Effective Dielectric Permittivity Research Articles

The electronic structures, elastic constants, dielectric permittivity, phonon spectra, thermal properties and optical response of monolayer zirconium dioxide: A first-principles study

Zirconium dioxide (ZrO2) with great mechanical and dielectric properties can be widely used in both structural and functional material fields. In this paper, the first-principles calculations are performed to investigate the geometrical and electronic structures, elastic constants and mechanical stability, Born effective charges and dielectric permittivity, phonon spectra and thermal properties, as well as optical response of monolayer ZrO2. A wide indirect bandgap of 4.41 eV and a p-d hybridized ionic bond are presented in monolayer ZrO2, instead of a zero bandgap and a sp2 hybridized covalent bond in grapheme. Both the large in-plane stiffness of 209.59 N/m and small Poisson's ratio of 0.24 imply monolayer ZrO2 a high strength in-plane and good flexibility out-plane. A strong dynamic charge transfer from Zr to O is existed in monolayer ZrO2. The large bandgap and dielectric permittivity, mechanical and dynamical stability show monolayer ZrO2 a promising two dimensional dielectric material. The high and sharp absorption peaks in the ultra-violet spectral region suggest monolayer ZrO2 a potential application as a ultra-violet detector.

Діелектрометрія гідратації флавінмононуклеотиду і ДНК

Subject and Purpose. The elucidation of the molecular mechanisms of action of biomolecules is necessary for the development of state-of-the-art means of diagnosing and treatment. Dielectric studies in the millimeter wave range are effective for puzzling out the nature of the interaction of biomolecules with a surrounding aqueous solvent. Flavin mononucleotide (FMN), which can kill microorganisms and destroy cancer cells, is of particular interest. The aim of the work is to recognize hydration effects (changes in the state of water molecules) in FMN solutions. Methods and Methodology. The complex dielectric permittivity (CDP) is measured in the EHF range. Knowing the difference between the CDP of FMN solution and the CDP of water we find the difference, D es , between the effective dielectric permittivities in terms of the Debye theory of polar liquids. Since the relaxation time of dipoles of bound water is one or two orders of magnitude longer than that of free water, the amount of the difference D es characterizes the hydration of biomolecules. At low concentrations, this difference is proportional to the number of bound water molecules. Results. It has been shown that approximately18 water molecules are bound to the FMN molecule. Groups of atoms as the most probable hydration centers (primarily due to the hydrogen bonds) have been indicated. As the pH decreases, the number of water molecules bound to the Flavin mononucleotide increases to 21. The study of the FMN–DNA solution has shown that one nucleotide accounts for 25–26 bound water molecules in total. However, composing hydration numbers assumes a quantity of components less than 20. An assumption is made that the additional components are due to the cooperative nature of the hydration, leading to the fact that even if some solvent molecules do not come into a direct contact with hydration centers, they are under the influence of biomolecules all the same. Conclusion. Extremely-high-frequency dielectrometry is an effective method of research into the interaction of biomolecules with a water-ionic solvent. A FMN hydration model has been proposed, which indicates probable hydration centers and tells a measure of their effect on the solvent. It has been found that the FMN with DNA interaction increases the number of bound water molecules per one nucleotide of the DNA. The obtained results have been compared to the existing models of the DNA with FMN interaction.

Features of Reflection from the Layer of Hyperbolic Metamaterial

In this paper, we investigate the features of reflection of a plane elliptically polarized electromagnetic wave fallen from isotropic transparent dielectric on a layer of hyperbolic metamaterial with optical axis in an interface and main effective dielectric permittivities less than the permittivity of dielectric. The cases are analyzed when one (ordinary or extraordinary) wave or two waves, exited inside the layer, are inhomogeneous. It is shown that for certain sub-wavelength thickness of the layer one can choose the angles of incidence for which two inhomogeneous (evanescent) waves existing in this layer totally tunnel through it. It is established that, in contrary to ordinary uniaxial crystal, maximal value of reflection coefficient from the metamaterial layer approaches the unity when the thickness increases.

Local negative permittivity and topological phase transition in polar skyrmions.

