Dielectric Constant Mismatch Research Articles

Oxygen vacancy related hole fast trapping in high mobility cubic-Ge/ZrO2 interface

Ge has the potential to replace Si as the future field-effect transistors channel material due to its superior hole mobility, and cubic zirconia with high dielectric constant and small lattice mismatch can be selected as its oxide layer. At present, the mechanism of charge trapping caused by defects in the Ge oxide layer interface and bulk of such device has not been accurately analyzed. In our work, we have constructed the cubic Ge/ZrO2 interface, and studied the electronic structure and hole trapping characteristics of the interface structure by first-principles hybrid-functional calculations with Marcus theory. According to research oxygen vacancies with the different distances (abbr. d O-int) away from the Ge substrate, we confirm that the oxygen vacancy can act as a fast trap center to capture the hole of the valence band maximum (VBM) from Ge, resulting in the ultrafast or fast transient charge trapping in the high-k gate dielectric. We found that, when a given range of applied electric field, the hole trap is ultrafast with capture time of 10−6–10−5 when d O-int is within the range of 2–7 , and there is a 2–3 order of magnitude increases in capture time as d O-int exceeds 7 with the maximum capture cross section reducing substantially. Here, our work provides a clear and reasonable description of the distance-dependent hole trapping process at the Ge/high-k dielectrics metal-oxide-semiconductor (MOS) devices and provides significant support for solving the reliability problem of microelectronic devices.

High Energy Storage Performance of PMMA Nanocomposites Utilizing Hierarchically Structured Nanowires Based on Interface Engineering.

To overcome the inherent high hysteresis loss of ferroelectric polymer-based nanocomposites, non-ferroelectric linear dielectric poly(methyl methacrylate) (PMMA) is adopted as the polymer matrix for high discharge efficiency. At the same time, slender ferroelectric BaTiO3 nanowires (BT NWs) with a high dielectric constant are selected as the nanofiller for high energy density. To avoid the agglomeration of BT NWs and enhance the strength of interfaces, dopamine is used as organic coatings to tailor the interface. The BT@dopa NWs/PMMA nanocomposites exhibit excellent interface compatibility between the BT NWs and PMMA matrix and a very good microstructure uniformity. Based on this, hierarchically structured BT@SiO2@dopa NWs are designed and prepared to overcome the uneven electric field distribution at the interface, resulting from the dielectric constant mismatch. The discharged energy density (Ue) can be largely enhanced from 3.76 J/cm3 for pure PMMA films to 11.78 J/cm3 for PMMA-based nanocomposites by incorporating 5.0 wt % BT@SiO2@dopa NWs. In addition, a high discharging efficiency (η) of 91% is obtained simultaneously in the nanocomposites. Both experimental and theoretical simulations demonstrate that the double core-shell structure nanowire fillers can effectively alleviate the local field distortion, inhibit leakage current, and suppress remnant electric displacement, leading to the high Ue and η. These findings are significant in facilitating the development of high-performance film dielectric capacitor materials using PMMA-based nanocomposites toward high energy storage density.

Surface polarization effects in confined polyelectrolyte solutions

Understanding nanoscale interactions at the interface between two media with different dielectric constants is crucial for controlling many environmental and biological processes, and for improving the efficiency of energy storage devices. In this contributed paper, we show that polarization effects due to such dielectric mismatch remarkably influence the double-layer structure of a polyelectrolyte solution confined between two charged surfaces. Surprisingly, the electrostatic potential across the adsorbed polyelectrolyte double layer at the confining surface is found to decrease with increasing surface charge density, indicative of a negative differential capacitance. Furthermore, in the presence of polarization effects, the electrostatic energy stored in the double-layer structure is enhanced with an increase in the charge amplification, which is the absorption of ions on a like-charged surface. We also find that all of the important double-layer properties, such as charge amplification, energy storage, and differential capacitance, strongly depend on the polyelectrolyte backbone flexibility and the solvent quality. These interesting behaviors are attributed to the interplay between the conformational entropy of the confined polyelectrolytes, the Coulombic interaction between the charged species, and the repulsion from the surfaces with lower dielectric constant.

