Steady State Electron Density Research Articles

Free vibration of a bi-layered composite plate of a piezoelectric semiconductor and a piezoelectric dielectric

We study the free vibration of a piezoelectric bi-layered plate composed of a piezoelectric semiconductor (PS) layer and a piezoelectric dielectric (PD) layer. The macroscopic theory of a PS consisting of the conventional theory of piezoelectricity and the drift-diffusion theory of semiconductors is used. The nonlinear equations for drift currents of electrons and holes are linearized for small perturbation of carrier density. The first-order zigzag approximation for in-plane displacements and electric potential through the thickness of each layer is used. Interfacial continuity of the displacement, the transverse shear stress, and the electric potential between the PS layer and the PD layer is ensured, which is very important and also experienced by layered structures. The number of independent unknown variables is reduced from 11 to 5 by using the interfacial continuity and the zero shear stress conditions at the top and bottom surfaces. The governing equation and corresponding boundary condition are derived using Hamilton’s principle. An analytical solution of a simply supported composite plate is obtained. The effects of steady-state electron density, axial force, and the geometric parameters on the vibration frequency and modes are discussed. The obtained results may be useful for further theoretical analysis of PS composites and practical application of piezotronic devices made from PS and PD materials.

Enhancement of Charge Transport of a Dye-Sensitized Solar Cell Utilizing TiO2 Quantum Dot Photoelectrode Film

A dye-sensitized solar cell (DSC) is the third generation of solar technology, utilizing TiO2 nanoparticles with sizes of 20–30 nm as the photoelectrode material. The integration of smaller nanoparticles has the advantage of providing a larger surface area, yet the presence of grain boundaries is inevitable, resulting in a higher probability of electron trapping. This study reports on the improvement of charge transport through the integration of quantum dot (QD) TiO2 with a size of less than 10 nm as the dye absorption photoelectrode layer. The QD TiO2 samples were synthesized through sol–gel and reflux methods in a controlled pH solution without surfactants. The synthesized samples were analyzed using microscopic, diffraction, absorption, as well as spectroscopic analyses. A current–voltage and impedance analysis was used to evaluate the performance of a DSC integrated with synthesized TiO2 as the photoelectrode material. The sample with smaller crystallite structures led to a large surface area and exhibited a higher dye absorption capability. Interestingly, a DSC integrated with QD TiO2 showed a higher steady-state electron density and a lower electron recombination rate. The shallow distribution of the trap state led to an improvement of the electron trapping/de-trapping process between the Fermi level and the conduction band of oxide photoelectrode material, hence improving the lifetime of generated electrons and the overall performance of the DSC.

Space charge saturation in multipactor discharges with parallel magnetic field

Numerical and theoretical models were developed to investigate space-charge effects in a multipactor discharge with an applied magnetic field. The magnetic field was assumed to be parallel to the RF electric field, and only the space-charge effects in the direction perpendicular to the fields were studied. Inclusion of space-charge effects allowed investigation of a saturation mechanism driven by electron diffusion. Numerical simulations showed that saturation of the multipactor discharge could be reached as diffusion balanced the electron production due to secondary emission. It was found that the steady state density reaches a limit as RF voltage was increased. Fluid equations for the electron motion were used to describe electron diffusion and derive an expression for the steady-state electron density in a typical multipactor discharge. It was found that in the presence of a magnetic field, electrons may be trapped by executing a slow rigid-rotor rotation, as described by the Brillouin condition. The electron behavior was found to be dynamic, satisfying the electron trapping condition while the electrons were moving between electrodes and violating the Brillouin condition near the electrodes leading to larger diffusion losses. Numerical simulations show that Brillouin trapping due to an applied parallel magnetic field can significantly increase saturation density by up to two orders of magnitude.

Helicon normal modes in Proto-MPEX**This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or

The Proto-MPEX helicon source has been operating in a high electron density ‘helicon-mode’. Establishing plasma densities and magnetic field strengths under the antenna that allow for the formation of normal modes of the fast-wave are believed to be responsible for the ‘helicon-mode’. A 2D finite-element full-wave model of the helicon antenna on Proto-MPEX is used to identify the fast-wave normal modes responsible for the steady-state electron density profile produced by the source. We also show through the simulation that in the regions of operation in which core power deposition is maximum the slow-wave does not deposit significant power besides directly under the antenna. In the case of a simulation where a normal mode is not excited significant edge power is deposited in the mirror region.

