Macroscopic Electric Field Research Articles

Deposition of Carbon Nanotubes on Silicon for Field Emission Application

This paper discusses some results about Single-Walled Carbon Nanotubes electrophoretically deposited on silicon in order to fabricate Field Emission devices. Field emission characterization was executed into a high vacuum chamber. Emission characteristics from distinct samples were compared by extracting the macroscopic electric field from V-d curves and applying it as a quality parameter. It was demonstrated that carbon nanotubes on the Si surface could improve approximately 8 times the field emission properties of devices.

The influence of electric fields on nanostructures—Simulation and control

Macroscopic electric fields may be used to influence or even control the shape evolution of single layer islands on a crystalline surface. We validate a phase-field approach to simulate such shape evolutions by comparison with simulations based on a sharp interface approach. The phase-field model is then used to formulate and discretize the control problem of finding an electric field which drives an island to a predefined shape. Some first results of this approach are presented.

Femtosecond laser excitation of coherent optical phonons in ferroelectricLuMnO3

We have used femtosecond pump-probe spectroscopy to excite and probe coherent optical phonon vibrations in single crystals of hexagonal ferroelectric ${\text{LuMnO}}_{3}$. An optical phonon mode of ${A}_{1}$ symmetry was coherently excited with 25 fs pump-laser pulses $(\ensuremath{\lambda}\ensuremath{\approx}800\text{ }\text{nm})$. The phonon mode, involving Lu ion motion along the $c$ axis, was identified as the soft mode driving the ferroelectric transition. The excitation mechanism was determined to be purely displacive in nature due to resonant excitation of a narrow intra-atomic ${d}_{xy,{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\rightarrow}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ transition in Mn. The lifetime of the $\text{Mn}\text{ }{d}_{xy,{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\rightarrow}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ excitation was measured to be 0.8 ps. A remarkable reversal of the sign of the oscillation amplitude ($\ensuremath{\pi}$ phase shift) of the reflectivity curve was observed upon comparing longitudinal-optical (LO) with transverse-optical (TO) mode geometries. The phase reversal is attributed to the macroscopic electric depolarization field accompanying infrared-active longitudinal phonon modes but absent in TO modes. In addition to the direct effect of the ion motion on the optical properties, which is the same in LO and TO modes, the longitudinal depolarization field of the LO mode gives rise to an additional modulation of the refractive index via the linear electro-optic effect which dominates the optical response.

Updated references for the structural, electronic, and vibrational properties of TiO2(B) bulk using first-principles density functional theory calculations

To get updated references on the structural, electronic, and vibration properties of the metastable TiO(2)(B) compound, infrared and Raman spectra of TiO(2)(B) are computed within the density functional theory framework and all active modes are assigned. Phonons and their possible coupling with the macroscopic electric fields resulting from the long-range interactions of instantaneous local dipoles (due to nuclear vibrations) in polar solids are taken into account through supercell calculations and longitudinal optical-transversal optical splitting corrections. Full structural relaxations using conventional density functional theory and hybrid functionals with localized Gaussian-type orbitals or plane-wave basis sets reveal a similar deviation of the local Ti environment compared to the TiO(2)(B) structural refinements reported so far. Such deviations are shown to be significant from those computed for anatase using the same method, thus yielding distinguishable spectroscopic responses for the two polymorphs.

3D modeling of the total electric field induced by transcranial magnetic stimulation using the boundary element method

Transcranial magnetic stimulation (TMS) delivers highly localized brain stimulations via non-invasive externally applied magnetic fields. This non-invasive, painless technique provides researchers and clinicians with a unique tool capable of stimulating both the central and peripheral nervous systems. However, a complete analysis of the macroscopic electric fields produced by TMS has not yet been performed. In this paper, we addressed the importance of the secondary E-field created by surface charge accumulation during TMS using the boundary element method (BEM). 3D models were developed using simple head geometries in order to test the model and compare it with measured values. The effects of tissue geometry, size and conductivity were also investigated. Finally, a realistically shaped head model was used to assess the effect of multiple surfaces on the total E-field. Secondary E-fields have the greatest impact at areas in close proximity to each tissue layer. Throughout the head, the secondary E-field magnitudes typically range from 20% to 35% of the primary E-field's magnitude. The direction of the secondary E-field was generally in opposition to the primary E-field; however, for some locations, this was not the case (i.e. going from high to low conductivity tissues). These findings show that realistically shaped head geometries are important for accurate modeling of the total E-field.

