Dielectric Screening Effect Research Articles

Manganese doping influence on the plasmon energy of nickel films

Manganese doping in nickel films capped with copper have been prepared by evaporation in vacuum. The films are composed of grains with an average diameter of ~20nm from scanning electron microscope scans. Optical absorption is measured over a wavelength range of 190–450nm. Two plasmon peaks are observed at 3.30eV and 4.45eV for a range of concentrations of films. The 4.45eV peak is a bulk plasmon peak that is enhanced by increasing the manganese in nickel. The 3.30eV peak is a surface plasmon peak that increases in width or strength of plasmon resonance with increasing concentration of manganese. This may be a combination effect of charge carrier concentration and dielectric screening from the reformed electronic band structure caused by manganese doping. By adding manganese into nickel, the ferromagnetic order is further destroyed as a transition into a spin glass occurs. This spin glass behavior is seen in a coercivity measurement at 4K where the coercivity drops precipitously as the doping concentration increases.

Optical Properties of Single-Walled Carbon Nanotubes Separated in a Density Gradient; Length, Bundling, and Aromatic Stacking Effects.

Single-walled carbon nanotubes (SWNTs) are promising materials for in vitro and in vivo biological applications due to their high surface area and inherent near infrared photoluminescence and Raman scattering properties. Here, we use density gradient centrifugation to separate SWNTs by length and degree of bundling. Following separation, we observe a peak in photoluminescence quantum yield (PL QY) and Raman scattering intensity where SWNT length is maximized and bundling is minimized. Individualized SWNTs are found to exhibit high PL QY and high resonance-enhanced Raman scattering intensity. Fractions containing long, individual SWNTs exhibit the highest PL QY and Raman scattering intensities, compared to fractions containing single, short SWNTs or SWNT bundles. Intensity gains of approximately ~1.7 and 4-fold, respectively, are obtained compared with the starting material. Spectroscopic analysis reveals that SWNT fractions at higher displacement contain increasing proportions of SWNT bundles, which causes reduced optical transition energies and broadening of absorption features in the UV-Vis-NIR spectra, and reduced PL QY and Raman scattering intensity. Finally, we adsorb small aromatic species on "bright," individualized SWNT sidewalls and compare the resulting absorption, PL and Raman scattering effects to that of SWNT bundles. We observe similar effects in both cases, suggesting aromatic stacking affects the optical properties of SWNTs in an analogous way to SWNT bundles, likely due to electronic structure perturbations, charge transfer, and dielectric screening effects, resulting in reduction of the excitonic optical transition energies and exciton lifetimes.

Dielectric screening and band-structure effects in low-energy photoemission

Nonlocal response of the surface to the incident light is included into an ab initio one-step photoemission theory. Surface-state normal emission spectra from Be(0001) and Al(100) are calculated by a full-potential scattering method and are found to agree well with the experiment in a wide energy range. The total exciting field is obtained within the random-phase approximation for jellium as well as for one-dimensional crystal models of the two surfaces. Material dependence of the multipole plasmon mode and strong effect of band structure on the photoyield is discussed.

Effects of environmental dielectric screening on optical absorption in carbon nanotubes

Effects of environmental dielectric screening on optical absorption spectrum are studied in carbon nanotubes within a k · p scheme. The screening is sensitive to the effective distance between the nanotube and the dielectric medium. For material surrounding the nanotube, the band gap decreases with the increase of the dielectric constant, but the energy of the ground exciton exhibits only a slight decrease for effective distance comparable to interlayer spacing of bulk graphite. For dielectric material inside the nanotube, effects are much weaker because of anti-screening.

Theory of Electrochemistry for Nanometer-Sized Disk Electrodes

It has been shown previously that conventional voltammetric theories may become inapplicable at electrodes of nanometer scale due to enhanced effects of the diffuse double layer on the interfacial charge transport and transfer processes (Anal. Chem. 1993, 65, 3343; J. Phys. Chem. B 2006, 110, 3262). As well as the diffuse double layer effects, we show in present study that the voltammetric responses of nanometer-sized electrodes would differ from the macroscopic electrodes due to significant edge effects of dielectric screening and electron tunneling if the electrode has planar geometries. These nanoedge effects arise because of the comparable size of the electrode with the dipole molecules and the effective electron tunneling distance. Models for these nanoedge effects are developed and combined with Poisson−Nernst−Planck theory and Marcus electron-transfer theory to describe the voltammetric characteristics of nanometer-sized disk electrodes. Marcus theory instead of Butler−Volmer theory is used to desc...

