Bulk Dielectric Constant Research Articles

Atomic Layer Growth of Rutile TiO2 Films with Ultrahigh Dielectric Constants via Crystal Orientation Engineering.

In general, the electronic and optical properties of oxide films can significantly benefit from highly textured crystallinity. However, oxide films grown by atomic layer deposition (ALD), a powerful technique for the synthesis of high-quality, nanoscale thin films, usually exhibit amorphous or randomly oriented polycrystalline phases. Here, we demonstrate the growth of highly textured rutile phase ALD TiO2 films through rational substrate design. Both a- and c-axis preferentially oriented TiO2 films are obtained by varying the lattice parameters of the initial ALD growth surface. Under optimized conditions, we find that it is possible to deposit high-quality, c-axis preferentially aligned TiO2 films with a bulk dielectric constant approaching 185, rivaling the single crystal limit. These films display a remarkably high dielectric constant of 117 despite thin thickness of 5.2 nm. Moreover, the addition of a single doping sequence of Al2O3 successfully suppresses leakage currents to levels compatible with modern dynamic random access memory cells, all the while maintaining the high bulk dielectric constant of 137. These results clearly highlight the prospect of utilizing crystal orientation engineering in ALD thin films for emerging semiconductor devices.

Characterizations of structural, optical, and dielectric properties of oxynitride perovskites ACa0.2M0.8O2.6N0.4 (A= Sr, Ba; M = Nb, Ta)

Perovskite-type oxynitrides ACa0.2M0.8O2.6N0.4 (A = Sr, Ba; M = Nb, Ta) were synthesized via the ammonolytic reaction between A5M4O15 and CaCl2, where the Ca2+ insertion and O2−/N3− substitution occurred cooperatively. In terms of the average structure, SrCa0.2Nb0.8O2.6N0.4 and SrCa0.2Ta0.8O2.6N0.4 belong to the orthorhombic Pnma space group, and BaCa0.2Nb0.8O2.6N0.4 and BaCa0.2Ta0.8O2.6N0.4, the primitive cubic Pm3‾m group. The comparison between the experimental lattice volume and the summed ionic volume suggested that ACa0.2M0.8O2.6N0.4 have higher degrees of ionicity than AM'0.2M0.8O3−xNx (M′ = Li, Mg, Mn), but are more covalent than ANa0.2M0.8O2.8N0.2. Despite the significant mismatches of size and charge between Ca2+ and Nb5+ (or Ta5+), no cation ordering was detected on the octahedral site. On the other hand, the O/N distribution appeared to depend on the bonding geometry around the anion sites in a way N favors the straighter bonding connectivity of M−N−M. The band gap energies of ACa0.2M0.8O2.6N0.4 were estimated to be 1.9–2.25 eV depending on A and M. The band gaps and color properties of AMO2N and AM'0.2M0.8O3−xNx (A = Sr, Ba; M = Nb, Ta; M′ = Li, Na, Mg, Ca, Mn) are compared. Thermogravimetry and differential scanning calorimetry were conducted in the air to assess the oxynitride stability. The electrical behaviors were studied by the equivalent circuit analysis of the impedance spectrum using compacted polycrystalline specimens, where BaCa0.2Ta0.8O2.6N0.4 was found to possess a bulk dielectric constant of 4550 along with an electrical conductivity of ≈10−6 S/cm at 27 °C. It remains, however, necessary to assess the extrinsic effects arising from the non-ideal sintering to interpret thoroughly the electrical property of BaCa0.2Ta0.8O2.6N0.4.

Effect of solid loading on phase composition, microstructure, mechanical and dielectric properties of fused silica ceramics by gecasting

