Showing papers on "Charge density published in 1987"

Theoretical study of band offsets at semiconductor interfaces

Abstract: We present a first-principles approach to deriving the relative energies of valence and conduction bands at semiconductor interfaces, along with a model which permits a simple interpretation of these band offsets. Self-consistent density-functional calculations, using ab initio nonlocal pseudo-potentials, allow us to derive the minimum-energy structure and band offsets for specific interfaces. Here we report results for a large number of lattice-matched interfaces, which are in reasonable agreement with reported experimental values. In addition, our systematic analysis leads to the important conclusions that, for the cases considered, the offsets are independent of interface orientation and obey the transitivity rule, to within the accuracy of our calculations. These are necessary conditions for the offsets to be expressible as differences between quantities which are intrinsic to each of the materials. Based on the information obtained from the full interface calculations, we have developed a new and simple approach to derive such intrinsic band offsets. We define a reference energy for each material as the average (pseudo)potential in a “model solid,” in which the charge density is constructed as a superposition of neutral (pseudo)atomic densities. This reference depends on the density of each type of atom and the detailed form of the atomic charge density, which must be chosen consistently for the different materials. The bulk band structures of the two semiconductors are then aligned according to these average potential positions. For many cases, these model lineups yield results close to those obtained from full self-consistent interface calculations. We discuss the comparison with experiments and with other model theories.

Electrostatic interaction of a solute with a continuum. Improved description of the cavity and of the surface cavity bound charge distribution.

Abstract: Algorithms for a finer description of cavities in continuous media and for a more efficient selection of sampling points on the cavity surface are described. Applications to the evaluation of solute surface and volume and to the calculation of the solute-solvent electrostatic interaction energy, as well as of the cavitation energy are shown as examples.

Adsorption of a polyelectrolyte chain to a charged surface

Abstract: The criteria for the adsorption of a polyelectrolyte chain in a salt solution on a uniformly charged planar membrane are derived using mean field arguments. Explicit formulas are obtained to describe the adsorption characteristics for varying surface charge density, charge on the polymer, Debye screening length κ−1, chain length L, and temperature T. The adsorption can be tuned using any one of these parameters. When T is the tuning variable, for example, the chain is adsorbed at T

Photorefractive properties of strontium‐barium niobate

Abstract: We have grown and optically characterized strontium‐barium niobate crystals, including both undoped and cerium‐doped crystals having two different Sr/Ba ratios (61/39 and 75/25). By measuring the coupling of two optical beams in the crystals, we have determined the following photorefractive properties: the effective density, sign, and spectral response of the dominant charge carrier, the grating formation rate, dark conductivity, and carrier diffusion length. We find that electrons are the dominant photorefractive charge carriers in all of our samples; the typical density of photorefractive charges is ∼1×1016 cm−3 in the undoped samples. The grating formation rate increases with intensity, with a slope of ∼0.3 cm2/(W s) over an intensity range of ∼1–15 W/cm2 in undoped samples. Cerium doping improves both the charge density (increased by a factor of ∼3) and the response rate per unit intensity (∼5 times faster).

Quantum simulation study of the hydrated electron

Abstract: An excess electron in a sample of classical water molecules at room temperature has been simulated using path integral techniques. The electron–water interaction is modeled by a pseudopotential with effective core repulsion and further terms for the Coulomb interaction and polarization effects. Various discretizations of the electron path, up to 1000 points, are examined. The charge distribution of the electron is found to be compact and to occupy a cavity in the water, in agreement with the conventional picture. The solvation shell structure is similar to that of relatively large solvated atomic anions, but the radial electron‐solvent correlations are largely smeared out due to fluctuations of the electronic density distribution. In parts of the simulation the structure of the first solvation shell corresponds on the average to the structure proposed for hydrated electrons by Kevan. The computed solvation energy and the estimated energy of the first optical excitation agree reasonably well with experimental data.

