nanowire field-effect transistor coupled to the interior of a cell by means of a hollow silicon dioxide nanotube can detect changes in the electric potential of the intracellular fluid.

Mapping Dirac quasiparticles near a single Coulomb impurity on graphene

Abstract: In metals, the Coulomb potential of charged impurities is strongly screened, but in graphene, the potential charge of a few-atom cluster of cobalt can extend up to 10 nm. By measuring differences in the way electron-like and hole-like Dirac fermions are scattered from this potential, the intrinsic dielectric constant of graphene can be determined.

The effects of transient, localized electric fields on equatorial electron acceleration and transport toward the inner magnetosphere

Abstract: [1] Motivated by recent observations of intense electric fields and elevated energetic particle fluxes within flow bursts beyond geosynchronous altitude (Runov et al., 2009, 2011), we apply modeling of particle guiding centers in prescribed but realistic electric fields to improve our understanding of energetic particle acceleration and transport toward the inner magnetosphere through model-data comparisons. Representing the vortical nature of an earthward traveling flow burst, a localized, westward-directed transient electric field flanked on either side by eastward fields related to tailward flow is superimposed on a nominal steady state electric field. We simulate particle spectra observed at multiple THEMIS spacecraft located throughout the magnetotail and fit the modeled spectra to observations, thus constraining properties of the electric field model. We find that a simple potential electric field model is capable of explaining the presence and spectral properties of both geosynchronous altitude and “trans-geosynchronous” injections at higher L-shells (L > 6.6 RE) in a manner self-consistent with the injections' inward penetration. In particular, despite the neglect of the magnetic field changes imparted by dipolarization and the inductive electric field associated with them, such a model can adequately describe the physics of both dispersed injections and depletions (“dips”) in energy flux in terms of convective fields associated with earthward flow channels and their return flow. The transient (impulsive), localized, and vortical nature of the earthward-propagating electric field pulse is what makes this model particularly effective.

Stationary resistive field distribution along epoxy resin insulators in air under DC voltage

Abstract: The stationary resistive field distribution on the gas-solid interface along epoxy resin insulators is theoretically and experimentally investigated. Due to the phenomenon that the charge carriers necessary for the conduction process originate from natural ionization and a constant generation rate is assumed, the resulting electric conductivity of gases under dc voltage stress is nonlinearly depending on the field strength. A simulation model which considers the relevant characteristic is used to calculate the electric field in gas-solid insulation systems. The influence of the ion pair generation rate of the gas, the electric field strength and the surface conductivity of an insulator on the resistive field distribution is investigated. The results are compared to simulations, in which the electric conductivity of the gas is assumed to be constant. The simulations are verified by measurements of the surface potential along an epoxy resin insulator under dc voltage.

Errata to “Surface-Potential-Based Drain Current Analytical Model for Triple-Gate Junctionless Nanowire Transistors”

Abstract: This paper proposes a drain current model for triple-gate n-type junctionless nanowire transistors. The model is based on the solution of the Poisson equation. First, the 2-D Poisson equation is used to obtain the effective surface potential for long-channel devices, which is used to calculate the charge density along the channel and the drain current. The solution of the 3-D Laplace equation is added to the 2-D model in order to account for the short-channel effects. The proposed model is validated using 3-D TCAD simulations where the drain current and its derivatives, the potential, and the charge density have been compared, showing a good agreement for all parameters. Experimental data of short-channel devices down to 30 nm at different temperatures have been also used to validate the model.

Large Signal Stability Analysis of 'More Electric' Aircraft Power Systems with Constant Power Loads

Abstract: This paper presents a detailed analysis of the dynamic behaviour of a hybrid ac-dc electric power system under large disturbances. The system under study is representative of a power distribution network which is currently being developed for future "more electric" aircraft (MEA). Brayton-Moser mixed potential is employed along with Lyapunov stability theorems to determine an analytical estimation of the large-signal stability boundary of the system. Extensive time- domain simulations using detailed behavioural models of the power system components are undertaken. It is shown that the analytically derived region of attraction agrees reasonably well with that obtained from the time-domain simulation. The predicted instability is experimentally validated.