Enhancement of carrier-collection by electrospun Nb-doped TiO$$_2$$ nanofiber-network in photoelectrode of dye-sensitized solar cells

Conductive nanofiber(NF)-network in nano scale is expected to act as a collector of excited carriers in a medium with short diffusion length of carriers. For the effective usage of conductive NF-network in mesoporous media, we tried to actualize the composite structure of electrospun NF-network of Nb-doped TiO $$_2$$ (TNO) in a mesoporous TiO $$_2$$ layer keeping the network and its direct connection to conductive substrate. The effects of inserted NF-network with different mesh size were examined for the performance of dye-sensitized solar cells. By the insertion of NF-network, the power conversion efficiency $$\eta$$ was increased by 29.3% resulting from the increase of the short circuit current density $$J_\text{sc}$$ by 31.3%. From the time-resolved spectra of photovoltage and photocurrent, and electrochemical impedance spectra, it was found that photoelectrodes with NF-network had long lifetime of the electrons $$\tau _\text{e}$$ , small recombination rate of electrons $$k_\text{eff}$$ , and large steady state electron density $$n_\text{s}$$ . The enhancement of photocurrent and the external quantum efficiency also shows that TNO NF-network acts as a good carrier collector with the low leakage rate of photoexcited carriers.

Preparation of Vertically Aligned ZnO/TiO2Core-Shell Composites for Dye-Sensitized Solar Cells

Vertically aligned ZnO/TiO2(VA-ZnO/TiO2) core-shell composites deposited on ZnO-seeded indium tin oxide (ITO) glasses have been synthesized by a chemical bath deposition approach for growing one-dimensional ZnO structure followed by a spin procedure for coating TiO2on the surface of ZnO structure. The influences of the cycles of spin coating of TiO2(CSCT) on the properties of VA-ZnO/TiO2and performances of the assembled DSSCs were studied. The power conversion efficiency of the VA-ZnO/TiO2-based DSSC measured under illumination of 100 mW/cm2and AM 1.5 can reach 0.81%, representing 93% improvement when compared with that of the pristine VA-ZnO electrode (0.42%). The intensity-modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) were applied to study the kinetics and interfacial transfer of the photogenerated electrons. Both the photocurrent and power conversion efficiency correlate well with the steady state electron density. Enlargement in surface area and dye adsorption, suppression of dissolution of Zn2+, diminishment in electron recombination, and fast transfer of injected electrons from excited dyes to TiO2conduction bands arising from coating TiO2on VA-ZnO are regarded as the predominant causes for this improvement.

First-principles scattering-state approach for nonlinear electrical transport in nanostructures

We present an ab initio scattering-state method for calculating the electrical transport properties of nanostructures at a finite bias voltage. A typical system of interest consists of two semi-infinite crystalline metal probes and a nanostructure (e.g., an atomic wire or a molecule) placed between the probes. The two metal probes have different chemical potentials at a finite bias voltage. The electronic structure of the system is described by the Kohn-Sham density functional method, with appropriate boundary conditions imposed on the electronic density and potential inside the metal probes. We expand the electronic wave functions with pseudoatomic orbitals and directly solve the self-consistent Kohn-Sham equation to obtain the transmission probabilities of electrons incident from either of the probes. The current through the molecule or nanostructure is then obtained by integrating the transmission between the two chemical potentials. Our scattering-state method provides stable and efficient algorithms for the complex bands, scattering-state wave functions, and the nonequilibrium steady-state electron density. As illustrations of the method, we present the calculated electrical transport properties of a defective carbon nanotube, a four-carbon atomic chain, and a benzene-dithiol molecular junction.