In this study, the fused silica slurries with an ultra-high solid loading of 85 vol%, low viscosity of 0.34 Pa s at 99.4 s−1, and good suspension stability were successfully prepared using two-gradation fused silica raw powders by optimizing the preparation processes. Owing to the inherent characteristics of gelcasting and good dispersion stability of resulting fused silica slurries, all the fused silica ceramics prepared with various solid loadings exhibited a homogeneous bimodal size distribution microstructure after pressureless sintering at 1250 °C for 4 h. Furthermore, a small amount of nanoscale cristobalite was precipitated from fused silica, and solid loading had no significant effect on the crystallization kinetics of cristobalite. A significant increase in solid loading effectively facilitated the densification of fused silica ceramics, enhanced bulk density, decreased sintering shrinkage, and strengthened mechanical properties. However, the increase in solid loading deteriorated dielectric properties within the range of 2–18 GHz. When the solid loading was 85 vol%, the bulk density, linear shrinkage, flexural strength, dielectric constant, and dielectric loss tangent were 2.12 g/cm3, 1.1 %, 72.3 MPa, 3.91–4.36, 0.0081–0.0171, respectively. These properties were favorable making the materials potential applications in the field of microwave transmission.

Charge Inversion in 1:1 Electrolytes: Analyzing the Energetics.

The effects of bulk concentration, surface charge density, ionic diameter, and bulk dielectric constant on charge inversion in 1:1 electrolyte systems are investigated. The framework of the classical density functional theory is used to describe the mean electrostatic potential and the volume and electrostatic correlations, which combine to define the adsorption of ions at a positively charged surface. Our results show that a decrease in the dielectric constant, in particular, creates charge inversion for 1:1 electrolytes by amplifying both the electrostatic potential and the screening component (which is generally much larger than the excluded-volume component). Local electrical potential inversion can occur even for moderate concentrations and surface charges. These findings are especially significant for ionic liquids and systems with organic molecules as solvents, as these generally have a dielectric constant much smaller than water.

A Performance Analysis of Soil Dielectric Models over Organic Soils in Alaska for Passive Microwave Remote Sensing of Soil Moisture

Passive microwave remote sensing of soil moisture (SM) requires a physically based dielectric model that quantitatively converts the volumetric SM into the soil bulk dielectric constant. Mironov 2009 is the dielectric model used in the operational SM retrieval algorithms of the NASA Soil Moisture Active Passive (SMAP) and the ESA Soil Moisture and Ocean Salinity (SMOS) missions. However, Mironov 2009 suffers a challenge in deriving SM over organic soils, as it does not account for the impact of soil organic matter (SOM) on the soil bulk dielectric constant. To this end, we presented a comparative performance analysis of nine advanced soil dielectric models over organic soil in Alaska, four of which incorporate SOM. In the framework of the SMAP single-channel algorithm at vertical polarization (SCA-V), SM retrievals from different dielectric models were derived using an iterative optimization scheme. The skills of the different dielectric models over organic soils were reflected by the performance of their respective SM retrievals, which was measured by four conventional statistical metrics, calculated by comparing satellite-based SM time series with in-situ benchmarks. Overall, SM retrievals of organic-soil-based dielectric models tended to overestimate, while those from mineral-soil-based models displayed dry biases. All the models showed comparable values of unbiased root-mean-square error (ubRMSE) and Pearson Correlation (R), but Mironov 2019 exhibited a slight but consistent edge over the others. An integrated consideration of the model inputs, the physical basis, and the validated accuracy indicated that the separate use of Mironov 2009 and Mironov 2019 in the SMAP SCA-V for mineral soils (SOM <15%) and organic soils (SOM ≥15%) would be the preferred option.

Interactions between Small Inorganic Ions and Uncharged Monolayers on the Water/Air Interface.

The interaction of several simple electrolytes with uncharged insoluble monolayers is studied on the basis of tensiometric and potentiometric data for the surface electrolyte solution|air. The induced adsorption of electrolyte on the monolayer is determined via a combination of data for equilibrium spreading pressure and surface pressure versus area isotherms. We show that the monolayer-induced adsorption of electrolyte is not only strongly ion-specific but also surfactant-specific. The comparison between the ion-specific effects on a carboxylic acid monolayer at low pH and an ester monolayer shows that the anion series follows the same order while the cation series reverses. The effect of the electrolyte on the chemical potential of the monolayer shows attraction between the surfactant and the ions at low monolayer densities, but at high surface densities, repulsion seems to come into play. In nearly all investigated cases, a maximum of monolayer-induced electrolyte adsorption is observed at intermediate monolayer densities. This suggests specific interactions between the surfactant headgroup and the ions. The Volta potential data for the monolayers are analyzed on the basis of the equations of quadrupolar electrostatics. The analysis suggests that the ion-specific effect on the Volta potential is due to the ion-specific decrement of the bulk dielectric constant of the electrolyte solution. Moreover, we present evidence that in most cases the effect of the electrolyte on the orientation of the adsorbed dipoles cannot be neglected. Instead, the change in the ion distribution in the electric double layer seems to have a small effect on the Volta potential.

