A simple approach to electronic charge induction in atomistic simulations

Summary A scheme is presented for periodically updating atom-centered partial charges due to changing local electric fields that arise during the course of a molecular simulation. The effects of electrostatic induction, due to polarization of the local electron density of bonded atom pairs, are realized through the approximation of the bond polarizability tensor as being cylindrically symmetric and directed along the bond axis, with negligible lateral polarizability. This simplification allows the reduction of polarization effects to a scalar bond polarization parameter, and subsequent partitioning of the local electron density into charge modifiers located at atom centers. The induced charges assigned to each atom are determined self-consistently. This simplification permits a reasonable approximation of both the magnitude and direction of molecular dipole moments and of molecular polarizabilities in the case of small-molecule fluorocarbons. The model is extended to the calculation of atom-centered partial atomic charges for two conformations of poly(vinylidenefluoride). The proposed method fits well into the framework of established atomistic simulation techniques based on Coulombic nonbonded atomic interactions.     

ENDOFULLERENE POLARIZATION AND DYNAMICS IN ELECTRIC FIELD: THE QUASICLASSIC APPROACH

The quasiclassical approach has been used to calculate a fullerene and an endofullerene molecular interaction with electric fields. A physical model for an endofullerene molecular polarizability in electric fields has been developed and equations describing the interaction have been deduced.     

Modeling of a carbon nanotube ultracapacitor

The modeling of carbon nanotube ultracapacitor (CNU) performance based on the simulation of electrolyte ion motion between the cathode and the anode is described. Using a molecular dynamics (MD) approach, the equilibrium positions of the electrode charges interacting through the Coulomb potential are determined, which in turn yield the equipotential surface and electric field associated with the capacitor. With an applied ac voltage, the current is computed based on the nanotube and electrolyte particle distribution and interaction, resulting in the frequency-dependent impedance Z(ω). From the current and impedance profiles, the Nyquist and cyclic voltammetry (CV) plots are then extracted. The results of these calculations compare well with existing experimental data. A lumped-element equivalent circuit for the CNU is proposed and the impedance computed from this circuit correlates well with the simulated and measured impedances.     

Atomic-scale study of electric dipoles near charged and uncharged domain walls in ferroelectric films

Ferroelectrics are materials exhibiting spontaneous electric polarization due to dipoles formed by displacements of charged ions inside the crystal unit cell. Their exceptional properties are exploited in a variety of microelectronic applications. As ferroelectricity is strongly influenced by surfaces, interfaces and domain boundaries, there is great interest in exploring how the local atomic structure affects the electric properties. Here, using the negative spherical-aberration imaging technique in an aberration-corrected transmission electron microscope, we investigate the cation-oxygen dipoles near 180 degrees domain walls in epitaxial PbZr(0.2)Ti(0.8)O(3) thin films on the atomic scale. The width and dipole distortion across a transversal wall and a longitudinal wall are measured, and on this basis the local polarization is calculated. For the first time, a large difference in atomic details between charged and uncharged domain walls is reported.     

Study of electric dipole and space charge characteristics of Al/C12TCNQ/Al by means of thermally stimulated current measurements

Electric dipole and space charge characteristics of Al/2-dodecyl-7, 7, 8, 8-tetracyanoquinodimethane (C₁₂TCNQ)/Al were studied by means of thermally stimulated current (TSC) measurements. The TSC curve of Al/C₁₂TCNQ/Al had two peaks, one at a temperature below 80 °C, which is due to the depolarization of electric dipoles, and the other, at a temperature above 80 °C, which is due to the movement of space charges in the C₁₂TCNQ Langmuir-Blodgett (LB) films. The polarization direction of the electric dipoles in the C₁₂TCNQ LB films was from the hydrophilic group to the hydrophobic group.     

Molecular simulation study on the effect of trapped charges on ferroelectric switching in β-phase PVDF crystals

Based on the energy-minimization method in molecular simulation, the effect of trapped charges on ferroelectric switching in β-phase PVDF Crystals was investigated. Our simulation indicated that the barrier height, caused by quasi-hexagonal-symmetry lattice field, was too low to well stabilize the orientations of molecular dipoles. However, the interaction between trapped charges and molecular chains would greatly increase the barrier height to build up a much more steady polarization state.     

