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Poly(vinylidene fluoride-trifluoroethylene) is a ferroelectric copolymer. It is considered a promising candidate for sensors, nonvolatile memory applications, and energy harvesting. However, imprint, a phenomenon associated with ferroelectric polarization, changes reversibly the material properties over time. In particular, imprint results in an increase of the ferroelectric polarization switching time and the coercive field, as well as in the decay of the remanent polarization and the permittivity. We present a concept to explain imprint and present experimental evidence supporting this concept. In this concept, an internal electric field arises as a consequence of the interaction between ferroelectric dipoles and relaxational dipoles present in the crystalline and the amorphous phase, respectively. We use an extended Weiss mean field approach and show that this internal field results in the experimentally observed imprint. In addition, we show that the imprint can be largely suppressed by increasing the copolymer's crystallinity. [ABSTRACT FROM AUTHOR]

We simulated dielectric nanospheres with a microscopic local field method and compared the results to the macroscopic mean field theory. From Maxwell's equations we find uniform local fields inside a sphere exposed to an electric field E, which was homogeneous before the sphere was brought in. The present paper raises the question about the validity of the macroscopic assumption. The dielectric spheres consist of neutral atoms on cubic lattice sites which show induced electronic polarization. For reasons of symmetry, the local field caused by the dipoles in the center of the sphere should be different from those at off-center locations resulting in an inhomogeneous polarization. This question about the validity is addressed by the microscopic method of local fields. With regard to dielectric nanospheres or other geometrical shapes which are used for nanodielectric composites the application of the microscopic local field method can calculate the actual local fields and polarizations whereas a macroscopic mean field calculation can yield misleading results. Index Terms--computational materials sciences, dielectric nanospheres, local field, Lorentz field, dielectric polarization, induced dipoles, dipole-dipole interaction, nanodielectrics, method of image dipoles, electrode effects

Using the scanning Kelvin probe the potential distribution on the surface of an ionconducting polymer (polyethylene oxide) can be determined. Due to space charge accumulations a voltage applied between two electrodes causes a strongly nonlinear potential distribution in the samples. The curvature of the potential distribution indicates negative mobile ions in the samples. During the measurement the dc currents in the samples are observed to verify the steady state of the potential distribution. Additionally the measurement results are compared to simulations performed with a three-dimensional hopping model. These simulations show qualitatively the same results as the experiments. Index Terms--Scanning Kelvin probe, surface potential measurement, space charge profiles, three-dimensional hopping model.

Demonstrations reveal that the image charges influence the dynamic and the static properties of the polyethylene oxide (PEO) samples is presented. Simulations using a discrete hoping model with deterministic and probabilistic components yield qualitatively the experimentally found results.

The effect of hydrogen absorption on the bulk of amorphous SiO2 thin films is investigated.

Imprint refers to a degradation mechanism in ferroelectrics. We studied effects of imprint in poly(vinylidene-trifluoroethylene) metal-ferroelectric-metal capacitors and found a common functional time dependence of imprint properties. The coercive field, the switching time, the permittivity, and the remanent dielectric displacement change linearly as a function of the logarithm of time after a ferroelectric polarization reversal. These changes were found to be independent of the sample thickness. Therefore, we propose imprint to be caused by mechanisms in the bulk of the ferroelectric. We additionally present by means of a feedback model based on a Weiss mean field approach a correlation between the change of the ferroelectric switching time and the change of the coercive voltage. [ABSTRACT FROM AUTHOR]

Polarization switching is simulated using a model based on a sequence of single dipole flips. The single dipole flips are assumed to be thermally activated with transition rates depending on the local field. The time to switch a single dipole depends on the deterministic transition rate and on a probabilistic factor. In each step, the dipole with the shortest flip time is switched. We investigate one dimensional dipole chains as well as two and three dimensional systems based on the barium titanate structure that comprises single charges fluctuating in double well potentials and induced dipoles. The two and three dimensional simulations yield intrinsic dead layers close to the electrodes that can not be switched even in very strong fields. These non switchable layers are nuclei for the domain wall motion and thus nuclei for the switching process. The switching time of the system decreases faster than exponential for low fields with increasing field. This decrease slows down for higher fields. Furthermore, we found intrinsic dead layers around a defect. [ABSTRACT FROM AUTHOR]