Dielectric strength of glass fibre fabric reinforced epoxy by nano-Al2O3

Filling epoxy resin in glass fibre fabric with surface modified nano-Al 2 O 3 as a means of increasing the dielectric strength is investigated in this work. The 50 Hz breakdown strength with 3 wt% nano-Al 2 O 3 is shown to be 10% higher than the neat epoxy/glass fibre material and 4% higher than the neat epoxy. An improved interface bonding between the glass fibre and the epoxy is evidenced by SEM characterization. Based on a multilayer dielectric model, it is found that introducing nano-Al 2 O 3 to the epoxy resin can efficiently alleviate the dielectric constant mismatch between the epoxy and the epoxy-integrated glass fibre layer, which results in an improvement in the dielectric strength.

Synthesis and electrorheological properties of hierarchical and core-shell MoS2@TiO2 nanocomposite

MoS2@TiO2 nanocomposite with core-shell and hierarchical structure was synthesized using a simple and effective two-step method. First, 3D radically oriented MoS2 nanospheres were initially synthesized using a simple and convenient hydrothermal method to provide a large surface and active site for the uniform TiO2 growth on MoS2. Then, the structure, morphology, composition and electrorheological (ER) properties were characterised using an X-ray powder diffractometer, Fourier infrared spectrometer, transmission electron microscopy, scanning electron microscope with an attached energy-dispersive spectrometer, X-ray photoelectron spectroscopy and ER rheometer. The MoS2@TiO2 nanocomposite was dispersed into insulating silicone oil to form an ER fluid at a concentration of 10 ​wt %, which shown an enhanced ER efficiency by combining the useful of two dimensional MoS2 multistage structures and coating TiO2 shell to control conductivity. The large dielectric constant mismatch and short relaxation time of ER fluid was observed, which resulted in the creation of multiple polarizations and enhanced ER properties.

Recombination energy for negatively charged excitons inside type-II core/shell spherical quantum dots

The recombination energy of isolated neutral exciton and that of isolated negatively charged exciton inside a type-II core/shell spherical quantum dot are studied. Our investigation considers the charge-carriers effective mass discontinuity at the surface contact between the core and shell materials. Although our model omits the effect of the surface polarization, the dielectric-constant mismatch at the nanodot boundaries was taken into account. In order to achieve the exciton and negative trion energies, we proceed by a variational calculation in the framework of the envelope approximation. Our results reveal a strong correlation between the nanodot morphology and the energy spectrum of the neutral and negatively charged exciton.

Decorating TiO2 Nanowires with BaTiO3 Nanoparticles: A New Approach Leading to Substantially Enhanced Energy Storage Capability of High-k Polymer Nanocomposites.

The urgent demand of high energy density and high power density devices has triggered significant interest in high dielectric constant (high-k) flexible nanocomposites comprising dielectric polymer and high-k inorganic nanofiller. However, the large electrical mismatch between polymer and nanofiller usually leads to earlier electric failure of the nanocomposites, resulting in an undesirable decrease of electrical energy storage capability. A few studies show that the introduction of moderate-k shell onto a high-k nanofiller surface can decrease the dielectric constant mismatch, and thus, the corresponding nanocomposites can withstand high electric field. Unfortunately, the low apparent dielectric enhancement of the nanocomposites and high electrical conductivity mismatch between matrix and nanofiller still result in low energy density and low efficiency. In this study, it is demonstrated that encapsulating moderate-k nanofiller with high-k but low electrical conductivity shell is effective to significantly enhance the energy storage capability of dielectric polymer nanocomposites. Specifically, using BaTiO3 nanoparticles encapsulated TiO2 (BaTiO3@TiO2) core-shell nanowires as filler, the corresponding poly(vinylidene fluoride-co-hexafluoropylene) nanocomposites exhibit superior energy storage capability in comparison with the nanocomposites filled by either BaTiO3 or TiO2 nanowires. The nanocomposite film with 5 wt % BaTiO3@TiO2 nanowires possesses an ultrahigh discharged energy density of 9.95 J cm-3 at 500 MV m-1, much higher than that of commercial biaxial-oriented polypropylene (BOPP) (3.56 J cm-3 at 600 MV m-1). This new strategy and corresponding results presented here provide new insights into the design of dielectric polymer nanocomposites with high electrical energy storage capability.

