Ferroelectric Vinylidene Fluoride Research Articles

Influence of physical aging of ferroelectric vinylidene fluoride copolymer films on their structural and electrophysical characteristics

AbstractGeneral problems of structural changes which occur at storage of two‐phase crystalline polymers with a metastable structure at room conditions are considered on the example of a vinylidene fluoride ‐ hexafluoropropylene copolymer. The physical aging occurs in flexible‐chain crystalline polymers (polyethylene, fluoropolymers under consideration, etc.), where, due to low glass transition temperatures, the liquid‐like dynamics of amorphous phase chains is realized at room temperature through cooperative micro‐Brownian motions with short relaxation times. Taking into account that covalent‐bound sections of the chains of the amorphous phase can enter the crystallites, the noted mobility may initiate changes in the size of the latter. Such a possibility is proved by the example of the noted copolymer. At low‐temperature crystallization of a vinylidene fluoride ‐ hexafluoropropylene copolymer from a solution the formed α‐phase crystals have little perfection. The size of the crystals increases when the films are stored in room conditions. Because of the crystal polymorphism, at the same time a certain fraction of γ‐phase crystals which are present in initial films undergo a polymorphic transformation γ → α. These processes lead to an increase in crystallinity. Moreover, during such processes additional structuring is observed, which is reduced to the displacement of various kinds of intra‐chain defects into the amorphous phase (and especially into the surface). Since the copolymers under study are ferroelectrics, they were studied by piezo force microscopy. It was found that despite the crystallization predominantly in nonpolar α‐phase, piezo force microscopy revealed a domain structure, which formation mechanism is discussed. The structural changes at physical aging of the films affect the character of the noted domain structure. Thus, it is suggested that the mechanism of the described structural changes is realized through the developed cooperative mobility of the chains in the amorphous phase, which characterizes the processes of rotational diffusion.

On the nature of hysteresis phenomena at low-voltage polarization of crystalline ferroelectric polymers

Hysteresis phenomena were observed under electric fields two orders of magnitude lower than the coercive ones in ferroelectric copolymer vinylidene fluoride–tetrafluoroethylene. From the field dependences of the electrical displacement hysteresis, the volumetric energy density of electric loss was calculated. Its value increases with increasing amplitude of the sawtooth external field and is higher in case of lower external field frequency. Complex dielectric permittivity measurements indicate that the noted high-voltage hysteresis phenomena at polarization should be related to the process of space charge formation, which is due to the presence of heterogeneity in the copolymer under study.

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.

Structural aspects of the high-temperature space charge relaxation in ferroelectric VDF/TFE 94/6 copolymer

Structural changes in isotropic films of ferroelectric vinylidene fluoride (VDF)/tetrafluoroethylene (TFE) 94/6 copolymers in the metastable state have been studied on heating. The metastable state was ascribed to γ-crystals. The dielectric relaxation spectroscopy was employed to study space charge relaxation processes. Two relaxations were observed at high temperatures. One of them was attributed to relaxation of space charge accumulated on the Al electrode which may be partially blocked due to new functional groups formed in the polymer surface. Recrystallization of the film from the melt is accompanied by small α-crystals formation. This results in a decrease in activation energy of the space-charge relaxation process.

B-doped diamond field-effect transistor with ferroelectric vinylidene fluoride–trifluoroethylene gate insulator

A B-doped diamond field-effect transistor (FET) with a ferroelectric vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymer gate insulator was fabricated. The VDF–TrFE film deposited on the B-doped diamond showed good insulating and ferroelectric properties. Also, a Pt/VDF–TrFE/B-doped diamond layered structure showed ideal behavior as a metal–ferroelectric–semiconductor (MFS) capacitor, and the memory window width was 11 V, when the gate voltage was swept from 20 to −20 V. The fabricated MFS-type FET structure showed the typical properties of a depletion-type p-channel FET and a maximum drain current density of 0.87 mA/mm at room temperature. The drain current versus gate voltage curves of the proposed FET showed a clockwise hysteresis loop owing to the ferroelectricity of the VDF–TrFE gate insulator. In addition, we demonstrated the logic inverter with the MFS-type diamond FET coupled with a load resistor, and obtained the inversion behavior of the input signal and a maximum gain of 18.4 for the present circuit.