Electroabsorption as a Probe of Electrons Injected into Nanocrystalline SnO2/TiO2 Core/Shell Thin Films

Core/shell nanoparticles comprised of a SnO2 core and a TiO2 shell, SnO2/TiO2, present in a mesoporous thin film, are of practical interest for applications in dye-sensitized water splitting as they outperform sensitized materials based on SnO2 or TiO2 (anatase or rutile polymorphs) for the water oxidation half-reaction. Here we report electroabsorption studies designed to quantify the surface electric field and the ability of the shell to screen an electric field from a surface anchored dye. The metal-to-ligand charge transfer (MLCT) absorbance of [Ru(bpy)2(4,4′-(PO3H2)2-2,2′-bipyridine)](PF6)2, abbreviated RuP, was indeed sensitive to electron injection into the oxide support. When a fixed charge of 1 mC was injected, the electric field magnitude (in MV/cm) increased in the order 0.15 TiO2 (rutile) < 0.35 TiO2 (anatase) ≪ 1.4 SnO2 (rutile). This order tracks the reported bulk dielectric constants of the oxide materials. Comparative studies with SnO2/TiO2 core/shell materials, fabricated by atomic layer deposition (ALD) of a TiO2 shell on a mesoporous SnO2 thin film, diminished the field strength by about a factor of 4 to values less than or about equal to that measured for anatase TiO2. Heat treating some SnO2/TiO2 core/shell materials at 450 °C resulted in a crystalline rutile TiO2 shell that screened the field most effectively, more so even than rutile TiO2 alone. Taken together, the data indicate that injected electrons reside within the SnO2 core region.

Interface dielectric constant of water at the surface of a spherical solute

Interface dielectric constant is used to quantify polar response of water interfacing a spherical solute. This interfacial parameter, affected by the interfacial structure within about two hydration layers, is fundamentally distinct from the bulk dielectric constant (a material property). Molecular dynamics simulations are used to extract the interface dielectric constant from fluctuation relations correlating the dipole moment of the interfacial layer with the medium electrostatics. For a probe ion, one has to calculate cross-correlations between the hydration shell dipole moment and the electrostatic potential, while cross-correlations between the shell dipole moment and the electrostatic field are required for a probe dipole. All protocols produce dielectric constants of water interfacing a nonpolar solute significantly below the bulk value. We analyze corrections imposed on the fluctuation relations by protocols using periodic boundary conditions with Ewald sums to compute electrostatic interactions. These corrections are insignificant for typical simulation protocols.

Self-assembled monolayers on porous low-k dielectrics by decyltrimethoxysilane vapor treatment: A perspective from electrical characteristics and time-dependence-dielectric-breakdown reliability

Self-assembled monolayer (SAM) is an attractive technology for back-end-of–line interconnects of integrated circuits for advanced technological nodes. In this study, SAMs were formed on the porous low-k films by decyltrimethoxysilane (DTMOS) in vapor phase at different temperatures. The effects of the formation SAMs on the electrical characteristics and reliability of the porous low-k films were characterized. With DTMOS vapor treatment at 50°C∼100°C, SAMs were formed onto the porous low-k films, thus enhancing the dielectric breakdown field, failure time, and Cu barrier capacity. Additionally, DTMOS vapor treatment at higher temperatures resulted in larger enhancement and more thermal stability. The formation SAMs was demonstrated to effectively repair the damaged porous low-k film, thus reducing the dielectric constant; although, the bulk dielectric constant was higher than that of the pristine film. To sum up, this study provided a promising SAMs processing derived from DTMOS vapor phase, for ensuring better integrity for the porous low-k films in the advanced technological nodes.

