Electrostatic Potential Field Research Articles

Ab initio quantum chemical calculation of electron density, electrostatic potential, and electric field of biomolecule based on fragment molecular orbital method

AbstractEfficient quantum chemical calculations of electrostatic properties, namely, the electron density (EDN), electrostatic potential (ESP), and electric field (EFL), were performed using the fragment molecular orbital (FMO) method. The numerical errors associated with the FMO scheme were examined at the HF, MP2, and RI‐MP2 levels of theory using 4 small peptides. As a result, the FMO errors in the EDN, ESP, and EFL were significantly smaller than the magnitude of the electron correlation effects, which indicated that the FMO method provides sufficiently accurate values of electrostatic properties. In addition, an attempt to reduce the computational effort was proposed by combining the FMO scheme and a point charge approximation. The error due to this approximation was examined using 2 proteins, prion protein and human immunodeficiency virus type 1 protease. As illustrative examples, the ESP values at the molecular surface of these proteins were calculated at the MP2 level of theory.

Solitary wave solutions of two-dimensional nonlinear Kadomtsev–Petviashvili dynamic equation in dust-acoustic plasmas

Nonlinear two-dimensional Kadomtsev–Petviashvili (KP) equation governs the behaviour of nonlinear waves in dusty plasmas with variable dust charge and two temperature ions. By using the reductive perturbation method, the two-dimensional dust-acoustic solitary waves (DASWs) in unmagnetized cold plasma consisting of dust fluid, ions and electrons lead to a KP equation. We derived the solitary travelling wave solutions of the two-dimensional nonlinear KP equation by implementing sech–tanh, sinh–cosh, extended direct algebraic and fraction direct algebraic methods. We found the electrostatic field potential and electric field in the form travelling wave solutions for two-dimensional nonlinear KP equation. The solutions for the KP equation obtained by using these methods can be demonstrated precisely and efficiency. As an illustration, we used the readymade package of Mathematica program 10.1 to solve the original problem. These solutions are in good agreement with the analytical one.

Fano factor for Dirac electrons in a supperlattice of normal/ferromagnetic/normal silicene junction

We investigate the electron transport in the presence of electric field and gate potential in a supperlattice of normal/ferromagnetic/normal silicene junction. We compute the total conductance and Fano factor using the transfer matrix method and Landauer-Buttiker formula. The total conductance and Fano factor of the silicene contain interesting information of the transport properties of the charge carriers. The dependence of the Fano factor behavior on the electric field strength, the gate potential, the thickness of the ferromagnetic region and more importantly the dependence on the number of barriers have been plotted. Our aim is to achieve a more accurate picture of the dependence of the Fano factor on parameters mentioned above. In this junction, Fano factor oscillates with the thickness of the ferromagnetic region, the electric field strength and the gate voltage in the ferromagnetic regions. Also we found that diffusive transport (F=1/3) occurs by taking large enough length of the ferromagnetic regions and tiny electric field strength. Another remarkable point is that Fano factor attains the full Poissonian value (F=1), by controlling the electric field strength and the length of the ferromagnetic regions. We see that with remaining intact the conductance, we can change the transport from Poissonian to diffusive transport by controlling the length of the ferromagnetic regions. However, these findings occur exactly in the case of ΔzE=0.5 when the number of barriers is large enough. Moreover, with considering dependence of the Fano factor on the electrostatic potential and electric field strength, we have proved that these parameters are controllable parameters on the kind of transport. It is said that Fano factor is very sensitive to the mentioned parameters and can be controlled by these parameters. In fact, we show that the value of Fano factor is a valuable tool for distinguishing the behavior of transport whereas this kind of information cannot be extracted from the conductance.

Intercalation Driven Porosity Effects in Coupled Continuum Models for the Electrical, Chemical, Thermal and Mechanical Response of Battery Electrode Materials

We present a coupled continuum formulation for the electrostatic, chemical, thermal and mechanical processes in battery materials. Our treatment applies on the macroscopic scale, at which electrodes can be modeled as porous materials made up of active particles held together by binders and perfused by the electrolyte. Starting with the description common to the field, in terms of reaction-transport partial differential equations for ions, variants of the classical Poisson equation for electrostatics, and the heat equation, we add mechanics to the problem. Our main contribution is to model the evolution of porosity as a consequence of strains induced by intercalation, thermal expansion and mechanical stresses. Recognizing the potential for large local deformations, we have settled on the finite strain framework. We present a detailed computational study of the influence of the dynamically evolving porosity, upon ion distribution, electrostatic potential fields, charge-discharge cycles and mechanical force generated in the cell.