H-terminated diamond field effect transistor with ferroelectric gate insulator

An H-terminated diamond field-effect-transistor (FET) with a ferroelectric vinylidene fluoride (VDF)-trifluoroethylene (TrFE) copolymer gate insulator was fabricated. The VDF-TrFE film was deposited on the H-terminated diamond by the spin-coating method and low-temperature annealing was performed to suppress processing damage to the H-terminated diamond surface channel layer. The fabricated FET structure showed the typical properties of depletion-type p-channel FET and showed clear saturation of the drain current with a maximum value of 50 mA/mm. The drain current versus gate voltage curves of the proposed FET showed clockwise hysteresis loops due to the ferroelectricity of the VDF-TrFE gate insulator, and the memory window width was 19 V, when the gate voltage was swept from 20 to −20 V. The maximum on/off current ratio and the linear mobility were 108 and 398 cm2/V s, respectively. In addition, we modulated the drain current of the fabricated FET structure via the remnant polarization of the VDF-TrFE gate and obtained an on/off current ratio of 103 without applying a DC gate voltage.

Ferroelectric properties of vinylidene fluoride/tetrafluoroethylene copolymer thin films consisting of needle-like crystals

Unique ferroelectric vinylidene fluoride (VDF)/tetrafluoroethylene (TeFE) copolymer thin films consisting of needle-like crystals were formed using Au-sputtered substrates. The VDF/TeFE films with a mixing ratio of mol % were melted at 170 °C for 30 min and then recrystallized at 116 °C for 300 min. The molecules in the resultant needle-like crystals had a β-phase form and showed 200/110 orientation, while the direction of each needle-like crystal was random in-plane. The remanent polarization of the 450-nm-thick VDF/TeFE films consisting of the needle-like crystals were estimated to be 62 mC/m2 from the electric displacement vs electric field (D–E) hysteresis measurement at the applied field of 156 MV/m and the frequency of 10 Hz. The remanent polarization of 62 mC/m2 was approximately 50% larger than that of conventional VDF/TeFE films consisting of plate-like crystals.

Polarization induced resistance switching effect in ferroelectric vinylidene-fluoride/trifluoroethylene copolymer ultrathin films

We observed a clear polarization reversal-induced resistance switching effect in ferroelectric Vinylidene-fluoride (VDF)/Trifluoroethylene (TrFE) copolymer thin films. Pt and Au were used as the bottom and top electrodes, respectively, and the thickness of the VDF/TrFE copolymer film was adjusted to be 10 nm. The conduction current was 100 times higher in the case of the spontaneous polarization of the VDF/TrFE film towards the Au electrode than that in the case of the opposite direction. This resistance switching was confirmed to be reproducible after 10 successive polarization reversals.

Influence of Dipolar Interactions in Ferroelectric Vinylidene Fluoride Copolymers on their Structure and Low-Temperature Molecular Mobility

Dielectric, IR spectroscopy and DSC measurements were made over a wide range of temperatures and frequencies in copolymers of vinylidene fluoride (VDF) with tetra- (TFE) and trifluoroethylene (TrFE) of different composition. Experimental results show that decrease of VDF content in VDF/TFE copolymers is accompanied by decrease of activation energy of kinetic units in the glassy state, though relaxation times of cooperative mobility in amorphous phase differ insignificantly. Substitution of TFE with TrFE in copolymers with the same VDF content increases activation energy values for the local mobility. The results of IR spectroscopy suggest that VDF/TFE copolymers with low VDF content have lower crystallinity, which may be explained by the secondary crystallization processes and absence of long isomers in planar zigzag conformation.

