Showing papers on "Charge density published in 2009"

Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions.

Abstract: We present a new continuum solvation model based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent. The model is called SMD, where the “D” stands for “density” to denote that the full solute electron density is used without defining partial atomic charges. “Continuum” denotes that the solvent is not represented explicitly but rather as a dielectric medium with surface tension at the solute−solvent boundary. SMD is a universal solvation model, where “universal” denotes its applicability to any charged or uncharged solute in any solvent or liquid medium for which a few key descriptors are known (in particular, dielectric constant, refractive index, bulk surface tension, and acidity and basicity parameters). The model separates the observable solvation free energy into two main components. The first component is the bulk electrostatic contribution arising from a self-consistent reaction field treatment that involves the solution of the nonho...

A grid-based Bader analysis algorithm without lattice bias

Abstract: A computational method for partitioning a charge density grid into Bader volumes is presented which is efficient, robust, and scales linearly with the number of grid points. The partitioning algorithm follows the steepest ascent paths along the charge density gradient from grid point to grid point until a charge density maximum is reached. In this paper, we describe how accurate off-lattice ascent paths can be represented with respect to the grid points. This improvement maintains the efficient linear scaling of an earlier version of the algorithm, and eliminates a tendency for the Bader surfaces to be aligned along the grid directions. As the algorithm assigns grid points to charge density maxima, subsequent paths are terminated when they reach previously assigned grid points. It is this grid-based approach which gives the algorithm its efficiency, and allows for the analysis of the large grids generated from plane-wave-based density functional theory calculations.

Performance of SM6, SM8, and SMD on the SAMPL1 test set for the prediction of small-molecule solvation free energies.

Abstract: The SM6, SM8, and SMD quantum mechanical aqueous continuum solvation models are applied to predict free energies of aqueous solvation for 61 molecules in the SAMPL1 test set described elsewhere (Guthrie. J. Phys. Chem. B 2009, 113, 4501−4507). For direct comparison to other models, frozen geometries, provided by Guthrie, were used together with the M06-2X density functional and the 6-31G(d) basis set. For the bulk electrostatic component of the solvation free energy, SM6 and SM8 employ a generalized Born model that uses polarized discrete partial atomic charges to model the electron density, with these charges being calculated by the CM4 and CM4M class IV charge models, respectively; SMD uses the polarized continuous quantum mechanical charge density. If five sulfonylureas are removed from the SAMPL1 set, the root-mean-square deviations (RMSDs) of SM6, SM8, and SMD on the remaining 56 molecules are 2.4, 2.6, and 2.5 kcal mol−1, respectively. The SM6, SM8, and SMD RMSDs on the five sulfonylureas are 14.2, ...

The nature of halogen...halogen interactions: a model derived from experimental charge-density analysis.

Abstract: Slightly attractive: The attractive and anisotropic nature of the ClCl interaction in C(6)Cl(6) is experimentally demonstrated from an expansion of the electron density rho(r) around the chlorine nuclei. The interaction is explained in a model in which there is a bonding attraction involving electron-deficient (see picture, blue) and electron-rich (red) regions of adjacent Cl atoms.

Charge Transfer Chemical Doping of Few Layer Graphenes: Charge Distribution and Band Gap Formation

Abstract: The properties of few layer (one layer (1 L) to four layer (4 L)) graphenes doped by adsorption and intercalation of Br2 and I2 vapors are investigated. The Raman spectra of the graphene G vibrations are observed as a function of the number of layers. There is no evidence for chemical reaction disrupting the basal plane π electron conjugation. Adsorption of bromine on 1 L graphene creates a high doped hole density, well beyond that achieved by electrical gating with an ionic polymer electrolyte. In addition, the 2D Raman band is completely quenched. The 2 L bilayer spectra indicate that the doping by adsorbed I2 and Br2 is symmetrical on the top and bottom layers. Br2 intercalates into 3 L and 4 L graphenes. The combination of both surface and interior doping with Br2 in 3 L and 4 L creates a relatively constant doping level per layer. In contrast, the G spectra of 3 L and 4 L with surface adsorbed I2 indicate that the hole doping density is larger on the surface layers than on the interior layers and tha...

