Low-frequency Dielectric Constant Research Articles

Breakdown of the Static Dielectric Screening Approximation of Coulomb Interactions in Atomically Thin Semiconductors.

Coulomb interactions in atomically thin materials are remarkably sensitive to variations in the dielectric screening of the environment, which can be used to control exotic quantum many-body phases and engineer exciton potential landscapes. For decades, static or frequency-independent approximations of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, have been sufficient. These approximations were first applied to quantum wells and were more recently extended with initial success to layered transition metal dichalcogenides (TMDs). Here, we use charge-tunable exciton resonances to investigate screening effects in TMD monolayers embedded in materials with low-frequency dielectric constants ranging from 4 to more than 1000, a range of 2 orders of magnitude larger than in previous studies. In contrast to the redshift predicted by static models, we observe a blueshift of the exciton resonance exceeding 30 meV in higher dielectric constant environments. We explain our observations by introducing a dynamical screening model based on a solution to the Bethe-Salpeter equation (BSE). When dynamical effects are strong, we find that the exciton binding energy remains mostly controlled by the low-frequency dielectric response, while the exciton self-energy is dominated by the high-frequency one. Our results supplant the understanding of screening in layered materials and their heterostructures, introduce a knob to tune selected many-body effects, and reshape the framework for detecting and controlling correlated quantum many-body states and designing optoelectronic and quantum devices.

Building Water Models Compatible with Charge Scaling Molecular Dynamics.

Charge scaling has proven to be an efficient way to account in a mean-field manner for electronic polarization by aqueous ions in force field molecular dynamics simulations. However, commonly used water models with dielectric constants over 50 are not consistent with this approach leading to "overscaling", i.e., generally too weak ion-ion interactions. Here, we build water models fully compatible with charge scaling, i.e., having the correct low-frequency dielectric constant of about 45. To this end, we employ advanced optimization and machine learning schemes in order to explore the vast parameter space of four-site water models efficiently. As an a priori unwarranted positive result, we find a sizable range of force field parameters that satisfy the above dielectric constant constraint providing at the same time accuracy with respect to experimental data comparable with the best existing four-site water models such as TIP4P/2005, TIP4P-FB, or OPC. The present results thus open the way to the development of a consistent charge scaling force field for modeling ions in aqueous solutions.

The effect of fluorination on the low and high frequency dielectric constants of non-polymeric organic semiconductors – towards homojunction solar cells

Fluorination of a non-polymeric donor–acceptor–acceptor–donor organic semiconductor leads to an increase in the thin-film low frequency and decrease in the optical frequency dielectric constant relative to the protonated material.

Оценки диэлектрических и оптических характеристик бинарных 3D- и 2D-соединений IV группы

Using the Harrison’s bond orbital model for the IV group 3D and 2D com-pounds analytical expressions of the high- and low-frequency dielectric suscepti-bilities and dielectric constants, linear electrooptical coefficient, photoelastic con-stants and pressure dependences of the dielectric constants are obtained.

Estimates of the dielectric and optical characteristics for the IV group 3D- and 2D-binary compounds

Using the Harrison's bond orbital model for the IV group 3D- and 2D-compounds analytical expressions of the high- and low-frequency dielectric suscepti-bilities and dielectric constants, linear electrooptical coefficient, photoelastic con-stants and pressure dependences of the dielectric constants are obtained. Keywords:: dielectric susceptibility and permittivitiy, photoelectric coefficient.

Dielectric behavior of nanocrystalline Cu-Fe ferrites synthesized by plasma arc discharge process

In this study, nanocrystalline Cu–Fe ferrite powders with the chemical composition of CuxFe1-xFe2O4 (where 0 ≤ x ≤ 0.5) were synthesized via the plasma arc discharge process. Regarding the phase analysis, a spinel structure of the nanocrystalline Cu–Fe ferrite phase has been developed successfully in all synthesized compositions. Dielectric properties were measured in the 100 Hz to 10 MHz range using an LCR meter. Results show that the substitution of Cu for Fe ions leads to a substantial increase in the dielectric constant due to the migration of some Fe3+ ions from tetrahedral to octahedral sites and the formation of Cu+/Cu2+ ion pairs in the octahedral sites contributing to the hopping process. The results also indicate that the low-frequency dielectric constant is influenced by grain boundary capacitance while grain capacitance plays a major contribution to the dielectric constant in the high-frequency region.

