Local Permittivity Research Articles

Li-ion solvation structure at electrified solid-liquid interface: Understanding solvation structure dynamics and its role in electrochemical energy storage through binary ethylene carbonate and dimethyl carbonate solvent.

Binary solvent electrolytes can provide interpretations for designing advanced electrolytes of next generation batteries. This study investigates the adsorption mechanisms of solvated lithium ions in binary solvents near charged electrodes. Molecular dynamic simulations are performed for lithium hexafluorophosphate (LiPF6) in ethylene carbonate and dimethyl carbonate (EC:DMC) solvent sandwiched between two electrodes. Results show that lithium ions form a tetrahedral solvation structure with two EC and two DMC molecules. The solvated lithium ion shows anti-electrostatic interaction with electrodes. This can be attributed to the electrostatic attraction of the polar end of the DMC molecule, which keeps the cation anchored to the positive electrode. Meanwhile, the solvation structure adopts a fix orientation at the negative electrode, which leads to unchanged electrostatic interaction at high charge density. Finally, EC molecules are swapped by DMC molecules near the negative electrode at high charge density. This leads to a decrease in local relative permittivity and, therefore, a decrease in differential capacitance. The differential capacitance of the positive electrode continuously decreases with increasing charge density. This is caused by the partial anchoring of solvent molecules holding the cations, which cancels the adsorption of anions near the positive electrode. This study provides insights into designing better electrolytes for efficient battery performance.

Dynamic permittivity of confined water under a static background field.

Molecular and collective reorientations in interfacial water are by-and-large decelerated near surfaces subjected to outgoing electric fields (pointing from surface to liquid, i.e., when the surface carries positive charge). In incoming fields at negatively charged surfaces, these rates show a nonmonotonic dependence on field strength where fastest reorientations are observed when the field alignment barely offsets the polarizing effects due to interfacial hydrogen bonding. This extremum coincides with a peak of local static permittivity. We use molecular dynamics simulations to explore the impact of background static field on high frequency AC permittivity in hydration water under an electric field mimicking the conditions inside a capacitor where one of the confinement walls is subject to an outgoing field and the other one to an incoming field. At strong static fields, the absorption peak undergoes a monotonic blue shift upon increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under an incoming field (the negative capacitor plate) features a red shift coinciding with maximal static-permittivity and reorientation-rate. The shift is mostly determined by the variation of the inverse static dielectric constant as proposed for mono-exponentially decaying polarization correlations. Conversely, hydration water at the opposite (positively charged) surface features a monotonic blue shift consistent with conventional saturation. The sensitivity of absorption peaks on the field suggests that surface charge densities could be deduced from sub-THz dielectric spectroscopy experiments in porous materials when interfaces accommodate a major fraction of water contained in the system.

Water Transport Management in a Bipolar Membrane for CO2 Electrolysis Application