(Invited) Dielectric Screening of Ionic Liquids and Implications for Their Capacitance, Solvation, and Structural Properties

Development and utilization of ionic liquids for applications as electrolytes and solvents requires understanding their electrostatic interactions with solutes, co-solvents, and interfaces. In this regard, the bulk dielectric constant of an ionic liquid is of little use, but rather the dielectric response must be characterized at atomistic lengthscales. We will discuss the dielectric response of ionic liquids as computed from molecular dynamics simulations and statistical mechanical theory. We will then describe how the dielectric properties of ionic liquids are connected with observed trends in interfacial capacitance, solvation, and liquid structure. Comparisons will be made to aqueous electrolytes and molten salts, and similarities and differences will be discussed.

An improved model and field calibration technique for measuring liquid water content in unfrozen and frozen soils with dielectric probes

AbstractDielectric soil moisture probes can be used to obtain frequently logged field observations of volumetric water content, but because this is an indirect method, it is challenging to ensure that you have a suitable calibration relationship and parameters. Challenges are associated with (a) limitations in the probes’ ability to accurately measure the soil bulk dielectric constant and (b) limitations in obtaining reliable direct observations to calibrate the probe against, due to the spatial heterogeneity in the field. Furthermore, in soils that freeze, we do not have a robust approach to account for the effect of ice on the instrument. In this study, we propose a calibration relationship for all dielectric probes that is physically based, parsimonious, accounts for errors in the bulk dielectric constant, and quantifies the uncertainty in the liquid water content caused by the presence of ice. We show that our relationship has a better performance and more realistic parameter values than existing approaches. To calibrate a dielectric probe in the field, we show that it is necessary to account for spatial heterogeneity in samples taken for calibration. There is value in taking simultaneous measurements of the bulk dielectric constant (using the same dielectric probe as is being calibrated) and gravimetric samples for water content at a number of points in space. Taking too few samples is likely to be misinformative—we show that it is better to not calibrate the probe at all than to calibrate it with too few data points.

Investigating the dielectric constant of barium titanate in a polymer-matrix nanocomposite

Barium titanate (BTO) is a ferroelectric material used in capacitors because of its high bulk dielectric constant. However, the impact of the size of BTO on its dielectric constant is not yet fully understood and is highly contested. Here, we present an investigation into the dielectric constant of BTO nanoparticles with diameters ranging between 50 and 500 nm. BTO nanoparticles were incorporated into acrylonitrile butadiene styrene and injection molded into parallel plate capacitors, which were used to determine nanocomposite dielectric constants. The dielectric constants of BTO nanoparticles were obtained by combining experimental measurements with computational results from COMSOL simulations of ABS-matrix nanocomposites containing BTO. The dielectric constant of BTO was observed to be relatively constant at nanoparticle diameters as small as 200 nm but sharply declined at smaller nanoparticle sizes. These results will be useful in the development of improved energy storage and power conditioning systems utilizing BTO nanoparticles.Graphical abstract

Electrical image potential and solvation energies for an ion in a pore in a metallic electrode or in a nanotube.

Electrical image potentials can be important in small spaces, such as nanoscale pores in porous electrodes, which are used in capacitive desalination and in supercapacitors, as argued by Bazant's group at Massachusetts Institute of Technology. It will be shown here that inside pores in porous metallic materials the image potentials can be considerably larger than near flat walls, as a result of the fact that the dielectric constant for an electric field perpendicular to a wall is much smaller than the bulk dielectric constant of water. Calculations will be presented for the image potential in spherical and cylindrically shaped pores. The calculations for cylindrical pores can also be applied to nanotubes. It was believed for a long time, on the basis of molecular dynamics simulations, that in order to push a salt solution through a small radius nanotube, work must be done against the solvation energy of the ions, which is larger inside a narrow nanotube than it is in the bulk. The relatively large image charge potential energy in narrow nanotubes, however, tends to oppose this increase in the solvation energy. The degree to which the image potential facilitates the flow of the salt ions into nanotubes will be discussed.