BSIM-IMG: A Compact Model for Ultrathin-Body SOI MOSFETs With Back-Gate Control

Abstract: In this paper, we present an accurate and computationally efficient model for circuit simulation of ultrathin-body silicon-on-insulator MOSFETs with strong back-gate control. This work advances previous works in terms of numerical accuracy, computational efficiency, and behavior of the higher order derivatives of the drain current. We propose a consistent analytical solution for the calculation of front- and back-gate surface potentials and inversion charge. The accuracy of our surface potential calculation is on the order of nanovolts. The drain current model includes velocity saturation, channel-length modulation, mobility degradation, quantum confinement effect, drain-induced barrier lowering, and self-heating effect. The model has correct behavior for derivatives of the drain current and shows an excellent agreement with experimental data for long- and short-channel devices with 8-nm-thin silicon body and 10-nm-thin BOX.

Thermodynamic relation between voltage-concentration dependence and salt adsorption in electrochemical cells.

Abstract: Electrochemical cells containing two electrodes dipped in an ionic solution are widely used as charge accumulators, either with polarizable (supercapacitor) or nonpolarizable (battery) electrodes Recent applications include desalination ("capacitive deionization") and energy extraction from salinity differences ("capacitive mixing") In this Letter, we analyze a general relation between the variation of the electric potential as a function of the concentration and the salt adsorption This relation comes from the evaluation of the electrical and mechanical energy exchange along a reversible cycle, which involves salt adsorption and release by the electrodes The obtained relation thus describes a connection between capacitive deionization and capacitive mixing We check this relation with experimental data already reported in the literature, and moreover by some classical physical models for electrodes, including polarizable and nonpolarizable electrodes The generality of the relation makes it very useful in the study of the properties of the electric double layer

Ionization in elliptically polarized pulses: Multielectron polarization effects and asymmetry of photoelectron momentum distributions

Abstract: In the tunneling regime we present a semiclassical model of above-threshold ionization with inclusion of the Stark shift of the initial state, the Coulomb potential, and a polarization induced dipole potential. The model is used for the investigation of the photoelectron momentum distributions in close to circularly polarized light, and it is validated by comparison with ab initio results and experiments. The momentum distributions are shown to be highly sensitive to the tunneling exit point, the Coulomb force, and the dipole potential from the induced dipole in the atomic core. This multielectron potential affects both the exit point and the dynamics, as illustrated by calculations on Ar and Mg. Analytical estimates for the position of the maximum in the photoelectron distribution are presented, and the model is compared with other semiclassical approaches.

Numerical simulation of nanosecond-pulse electrical discharges

Abstract: Recent experiments with a nanosecond-pulse, dielectric barrier discharge at the stagnation point of a Mach 5 cylinder flow have demonstrated the formation of weak shock waves near the electrode edge, which propagate upstream and perturb the bow shock. This is a promising means of flow control, and understanding the detailed physics of the conversion of electrical energy into gas motion will aid in the design of efficient actuators based on the concept. In this work, a simplified configuration with planar symmetry was chosen as a vehicle to develop a physics-based model of nanosecond-pulse discharges, including realistic air kinetics, electron energy transport, and compressible bulk gas flow. A reduced plasma kinetic model (23 species and 50 processes) was developed to capture the dominant species and reactions for energy storage and thermalization in the discharge. The kinetic model included electronically and vibrationally excited species, and several species of ions and ground state neutrals. The governing equations included the Poisson equation for the electric potential, diffusion equations for each neutral species, conservation equations for each charged species, and mass-averaged conservation equations for the bulk gas flow. The results of calculations with this model highlighted the path of energy transfer in the discharge. At breakdown, the input electrical energy was transformed over a time scale on the order of 1?ns into chemical energy of ions, dissociation products, and vibrationally and electronically excited particles. About 30% of this energy was subsequently thermalized over a time scale of 10??s. Since the thermalization time scale was faster than the acoustic time scale, the heat release led to the formation of weak shock waves originating near the sheath edge, consistent with experimental observations. The computed translational temperature rise (40?K) and nitrogen vibrational temperature rise (370?K) were of the same order of magnitude as experimental measurements (50?K and 500?K, respectively), and the approach appears promising for future multi-dimensional calculations. The effectiveness of flow control actuators based on nanosecond-pulse, dielectric barrier discharges is seen to depend crucially on the rapid thermalization of input energy, in particular the rate of quenching of excited electronic states and the rate of electron?ion recombination.