Molecular dynamics simulation of the electrochemical interface between a graphite surface and the ionic liquid [BMIM][PF6]

Abstract: The structure of the electrical double layer in the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) near a basal plane of graphite was investigated by molecular dynamics simulation. The calculations were performed both for an uncharged graphite surface and for positively and negatively charged ones. It is found that near an uncharged surface the ionic liquid structure differs from its bulk structure and represents a well-ordered region, extending over ∼20 A from the surface. Three dense layers of ca 5 A thick are clearly observed at the interface, composed of negative ions and positively charged rings. It is established that in the first adsorption layer the imidazolium ring in the [BMIM]+ cation tends to be arranged in parallel to the graphite surface at a distance of 3.5 A. The [PF6]− anion is oriented in such a way that the phosphorus atom is at a distance of 4.1 A from the surface and triplets of fluorine atoms form two planes parallel to the graphite surface. Ions adsorbed at the uncharged surface are arranged in a highly defective 2D hexagonal lattice and the corresponding lattice spacing is approximately four times larger than that of the graphene substrate. The influence of the electrode potential on the distribution of electrolyte ions and their orientation has also been investigated. Increase in the electrode potential induces broadening of the angle distribution of adsorbed rings and a shift of the most probable tilt angle towards bigger values. It was shown that there are no adsorbed anions on the negatively charged surface (σ = −8.2 μC cm−2), but the surface concentration of adsorbed cations on the positively charged surface (σ = +8.2 μC cm−2) has a nonzero value. In addition, the influence of the surface charge (±σ) on the volume charge density and electric potential profiles in an electrolyte was studied. The differences in the cation and anion structure result in the fact that the integral capacitance of the electrical double layer depends on the electrode polarity and equals C = 4.6 μF cm−2 at σ = −8.2 μC cm−2 and C = 3.7 μF cm−2 at σ = +8.2 μC cm−2.

The electronic structure of oxygen atom vacancy and hydroxyl impurity defects on titanium dioxide (110) surface.

Abstract: Introducing a charge into a solid such as a metal oxide through chemical, electrical, or optical means can dramatically change its chemical or physical properties. To minimize its free energy, a lattice will distort in a material specific way to accommodate (screen) the Coulomb and exchange interactions presented by the excess charge. The carrier-lattice correlation in response to these interactions defines the spatial extent of the perturbing charge and can impart extraordinary physical and chemical properties such as superconductivity and catalytic activity. Here we investigate by experiment and theory the atomically resolved distribution of the excess charge created by a single oxygen atom vacancy and a hydroxyl (OH) impurity defects on rutile TiO(2)(110) surface. Contrary to the conventional model where the charge remains localized at the defect, scanning tunneling microscopy and density functional theory show it to be delocalized over multiple surrounding titanium atoms. The characteristic charge distribution controls the chemical, photocatalytic, and electronic properties of TiO(2) surfaces.

Microstructure and Capacitance of the Electrical Double Layers at the Interface of Ionic Liquids and Planar Electrodes

Abstract: We report on the molecular dynamics simulations of the electrical double layers (EDLs) at the interface of ionic liquids [BMIM][NO3] and planar electrodes. Simulations confirm that a Helmholtz-like interfacial counterion layer exists when the electrode charge density is negative or strongly positive, but the counterion layer is not well-defined when the electrode charge density is weakly positive. The thickness of the EDL, as inferred from how deep the charge separation and orientational ordering of the ions penetrate into the bulk ILs, is about 1.1 nm. The liquid nature of the IL and the short-range ion−electrode and ion−ion interactions are found to significantly affect the structure of the EDL, particularly at low electrode charge densities. Charge delocalization of the ions is found to affect the mean force experienced by the ions and, thus, can play an important role in shaping the EDL structure. The differential capacitance of the EDLs is found to be nearly constant under negative electrode polariza...

Nuclear charge radii of 7,9,10Be and the one-neutron halo nucleus 11Be.

Abstract: Nuclear charge radii of ;{7,9,10,11}Be have been determined by high-precision laser spectroscopy. On-line measurements were performed with collinear laser spectroscopy in the 2s_{1/2}-->2p_{1/2} transition on a beam of Be+ ions. Collinear and anticollinear laser beams were used simultaneously, and the absolute frequency determination using a frequency comb yielded an accuracy in the isotope-shift measurements of about 1 MHz. Combining this with accurate calculations of the mass-dependent isotope shifts yields nuclear charge radii. The charge radius decreases from 7Be to 10Be and then increases for the halo nucleus 11Be. When comparing our results with predictions of ab initio nuclear-structure calculations we find good agreement. Additionally, the nuclear magnetic moment of 7Be was determined to be -1.3995(5)micro_{N} and that of 11Be was confirmed with an accuracy similar to previous beta-NMR measurements.