Transfer learning for materials informatics using crystal graph convolutional neural network

For successful applications of machine learning in materials informatics, it is necessary to overcome the inaccuracy of predictions ascribed to insufficient amount of data. In this study, we propose a transfer learning using a crystal graph convolutional neural network (TL-CGCNN). Herein, TL-CGCNN is pretrained with big data such as formation energies for crystal structures, and then used for predicting target properties with relatively small data. We confirm that TL-CGCNN can improve predictions of various properties such as bulk moduli, dielectric constants, and quasiparticle band gaps, which are computationally demanding, to construct big data for materials. Moreover, we quantitatively observe that the prediction of properties in target models via TL-CGCNN becomes more accurate with an increase in size of training dataset in pretrained models. Finally, we confirm that TL-CGCNN is superior to other regression methods in the predictions of target properties, which suffer from small amount of data. Therefore, we conclude that TL-CGCNN is promising along with compiling big data for materials that are easy to accumulate and relevant to the target properties.

Exploring the electron-hole transport nature and optoelectronic properties of 4-nitro-4′-amino-azobenzene-based dyes

Charge transport and electronic properties of 4-nitro-4′-amino-azobenzene compounds with donor-acceptor backbone were examined at the molecular level. Optical properties of all the studied six azobenzene dyes were also explored at the solid-state bulk level, i.e., dielectric constants, conductivity, refractive index, and extinction coefficient values. The introduction of benzyl group enhances whereas propanenitrile diminishes the values for real and imaginary dielectric functions. Moreover, the substitution of 3-methoxy-3-oxopropyl and propanenitrile at amino group decreases the real conductivity values; however, bis(3-methoxy-3-oxopropyl) improves these values. These 4-nitro-4′-amino-azobenzene compounds showed good refractive indices in the energy range 0–2 eV which revealed that these dyes would be efficient to generate healthier outputs with lower energy inputs. The effect of substituents was also investigated on transfer integrals, frontier molecular orbitals (FMOs), reorganization energies, electron affinity (EA), and ionization energy (IE). The ground-state geometries were optimized by density functional theory at B3LYP/6-31G** level to probe the molecular level properties. The same level of theory was used for the cation and anion optimizations. The hole and electron transfer nature of azo dyes was investigated by evaluating the electron/hole transfer integrals (Velectron/Vhole) as well as hole and electron reorganization energy values. The results exposed that superior Vhole values of Azo2 and Azo4 might lead to these dyes as p-type contenders. Moreover, larger Velectron values of Azo1, Azo3, Azo5, and Azo6 might lead to these dyes as n-type competitors. These results revealed that inspected Azo dyes would be able to serve as competent hole and electron transfer materials within versatile organic semiconductors.

Atomistic description of plasmonic generation in alloys and core shell nanoparticles.

Using the extended discrete interaction model we investigate the tunabilty of surface plasmon resonances in alloys and core-shell nanoparticles made from silver and gold in the small (1-15 nm) nanoscale regime where classical models based on the bulk dielectric constant may not apply. We show that the surface plasmon resonance of these alloys and core-shell particles to a large extent follow Vegard's law irrespective of the geometry of the nanoparticle. The evolution of the polarizability with size demonstrates a highly non-linear behaviour of the polarizability with the ratio of the constituents and geometry in alloys and core-shell nanoparticles, with the exception of the longitudinal surface plasmon resonance in nanorods and, partly, nanodisc alloys. We here show that the non-linear behaviour can be explained in terms of the difference in polarizability of the mixing constituents and local effects causing a quenching of the dipoles for geometries with a low aspect ratio. A thorough statistical investigation reveals that there is only a small dependence of the surface plasmon resonance on atomic arrangement and exact distribution in a nanoparticle and that the standard deviation decreases rapidly with the size of the nanoparticles. The physical ground for the random distribution algorithm for alloys in discrete interaction models is explained in detail and verified by a statistical analysis. For nanoparticles below 4 nm a sampling strategy is recommended.

Radar Sounding of Subsurface Structure in Eastern Coprates and Capri Chasmata, Mars

AbstractWe surveyed the subsurface structure in eastern Coprates and Capri Chasmata in the equatorial region using high‐resolution visible images, digital terrain models, and radar sounding data. We identified subsurface reflectors in four areas of the chasmata. At the stratigraphic exposure on the chasmata walls, the corresponding depth of the reflector is ∼60 m. The bulk dielectric constants of the layers above the reflectors are calculated as 3.4–4.0, suggesting a rock‐air mixture with ∼39.3% and 46.1% porosity or a rock‐air‐ice mixture with &lt;21.2% water ice fraction. This high porosity corresponds to nonwelded and unconsolidated sediments emplaced by aeolian, fluvial, and volcanic activities. If water ice actually exists, further studies and discussions are required for the mechanism to maintain it within low latitudes.