Field electron emission from bunchy flake-like nano-carbon films

This paper reports that bunchy flake-like nano-graphite crystallite films (BNGCFs) were deposited on Si substrates by using the microwave chemical vapour deposition technique. Furthermore the BNGCFs were characterized by x-ray diffraction spectra, scanning electron microscopy, Raman spectra and field emission (FE) I–V measurements, and a lowest turn-on field of 1.5 V/μm, and a high average emission current density of 30 mA/cm2 at a macroscopic electric field of 8.0 V/μm were obtained. The J–E data did not follow the original Fowler–Nordheim (F–N) relation since they were not well represented in the F–N plot by a straight line. A model considering the F–N mechanism, and the statistic effects of FE tip structures has been applied successfully to explain all the FE data observed for E < 8.8 V/μm.

General formulation for quantum macroscopic nonlinear optics

We present a general formulation for quantum macroscopic nonlinear optics, which can be considered as a fully quantized version of its semiclassical counterpart in which the electric field is treated as the classical variable. Our formulation begins with the fundamental minimal-coupling Lagrangian, which is then transformed into the multipolar Lagrangian. After a quantum preservative expansion by using a Hamiltonian decomposition proposed in this paper, we have found the formal relations between the Heisenberg operators of macroscopic polarization density and macroscopic electric field. Finally, the linearized quantum effects for nonlinear optics are also discussed.

Diffusioosmotic flow of electrolyte solutions in fibrous porous media at arbitrary zeta potential and double-layer thickness

The diffusioosmosis of electrolyte solutions in the fibrous medium constructed by a homogeneous assemblage of parallel, charged, circular cylinders caused by a uniform concentration gradient prescribed in their axial direction is studied theoretically. The electric double layers adjacent to the cylinders may have an arbitrary thickness. A unit cell model is used to account for the interaction effect among the cylinders. The electrostatic potential distribution in the fluid phase is determined with an analytical approximation to the solution of the Poisson–Boltzmann equation. By solving the fluid momentum equation with the constraint of no net electric current arising from the co-current diffusion, electric migration, and diffusioosmotic convection of the electrolyte ions, the macroscopic electric field and the fluid velocity in the axial direction induced by the applied electrolyte concentration gradient are obtained semi-analytically as functions of the radial position in a cell in a self-consistent manner. The magnitude and direction of the diffusioosmotic flow relative to the concentration gradient are determined by the combination of the porosity of the array of cylinders, the zeta potential of the cylinders, the properties of the electrolyte solution, and other relevant factors. The fluid velocity generally increases with increasing porosity of the array of cylinders, but there are exceptions. The effects of the radial distribution of the induced electric field and of the ionic convection in the double layers on the diffusioosmotic flow are significant.

Radiative recombination in type-II ZnSe/BeTe heterostructures at high densities of free carriers

We have studied in detail the photoluminescence in type-II ZnSe/BeTe structures with large band offsets at high optical excitation levels. Our goal is a qualitative and quantitative description of the effect of a high carrier density on the spectral and kinetic properties of the photoluminescence in such structures. We have established that a high photoexcitation density leads to a significant spectral rearrangement of the spatially indirect radiative recombination, which also depends on the thickness of the structure layers. The spectral and kinetic characteristics of the indirect optical transitions in narrow and wide structures have been found to differ qualitatively. Our numerical calculations of the effect of macroscopic electric fields on the energy spectrum of free carriers at high densities of spatially separated charges, n > 1013 cm−2, give a qualitative and quantitative description of the experimentally observed photoluminescence properties. The calculations also show that the degree of localization of the above-barrier hole states grows significantly with n under conditions of increasing band bending, which should cause a weakening of the charge separation, especially in wide structures. In an experiment, the effect manifests itself in an enhancement of the spatially direct transition at a simultaneous weakening of the indirect transition both with increasing structure width and with increasing n.