Effects of Coulomb blockade on the photocurrent in quantum dot infrared photodetectors

We present theoretical studies of the effects of Coulomb blockade on the photocurrent of quantum dot infrared photodetectors within the Anderson model with two localized levels coupled with the electromagnetic field. We use the Keldysh Green function method to calculate the photocurrent. The energy levels, on-site Coulomb energy, and coupling parameters between leads and quantum dot states, as functions of the applied field, are evaluated within an effective mass model. It is found that the Coulomb interaction and level mixing in the many-body open system lead to a double-peak spectrum for the intraband transition. The center of gravity of the spectrum is redshifted as the applied bias increases, which competes with the blueshift caused by the Stark effect. Furthermore, the photocurrent is found to be a nonlinear function of the steady-state electron density of the quantum dot, in sharp contrast to quantum well infrared photo-detectors.

Interaction of Gravity Waves with Ionospheric Plasma: Implications for Jupiter's Ionosphere

The J0-ingress radio occultation of the Galileo orbiter by Jupiter exhibits a system of well-defined, regularly spaced electron layers in the altitude range where the presence of gravity waves have been previously inferred. Based on the terrestrial analog of sporadic E and spread F ionospheric layers, we argue that the observed layers are a result of dynamical processes rather than chemistry. We consider the impact of upward propagating gravity waves on the plasma distribution in a H + dominated ionosphere. The relevant physics is discussed and illustrated with an analytic, small-amplitude model. A time-dependent, 2D, large-amplitude model is developed to simulate the observed large excursions in the J0-electron density profile. We show that gravity waves with parameters consistent with the thermal structure of Jupiter's upper atmosphere are capable of creating large peaks in the electron density similar to the observed ones. The ionospheric response is extremely anisotropic with respect to the direction of wave propagation. We demonstrate that the location of the J0-ingress radio occultation on Jupiter favors large ionospheric response for waves propagating along the magnetic meridian. A wave driven plasma flux results in plasma removal above the altitude of maximum ionospheric response and plasma deposition in the region below, significantly modifying the initial steady state electron density profile.

Negative ion densities in chlorine- and boron trichloride-containing inductively coupled plasmas

The chlorine negative ion density and the electron density in chlorine- and boron trichloride-containing inductively coupled plasmas were investigated experimentally. Measurements were performed in a GEC reference cell operated in the inductively coupled mode. The chlorine negative ion density was measured as an excess electron density produced by photodetachment of electrons from the negative ions by 266 nm radiation from a frequency-quadrupled Nd:YAG laser. Both the excess electron density and the steady-state electron density were measured using a microwave interferometer. Various gas mixtures were investigated including pure Cl2, pure BCl3, combinations of the two, and combinations of these gases with N2 and Ar. Cl− densities up to 4×1011 cm−3 were measured depending on the gas mixture. The ratio of electron density to Cl− density was as high as 3 in Cl2 mixtures, and up to 5 in BCl3 mixtures. The plasma was probed at lower photon energy (355 nm) to photodetach electrons from other potential negative ions present in the plasma: to within the sensitivity of the measurement (1.3×108 cm−3), no other negative ions such as BClx− or Cl2− ions were detected. Significant interactions between Cl2 and Ar were observed as determined by changes in the Cl− density. The Cl− density also changed when BCl3 was added to Cl2 discharges. Addition of Ar or N2 to Cl2 plasmas had little effect on Cl− density, but nitrogen addition had very dramatic effects when added to BCl3, causing a factor of 2 increase in the Cl− density.

Electron and metastable densities in parallel-plate radio-frequency discharges

Electron densities were measured in continuous and pulse-modulated, 13.56-MHz, helium and argon discharges. These measurements were made in a symmetrically driven Gaseous Electronics Conference Reference Cell. Pulse modulation of the applied potential allowed observation of the time required for the electron density to achieve steady state. In general, helium discharges reached steady state in approximately 1.5 ms, taking three to ten times longer than argon discharges under similar operating conditions. As much as a threefold increase of the electron density was observed in the afterglow of a pulse-modulated helium discharge, indicative of large metastable densities. Absorption spectroscopy was used to measure the helium singlet and triplet metastable densities throughout the volume of the discharge. For a 1.0-Torr discharge, helium triplet metastable densities at the center of the discharge were as large as 2.5×1011 cm−3 while the peak singlet density was 3.0×1010 cm−3. The steady-state electron density varied from 3.8×1010 cm−3 at 50 V to 2.3×1011 cm−3 at 200 V for the 1.0-Torr helium discharge.

