Vinylidene Fluoride-trifluoroethylene Copolymer Research Articles

Pyroelectric effect in composites based on copolymer P(VDF-TrFE) and ferroelectric ceramics BPZT

The pyroelectric response of composite polymer films based on poly(vinylidene fluoride-trifluoroethylene) copolymer (P(VDF-TrFE)) and powdered ceramic ferroelectric of barium lead zirconate titanate (BPZT) was investigated. The appearance of a near-surface layer with high pyroelectric activity was observed during the polarization of composite material samples in the electric field of corona discharge.

Contributions of distinctive dynamic processes to dielectric response of a relaxorlike reduced poly(vinylidene fluoride-trifluoroethylene) copolymer

Dynamic processes in relaxorlike reduced polyvinylidene fluoride-trifluoroethylenePVDF-TrFE copolymer have been studied by measurements of the temperature and frequency-dependent linear and thirdorder nonlinear dielectric response. Dielectric analysis of relaxor polymers has, namely, been incomplete until now, i.e., data interpretation was complicated by the fact that two similar types of dynamics, relaxorlike behavior in the crystalline part of the system and glass-to-rubber transition in the amorphous matrix, take place in the same temperature range and thus superimpose in the detected response. On the other hand, here, in the PVDF-TrFE system, synthesized via reductive dechlorination from the polyvinylidene fluoridechlorotrifluoroethylene, relaxor peaks take place at much higher temperatures, thus both dynamics can be investigated separately. Analysis of the experimental data reveals that the relaxorlike dielectric dynamics in the reduced PVDF-TrFE copolymer, which is now not influenced by processes in the amorphous matrix, indeed is almost identical to that observed in classical relaxor systems and, furthermore, reminiscent of the dynamic behavior observed in various spin glasses. Also, a clear indication was obtained that all polar dynamic processes terminate down to 100 K and that below this temperature only nonpolar contributions govern the dielectric response of the reduced PVDF-TrFE copolymer.

Specific features of the formation of polymer-dye systems based on nanostructured polymer matrices prepared by solvent crazing

Specific features of the formation of polymer-dye systems based on various nanostructured polymer matrices prepared by the method of solvent crazing are discussed. In the general case, the formation of polymer-dye composites includes four main stages: sorption of dye molecules by the highly disperse fibrillar material of crazes, shrinkage of the polymer composite due to the removal of the solvent, migration of dye molecules from their localized sites on the surface of fibrils, and healing of the structure of crazes (internal interfacial boundaries) under thermal treatment. Analysis of the migration of dye molecules in the polymer matrix includes the following assumptions: first, a metastable (nonequilibrium) state of the system after solvent crazing and introduction of dye molecules into the fibrillar craze material and, second, the statement according to which both the depth and direction of the above migration processes are controlled by the free energy of mixing of components. For amorphous glassy systems (PVC, PS, PC), healing of the fibrillar craze material (after shrinkage and removal of the solvent) is observed. In the case of semicrystalline polymers (PP, vinylidene fluoride-trifluoroethylene copolymer) and amorphous crystallizable polymer matrices (PET), the intensity of healing upon thermal treatment decreases due to the presence of crystalline regions, which slow down the motion of macromolecules.

Phase transitions in ultrathin ferroelectric polymer films and nanocrystals

The dielectric dispersion of nanocrystals of vinylidene fluoride—trifluoroethylene copolymer of 70/30 composition was investigated. A phase transition was revealed in ferroelectric nanocrystals at T = 100°C. A wide temperature hysteresis was observed, indicating a first-order phase transition.

Intrinsic electrocaloric effects in ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymers: Roles of order of phase transition and stresses

The electrocaloric effects accompanied with the ferroelectric to paraelectric phase transitions in poly(vinylidene fluoride-trifluoroethylene) are investigated within the Landau–Devonshire theory. Just changing the nature of the phase transition from the first-order to the second-order reduces the isothermal entropy change, adiabatic temperature change and refrigerant capacity. The isothermal entropy change in the second-order transition is about one half of that in the first-order one, which is confirmed by experiments and is also consistent with the magnetocaloric counterpart. Converting to be film also leads to the reduction in electrocaloric effects, generally ascribed to the decrease of pyroelectric coefficients.