Topological solitons such as magnetic skyrmions have drawn attention as stable quasi-particle-like objects. The recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened up new vistas to explore topology, emergent phenomena and approaches for manipulating such features with electric fields. Using macroscopic dielectric measurements, coupled with direct scanning convergent beam electron diffraction imaging on the atomic scale, theoretical phase-field simulations and second-principles calculations, we demonstrate that polar skyrmions in (PbTiO3)n/(SrTiO3)n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion. This enhances the effective dielectric permittivity compared with the individual SrTiO3 and PbTiO3 layers. Moreover, the response of these topologically protected structures to electric field and temperature shows a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity. Pulsed switching measurements show a time-dependent evolution and recovery of the skyrmion state (and macroscopic dielectric response). The interrelationship between topological and dielectric properties presents an opportunity to simultaneously manipulate both by a single, and easily controlled, stimulus, the applied electric field.

Changes in the structural and electrophysical properties of Ba0.8Sr0.2TiO3 films with decreasing thickness

Abstract Comparative studies of structural and electrophysical properties were performed for Ba0.8Sr0.2TiO3 (BSTO) films with the thickness of 100 nm, 120 nm and 350 nm. The surface morphology, the domain structure and local polarization switches were explored in the ferroelectric phase whereas high-frequency capacitance–voltage characteristics were measured in the paraelectric phase at 121°С. It was shown that good crystalline properties of the BSTO surface and grains are preserved until the thickness of ceramic layers decrease to 100 nm. The surface roughness did not exceed 2 – 4 nm and the average radius grains was in the range of 64 – 87 nm. Polarization domains induced with a microscope tip were stable and are conserved for several hours. Electrophysical properties of Ba0.8Sr0.2TiO3 films in the paraelectric phase strongly depend on the thickness of layers of BSTO ceramics. Effective dielectric permittivity of BSTO in the limit of weak fields decreases when the film thickness decreases from 350 nm to 100 nm from 920 to 400.

Homogenization of Maxwell\u2019s equations in a layered system beyond the static approximation

The propagation of electromagnetic waves through a disordered layered system is considered in the paradigm of the homogenization of Maxwell’s equations. Although the accuracy of the effective dielectric permittivity and/or magnetic permeability is still unclear outside the static approximation, we show that the effective wave vector can be correctly introduced even in high-frequency cases. It is demonstrated that both the real and imaginary parts of the effective wave vector are self-averaging quantities connected by the Kramers–Kronig relations. We provide a unified approach to describe the propagation and localization of electromagnetic waves in terms of the effective wave vector. We show that the effective wave vector plays the same role in describing composite materials in electrodynamics as the effective dielectric permittivity does in statics.

Homogenization method for one-dimensional photonic crystals with magnetic and chiral inclusions

We have theoretically investigated the electromagnetic properties for one-dimensional (1D) photonic crystals with magnetic and artificial chiral inclusions in the quasi-static limit, that is when the size of the unit cell of the crystal is small with respect to the wavelength of the operating wave. We suggest a homogenization theory to determine the effective tensors of the optical response, to achieve this objective, we apply the Bloch’s plane waves method to describe the electromagnetic modes that can propagate in the periodic structure under consideration. Subsequently, the Maxwell’s “microscopic” equations are homogenized replacing the Bloch waves by plane waves that attenuate the fast oscillations of the electromagnetic fields within the unit cell (macroscopic level), i.e., the average-macroscopic electromagnetic fields are determined by the component corresponding to the null reciprocal lattice vector in the expansion in plane waves. The numerical implementation of our theory of homogenization allow us to study the effective bianisotropic electromagnetic response (effective dielectric permittivity, magnetic permeability and electric-magnetic coupling tensors, this last is described by an effective chiral parameter) for 1D photonic crystals whose constituents in its unit cell are a dielectric layer and another magnetic (both isotropic and anisotropic) or chiral inclusion layer. The results are illustrated and discussed by the effective parameters as a function of the filling fraction of the inclusion and show for each case of homogeneous effective medium different components of anisotropy in the electromagnetic response of permittivity, permeability and chirality with the increase of the filling fraction. Besides, the behaviors of the obtained graphs agree well with Rytov’s formulas of effective medium. The relevant results of this theory will be very useful for the study and better understanding of the nature and design of metamaterials with predetermined anisotropic optical properties.

Orientational Ordering of Tree Elements in the Dielectric Permittivity Model of a Forest Stand

The dielectric characteristics of trees are studied for a dielectric permittivity model of a forest stand of L1 band signals of navigation satellites. The contribution of the orientational ordering of tree elements and of the wood anisotropy to the effective wood dielectric permittivity e has been estimated. The crown layer has the height-depend anisotropy ∆e and the orientational ordering of branch tensor elements. The anisotropy gradients in the layer of trunks are determined by the height-dependent wood volume of a single tree and the tree height distribution in a large forest.