Dielectric constant of disordered phases of the smallest monoalcohols: Evidence for the hindered plastic crystal phase

With gradual temperature increase in premelting regions of solid phase of methanol and high pressure phase of ethanol, and using novel procedure of separation of electrode polarization effects, we are able to register the contribution of relaxation process to low-frequency dielectric constant. This contribution is about half the liquid’s dielectric constant near temperature of solidification, and is almost an order of magnitude higher than reported earlier for ambient pressure phase of methanol. As opposed to dielectric constant of water at ambient pressure, which does not change much during crystallization, our finding indicates the hindrance of molecule rotation in orientationally disordered phases of monoalcohols. Similar dielectric responses of ambient pressure methanol and high-pressure phase of ethanol imply existence of hindered plastic crystal phase of ethanol (not observed at low pressures). We also have found some dynamic disorder in nominally fully ordered phase of these monoalcohols.

Диэлектрические и оптические свойства кубических монокристаллов SiC, GeC и SnC: модельные оценки

Within the scope of the Harrison’s bond orbital model analytical expressions of the high- and low-frequency dielectric susceptibilities and dielectric constants, linear electrooptical coefficient, photoelastic constants and pressure dependences of the dielectric constants are obtained for the cubic carbides of the group IV elements.

Dielectric and optical properties of the cubic SiC, GeC and SnC monocrystals: Model estimations

Within the scope of the Harrison's bond orbital model analytical expressions of the high- and low-frequency dielectric susceptibilities and dielectric constants, linear electrooptical coefficient, photoelastic constants and pressure dependences of the dielectric constants are obtained for the cubic carbides of the group IV elements. Keywords: dielectric susceptibility, dielectric constants, electrooptical coefficient, photoelastic constant.

Dielectric permittivity of organosilicate glass thin films on a sapphire substrate determined using time-domain THz and Fourier IR spectroscopy

The engineering of dielectrics such that they have low dielectric constants is an important challenge in the semiconductor industry. The IR absorption and THz relaxation make a significant contribution to the low-frequency dielectric constant of porous organosilicate glass (OSG) thin films used in the interconnects of integrated circuits as low-k dielectrics. Determination of the dielectric contributions with the aid of the electric dipole absorption bands in the THz region has certain methodological limitations owing to the effect of the substrate. To overcome this problem, we used a c-cut sapphire plate as a substrate, which is optically isotropic and transparent in the THz range. The parameters of both the IR and THz absorption bands in the OSG films were measured using Fourier IR and pulsed THz spectroscopy and analysed using the classical oscillator model. The contribution of the THz relaxation is responsible for approximately 15%–17% of the value of the low-frequency dielectric constant.

Systematic characterization of THz dielectric properties of multi-component glasses using the unified oscillator model

A terahertz (THz) dielectric property characterization method based on a unified single oscillator model has been developed and applied to a variety of multi-component silicate oxide glasses. The experimental values of dielectric constant determined by THz time-domain spectroscopy (TDS) in the sub-THz region have been confirmed to agree well with the values calculated by the single oscillator model which incorporates the local field effects and the material’s ionicity. This has provided a unified formulation that enables systematic determination of the key physical parameters solely from the high-frequency (optical) and low-frequency (sub-THz) dielectric constants and characteristic resonance frequency in the (sub-)THz region. The low-frequency dielectric constant has been demonstrated to be fully determined by a single parameter of the microscopic total susceptibility. Also, the polarization ionicity, which is defined by the ionic fraction in the microscopic total susceptibility, has been found to be a good indicator to represent the ionic nature of the material. Through this analysis, an increasing trend of the effective ionic charge has been found in high-dielectric constant glasses such as oxyfluorosilicate glasses, and the physical mechanism of their dielectric constant enhancement has been discussed. The present method is expected to be applied to design and characterize dielectric properties of a wide range of multi-component glasses and other isotropic, insulating materials.

Structural, elastic, optical and dielectric properties of Li0.5Fe2.5 O4 nanopowders with different particle sizes

LiFe5O8 (Li0.5Fe2.5O4) nanopowders were synthesized by the sol-gel auto-combustion method and were annealed at different temperatures of 500, 600, 700, and 1100 °C. The spinel phase formation of these samples has been confirmed by X-ray powder diffraction (XRD) technique and Fourier transform infrared (FTIR) spectroscopy. XRD analysis revealed order to disorder phase transition of Li0.5Fe2.5O4 during the annealing temperature. This structural transition was also confirmed by differential scanning calorimetry (DSC). The scanning electronic microscopy images reveal that nanoparticles size increases from 9 nm to 500 nm with increasing annealing temperature. The influence of particle size on elastic parameters has been investigated by FTIR spectroscopy. The band gap energy estimated by UV–vis spectroscopy of LiFe5O8 nanoparticles decreased from 1.41 to 1.27 eV when the annealing temperature increases. Furthermore, impedance spectroscopy measurements have been carried out over large frequency and temperature ranges. The conductivity spectrum indicates that the tested specimens are defective. It also shows that the conductivity runs to frequency according to Jonscher’s law. Static conductivity responds to temperature in conformity with a small polaron hopping model whereas dispersive one evaluates according to correlated hopping model. The conductivity gets mixed (electronic and ionic) only when samples were annealed at 700 °C and 1100 °C. According to the Nyquist diagram, each sample is capacitive and resistive. Increasing annealing temperature led to major improvements in permittivity, and a giant low-frequency dielectric constant was observed in the sample annealed at 1100 °C. The total loss (ɛ″) is governed by the conduction mechanism and described by Giuntini's theory. The improved electrical, optical, and elastic properties indicates the potential application LiFe5O8 nanoparticles in future multifunctional devices (microelectro-mechanical systems (MEMS), optoelectronic and photovoltaic, photocatalytic activity under visible light, bolometer, low temperature co-fired ceramics (LTCC) and gas sensor applications.).