Graphene-based supercapacitors in the parallel-plate electrode configuration: Ionic liquids versus organic electrolytes

Abstract: Supercapacitors with two single-sheet graphene electrodes in the parallel plate geometry are studied via molecular dynamics (MD) computer simulations. Pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF−4) and a 1.1 M solution of EMI+BF−4 in acetonitrile are considered as prototypes of room-temperature ionic liquids (RTILs) and organic electrolytes. Electrolyte structure, charge density and associated electric potential are investigated by varying the charges and separation of the two electrodes. Multiple charge layers formed in the electrolytes in the vicinity of the electrodes are found to screen the electrode surface charge almost completely. As a result, the supercapacitors show nearly an ideal electric double layer behavior, i.e., the electric potential exhibits essentially a plateau behavior in the entire electrolyte region except for sharp changes in screening zones very close to the electrodes. Due to its small size and large charge separation, BF−4 is considerably more efficient in shielding electrode charges than EMI+. In the case of the acetonitrile solution, acetonitrile also plays an important role by aligning its dipoles near the electrodes; however, the overall screening mainly arises from ions. Because of the disparity of shielding efficiency between cations and anions, the capacitance of the positively-charged anode is significantly larger than that of the negatively-charged cathode. Therefore, the total cell capacitance in the parallel plate configuration is primarily governed by the cathode. Ion conductivity obtained via the Green-Kubo (GK) method is found to be largely independent of the electrode surface charge. Interestingly, EMI+BF−4 shows higher GK ion conductivity than the 1.1 M acetonitrile solution between two parallel plate electrodes.

Ionospheric and thermospheric variations associated with prompt penetration electric fields

Abstract: [1] This paper presents a comprehensive modeling investigation of ionospheric and thermospheric variations during a prompt penetration electric field (PPEF) event that took place on 9 November 2004, using the Thermosphere-Ionosphere-Mesosphere Electrodynamic General Circulation Model (TIMEGCM). The simulation results reveal complex latitudinal and longitudinal/local-time variations in vertical ion drift in the middle- and low-latitude regions owing to the competing influences of electric fields and neutral winds. It is found that electric fields are the dominant driver of vertical ion drift at the magnetic equator; at midlatitudes, however, vertical ion drift driven by disturbance meridional winds exceeds that driven by electric fields. The temporal evolution of the UT-latitude electron density profile from the simulation depicts clearly a super-fountain effect caused by the PPEF, including the initial slow-rise of the equatorial F-layer peak height, the split of the F-layer peak density, and the subsequent downward diffusion of the density peaks along magnetic field lines. Correspondingly, low-latitude total electron content (TEC) becomes bifurcated around the magnetic equator. The O/N2column density ratio, on the other hand, shows very little variations during this PPEF event, excluding composition change as a potential mechanism for the TEC variations. By using realistic, time-dependent, high-latitude electric potential and auroral precipitation patterns to drive the TIMEGCM, the model is able to successfully reproduce the large vertical ion drift of ∼120 m/s over the Jicamarca incoherent radar (IS) in Peru, which is the largest daytime ion drift ever recorded by the radar. The simulation results are validated with several key observations from IS radars, ground GPS-TEC network, and the TIMED-GUVI O/N2column density ratio. The model-data intercomparison also reveals some deficiencies in the TIMEGCM, particularly the limitations imposed by its upper boundary height as well as the prescribed O+ flux.