Frank constants calculation method for erbium-based liquid crystal

The orienting effect of a magnetic field on an erbium-based liquid crystal complex was studied by a capacitance method. The dependence of the effective dielectric permittivities of the complex on the magnetic field is obtained. A theoretical approach and a numerical method for determining the Frank elastic constants are proposed based on the experimental dependence of the effective values of the permittivity on various magnetic fields.

Complex Effective Dielectric Permittivity and Characteristic Impedance of Tunable Coplanar Line

An analysis of complex dielectric permittivity and characteristic impedance of micromechanically tunable coplanar line is presented. The coplanar line parameters tuning is achieved by signal line electrode movement above the substrate or the dielectric plate above the surface of line electrodes. A reconfiguration of electromagnetic field with complex nature occurs as a result of such movement in the line. It is described in terms of effective permittivity and characteristic impedance. We studied an influence of physical and geometrical parameters of the line on characteristics of effective permittivity tuning and change in characteristic impedance and line loss. It is found that proposed method for line tuning parameters allows us to obtain a high sensitivity to movement for effective parameters, wherein the level of losses in the line is not deteriorated, and under certain conditions are reduced. These results make it possible to design high-quality tunable resonant elements and phase shifters based on micromechanically controlled coplanar line.

Theoretical investigation of enhanced sensing property in 1D TiO2/SiO2 periodic layers containing a defect layer of the nanocomposite with different radii of silver nanoparticles in the host liquid crystal

In this paper, we have theoretically investigated the effective dielectric permittivity of the nanocomposite (NC) layer of liquid crystal (LC) with silver nanoparticles at different radii using the Maxwell-Garnett model. The optical properties of a one-dimensional periodic structure (1DPS) of TiO2/SiO2 dielectric materials containing defect NC layer, (TiO2|SiO2)3|NC|(TiO2|SiO2)3, with different radii of silver nanoparticles studied theoretically using the well-known transfer matrix method (TMM). The effective permittivity of NC for = 0° and 90° orientation angles of LC is calculated with varying sizes of the nanoparticles because the size of the silver nanoparticles affects the surface plasmon resonance (SPR). The transmission of defect mode wavelength peaks for 0° and 90° orientation angle of the LC is studied for different radii of nanoparticles. Besides this, we have also studied the absorption characteristics of the considered NC defect structure for different orientation of the LC with varying radii of the nanoparticles.

Dissipative particle dynamics for systems with polar species: Interactions in dielectric media.

In this work, we develop a method for simulating polar species in the dissipative particle dynamics (DPD) method. The main idea behind the method is to treat each bead as a dumb-bell, i.e., two sub-beads kept at a fixed distance, instead of a point-like particle. The relation between the bead dipole moment and the bulk dielectric permittivity was obtained. The interaction force of single charges in polar liquid showed that the effective dielectric permittivity is somewhat smaller than that obtained for the bulk case at large separation between the charges. In order to understand the reasons behind the observed drop in the dielectric permittivity, we calculate the electric field of an isolated charge in a polar liquid; no permittivity drop is observed for this case. We can assume that the behavior observed for the force is due to the fact that the probing point is always associated with the charged bead, which is a force center, which essentially leads to a non-homogeneous density distribution around it on average; this is not the case when the field is measured. The interaction of a single charge with an interface between two liquids with different permittivities was studied after that; the model is found to correctly reproduce the "mirror image" effects. Finally, we show why it is necessary to treat the polar species in DPD explicitly by investigating the behavior of a charged colloidal particle at a liquid-liquid interface.

Homogenization of time-harmonic Maxwell’s equations in nonhomogeneous plasmonic structures

We carry out the homogenization of time-harmonic Maxwell’s equations in a periodic, layered structure made of two-dimensional (2D) metallic sheets immersed in a heterogeneous and in principle anisotropic dielectric medium. In this setting, the tangential magnetic field exhibits a jump across each sheet. Our goal is the rigorous derivation of the effective dielectric permittivity of the system from the solution of a local cell problem via suitable averages. Each sheet has a fine-scale, inhomogeneous and possibly anisotropic surface conductivity that scales linearly with the microstructure scale, d. Starting with the weak formulation of the requisite boundary value problem, we prove the convergence of its solution to a homogenization limit as d approaches zero. The effective permittivity and cell problem express a bulk average from the host dielectric and a surface average germane to the 2D material (metallic layer). We discuss implications of this analysis in the modeling of plasmonic crystals.

