Dipole Reorientation Research Articles

Dynamics of poly(vinyl butyral) studied using dielectric spectroscopy and 1H NMR relaxometry.

Dielectric Spectroscopy (DS) and 1H Fast Field-Cycling (FFC) NMR relaxometry were applied for understanding the dynamic behavior of the amorphous ter-polymer poly(vinyl butyral) (PVB) across the glass transition temperature (Tg = 70 °C by Differential Scanning Calorimetry). Above Tg, main chain segmental motions (α relaxation) were detected and characterized using both DS and FFC NMR relaxometry. The correlation times extracted by the analysis of DS and FFC NMR relaxometry data agreed within a factor of three and showed a Vogel-Fulcher-Tammann temperature dependence, with an associated Tg of 69 °C and a fragility of 155 for PVB glass. Below Tg, a secondary process (β relaxation) was revealed by DS, and was ascribed to reorientations of the vinyl alcohol dipoles due to local twisting motions with an associated activation barrier of 11 kcal mol-1. The β process was also found to contribute to 1H NMR relaxation above Tg.

Two-step kinetic model of the self-assembly mechanism for diphenylalanine micro/nanotube formation.

Peptide nanostructures compose a new class of materials that have gained attention due to their interesting properties. Among them, nanotubes of diphenylalanine (FF) and its analogues have been one of the most studied structures in the last few years. Their importance originates from the need to better understand the formation of β-amyloid fibrils which are associated with Alzheimer's disease. In this work, the FF self-assembly process was probed using time-resolved Raman microscopy. The changes in the Raman spectra are followed over time after injecting water into a FF-film until micro/nanotubes (MNTs) are formed. Specific features of the Raman spectra clearly suggest that FF-molecules after water injection form an intermediate species before forming FF-MNTs. The broad Raman bands observed for the intermediate species suggest the presence of very heterogeneous structures based on FF. The FF-MNTs appear almost instantaneously (detected via the rise of the typical Raman bands of FF-MNTs at 761, 1249 and 1426 cm-1) after the intermediate structures are formed. This delayed formation of FF-MNTs supports a nucleation process. The formation via nucleation of FF-MNTs is further corroborated by a simulation of the Raman spectra based on a 2-step kinetic model and the respective vibrational Raman modes are identified using Density Functional Theory vibrational calculations. Our results indicate that the driving force for the FF-MNT patterning process is the electric dipole re-orientation originating from the FF dipeptide unit connectivity over time.

Poly(vinyl alcohol) as a gas accumulation layer for an organic field-effect transistor ammonia sensor

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(vinyl alcohol) (PVA) as a gas accumulation layer were fabricated. The results showed the percentage responses of saturation current (Ion) was 50.3% under 0.5ppm NH3, which is over one order of magnitude higher than that of pure PMMA dielectric. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH3 gas. Moreover, the percentage responses of Ion would increase to 66.9% and 30.2% under 0.5ppm and 0.2ppm NH3 after applied a gate voltage pulse, respectively. The sensing properties of these OFET gas sensors can realize absorption and desorption with opposite gate bias, exhibiting non-volatile states of Ion and VT under the programming-erasing bias as used for memory elements. By analyzing the morphologies of dielectric and organic semiconductor, and the electrical characteristics of OFET sensors, the interaction between NH3 and PVA will be strengthened under the influence of the applied electrical field on the hydroxyl dipoles. The bias-induced re-orientation of the hydroxyl dipoles can modulate the influence of NH3 on the trapping (absorption) and detrapping (desorption) processes of charge carriers at the pentacene/PVA interface.

Rapid stability of ferroelectric polarization in the Ca, Ce hybrid doped BaTiO3 ceramics.

In this work, we report a rapid stability phenomenon of ferroelectric polarization in the Ca, Ce hybrid doped BaTiO3 ceramics (BCaxT+BTCe8) (x = 10, 20, 24, 30 mol%) prepared by separate doping Ca2+ and Ce4+ ions. Double hysteresis loops are identified in the aged BCaxT+BTCe8 samples; meanwhile, the polarization of these loops present a rapid decrease within very short aging time (about 1 h), and then the polarization remains almost unchanged over the followed ~1000 h. This phenomenon is not reported in previous researches. Raman scattering spectrum indicates that oxygen vacancies are generated because of Ca2+ ions entering into Ti sites partly in the BCaxT+BTCe8 samples, and then the oxygen vacancies are quantitatively characterized by half of the Ce3+ content through the XPS test. The emergence of the aging phenomenon is explained through the defect dipole reorientation mechanism. The larger radius of Ca2+ ions is further discussed as a possible reason for the rapid stability phenomenon of ferroelectric polarization. It may provide an effective design method from the viewpoint of the ionic radius to accelerate polarization stability, and thus to facilitate the possible practical applications of the aging effect.