Stability analysis solutions for nonlinear three-dimensional modified Korteweg–de Vries–Zakharov–Kuznetsov equation in a magnetized electron–positron plasma

The nonlinear three-dimensional modified Korteweg–de Vries–Zakharov–Kuznetsov ​(mKdV–ZK) equation governs the behavior of weakly nonlinear ion-acoustic waves in magnetized electron–positron plasma which consists of equal hot and cool components of each species. By using the reductive perturbation procedure leads to a mKdV–ZK equation governing the oblique propagation of nonlinear electrostatic modes. The stability of solitary traveling wave solutions of the mKdV–ZK equation to three-dimensional long-wavelength perturbations is investigated. We found the electrostatic field potential and electric field in form traveling wave solutions for three-dimensional mKdV–ZK equation. The solutions for the mKdV–ZK equation are obtained precisely and efficiency of the method can be demonstrated.

Three-dimensional nonlinear modified Zakharov–Kuznetsov equation of ion-acoustic waves in a magnetized plasma

The nonlinear three-dimensional modified Zakharov–Kuznetsov (mZK) equation governs the behavior of weakly nonlinear ion-acoustic waves in a plasma comprising cold ions and hot isothermal electrons in a presence of a uniform magnetic field. By using the reductive perturbation procedure leads to a mZK equation governing the propagation of ion dynamics of nonlinear ion-acoustic waves in a plasma. The mZK equation has solutions that represent solitary traveling waves. We found the electrostatic field potential and electric field in form traveling wave solutions for three-dimensional mZK equation. The solutions for the mZK equation are obtained precisely and efficiency of the method can be demonstrated. The stability of solitary traveling wave solutions of the mZK equation to three-dimensional long-wavelength perturbations are investigated.

Phase contrast STEM for thin samples: Integrated differential phase contrast

It has been known since the 1970s that the movement of the center of mass (COM) of a convergent beam electron diffraction (CBED) pattern is linearly related to the (projected) electrical field in the sample. We re-derive a contrast transfer function (CTF) for a scanning transmission electron microscopy (STEM) imaging technique based on this movement from the point of view of image formation and continue by performing a two-dimensional integration on the two images based on the two components of the COM movement. The resulting integrated COM (iCOM) STEM technique yields a scalar image that is linear in the phase shift caused by the sample and therefore also in the local (projected) electrostatic potential field of a thin sample. We confirm that the differential phase contrast (DPC) STEM technique using a segmented detector with 4 quadrants (4Q) yields a good approximation for the COM movement. Performing a two-dimensional integration, just as for the COM, we obtain an integrated DPC (iDPC) image which is approximately linear in the phase of the sample. Beside deriving the CTFs of iCOM and iDPC, we clearly point out the objects of the two corresponding imaging techniques, and highlight the differences to objects corresponding to COM-, DPC-, and (HA) ADF-STEM. The theory is validated with simulations and we present first experimental results of the iDPC-STEM technique showing its capability for imaging both light and heavy elements with atomic resolution and a good signal to noise ratio (SNR).

A low subthreshold swing tunneling field effect transistor for next generation low power CMOS applications

We propose a low subthreshold swing transistor architecture called Negative Capacitance Single Gate Silicon-On-Insulator Tunneling Field Effect Transistor (NC-SG-SOI-TFET) and present an analytical model to characterize its performance. Electrostatic potential distribution and electric field intensity in the channel region are obtained by solving the Poisson equation, and the drain current is calculated using the band-to-band carrier generation rate. An additional layer of ferroelectric oxide is used to obtain the negative capacitance. Effect of ferroelectric oxide is incorporated using one-dimensional Landau formalism. Through two dimensional theoretical analysis, we show that the proposed device has superior performance over traditional TFETs in terms of subthreshold swing and short channel effects. For example, a subthreshold swing of 11.82mV/decade and operating voltage of 0.65V for a drain current of 10−8A/µm have been obtained. The physics behind the improved performance is discussed based on the presented model. The analytical model would also be instrumental in designing and optimizing such devices avoiding complexities and cost of numerical models.

