Showing papers on "Charge density published in 1992"

The single-electron transistor

Abstract: The discovery of periodic conductance oscillations as a function of charge density in very small transistors has led to a new understanding of the behavior of electrons in such small structures. It has been demonstrated that, whereas a conventional transistor turns on only once as electrons are added to it, submicronsize transistors, isolated from their leads by tunnel junctions, turn on and off again every time an electron is added. This unusual behavior is primarily the result of the quantization of charge and the Coulomb interaction between electrons on the small transistor. However, recent experiments demonstrate that the quantization of energy is important as well.

Impact of illumination level and oxide parameters on Shockley–Read–Hall recombination at the Si‐SiO2 interface

Abstract: The experimentally observed dependence of effective surface recombination velocity Seff at the Si‐SiO2 interface on light‐induced minority carrier excess concentration is compared with theoretical predictions of an ‘‘extended Shockley–Read–Hall (SRH) formalism.’’ The calculations of SRH‐recombination rates at the Si‐SiO2 interface are based on the theory of a surface space charge layer under nonequilibrium conditions and take into account the impact of illumination level, gate metal work function, fixed oxide charge density, and the energy dependence of capture cross sections σn, σp and interface state density Dit. Applying this theory to p‐type silicon surfaces covered by high quality thermal oxides, the experimentally observed strong increase of Seff with decreasing minority carrier excess concentration could quantitatively be attributed to the combined effect of the σn/σp ratio of about 1000 at midgap and the presence of a positive fixed oxide charge density Qf of about 1×1011 charges/cm2. Due to the f...

Contact Electrification and Adhesion Between Dissimilar Materials

Abstract: Simultaneous measurements of surface force and surface charge demonstrate strong attraction due to the spontaneous transfer of electrical charge from one smooth insulator (mica) to another (silica) as a result of simple, nonsliding contact in dry nitrogen. The measured surface charge densities are 5 to 20 millicoulombs per square meter after contact. The work required to separate the charged surfaces is typically 6 to 9 joules per square meter, comparable to the fracture energies of ionic-covalent materials. Observation of partial gas discharges when the surfaces are approximately 1 micrometer apart gives valuable insight into the charge separation processes underlying static electrical phenomena in general.

Resonance interactions in acyclic systems. 3. Formamide internal rotation revisited. Charge and energy redistribution along the C-N bond rotational pathway

Abstract: The changes that occur during the rotation of the amino group of formamide have been studied in some detail. Geometry optimizations at the MP2/6-31G* level confirmed the relatively large increase in C-N bond length but small decrease in the C-0 length on going from the planar structure to the rotational transition state. A calculation of the force constants for formamide and for the transition state showed that the carbonyl force constant changed relatively little, but the C-N constant changed by about 30%. The path followed in the rotation was studied starting with the saddle point geometry and following it computationally down to the ground state. The geometrical changes are discussed. The electron populations were calculated for a number of structures along the reaction coordinate by numerical integration of the charge density within uniquely defined atomic volumes. The oxygen population was little affected by the rotation and the main charge shift was between carbon and nitrogen. The electrostatic potentials for the structures also were examined and converted to effective charges for spherically symmetrical atoms. All of the analyses indicated that essentially all of the interactions leading to the rotational barrier originate in the C-N bond and that the oxygen does not participate to a significant extent. The direction of the charge shift between C and N was in opposite directions for the electron populations derived by integration of the charge density, and by fitting the electrostatic potentials. However, this was due to the difference in the definition of the atoms, being anisotropic in the first case and spherically symmetrical in the second. All of the observations can be rationalized on the basis of the assumption that stabilization of the lone pair on nitrogen is the most important factor in determining both structures and energies.

Molecular orbital cluster model study of bonding and vibrations of CO adsorbed on MgO surface

Abstract: The interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg2+ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas-phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field–dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the CO molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ωe, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal–CO bond as arising from σ donation and π back donation.

Relativistic density-dependent Hartree approach for finite nuclei.

Abstract: We develop a relativistic density-dependent Hartree approach for finite nuclei, where the coupling constants of the relativistic Hartree Lagrangian are made density dependent and are obtained from the relativistic Brueckner-Hartree-Fock results of nuclear matter. The calculated results on binding energies and root mean square radii of {sup 16}O and {sup 40}Ca agree very well with experiment. The charge densities from electron scattering are also calculated and their dependence on the nucleon-nucleon interaction is discussed in relation with nuclear matter properties.