(pi, pi) electronic order in iron arsenide superconductors.

Abstract: The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba(1-x)K(x)Fe(2)As(2) is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (pi, pi) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).

An explanation for the charge on water’s surface

Abstract: Measurements with different techniques point to a strong affinity of hydroxide ions for interfaces between water and hydrophobes, but some spectroscopic experiments do not detect excess hydroxide at the interface, while others do. Hydroxide ions are unusual in that they reduce the relative permittivity of an electrolyte solution more than other monovalent, monatomic ions. This implies that they suppress the collective dipole-moment fluctuations of nearby waters. We show that the absence of these fluctuations leads to a Hamaker-like force on the hydroxide ion that attracts it to regions where dipole-moment fluctuations are smaller than in bulk water, in other words, to regions of low relative permittivity. We show also that there is no contradiction between the picture of the basic, negatively charged interface and spectroscopic measurements. This is, in part, because the hydroxides are mostly below the outermost molecular layers. By combining a simple model for this fluctuation force with a modified Poisson–Boltzmann equation, we reproduce the dependence of the ζ-potential on pH, including the low isoelectric point, the approximate magnitude of the experimental surface charge density, and the Jones–Ray data for the dependence of surface tension on electrolyte concentration. We discuss also the apparent contradiction between molecular-dynamics simulations that deny and experiments that support a basic, negatively charged interface.

High-field transport and velocity saturation in graphene

Abstract: High-field transport in graphene is studied by the Monte Carlo simulation. The results indicate velocity and current saturation in agreement with a recent experiment [I. Meric, M. Y. Han, A. F. Young, B. Oezyilmaz, P. Kim, and K. Shepard, Nat. Nanotechnol. 3, 654 (2008)]. The saturation current scales as the square root of the charge density, or equivalently, the square root of the gate overdrive voltage, which is qualitatively different from silicon field-effect transistors. By analytical fitting to the numerical simulation results, a simple expression of the field-dependent mobility is obtained at different strengths of charged impurity scattering.

Can the Formation of Pharmaceutical Cocrystals Be Computationally Predicted? 2. Crystal Structure Prediction.

Abstract: We report a multistage lattice energy minimization methodology for generating stable packing arrangements of cocrystals containing flexible molecules. In the first approximation, the intermolecular electrostatic interactions are modeled with atomic charges and the molecular deformation energy is interpolated over a set of precomputed quantum mechanical values. At subsequent stages, the accuracy is improved by first using analytically rotated and then conformation-dependent multipole moments, computed from the isolated-molecule charge density, and "on-the-fly" quantum mechanical calculations to compute the intramolecular deformation energy. This multistage approach increases the efficiency of the search and establishes the molecule-dependent error due to the atomic charge representation of the charge density and the neglect of the conformational dependence of atomic multipole moments. The methodology is used to study the lattice energy landscapes of the cocrystals of 4-aminobenzoic acid with 2,2'-bipyridine and 4-nitrophenylacetic acid, as well as the single-component crystals. All single-component, experimentally determined crystal structures within the scope of the search were found at, or very close to, the global minimum. The experimental cocrystal with 2,2'-bipyridine is also predicted to be among the most stable packing arrangements. On the contrary, the lattice energy landscape of the cocrystal with 4-nitrophenylacetic acid contains several low energy structures that are more stable than the experimentally observed form and have different hydrogen bonding motifs. Overall, the methodology can provide worthwhile crystal energy landscapes for multicomponent organic solids and thereby contribute to understanding cocrystal formation.

High Carrier Densities Achieved at Low Voltages in Ambipolar PbSe Nanocrystal Thin-Film Transistors

Abstract: Efficient transport of both electrons and holes with high carrier densities is a requirement for obtaining light-emitting transistors from films of colloidal semiconductor nanocrystals. Such devices offer an approach to efficient electrically pumped nanocrystal lasers with tunable emission. Here, we report a low-voltage ambipolar thin-film transistor that features high carrier mobility and high induced carrier density by combining a PbSe nanocrystal film with a high-capacitance ion-gel gate dielectric layer (∼22 and ∼9 μF/cm2 for electron and hole accumulation, respectively). At operation voltages below 2.5 V, electron and hole densities higher than ∼1014 carriers/cm2 could be achieved in the PbSe nanocrystal film, which corresponds to ∼3 electrons or holes per particle. Carrier mobilities were also dependent on charge density and were as high as 0.4 and 0.02 cm2/(V s) for electrons and holes, respectively.

