Ferroelectric Copolymers Of Vinylidene Fluoride Research Articles

Solution-processed ferroelectric polymer nanocrystals

Since the first demonstrations of ferro- and piezoelectricity in solution-processed ferroelectric polymer thin films research activities have been stimulated in both academia and industry. Several applications have been reported using ferroelectric polymer thin films as bistable polarization switches in memory devices or as transducers in (micro-)electromechanical devices. In light of the continuing miniaturization of such devices, nanoscaled structures of ferroelectric polymers such as nanomesas are attracting increasing interest. However, nanopatterning ferroelectrics, to develop stable and reliable ferroelectric nanomesas remains challenging. In this work, we demonstrate the fabrication of solution-processed ferroelectric polymer nanocrystals, i.e. ferroelectric nanomesas, and verify their ferroelectric activity. By using the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)), we are able to isolate crystalline domains. Morphology, molecular conformation, (nano-)mechanical and electromechanical properties of the nanocrystals are measured, yielding comparable results to those observed in uniform, continuous thin films. Therefore, synthesizing self-assembled ferroelectric polymer nanocrystals from polymer solution by spin-casting represents a promising approach for the realization of ferroelectric nanomesas.

Multi-scale characterisation of a ferroelectric polymer reveals the emergence of a morphological phase transition driven by temperature

Ferroelectric materials exhibit a phase transition to a paraelectric state driven by temperature - called the Curie transition. In conventional ferroelectrics, the Curie transition is caused by a change in crystal symmetry, while the material itself remains a continuous three-dimensional solid crystal. However, ferroelectric polymers behave differently. Polymeric materials are typically of semi-crystalline nature, meaning that they are an intermixture of crystalline and amorphous regions. Here, we demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) strongly affects its Curie transition, as not only a change in crystal symmetry but also in morphology occurs. We demonstrate, by high-resolution nanomechanical measurements, that the semi-crystalline microstructure in the paraelectric state is formed by crystalline domains embedded into a softer amorphous phase. Using in situ X-ray diffraction measurements, we show that the local electromechanical response of the crystalline domains is counterbalanced by the amorphous phase, effectively masking its macroscopic effect. Our quantitative multi-scale characterisations unite the nano- and macroscopic material properties of the ferroelectric polymer P(VDF-TrFE) through its semi-crystalline nature.

Formation of Piezo- and Pyroelectric Matrices with the Use of Nanoprofiled Silica

In this work, a process solution for the formation of an array of piezo- and pyroelectric structures with the use of a matrix of nanoprofiled silica is demonstrated. It is proposed to use plasma etching through a hard mask of porous anodic alumina for the formation of the matrix. Nanoparticles of a ferroelectric copolymer of vinylidene fluoride with trifluoroethylene are formed in the pores of the matrix. The results of measurements of the piezoelectric and pyroelectric responses of this structure are presented.

Influence of Dye Molecules on the Polarization of Ferroelectric Vinylidene Fluoride Copolymer

Electrical, electromechanical, and structural–optical properties of films of vinylidene fluoride copolymer with tetrafluoroethylene TFE, doped with Rhodamine 6G dye, have been investigated. It is found that the conductivity increases in doped films; at the same time, it “anomalously” decreases with an increase in the field. The hysteresis loop of local piezoelectric response has an asymmetric shape, which is related to the difference in the local field when its polarity changes. According to the data of absorption and luminescence spectra, the dye (at chosen concentrations) exists in form of, at least, monomers and dimers. It is shown by IR spectroscopy that hydrogen bonds can be formed between a dye molecule and vinylidene fluoride units located in amorphous regions. A model is formulated, which provides a correlation between the electrical and structural properties of the doped films.

An effect of the film texture on high-voltage polarization and local piezoelectric properties of the ferroelectric copolymer of vinylidene fluoride

Vinylidene fluoride and tetrafluoroethylene copolymer 94/6 texturing has been carried out by one-axis drawing at Td = 75 °C up to the ratio of 6 with subsequent isometric tempering at 130 °C. Data of X-ray diffraction and IR-spectroscopy indicate partial polymorph transition from the metastable γ-phase to the polar β-modification with long segments in the conformation of the planar zigzag. The surface topography data show that the oriented film roughness turns out to be almost three times lower than that of the isotropic one. High-resolution vector piezoresponse force microscopy (PFM) has been used to investigate topography and ferroelectric domains in polymer films for visualization out-of-plane and in-plane polarization components. It is shown that, in the oriented sample, both crystallinity and the degree of polar β-phase crystal perfection are higher. It was found that conductivity “abnormal” dropping takes place at the stage of intensive Pr rising with the field growing. This fact is explained by quick trapping of charge carriers by polar planes of crystals with increasing the effective trap area.