Generating high dielectric constant blends from lower dielectric constant dipolar polymers using nanostructure engineering

It is a great challenge in dielectric polymers to achieve a high dielectric constant while maintaining low dielectric loss and high operating temperatures. Here we report that by blending two glassy state dipolar polymers i.e., poly(arylene ether urea) (PEEU, K=4.7) and an aromatic polythiourea (ArPTU, K=4.4) to form a nanomixture, the resulting blend exhibits a very high dielectric constant, K=7.5, while maintaining low dielectric loss (<1%). The experimental and computer simulation results demonstrate that blending these dissimilar dipolar polymers causes a slight increase in the interchain spacing of the blend in its glassy state, thus reducing the barriers for reorientation of dipoles in the polymer chains along the applied electric field and generating a much higher dielectric response than the neat polymers.

Heterogeneity during Plasticization of Poly(vinylpyrrolidone): Insights from Reorientational Mobility of Single Fluorescent Probes.

While dynamics of single-molecule (SM) fluorescent probes have been used to investigate the structure and relaxation processes in polymers near the glass transition temperature (Tg), it is difficult to perform SM imaging at elevated temperatures which restricts such studies to a limited number of polymers for which Tg is close to room temperature (RT). Plasticization, solvent (or additive) induced lowering of Tg, offers an alternate avenue to access various effective temperatures in the glassy and rubbery phases of polymers under ambient conditions. By investigation of the reorientational propensity of individual Rhodamine 6G (Rh6G) probes, which is governed by rigidity/dynamics of the polymer cavities, we have explored the extent of spatiotemporal heterogeneity during moisture induced plasticization of poly(vinylpyrrolidone) (PVP), far below and near (below and above) bulk Tg. Lack of any probe reorientation suggests that the matrix remains extremely rigid up to a certain level of hydration, as expected for probes buried deep within the glassy state. At intermediate levels of hydration, SMs undergo a wide variety of rotational dynamics ranging from being static/wobbling motion to slow, hindered large-angle reorientation, as well as facile, intermittently hindered fast rotation, which reflects that swelling/softening of network cavities is spatiotemporally extremely diverse as the effective Tg approaches RT. SM probes exhibit temporally nonuniform rotational mobility even at relatively high moisture contents of the matrix beyond which probes can undergo translational motion, which indicates that relatively slow time scale polymer segmental motion can be operational for plasticized PVP (in the rubbery state). Our inferences are supported by the non-Gaussian nature of angular jump distributions for dipolar reorientation, similar to those reported for translational diffusion of SM tracers in polymers and cellular media, suggesting the existence of slow time-varying local environmental changes around individual probe molecules during plasticization.

Dielectric relaxation in AC powder electroluminescent devices

Abstract The dielectric properties of AC powder electroluminescent devices were measured and analyzed using complex impedance spectroscopy to determine the relaxation processes occurring within the devices. The relaxation processes identified were ascribed to the electrode polarization caused by ion accumulation at the electrode/resin interfaces, the Maxwell-Wagner-Sillars effects at the (ZnS or BaTiO3) particle/resin interfaces, and the dipolar reorientation of polymer chains in the resin matrix. Each relaxation process was represented by its corresponding equivalent circuit component. Space charge polarization at the electrodes were represented by a Warburg element, a resistor, and a constant phase element. The resin matrix, ZnS/resin and BaTiO3/resin interfaces could each be modeled by a resistor and a capacitor in parallel. The simulated equivalent circuits for three different printed structures showed good fitting with their experimental impedance results.