Bipolar membranes (BPMs) offer significant potential for electrochemical technologies, especially in CO2 electrolysis. Their unique ability to create pH gradients provides optimum chemical microenvironments at the cathode and anode for efficient, cheap and sustainable CO2 electrolysis. However, state-of-the-art BPMs are limited in their performance due to phenomena such as membrane degradation and membrane dehydration leading to low ionic conductivity, and hence their complete potential for various electrochemical technologies is yet to be realized. Previous studies have identified water transport as one of the main factors limiting the performance of BPM. A multi-physics model of BPM can serve as a useful tool to gain a comprehensive understanding of different performance limiting factors and thereafter propose solutions to overcome these limitations. The goal of the present study is to provide model based design guidelines for a BPM that facilitates efficient water transport, making it a suitable candidate for integration into a zero-gap CO2 reduction electrolyzer operating at industrial-grade performance levels.We have built a 1-D continuum model of a BPM kept in an aqueous (bi)carbonate electrolyte solution, incorporating physical phenomena such as water-dependent transport of ions and neutral species, field-enhanced homogeneous reactions (water-dissociation and acid-base reactions) along with CO2 phase transfer reaction at the BPM-aqueous electrolyte interfaces. The continuum model employs Poisson-Nernst-Planck equations to resolve local chemical species concentration. The effect of the electric field on the equilibrium dissociation constants of various charge-generating homogenous reactions is incorporated. This becomes important especially at interfaces within the domain where a sharp increase in electric field is observed. Unlike previous modeling studies, we also incorporate mass-conservation equation for water transport to resolve local water concentration inside the BPM. Local water concentration influences the transport of species in the BPM through their water-dependent activity and diffusivity. The electric potential distribution is resolved by incorporating water-dependent local permittivity of the BPM.Our model investigates both the reverse and forward bias operational modes of the BPM. In alignment with the experimental observation, in the reverse bias regime, the model identifies a distinct initial phase where current density is limited, with the majority of the current carried by protons and hydroxide ions at low overpotentials. As the potential increases, there's a rapid increase in current density, indicating a breakdown regime caused by an electric field-enhanced water dissociation reaction at the BPM interface. At even higher potentials, the polarization curve enters a third phase where the current density saturates characterized by water transport limitation. This aspect of the BPM polarization curve is uniquely captured by our model. Conversely, in the forward bias mode, as expected we do not observe any limitations due to mass transport, since water is generated at the bipolar junction. Here, the model forecasts a saturation of CO2 in the electrolyte, leading to gas bubbles forming at the BPM interface. To address the issue of water transport limitation, we conduct various parametric studies focusing on different membrane characteristics, such as individual layer thicknesses and their ion exchange capacities. The outcomes of these analyses provide crucial guidelines for optimizing these parameters and enhancing water management in BPM-based systems.

In-plane distribution of huge local permittivity of KTa1-xNbxO3 estimated from local phase transition temperatures and spatially averaged permittivity.

Potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystals have a very large relative permittivity εr (>104) just above the paraelectric to ferroelectric phase transition temperature (TC). The quadratic electro-optic coefficient and the electro-strictive coefficient are also very large because of their proportionality to εr2. However, the local relative permittivity can easily vary spatially due to the incongruently melting nature of KTN. In this study, we quantitatively estimated the in-plane distribution of the huge local relative permittivity of KTN. First, we measured the spatial distribution of TC using scanning nonlinear dielectric microscopy, then deposited the electrodes and measured the temperature dependence of the spatially averaged permittivity using an LCR meter. Following that, we evaluated the spatial distribution of the huge local permittivity from the combination of the spatial distribution of TC and the spatially averaged permittivity. Finally, we measured the deflection angle of light to confirm the validity of the εr estimation procedure. The maximum error for the estimated permittivity was estimated to be around 3.3%.

Single-Use, Metabolite Absorbing, Resonant Transducer (SMART) Culture Vessels for Label-Free, Continuous Cell Culture Progression Monitoring.

Secreted metabolites are an important class of bio-process analytical technology (PAT) targets that can correlate to cell conditions. However, current strategies for measuring metabolites are limited to discrete measurements, resulting in limited understanding and ability for feedback control strategies. Herein, a continuous metabolite monitoring strategy is demonstrated using a single-use metabolite absorbing resonant transducer (SMART) to correlate with cell growth. Polyacrylate is shown to absorb secreted metabolites from living cells containing hydroxyl and alkenyl groups such as terpenoids, that act as a plasticizer. Upon softening, the polyacrylate irreversibly conformed into engineered voids above a resonant sensor, changing the local permittivity which is interrogated, contact-free, with a vector network analyzer. Compared to sensing using the intrinsic permittivity of cells, the SMART approach yields a 20-fold improvement in sensitivity. Tracking growth of many cell types such as Chinese hamster ovary, HEK293, K562, HeLa, and E. coli cells as well as perturbations in cell proliferation during drug screening assays are demonstrated. The sensor is benchmarked to show continuous measurement over six days, ability to track different growth conditions, selectivity to transducing active cell growth metabolites against other components found in the media, and feasibility to scale out for high throughput campaigns.