Direct measurement of the electron density in electron-beam-irradiated Ar-F2gas mixtures by time-resolved interferometry

The secondary-electron density in electron-beam-irradiated Ar-F/sub 2/ gas mixtures has been measured for the first time using time-resolved interferometry at 9.6 ..mu..m. Measurements were performed for gas mixtures with various F/sub 2/ concentrations between 0.0025% and 2% and total pressures between 0.5 and 2 atm. The e-beam pulse was 350 nsec long at an energy of 150 keV and current density ranging between 0.025 and 6.3 A/cm/sup 2/. From the dependence of the measured steady-state electron density on fluorine concentration, the rate constant for electron attachment to fluorine at zero electric field has been determined for the first time. For a given e-beam current density, the attachment rate constant is found to be a decreasing function of the fluorine concentration, while for a given F/sub 2/ concentration it is found to be an increasing function of the e-beam current density. The results are interpreted as a manifestation of the variation of the electron energy distribution as a function of F/sub 2/ concentration and e-beam current density. The predictions of a theoretical kinetic model for calculation of the electron distribution function, taking into account the removal of slow electrons by attachment, are compared with the experimental results. It is concluded thatmore » for the range of the experimental parameters presented above, the net effect of adding fluorine to the gas mixture is an increase in the average electron energy, caused by removal of low-energy electrons by attachment.« less

Effect of diffusion on electron density across the nonequilibrium MHD channel

This paper presents an investigation of the effect of thermal diffusion of charge carriers on the electron density in a nonequilibrium MHD generator. The source of diffusion is the inhomogeneity in the background temperature and/or the nonuniform heating of electrons across the MHD channel cross section on account of the velocity boundary layer. The continuity equation of electrons has been solved in conjunction with the diffusion velocity equation to obtain the steady state electron density distribution in an MHD generator. As a result of diffusion, the electron density is found to increase with the distance from the channel axis, while the reverse is predicted from the assumption of local equilibrium. Further, it is seen that the increase in the values of Hall parameter, boundary layer thickness and the flow velocity enhances the effect of diffusion. Decrease in the wall temperature is also found to have a similar effect. The diffusion of the carriers is most prominent in the segmented geometry. It is concluded that overall electron density gets enhanced if recombination and diffusion times are of the same order.

On the behaviour of the polar ionosphere under the influence of incident electron flux

The effect of incident electron flux on the behaviour of the winter polar ionosphere is investigated by using Green's function and integral equations. The treatment assumes a one spatial dimensional model and includes diffusion, recombination and production from the electron flux. Different incident electron energy spectrums in the soft electron zone outside the polar cavity have been used to compute the steady state electron density profiles. The effect of UT control of the diurnal variation in the incident electron flux is analyzed using the time variation in the incident energy flux given by Maehlum. The results show a UT variation in the foF2 consistent with observations. It will be shown that UT variation in the electron flux can also produce large variations in hmF2 which, however, must show some correlation with foF2 in the absence of other effects.

Winds and electromagnetic drifts in the equatorial F2-region

Previous calculations of steady-state electron density distributions in the equatorial ionosphere are extended to study the effect of variations in the electromagnetic drift velocity. It is found that a very small downward drift is sufficient to destroy the equatorial anomaly. The effects of meridional winds in producing distributions which are asymmetrical about the Equator are studied, and from a comparison with experimental data it is suggested that such winds can well account for the asymmetrical distributions found at the solstices. The effects of asymmetrical photoionization and heating of the neutral air are insufficient to explain these solstice results.

Production rates and electron densities in the lower ionosphere due to solar cosmic rays

The ratio, ψ, of Q (the production rate of electron-ion pairs) to N 2 (the steady-state electron density squared) has been calculated for solar cosmic ray event conditions over the altitude range 40 < Z < 80 km. The calculations were accomplished by determining the production rate from direct measurements of the particle spectra and using electron density values determined by multifrequency riometers. The ratio, ψ, is shown as a function of altitude and of the number density of neutral molecules. A comparison with theoretical values for ψ shows a disagreement not easily explained.