PAC investigations of radiation damage annealing in In implanted ZnO

The structural and electronic environment about implanted radioactive ¹¹¹In(→¹¹¹Cd) probe atoms as a function of annealing temperature in a single crystal of ZnO(0 0 0 1) has been monitored on an atomic scale using perturbed angular correlation technique, a nuclear hyperfine method. This technique is based upon the hyperfine interaction of the nuclear electric quadrupole moment or magnetic moment of the probes, respectively, with the electric field gradient or magnetic hyperfine field arising from the extra-nuclear electronic charges and spin distributions. The probe atoms ¹¹¹In were recoil-implanted at room temperature following heavy-ion nuclear reactions. The electric quadrupole interaction was measured at room temperature for as-implanted and annealed samples. The thermal annealing in ambient nitrogen up to 1000 °C showed a progressive reduction of disorder around the probe atom as evidenced via continual decrease in width of the distribution of quadrupole interaction frequencies. Present measurements suggested that annealing at 800 °C for 30 min in flowing nitrogen is enough to produce an optimum recovery of crystallinity. After annealing of radiation damage at 1000 °C we observed an axially symmetric electric field gradient which is characterized by the unique quadrupole interaction frequency of 30.6(3) MHz and a frequency distribution of width nearly zero. The observed electric field gradient was attributed to substitutional incorporation of probe atoms at cation-sites of ZnO. In contrast to annealing in ambient nitrogen at 1000 °C, air annealing of ¹¹¹In implanted ZnO samples revealed change in local stoichiometry about probe atoms which is attributed to the internal oxidation of the indium probes. The measured electric field gradient and asymmetry parameter at cation-sites of ZnO have been compared with theoretical calculations using a simple point charge model.     

Observation and cancellation of a perturbing dc stark shift in strontium optical lattice clocks

We report on the observation of a dc Stark frequency shift at the 10-(13) level by comparing two strontium optical lattice clocks. This frequency shift arises from the presence of electric charges trapped on dielectric surfaces placed under vacuum close to the atomic sample. We show that these charges can be eliminated by shining UV light on the dielectric surfaces, and characterize the residual dc Stark frequency shift on the clock transition at the 10-(18) level by applying an external electric field. This study shows that the dc Stark shift can play an important role in the accuracy budget of lattice clocks, and should be duly taken into account.     

Classification and analysis of excitations in a coupled finite nanoparticle cylinder-shell lattice

We present a general model for computing an optical response function of a finite shell lattice of semiconducting or metallic nanoparticles. Within a second quantization formalism, a cylindrical shell of induced, coupled dipoles is considered in the presence of an external electric field. Numerical analysis of the eigenmodes and quantum mechanical response function allow us to identify resonator effects due to constructive interferometric interaction of the light to the dipole lattice. Adjusting the wavelength of the external electric field, a coherent resonance excitation is possible for a fixed parameter of the cylinder radius.     

FCPAC: Fast calculation of partial atomic charges using strictly localized molecular orbitals

Most force fields of molecular mechanics use constant partial atomic charges whereas it is now admitted that those charges strongly vary with the environment. Charge variations are particularly important in hydrogen bonds where polar entities come close together. Moreover, organizing H-bonds in networks, like in secondary structures of proteins, go with cooperative effects which lead to an overall electrostatic stabilization of the system. This is why variable atomic charges are required to correctly mimic these effects. Different methods of charge calculation were developed, most use polarizable dipole or electronegativity equalization. In this work, we propose a semi-empirical method (fast calculation of partial atomic charges (FCPAC)) which derives partial atomic charges from strictly localized molecular orbitals. The approximations enable to treat molecular systems with hundreds of atoms within reasonable delays. Results reported here show that charges calculated with FCPAC are similar to charges derived from Mulliken’s population analysis of a restricted Hartree–Fock wave function using the split valence basis set 6-31G and including polarization orbitals p and d respectively for hydrogen and heavy atoms, RHF/6-31G(d,p). Comparison with ab initio calculation was also performed on a system composed of five formamide molecules disposed in the same geometry as peptide bonds in an helix. Charges variations are similar and suggest that FCPAC is suitable for quantitative estimation of cooperative effects.     