Consolidation of Charged Colloids during Drying

We consider the drying of latex dispersions containing submicrometer-sized particles dispersed in water. It is well known that the consolidation of colloidal particles is influenced by a number of factors such as particle size and shape and interparticle potential. In this work, we focus on the effect of surface charge on the consolidation front. Recent experimental and theoretical investigations on the sedimentation of charged colloidal spheres have shown that the large mass difference between noninteracting colloids and ions sets up a macroscopic electric field, thereby enhancing the diffusivity of the particles and resulting in an inflated sedimentation profile. Our experimental measurements of the concentration profile during drying-induced consolidation also reveal similar charge effects. We present a model for the consolidation of charged particles that accounts for the presence of an induced external electric field. As expected, the predicted particle diffusivity is enhanced by the onset of the electric field at low particle concentration. Fluorescence and bright-field microscopy were used to detect the particle concentration variation in a dispersion dried in a capillary, and the measured profile agrees with the prediction confirming the influence of particle charge on consolidation.

Renormalization of the band gap in highly photoexcited type-II ZnSe/BeTe structures

For the type-II ZnSe/BeTe heterostructures, a large (∼0.1 eV) red shift of the edge of interband recombination in the ZnSe layers is observed at high densities of spatially separated photoexcited electrons and holes (∼1013 cm−2). The observed magnitude of renormalization of the band gap exceeds the magnitudes predicted by the multiparticle theory for dense type-I electron-hole systems at the same concentrations of two-dimensional charge carriers. Numerical calculations show that macroscopic electric fields induced by separated charges have a profound effect on the energy of direct transitions in type-II structures, resulting in an additional decrease in the energy of the transitions. In wide structures, where the ZnSe layer thickness is ≳ 15 nm, the renormalization effect is less pronounced. This is attributed to incomplete spatial separation of photoexcited charge carriers in the case of profound band bending and, thus, to the less-pronounced effect of electric fields.

Effects of Spatial Dispersion on Reflection From Mushroom-Type Artificial Impedance Surfaces

In a spatially dispersive medium, the electric dipole moment of an inclusion cannot be related to the macroscopic electric field through a local relation. Several recent works have emphasized the role of spatial dispersion in wire media, and demonstrated that arrays of parallel metallic wires may behave very differently from a uniaxial local material with negative permittivity. Here, we investigate the effect of spatial dispersion on reflection properties of the mushroom structure introduced by Sievenpiper, based on local and nonlocal homogenization methods. The objective of this paper is to clarify the role of spatial dispersion in the mushroom structure and demonstrate that, under some conditions, it is suppressed. The metamaterial substrate, or metasurface is modeled as a wire medium covered with an impedance surface. Surprisingly, it is found that, in such a configuration, the effects of spatial dispersion may be nearly suppressed when the slab is electrically thin, and that the wire medium can be modeled very accurately using a local model.

Thermoelectric effect on charged colloids in the Hückel limit

We study the thermophoretic coefficient D(T) of a charged colloid. The non-uniform electrolyte is characterized in terms of densities and diffusion currents of mobile ions. The hydrodynamic treatment in the vicinity of a solute particle relies on the Hückel approximation, which is valid for particles smaller than the Debye length, a << [Formula: see text] . To leading order in the parameter a/[Formula: see text] , we find that the coefficient D(T) consists of two contributions, a dielectrophoretic term proportional to the permittivity derivative dvarepsilon/dT , and a Seebeck term, i.e., the macroscopic electric field induced by the thermal gradient in the electrolyte solution. Depending on the particle valency, these terms may take opposite signs, and their temperature dependence may result in a change of sign of thermophoresis, as observed in several recent experiments.