Effect of the chemical functionalization on charge transport in carbon nanotubes at the mesoscopic scale.

Abstract: We present first-principles calculations of quantum transport in chemically functionalized metallic carbon nanotubes with lengths reaching the micrometer scale and random distributions of functional groups. Two typical cases are investigated, namely, a sp2-type bonding between carbene groups (CH2) and the nanotube sidewalls and a sp3-type bonding of nanotubes with paired phenyl groups. For similar molecular coverage density, charge transport is found to range from a quasi-ballistic-like to a strongly diffusive regime, with corresponding mean free paths changing by orders of magnitude depending on the nature of the chemical bonding.

Minimization of Electrostatic Free Energy and the Poisson–Boltzmann Equation for Molecular Solvation with Implicit Solvent

Abstract: In an implicit-solvent description of the solvation of charged molecules (solutes), the electrostatic free energy is a functional of concentrations of ions in the solvent. The charge density is det...

Nuclear 'bubble' structure in Si-34

Abstract: Bubble nuclei are characterized by a depletion of their central density. Their existence is examined within three different theoretical frameworks: the shell model and nonrelativistic and relativistic microscopic mean-field approaches. We analyze 34Si and 22O as possible candidates for proton and neutron bubble nuclei, respectively. In the case of 22O, we observe a significant model dependence, thereby calling into question the bubble structure of 22O. In contrast, an overall agreement among the models is obtained for 34Si. Indeed, all models predict a central proton density depletion of about 40% and a central charge density depletion of 25%– 30%. This result provides strong evidence in favor of a proton bubble in 34Si.

Transport Coefficients at Zero Temperature from Extremal Black Holes

Abstract: Using the AdS/CFT correspondence we study transport coefficients of a strongly-coupled (2 +1)-dimensional field theory at {\it zero} temperature and finite charge density. The field theory under consideration is dual to the extremal Reissner-Nordstrom AdS_4 black hole in the bulk. We show that, like the cases of scalar and spinor operators studied in \cite{Faulkner:2009wj}, the correlators of charge (vector) current and energy-momentum (tensor) operators exhibit scaling behavior at low frequency. The existence of such low frequency behavior is related to the fact that the near-horizon geometry of the extremal black hole background has an AdS_2 factor. We carefully calculate the shear viscosity (at zero temperature) and show that the ratio of the shear viscosity to the entropy density takes the value of 1/4\pi. Because of the AdS_2 factor, we argue that this result stays the same for all d-dimensional boundary field theories dual to the extremal Reissner-Nordstrom AdS_{d+1} black holes. Also, we compute the charge conductivity at zero temperature. The limiting behavior of the conductivity for small frequencies is also attributed to the near horizon AdS_2 factor and is argued to hold regardless of the dimension of the zero-temperature boundary field theory. Finally, using the extremal dyonic AdS_4 black hole as the background, we extract the conductivity in the presence of a constant magnetic field.

First-principles studies of intrinsic point defects in magnesium silicide

Abstract: We have studied intrinsic point defects in magnesium silicide, Mg2Si, by density-functional theory. Evaluating the formation energies of point defects, we show that n-type electric conductivity of Mg2Si originates from formations of positively charged Mg ions at interstitial sites, regardless of the chemical composition in crystal growth. Moreover, we have calculated the Born effective charge tensors and the valence charge density distribution. They show Mg2Si is an ionic crystal composed of Mg2+ and Si4− which have very different ionic radii, 0.6 A and 2.1 A, respectively. We have concluded that the unfavorable antisite defect, MgSi, is due to the dissimilar ionic radii.