Engineered substrates for metasurface antennas

This letter explores the advantages of additively manufactured substrates with spatially varying electromagnetic properties. These engineered substrates will be constructed using space filling curves (SFC) of various orders. New advanced manufacturing systems such as the nScrypt 3Dn-300, have enabled the rapid fabrication of these SFC substrates. This letter will apply the engineered SFC substrate to the design and fabrication of metasurface antennas. By utilising a SFC to vary the local substrate permittivity, along with the printed conductive patch dimensions, the range of achievable surface impedances can be greatly expanded. This enlarged design space will be leveraged to yield increased gain for a given metasurface antenna size. Methods to characterise the substrate permittivity and conductive ink are discussed along with a complete description of the metasurface antenna design, fabrication and validation process.

Polarization Sensitivity in Scattering-Type Scanning Near-Field Optical Microscopy—Towards Nanoellipsometry

Electric field enhancement mediated through sharp tips in scattering-type scanning near-field optical microscopy (s-SNOM) enables optical material analysis down to the 10-nm length scale and even below. Nevertheless, the out-of-plane electric field component is primarily considered here due to the lightning rod effect of the elongated s-SNOM tip being orders of magnitude stronger than any in-plane field component. Nonetheless, the fundamental understanding of resonantly excited near-field coupled systems clearly allows us to take profit from all vectorial components, especially from the in-plane ones. In this paper, we theoretically and experimentally explore how the linear polarization control of both near-field illumination and detection can constructively be implemented to (non-)resonantly couple to selected sample permittivity tensor components, e.g., explicitly to the in-plane directions as well. When applying the point-dipole model, we show that resonantly excited samples respond with a strong near-field signal to all linear polarization angles. We then experimentally investigate the polarization-dependent responses for both non-resonant (Au) and phonon-resonant (3C-SiC) sample excitations at a 10.6 µm and 10.7 µm incident wavelength using a tabletop CO2 laser. Varying the illumination polarization angle thus allows one to quantitatively compare the scattered near-field signatures for the two wavelengths. Finally, we compare our experimental data to simulation results and thus gain a fundamental understanding of the polarization’s influence on the near-field interaction. As a result, the near-field components parallel and perpendicular to the sample surface can be easily disentangled and quantified through their polarization signatures, connecting them directly to the sample’s local permittivity.

Dirac points, new photonic band gaps, and effect of magnetically induced transparency in dichroic cholesteric liquid crystals with wavelength-dependent magneto-optical activity parameter.

We investigated the properties of cholesteric liquid crystals (CLCs) being in an external static magnetic field directed along the helix axis. We considered a dichroic CLC, that is, CLC with parameters ReΔ=Reɛ_{1}-Reɛ_{2}/2=0 and ImΔ=Imɛ_{1}-Imɛ_{2}/2≠0, where ɛ_{1,2} are the principal values of the local dielectric permittivity tensor. We have shown that in the case of the wavelength dependence of the magneto-optic activity parameter, new features appear in the optics of dichroic CLCs, in particular, in this case new Dirac points appear. Dirac points are points where there is an intersection of any two wave vector curves (they degenerate) and a linear law of the wave vector dependence on the frequency near these points. And moreover, at some Dirac points photonic band gaps (PBGs) appear; at others, lines of magnetically induced transparency (MIT), that is, a full transmission band appears, in an absorbing medium. In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field, three PBGs of different nature appear: a transmittance band, two narrow lines of MIT, and at others a broadband MIT, etc. This system is nonreciprocal, and the nonreciprocity changes over a wide range. It is observed both for reflection and transmittance and for absorption. The soft-matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can be applied as tunable narrow-band or broadband filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and in other devices.

Detection of defects in polyethylene pipes using open microwave coaxial line resonator sensor

A new method using a microwave coaxial line resonator sensor for the non-destructive evaluation of defects in polyethylene pipes is presented. The sensor consists of a coaxial line section and an extended section of circular waveguide. One end is short-circuited and the other end is open to free space. The sensor aperture is designed to conform to the curvature of the pipe outer surface, so that the common problem of the standoff distance effect by open-ended waveguide scanning is well circumvented. The presence of defects causes local permittivity and volume changes, leading to variations of the resonance frequency and quality factor due to material perturbation. The detection principle is confirmed by electromagnetic simulation. From the experiments, it is shown that the proposed sensor can perform both detection and classification of flat-bottom holes. The minimum diameter of the hole that can be detected is 1 mm, which is lower than that of a hole required in the standard practice. This method can also effectively detect axial defects and determine the lengths. In addition, an open-ended waveguide is used to detect axial holes, and the results were compared with those of coaxial line resonator. The sensor system presented here has the advantages of low cost, convenience for on-site detection and quantitative analysis.