Glass transition of Langmuir-Blodgett polymer films

The dielectric dispersion of vinylidene fluoride-trifluoroethylene copolymer P(VDF/TrFE) (70/30) films obtained by the Langmuir-Blodgett technique was investigated in the low-temperature range from 20 to −100°C. The thickness of the explored films was 40 nm. A glass-transition range from −25 to −40°C was detected; this indicates the presence of an amorphous phase in the LB films.

Phase transition in ferroelectric nanocrystals of the poly(vinylidene fluoride-trifluoroethylene) copolymer

The dielectric dispersion of nanocrystals of the poly(vinylidene fluoride-trifluoroethylene) copolymer with the 70/30 composition has been studied. It has been found that the phase transition in ferroelectric nanocrystals takes place at T = 100°C. The observed broad temperature hysteresis indicates the occurrence of a first-order phase transition.

Effects of Chemical Treatments on the Electrical Behaviors of Ferroelectric Poly(vinylidene fluoride-trifluoroethylene) Copolymer for Nonvolatile Memory Device Applications

Ensuring the sound ferroelectric behaviors of the ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] after various fabrication processes routinely performed for the device integration is very important for the practical nonvolatile memory applications using P(VDF-TrFE). In this work, the changes in the electrical and physical properties of the P(VDF-TrFE) thin film were investigated, assuming that some chemical treatments are involved in the fabrication processes for P(VDF-TrFE)-based memory devices. Although the treatments in the conventional developer and acids fortunately caused no critical damage to the film, it was found that the use of some resist strippers provides harsh conditions to the film. They caused the degradation in the crystallinity and surface roughness, and hence large increases in coercive field and leakage current were observed. Among the chemicals tested in this work, we proposed 1-methoxy-2-propanol as the best resist stripper. Metal–ferroelectric–metal capacitors and metal–ferroelectric–insulator–semiconductor diodes were also fabricated using the proposed patterning methods, in which no critical degradation in ferroelectric memory effects was observed when the size of capacitor decreased to 30 ×30 µm2. We conclude that these experimental studies can give us a useful solution to realizing memory arrays and related integrated circuits by exploiting P(VDF-TrFE)-based nonvolatile memory devices.

Polarization switching kinetics of ferroelectric nanostructures of vinylidene fluoride-trifluoroethylene copolymer

The polarization switching kinetics of ferroelectric polymer Langmuir-Blodgett films and copolymer nanocrystals (nanomesas) was investigated using piezoresponse force microscopy. The nanocrystals were prepared by self-organization from Langmuir-Blodgett films of a 70% vinylidene fluoride and 30% trifluoroethylene copolymer. The polarization switching time exhibits an exponential dependence on reciprocal voltage and decreases with temperature being consistent with the nucleation switching dynamics.

Effects of Annealing on the Optical and Electronic Properties of Poly (vinylidene fluoride-trifluoroethylene) Copolymer Thin Films

Thin films of vinylidene fluoride with trifluoroethylene [P(VDF-TrFE)] copolymer have been deposited onto bare Si and SiO2 by spin casting from methylethylketone solutions. The structures and optical and electronic properties for P(VDF-TrFE) films after vacuum and forming gas annealing were studied. The degree of structural order and the crystallinity determined by X-ray diffraction were increased with the thermal annealing time. Ellipsometry spectroscopy were employed to investigate the changes in the thickness, refractive indices n, and the anisotropic properties. The results reflected that the n was increased with the thermal annealing time and temperature, and the anisotropy of the annealed films was strengthened with the annealing time and temperature. Metal-polymer-(oxide)-semiconductor capacitors were used to measure the static dielectric constant K and interfacial electronic properties of P(VDF-TrFE) on Si which the K increase with film thickness and thermal annealing. The interface at Al-P(VDF-TrFE) and/or P(VDF-TrFE)-Si affect the K value which is sensitive for films thinner than 120 nm. The Si-P(VDF-TrFE) interface quality determined using capacitance-voltage and current-voltage measurementswas found to be improved after forming gas annealed.