Theory of quasiparticle scattering in a two-dimensional system of helical Dirac fermions: Surface band structure of a three-dimensional topological insulator

Abstract: We study the quasiparticle interference QPI patterns caused by scattering off nonmagnetic, magnetic point impurities, and edge impurities, separately, in a two-dimensional helical liquid, which describes the surface states of a topological insulator. The unique features associated with hexagonal warping effects are identified in the QPI patterns of charge density with nonmagnetic impurities and spin density with magnetic impurities. The symmetry properties of the QPI patterns can be used to determine the symmetry of microscopic models. The Friedel oscillation is calculated for edge impurities and the decay of the oscillation is not universal, strongly depending on Fermi energy. Some discrepancies between our theoretical results and current experimental observations are discussed. DOI: 10.1103/PhysRevB.80.245317

Multiband Mobility in Semiconducting Carbon Nanotubes

Abstract: We present new data and a compact mobility model for semiconducting single-wall carbon nanotubes, with only two adjustable parameters, the elastic and inelastic collision mean free paths at 300 K. The mobility increases with diameter, decreases with temperature, and has a more complex dependence on charge density. The model and data suggest that the room temperature mobility does not exceed 10 000 cm2/Vmiddots at high carrier density (n > 0.5 nm-1) for typical single-wall nanotube diameters, due to the strong scattering effect of the second subband.

Residual charge distribution of positive surface streamer

Abstract: The measuring system of charges accumulated on an insulating plate is developed with an electrostatic probe whose sensing electrode is of 0.5 mm diameter. In the inverse calculation from the probe outputs to the charge distribution, Tikhonov's regularization technique is effectively used to suppress the excessive amplification of the noise. In the case of measuring a 2 mm thick PMMA plate, the spatial resolution of the measuring system reaches 1.0 mm. With this system, the residual surface charge distribution on an insulating plate just after the occurrence of a positive surface discharge at 25 kPa (190 Torr) air is measured. On the head of a positive surface streamer, there remains 10−11 C surface charge, which satisfies the criterion of an electron avalanche-to-streamer transformation. The charge density across a streamer takes a minimum on its centre and a maximum on its sheath. This fact suggests that a quasineutral channel exists in the centre of a streamer, and positive ions remain on the sheath of it.

Electric field induced magnetic anisotropy in a ferromagnet.

Abstract: We report the first observation of a transient all electric field induced magnetic anisotropy in a thin film metallic ferromagnet. We generate the anisotropy with a strong ( � 10 9 V=m) and short (70 fs) ~ E-field pulse. This field is large enough to distort the valence charge distribution in the metal, yet its duration is too brief to change the atomic positions. This pure electronic structure alteration of the sample generates a new type of transient anisotropy axis and strongly influences the magnetization dynamics. The successful creation of such an anisotropy opens the possibility for all ~ E-field induced magnetization reversal in thin

Electronic structure and optical properties of ZnSiO3 and Zn2SiO4

Abstract: The electronic structure and optical properties of orthorhombic, monoclinic, and rhombohedral (corundum type) modifications of ZnSiO3, and of rhombohedral, tetragonal, and cubic (spinel type) modifications of Zn2SiO4 have been studied using ab initio density functional theory calculations. The calculated fundamental band gaps for the different polymorphs and compounds are in the range 2.22–4.18 eV. The lowest conduction band is well dispersive similar to that found for transparent conducting oxides such as ZnO. This band is mainly contributed by Zn 4s electrons. The carrier effective masses were calculated and compared with those for ZnO. The topmost valence band is much less dispersive and contributed by O 2p and Zn 3d electrons. From the analysis of charge density, charges residing in each site, and electron localization function, it is found that ionic bonding is mainly ruling in these compounds. The calculated optical dielectric tensors show that the optical properties of ZnSiO3 and Zn2SiO4 are almost...

Magnitude of Finite‐Nucleus‐Size Effects in Relativistic Density Functional Computations of Indirect NMR Nuclear Spin–Spin Coupling Constants

Abstract: A spherical Gaussian nuclear charge distribution model has been implemented for spin-free (scalar) and two-component (spin-orbit) relativistic density functional calculations of indirect NMR nuclear spin-spin coupling (J-coupling) constants. The finite nuclear volume effects on the hyperfine integrals are quite pronounced and as a consequence they noticeably alter coupling constants involving heavy NMR nuclei such as W, Pt, Hg, Tl, and Pb. Typically, the isotropic J-couplings are reduced in magnitude by about 10 to 15 % for couplings between one of the heaviest NMR nuclei and a light atomic ligand, and even more so for couplings between two heavy atoms. For a subset of the systems studied, viz. the Hg atom, Hg(2) (2+), and Tl--X where X=Br, I, the basis set convergence of the hyperfine integrals and the coupling constants was monitored. For the Hg atom, numerical and basis set calculations of the electron density and the 1s and 6s orbital hyperfine integrals are directly compared. The coupling anisotropies of TlBr and TlI increase by about 2 % due to finite-nucleus effects.