Diffraction effects in the propagation of radiation in polarizable layers

The very low transmission of light through holes smaller than the wavelength has been found to be enhanced for subwavelength apertures in metallic surfaces with periodic corrugations. This effect has been attributed to the interaction of light with surface plasmons. Similar effects obtained subsequently for non-metallic surfaces have been attributed to evanescent waves on the surface produced by the diffracted Bloch waves from different points in the array. We present an exact solution of Maxwell's equations in the discrete dipole approximation (DDA) for a periodic array of polarizable point dipoles in a layer. Metallic as well as non metallic layers are described. When the wavelength is smaller than the lattice period there is a Bragg's scattered wave, while for subwavelength conditions an evanescent wave on the surface appears. The transmission/reflection coefficients are found to oscillate as a function of frequency, with resonances occurring in a broad range of frequencies depending on the polarizability, at which the evanescent field is enhanced. A detailed study is presented for nanostructured arrays. We find that this model agrees with features observed in experiments through hole arrays supporting the role played by diffraction during light transmission through such arrays without invoking surface plasmons and providing a base to analyze more complex geometries.     

Frequency domain control of quantum state singlet/triplet character and prospects for an all-optical spin switch

In recent years, quantum optics has been expanded from atomic to molecular systems despite much weaker oscillator strengths and complex relaxation pathways that have presented serious challenges in the past. The richness of molecular excitation pathways and the variety of molecular interactions has made it possible to develop novel applications in this field. We have demonstrated how the Autler–Townes effect can be used to control molecular angular momentum alignment and how the Autler–Townes split line shape, combined with accurate control-laser electric field amplitude measurement, can be used to map the absolute magnitude of the molecular internuclear distance dependent electronic transition dipole moment function. In addition, the electric field amplitude in the control laser Rabi frequency can be used as a ‘tuning’ mechanism for the mixing coefficients of energy levels that are weakly perturbed by the spin–orbit interaction, i.e. to control the valence electron spin polarization and the spin multiplicity of molecular quantum. We propose an extension of this control mechanism to an all-optical spin switch.     

Microscopic theory of impurities in liquid helium. III

The theory of the polaron based on path integrals is used to discuss the motion of an impurity in liquid helium. A model Hamiltonian is introduced in which the interaction between the phonons and the impurity depends on two parameters. The mobility equations are solved in two approximations for the recoil motion of the ion. The real part of the frequency-dependent mobility comes from the radiation of phonons as well as the scattering of phonons. We find three possible solutions for the frequency-dependent mobility using one of these approximations. This corresponds to the impurity having three different effective masses. The motion of an ion in a high electric field is calculated. For velocities greater than the Landau critical velocity the ion continuously radiates rotons. The motion of the ion for velocities less than the critical velocity is also calculated.     

Non-vanishing ponderomotive AC electrophoretic effect for particle trapping

We present here a study on overlooked aspects of alternating current (AC) electrokinetics-AC electrophoretic (ACEP) phenomena. The dynamics of a particle with both polarizability and net charges in a non-uniform AC electric trapping field is investigated. It is found that either electrophoretic (EP) or dielectrophoretic (DEP) effects can dominate the trapping dynamics, depending on experimental conditions. A dimensionless parameter γ is developed to predict the relative strength of EP and DEP effects in a quadrupole AC field. An ACEP trap is feasible for charged particles in 'salt-free' or low salt concentration solutions. In contrast to DEP traps, an ACEP trap favors the downscaling of the particle size.     

Non-linear dielectric properties of aluminium/monomolecular layers of calcium behenate/aluminium structures

We studied the non-linear dielectric properties of aluminium/monomolecular layers of calcium behenate/aluminium structures at high electric induction (D > 10^?3 C m^?2). These properties generally arise from the motion of electric charges that are trapped in both the alumina at the surface of the electrodes and in the monolayers and result in a hyperpolarizability. However, in some particular cases in which dipolar relaxation mechanisms were present we observed a decrease in the polarizability. This decrease is due to a Langevin-Debye-type saturation at high electric field. Since the mechanical compressibility of the monomolecular layers is small (1.6 × 10^?10 N m^?2), electrostatic compressibility and the electrostriction phenomena are of minor importance.The introduction of small quantities of water produces a marked increase in the non-linear phenomena which may mask all the other phenomena.     