Enhanced actuation performance of piezoelectric fiber composites induced by incorporated BaTiO3 nanoparticles in epoxy resin

Piezoelectric fiber composites (PFCs) have attracted much interest owing to their flexibility and toughness compared with conventional monolithic piezoceramic wafers. The free strain values and actuation property of PFCs strongly depend on the active electric field applied in Pb(Zr1−xTix)O3 (PZT) fibers. Reducing the dielectric constant mismatch between PZT fiber and the assembling epoxy resin would greatly increase the active electric field in PZT fiber. Therefore, BaTiO3 (BT) nanoparticles were introduced into the epoxy resin to enhance the dielectric constant. Homogeneous dispersion of BT nanoparticles and tight adhesion with the epoxy resin were achieved through a surface modification by dopamine. The maximum dielectric constant of dopamine modified BT/epoxy (BT@Dop/epoxy) nanocomposites was 10.38 with 12 wt% BT@Dop content at 1 kHz. The maximum free strain of PFCs reached 1820 ppm with 6 wt% BT@Dop content, while PFCs assembled by pure epoxy showed 790 ppm at the same processing condition. The tip displacement of cantilever beam actuated by PFCs reached the peak of 19 mm at the resonance frequency with 6 wt% BT@Dop, which was improved by 90% comparing to PFCs with pure epoxy.

Polarization effects on spectra of spherical core/shell nanostructures: Perturbation theory against finite difference approach

Poisson equation is solved analytically in the case of a point charge placed anywhere in a spherical core/shell nanostructure, immersed in aqueous or organic solution or embedded in semiconducting or insulating matrix. Conduction and valence band-edge alignments between core and shell are described by finite height barriers. Influence of polarization charges induced at the surfaces where two adjacent materials meet is taken into account. Original expressions of electrostatic potential created everywhere in the space by a source point charge are derived. Expressions of self-polarization potential describing the interaction of a point charge with its own image–charge are deduced. Contributions of double dielectric constant mismatch to electron and hole ground state energies as well as nanostructure effective gap are calculated via first order perturbation theory and also by finite difference approach. Dependencies of electron, hole and gap energies against core to shell radii ratio are determined in the case of ZnS/CdSe core/shell nanostructure immersed in water or in toluene. It appears that finite difference approach is more efficient than first order perturbation method and that the effect of polarization charge may in no case be neglected as its contribution can reach a significant proportion of the value of nanostructure gap.

Ground state energy and wave function of an off-centre donor in spherical core/shell nanostructures: Dielectric mismatch and impurity position effects

Ground state energy and wave function of a hydrogen-like off-centre donor impurity, confined anywhere in a ZnS/CdSe spherical core/shell nanostructure are determined in the framework of the envelope function approximation. Conduction band-edge alignment between core and shell of nanostructure is described by a finite height barrier. Dielectric constant mismatch at the surface where core and shell materials meet is taken into account. Electron effective mass mismatch at the inner surface between core and shell is considered. A trial wave function where coulomb attraction between electron and off-centre ionized donor is used to calculate ground state energy via the Ritz variational principle. The numerical approach developed enables access to the dependence of binding energy, coulomb correlation parameter, spatial extension and radial probability density with respect to core radius, shell radius and impurity position inside ZnS/CdSe core/shell nanostructure.

Self-assembly of colloidal particles into strings in a homogeneous external electric or magnetic field

Colloidal particles with a dielectric constant (magnetic susceptibility) mismatch with the surrounding solvent acquire a dipole moment in a homogeneous external electric (magnetic) field. The resulting dipolar interactions can lead to aggregation of the particles into string-like clusters. Recently, several methods have been developed to make these structures permanent. However, especially when multiple particle sizes and/or more complex shapes than single spheres are used, the parameter space for these experiments is enormous. We therefore use Monte Carlo simulations to investigate the structure of the self-assembled string-like aggregates in binary mixtures of dipolar hard and charged spheres, as well as dipolar hard asymmetric dumbbells. Binary mixtures of spheres aggregate in different types of clusters depending on the size ratio of the spheres. For highly asymmetric systems, the small spheres form ring-like and flame-like clusters around strings of large spheres, while for size ratios closer to 1, alternating strings of both large and small spheres are observed. For asymmetric dumbbells, we investigate both the effect of size ratio and dipole moment ratio, leading to a large variety of cluster shapes, including chiral clusters.