Uniqueness in an inverse boundary problem for a magnetic Schr\"odinger operator with a bounded magnetic potential

Abstract: We show that the knowledge of the set of the Cauchy data on the boundary of a bounded open set in $\R^n$, $n\ge 3$, for the magnetic Schrodinger operator with $L^\infty$ magnetic and electric potentials determines the magnetic field and electric potential inside the set uniquely. The proof is based on a Carleman estimate for the magnetic Schrodinger operator with a gain of two derivatives.

Molecular theory of weak polyelectrolyte thin films

Abstract: In this work, we extend the recently developed Molecular Theory of Weak Polyelectrolyte Gels to study hydrogel films. This approach explicitly accounts for all of the physicochemical interactions determining the thermodynamic equilibrium of these films and it incorporates molecular details of the polymer network. In particular, we applied this theoretical framework to investigate the response of a thin film of cross-linked hydrophilic polyacid chains to variations in external stimuli such as pH, salt concentration and applied electric potential. Swelling of the polyacid gel film is a continuous but sharp transition that occurs in a narrow range of bulk pH, around the pKa of the acid monomers. The width of this transition range depends on the salt concentration. The gel swells if the bulk pH is larger than the pKa of the monomers and it collapses otherwise. Swelling, however, is not due to a high degree of dissociation in the network, since the swollen gel can have very low degree of electric charge, but the result of the complex balance between the acid–base equilibrium, the gel molecular organization, and the resulting electrostatic interactions. The region close to the surface (up to a few nanometres-thick), the center of the gel, and the interface between the film and the solution have different chemical compositions, which are each different from that of the bulk solution in equilibrium with the film. In particular, the pH in all these regions can be controlled by changing the bulk pH and salt concentration. In addition, there is a gradient of pH going from the solution inside the film, whose magnitude can be tuned by varying bulk pH and salt concentration. In the region near the surface, both the pH and total charge density can be controlled by applying an electric potential. The thin film behaves as an electric insulating material. We calculated the potential of mean force for the insertion of charged nanoparticles inside the hydrogel film. Depending on the electric charge and size of the nanoparticle, there can be an attractive well or a repulsive barrier of several kBT for the nanoparticle to enter the gel from the solution. These findings are relevant in the design of a variety of functional devices using hydrogel films.

Polyelectrolyte adsorption and layer-by-layer assembly: Electrochemical control

Abstract: Thin films formed via the adsorption or layer-by-layer assembly of charged polymers are important in many sensing, energy, and biomedical applications. When the underlying substrate is a (semi)conductor, the opportunity exists to influence film formation and film properties through an applied electric potential. The recent literature on electrochemical influence of polyelectrolyte-based films is reviewed, with a focus on monolayer and multilayer film assembly and disassembly. Of particular interest are monolayer films grown to a tailored thickness on the 10–100 nm scale, and polyelectrolyte multilayer films controllably disassembled, upon application of a modest electric potential. Experimental observations are discussed in terms of governing factors such as interfacial pH and ionic composition, counter-ion correlations, charge regulation, dielectric discontinuity, and short-range polymer–polymer interactions. Recent modeling efforts are also briefly addressed.

Polarization independent blue-phase liquid crystal cylindrical lens with a resistive film

Abstract: We propose a new electrode design for polarization-independent cylindrical lens using a polymer-stabilized blue phase liquid crystal (BPLC). The top electrode is coated with a transparent and resistive film to generate linearly varying electric potential from center to edge; while the bottom iridium tin oxide electrode has a constant potential. Therefore, the vertical electric field across the BPLC layer varies linearly over the lens aperture and a desired parabolic phase profile is obtained automatically according to the Kerr effect. Simulation results show that this simple device is polarization independent and it has parabolic-like phase profile in a large tuning range.