A critical phenomenon of phase transition in hydroxyapatite investigated by thermally stimulated depolarization currents

AbstractWe observed a critical phenomenon of ferroelectric transition in hydroxyapatite (HAp) through measuring the thermally stimulated depolarization current of polycrystalline monoclinic HAp. Two peaks attributable to hydroxide (OH−) ion dipole reorientations were observed. The one was very sharp, the other displayed a broad peak ranging from 350 to 550 K. While the broad peak can be attributed to the diffusive reorientation motions of OH− ion dipoles, the sharp peak, related to the ferroelectric phase transition, implies the presence of ferroelectricity in HAp. The sharp peak was found near 380 K, the phase‐transition temperature from the monoclinic to hexagonal is ascribed to a critical phenomenon caused by the ferroelectric behavior of OH− ion dipole reorientations.

Elastic and anelastic relaxation behaviour of perovskite multiferroics II: PbZr0.53Ti0.47O3 (PZT)\u2013PbFe0.5Ta0.5O3 (PFT)

Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr0.53Ti0.47O3–PbFe0.5Ta0.5O3 (PZT–PFT) have been assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr0.53Ti0.47O3–PbFe0.5Nb0.5O3 (PZT–PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe0.5Ta0.5O3. As for PZT–PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic–tetragonal–monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel–Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT–PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales.

Molecular Rotors Built in Porous Materials.

Molecules and materials can show dynamic structures in which the dominant mechanism is rotary motion. The single mobile elements are defined as "molecular rotors" and exhibit special properties (compared with their static counterparts), being able in perspective to greatly modulate the dielectric response and form the basis for molecular motors that are designed with the idea of making molecules perform a useful mechanical function. The construction of ordered rotary elements into a solid is a necessary feature for such design, because it enables the alignment of rotors and the fine-tuning of their steric and dipolar interactions. Crystal surfaces or bulk crystals are the most suitable to adapt rotors in 2D or 3D arrangements and engineer juxtaposition of the rotors in an ordered way. Nevertheless, it is only in recent times that materials showing porosity and remarkably low density have undergone tremendous development. The characteristics of large free volume combine well with the virtually unhindered motion of the molecular rotors built into their structure. Indeed, the molecular rotors are used as struts in porous covalent and supramolecular architectures, spanning both hybrid and fully organic materials. The modularity of the approach renders possible a variety of rotor geometrical arrangements in both robust frameworks stable up to 850 K and self-assembled molecular materials. A nanosecond (fast dynamics) motional regime can be achieved at temperatures lower than 240 K, enabling rotor arrays operating in the solid state even at low temperatures. Furthermore, in nanoporous materials, molecular rotors can interact with the diffusing chemical species, be they liquids, vapors, or gases. Through this chemical intervention, rotor speed can be modulated at will, enabling a new generation of rotor-containing materials sensitive to guests. In principle, an applied electric field can be the stimulus for chemical release from porous materials. The effort needed to obtain strong dipoles that are noncentrosymmetrically mounted onto rotors and do not hamper rotational motion is a further aspect of this research activity. Thus, materials showing dielectric properties in response to applied electric fields have been fabricated. This may lead to challenging materials that are promptly responsive to an applied electric field, altering the ferroelectric or antiferroelectric ground state by fast dipole reorientation when subjected to electric polarization.

ZnO nanoparticles' decorated reduced-graphene oxide: Easy synthesis, unique polarization behavior, and ionic conductivity

This work's focus is on the easy synthesis, elucidation of material characteristics and unique polarization behavior of ZnO nanoparticles' decorated r-GO. An optimized molecular-level mixing technique is used to synthesize ZnO decorated r-GO. The composite is found to exhibit high remnant polarization while its ionic conductivity follows the Arrhenius law. The observed polarization behavior is attributed to the lattice parameter disorder in ZnO combined with the presence of an appropriate number of functional groups in the composites. Moreover, a slight increase in r-GO content in the composites can increase the remnant polarization by one order of magnitude (i.e., from 0.062μC/cm2 to 0.232μC/cm2) under the influence of an external electric field (~1.6kV/cm). Experimental observations clearly indicate that the functionalized r-GO plays an important role (i.e., it facilitates easy dipolar alignment and reorientation) in polarization and increasing the number of stable polarization states with respect to the applied field direction.