Thermal Barkhausen effect is observed in poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] copolymer films for the first time. Barkhausen pulses arise at phase transition close to the Curie point during both heating and cooling without an external electric field. Three types of current pulses are obtained associated with nucleation and domain wall motion. The switched dipole number under one pulse is generally much less than 1% of the sample's total dipoles. [ABSTRACT FROM AUTHOR]

Polarization relaxation effects and charge injection in poly(vinylidene fluoride/trifluoroethylene) thin films are investigated in a thickness range between 60 nm and 400 nm. In time domain measurements of polarization reversal, the switching transients exhibit two stages: a rapid increase of polarization followed by a continuous slow increase lasting for several decades. This continuous increase of the polarization corresponds to a t-α (Kohlrausch law) behaviour of the current density in the long time range. For a constant electric field, these currents in the long time range are independent of the sample thickness as expected for a relaxational volume polarization effect. Furthermore, the isochronal polarization and depolarization currents saturate at high fields. We described the experiments using an asymmetric double well potential model with a distribution of relaxation times in which dipoles are fluctuating thermally activated between the two minima. As temperature increases, charge injection sets in. The charging currents then deviate from the t-α behaviour and become constant in time. The discharging currents are found to be a superposition of dipole relaxational currents and space charge relaxational currents. In the short time range, only the dipole relaxational currents are found. After several seconds, a space charge relaxation current with a pronounced minimum in time sets in, whose time constant shifts to shorter values with increasing temperature. [ABSTRACT FROM AUTHOR]

For ferroelectrics with a phase transition of the first order a double hysteresis loop of the polarization can be found close to TC. This behavior is explained using the Weiss field model taking into account that strain in the ferroelectric is induced by the polarization via the piezoeffect. The Weiss field model is looked at as a feedback loop where the polarization influences itself by the electrostatic interaction between the constituent dipoles. [ABSTRACT FROM AUTHOR]

The electronic structure and self-polarization of P(VDF-TrFE) Langmuir-Blodgett nanofilms were analyzed under temperature-driven phase transitions, according to their thickness, composition, and structural conformation. Both thermo-stimulated exoelectron emission (TSEE) spectroscopy and computational simulation, including quantum-chemical calculations from first principles, were carried out. PVDF and composite P(VDF-TrFE) (70:30) molecular chains as Trans and Gauche conformers, as well as crystal cells, were modeled for these TSEE analyses. The quantum-chemical calculations and the computational simulation were based on the density functional theory (DFT) as well as semi-empirical (PM3) methods. It was demonstrated that the energy of electron states, as well as the total energies of the studied P(VDF-TrFE) molecular clusters during phase transformation, is influenced by electron work function and electron affinity. Analysis was performed by combining TSEE experimental data with the computational data of the molecular models, demonstrating the effectiveness of this joint approach. For the first time, TSEE was used for contactless measurements of nanofilm polarization, and characterization of the phase transition. The proposed new method can be widely applied in nanobiomedicine, particularly in development of new bone bio-implants, including built-in sensors (new smart nanotechnology). [ABSTRACT FROM AUTHOR]

The switching process in ferroelectrics with a maximum in time for the switching current as observed by Merz [Phys. Rev. 95, 690 (1954)] is simulated. To this end, the Weiss molecular mean-field model is combined with the fluctuation of dipoles in double-well potentials. A first-order rate equation for the polarization with time-dependent coefficients results. This equation is solved for the time-dependent switching process, as well as for the static polarization hysteresis loop. [ABSTRACT FROM AUTHOR]