Correlated changes in meso- and nano-scale hierarchical structure of Vinylidene fluoride-trifluoroethylene random copolymers investigated by simultaneous measurements of DSC and SAXS/WAXD utilizing synchrotron radiation

Hierarchical structural features of Vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers during the phase transition have been successfully investigated in detail by in-situ simultaneous measurements of DSC and SAXS/WAXD utilizing synchrotron radiation. In a heating process at a constant rate of 1 °C/min, a broad endothermic peak was measured for a VDF-TrFE copolymer from ca. 75°C to ca. 135°C. On the other hand, WAXD data indicated that the phase transition from the crystal form II to the high-temperature (HT) phase began to occur at ca. 105°C. With an increase in temperature from ca. 75° C to ca. 105° C (temperature region A), lamellar staking distance (long period) and lamellar thickness evaluated in SAXS data analyses slightly increased though the crystallite size (the lateral dimension of the lamellae) was almost constant. The (110) reflections of the crystal form II gradually decreased in integrated intensity and scattering from the amorphous region increased instead, which implied that lamellae in crystal form II became partially disordered before the phase transition.

Polarization switching kinetics of ferroelectric nanomesas of vinylidene fluoride-trifluoroethylene copolymer

The polarization switching kinetics of ferroelectric polymer nanomesas was investigated using piezoresponse force microscopy. The nanomesas were made by self-organization from Langmuir–Blodgett films of a 70% vinylidene fluoride and 30% trifluoroethylene copolymer. The polarization switching time exhibits an exponential dependence on reciprocal voltage that is consistent with nucleation-type switching dynamics.

Dielectric spectroscopy of ultrathin ferroelectric polymer films

The dielectric dispersion of poly(vinylidene fluoride-trifluoroethylene) copolymer films prepared using the Langmuir-Blodgett technique is studied. All films of different compositions and thicknesses undergo a Debye-type dielectric relaxation. The phenomenon of critical slowing down is investigated. It is found that the relaxation time τ and the kinetic coefficient ξ depend on the thickness of the film.

Influence of the relative humidity on the properties of ferroelectric poly(vinylidene fluoride-trifluoroethylene)

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.

Ferroelectric switching dynamics in VDF-TrFE copolymer thin films spin coated on Si substrate

Simultaneous measurements of the charge Q and the capacitance C were performed for an MFS capacitor with Au-(vinylidene fluoride-trifluoroethylene copolymer)-(n-Si) structure using a double-frequency voltage consisting of a low-frequency high voltage and a high-frequency sinusoidal low voltage. The use of a triangular high voltage yields asymmetrical Q-V and C-V hysteresis loops that support the full ferroelectric polarization reversal occurring in close relation to charge compensation in the n-Si layer. The use of a rectangular high voltage reveals the details of asymmetric switching dynamics. Polarization switching toward the positive side starts with the loss of the depletion layer and progresses rapidly owing to the accumulation of majority carriers to be completed at a time analogous to that for the case of a metal-ferroelectric-metal capacitor. On the other hand, that toward the negative side is markedly impeded by depletion layer formation but is eventually completed via a constant-current process at a time given by the ratio of switched polarization and constant current. On the basis of a linear relationship between the constant current and the depletion layer width, we conclude that the switching dynamics under a negative voltage is controlled by the rate of minority carrier generation in the depletion layer.

Domain stabilization effect of interlayer on ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin film

A charge injection material, poly(3,4-ethylene dioxythioohene)-poly(styrene sulfonic) acid, is introduced as a buffer layer between metal electrodes and a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer ultrathin film. The buffer layer not only prevents the reactions between P(VDF-TrFE) layer and the top electrode during metallization process, which leads to the loss of polarization, but also supplies the amount of charges needed for compensation of the ferroelectric dipoles to stabilize the domain during switching process so that the fatigue property was improved tremendously. The sandwiched structure shows prominent ferroelectric properties in a 50 nm P(VDF/TrFE) film. A coercive electric field of 52 MV/m and a remnant polarization of 88 mC/m2 are recorded. With two additional organic interlayers, 33 nm thick in total, the coercive field decreases. After more than 1×107 cycles of switching, the polarization remains as high as 68 mC/m2. Moreover, the cell still has good ferroelectric properties at 60 °C. Even as the thickness of P(VDF/TrFE) film is down to 25 nm, a remnant polarization of 54 mC/m2 is obtained.

Low Voltage Operation of Nonvolatile Ferroelectric Capacitors Using Poly(vinylidene fluoride-trifluoroethylene) Copolymer and Thin Al[sub 2]O[sub 3] Insulating Layer

We have successfully demonstrated low voltage operation of metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)]/Al 2O3 /p-Si(100) structures. P(VDF-TrFE) copolymer thin films (thickness of 20 nm) prepared by the spin-coating method and annealed at 140°C showed good ferroelectric behavior with a remnant polarization of 1 μC/cm 2 at a voltage sweep of ±4 V. Remarkable reduction in leakage current was realized by introducing a 5 nm thick Al 2 O 3 layer grown by atomic layer deposition. In capacitance-voltage measurements, MFIS capacitors showed rectangular-shaped hysteresis curves, attributed to the ferroelectric material, in which the memory window width was 1.5 V, even at applied voltages of ±3V.