On the electrostatic potential in crystalline systems where the charge density is expanded in Gaussian functions

Abstract: Methods for the evaluation of the electrostatic potential in systems which are periodic in three dimensions, where the electronic charge distribution is expanded as a linear combination of Gaussian functions of arbitrary quantum number, are described. An ‘exact’ method based on the Ewald potential function is derived, and an approximate scheme using a distributed point multipole representation of the charge distribution is then presented. Recursion formulae enable Cartesian and thence spherical derivatives (of any order) of the potential to be computed. Related procedures for the evaluation of the Fock matrix elements and the coulombic contribution to the total energy per unit cell are described. Tests of the exact and approximate procedures establish their relative accuracy and cost in the MgO, Si and Si12O24 systems in the rock-salt, diamond and chabazite structures, respectively. Applications include a study of the electric field gradient at the C, N and O sites in urea (crystalline and molecular), and...

Critical conditions for the binding of polyelectrolytes to small oppositely charged micelles

Abstract: Mixed micelles associate with polyelectrolytes of opposite charge to form soluble or insoluble complexes. The interaction takes place abruptly-signaled by a sudden enhancement in scattering-upon an increase of micelle surface charge density, an increase in polymer linear charge density, or a decrease in ionic strength, and involves a microscopic or macroscopic phase change. Because the surface charge state and the dimensions of the mixed micelles may be varied over a wide range, the polyion-micelle system provides a useful model for polyelectrolyte-colloid interactions

Towards Very Large-Scale Electronic-Structure Calculations

Abstract: We present a new approach to density functional theory, which does not require the calculation of Kohn-Sham orbitals. The computational workload required by our method—which is based on the calculation of selected elements of the Green's function—scales linearly with the volume of the system, thus opening the way to first-principles calculations for very large systems. Some of the problems which still hinder the achievement of this goal are discussed, and possible solutions are outlined. As an application, we calculate the charge density of a model silicon supercell containing 64 atoms slightly displaced at random from equilibrium.

Direct determination of self-consistent total energies and charge densities of solids: A study of the cohesive properties of the alkali halides.

Abstract: A recently proposed method for directly determining the self-consistent total energies and charge densities of solids is used to study the cohesive properties of all the alkali halides. The calculated lattice parameter, bulk modulus, and dissociation energy of each compound are reported and compared with the corresponding experimental data. The results have a good accuracy and their analysis gives some insights on possible improvements of the approximations that we have used. The relativistic contributions to the various properties have been evaluated by performing fully relativistic calculations for the heaviest compounds: The most relevant effect is an increase of the dissociation energies of the cesium halides which, for CsI, amounts to 7% of the experimental value.

Periodic ab initio Hartree-Fock calculations of the low-symmetry mineral kaolinite

Abstract: Periodic ab initio Hartree-Fock LCAO calculations have been carried out on the P1 symmetry clay mineral kaolinite, Al{sub 2}Si{sub 2}O{sub 9}H{sub 4} using STO-3G and modified 6-21G basis sets. The three structural degrees of freedom associated with the inner hydrogen atom of this compound have been optimized using the STO-3G basis. The equilibrium position of the inner hydrogen (described relative to the adjoining O-H bond) is predicted to be approximately parallel to the (ab) plane (forming an angle of +3.1{degrees}) with an O-H bond length of 0.99 {angstrom} and the two Al-O-H angles of 107.7{degrees} and 107.4{degrees}, respectively. The physical properties of the system were obtained using the 6-21G basis at the geometry deduced from the STO-3G calculations. The total valence density of states has been computed. Projected densities of states have been evaluated for the chemically distinct sets of elements. It is found that the majority of the valence states are composed of oxygen 2s and 2p atomic orbitals with overlap from Si and Al 3s and 3p atomic orbitals. Crystal charge densities and density deformation maps have been computed in the basal plane of the silicate ring system. The calculations have revealed small charge densities in the centermore » of the ditrigonal cavity (10{sup {minus}4}-10{sup {minus}5} e/bohr{sup 3}) and an oblate distortion of the oxygen charge density directed toward the center of the ring system. Electrostatic potential maps have been computed in the chemically accessible regions of the structure near the ditrigonal cavities. These maps indicate that the center of the cavity is at a negative potential relative to nearby silicon positions. Analogous calculations have been carried out perpendicular to the layers of the material. The results of these calculations reveal extensive interlayer hydrogen bonding. 21 refs., 11 figs., 2 tabs.« less

Charge dynamics in irradiated polymers

Abstract: Recent progress in the field of charge storage and charge transport in polymers irradiated with partially penetrating electron beams is discussed. The analytical model used to describe these phenomena takes the following physical parameters and processes into consideration: charge and energy deposition profiles, ohmic relaxation due to the radiation-induced conductivity, charge drift due to a finite carrier mobility, deep trapping without release, and trap filling due to a finite trap density. The model calculations show the effect of the various parameters on the shape of the charge distribution and on its motion. Experiments with the laser-induced pressure-pulse method on several high-resistivity polymers yield the location of the peak of the charge layers and their shape as function of injected charge density time and annealing conditions. The experimental results are interpreted with an analytical model and realistic values of the required physical parameters. Conclusions about charge dynamics in irradiated polymers are drawn and compared with those of earlier investigations. >