Uniaxially aligned nucleation of vinylidene fluoride oligomer single-crystals on highly ordered ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymer

Uniaxially aligned single-crystals of ferroelectric vinylidene fluoride (VDF) oligomer were obtained on a highly-oriented ultra-thin film of vinylidene fluoride–trifluoroethylene copolymer P(VDF/TrFE). Trapezoidal shaped plate-like VDF oligomer crystals with 20nm thickness were selectively grown perpendicularly on P(VDF/TrFE) lamella crystals as a structural template. Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) and synchrotron two-dimensional grazing incidence X-ray diffraction (2D-GIXD) revealed that the molecular chain axis (c-axis) of P(VDF/TrFE) oriented to the friction transfer direction with the a-axis perpendicular to the film surface, and the friction-transferred P(VDF/TrFE) film induced the uniaxially aligned nucleation of the VDF oligomer with an epitaxial relationship (010)[001]VDF-oligomer//(100)[001]P(VDF/TrFE) in initial crystal growth of VDF oligomer.

Structural and Ferroelectric Characterization of Uniaxially Oriented Vinylidene Fluoride Oligomer Thin Films

A uniaxially oriented film of ferroelectric vinylidene fluoride (VDF) oligomer was fabricated by vacuum deposition on a friction-transferred vinylidene fluoride–trifluoroethylene copolymer; i.e., P(VDF/TrFE), film as a structural template. Fourier transform infrared spectroscopy and scanning electron microscopy measurements revealed that the ultra thin P(VDF/TrFE) film induced the formation of the ferroelectric form I crystal phase and the uniaxial orientation of what to the VDF oligomer by epitaxial growth. The VDF oligomer/P(VDF/TrFE) films exhibit ferroelectric hysteresis loops with a coercive field and a remanent polarization of 150 MV/m and 77 mC/m2, respectively; however, VDF oligomer films without a structural template showed no ferroelectric properties.

Solution‐derived ferroelectric vinylidene fluoride oligomer thin films with large polarization

AbstractDense and uniform vinylidene fluoride (VDF) oligomer thin films with a highly polar β phase were prepared for the first time by a low‐cost and scalable solution casting approach, after treatments of substrate surface functionalization and hot‐pressing. Introducing hydrated salt in the precursor solution effectively promoted the ferroelectric β phase. The VDF oligomer thin films obtained with short molecular chains exhibited high crystallinity and high remnant polarization (91 mC m−2), which is larger than both the polymer and copolymer counterpart films. The reasons for the observed low dielectric constant at low electric field, despite its larger polarization at high field, and the relatively high coercive field are discussed on the basis of the distinct structural characteristics of VDF oligomers. The low polar bulky end‐groups and difficulties in kink formation and propagation may result in the observed low dielectric constant at low electric field and the high coercive field. Copyright © 2011 Society of Chemical Industry

Polarization fatigue in ferroelectric vinylidene fluoride and trifluoroethylene copolymer thin films

In recent years polarization fatigue in ferroelectric polymers has attracted much attention due to their potential use in nonvolatile memories, though the mechanism is still an open question. Here we mainly focus on the experimental observations on two physical phenomena occurring during fatigue process: The frequency dependence of the fatigue rate and the change in the coercive field with increased fatigue cycles. The frequency dependence of fatigue rate shows that at lower fatigue frequencies the rate obeys a universal scaling behavior with respect to N/f, where N is the fatigue cycles and f is the fatigue frequency, while at much higher frequencies this scaling behavior is broken and the fatigue rate is N/f dependent. In addition, both the increase and the decrease of the coercive field are observed, which indicates that the change in the coercive field should be dominated by different mechanisms. Based on the understanding of the crucial influence of space charges on electrical fatigue, we give a detailed discussion on both phenomena.

Switching Dynamics in Ferroelectric Vinylidene Fluoride–Trifluoroethylene Copolymer Thin Film with α,ω-Dihexylsexithiophene Semiconductor Layer

Ferroelectric switching dynamics in thin vinylidene fluoride–trifluoroethylene copolymer films with an organic semiconductor α,ω-dihexylsexithiophene layer have been investigated on the basis of the measurements of the electrical properties including D–E hysteresis loop, switching characteristics, and capacitance changes. The authors elucidate that the full polarization reversal can be accomplished and the depletion is maintained at zero bias field. The time domain measurement together with capacitance measurements enables the evaluation of the close relationship between polarization reversal and carrier behavior in semiconductor layer. Polarization reversal toward the negative side under negative voltage progresses rapidly owing to the accumulation of majority carriers. Polarization reversal toward the positive side proceeds through two-step reversals. The mechanism of the two-step switching is explained by the depolarization field induced by the uncompensated polarization charge due to the formation of the depletion layer.