Theoretical study of BN, BP and BAs at high pressures

Abstract: The present work employs the total-energy pseudopotential technique to calculate the relative stabilities at high pressures of the zinc-blende \ensuremath{\beta}-Sn, and rocksalt structures in the series of compounds BN, BP, and BAs. For compressed volumes these compounds are shown to favor the rocksalt phase over \ensuremath{\beta}-Sn, even though BP and BAs are the least ionic of the III-V zinc-blende compounds. This indicates that the stability of the rocksalt structure under pressure depends on more than just the ionicity factor. The structural preference is analyzed in terms of charge densities. The charge rearrangements at different volumes are also studied. We find that for BN the charge associated with B decreases with volume compression while the charge associated with N increases. In BP and BAs the opposite trend is observed, with P and As behaving like B in BN. BN appears to behave like a typical III-V compound semiconductor in its charge distribution while BP and BAs are anomalous cases which can be characterized by reversing the standard assignments for the anions and cations in these compounds.

Mechanistic approaches to interactions of electric and electromagnetic fields with living systems

Abstract: Ions and Membrane Surfaces.- Ionic Processes at Membrane Surfaces: The Role of Electrical Double Layers in Electrically Stimulated Ion Transport.- Membrane Transduction of Low Energy Level Fields and the Ca++ Hypothesis.- Electrochemical Kinetics at the Cell Membrane: A Physicochemical Link for Electromagnetic Bioeffects.- Modification of Charge Distribution at Boundaries between Electrically Dissimilar Media.- The Role of the Magnetic Field in the EM Interaction with Ligand Binding.- Cyclotron Resonance in Cell Membranes: The Theory of the Mechanism.- Experimental Evidence for Ion Cyclotron Resonance Mediation of Membrane Transport.- Frequency and Amplitude Dependence of Electric Field Interactions: Electrokinetics and Biosynthesis.- Macromolecules.- The Influence of Surface Charge on Oligomeric Reactions as a Basis for Channel Dynamics.- Internal Electric Fields Generated by Surface Charges and Induced by Visible Light in Bacteriorhodopsin Membranes.- Interaction of Membrane Proteins with Static and Dynamic Electric Fields via Electroconformational Coupling.- Interactions Between Enzyme Catalysis and Non Stationary Electric Fields.- Patterns of Transcription and Translation in Cells Exposed to EM Fields: A Review.- Interaction of Electromagnetic Fields with Genetic Information.- Membrane Matrix.- Transient Aqueous Pores: A Mechanism for Coupling Electric Fields to Bilayer and Cell Membranes.- Electrorotation - The Spin of Cells in Rotating High Frequency Electric Fields.- Membranes, Electromagnetic Fields and Critical Phenomena.- Field Effects in Experimental Bilayer Lipid Membranes and Biomembranes.- Fusogenic Membrane Alterations Induced by Electric Field Pulses.- Integrated Systems.- Some Possible Limits on the Minimum Electrical Signals of Biological Significance.- Electrostatic Fields and their Influence on Surface Structure, Shape and Deformation of Red Blood Cells.- Cell Surface Ionic Phenomena in Transmembrane Signaling to Intracellular Enzyme Systems.- Low Energy Time Varying Electromagnetic Field Interactions with Cellular Control Mechanisms.- The Mechanism of Faradic Stimulation of Osteogenesis.- The Role of Calcium Ions in the Electrically Stimulated Neurite Formation in Vitro.- On the Responsiveness of Elasmobranch Fishes to Weak Electric Fields.- Contributors.