Static polarizability of carbon nanotubes: ab initio independent-particle calculations

An ab initio study of the static polarizability of the carbon nanotubes within density functional theory in local density and independent-particle approximations has been performed. Full-potential projected augmented wave method was used. Specifically, the static dielectric function (0) and electric polarizability (0) of a number of the armchair [(3,3),(5,5),(10,10),(15,15),(20,20)], zigzag [(5,0),(10,0),(15,0),(16,0),(20,0)] and chiral [(4,2),(6,2),(6,4),(8,4),(10,5)] carbon nanotubes have been calculated. The underlying atomic structure of the carbon nanotubes was determined theoretically. The calculated static polarizability (0) for the nanotubes is rather anisotropic with (0) for electric field parallel to the tube axis ( ) being two to three times larger than that for electric field perpendicular to the tube axis ( ). This anisotropy is further enhanced by up to a factor of five when the depolarization effects for is taken into account by a classical correction. For both electric field polarizations, (0) is roughly proportional to the square of the tube diameter, suggesting that it is independent of the chirality and the details of the electronic structure of the nanotubes.     

Refractive index of polymethylmethacrylate oriented by fluid temperature under electrical field

We prepared micron and submicron polymethylmethacrylate (PMMA) layers by the spin-coating method. We investigated the possibility to orientate polymer dipoles in electric field in the glass transition area (T _g) and the fluid temperature of PMMA with the aim to increase its refractive index (n) after the layer is cooled below T _g. We have studied the effect of electric field (up to 12 kV cm⁻¹) on change of surface morphology of the layer, dependence of n and contact angle (surface wettability) on the field and dependence of layers orientation on orientation of electric field. The surface morphology was examined using atomic force microscopy (AFM), contact angles were measured by goniometer, film thickness was measured by profilometer, refractive index of films was determined using refractometer. The change of refractive index as dependent on the PMMA layer orientation in electric field depends on temperature and electric field. The highest change in n was found for electric field 11 kV cm⁻¹. The change in contact angle (wettability) on surface of an orientated PMMA layer confirms the dipoles orientation in electric field unambiguously. The orientation of layers causes a “slight” change in their morphology and a “slight” increase of surface roughness only for one direction of field effect. Change in colour for oriented layers does not depend on orientation of electric field.     

Carbon nanotubes as electrodes for dielectrophoresis of DNA

Dielectrophoresis can potentially be used as an efficient trapping tool in the fabrication of molecular devices. For nanoscale objects, however, the Brownian motion poses a challenge. We show that the use of carbon nanotube electrodes makes it possible to apply relatively low trapping voltages and still achieve high enough field gradients for trapping nanoscale objects, e.g., single molecules. We compare the efficiency and other characteristics of dielectrophoresis between carbon nanotube electrodes and lithographically fabricated metallic electrodes, in the case of trapping nanoscale DNA molecules. The results are analyzed using finite element method simulations and reveal information about the frequency-dependent polarizability of DNA.     

Ab-initio and DFT methodologies for computing hyperpolarizabilities and susceptibilities of highly conjugated organic compounds for nonlinear optical applications

In this talk, after a short introduction on the methodologies used for computing dipole polarizability (α), second and third-order hyperpolarizability and susceptibility; the results of theoretical studies performed on density functional theory (DFT) and ab-initio quantum mechanical calculations of nonlinear optical (NLO) properties for a few selected organic compounds and polymers will be explained. The electric dipole moments (μ) and dispersion-free first hyperpolarizabilities (β) for a family of azo-azulenes and a styrylquinolinium dye have been determined by DFT at B3LYP level. To reveal the frequency-dependent NLO behavior, the dynamic α, second hyperpolarizabilities (γ), second (χ⁽²⁾) and third-order (χ⁽³⁾) susceptibilites have been evaluated using time-dependent HartreeFock (TDHF) procedure. To provide an insight into the third-order NLO phenomena of a series of pyrrolo-tetrathiafulvalene-based molecules and pushpull azobenzene polymers, two-photon absorption (TPA) characterizations have been also investigated by means of TDHF. All computed results of the examined compounds are compared with their previous experimental findings and the measured data for similar structures in the literature. The one-photon absorption (OPA) characterizations of the title molecules have been theoretically obtained by configuration interaction (CI) method. The highest occupied molecular orbitals (HOMO), the lowest unoccupied molecular orbitals (LUMO) and the HOMO–LUMO band gaps have been revealed by DFT at B3LYP level for azo-azulenes, styrylquinolinium dye, push–pull azobenzene polymers and by parametrization method 6 (PM6) for pyrrolo-tetrathiafulvalene-based molecules.