Ferroelectric Polymer Thin Films Research Articles

Three dimensional phase field study on the thickness effect of ferroelectric polymer thin film

The electromechanical behavior of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] ferroelectric thin film was investigated using the three dimensional (3D) phase-field method. Various energetic contributions, including elastic, electrostatic, and domain wall energy were taken into account in the variational functional of the phase field model. Evolution of the microscopic domain structures of P (VDF-TrFE) polymer film was simulated. Effects of the in-plane residual stress, the film thickness and externally applied electric bias field on the electromechanical properties of the film were explored. The obtained numerical results showed that the macroscopic responses of the electric hysteresis loops are sensitive to the residual stress and electric bias field. It was also found that thickness has a great effect on the electric hysteresis loops and remanent polarization.

Huge electrocaloric effect in Langmuir–Blodgett ferroelectric polymer thin films

In this work, we studied polyvinylidene fluoride (PVDF)-based ferroelectric copolymer and terpolymer films grown, by the Langmuir–Blodgett (LB) technique, on polyimide substrates. The LB technique is believed to improve the film quality, i.e. to confer better crystallinity and less parasitic amorphous phase. As a consequence, a reversible adiabatic temperature change ΔT up to 21 K is demonstrated. The large electrocaloric effects of LB films close to room temperature suggest the potential for applying LB films in cooling systems.

Nanoscale domain growth dynamics of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin films

Nanoscale domain growth dynamics of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin films were investigated by piezoresponse force microscopy. A 90 nm thick ferroelectric polymer thin films were fabricated on Au substrate by spin-coating method. The domain size of nanoscale dot pattern was linearly proportional to logarithmic value of the pulse width. However there was a significant asymmetry in nucleation and lateral domain growth depending on the voltage polarity, which implies the existence of the preferred polarization states. The obtained activation field indicates the nucleation-limited domain switching behaviors of ferroelectric polymer thin films.

Electrospray induced ferroelectricity in poly(vinylidene fluoride) thin films

Ferroelectric polymer thin films are interesting for applications in flexible and lightweight electronic equipment. Among the ferroelectric polymers, poly(vinylidene fluoride) (PVDF) possesses the highest theoretical ferroelectric moment. However, development of single-step production methods of PVDF thin films containing a significant ferroelectric moment is difficult, due to the large number of processing requirements that need to be fulfilled simultaneously: film deposition, ferroelectric β-phase formation, and dipole alignment. Nonetheless, electrospray deposition appears to be a promising method. The structure of electrospray-deposited films has been studied by infrared absorption and the ferroelectricity of the films has been determined by AFM methods and by macroscopic I–V measurements. Films prepared with solutions of 0.05 wt% PVDF in DMF consist solely of small ferroelectric β-phase crystallites embedded in amorphous phase. The formation and alignment of β-phase dipoles occur progressively during electrospray deposition because of the ion current present in electrospray. Moreover, if the non-ferroelectric α-phase is initially present in the film, it disappears progressively as a function of spray time. Thus, electrospray can provide PVDF thin films with a level of ferroelectricity depending on the spray current and on the spray time.

Recent advances in ferroelectric polymer thin films for memory applications

An overview of ferroelectric VDF/TrFE copolymer thin films is presented based on our recent work on their phase transition behavior and polarization switching dynamics. These properties originate in the hierarchical structures consisting of monomer dipoles, all-trans conformation, parallel chain packing and crystalline orientation in thin films. Successive loss of intermolecular and intramolecular orders yields multiple phase transitions from a ferroelectric phase to a molten phase via anti-ferroelectric and paraelectric phases. Polarization switching dynamics are shown to depend on a variety of factors not only applied field strength but also electrode, substrate, thickness and wave form of applied field. These results are discussed in relation to their hierarchical structures as well as nonvolatile memory applications.

Multiscale kinetic model for polarization switching in ferroelectric polymer thin films

Polarization switching in ferroelectric polymers is studied using a multiscale framework. A continuum Landau-Ginzburg-Devonshire model for a first-order phase transition is parametrized for ideal all trans chains of P(VDF-TrFE) (70:30) copolymers using data obtained from molecular-dynamics (MD) simulations. Thermal fluctuations and kinetics are accounted for by using a time-dependent Ginzburg-Landau model where the length and time scales, as well as the thermal noise amplitude, are also set from MD simulations. This method is used to investigate the nature of polarization switching in ferroelectric polymers and to test recent claims that ultrathin ferroelectric polymer films undergo intrinsic switching. Our simulations show that for a defect-free system, domain nucleation due to thermal fluctuations prevents homogeneous switching of the polarization, even at very small thicknesses. However, this nucleation does not substantially decrease the coercive field compared to the intrinsic value.

Restricted domain growth and polarization reversal kinetics in ferroelectric polymer thin films

A combination of microscopic and macroscopic methods enables an insight into polarization reversal in the copolymer of vinylidene fluoride and trifluoroethylene. Piezoforce microscopy together with switching charge measurements suggests that the polarization reversal is impeded by the restricted geometry of the domain growth. This effect limits the applicability of the conventional switching model of Kolmogorov-Avrami to the first stage of the polarization reversal. The interface-adjacent passive layer is found to impact the switching properties considerably resulting in a retardation of the polarization reversal. Furthermore, an extraordinary dielectric constant increase is observed in the films with a passive layer due to an additional domain wall contribution.