Charged dust in the Earth's mesopause; effects on radar backscatter

Abstract: We discuss to what extent small scale density inhomogeneities in the dust distribution may influence radar backscatter in the summer mesopause. We show for a reasonable range of parameters that falling dust interacting with a neutral gas vortex cannot penetrate to the centre of the vortex. The size of the hole in the dust space density distribution around the vortex centre depends on the vortex size and rotation speed and on the free fall velocity of the dust. If the dust contains a non-negligible fraction of the space charge (this requires that the dust is charged by the photoelectric effect), a substantial gradient in the positive charge density across the dust hole edge results. The positive charge density profile in the thin boundary layer will depend on the size distribution of the dust grains. A corresponding gradient in the electron density will appear and we discuss the conditions under which it will lead to a radar reflection of the magnitude observed in the summer polar mesosphere (PMSE - Polar Mesospheric Summer Echoes). This mechanism appears to have the potential of explaining the observed characteristics of the radar echoes such as the strong wavelength dependence and the narrow and sometimes complex and composite spectral signal profiles.

Contact potentials of solution interfaces : phase equilibrium and interfacial electric fields

Abstract: Calculations of the electric fields at the water liquid-vapor interface are used to motivate a general discussion of the electrostatic potential differences between conducting phases in equilibrium with respect to independent transport of charged species. A formula which requires only structural in formation on the interfacial charge density profile is presented for this contact (or surface ) potential.

Molecular‐dynamics study of adiabatic proton‐transfer reactions in solution

Abstract: A molecular‐dynamics study of adiabatic proton transfer between two ions in a polar solvent is presented. The proton is treated as a quantum particle in three dimensions and the polar solvent is composed of classical rigid, dipolar molecules. The coupled Schrodinger and Newton’s equations are solved to determine the proton charge density and solvent configuration. The rate coefficient for the proton transfer is computed from correlation function expressions and corrections to transition‐state theory due to recrossing of a free‐energy barrier are determined. The simulation results are compared with a simple two‐state model.

Surface Charge—Induced Ordering of the Au(111) Surface

Abstract: Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x radical3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 +/- 0.02 electron per atom in all three solutions.

On the mapping of electrostatic properties from the multipole description of the charge density.

Abstract: A method is presented to calculate the electrostatic potential, the electric field and the electric field gradient in a crystal from the atomic multipole expansion of the experimental charge density, as described by the Hansen-Coppens formalism [Hansen & Coppens (1978), Acta Cryst. A34, 909–921]. The electrostatic properties are expressed in terms of the positions and the charge-density parameters of the individual atoms. Contributions due to the procrystal charge density and the deformation charge density are compared. The method is illustrated by the calculation of the electrostatic potential maps of fully deuterated benzene and of iron(II) tetraphenylporphyrin.

Experimental and theoretical study of the charge density in 2‐methyl‐4‐nitroaniline

Abstract: The in‐crystal molecular dipole moment of the nonlinear optical material 2‐methyl‐4‐nitroaniline has been determined from a charge density analysis of x‐ray diffraction data. The results indicate a considerable enhancement of the free molecule dipole moment, due to the crystal field. The analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. Careful consideration must also be given to the treatment of hydrogen atoms, in the absence of complementary neutron diffraction data. An analysis of the deformation density and Laplacian of the charge density proves useful for revealing weak hydrogen bonding effects. Ab initio calculations at the Hartree–Fock double‐ζ level are reported for the molecule 2‐methyl‐4‐nitro‐aniline, with and without an applied electric field. In the former case, the magnitude and direction of the applied field were determined by a dipole lattice sum, to assess the magnitude of crystal field effects. The effect was...

Interactions between primitive electrical double layers

Abstract: A free energy density functional theory is applied to predict the electrostatic force between charged surfaces immersed in the restricted primitive electrolyte, in which ions are represented by charged hard spheres and solvent by a uniform dielectric continuum. The particle correlation due to hard‐core exclusions is incorporated in the nonlocal density functional of inhomogeneous hard sphere fluid. The ion–ion electrostatic correlation is treated in the mean spherical approximation. The surface force is found to depend strongly on the electrolyte concentration, surface charge density, and valency of the counterions. Attractive forces are often observed, especially in electrolytes containing divalent counterions. The maximum of the attraction can be an order of magnitude stronger than the van der Waals force evaluated at the same surface separation. The prediction of the classical Poisson–Boltzmann theory that the surface force is always repulsive is qualitatively correct only for monovalent electrolytes a...