Scanning Quantum Dot Microscopy.

We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

Potential‐based threshold voltage and subthreshold swing models for junctionless double‐gate metal‐oxide‐semiconductor field‐effect transistor with dual‐material gate

SummaryOn the basis of quasi‐two‐dimensional solution of Poisson's equation, an analytical threshold voltage model for junctionless dual‐material double‐gate (JLDMDG) metal‐oxide‐semiconductor field‐effect transistor (MOSFET) is developed for the first time. The advantages of JLDMDG MOSFET are proved by comparing the central electrostatic potential and electric field distribution with those of junctionless single‐material double‐gate (JLSMDG) MOSFET. The proposed model explicitly shows how the device parameters (such as the silicon thickness, oxide thickness, and doping concentration) affect the threshold voltage. In addition, the variations of threshold voltage roll‐off, drain‐induced barrier lowering (DIBL), and subthreshold swing with the channel length are investigated. It is proved that the device performance for JLDMDG MOSFET can be changed flexibly by adjusting the length ratios of control gate and screen gate. The model is verified by comparing its calculated results with those obtained from three‐dimensional numerical device simulator ISE. Copyright © 2015 John Wiley & Sons, Ltd.

Experimental and numerical studies on the sample area and skin effect of the three-rod time domain reflectometry probe

The sample area and skin effect of the widely used three-rod time domain reflectometry (TDR) probe is not fully understood. The seepage analysis software was adopted to compute the electrostatic potential field surrounding the TDR probe and a quantitative assessment of the sample area and skin effect for probes that varied in design with respect to rod diameter (D) and rod spacing (L) was conducted. The effectiveness of the numerical method was demonstrated by experiments designed to determine the extent of the sample area for the three-rod probe with a rod diameter of 6 mm and a rod spacing of 30 mm. The main findings from the experimental and numerical studies are as follows. The electrical conductivity of the test medium has a negligible effect on the sample area of the three-rod probe when it is limited within 150 ms/m. The sample area (Sf = 90%) for the three-rod probe can be represented as an ellipse with a semi-major axis length equal to L + D/2. The magnitude of the sample area is predominantly determined by the rod spacing for the dimension ranges under consideration (D = 4–8 mm; L = 15–60 mm), and it can be predicted by a quadratic equation. The extent of the skin effect (Sf = 50%/Sf = 90%) for the three-rod probe is dependent on the ratio of the rod spacing to the rod diameter (L/D) and the quantitative skin effect can be also evaluated by a quadratic equation. On the basis of these findings, a design procedure is proposed to determine the optimal rod diameter (D) and rod spacing (L) for a three-rod TDR probe.

(Invited) Charge-Based Modelling of the Channel Current in Organic Field Effect Transistors Considering Injection Effects

Models for the current calculation in organic field-effect transistors (OFETs) are usually based on the typical MOSFET equations. Two different independent equations connected by the threshold voltage are used, in which the threshold voltage is usually a fitting parameter without relation to physical parameters hence the impact of their variability on the threshold voltage is not clear. We present a modeling approach for the device current in OFETs which is physically- and charge-based with a continuous equation for the channel current in organic field-effect transistors. It is assumed that the conducting channel of an OFET is reduced to a small number of monolayers of a specific thickness dm where the accumulated charge is uniformly distributed [1] as well as that all traps are filled in the on-state of the device (trapped charges qNt). The accumulated mobile charge per area Q’m is calculated and is a function of that specific thickness dm, the density of shallow traps Nc, and the surface potential ΦC. While calculating the mobile charge Q’m via the Poisson’s equation it gets obviously that there is no closed form solution for the surface potential ΦC. So it is solved once numerically for a given voltage VGS=V0 and kept constant for a given technology. The value of that (arbitrarily) chosen voltage within the operating regime and the corresponding mobile charge Q’m0 have almost no influence on the final result of the current and the threshold voltage. Closed form solutions for mobile charges at the source/drain end of the channel have been derived [2] and are used for the calculation of the channel current IDS/channel considering a diffusion and a drift part of the current. In the presented approach the mobile charges densities are a function of VGS, Q’m0, and V0. An expression of the threshold voltage Vthis also found. The injection effects occurring at the metal-semiconductor-junction are considered in the calculation of the total channel current as well and are mainly dominated by the mismatch of the metal work function and the semiconductor HOMO level (p-type semiconductor). Tunneling and thermionic currents are considered to be the main current contributions at the metal-semiconductor-junction and are calculated via a 2D solution of the electrostatic potential and electric field within the channel area [3]. Parasitic source and drain resistances are considered as well. The model is verified with measurements on OFETs fabricated with small molecules (Fig. 1), implemented in Verilog-A, and currently tested for circuit design.