Scanning Probes of Nonlinear Properties in Complex Materials

Nonlinear materials have found wide applications in electronic devices. Of special importance and interest are those with varistor-type grain boundaries and those displaying large piezoelectric and electrostrictive responses to external fields. To access the local properties of these materials, we have designed two new contact mode scanning probe techniques. The first technique referred to as nanoimpedance microscopy/spectroscopy, which incorporates impedance spectroscopy with a conducting AFM tip as an electrode, has been demonstrated on measuring local grain boundary properties of a ZnO varistor, as well as the contact quality between AFM tip and a ferroelectric sample. The other technique referred to as second harmonic piezoresponse force microscopy developed to measure the electrosctriction has been demonstrated on ferroelectric polymer thin film. A theoretical model has been presented to describe the contrast formation of the second harmonic as well as the first harmonic (piezoresponse) images and to explain the observed hysteretic field dependence of the second harmonic amplitude signal.

Characterization of Nanoscale Domains in Ferroelectric Polymer Thin Films by Scanning Probe Microscopy

Nanoscale domains have been formed and detected in ferroelectric polymer thin films using scanning probe techniques. The “erasing” and “rewriting” of local domains are also demonstrated and discussed in this article. The results suggest that ferroelectric polymer is a promising ultrahigh-density information storage medium.

Structures and local electrical properties of ferroelectric polymer thin films in thermal process investigated by dynamic-mode atomic force microscopy

Local changes in structure and surface potential of ferroelectric copolymer thin films in thermal processes were directly investigated by dynamic-mode atomic force microscopy. The copolymer thin films formed on graphite substrates consist of rod-like grains when they are annealed at a temperature between the Curie point and the melting point. Atomic force microscopy (AFM) observations revealed that the fine structures in the rod-like grains suddenly disappeared and that the grains turned smooth at approximately 120°C during the heating process. They in turn gradually reappeared from 85 to 70°C during the cooling process. These two transition temperatures approximately corresponded to the Curie points of bulk copolymer. We also measured the local surface potential variation in the heating process by Kelvin force microscopy. These results show the ferroelectric phase transition is accompanied by the changes of both the internal structure and the surface potential of the grains.

Molecular Ferroelectricity of Vinylidene Fluoride Oligomer Investigated by Atomic Force Microscopy

Nanometer-scale electrical properties of local ferroelectric domains formed in thin films of newly synthesized vinylidene fluoride (VDF) oligomer were investigated by atomic force microscopy (AFM). Local poling and observation of the piezoelectric response revealed that the polarized domains were reversibly formed and erased in a nanometer-thick film by applying dc or pulse voltages between the electrically conductive AFM tip and the bottom electrode. The formed domain size depends on the pulse poling conditions and increases when the magnitude and the duration of a pulse voltage are increased. A local domain with a diameter of 65 nm was successfully created. The results in this work are comparable to those of previous studies performed on ferroelectric polymer thin films, suggesting that this material is one of the promising candidates for ferroelectric applications such as high-density data storages, and for molecular controls of ferroelectric properties.

Critical thickness of crystallization and discontinuous change in ferroelectric behavior with thickness in ferroelectric polymer thin films

We report on the observation of the critical thickness of crystallization of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer thin films, which were solution spun cast on platinum coated silicon wafer. The effect occurs at about 100 nm thickness, which is significantly above any currently known spatial dimensions of the polymer, so that for films at thickness below about 100 nm, the crystallization process is strongly hindered, resulting in a low crystallinity in these films. This low crystallinity leads to a large and discontinuous change of the dielectric constant and ferroelectric polarization in the films below the critical thickness.

Nanometer-Scale Characterization of Ferroelectric Polymer Thin Films by Variable-Temperature Atomic Force Microscopy

The morphological changes of ferroelectric polymer thin films during the annealing process were directly imaged by variable-temperature atomic force microscopy (AFM). The growing process of the crystallites underlying the surface amorphous layer was observed as well as the morphological changes using intermittent-contact AFM. We found that fine structures appeared in the rod like grains at a temperature corresponding to the Curie point of the bulk copolymer in the cooling process. In addition, local piezoelectric response was mapped in order to investigate the relationship between the structural change and the ferroelectric phase transition. The local polarization prepared at room temperature disappeared after the film was heated above 120°C, which agreed with both the Curie temperature and the temperature of the observed structural change.

A Metal-Insulator-Semiconductor (MIS) Device Using a Ferroelectric Polymer Thin Film in the Gate Insulator

A nonvolatile MIS memory device using a ferroelectric polymer thin film in the gate insulator is proposed. In the gate electrode of the device, a ferroelectric polymer thin film is sandwiched between two insulator films to prevent carrier injection into the polymer thin film. Al-SiO2-P (VDF/TrFE)-SiO2-Si capacitors were fabricated to evaluate the basic characteristics of the device by C-V measurement, and ferroelectric polarization reversal was observed in the capacitors. Based on the C-V measurements, MIS transistors were fabricated using a process which virtually self-aligns the effective gate area to the source/drain. It was shown that the MIS transistor could be electrically programmed and erased. The on/off ratio of the transistor was greater than 106.