Surface-Enhanced Raman Spectroscopy on Hybrid Graphene/Gold Substrates near the Percolation Threshold.

Graphene is a promising platform for surface-enhanced Raman spectroscopy (SERS)-active substrates, primarily due to the possibility of quenching photoluminescence and fluorescence. Here we study ultrathin gold films near the percolation threshold fabricated by electron-beam deposition on monolayer CVD graphene. The advantages of such hybrid graphene/gold substrates for surface-enhanced Raman spectroscopy are discussed in comparison with conventional substrates without the graphene layer. The percolation threshold is determined by independent measurements of the sheet resistance and effective dielectric constant by spectroscopic ellipsometry. The surface morphology of the ultrathin gold films is analyzed by the use of scanning electron microscopy (SEM) and atomic force microscopy (AFM), and the thicknesses of the films in addition to the quartz-crystal mass-thickness sensor are also measured by AFM. We experimentally demonstrate that the maximum SERS signal is observed near and slightly below the percolation threshold. In this case, the region of maximum enhancement of the SERS signal can be determined using the figure of merit (FOM), which is the ratio of the real and imaginary parts of the effective dielectric permittivity of the films. SERS measurements on hybrid graphene/gold substrates with the dye Crystal Violet show an enhancement factor of ~105 and also demonstrate the ability of graphene to quench photoluminescence by an average of ~60%.

The effect of temperature and graphene concentration on the electrical conductivity and dielectric permittivity of graphene–polymer nanocomposites

Several recent experiments have revealed the remarkable influence of temperature and graphene concentration on the effective electrical properties of graphene–polymer nanocomposites, but no theory seems to exist at present to quantify such dependence. In this work, we develop a novel micromechanics-based homogenization scheme to connect the microstructural features of constituent phases to the temperature-dependent macroscopic conductivity and permittivity for the nanocomposites. The key microstructural features include the graphene volume concentration, temperature-dependent electrical properties of constituent phases, percolation threshold, imperfect mechanical bonding effect with temperature-degraded interlayer, and the temperature-dependent electron tunneling and Maxwell–Wagner–Sillars polarization. We consider the activation of free electrons and polarization of molecules to write the constitutive equations of polymer, and the collision and vibration probabilities to write those of graphene. We highlight the developed theory with a direct comparison to the experimental data of rGO/epoxy nanocomposites over the temperature range from 293 to 353 K. It shows that before the percolation threshold, the effective electrical conductivity and dielectric permittivity markedly increase with temperature, but after the percolation threshold, the influence of temperature diminishes significantly. In the latter case, the effective permittivity increases only slightly, while the conductivity exhibits an opposite trend.

Temperature dependent conductivity and broadband dielectric response of precursor-derived Nb2O5

Different polymorphs of niobium pentoxide (Nb2O5) were synthesized using niobium ethoxide as a precursor by varying the pyrolyzing temperature. The room temperature X-ray diffractograms revealed the irreversible phase evolution from amorphous to pseudohexagonal (823 K) to orthorhombic (1023 K) and to monoclinic crystal structure (1223 K). While phase evolution was also confirmed by thermogravimetry and dilatometry, Raman spectroscopy clearly suggested complete elimination of free carbon in the pyrolysed ceramics. The sintering conditions were optimized to produce a highly dense (>95%) thermodynamically stable monoclinic Nb2O5. The electrical properties of stable monoclinic Nb2O5 sample were thoroughly studied. The monoclinic Nb2O5 was found to have a dielectric constant of around 28 with a dielectric loss of 0.008 at room temperature and at 100 kHz. At low measurement frequencies, an anomalous increase in the effective dielectric permittivity with increasing temperature was observed. Large values of the ε’ are associated with polarization due to the accumulation of free electrons at the grain boundaries. An analysis of the dispersion curves of Nb2O5 revealed that two relaxation processes are responsible for the observed anomalies, and the temperature dependencies of their parameters (dielectric strength, relaxation time and spectrum broadening parameter) were determined. The low-frequency process (relaxation time τ0 ~ 0.45 s), which makes the largest contribution to the dielectric constant, was apparently due to the inhomogeneous conductivity of ceramics. It was revealed that the DC conductivity of ceramics has thermoactivation character with activation energy of about 660 meV and was determined by the oxygen vacancies.