Modeling marine mud: A four phase system

Fine-grained marine sediment consists of up to four phases: (1) clay Minerals often with some silt and sand, (2) saline water, (3) free gas, and (4) organic matter (OM). Shallow-water, shelf, and slope muds often have all four phases present. The clay minerals are the “building blocks” of mud deposits; these consist of multi-plate face-to-face domains. There are only three possible modes of domain contact (termed signatures): (1) edge-to-face (EF), (2) edge-to-edge (EE), and (3) face-to-face (FF), usually offset. The domain faces are negatively charged and have a large surface area and the edges are both negative and positive charged. OM is usually expressed as total organic carbon (TOC) in percent total dry mass and can range from <0.5% to several percent and the OM can occupy a significant portion of pore volume. OM is usually attached to clay domain signatures and faces with the location partially dictated by electrostatic potential energy fields. When several percent TOC is present, the OM drives up water contents and porosity considerably. When free gas is present, it replaces part of the seawater in the pore space. Several published organo-clay fabric models for mud deposits may improve prediction of acoustic properties.

Comparison of Methods for Computation of Electron Optical System Potential Distribution

To calculate the electrostatic field of electron gun was used software ANSYS. ANSYS is based on FEM. Model of electron gun is complicated enough, so we used submodeling. Test precision of calculated electrostatic field potential values was used simple immersion lens. The computed values by FEM using submodeling were in comparison with values computed in analytical way. They were very close, when the finite element size of submodel was four times smaller than the element size of coarse model. Ill.10, bibl. 4. (in Lithuanian; summaries in Lithuanian, English, Russian).

Coulomb impurity effect on electrically induced Dirac bound states in graphene

Using the massless Dirac–Weyl model for monolayer graphene sheet, we study the low-lying spectra of a single Dirac electron system bound to an on-center positively charged Coulomb impurity, under both electrostatic potential and magnetic field. Numerical results obtained from diagonalization show that, the increase of the electrostatic potential causes the low-lying states to evolve from one Landau-type plateau to the higher ones, and the whole spectra exhibit similar features with slight shifts in eigenenergies when the on-center impurity is considered. Electrostatic-potential dependent optical spectra with their corresponding absorption coefficients as functions of incident photon energies for transitions between low-lying states are presented.

Electrophoresis of electrically neutral porous spheres induced by selective affinity of ions.

I investigate the possibility that electrically neutral porous spheres electrophorese in electrolyte solutions with asymmetric affinity of ions to spheres on the basis of electrohydrodynamics and the Poisson-Boltzmann and Debye-Bueche-Brinkman theories. Assuming a weak electric field and ignoring the double-layer polarization, I obtain analytical expressions for electrostatic potential, electrophoretic mobility, and flow field. In the equilibrium state, the Galvani potential forms across the interface of the spheres. Under a weak electric field, the spheres show finite mobility with the same sign as the Galvani potential. When the radius of the spheres is significantly larger than the Debye and hydrodynamic screening length, the mobility monotonically increases with increasing salinity.