Discovery of highly polarizable semiconductors BaZrS3 and Ba3Zr2S7

There are few known semiconductors exhibiting both strong optical response and large dielectric polarizability. Inorganic materials with large dielectric polarizability tend to be wide-band gap complex oxides. Semiconductors with strong photoresponse to visible and infrared light tend to be weakly polarizable. Interesting exceptions to these trends are halide perovskites and phase-change chalcogenides. Here we introduce complex chalcogenides in the Ba-Zr-S system in perovskite and Ruddlesden-Popper structures as a new family of highly polarizable semiconductors. We report the results of impedance spectroscopy on single crystals that establish BaZrS3 and Ba3Zr2S7 as semiconductors with low-frequency relative dielectric constant (${\epsilon}_0$) in the range 50 - 100, and band gap in the range 1.3 - 1.8 eV. Our electronic structure calculations indicate the enhanced dielectric response in perovskite BaZrS3 versus Ruddlesden-Popper Ba3Zr2S7 is primarily due to enhanced IR mode-effective charges, and variations in phonon frequencies along $\langle 001 \rangle$; differences in the Born effective charges and the lattice stiffness are of secondary importance. This combination of covalent bonding in crystal structures more common to complex oxides results in a sizable Fr\"ohlich coupling constant, which suggests that charge carriers are large polarons.

Hydrothermal synthesis of Ba1−xLaxTiO3 (x = 0.2, 0.4, 0.6, & 0.8) nanorods: structure, morphology, optical band gap, and dielectricity behavior

A series of Ba1−xLaxTiO3 (x = 0.2, 0.4, 0.6, & 0.8) nanorods were synthesized through hydrothermal method. The X-ray diffraction patterns indicated the cubic phases for x = 0.2 – 0.8 contents. The TEM pictures revealed the formation of nanorods of high length. The Tauc’s plots showed the wide band gap (Eg) for x = 0.2 & 0.6 compositions while the narrow band gap was noticed for x = 0.4 & 0.8 contents. At low-frequency (dielectric constant = − 1051.4 at log ω = 2.4627) values, the negative dielectric constant and dielectric loss were observed at x = 0.2. This behavior was similar to the metamaterial. At 8 MHz frequency, the high dielectric constant of 35.7 was noticed for x = 0.2. In addition, the frequency dependence of ac-electrical conductivity was also studied.

Realistic dielectric response of high temperature sintered ZnO ceramic: a microscopic and spectroscopic approach.

High temperature sintering (1200–1400 °C) has been performed on ZnO ceramics. An X-ray Absorption Fine Structure (XAFS) study shows that high sintering temperature introduces a constant amount of VO and VZn defects without any significant effect on the crystal or electronic structure of Wurtzite ZnO. The combined effects of grain boundaries and voids are considered responsible for the apparent colossal dielectric constant (ε′) > 104 at low frequency (∼102 Hz) for all the sintered ZnO ceramics. The superior contact among grains of the ZnO-1200 sample enhances both the interfacial and orientational polarization of the Zn2+–VO dipoles, which results in the increase of low and high frequency dielectric constants (ε′) and the corresponding dielectric loss (tan δ) also increases. On the other hand, high temperature sintering of ZnO at 1300 °C and 1400 °C introduces voids at the expense of reduced grain and grain boundary contact areas, thus affecting both the interfacial and orientational polarization with corresponding reduction of dielectric constant (ε′) and dielectric loss. Orientational polarizations due to Zn2+–VO dipoles are suggested to remain fixed and it is the microstructure which controls the dielectric properties of high temperature sintered ZnO ceramics.

Microscopic approach for low-frequency dielectric constant of liquid water

ABSTRACT A theoretical model has been proposed for interpreting the dispersion of the low-frequency dielectric constant of liquid water with two separate arguments. The first argument relates to dipole orientation in the water system and the second one concerns motions of ions towards the electrodes. It reveals that the compensation between these two arguments leads to the isopermittive point, at which the water dielectric constant holds constant in the range of temperature from 301 K to 313 K. The existence of the isopermittive point is also highlighted under the view from the basis of thermodynamics. We show that the isopermittive point only exists in a narrow region of temperature. The changes in enthalpy and Gibbs free energy are estimated, using van’t Hoff equation, for the water system in the equilibrium. Our theoretical model can help in the perspective research on dynamic processes happening in live cells around the crossing point.