Molecular mobility and structuring in textured films of the ferroelectric copolymer of vinylidene fluoride with tetrafluoroethylene

The processes of molecular mobility in vinylidene fluoride–tetrafluoroethylene ferroelectric copolymer–based films textured via uniaxial drawing are studied. It is found that the cooperative mobility in the amorphous regions of the copolymer is described by the Vogel–Fulcher equation, whose parameters are sensitive to the drawing conditions. It is shown that a decrease in the drawing temperature causes a reduction in the effective activation energy of the above-mentioned motion. In accordance with the X-ray diffraction data, this circumstance may be explained by an increase in the packing density of chains in interlamellar spaces. This effect is partially provided by deformation-induced processes of formation of small and defective crystals of the paraelectric or antiferroelectric phase in spaces between neighboring lamellar crystals at the primary stage of crystallization. The relationship between the value of the orientational polarization in oriented films and the topology of structure formation in microfibrils is discussed. It is observed that enlargement of the interlamellar space (filled with a liquidlike amorphous phase) between neighboring crystals in microfibrils promotes an increase in the orientational polarization of chains in the amorphous phase.

An effect of the electrode material on space charge relaxation in ferroelectric copolymers of vinylidene fluoride

Processes of relaxation of space charges formed by impurities carriers in isotropic films of vinylidene fluoride and tetrafluoroethylene copolymers of the composition 71/29 and 94/6 were studied. Al and Au symmetric electrodes deposited by evaporation in vacuum have been used. In the case of Al electrodes at temperatures above 100 °C, giant low frequency dielectric dispersion was observed, while it is absent in films with Au electrodes. Causes of this phenomenon were studied by the X-ray photoelectron spectroscopy. It was shown that at Al deposition, new functional groups, such as Al-C, Al-F, and Al2O3, which are not characteristic of the copolymer film surface, formed. They were supposed to be traps for impurity carriers and because of this the electrode became partially blocked. This led to appearance of the giant electrode polarization on the metal-polymer boundary, which did not take place in the case of Au electrodes. Parameters of the Au4f line for the copolymer with different contents of fluorine atoms in the chain were analyzed. An increase in the number of these atoms was shown to result in the line shift to higher energies. This phenomenon was associated with an increase in the shift of the electron density from Au atoms to the F ones which has a high affinity to electrons.

Local piezoelectric response, structural and dynamic properties of ferroelectric copolymers of vinylidene fluoride–tetrafluoroethylene

The characteristics of the local piezoelectric response of isotropic films of copolymers of vinylidene fluoride (VDF) were compared with 6 and 29 mol% tetrafluoroethylene (TFE) obtained by crystallization from a solution in acetone. Time dependence of the electric displacement response was analyzed after switching of the spontaneous polarization. A copolymer with a higher content of tetrafluoroethylene is characterized by higher values of electrical displacement and piezoelectric response. For interpretation of this fact, we used molecular mobility in amorphous phase dates. It is shown that the activation energy of local and cooperative liquid-like (in the amorphous phase) mobility is markedly lower in the copolymer with a higher content of TFE. Under identical conditions of the crystallization, both films of the copolymers lead to the formation of larger crystals of the polar phase and magnitude of a “long” period at a high content of copolymer of TFE. It is postulated that these structural parameters are responsible for the stable value of residual local piezoelectric activity. It is found that the rapid decay of the signal in the local piezoresponse of polarized films is controlled by the activation energy of the local and cooperative dynamics chains of the amorphous phase.

Polarization of block textured films of ferroelectric copolymers of vinylidene fluoride with tetrafluoroethylene in an external field

The polarization of oriented films of the ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene in a sinusoidal electric field is studied. In low fields, the copolymer behaves as a linear dielectric whose permittivity nonlinearly increases with the amplitude of the alternating field. This behavior is related to the preferential orientation of the dipole moment of the chain along the normal to the surface and to the monocrystal-structure formation. In higher fields, the hysteresis behaves formally coincidently with the behavior of an antiferroelectric. The important role of through stressed chains in the amorphous phase in the initiation of domains of a new direction in polar crystals is observed. Increased adsorption of water molecules on the surface of a polarized film is found and is attributed to the formation of the final density of the stable polarization charge on it.