Symmetric Ferroelectric Switching in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

We report the observation of asymmetric switching dual peaks in ferroelectric copolymer films. These dual peaks occurs when the poling electric field is just below the coercive field and can be removed by continuous application of high enough switching voltage. Our experimental observations can be explained by the injection and the redistribution of space charges in ferroelectric films.

The Strong Dependence of Polarization Fatigue on Poling-Voltage Conditions in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

Recently, polarization fatigue in ferroelectric polymers has attracted much attention due to their potentials in nonvolatile memory devices, although the mechanism behind is still an open question. In this letter, we report the strong dependence of polarization fatigue on poling voltages, which indicates that the application of a poling electric field slightly below the coercive field can greatly degrade the fatigue endurance. This phenomenon may be directly related to the fatigue mechanism, and its origin is discussed based on the understanding of the injection and redistribution of space charges in ferroelectric films.

Crystal and Layer Structures of Ferroelectric Oligomer Thin Films

The crystal structure of ferroelectric vinylidene fluoride (VDF) oligomer [CF3(CH2CF2)nI] (n = 12) thin films has been investigated in detail. Films with molecular orientations parallel and perpendicular to the silicon substrate were prepared by deposition at substrate temperatures of 120 K (low-Ts) and 310 K (high-Ts), respectively. X-ray diffraction revealed a base-centered monoclinic structure with space group Cm and lattice parameters a = 0.488 nm, b = 0.865 nm, and c = 3.137/sin β nm (the monoclinic angle β could not be evaluated). The average density of both films determined from X-ray reflectivity (2.0−2.1 g/cm3) was lower than that calculated from the lattice parameters (2.42 g/cm3) due to the surface topology. Small crystal grains are loosely packed in the low-Ts film, while large hexagonal grains increase the surface roughness in the high-Ts film. In both films, furthermore, the even lower density (1.5−1.7 g/cm3) near the film−substrate interface suggests that the molecules are aligned vertically in this region.

Microscopic characterization of polarization fatigue in ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

Recently studies on electric fatigue in ferroelectric polymers have drawn much attention due to the potential application of ferroelectric polymers in nonvolatile memory devices. Compared to inorganic ferroelectrics, ferroelectric polymers are mostly semi-crystalline with a coexistence of crystalline phase and amorphous phase. To well understand electric fatigue in ferroelectric polymers, it is necessary to determine the different contributions from crystallites, amorphous phase and even boundaries between crystallites to polarization fatigue. Here we introduce a SPM-based technique to characterize electric fatigue in nanometer scale. Microscopic fatigue is determined by the attenuation of voltage-excited local vibration amplitudes at two kinds of local structures (the crystallites and the boundaries between crystallites), and, for comparison, macroscopic electric fatigue characteristics are also recorded by a standard Sawyer–Tower circuit. Our observations indicate that the attenuation rate of vibration amplitude on crystallites is much slower than that at boundaries. Abnormal increase of vibration amplitude and slim local butterfly loops are observed at some of boundaries in deeply fatigued films.

The possibility of polarization recovery in fatigued ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

Simple treatments were made to study the possibility of polarization recovery in fatigued P(VDF–TrFE) films. Annealing at a temperature below or above the Curie point of ferroelectric films would not improve the ferroelectric property of fatigued films, while a long-time rest would induce partially or even fully polarization recovery, though P–E loops obtained from recovered films showed a rounded shape. Some discussions were also given to explain our observations.

Observation of asymmetric ferroelectric switching in fatigued ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

In this paper, we illuminated the observation of asymmetric ferroelectric switching phenomena in fatigued P(VDF-TrFE) copolymer films. Two types of asymmetric switching were observed: one was the occurrence of switching dual peaks obtained from films with asymmetric electrode structure and the other was the jump of remanent polarization during fatigue process, as was obtained mostly from films with asymmetric electrodes and occasionally from films with symmetric electrodes. No evidence indicated the dependence between the switching dual peaks and the jump of remanent polarization. The possible mechanism of such asymmetric phenomena was also discussed.