Charge density topological study of bonding in lithium clusters

Abstract: The topological behaviour of the electron density (ρ) derived from correlated wavefunctions is analyzed for Li2, Li4(D2h), Li5(C2v), and Li6(D3h) planar clusters considered in their optimal geometry. The topology ofρ of Li2 shows an unusual maximum located at the midpoint of the Li-Li equilibrium distance. The occurrence of maxima ofρ at positions other than nuclei (characteristic also for planar Li4, Li5, and Li6 clusters) implies the existence of molecular subspaces (bounded by zero-flux surfaces in the gradient ofρ at each point of the surface) which do not enclose a nucleus but still satisfy the virial theorem. This result provides a generalization of Bader's quantum theory of atoms in molecules to systems in which electrons behave partially as mobile metallic electrons. Maxima ofρ preferentially occur within the triangles (two in Li4, two in Li5 and three in Li6), while the number of maxima at the Li-Li midpoint is minimized: they are present only when the existence of a maximum within a triangle is not allowed because of the non suitable formal valence of the Li atoms involved. All the cluster atoms are bonded to “attractors” associated with the unusualρ maxima, but they are not directly bonded to each other. The cluster stability is found to be dependent on the number and kind ofρ maxima. The topological analysis clearly differentiates between Li atoms which occupy different coordination positions within the cluster in terms of their local and average properties. In particular, the degree ofsp hybridization is markedly different for Li atoms with two, three or four nearest neighbors. This implies that a unique definition of a reference valence state for atoms in clusters is impossible. As a consequence, the use of standard electron density difference maps for the description of the charge accumulation and depletion process which ensues the chemical bonding, appears rather questionable.

Theoretical study of the structure, lattice dynamics, and equations of state of perovskite-type MgSiO3 and CaSiO3

Abstract: The structural distortions, lattice dynamics, and equations of state of the high-pressure perovskite phases of MgSiO3 and CaSiO3 are examined with a parameter-free theoretical model. A theoretical ionic description of the crystal charge density is constructed from shell-stabilized ions, whose wavefunctions are calculated from Hartree-Fock theory. The short-range forces are then calculated in the pairwise-additive approximation from modified electron gas theory. The resulting many-body-corrected pair potentials are used to study the lattice dynamics in the quasiharmonic approximation. The cubic structure of MgSiO3 perovskite (Pm3m) is found to be dynamically unstable at all pressures, with imaginary quasiharmonic phonons occurring at the edge of the Brillouin zone. In contrast, the cubic phase of CaSiO3 perovskite is found to be stable at low pressures but becomes dynamically unstable at ∼ 109 GPa (1.09 Mbar). Energy minimization of MgSiO3 in an orthorhombic cell (Pbnm) is performed to obtain a distorted perovskite structure that is dynamically stable. The calculated unit cell parameters at zero pressure and room temperature are within 2 percent of those determined by x-ray diffraction. The theoretical equation-of-state calculations predict a lower compressibility and thermal expansivity for the two silicate perovskites than does the available experimental data on these compounds. Extensions of the present ionic model for more accurate predictions will require the inclusion of polarization of charge density and vibrational anharmonicity.

The shape of molecular charge distributions: Group theory without symmetry

Abstract: A group theoretical framework is proposed for a detailed characterization of the shapes of electronic charge distributions of general, asymmetric molecules. The proposed shape groups are the homology and cohomology groups of charge density contour surfaces. These shape groups depend on two real parameters, the charge density value a for the contour and a curvature parameter b. The two-parameter family of various homology groups and cohomology groups of charge density contour surfaces is independent of the symmetry properties of the molecules and gives a concise description of the dominant shape characteristics. For any fixed parameter value b these groups may change only at specific charge density values, characteristic to the given molecule. On the other hand, for a fixed-charge density contour the group changes induced by a change in the curvature parameter b provide a description of the fine details of the shape of the electron density. The changes in the structure of these groups follow strict algebraic relations, that provide a quantitative measure for shape-similarity between various molecules. The two-parameter shape group method is an extension of an earlier method proposed for biochemical applications.

Method for calculating the electronic structures of large molecules; helical polymers

Abstract: We present a self‐consistent one‐electron scheme for calculating ground‐state properties of large systems with complex boundaries. It is based on linear muffin‐tin orbitals (LMTO’s) and the density functional formalism in its local approximation. A multiple‐κ LMTO basis set is used. No shape approximations, neither for the potential nor for the charge density, are made. Outside the spheres the charge density is fitted to a series of atom‐centered Hankel functions and the two‐ and three‐center integrals used for the overlap and Hamiltonian matrices, as well as for the charge density fit, are performed analytically. Inside the spheres the non‐muffin‐tin part of the charge density is treated by spherical‐harmonics expansions. It is shown how the method can be applied to helical polymers. Test calculations on the N2 molecule are reported.

Surface states of excess electrons on water clusters.