Structure and Ferroelectric Properties of Nanoimprinted Poly(vinylidene fluoride-ran-trifluoroethylene)

Nanoimprint lithography (NIL) was used to shape thin films of a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (PVDF-TRFE), using a variety of molding shapes and imprinting conditions. The morphology of the layers was characterized by atomic force microscopy (AFM), and preferential orientation of the crystallographic axes was monitored by infrared microspectroscopy; in addition, the local ferroelectric properties were obtained by piezoresponse force microscopy (PFM). When the sample is imprinted in its paraelectric phase in conditions leading to complete confinement, in cavities of size lower than the natural lamellar length observed in a continuous thin film, the crystallographic a axis aligns preferentially parallel to the substrate, and the crystalline lamellae are of significantly reduced length. These characteristics translate in a strongly decreased coercive field and accelerated ferroelectric switching, which is in part ascribed to the improved coupling between the electric field and the properly oriented dipole moments. When decreasing the confinement either by leaving a residual film connecting the nanopillars, or by increasing the lateral size of the nanopillars above the natural lamellar length, or by using line molds where confinement only exists in one direction, or by using continuous films, the preferential orientation becomes less visible and the lamellar length increases, resulting in increased coercive and switching fields. Interestingly, the average length of the crystalline lamellae tends to correlate with the value of the coercive field. Finally, if the sample is imprinted in the melt, a flat-on setting of the crystalline lamellae ensues, with a vertical chain axis which is unfavorable for ferroelectric properties probed with a vertical electric field.

Heterogeneous chemical crosslinking and high-voltage polarization in a ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene

The low-temperature chemical crosslinking of a copolymer of vinylidene fluoride and tetrafluoroethylene was studied through various physicochemical methods. The reaction was conducted in solution in the presence of diethylenetriamine as a crosslinking agent. The penetration of the mixture molecules and the crosslinking agent only in the amorphous phase of the copolymer was provided via selection of the ratio between a good solvent (dimethylformamide) and a poor solvent (ethanol). Owing to this, the crosslinking reaction occurred in the amorphous phase and hardly involved the crystals. This outcome was confirmed by wide-angle X-ray diffraction data. The structural and chemical changes in the amorphous phase during crosslinking were recorded with the use of IR spectroscopy and differential scanning calorimetry. It is shown that crosslinking is accompanied by the formation of C=C bonds in the copolymer chains. The study of high-voltage polarization and conductivity during exposure to bipolar rectangular pulses suggested that crosslinking leads to an increase in the carrier concentration. It was found that the surface potentials in the films increase with an increase in the number of high-voltage pulses applied to the sample. This circumstance is attributed to the fact that the double bonds formed in the copolymer chains can effectively trap negatively charged carriers.

The Evolution of Activation Field with Temperature in Ferroelectric Copolymer of Vinylidene Fluoride and Trifluoroethylene Films

Vinylidene fluoride and trifluoroethylene [P(VDF-TrFE)] ferroelectric thin film was spin-coated on Au-coated polyimide substrate and its polarization reversal was investigated by analyzing the evolution of activation field (α) with temperature. Although α is nearly a constant between 300 and 330 K, it increases linearly when temperature rates between 200 and 230 K, and between 230 and 300 K. On the other hand, the intensity of X-ray diffraction for P(VDF-TrFE) films from 200 to 330 K indicates that glass-transition process plays a significant role in both the microstructure and the polarization reversal of P(VDF-TrFE) copolymer.

Electrical control of photoluminescence wavelength from semiconductor quantum dots in a ferroelectric polymer matrix

We report controllable tuning of the room temperature photoluminescence band of CdSe semiconductor quantum dots embedded in thin films of ferroelectric copolymer of vinylidene fluoride and trifluoroethylene made by Langmuir-Blodgett deposition. The high breakdown strength of the polymer permits the application of electric fields of up to 400 MV/m and results in a shift in the photoluminescence peak by up to 9 nm, nearly half the fluorescence band width. Moreover, we found that at these high electric fields, the Stark effect exhibits unusual fourth power dependence.

Polarization switching at the nanoscale in ferroelectric copolymer thin films

The polarization switching kinetics were measured at the nanoscale in continuous thin films of a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene. The dependence of the switching rate on voltage for a 54-nm thick film exhibits extrinsic nucleation and domain-growth type kinetics with no true threshold coercive field, and is qualitatively different from the behavior of an 18-nm thick film, which exhibits intrinsic switching kinetics, and a true threshold field. The results are consistent with studies of thin film capacitors of much larger area and with a recent refinement of the theory of the critical size for intrinsic switching.