Porosity-dependent percolation threshold and frequency-dependent electrical properties for highly aligned graphene-polymer nanocomposite foams

Highly aligned graphene-polymer nanocomposite foams are of great interest due to their excellent electrical conductivity and dielectric permittivity along the aligned direction while maintaining flexibility and lightweight, but no theory seems to exist to quantify their frequency-dependent phenomena at present. In this work, we develop a two-scale effective-medium theory under the complex setting to study the porosity-dependent percolation threshold and the frequency-dependent electrical properties for such nanocomposite foams over a wide range of alternating current (AC) frequency. The key features of the theory include the maximum distribution angle, graphene volume concentration, percolation threshold, porosity, Maxwell-Wagner-Sillars polarization, electronic tunneling, and the frequency-dependent Dyre electron hopping and Debye dielectric relaxation at the interface. We highlight the developed theory with a direct comparison to experimental data of highly aligned graphene/epoxy nanocomposite foam over the frequency range from 102 to 106 Hz. It shows that the graphene percolation threshold decreases markedly with respect to porosity. In addition, the effective electrical conductivity and dielectric permittivity of highly aligned graphene-polymer nanocomposite foam, including the in-plane and out-of-plane ones, increase and decrease with respect to AC frequency, respectively.

Single-Stage Formation of Film Polymer Composites in Supercritical Colloid Solutions of Nanoparticles Obtained by Laser Ablation

Abstract—A review of earlier studies is presented together with new results on the creation of highly concentrated colloid solutions with nanoparticles of noble metals (gold and silver) in a supercritical carbon dioxide (SC–CO2) medium and the development of a single-stage method for the production of polymer nanocomposites. The effect of the SC–CO2 density on the dynamics of the formation and decomposition of supercritical fluids (SCFs) of silver and gold nanocolloids in the process of pulsed laser ablation and after its termination is studied. The possibility of the formation of colloids from bimetallic nanoparticles was considered. The effective dielectric permittivity of a colloid solution of silver nanoparticles in SC–CO2 is calculated by the numerical modeling of extinction spectra in the dipole approximation at various densities, with consideration for the size of nanoparticles and the degree of their asphericity. The formation of various film nanocomposites based on a porous copolymer polytetrafluoroethylene—vinylidenefluoride with ruby nanoparticles and two modifications of pure porous polytetrafluoroethylene with silver nanoparticles in the same SCF reactor and with a single technological process is considered.

Microwave refraction coefficient in nanocomposites and nanostructures in magnetic field

Microwave refraction coefficient is studied for two types of composites and for metallic (CoFe)/Cu nanostructures. Nanocomposites of the first type are prepared on the base of artificial opal matrices with embedded particles of nickel-zinc ferrite and metallic cobalt. The introduced particles are sized from 5 to 60 nm. The composite material of the second type is prepared of permalloy flakes sized about 2 – 50 μm placed into epoxy amine matrix. At frequencies of centimeter and millimeter waveband the magnetic field dependences of transmission and reflection coefficients have been measured. The complex refractive coefficients are obtained as a function of magnetic field. The complex refraction coefficient can be expressed via the effective dielectric permittivity and magnetic permeability. Analysis of magnetic field dependences of refraction coefficient shows that strong variations are observed near ferromagnetic resonance. For metallic nanostructures there are also variations of the refraction coefficient caused by giant magnetoresistive effect.

Hyperbolic Metamaterials at Microwaves With Stacked Inductive Coiled-Wire Arrays

Metasurfaces made of arrayed wires are inductive only for propagating waves and become capacitive when illuminated by evanescent waves. We show how to extend the inductive behavior of such metasurfaces for the case of illumination by plane waves with large wavenumber, i.e., for evanescent waves. The proposed metasurface exhibits an effective surface inductance that is extended to evanescent waves with large transverse wavenumber using coiled wires. Metasurfaces with inductive response over a wide spatial spectrum can be used for the realization of hyperbolic metamaterials (HMs) at microwave frequencies. Stacking such inductive metasurfaces separated by thin dielectric spacers leads to a negative real part of the in-plane, effective dielectric permittivity, and thus, to hyperbolic wavevector dispersion at microwave frequencies. We establish the requirements for such hyperbolic dispersion condition using arrays of coiled wires, also accounting for losses. We show that the negative in-plane real part of dielectric permittivity stretches up to transverse wavenumbers that are several times larger than the free-space wavenumber, over a broad frequency bandwidth. The theory and important design analytic formulas are presented to demonstrate the inductive behavior, as well as the propagative nature of waves in the proposed HM, in agreement with full-wave simulations.