Ferroelectric behaviour of poly(vinylidene fluoride/trifluoroethylene) copolymer thin films has been investigated through polarization switching measurements. Samples prepared by Langmuir-Blodgett (LB) deposition in the range of 37-139 nm and samples prepared by spin coating in the range of 53-327 nm exhibit almost the same switching behaviour. Reducing the sample thickness, the hysteresis loops tend to slant and the switching transients become broader. The coercive field increases with decreasing sample thickness, whereas the remanent polarization is nearly independent of film thickness. The switching time decreases with increasing field and it is almost independent of thickness down to 69 nm in LB films and 71 nm in spun films. Slower switching is observed for a thickness below 60 nm in both LB films and spun films. This could be due to the depolarization field induced by the surface oxidation layers of Al electrodes which becomes the more pronounced the thinner the sample is. Furthermore, retention properties and fatigue endurance of both kinds of films are studied which show a similar decay tendency. The above results suggest thin films in the range of 60-100 nm are promising candidates for technical applications. [ABSTRACT FROM AUTHOR]

Metal-insulator-semiconductor structures with thin films of the ferroelectric copolymer poly(vinylidene fluoride trifluoroethylene) on oxidized n-silicon exhibit negative capacitances, i.e., inductivelike behavior. The magnitude of negative capacitances depends besides on the layer properties also on frequency and bias. Additional time domain measurements confirm the inductive character. A qualitative model for this behavior based on space charge limited currents is briefly discussed. [ABSTRACT FROM AUTHOR]

Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymers were prepared by the Langmuir–Blodgett technique. These films show a hysteresis loop of the polarization versus the applied electrical field in a dry atmosphere which can be attributed to a ferroelectric phase. By increasing the relative humidity in the surrounding atmosphere, water molecules are absorbed by the samples yielding quasi-free charge carriers. These charges can fluctuate between the electrodes of the samples and build a space charge polarization superimposed to the ferroelectric polarization. Therefore, the hysteresis loops at low frequencies, where the quasi-free charges can contribute to the total polarization, are broadened and the measured remanent polarization is increased. Also, the real and the imaginary parts of the dielectric permittivity of the samples in the low frequency range are increased. Additionally, the transient decay of the retained charge is more pronounced for higher relative humidities. Furthermore, the transient desorption of the water from the polymer is measured by detection of the rapidly changed relative humidity. However, ellipsometric measurements show only a small increase in the sample thickness and no significant change in the refractive index due to the absorption of water. [ABSTRACT FROM AUTHOR]

Metal-silicondioxide-silicon structures with palladium electrodes, instead of standard gold or aluminum electrodes, show an increasing dielectric permittivity of the SiO[sub 2] films with increasing ambient hydrogen pressure. Measurements of the complex dielectric permittivity ε[sub SiO[sub 2]](f) in dependence on the hydrogen pressure are carried out at room temperature. The broad relaxational response spectra of the thin films result from a volume polarization effect. Correspondingly, the contribution of absorbed hydrogen to the dielectric permittivity is modeled by proton fluctuations in coulombic double-well potentials between pairs of oxygen ions. The broad distribution of relaxation times can be attributed to the distribution of interatomic distances between the oxygens within the amorphous SiO[sub 2]. From the experimental dielectric response spectra, a pair distribution function (PDF) of oxygens is derived. This PDF is similar to PDFs from literature, which originate from theoretical lattice simulations of amorphous SiO[sub 2]. As a quantitative result, we estimate the number of relaxation centers, i.e., oxygen pairs occupied by a proton, which contribute to the dielectric permittivity of the SiO[sub 2] thin films in dependence on the hydrogen pressure. [ABSTRACT FROM AUTHOR]

Dynamic Monte-Carlo simulations of ionic space charge relaxations are carried out using a three-dimensional model for thermally activated ion hopping in a multiwell energy structure. In this model a solid polymer electrolyte is embedded between two ideal blocking electrodes. The polymer is subdivided into 100 × 100 × 100 lattice cells. Positive ions (typically 1000) are distributed on the cells. To provide charge neutrality a negative background charge, constant in space and time, is introduced. The positive ions are able to hop between neighboring cells, surmounting energy barriers of distributed heights. The barrier heights consist of an intrinsic part due to the polymer structure, a part due to the Coulomb interaction of the ions, and a part due to the externally applied field. To calculate the interaction between the ions and the electrodes a method of images is used. Periodical boundary conditions are used for those lattice surfaces which are not in contact with the electrodes. The ionic space charge polarization process is simulated as dependent on the time, the sample thickness, the ion concentration, and the externally applied voltage. The polarization current after a step of the electric field shows dispersion due to distributed energy barrier heights in the short time range and a Kohlrausch behavior due to image charges in the long time range. [ABSTRACT FROM AUTHOR]