Numerical simulation of the coupled multiphysics fields and reactions during the microwave pyrolysis of wood particles

Microwave heating has gained significant attention as an alternative to conventional methods for biomass pyrolysis. To investigate the electromagnetic-thermal-hydraulic-chemical processes during biomass pyrolysis under microwave irradiation, a three-dimensional comprehensive pyrolysis model was developed. The model numerically described the physicochemical processes of the electromagnetic wave propagation in the reactor, heat transfer and fluid flow inside porous wood particles, pyrolysis reactions, and phase changes. The model was validated by comparing the temperature profiles of biomass samples under varying microwave powers against literature data. The modeling results demonstrated that there was a strong interaction between the distributions of the microwave electromagnetic fields in the cavity and that inside the wood particles; these distributions changed with the size, shape, and location of the biomass sample in the cavity. Using the model, the thermal response of wood particles to microwave radiation was determined. The predicted results showed that a millimeter-sized wood particle was large enough to straddle multiple high-energy microwave regions in the cavity, resulting in more than one intraparticle hotspots. The transient dielectric loss inside wood particles is nonuniform due to the enhancement of the local relative permittivity by the local hotspots. The maximum permeability within the wood particles increases by an order of magnitude during microwave pyrolysis at 1 kW. The hotspots have a considerable influence on the structural homogeneity and stability of biochar due to the formation of large thermal and expansion stresses around the hotspot locations. Moreover, the energy efficiency (ηe) of converting microwave electromagnetic energy into the thermal energy of wood particles is sensitive to the input microwave power and particle shape. For instance, ηe ranges from 12.44% to 36.15% under the microwave powers of 0.8–1 kW for the pyrolysis of a cylindrical wood particle with aspect ratios of 0.25–2. This work provides the fundamental technical support for the industrial application of microwave pyrolysis.

Disordering of Starch Films as a Factor Influencing the Release Rate of Biologically Active Substances.

The release of a spin probe (nitroxide radical) from polymer films was studied by electron paramagnetic resonance (EPR). The films were fabricated from starch having different crystal structures (A-, B-, and C-types) and disordering degrees. Film morphology (analysis of the scanning electron microscopy (SEM)) depended on the presence of dopant (nitroxide radical) to a larger extent rather than on crystal structure ordering or polymorphic modification. The presence of nitroxide radical led to additional crystal structure disordering and reduced the crystallinity index from the X-ray diffraction (XRD) data. Polymeric films made of amorphized starch powder were able to undergo recrystallization (crystal structure rearrangement), which manifested itself as an increase in crystallinity index and phase transition of the A- and C-type crystal structures to the B-type one. It was demonstrated that nitroxide radical does not form an individual phase during film preparation. According to the EPR data, local permittivity of starch-based films varied from 52.5 to 60.1 F/m, while bulk permittivity did not exceed 17 F/m, which demonstrates that local concentration of water is increased in the regions near the nitroxide radical. The mobility of the spin probe corresponds to small stochastic librations and is indicative of the strongly a mobilized state. The application of kinetic models made it possible to find out that substance release from biodegradable films consists of two stages: matrix swelling and spin probe diffusion through the matrix. Investigation of the release kinetics for nitroxide radical demonstrated that the course of this process depends on the type of crystal structure of native starch.