Ferroelectric Polymer Gate Transistor as a Model System for Exploring the Mechanisms of the Retention Loss

An attractive combination of properties of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)), including a relatively high spontaneous polarization and low dielectric constant as well as low processing temperature makes this material useful for studying the ferroelectric gate operation for 1T nonvolatile ferroelecetric memory applications. Here we explore a silicon-based ferroelectric field effect transistor with P(VDF-TrFE) gate showing a persistent switching of the drain current with the “on”/“off” current ratio of 103–102 and retention exceeding 5 days. The physical mechanism of the retention loss has been addressed by monitoring the drain current relaxation in combination with the time-resolved piezo-force scanning probe microscopy. The results suggest that the retention loss is controlled by the polarization screening due to the charge injection into the interface-adjacent layer rather than the polarization loss due to the depolarization effect.

Retention in nonvolatile silicon transistors with an organic ferroelectric gate

A silicone-based one-transistor nonvolatile memory cell has been implemented by integration of a ferroelectric polymer gate on a standard n-type metal oxide semiconductor field effect transistor. The polarization reversal in the gate results in a stable and reproducible memory effect changing the source-drain current by a factor 102–103, with the retention exceeding 2–3 days. Analysis of the drain current relaxation and time-resolved study of the spontaneous polarization via piezoforce scanning probe microscopy indicates that the retention loss is controlled by the interface-adjacent charge injection rather than the polarization instability. A semiquantitative model describes the time-dependent retention loss characterized by an exponential decay of the open state current of the transistor. The unique combination of properties of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene, including an adequate spontaneous polarization and low dielectric constant as well as rather benign processing demands, makes this material a promising candidate for memories fully compatible with silicon technology.

Radiation-Induced Modification of P(VDF/TrFE) Copolymers Studied by ESR and Vibrational Spectroscopy

Radical species produced in ferroelectric copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE) copolymers with (50/50) and (75/25) composition by fast-electron irradiation were studied by electron spin resonance (ESR). The irradiation was performed in both the para- and ferroelectric phases. The ESR spectra were found to be rather complex and changed with time. Three types of radicals were identified, e.g., the peroxy radical, and alkyl in-chain –CF2–C•H–CF2– and end-chain –C•H2 radicals. The decays of the radicals have a similar tendency and show a faster decaying process within a short period followed by a slower decaying process. The fast decay is related to the recombination reaction of alkyl radicals, whereas the slower one is due to the peroxy radicals recombination. In poly(VDF/TrFE) (75/25) copolymer irradiated in the paraelectric phase at T* = 420 K the decay of radicals, which were identified as the long-lived polyenyl radicals –CH2–(CF=CH)n–C•F–CH2–, is completed within one week and then the concentration of these radicals remains constant. The delocalization of unpaired electrons over up to five C=C bonds was estimated from the infrared studies.

Structure formation in crystallizing ferroelectric polymers

The structure of ferroelectric copolymers of vinylidene fluoride with hexafluoropropylene is investigated using wide-angle x-ray diffraction. It is shown that the crystallization of extruded films of vinylidene fluoride-hexafluoropropylene copolymers brings about the formation of the nonpolar α modification and, possibly, the polar αp modification. An increase in the hexafluoropropylene content in the copolymer leads to a decrease in the degree of perfection of the α (αp) phase, on the one hand, and a reduction of its volume fraction, on the other hand.

Dielectric Response and Conformational Disorder in Polymer Relaxors

Ferroelectric copolymers of vinylidene fluoride and trifluoroethylene irradiated with fast electrons were found to exhibit dispersive dielectric response, characteristic of ferroelectric relaxors and giant electrostriction. Here we report the results of our ESR, IR and Raman spectroscopy studies of radiation damage to the copolymers and relate the effects to the conformational disorder and dispersive dielectric response.

Nematic Liquid Crystals Dispersed in a Ferroelectric Copolymer of Vinylidene Fluoride and Trifluoroethylene

We constructed polymer-dispersed liquid crystals which consisted of nematic liquid crystal 4'-n-pentil-4-cyanobiphenyl (5CB) and a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene [P(VDF/TrFE)] by using a solvent-induced phase separation method. The hysteresis between the dielectric displacement and the external electric field was observed simultaneously with the electrooptical behavior in the poling process of the sample. The memory of the applied electric field appeared as a peak shift in the electrooptical dependence. This memory decayed on the order of 100 s. We consider that this decay was due to the screening of the spontaneous polarization of P(VDF/TrFE) by the redistribution of conductive charges in the crystal region of P(VDF/TrFE).