Exciton states and interband optical transitions in wurtzite InGaN/GaN quantum dot nanowire heterostructures

Within the framework of the effective-mass approximation, the exciton states and interband optical transitions in In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated using a variational method, in which the important built-in electric field (BEF) effects, dielectric-constant mismatch and three-dimensional confinement of the electron and hole in In x Ga 1− x N QDs are considered. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron-hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the exciton oscillator strength.

Nonlinear Optical Properties of Semiconductor Nanowires

The orientation dependency of nonlinear optical effects, including the second-harmonic generation (SHG) and third-harmonic generation (THG), as well as two-photon luminescence, in semiconductor nanowires (NW) are described. The mismatch of dielectric constants between nanostructures and their environment governs the rise of optical nonlinearities causing SHG even in materials with a high symmetry crystal lattice that would not generate second harmonic in the bulk state. Due to the depolarization effects, the intensity of the optical electric field inside illuminated NWs depends dramatically on their orientation related to the exciting light polarization. As a result, rotation of the light polarization causes giant angular dependency of all the mentioned optical phenomena, with the maximum relative amplitude at the electric-field polarization oriented along the NWs. Simultaneous measurements of two-photon-induced luminescence, SHG and THG in ZnSe NWs are presented. In accordance with the theory, the angular dependencies of nonlinear phenomena were observed and influence of the environmental dielectric constant on the magnitude of the nonlinear signals was demonstrated. The relative amplitudes of the angular dependencies were also significantly influenced by NW bending and nonuniformity.

Computational study of filler microstructure and effective property relations in dielectric composites

Phase field modeling and computer simulation is employed to study the relations between filler microstructures and effective properties of dielectric composites. The model solves electrostatic equations in terms of polarization vector field in reciprocal space using a fast Fourier transform technique and parallel computing algorithm. Composites composed of linear constituent phases of different dielectric constants are considered. Interphase boundary conditions are automatically taken into account without explicitly tracking interphase interfaces in the composites. Various factors associated with filler microstructures are systematically investigated, including dielectric constant mismatch between fillers and matrix, particle size, shape, orientation, volume fraction, and spatial arrangement as well as directional alignment. Heterogeneous distributions of polarization, charge density, and local electric field are calculated for each composite microstructure, based on which effective dielectric constant and dielectric anisotropy of the composites are determined. It is found that electrostatic interactions among high-dielectric-constant fillers embedded in low-dielectric-constant matrix play critical roles in determining the composite properties, which sensitively depend on filler arrangement and, especially, directional alignment into fibrous microstructures (chains). Such microstructurally engineered composites, whose fillers are not randomly dispersed, exhibit strong dielectric anisotropy despite all constituent components being isotropic.

Exciton states and optical transitions in InGaN/GaN quantum dot nanowire heterostructures: Strong built-in electric field and dielectric mismatch effects

Considering the strong built-in electric field (BEF), dielectric-constant mismatch and 3D confinement of the electron and hole, the exciton states and interband optical transitions in [0 0 0 1]-oriented Ga-rich wurtzite In x Ga 1− x N/GaN strained quantum dot (QD) nanowire heterostructures are investigated theoretically using a variational approach under the effective mass approximation. We find that the strong BEF gives rise to an obvious reduction of the effective band gap of QDs and leads to a remarkable electron–hole spatial separation. The BEF, QD height and radius, and dielectric mismatch effects have a significant influence on exciton binding energy, electron interband optical transitions, and the radiative decay time. Our calculations show that the radiative decay time of the redshifted transitions is large and increases almost exponentially when the QD height increases, which is in good agreement with the previous experimental and theoretical results.