Electronic continuum model for molecular dynamics simulations of biological molecules.

Electronic polarizability is an important factor in molecular interactions. In the conventional force fields such as AMBER or CHARMM, however, there is inconsistency in how the effect of electronic dielectric screening of Coulombic interactions, inherent for the condensed phase media, is treated. Namely, the screening appears to be accounted for via effective charges only for neutral moieties, whereas the charged residues are treated as if they were in vacuum. As a result, the electrostatic interactions between ionized groups are exaggerated in molecular simulations by the factor of about 2. The discussed here MDEC (Molecular Dynamics in Electronic Continuum) model provides a theoretical framework for modification of the standard non-polarizable force fields to make them consistent with the idea of uniform electronic screening of partial atomic charges. The present theory states that the charges of ionized groups and ions should be scaled; i.e. reduced by a factor of about 0.7. In several examples, including the interaction between Na (+) ions, which is of interest for ion-channel simulations, and the dynamics of an important salt-bride in Cytochrome c Oxidase, we compared the standard non-polarizable MD simulations with MDEC simulations, and demonstrated that MDEC charge scaling procedure results in more accurate interactions. The inclusion of electronic screening for charged moieties is shown to result in significant changes in protein dynamics and can give rise to new qualitative results compared with the traditional non-polarizable force fields simulations.

Environment Effects on Excitons in Semiconducting Carbon Nanotubes

Effects of environmental dielectric screening on excitons in carbon nanotubes are studied within a k · p scheme and a continuum model. They are shown to be sensitive to the effective distance between the nanotube and the dielectric medium relative to the diameter. For material surrounding the nanotube, the band gap decreases with the increase of the dielectric constant, but the energy of the ground exciton exhibits only a slight decrease for effective distance comparable to interlayer spacing of bulk graphite. The binding energy of excited exciton states disappears rapidly. For dielectric material inside the nanotube, effects are much weaker and excited exciton states remain as bound states even for very large dielectric constant.

Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interfaces.

EFFECT OF DIELECTRIC SCREENING ON THE DIAMAGNETIC SUSCEPTIBILITY OF A DONOR IN LOW DIMENSIONAL SEMICONDUCTING SYSTEMS

The effect of spatial dielectric screening on the diamagnetic susceptibility (χ dia ) of a donor in Low Dimensional Semiconducting Systems like Quantum Well, Quantum Well Wire and Quantum Dot in the infinite barrier model has been computed and investigated within the effective mass theory using variational method. We observe that the effect of spatial dielectric screening on χ dia decreases with decrease of dimensionality of the system.

Tuning the Effective Fine Structure Constant in Graphene: Opposing Effects of Dielectric Screening on Short- and Long-Range Potential Scattering

We reduce the dimensionless interaction strength alpha in graphene by adding a water overlayer in ultrahigh vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30%, due to the background dielectric constant enhancement leading to a reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short-range impurities is decreased by almost 40%, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron-hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.

Optical study of porphyrin-doped carbon nanotubes

Porphyrin-doped carbon nanotubes in sodium dodecylbenzenesulfonate (NaDDBS) aqueous solution were obtained from NaDDBS/carbon nanotube aqueous dispersions. Several phonon-assisted absorption and recombination processes seem to occur, including one-phonon and two-phonon processes, and remain present even upon porphyrin doping. Power-law scaling of exciton binding energies and environmental dielectric screening effects are used to infer the doping from photoluminescence maps. The dielectric constant of the 5,10,15,20-tetrakis(4-trimethylammonium phenyl) porphyrin (H 2TTMAPP) in NaDDBS aqueous solution seems to be higher than the one of NaDDBS/aqueous solution apparently because the counterions have opposite net charges.