Common theoretical framework for quantum chemical solvent effect theories

Abstract: Quantum chemical solvent effect theories deal with the description of the electronic structure of a molecular subsystem embedded in a solvent or other molecular environment. The average reaction field theories, which describe electrostatic and polarization interactions between solute and solvent, can be formulated in terms of a nonlinear reaction potential operator. This operator depends on the one hand on the reaction potential function of the solvent, and on the other hand on the charge density operators, which appear in the solute-solvent interaction. The former quantity is determined by the physical model of the solvent (e.g. dielectric continuum, discrete model, crystal lattice, etc.). The charge density operator can be approximated at different levels, like exact, one-centered and multicentered multipolar forms. These two ingredients of the theory, the reaction potential response function and the specific charge density operator, define unequivocally different solvent effect models. Various versions of average reaction field models are critically reviewed on the basis of this common theoretical framework.

Experimental vs. theoretical topological properties of charge density distributions. An application to the l-alanine molecule studied by X-ray diffraction at 23 K

Abstract: By using an aspherical-atom formalism, the electrostatic potential and the dipole moment of l -alanine were recently derived from a 23 K X-ray diffraction study. This paper focusses on how the topological properties of the experimentally derived density differ from those obtained by ab initio wavefunctions of differing accuracy. Factors influencing the theoretical densities, such as basis-set quality, inclusion of electron correlation and crystal-field effects, are scrutinized and the theoretical results compared with those derived from optimum least-squares refined experimental densities and from the very crude independent-atom model. The experimental and theoretical charge distributions show the same number and the same type of critical points in the density field. A close agreement was found for bond critical point locations and density values for both covalent intramolecular interactions and intermolecular hydrogen bonds. Discrepancies in the density Laplacian field portraits are discussed and the subtle effects of the crystal field are pointed out.

Electrostatic Force and Absorption Current of Alumina Electrostatic Chuck

Abstract: Electrostatic chucks composed of alumina ceramics doped with 1.3 wt% TiO2 and 3.5 wt% Cr2O3 were fabricated. Evaluations of electrostatic force and absorption charge in various atmospheres were carried out. The electrostatic force in various atmospheres clarified that the electrostatic force is affected not only by the bulk resistance, but also by the surface resistance of the ceramics. Absorption charge measurements revealed that the surface resistance affects the charge distribution of the dielectric layer, and it is deduced that the large electrostatic force of the alumina electrostatic chuck in the vacuum atmosphere is explained by the space charge which concentrates on the dielectric layer and intensity contact potential difference.

Atomistic modelling of the metal/oxide interface with image interactions

Abstract: We calculate the interfacial energy and lowest energy relative position for an Ag (001)/MgO (001) interface. The dominant image terms and short-range repulsions are included in full, and the MgO ions are relaxed to equilibrium using the MIDAS code. An essential new feature is the suppression of charge density fluctuations with wave-vectors greater than a (Fermi wavevector) cutoff. Our results show that the powerful methods based on interatomic potentials, widely used for ionic systems, can be extended to metal/ionic interfaces.

Electrostatic models for polypeptides: can we assume transferability?

Abstract: In most simulations of flexible molecules, the electrostatic model, usually a set of atomic point charges, is assumed to be independent of conformation. The electrostatic models for large molecules are usually constructed by assuming that the atomic charge densities can be transferred from smaller model molecules. Both assumptions neglect polarization of the atoms by the rest of the molecule. We test the accuracy of these assumptions for peptide systems. This is done using ab initio charge densities for N-acetyl, N′-methylamide blocked derivatives of alanine (CH3CO · NHCHCH3CO · NHCH3) and diglycine [CH3CO ·(NHCH2CO)2· NHCH3] in different conformations. The electrostatic fields are calculated from sets of multipoles (charge, dipole, quadrupole etc.) at each atomic site, obtained from a distributed multipole analysis (DMA) of the wavefunction. Model DMAs can be constructed by transforming the atomic multipoles from the wavefunction of one conformer to represent the structure of another, or by transferring multipoles from a blocked single peptide to a polypeptide. The accuracy of the transferability assumptions are then tested by comparing the electrostatic potential around the molecule, as predicted by the models which assume transferability, with that calculated from the DMA of the entire molecule in the chosen conformation.Our results show that large errors in the electrostatic potential outside the molecule can result from the assumption that the charge density is independent of the (ϕ, ψ) torsion angles. However, the different models agree well on the positions (although not the relative energies) of the probable water binding sites. This implies that there is some limited utility in the crude approximation that the charge density is independent of conformation. However, model charge distributions for blocked diglycine transferred from the DMAs of blocked single peptides with the same torsion angles are much more successful, and provide a promising route forward to accurate electrostatic models for polypeptides.