Characterization of an asymmetric parallel plate radio-frequency discharge using a retarding field energy analyzer

Abstract: A retarding field energy analyzer is used to characterize an asymmetric, 13.56 MHz driven, capacitively coupled, parallel plate discharge operated at low pressure. The characterization is carried out in argon discharges at 10 and 20 mTorr where the sheaths are assumed to be collisionless. The analyzer is set in the powered electrode where the impacting ion and electron energy distributions are measured for a range of discharge powers. A circuit model of the discharge is used to infer important electrical parameters from the measured energy distributions, including electrode excitation voltages, plasma potential and sheath potentials. Analytical models of the ion energy distribution in a radio-frequency sheath are used to determine plasma parameters such as sheath width, ion transit time, electron temperature and ion flux. A radio-frequency compensated Langmuir probe is used for comparison with the retarding field analyzer measurements.

Fractional Diffusion Equation and the Electrical Impedance: Experimental Evidence in Liquid-Crystalline Cells

Abstract: The electrical impedance data of different nematic liquid-crystal cells are analyzed in the framework of a model in which the diffusion of mobile ions in the bulk is governed by a fractional diffusion equation of distributed order. The boundary conditions at the electrodes limiting the sample are described by an integro-differential equation governing the kinetic at the interface that embodies, in particular, the usual kinetic equation for describing the adsorption–desorption process at the electrodes but is expressed in terms of a temporal kernel that can be chosen to cover scenarios that are not suitably described within the usual framework of blocking electrodes. The analysis is carried out by supposing that the positive and negative ions have the same mobility and that the electric potential profile across the sample satisfies the Poisson’s equation. The results cover a rich variety of scenarios, including the ones connected to anomalous diffusion.

Charge transport properties of dielectrics revealed by isothermal surface potential decay

Abstract: Revealing the charge transport properties of space grade high insulation materials can benefit the mitigation of electrostatic discharges (ESD) on spacecraft. The charge transport properties of polyimide are investigated by an isothermal surface potential decay (ISPD) experiment under a simulated space environment chamber. After irradiated the sample by an electron gun, the 2-D surface potential distributions are measured by a non-contact potential probe. From the surface potential decay curves, we obtain current density at steady state ISPD. Analyzing the steady state current density against surface potential, we find two regimes. One is Ohmic regime and another is Space Charge Limited Current (SCLC) regime, which are separated at around -950 V with the sample thickness of 27 μm at 298 K. Ohmic resistivity and effective charge carrier mobility are calculated from these two regimes, respectively. In addition, the trap density of polyimide is derived from the SCLC theory, when the sample is charged to high initial surface potential.

Characteristics and mechanisms of surface charge accumulation on a cone-type insulator under dc voltage

Abstract: In order to study the phenomenon of surface charge accumulation on cone-type insulators, a surface charge measuring system is established, based on the electrostatic probe method. The surface potential distributions on a cone-type insulator are measured and the charge distributions are calculated based on the 3D field calculation by the surface charge method. The characteristics of charge distributions are compared under different voltage durations and voltage amplitudes. The results show that the charge accumulation comes to a steady state after 120 min. A threshold voltage effect is observed that charges seldom accumulate unless the applied voltage reaches a certain magnitude. The mechanisms of charge accumulation are discussed. The surface conduction and volume conduction do not seem to dominate the charge accumulation on the insulator. Partial discharges in gas may be the main sources of surface charges.

Electric Field Modulation of the Membrane Potential in Solid-State Ion Channels

Abstract: Biological ion channels are molecular devices that allow a rapid flow of ions across the cell membrane. Normal physiological functions, such as generating action potentials for cell-to-cell communication, are highly dependent on ion channels that can open and close in response to external stimuli for regulating ion permeation. Mimicking these biological functions using synthetic structures is a rapidly progressing yet challenging area. Here we report the electric field modulation of the membrane potential phenomena in mechanically and chemically robust solid-state ion channels, an abiotic analogue to the voltage-gated ion channels in living systems. To understand the complex physicochemical processes in the electric field regulated membrane potential behavior, both quasi-static and transient characteristics of converting transmembrane ion gradients into electric potential are investigated. It is found that the transmembrane potential can be adequately tuned by an external electrical stimulation, thanks to the unique properties of the voltage-regulated selective ion transport through a nanoscale channel.