Permittivity Threshold and Thermodynamics of Integer Charge-Transfer Complexation for an Organic Donor–Acceptor Pair

Molecular charge doping involves the formation of donor-acceptor charge-transfer complexes (CTCs) through integer or partial electron transfer; understanding how local chemical environment impacts complexation is important for controlling the properties of organic materials. We present steady-state and temperature-dependent spectroscopic investigations of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) complexed with the electron donor and hole transport material N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine (MPDA). Equilibrium formation constants (KCT) were determined for donor-acceptor pairs dissolved in a series of solvents covering a range of values of permittivity. A threshold for highly favorable complex formation was observed to occur at ϵ ∼ 8-9, with large (>104) and small (<103) values of KCT obtained in solvents of higher and lower permittivity, respectively, but with chloroform (ϵ = 4.81) exhibiting an anomalously high formation constant. Temperature-dependent formation constants were determined in order to evaluate the thermodynamics of complex formation. In 1,2-dichloroethane (ϵ = 10.36) and chlorobenzene (ϵ = 5.62), complex formation is both enthalpically and entropically favorable, with higher enthalpic and entropic stabilization in the solvent with higher permittivity. Complexation in chloroform is exothermic and entropically disfavored, indicating that specific, inner-shell solvent-solute interactions stabilize the charge-separated complex and result in a net increase in local solution structure. Our results provide insight into how modification to the chemical environment may be utilized to support stable integer charge transfer for molecular doping applications and requiring only modest changes in local permittivity.

Contact-Free, Passive, Electromagnetic Resonant Sensors for Enclosed Biomedical Applications: A Perspective on Opportunities and Challenges.

Inexpensive and accurate tools for monitoring conditions in enclosed environments (through garments, bandages, tissue, etc.) have been a long-standing goal of medicine. Passive resonant sensors are a promising solution for such wearable health sensors as well as off-body diagnostics. They are simple circuits with inherent inductance and capacitance (LC tank) that have a measurable resonant frequency. Changes in local parameters, e.g., permittivity or geometry, effect inductance and capacitance which cause a resonant frequency shift response. This signal transduction has been applied to several biomedical applications such as intracranial pressure, hemodynamics, epidermal hydration, etc. Despite these many promising applications presented in the literature, resonant sensors still do not see widespread adoption in biomedical applications, especially as wearable or embedded sensing devices. This perspective highlights some of the current challenges facing LC resonant sensors in biomedical applications, such as positional sensitivity, and potential strategies that have been developed to overcome them. An outlook on adoption in medicine and health monitoring is presented, and a perspective is given on next steps for research in this field.

Dielectric Properties of Polymer Nanocomposite Interphases Using Electrostatic Force Microscopy and Machine Learning.

Knowing the dielectric properties of the interfacial region in polymer nanocomposites is critical to predicting and controlling dielectric properties. They are, however, difficult to characterize due to their nanoscale dimensions. Electrostatic force microscopy (EFM) provides a pathway to local dielectric property measurements, but extracting local dielectric permittivity in complex interphase geometries from EFM measurements remains a challenge. This paper demonstrates a combined EFM and machine learning (ML) approach to measuring interfacial permittivity in 50 nm silica particles in a PMMA matrix. We show that ML models trained to finite-element simulations of the electric field profile between the EFM tip and nanocomposite surface can accurately determine the interface permittivity of functionalized nanoparticles. It was found that for the particles with a polyaniline brush layer, the interfacial region was detectable (extrinsic interface). For bare silica particles, the intrinsic interface was detectable only in terms of having a slightly higher or lower permittivity. This approach fully accounts for the complex interplay of filler, matrix, and interface permittivity on the force gradients measured in EFM that are missed by previous semianalytic approaches, providing a pathway to quantify and design nanoscale interface dielectric properties in nanodielectric materials.

Quantification of the dielectric constant of MoS2 and WSe2 Nanosheets by electrostatic force microscopy

The quantitative characterization of the local permittivity of transition metal dichalcogenide (TMD) materials is essential to estimate both their electrostatic properties and further achieve electronic applications. Electrostatic force microscopy (EFM) can achieve local, non-destructive, and high-resolution characterization of the dielectric properties of materials. However, in EFM, the probe geometry and long-range nature of electrostatic forces make the effective probe region that interacts with the specimen complex, posing daunting challenges to accurately measuring the dielectric property. Here, we present a systematic analysis of the impact of probe geometry on the EFM measurement and quantification of the dielectric constant. Then, a corrected methodology to partly remove such a geometric effect is developed. The proposed methodology has been validated by using finite element numerical calculations and successfully applied to quantitatively characterize the dielectric performance of MoS2 and WSe2 nanosheets. The results show that the dielectric constant determined by the corrected method is insensitive to the geometric parameters of the probe, especially the probe height and cantilever size. This work provides a more accurate and universal way to estimate the dielectric constants, and opens new opportunities in the study of the electrostatics of 2D materials that is essential for their promised applications.