Mismatch of dielectric constants at the interface of nanometer metal-oxide-semiconductor devices with high-K gate dielectric impacts on the inversion charge density

The comparison of the inversion electron density between a nanometer metal-oxide-semiconductor (MOS) device with high-K gate dielectric and a SiO2 MOS device with the same equivalent oxide thickness has been discussed. A fully self-consistent solution of the coupled Schrodinger–Poisson equations demonstrates that a larger dielectric-constant mismatch between the gate dielectric and silicon substrate can reduce electron density in the channel of a MOS device under inversion bias. Such a reduction in inversion electron density of the channel will increase with increase in gate voltage. A reduction in the charge density implies a reduction in the inversion electron density in the channel of a MOS device. It also implies that a larger dielectric constant of the gate dielectric might result in a reduction in the source–drain current and the gate leakage current.

Enhanced outcoupling of electroluminescence from ZnS:ErF3 thin films by a photonic crystal

Enhanced 1550 nm near-infrared emission due to a photonic crystal (PC) in zinc sulfide (ZnS) doped with erbium trifluoride (ErF3) has been studied in sputter deposited alternating current thin film electroluminescent (ACTFEL) devices. The honeycomb structured PC was fabricated into the ZnS:ErF3 layer by electron beam lithography and argon sputtering. A thin film of yttria-stabilized zirconia (YSZ) was deposited into the honeycomb structure of holes, creating the photonic crystal structure with a dielectric constant mismatch between the ZnS:ErF3 layer and the YSZ of approximately 20. The photonic band gaps were modeled for different values of dielectric mismatch and the honeycomb structure hole diameter and lattice spacing. For the 1550 nm wavelength, the lattice constant and hole diameter typically were ∼748 and ∼342 nm, respectively. The YSZ structured PC ACTFEL device showed a twofold increase in 1550 nm light intensity versus a device without a PC structure. These results could not be explained by clas...

Polarizability of spherical biological cells in the presence of localized surface charge distributions at the membrane interfaces

The electrical polarizability α(ω) of a biological cell in the presence of a layer of localized, partially bounded, charges at the two cell membrane interfaces has been calculated within the dipolar approximation. The cell is modeled in the light of the single-shell spherical model, but the results can be easily extended to shelled particles of more complex shape. Under the influence of an external electric field, the presence of these charge distributions, which added to the ones originated by the mismatch of the complex dielectric constants of the different media, produces a further dielectric relaxation, besides the one due to the usual Maxwell-Wagner effect. We explicitly find the contribution that must be added to the electrical polarizability α(ω) in order to take into account the surface electrical currents originated by the localized charges free to move on the membrane surfaces. Our results, maintaining their validity whatsoever the values of the surface charge distributions and, moreover, whatsoever the values of the membrane conductivity are, extend the applicability of the model recently proposed by Prodan [Biophys. J. 95, 4174 (2008)], who developed an analytical solution which offers reliable results only in the case of weak surface charge distributions and, moreover, for negligible small values of the membrane conductivity. Our approach, removing these constrains, represents a valuable improvement toward more realistic biological cell models and widens the use of dielectric relaxation methods to a larger class of biological systems.

Phase diagram of colloidal spheres in a biaxial electric or magnetic field

Colloidal particles with a dielectric constant mismatch with the surrounding solvent in an external biaxial magnetic or electric field experience an "inverted" dipolar interaction. We determine the phase behavior of such a system using Helmholtz free energy calculations in Monte Carlo simulations for colloidal hard spheres as well as for charged hard spheres interacting with a repulsive Yukawa potential. The phase diagram of colloidal hard spheres with inverted dipolar interactions shows a gas-liquid transition, a hexagonal ABC stacked crystal phase, and a stretched hexagonal-close-packed crystal. The phase diagram for charged spheres is very similar, but displays an additional layered-fluid phase. We compare our results with recent experimental observations.

The effect of dielectric constant mismatch on the binding energy of hydrogenic impurities in cylindrical quantum wires

The ground state binding energy dependency of hydrogenic impurities on dielectric constant mismatch is investigated in cylindrical quantum wires as a function of wire radius and alloy concentration. The calculations are done for a cylindrical GaAs/Ga 1− x Al x As system with a uniform external magnetic field applied parallel to the wire axis using the effective mass approximation and variational methods for finite confinement potentials. According to the obtained results, one can ascertain that in calculating binding energies omitting the dielectric mismatch of the system is justified for large wire radii. However, there is a sign change in contribution for the binding energy for smaller wire radii. Furthermore, the magnitude of the contribution is more pronounced for smaller wire radii and larger alloy concentrations.