Microwave dielectric relaxation spectroscopy studies on associative polar binary mixtures of nitrobenzene with primary alcohols

The microwave dielectric relaxation behavior of binary mixtures of nitrobenzene with primary alcohols namely ethanol, 1-propanol and 1-butanol has been studied at different temperatures using Time Domain Reflectometry method. The measured complex dielectric spectra of pure and mixture solutions have been fitted with Debye model using least square method. The obtained static dielectric and relaxation time constant values are plotted against mole fraction of solute. The important dielectric parameters such as excess dielectric constant, excess inverse relaxation time and effective Kirkwood correlation factors are computed and the same has been used to interpret the intermolecular interaction and dynamic change in the solution. Furthermore, the mixing effect of nitrobenzene in different chain length alcohols has been explored by concentration dependent excess dielectric constant values. The positive molar enthalpy values of mixtures indicate that the heat energy is absorbed owing to the process of dipole reorientation. Moreover, the heterogeneous interaction between solute and solvent molecular functional groups has been confirmed by equi-molar FT-IR spectra analysis.

Molecular Mechanism of Polarization and Piezoelectric Effect in Super-Twisted Collagen.

It has been known for decades that bone exhibits piezoelectric behavior. In recent years, it was directly proved that this effect stems from a polymeric matrix in bone, i.e., collagen fibrils. This effect in collagen is distinctly different from organic piezoelectric crystals, given the semicrystalline molecular structure of the collagen biopolymer. As such, the molecular mechanism of this electromechanical coupling effect in a realistic "super-twisted" model of collagen has been elusive. Herein, we present an investigation on the molecular mechanism of piezoelectric effect in collagen using full atomistic simulation based on the experimentally verified "super-twisted" microstructure of collagen. Our results reveal that collagen exhibits a uniaxial polarization along the long axis of the collagen fibril. In addition, the piezoelectric effect in collagen originates at the collagen molecule level and is due to the mechanical stress-induced reorientation and magnitude change of the permanent dipoles of individual charged and polar residues. A piezoelectric constant in the range of 1-2 pm/V (pC/N) is obtained from the simulation, which agrees well with the experimental data.

Thermodynamic and Molecular Dielectric Relaxation Studies of Polar–Polar Binary Mixtures Using Time Domain Reflectometry Technique

The complex dielectric spectra of nitrobenzene, 2-butanol and their binary mixtures were measured in the frequency range of 10 MHz–20 GHz at different temperatures using Time Domain Reflectometry. The static permittivities and relaxation times were extracted from the complex dielectric spectra of the pure compounds and binary mixtures by fitting to the Debye model using the least-squares fitting method. The derived static permittivity (e 0) and dielectric relaxation time (τ 0) values were used to calculate various dielectric parameters including the excess dielectric constant, effective Kirkwood correlation factor, excess inverse relaxation time and thermodynamic parameters. Excess dielectric parameters were fitted with the Redlich–Kister type polynomial equation. The result from dielectric analyses confirms the formation of a heterogeneous complex structure by association of unlike molecules. This hetero-molecule interactions produce a electric field in the mixtures and, as a result, the effective dipoles rotate faster. Molecular rotation and dipole reorientation motions in these complex system are discussed in terms of the molar activation entropy and enthalpy. Additionally, the hydrogen bond interaction between solute and solvent were confirmed by FT–IR spectral analysis.

Force field development and simulations of senior dialkyl sulfoxides.

Thermodynamics, structure, and dynamics of diethyl sulfoxide (DESO) and ethyl methyl sulfoxide (EMSO) were investigated using ab initio calculations and non-polarizable potential based molecular dynamics (MD) simulations. The additive pairwise force field (FF) for EMSO and DESO was proposed for the first time, preserving explicit compatibility with their most known homologue, DMSO. The simulations reveal similar structures and thermodynamic properties of DMSO, DESO and EMSO. However, the transport properties are significantly different: DESO and DMSO are less mobile and an order of magnitude more viscous. Furthermore, dipole reorientation in DESO and EMSO occurs ca. 2-4 times slower than in DMSO at room temperature. This observation favors applications of higher sulfoxides as cryoprotectants and provides a microscopic interpretation of the earlier experimental data.