As a model system for solid polymer electrolytes thin polyethylene oxide (PEO) films (0.6-30 µm) are prepared as A1-PEO-A1 and as A1-PEO-Si structures with blocking electrodes. Dielectric measurements (capacitance and loss angle) are carried out in a frequency range of 3 mHz ≤ f ≤ 1 MHz in atmospheres of different relative humidities (0%-75%) and at different temperatures (293-313 K). The nominal permittivity varies from 4 at high frequencies to up to 150 000 at low frequencies. We find true volume polarization in the high frequency range and a thermally activated space charge relaxation process in the low frequency range. Its time constant exhibits a remarkable dependence on the sample thickness, the relative humidity, the ion concentration, and the temperature. Due to the absorbed dipolar water molecules the activation energy of this process is decreased, leading to higher mobility of the ions. An increase in ion concentration also leads to a lower activation energy. The ions are able to move through the sample to the electrode interfaces and build up ionic space charge. We assume that at the interfaces oxide layers are formed, and these are almost impermeable to these ions. The space charge relaxation process has a broad frequency spectrum. The imaginary part of the nominal permittivity [Multiple line equation(s) cannot be represented in ASCII text e has a slope of β = -0.8 to 0.7 above the main relaxation frequency. In the time domain, after application of a voltage step a Kohlrausch behavior of the relaxation current is found. In the short time range we find a current of j ∞ t[sup -0.3] and in the long time range we find j ∞ t[sup -1.1]. A comparison of the frequency and the time domain by Fourier transform shows that the space charge relaxation process is time invariant for small applied voltages. Nonlinear properties that appear at higher voltages are investigated in the frequency domain and in the time domain. [ABSTRACT FROM AUTHOR]

The dielectric permittivity of amorphous SiO thin films could be increased by the evaporation of palladium electrodes. The evaporation of gold electrodes does not show a comparable effect. Gold electrodes reproduce the literature value for the dielectric permittivity of SiO. Because the preparation processes of both types of electrodes have been carried out in the same manner, the detected increase of the dielectric permittivity must be caused by the physical properties of the used evaporated material. A significant difference between gold and palladium is the ability of palladium to absorb large amounts of hydrogen. Therefore we assume in the case of evaporated Pd electrodes a diffusion process of hydrogen atoms into the insulator, which cause additional relaxation centers. The contribution of these relaxation centers to the polarization is measured as an enhanced dielectric permittivity of the SiO thin film. Measurements of the small signal response have been carried out in the frequency domain, the time domain, where a Kohlrausch law is found, and in dependence of the temperature. The large signal response was studied using isochronal time domain measurements. A double well potential model is discussed to interpret the experimental results quantitatively. It is shown that a distribution of relaxation times is caused by a distribution of distances between neighbored atoms in the amorphous state. c 2001 American Institute of Physics. [DOI: 10.1063/1.1384484]. [ABSTRACT FROM AUTHOR]

Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymers are prepared by a Langmuir-Blodgett technique and the films have displayed ferroelectric properties like hysteresis loops of the polarization and butterfly curves of the ac capacitance versus the applied field. The results have shown that the small signal nonswitching relaxational response resulting in the dielectric permittivity is coupled to the large signal ferroelectric behavior resulting in the hysteresis loops.

The important role of electrode effects in ion-conducting polymers in which a space charge polarization of mobile internal ions prevails is described. The static and dynamic charge distribution in ion-conducting polymers which is influenced by own image charges are explained by measurement data of the transient surface potential with the scanning Kelvin probe using polyethylene oxide doped with lithium perchlorate (PEO-LiCl[O.sub.4]).