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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]

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]

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]

The ferroelectric hysteresis loops of poly(vinylidene fluoride/trifluoethylene) [P(VDF–TrFE)] copolymer films are investigated as a function of external field and temperature. Starting from a plateau at the low frequency side the coercive field increases with increasing frequency. It exhibits a maximum in the kHz-range and decreases then. The remanent polarization is almost constant at low frequencies and decreases above the kHz-range. For a constant frequency, the coercive field increases with increasing the amplitude of the external field. Furthermore, the hysteresis loops at different temperatures at a given frequency and amplitude exhibit a linear decrease of coercive field with increasing temperature. A double hysteresis loop is observed close to the Curie point and a sharp jump of the remanent polarization is obtained in samples as thick as 600 nm indicating a first order phase transition. For samples as thin as 90 nm, the double hysteresis loop of the polarization is absent and the change of remanent polarization dependent on temperature is smoother. The above experimental results can be explained and simulated in the Weiss mean field model. The simulation results are compared with the experiments and show a good consistency. [ABSTRACT FROM AUTHOR]

Abstract: Ferroelectric polyvinylidenefluoride-trifluoroethylene copolymer films with different thicknesses are prepared by a solvent-cast technique, by spin coating, and by a horizontal Langmuir–Blodgett technique respectively. The frequency dependent dielectric permittivity of these films is investigated with varying sample thickness and varying temperature in the ferroelectric as well as in the paraelectric phase. A dielectric relaxation according to a Vogel–Tamman–Fulcher law of the relaxation times is found in all samples. However, the relaxation times extracted from the dielectric permittivity in the frequency range are not consistent with the relaxation times determined from the temperature range. An explanation for this behavior is given by a temperature dependent distribution of relaxation times. Additionally, in thin samples a second relaxation with a weak anomalous temperature dependence, i.e. an increasing relaxation time with increasing temperature, is observed at high frequencies. Detailed investigations show that this behavior can be attributed to an electrode effect. [Copyright &y& Elsevier]