Composites of silver nanoparticles and poly(vinylidene fluoride-trifluoroethylene)copolymer: Preparation as well as structural and electrical characterization

AbstractComposites comprising of polymers and metal nanoparticles are of great interest in regard to electronic and opto-electronic applications. The preparation of such nanocomposites with homogenously dispersed particles usually cannot be solved by mixing the polymer and the desired isolated colloids due to strong agglomeration tendency of the metallic nanoparticles. Consequently, nanocomposites with colloids have been prepared by synthesis of the inorganic particles in situ, for instance in solution, and then mixed with the polymer solution.Extensive attention has been given to the study of the plasmonic properties of noble metal nanoparticles as a result of their potential application as waveguides, photonic circuits, and sensors [1]. Surface plasmon polaritons are excited when electromagnetic radiation causes coherent oscillations of the conducting electrons of noble metal nanoparticles such as gold, silver or copper. The selective photon absorption and scattering allow the monitoring of the optical properties of the nanoparticles by conventional spectroscopic methods like UV-vis spectroscopy. Previous investigations show that the surface plasmon resonance frequency is extremely sensitive to the size, shape, and the surrounding dielectric environment of the nanoparticles [2].In order to obtain multifunctional composites electro-active polymers (EAP) can be chosen as matrix materials. EAPs such as polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) show a ferroelectric polarization accompanied with piezo- and pyroelectric properties. Both polymers are suitable for composite preparation as earlier studies have shown, e.g. performed on ceramic-polymer composites in order to optimize their piezo- and pyroelectric properties and to adjust their dielectric properties, respectively. Recently, PVDF with embedded metallic nanoparticles was studied regarding the kinetics of film preparation, dispersion and resulting properties [3].In this work, the influence of homogenously dispersed silver-nanoparticles in electro-active polymers such as PVDF and P(VDF-TrFE) has been investigated over a broad range of mass fractions of silver. For low silver nanoparticle content (up to 3wt.%) the surface plasmon polariton resonance peak can be observed in the blue spectral region. From the infrared spectra it is concluded that no significant degradation of the polymers occurs. Higher silver amounts cause the formation of fractal-like agglomerates. Thus, a high extinction cross section in the visible and infrared spectral range is found. Furthermore, the influence of the silver mass fraction to the thermal, electrical and dielectric properties of the nanocomposites is discussed in detail.[1] S. A. Maier, H. A. Atwater, J. Appl. Phys. 2005, 98, 011101-10. [2] J. J. Mock, et al, J. Chem. Phys. 2002, 116, 6755-6759.[3] J. Compton, et al, Makromol. Symp. 2007, 247, 182-189.

Comparative electrical bistable characteristics of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer based nonvolatile memory device architectures

We report the thermal and electrical bistable characteristics of ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE) (72∕28mol%)] thin films as a function of varying memory device architectures. Rectangular-shaped capacitance-voltage (C-V) hysteresis loops obtained using 100nm P(VDF-TrFE) films with a metal-ferroelectric-insulator-semiconductor (MFIS) diode architecture were more suitable for distinguishing the data-bit state compared with the symmetrical hysteresis observed using metal-ferroelectric-metal capacitors. Poly(4-vinyl phenol) used as a dielectric insulator in the MFIS prevented shifting of the C-V hysteresis curve toward the negative bias voltage.

Impact of Kr gas mixing in oxygen plasma etching of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer films

Oxygen plasma etching characteristics of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer films are investigated. It was found in MFM (M: metal; F: ferroelectric) capacitors that plasma damage effects to the ferroelectric properties were insignificant when Au metal masks were used. On the contrary, C-V (capacitance versus voltage) characteristics were significantly degraded in plasma-etched MFIS (I: insulator; S: semiconductor) diodes. The origin of this phenomenon is speculated to be degradation of the SiO2∕Si interface by energetic oxygen ions and then mixing of Kr gas to the oxygen plasma is attempted to decrease the plasma damage.