Excited Excitonic States in Single-Walled Carbon Nanotubes

Polarized photoluminescence excitation spectroscopy on individual SWNTs reveals not only the longitudinal and transverse E 11, E 22, and E 12 ground-state excitons but also excited excitonic states including the continuum. When heated, SWNTs are known to undergo a bandgap shift transition (BST), which effectively changes the nanotube dielectric environment. Here, we show that the entire spectrum of excitonic resonances blue shifts under this transition, with excited states showing larger shifts, approaching 100 meV for a 1 nm diameter nanotube. The excitonic binding energy, Coulomb self-energy correction, and dielectric shift under the BST are estimated. Analysis of this blue shift reveals the dominant effect of dielectric screening on SWNT excitonic states.

Effects of dielectric screening and position dependent effective mass on donor binding energies and on diamagnetic susceptibility in a quantum well

Abstract The binding energies of a shallow hydrogenic donor in the GaAs quantum well have been calculated using five different screening functions within the effective mass approximation. Both the infinite and finite confinements are used. Though all the screening functions yield results, which are nearly the same for all well dimensions studied, it is observed that the Resta dielectric function leads to smaller binding energies for well dimensions ≤100 A. The diamagnetic susceptibility of a hydrogenic donor has also been estimated and the results are in good agreement with the recently reported values. In addition, the position dependent effective masses are used for both confinements and the results for donor ionization energies and diamagnetic susceptibilities are shown to be appreciable for narrow well dimensions. Using the Resta dielectric function, pressure effects on donor ionization energies and diamagnetic susceptibilities are estimated for Q2D GaAs. It is found that while the ionization energies increase with pressure, | χ dia | decreases with pressure.

Dielectric screening effects in molten AgI–AgBr system

The screened effect in molten AgI–AgBr system is discussed by the inverse dielectric function 1/ϵ(q), which is newly obtained in terms of the charge–charge structure factor S ZZ (q) by the re-examined theory of dielectric screening in molten salts. S ZZ (q) and in due course, 1/ϵ(q) are obtained from structural properties of molten AgI–AgBr system by molecular dynamics (MD) simulations. The obtained 1/ϵ(q) is multiplied onto the attractive part in the inter-ionic potential between cation and anion to obtain the screened potential. The total screened potential is examined in terms of the potential of mean force.

Redshift and optical anisotropy of collectiveπ-volume modes in multiwalled carbon nanotubes

A combined study concerning localized electron energy-loss spectroscopy (EELS) and modeling of collective $\ensuremath{\pi}$-volume modes in multiwalled carbon nanotubes (MWCNT) is presented. The changing line width and eigenfrequency of the $\ensuremath{\pi}$-volume mode can be ascribed solely to optical anisotropy and ``cylindrical anisotropy.'' Optical anisotropy results from the weighting of various nearly degenerate and nondegenerate states allowed for the ${\mathbf{E}}_{\ensuremath{\Vert}c}$ and ${\mathbf{E}}_{\ensuremath{\perp}c}$ polarizations. Cylindrical anisotropy arises from a lowering of the symmetry arising from the nanotube geometry. The eigenfrequency of the $\ensuremath{\pi}$-volume mode corresponds to polarization eigenmodes of graphite, and not to new maxima in the joint density of states, since momentum transfer $\mathrm{\ensuremath{\Delta}}{\mathbf{q}}_{\ensuremath{\pi}}\ensuremath{\rightarrow}0$. Results are also included from multiwalled hexagonal-boron nitride nanotubes (MWBNNT). An accurate description of the $\ensuremath{\pi}$-volume mode in multiwalled nanotubes has not been attempted so far, and is essential to resolve coupled MWCNT $\ensuremath{\pi}$-surface features, which are usually obscured in spectra obtained in penetrating-beam geometry. Volume mode-extracted EEL spectra demonstrate eigenfrequency modification of coupled $\ensuremath{\pi}$-surface features in the presence of a MWCNT dielectric filling. It was found, owing to dielectric screening effects and smearing of the dipole mode, that aloof-beam EELS which is conventionally applied to surface plasmon investigations, cannot give this information.