Effect of electric displacement saturation on the hysteretic behavior of ferroelectric ceramics and the initiation and propagation of cracks in piezoelectric ceramics

Abstract: This paper presents a computational investigation of a proposed simplified account for electric displacement saturation on the hysteretic behavior of initially unpoled ferroelectric ceramics as well as on the initiation and propagation of cracks in poled ferroelectric ceramics within the linear regime of piezoelectricity. For the latter case, experimental observations suggest an odd dependency of the onset of crack initiation in these brittle materials on the orientation of the applied electric field with respect to their poling direction which contradicts theoretical results which propose an even dependency of the energy release rate on the applied electric field within the framework of anisotropic linear piezoelectricity. Electric non-linearities arising at regions of inhomogeneities such as inclusions or at the crack tip are proposed in the literature to avoid this discrepancy. Electric displacement saturation is one such non-linear effect which is investigated in this work. A simplified account of this effect is proposed based on an exponential saturation model of the identified material parameters which can be related to this non-linearity. Its advantage over the superposition of a complex function onto the singular solution of a crack within the framework of linear piezoelectricity lies in the straightforward extension of the proposed approach to problems where no analytical solutions exist. This is outlined based on its incorporation into a rate-dependent ferroelectric model accounting for polarization switching as well as based on its incorporation into a finite element framework capable of simulating the initiation and propagation of cracks in piezoelectric ceramics through strong discontinuities in the displacement field and the electric potential. It is shown that besides the determination of the crack initiation onset also the crack propagation direction is influenced by the appearance of saturation zones arising at the crack tip normal to the polarization direction. The numerically obtained crack paths are found to be close to the experimentally reported results.

Mechanism of heating of pre-formed plasma electrons in relativistic laser-matter interaction

Abstract: The role of the longitudinal ambipolar electric field, present inside a pre-formed plasma, in electron heating and beam generation is investigated by analyzing single electron motion in the presence of one electromagnetic plane wave and “V” shaped potential well (constant electric field) in a one dimensional slab approximation. It is shown that for the electron confined in an infinite potential well, its motion becomes stochastic when the ratio of normalized laser electric field a0, to normalized longitudinal electric field Ez, exceeds unity, i.e., a0/Ez≳1. For a more realistic potential well of finite depth, present inside the pre-formed plasma, the condition for stochastic heating of electrons gets modified to 1≲a0/Ez≲L, where L is the normalized length of the potential well. The energy of electron beam leaving such a potential well and entering the solid scales ∼a02/Ez, which can exceed the laser ponderomotive energy (∼a0) in the stochastic regime.

Floating-Gate Corner-Enhanced Poly-to-Poly Tunneling in Split-Gate Flash Memory Cells

Abstract: The poly-to-poly tunneling characteristics in the third-generation SuperFlash memory cell have been analyzed. It has been demonstrated that, even without a sharp floating-gate (FG) tip, the cell still demonstrates the main features of the erase process from previous SuperFlash generations, namely, corner (tip)-enhanced tunneling, asymmetry of the tunneling voltage in forward and reverse directions, strong localization of the tunneling process, and effective suppression of anode hole injection. Furthermore, a new method for measuring the tunneling voltage on a regular FG cell is described. The reliability aspects of corner-enhanced tunneling in the SuperFlash cell are also discussed.

Energy and force prediction for a nanosecond pulsed dielectric barrier discharge actuator

Abstract: A three-species physical model is presented for dielectric barrier discharge (DBD) actuator under atmospheric pressure. The governing equations are solved for temporal and spatial distribution of electric potential and charge species using the finite element based multiscale ionized gas flow code. The plasma model is loosely coupled with compressible Navier-Stokes equations through momentum and energy source terms. Two cases of rf powered and nanosecond pulsed barrier discharge actuators are simulated. Based on the imparted time average electrohydrodynamic force and power deposition to the neutral gas, the nanosecond pulsed DBD actuator creates significant pressure variations within few microseconds. These results are in reasonable agreement with recently reported experimental shadow images.