Schwinger mechanism and graphene

The Schwinger mechanism, the production of charged particle-antiparticle pairs in a macroscopic external electric field, is derived for $2+1$-dimensional theories. The rate of pair production per unit area for four species of massless fermions, with charge $q$, in a constant electric field $E$ is given by ${\ensuremath{\pi}}^{\ensuremath{-}2}{\ensuremath{\hbar}}^{\ensuremath{-}3/2}{\stackrel{\texttildelow{}}{c}}^{\ensuremath{-}1/2}(qE{)}^{3/2}$ where $\stackrel{\texttildelow{}}{c}$ is the speed of light for the two-dimensional system. To the extent undoped graphene behaves like the quantum field-theoretic vacuum for massless fermions in $2+1$ dimensions, the Schwinger mechanism should be testable experimentally. A possible experimental configuration for this is proposed. Effects due to deviations from this idealized picture of graphene are briefly considered. It is argued that with present day samples of graphene, tests of the Schwinger formula may be possible.

Macroscopic electrostatic potentials and interactions in self-assembled molecular bilayers: The case of Newton black films

We propose a very simple but "realistic" model of amphiphilic bilayers, simple enough to be able to include a large number of molecules in the sample but nevertheless detailed enough to include molecular charge distributions, flexible amphiphilic molecules, and a reliable model of water. All these parameters are essential in a nanoscopic scale study of intermolecular and long range electrostatic interactions. We also propose a novel, simple, and more accurate macroscopic electrostatic field for model bilayers. This model goes beyond the total dipole moment of the sample, which on a time average is zero for this type of symmetrical samples; i.e., it includes higher order moments of this macroscopic electric field. We show that by representing it with a superposition of Gaussians, it can be analytically integrated, and therefore its calculation is easily implemented in a molecular dynamics simulation (even in simulations of nonsymmetrical bi- or multilayers). In this paper we test our model by molecular dynamics simulations of Newton black films.

Generation of a Picosecond Runaway Electron Beam in a Gas Gap With a Nonuniform Field

The formation of a picosecond beam of runaway electrons in a gas-filled acceleration gap with a cold cathode and a strongly nonuniform electric field was investigated. The experimental data obtained were compared to those characteristic of the mode of generation and acceleration of electrons in vacuum. Voltage pulses of amplitudes up to -300 kV (in a traveling wave) whose minimum rise time and FWHM did not exceed 100-150 ps were applied to the cathode. The duration and amplitude of the current pulse of the picosecond runaway electron beam behind the anode foil were measured with high time resolution. The emission region of the beam in a gas-filled diode was determined experimentally. The time-of-flight method was used to investigate the acceleration mode of particles in the gap. Information about the part played by field emission in the initiation of the runaway electron beam has been obtained. It has been demonstrated that the point within the rise time of the accelerating voltage pulse at which the beam is injected into the gap correlates with the magnitude of the macroscopic electric field at the cathode emitting edge.

Diffusioosmosis of Electrolyte Solutions around a Circular Cylinder at Arbitrary Zeta Potential and Double-Layer Thickness