Effect of dielectric screening on the binding energies and diamagnetic susceptibility of a donor in a quantum well wire

AbstractThe effect of the dielectric screening on donor binding energies in a quantum wire has been estimated. Since the screening function is shown to be significant in a few phenomena like carrier transport, and not so in a few cases like the donor binding energy calculations, five different screening functions have been used in the calculations to see their relative merits. It is observed that the dielectric function obtained by Resta using the Thomas–Fermi approximation not only gives lower estimates of the ionization energies for all well widths (L ), but also is shown to vary very rapidly with L . The other functions used are those of Hermanson and Vinsome & Richardson. Using the results of the variational calculations for the finite barrier problem, the diamagnetic susceptibility of a donor as a function of L has also been estimated. Though experimental results are not available, we have compared our results for E ion with other theoretical estimates available in the literature. The agreement is found to be good. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of dielectric screening on positronium formation in acetone-acetic acid polar solvent blends

The influence of polarization of a medium in the vicinity of the positron-electron pair produced at the end of a fast-positron track on the formation of positronium (Ps) was considered. Weakening interaction between the positron and electron, polarization leads to a decrease in the Ps formation probability. The transience of Ps formation requires that the time delay of medium polarization should be taken into account. In turn, the magnitude of dielectric screening in acetone-acetic acid mixtures strongly depends on the ratio between the amounts of polar monomeric acetic acid and its nonpolar dimer. Sequentially taking account of these effects, it is possible to explain both the complex behavior of the Ps yield and the dependence of dielectric permittivity of the mixture on its composition.

Ionic pseudopotential for semiconductors without cut-off parameter for core-repulsion with application to Si

An ionic pseudopotential for semiconductors is proposed, which consists of a set of continuous exponential functions. Introduced damping and amplitude parameters into the pseudopotential are to be treated as adjustable. The most important features of the proposed pseudopotential is that (1) it has no sharp cut-off parameter for the core-repulsion and (2) it is continuous and has continuous derivatives to arbitrary order. The proposed pseudopotential is applied to Si and the adjustable parameters are determined so as to be consistent with the Si crystal empirical pseudopotential of high quality by taking a valence electron dielectric screening effect into account. The effectiveness of the proposed ionic pseudopotential is discussed by (1) comparing the calculated ionic energy levels of Si with experiments, (2) checking the consistency between the ionic and crystal pseudopotentials for Si, and so on.

The Effects of Structural and Microenvironmental Disorder on the Electronic Properties of Poly[2-methoxy,5-(2‘-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH−PPV) and Related Oligomers

In this study, electroabsorption (Stark) spectroscopy is used to determine the trace of the change in polarizability (tr ) and the absolute value of the change in dipole moment (|Δμ|) of the electroluminescent polymer poly[2-methoxy,5-(2‘-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH−PPV) and several model oligomers in solvent glass matrixes. We find a value of tr of ∼2000 A3 for the polymer and for a 9-ring substituted oligomer in both toluene and 2-methyl tetrahydrofuran matrixes at 77 K with smaller values being obtained for 3- and 5-ring unsubstituted oligomers. Although gas-phase calculations of tr using INDO/SCI yield values that are about a factor of 8 smaller than the experiment, excellent agreement is obtained when the effects of solid-state dielectric screening are included. Screening increases tr by bringing the energy gap between the 1Bu and mAg states into agreement with solid-state measurements. Substantial values of |Δμ| are observed experimentally both for the polymer and for the oligomers (6...

Effects of dielectric screening on the optical properties of metallic nanoshells

The time dependent density functional method for the calculation of optical properties of metallic nanoshells is extended to include the combined influence of a dielectric core and a dielectric embedding medium. The coupling between the polarization charges at the inner and outer surfaces of the shell is found to strongly influence the position of the plasmon resonances. The method is applied to gold nanoshells with gold sulfide cores in water for two different diameters, and the energies of the calculated plasmon resonances are found to be in good agreement with experimental measurements.