Comprehensive Approach to the Interpretation of the Electrical Properties of Film-Forming Molecules.

This paper presents a general protocol for the interpretation of the electric surface potential of Langmuir monolayers based on a three-layer capacitor model. The measured values were correlated with the results from DFT molecular dynamics simulations, and, as a result, the local dielectric permittivities and dipole-moment components of molecules organized in the monolayer were obtained. The main advantage of the developed approach is applicability to amphiphiles of any type; irrespective of the structure of the polar head as well as the molecular organization and inclination in the surface film. The developed methodology was successively applied to an atypical surface-active compound, perfluorodecyldecane, and its derivatives containing the hydroxyl, thiol, and carboxyl moiety. The following contributions to the apparent dipole moments connected with the reorientation of water molecules and local dielectric permittivities in the vicinity of polar and apolar molecule parts, respectively, were determined: μw/εw = −0.85 D, εp = 5.00, and εa = 1.80. Moreover, the investigated perfluorodecyldecane derivatives were comprehensively characterized in terms of their surface activity, film rheology, and effective surface dissociation equilibria. The proposed methodology may be crucial for the process of the design and the preliminary characterization of molecules for sensor and material science applications.

Eigenstates of Maxwell's equations in multiconstituent microstructures

The eigenstates of Maxwell's monochromatic equations in a multiconstituent composite medium are developed and used to expand a physical field that is created either by an externally given current density or by an incident external field. The local electric permittivity $\ensuremath{\kappa}(\mathbf{r})$, as well as the local magnetic permeability $\ensuremath{\mu}(\mathbf{r})$, are assumed to have uniform values in each constituent, but to differ in the different constituents.

A fabrication method for adaptive dielectric gradient insulating components

Dielectric gradient components have advantages in electric field mitigation and insulation improvement. In this paper, we propose a fabrication method for adaptive dielectric gradient components using in situ AC electric field, including the mechanism and the corresponding operational procedures for industrial applications. Based on the electric polarisation and self-assembly effect of the filler particles in the liquid matrix, the chain-like structure in the high field strength region is constructed to enhance the local permittivity and mitigate the maximum of the spatial electric field. The dielectric gradient basin insulator is prepared by this method, and its flashover voltage is increased by 12.8% compared with that of a homogeneous dielectric basin insulator, and the improvement is 20.8% when metal particles are present on the surface. The more non-uniform the initial electric field is, the greater the improvement in flashover voltage. This method is expected to promote the industrial application of dielectric gradient insulating components.

Performance enhancement strategies for surface plasmon resonance sensors in direct glucose detection using pristine and modified UiO-66: effects of morphology, immobilization technique, and signal amplification

Au nanoparticles encapsulated in UiO-66 MOF were hybridized with Au films to generate strong coupling between their plasmon resonances. Glucose absorbing to UiO-66 modulates the local permittivity, which can be detected by the SPR sensor.

Microwave impedance microscopy and its application to quantum materials

Materials in which quantum mechanical effects lead to novel properties on the macroscopic scale are of interest both fundamentally and for applications. For such quantum materials, the ability to measure local conductivity and permittivity with nanoscale spatial resolution, minimal sample preparation, in a contact-free fashion and under a wide range of external conditions (such as temperature and magnetic field) is highly desirable. To this end, microwave impedance microscopy, a scanning probe technique that measures tip–sample admittance (inverse of impedance) in a non-contact geometry at microwave frequencies, has matured over the past decade to become a tool that can do just that. This Technical Review describes its fundamental working principles and practical implementations, discusses its application to a wide range of quantum materials, including correlated, topological and 2D van der Waals materials, and outlines future opportunities in expanding the capabilities of microwave impedance microscopy.