Nanoporous Structural Science Developed by the MEM/Rietveld Method

Wide-spread functionalization research of Metal Organic Frameworks(MOFs) has brought rapid increase in variety of materials since the beginning of structural study in nanoporous of MOFs were made by SR(Synchrotron Radiation) powder diffraction using the MEM(Maximum Entropy Method)/Rietveld Method(Kitaura et al, 2002). The MEM/Rietveld method has successfully applied to refine the structural position of absorbed molecules and to investigate a bonding nature between the molecules and MOF's pore walls. Noise-resistance electron density mapping with incomplete data set was a key advantage of MEM to visualize unmodeled feature of molecules in nanoporous. Since then, the charge density studies by the MEM/Rietveld Method have uncovered various ordering structure of absorbed molecules into nanoporous more and more(Takata, 2008). Those findings ignited trends to design the nanoporous as the space to be functionalized. Recently, the MEM/Rietveld method has been further developed as the method to map an electrostatic potential and electric field(Tanaka 2006). This technique is making a progress in structural science of MOFs since the visualized electrostatic potential in the nanoporous ought to provide information of interplay between the molecule and the pore walls. The talk will present the recent progress and challenges of the MEM/Rietveld method to the structural science of the MOFs.

Application of maximum-entropy electrostatic potential in Materials Science.

Charge density (CD) studies by Maximum Entropy Method (MEM) (Sakata & Sato, 1990) from x-ray diffraction data have been widely applied to solve problems and questions in materials science during past two decades. Encapsulations of metal atoms (Takata et al, 1995), gas molecules, as well as protein molecules in the materials have been visualized as MEM CDs. The MEM CD technique is now regarded as a sophisticated technique for visualization in atomic scale. Electrostatic potential (EP) and electric field (EF) from x-ray diffraction data using MEM have been developed in 2006 (Tanaka et al, 2006). The EP & EF successfully applied to ferroelectric material PbTiO3 and a charge ordered manganite system. The method has huge potential in materials science since interaction in the non-atomic region can be visualized experimentally. One of the promising target for EP & EF analysis is host-guest systems, such as porous coordination polymers (PCPs), zeolites, clathrates as well as endohedral metallofullerenes[3]. In the case of host-guest systems, the guest atom(s) or molecule(s) are located in spatially wider sites in comparison to other type of materials. Therefore the detailed structural information in the spatially wider sites is one of the most important issues. In the present study, I present an application of MEM EP & EF analysis to host-guest related system, icosahedral B12 cluster materials and hydrogen adsorbed PCP. The EP studies clearly visualize doping sites in B12 based superconductor and adsorption sites in PCP. The EF enables us to estimate quantitative interaction from host to guest. The quantitative evaluation really bridges between experiment and theory in materials science.

Subthreshold behavior models for nanoscale junctionless double-gate MOSFETs with dual-material gate stack

By using a variable separation technique, an analytical model of the two-dimensional (2D) channel electrostatic potential for junctionless dual-material double-gate (JLDMDG) MOSFETs is derived from the 2D Poisson’s equation. On the basis of the 2D channel electrostatic potential and the current continuity equation, a subthreshold current model is obtained. The advantages of JLDMDG MOSFETs are proved by comparing the central electrostatic potential and electric field distribution with those of junctionless single-material double-gate (JLSMDG) MOSFETs. In addition, the influence of different device parameters (such as body thickness, oxide thickness, and the ratio of gate length) on subthreshold current and subthreshold slope is investigated. It is found that a smaller body thickness or gate oxide thickness or a longer control gate induces a better subthreshold performance. The data extracted from the developed model are in good accordance with simulation results obtained from DESSIS.

2D physics-based closed-form modeling of dopant-segregated Schottky barrier UTB MOSFETs

A 2D closed-form, analytical compact current model for long and short-channel Schottky barrier (SB) Multi-Gate MOSFETs is presented. The physics-based two-dimensional model for the electrostatic potential and electric field of a ultra-thin body (UTB) MOSFET is derived with the help of Poisson’s equation and the conformal mapping technique by Schwarz–Christoffel. Furthermore, simple closed-form current equations were derived by using analytical expressions for the tunneling and thermionic currents. Essential 2D effects on the currents are included in the model which are combined with diffusion effects in the channel region. A comparison of our 2D physics-based compact model is done versus measurement data for a dopant segregated fully-depleted Schottky barrier MOSFET.