Ferromagnetism and the effect of free charge carriers on electric polarization in the double perovskiteY2NiMnO6

The double perovskite Y_2NiMnO_6 displays ferromagnetic transition at Tc = 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (M_s) and also, validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P 21/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of frequency dependent dielectric constant and equivalent circuit analysis of impedance data takes into account the bulk contribution to total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T_c). Pyrocurrent measurements register a current flow with onset near Tc and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to re-orientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material

Atomic Structures of CH3NH3PbI3 (001) Surfaces

We report on the atomic structures of methylammonium (MA) lead iodide (CH3NH3PbI3) perovskite surfaces, based on a combined scanning tunneling microscopy and density functional theory calculation study. A reconstructed surface phase with iodine dimers, coexisting with the pristine zigzag phase, was found at the MA-iodine-terminated (001) surfaces of the orthorhombic perovskite films grown on Au(111) surfaces. The reorientation of surface MA dipoles, which strengthens the interactions with surface iodine anions, resulting in a slight energy reduction of 34 meV per unit cell, is responsible for the surface iodine dimerization. According to our calculation, the surface MA dipoles weaken the surface polarity and are therefore considered to be stabilizing the surface structures.

Effects of the Dielectric Response of Single-Component Liquids and Liquid Mixtures on Electrochemical Properties between Charged Plates

We study the dielectric properties of single-component liquids and liquid mixtures between charged plates and develop a modified Poisson–Boltzmann (PB) equation using Booth’s theory. Our theory accounts for the permanent dipole moment and polarizability of solvents and the hydrogen bonds between solvents. We demonstrate that these key parameters are crucial for capturing the reorientation of solvent dipoles under external electrostatic fields. Our results show that variations in the capacitance of ionic solutions can be nonmonotonic, whereas the conventional PB theory predicts monotonic behavior. Specifically, the effect of hydrogen bonding produces highly nonmonotonic behavior that depends on the surface charge density and applied voltage. The resultant capacitance can be enhanced by hydrogen bonding, but a significant decline in the capacitance can also occur as the applied voltage is increased. Furthermore, our novel finding is that field-induced changes in the dielectric value may cause the unconventi...

The ageing and de-ageing behaviour of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free piezoelectric ceramics

Ageing behaviour usually occurs in acceptor-doped piezoelectric materials (e.g., hard lead zirconate titanate) and exhibits the development of a pinched or shifted hysteresis loop over time. Although no pinched hysteresis loop was observed for lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 material, this study showed that the piezoelectric properties change over time in the poled state. The shift of the hysteresis loop along the electric field axis and the development of asymmetry in strain and permittivity hysteresis loop were observed during the ageing process. The origin of this ageing behaviour is proposed to be local defect dipoles and the migration of the charged defects to the grain boundaries. The reorientation of the defect dipole contributes to a fast but unstable ageing mechanism in this material while the migration of the charged defects contributes to a slow but more stable mechanism.

Separation of experimental 2D IR frequency-frequency correlation functions into structural and reorientation-induced contributions.

A vibrational transition frequency can couple to its environment through a directional vector interaction. In such cases, reorientation of the vibrational transition dipole (molecular orientational relaxation) and its frequency fluctuations can be strongly coupled. It was recently shown [Kramer et al., J. Chem. Phys. 142, 184505 (2015)] that differing frequency-frequency correlation function (FFCF) decays, due to reorientation-induced spectral diffusion (RISD), are observed with different two-dimensional infrared polarization configurations when such strong coupling is present. The FFC functional forms were derived for the situation in which all spectral diffusion is due to reorientational motion. We extend the previous theory to include vibrational frequency evolution (spectral diffusion) caused by structural fluctuations of the medium. Model systems with diffusive reorientation and several regimes of structural spectral diffusion rates are analyzed for first order Stark effect interactions. Additionally, the transition dipole reorientational motion in complex environments is frequently not completely diffusive. Several periods of restricted angular motion (wobbling-in-a-cone) may precede the final diffusive orientational randomization. The polarization-weighted FFCF decays are presented in this case of restricted transition dipole wobbling. With these extensions to the polarization-dependent FFCF expressions, the structural spectral diffusion dynamics of methanol in the room temperature ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate can be separated quantitatively from RISD using the experimental center line slope data. In addition, prior results on the spectral diffusion of water, methanol, and ethanol in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are re-examined to elucidate the influence of reorientation on the data, which were interpreted in terms of structural fluctuations.