PVDF composites with spherical polymer-derived SiCN ceramic particles have significantly enhanced low-frequency dielectric constants

We report a novel functional hybrid film fabricated by using polysilazane-derived SiCN ceramics as fillers and polyvinylidene fluoride (PVDF) as the matrix. Regular spherical-shaped and amorphous SiCN ceramic powders were fabricated via a facile solvothermal method combined with a polymer-derived ceramic (PDC) process. The dielectric properties of the SiCN/PVDF composites were investigated by broadband dielectric spectroscopy from 0.1 Hz to 106 Hz. The composites showed significantly enhanced low-frequency dielectric constants. When the SiCN loading content reached 40 vol%, the dielectric constant of the composites was approximately 360 at 0.1 Hz, which is 20 times the value of pure PVDF (18). The dielectric constant of the composites at 0.1 Hz fit the effective medium theory (EMT) model well. The results suggest that the SiCN/PVDF composites are promising dielectric materials and have potential use in electrical applications.

Effect of oxygen content on dielectric characteristics of Cr-doped LaTiOx**Project supported by the National Natural Science Foundation of China (Grant No. 51571006).

The ceramics La0.85Cr0.15TiOx and La0.7Cr0.3TiOx are prepared by conventional solid-state reaction method. The dielectric properties of Cr-doped LaTiOx as a function of frequency (0.1 kHz ≤ f ≤ 1 MHz) and temperature (77 K ≤ T ≤ 360 K) are studied. The blocks are then annealed in a flowing O2 or Ar/H2 to convert their oxygen content and the tests mentioned above are performed. The highly oxygenated samples exhibit extremely high low-frequency dielectric constants at room temperature (∼106). The results show that the oxygen stoichiometry could significantly influence the dielectric properties of Cr-doped LaTiOx.

Bias dependent conduction and relaxation mechanism study of Cu5FeS4 film and its significance in signal transport network

Here we have reported the electric and dielectric properties of a Bornite (Cu5FeS4) film using non-destructive complex impedance spectroscopy (CIS) in the frequency range 40 Hz–20 MHz and bias voltage range 0–0.8 V. A typical hydrothermal reaction was carried out to obtain the Cu5FeS4 material. Thereafter, we fabricated the film of the synthesized material and analysed its various properties as a function of frequency and dc bias voltages. The CIS analysis at room temperature (303 K) for different forward dc bias voltages shows that the bulk resistance plays a predominant role in conduction mechanism. The complex impedance (Nyquist) plots are well modelled with grain and grain boundary resistance by introducing proper ac equivalent circuit. The impedance loss spectra showed that the relaxation peak shifts towards the higher frequency with increasing bias voltages, which implies the possibility of a charge hopping between the localized charge states. Furthermore, we find that dielectric constant ( $${\varepsilon ^\prime }$$ ), dielectric loss ( $${\varepsilon ^{\prime \prime }}$$ ), electrical modulus ( $${{\text{M}}^*}$$ ) and ac/dc conductivity of the Cu5FeS4 film are strongly frequency dependent. The values of $${\varepsilon ^\prime }$$ and $${\varepsilon ^{\prime \prime }}$$ decreases whereas ac conductivity increases with increasing frequency. High frequency and low frequency dielectric constant of the material are found to be 3.48 and in the order of 104 respectively. Electrical modulus study is introduced for better explanation of conductivity relaxation phenomenon. Finally, we investigated the variation of ac/dc conductivity with forward dc bias voltages. This confirmed the presence of a hopping mechanism for electrical transport in our system, which can be best explained on the basis of jump relaxation model. Overall, this extensive study based on complex impedance spectroscopy demonstrates the dielectric relaxation behaviour of Cu5FeS4 film and also shed light on hopping induced conduction mechanism.

DC-bias and visible light effect on dielectric characteristics of La0.5Cr0.5TiO3+δ

La0.5Cr0.5TiO3+δ ceramic sample was prepared via traditional solid-state reaction route. Frequency and temperature dependence of dielectric permittivity were studied in the range of 102 ~ 106Hz and of 77 ~ 360K, respectively. It was observed that extraordinarily high low-frequency dielectric constants appeared at room temperature, and dielectric relaxation peaks shifted to higher temperature with increasing frequency. In the dc-bias studies, it was also found that the dielectric permittivity had obviously dc-bias dependence in low frequency, but independence as the frequency above 14kHz. Interestingly, the dielectric characteristics of the sample had obvious light dependence at room temperature within the measured frequency range. The results demonstrate that visible light improves the dielectric properties of the ceramic by means of I–V and complex impedance analysis.