The steady diffusioosmotic flow of an electrolyte solution around a charged circular cylinder caused by a constant concentration gradient imposed in the direction along the axis of the cylinder is investigated theoretically. The cylinder can have either a constant surface potential or a constant surface charge density of an arbitrary value. The electric double layer surrounding the cylinder can have an arbitrary thickness, and its electrostatic potential distribution is determined by an analytical approximation to the solution of the Poisson-Boltzmann equation. Solving a modified Navier-Stokes equation with the constraint of no net electric current arising from the cocurrent diffusion, electric migration, and diffusioosmotic convection of the electrolyte ions, the macroscopic electric field and the fluid velocity along the axial direction induced by the imposed electrolyte concentration gradient are obtained semianalytically as functions of the radial position in a self-consistent manner. The direction of the diffusioosmotic flow relative to the concentration gradient is determined by the combination of the zeta potential (or surface charge density) of the cylinder, the properties of the electrolyte solution, and other relevant factors. For a prescribed concentration gradient of an electrolyte, the magnitude of the fluid velocity generally increases with increasing distance from the surface of the cylinder, but there are exceptions. The effects of the radial distribution of the induced macroscopic electric field and of the ionic convection in the double layer on the diffusioosmotic flow are quite significant in practical situations.

Deuterium influence on the field emission from inorganic nanowires

We report the initial conditioning procedure and achieved properties of nanowires composed of molybdenum sulfur iodine and directly grown on the top of a macroscopic molybdenum wire. Such a wire with nanowires, which were mostly perpendicularly grown to the surface, was applied as a radial field emitter with a 20 mm2 geometric area positioned inside a cylindrical metal anode. A stable current density of ∼2 mA/cm2 was achieved at a relatively low macroscopic electric field. Simultaneous recording of gases released from the anode exhibits linear relationship with the dissipated power. The most unexpected result was a reversible field emission current dependence induced by varying the deuterium pressure within the range from 10−8 to 10−4 mbar. The maximum increase for a factor of 7 was registered. Possible mechanisms responsible for the observed phenomena are discussed.

The effect of diffusioosmosis on water transport in polymer electrolyte fuel cells

An analytical study of the effect of diffusioosmosis caused by the concentration gradient of hydrogen ions on the isothermal transport of water in a fully hydrated membrane of a polymer electrolyte fuel cell (PEFC) is presented. A capillary tube or slit with a negatively charged wall is chosen to model the nanopores of the membrane. The electric double layer adjacent to the capillary wall may have an arbitrary thickness relative to the capillary radius and its electrostatic potential distribution is determined as the solution of the Poisson–Boltzmann equation. Solving a modified Navier–Stokes equation, the fluid velocity in the axial direction of the capillary induced by the macroscopic electric field and protonic concentration gradient is obtained as a function of the radial position in closed forms. The results for the local and averaged electrokinetic velocities in the capillary show that the effect of diffusioosmosis on the water transport in the membrane of a PEFC can be significant in comparison with that of electroosmosis under low-potential-difference operations.

Stabilizing mechanism of the dipolar structure and its effects on formation of carriers in wurtzite {0001} films: InN and ZnO

The stabilization of the dipolar structure and its effects on carrier formation in wurtzite films are investigated through a case study of InN and ZnO {0001} films by means of the highly precise all-electron full-potential linearized augmented plane-wave method. The calculated total energies of both the wurtzite and graphitic structures demonstrate that the structural phase transition from the graphitic to wurtzite occurs when the thickness is beyond six (ten) bilayers in InN (ZnO). Our analysis of the calculated energies reveals that the phase transition is due to the competition between the bulk energy and the extra energy originating from the macroscopic electric field of the dipole structure. Further, an analysis of the electronic structure and charge densities in InN {0001} films reveals the screening role of the electric field. The screening effect causes a broadening of $\mathrm{N}\phantom{\rule{0.2em}{0ex}}2p$ states in the band structure by approximately $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and charge redistribution in each polarizable unit. For InN and ZnO {0001} films, we have also found by using pseudohydrogens to passivate the opposite surfaces that the screening has little effect on the surface states and on the work functions. In contrast, the calculated formation energies of defects in InN {0001} films (N vacancy and substitutional Mg) using both pristine and pseudohydrogen passivated surfaces show that the internal electric field of the dipolar structure in wurtzite films may affect the formation of carriers near the surface. Thus, these results provide some fundamental characteristics for the polar surfaces of wurtzite films.