The genuine ac-to-dc proton conductivity crossover of nafion and polymer dielectric relaxations as a fuel cell polarization loss

The non-ohmic behavior of Nafion electrical properties, i. e., the thickness and potential dependent conductivity, was studied in the impedance, dielectric and conductivity representations with the use of a special through-plane sample-holder in a 4-probe array. Such measurements allowed identifying the genuine ac-to-dc conductivity crossover frequency in Nafion, which occurs for f < 10-1 Hz. In addition, the minimization of the interfacial electrode/ionomer polarizations with the 4-probe setup permitted the determination of the bulk dc conductivity and dielectric constant of Nafion, which are σ ~ 0.03 Scm-1 and ε′ ~ 106 (T = 40 °C and RH = 100%), respectively. The colossal dielectric constant is shown to increase the Debye length of the electric double layer to values comparable to the membrane thickness. Therefore, the exponential increase of the proton conductivity with increasing both membrane thickness and electric potential are a result of canceling out the non-linear effects of electric double layer caused by the high dielectric permittivity of Nafion. The ac-to-dc conductivity crossover in H2/O2 fuel cell impedance curves takes place for f < 100 Hz and matches with the ex situ impedance spectroscopy study in excellent agreement, revealing a striking result: the potential dependent conductivity of Nafion requires extra fuel cell overpotential to overcome the electrode/ionomer interfacial polarization representing an additional polarization loss to polymer electrolyte fuel cells.

Doping mechanisms and dielectric properties of Ca-doped bismuth magnesium niobate pyrochlores

Here we report the doping mechanisms and dielectric properties of Ca-doped bismuth magnesium niobate (BMCN) pyrochlores synthesised using solid-state reaction at 1025 °C over 24–48 h. The extensive solid solution of (Bi3.36Mg0.64-xCax)(Mg1.28Nb2.72)O13.76 (0 ≤ x ≤ 0.7) pyrochlores formed with a fully indexed cubic structure, space group Fd3m and lattice constants, a = b = c in the range 10.5621 (17)−10.5332 (14) Å. The SEM analysis confirmed that the surface morphologies of BMCN pyrochlores were of irregular polyhedral grains and their crystallite sizes, as calculated by Scherrer and Williamson-Hall methods, were found to be in the range 46–70 nm and 38–75 nm, respectively. These thermally stable BMCN pyrochlores were highly insulating materials that showed a reproducible dielectric behaviour over heat-cool cycles and their activation energies were estimated to be in the range 1.17–1.47 eV. High bulk dielectric constants, ε’, 69–171 and low dielectric losses, tan δ, in the order of ~10−3 were recorded at 30 °C and 1 MHz; negative temperature coefficient of capacitance (TCC) in the range, (−319)−(-933) ppm/°C was measured over ~30–300 °C at 1 MHz.

Screening of Coulomb interactions in liquid dielectrics

The interaction of charges in dielectric materials is screened by the dielectric constant of the bulk dielectric. In dielectric theories, screening is assigned to surface charges appearing from preferential orientations of dipoles along the local field in the interface. For liquid dielectrics, such interfacial orientations are affected by the interfacial structure characterized by a separate interfacial dipolar susceptibility. We argue that dielectric properties of polar liquids should be characterized by two distinct susceptibilities responsible for local response (solvation) and long-range response (dielectric screening). A microscopic model for dipolar screening in polar liquids is developed here. It shows that the standard bulk dielectric constant is responsible for screening at large distances. The potential of mean force between ions in polar liquids becomes oscillatory at short distances. Oscillations arise from the coupling of collective longitudinal excitations (dipolarons) of the polar liquid with its interfacial density profile. Interfacial electrostatics demonstrates a crossover beyond the solute radius of ∼1 nm from the scaling laws roughly consistent with continuum expectations for small solutes to new scaling trends characterizing the much softer nano-scale solute–solvent interface. This crossover also marks the transition to a continuum-type electrostatic screening between ions, when short-distance oscillation of the potential of mean force become strongly damped. Screening oscillations are enhanced for more structured interfaces.