Abstract: Electron attachment to water clusters was explored by the quantum path-integral molecular-dynamics method, demonstrating that the energetically favored localization mode involves a surface state of the excess electron. The cluster size dependence, the energetics, and the charge distribution of these novel electron-cluster surface states are explored.

The Electrohydrodynamic Origin of Turbulence in Electrostatic Precipitators

Abstract: The functioning of an electrostatic precipitator in the light of previous studies on various regimes of electroconvection in both parallel and divergent electric fields is examined. Coupling between velocity and charge density fluctuations for both ions and charged particles is discussed. It is shown that for a certain diameter range of the particles, their nondimensional mobility parameter takes values similar to those characterizing ions in liquids. An experimental simulation using insulating liquids is proposed.

Preparation and properties of polystyrene model colloids: I. Preparation of surface-active monomer and model colloids derived therefrom

Abstract: Model colloids having known particle size and surface chemistry are highly desirable for use in a variety of studies, such as colloid stability, interactions between particles and walls, measurement of diffusion coefficients, and rheology. Independent control of particle size and surface functional group density cannot, however, be achieved by conventional emulsion polymerization methods. This article is concerned with a method by which this goal can be accomplished by means of a surface-active comonomer, sodium sulfodecylstyryl ether (SSDSE), which was synthesized through four-step sequences of reactions starting from coumarin. Several series of monodisperse polystyrene latexes of the same size have been prepared, covering a range of particle size with radius from 90 to 120 nm, polydispersity indices on the average of 1.005 to 1.015 and surface charge densities from ca. 1.0 to 10.0 μC/cm 2 . These latexes were synthesized by a two-step process. First, monodisperse latexes with low surface charge density were formed by means of conventional emulsion polymerization. Then, a seed copolymerization with the surface-active comonomer was used to control the surface charge density of the latex.

Modeling of corona characteristics in a wire-duct precipitator using the charge simulation technique

Abstract: The charge simulation technique has been adapted to model the electrostatic and the corona characteristics in clean air of a duct-type electrostatic precipitator. The study involves the evaluation of the electric potential, electric field, and charge density in the interelectrode space as a function of corona current. The results show good agreement with published experimental data. The method developed can be applied to other geometries in the presence of space charge. The commonly used assumption that the space charge affects the magnitude but not the direction of the electric field is shown to be inadequate for large values of corona current. Also, the effect of using different values for the mobility of negative ions is presented.

Low Temperature Growth of SiO2 Thin Film by Double-Excitation Photo-CVD

Abstract: A new technique for the low-temperature growth of SiO2 films has been developed by double-excitation photo-CVD. SiO2 films can be grown at 30~240°C from a mixture of Si2H6 and O2 by photo-CVD using a D2 lamp as a VUV light source, or by double-excitation using an additional Xe lamp or a low-pressure Hg lamp as a UV light source. The growth rate is 90 A/min in a photo-CVD at 30°C, while deposition does not occur in thermal CVD at 30°C. Concentrations of Si–OH and Si–H bonds can be considerably reduced by the use of photo-excitation. Particularly, double-excitation reduces the oxide-charge density to a large extent and also brings the dielectric constants close to those of thermally-grown SiO2. The minimum charge density is 5×1010 cm-2 in a film deposited at 240°C. The interface state density of a Si MOS diode is also reduced by double-excitation using a Hg lamp in an O2-excess atmosphere and annealing at 280°C. Its minimum density is 8×1010 cm-2eV-1.

Helium atom differential cross sections for scattering from single adsorbed CO molecules on a Pt(111) surface

Abstract: Differential cross sections have been measured for the scattering of helium atoms from isolated CO molecules on a Pt(111) surface. The cross sections reveal an oscillatory structure as a function of scattering angle extending to large momentum transfer on both sides of the specular peak. Using a hard hemisphere model to approximate the interaction potential, the data can be well reproduced by a hard core of radius about 2.5 A. This result is compared to the charge density profile of the adsorbed molecule and also to the gas phase interaction potential, and in both cases good agreement with the experimentally predicted classical turning points is found.

Transferred Charge in the Active Layer and EL Device Characteristics of TFEL Cells

Abstract: The relationship between transferred charge in the active layer of a thin-film electroluminescent (TFEL) cell and EL device characteristics has been investigated electrically. Making use of a charge density-voltage characteristic measuring system based on a Sawyer-Tower circuit, a general formula for the input power density was found to be given by twice the product of the frequency, the threshold voltage of the active layer, and the transferred-charge density. From this formula, the luminous efficiency could be estimated, once the dependence of the luminance upon the frequency and the transferred charge density was found. At a frequency smaller than the inverse of twice the luminous decay time, e.g. 500 Hz in the ZnS:Mn case, the luminance was found to be proportional to the product of frequency and transferred charge density. Hence, the luminous efficiency becomes constant with a value 2–2.5 times larger than that for 5 or 1 kHz drive conditions.

Neutron Spin Echo Study of Dynamic Correlations Near Liquid-Glass Transition

Abstract: Neutron spin echo measurements have been performed on the mixed ionic system Ca0.4K0.6(NO3)1.4 around the glass transition temperature Tg in order to determine the time dependence of the density-density correlation function q(t). This quantity plays a central role in recent mode-coupling theories of the glass transition in schematic models. On the liquid side of the transition our results reveal that there are two distinct slow relaxation processes. The faster one is situated in the 10-13-10-12s time domain. The second one (a) shows a rapid slowing down on T approaching the transition, which scales with the Stokes-Einstein diffusion constant (b) has a stretched exponential form exp (-(t/τ)β] with β 0.6 and (c) tends to become the non-ergodic fraction of the structure factor at the transition. The amplitude ratio of the second and the first relaxation steps varies between 3 and 6 in a smooth oscillatory fashion over the studied wave-number range of q = 0.1-2.1 A-1. These findings are in qualitative agreement with theoretical predictions, but only if the theoretically relevant glass transition temperature T0 is assumed to lie considerably above the phenomenological glass temperature Tg. This implies that the theory breaks down at relaxation times longer than some 10-5s. In contrast, the q dependence of the characteristic time τ of the slower relaxation step revealed a fully unexpected feature, a strong maximum at q0/2, where q0 is the position of the first peak in the structure factor. We propose that this slowing down of the mass density fluctuations reflects the maximum of the charge density structure factor. This new phenomenon is expected to allow the study of the static charge density correlations directly in the q space via the induced dynamic effects.

Applied Field and Total Dose Dependence of Trapped Charge Buildup in MOS Devices

Abstract: A rate equation for charge buildup which includes carrier sweep out, geminate recombination, hole/ electron trapping, and effects of internal fields is developed. The first moment of the resulting charge distribution is calculated to yield the midgap voltage shift as a function of irradiation time. The initial midgap voltage shift per dose and the maximum midgap voltage shift are derived. The field dependence of these quantities is shown to be a consequence of the field dependence of the hole/electron capture cross sections and geminate recombination escape probability The results of this formulation show that the E-1/2 decrease in the midgap shift per dose with field described in the literature is due to the decrease of the hole capture cross section with increasing applied field. The theory is validated by comparison with experimental results obtained on 225 A thermal oxide on p-type silicon test capacitors irradiated under bias at room temperature.

Negative surface charge density near heart calcium channels. Relevance to block by dihydropyridines.

Abstract: We have measured the density of negative surface charges near the voltage sensor for inactivation gating of (L-type) Ca channels in intact calf Purkinje fibers and in isolated myocytes from guinea pig and rat ventricles. Divalent cation-induced changes in the half-maximal voltage for inactivation were determined and were well described by curves predicted by surface potential theory. We measured shifts in inactivation induced by Ca, Sr, and Ba in the single cells, and by Sr in the Purkinje fibers. All of the data were consistent with an estimated negative surface charge density of 1 electronic charge per 250 A2. In addition, the data suggest that Ca, but neither Ba nor Sr, binds to the negative charges with an association constant on the order of 1 M-1. We find that divalent ion-induced changes in surface potential can account for most of the antagonism between these ions and Ca channel block by 1,4-dihydropyridines.