There are different kinds of novel properties and applications of polyvinylidene difluoride (PVDF)-based ferroelectric polymer films. Several issues associated with the structure, properties, and applications of PVDF-based ferroelectric polymer films are discussed. The main achievements of the research include high electric tunability of relaxor ferroelectric Langmuir–Blodgett (LB) terpolymer films, the creep process of the domain switching in poly(vinylidene fluoride-trifluoroethylene) ferroelectric thin films, transition from relaxor to ferroelectric-like phase in poly(vinylidene fluoride-trifluoroethylene -chlorofluoroethylene) terpolymer ultrathin films, abnormal polarization switching of relaxor terpolymer films at low temperatures, huge electrocaloric effect in LB ferroelectric polymer thin films, self-polarization in ultrathin LB polymer films, enhanced dielectric and ferroelectric properties in artificial polymer multilayers, and transition of polarization switching from extrinsic to intrinsic in ultrathin PVDF homopolymer films.
- Poly(vinylidene fluoride)
- Langmuir–Blodgett technique
- Ferroelectric polymer
- Ferroelectric relaxor
Polyvinylidene difluoride (PVDF), consisting of (–CF2–CH2–)n, with a carbon chain and hydrogen and fluorine atom on the two sides of carbon, respectively, is not a new synthesized material, which has been found in 50 years ago. In the beginning, PVDF was studied for its high dielectric permittivity and for the diversity of crystalline-phase types. About 10 or 15 years later, piezoelectricity and ferroelectricity properties were found in PVDF materials [1–6].
As it is known, PVDF was found in possession of four or more crystalline-phase types. It consists of the α-phase, β-phase, γ-phase, and σ-phase. Phase types are related to various molecular configurations. In these different phase types, the β-phase is of polar form and the α-phase is of antipolar form; the molecular configuration is shown in Fig. 1. The antipolar form α-phase PVDF can be transformed to the β-phase polar form by rapid cooling from the melting or stretching along the carbon chain [1,2,5].
The other two types of PVDF phases derive from the β-phase and α-phase. Recently, it has been proved that the σ-phase PVDF is also a polar form. When the poly(trifluoroethylene) is added into the PVDF, a new type of copolymer P(VDF-TrFE), consisting of –((–CF2–CH2)
The terpolymer derived from the P(VDF-TrFE) is the ferroelectric relaxor polymer P(VDF-TrFE-CFE), which adds chlorofluoroethylene into the P(VDF-TrFE) copolymer. PVDF-based polymer films possess many special properties: dielectric, ferroelectric, piezoelectric, pyroelectric properties and so on. Based on these properties, these films can be used as transducers, ferroelectric memory, gate of transistor, and uncooled infrared sensor. In the following paragraph, some special properties and potential applications of PVDF-based polymer films will be introduced.
There are several methods for preparing the PVDF-based films, for instance, spin coating, wire bar method, and Langmuir–Blodgett (LB) technique [7–9]. The mostly used film preparing method is spin coating, which is widely used for preparing the films with thickness larger than 30 nm. It is very difficult to get the ultrathin PVDF-based films based on spin coating and other normal methods. The LB method can be used to prepare the ultrathin two-dimensional ferroelectrics. In this chapter, the films of PVDF-based films mostly fabricated using LB technology. In 1995, S. P. Palto, L. M. Blinov, and V. M. Fridkin et al. began studies of ultrathin ferroelectric LB films of P(VDF-TrFE) copolymers with trifluoroethylene, P(VDF-TrFE) . In 2007, J.L. Wang et al. prepared the ultrathin P(VDF-TrFE-CFE) terpolymer LB films . Recently, we achieved the ultrathin PVDF homopolymer films with good ferroelectric performance. The detail of preparing the PVDF-based polymer films by LB technique is described as follows. The typical characteristic of the LB method is one monolayer (ML) at a time by repeatedly dipping a substrate into a liquid subphase coated with a monolayer of the desired polymer. The PVDF-based polymer should be firstly dissolved in dimethylformamide to form a dilute solution 0.01–0.05 wt%. The liquid subphase is ultrapure water with 18 MΩ/m. After the PVDF-based polymer solution is dropped into the water, about 30 min later, the PVDF-based polymer molecule chain will be floating on the surface of the water (as shown in Fig. 2a). In this case, the films are not uniform. The next step is pressing the bar of the LB technique until the surface pressure is up to 5 mN/m or less (as described in Fig. 2b). In this processing, if the surface is too large, the film on the water surface will collapse. The final step is shown in Fig. 2c, dipping the substrate horizontally on the surface of the water covered with monolayer PVDF molecule. Then raising the substrate slowly, one monolayer of PVDF ultrathin film is achieved. Repeating this process, the different thicknesses of PVDF-based film will be produced.
PVDF-based polymer films possess many special properties, for example, dielectric, ferroelectric, piezoelectric, and pyroelectric properties and so on. Based on these properties, the films can be used for preparing the transducers, ferroelectric memory, gate of transistor, and uncooled infrared sensor. In the following paragraph, some special properties and applications will be introduced.
3. Dielectric tunability properties of PVDF-based polymer
The dielectric tunability of the PVDF has been studied by Lu et al. in 2008; the huge tunability can be reached in the P(VDF-TrFE) copolymer films, but in the P(VDF-TrFE-CFE) relaxor terpolymer films, the tunability is lower than the copolymer’s . For the traditional way of the relaxor ferroelectrics, the electric tunabilities are very large. Since the film thickness is controlled on a molecule level, ferroelectric LB polymer films have shown some exceptional properties, such as excellent crystallinity, two-dimensional ferroelectricity, surprising giant breakdown voltage, etc.; it is expected that terpolymer films derived from the LB technology can provide a special microstructure to study the origin of the excellent properties of relaxor ferroelectric terpolymers. High-quality ultrathin films of both ferroelectric P(VDF-TrFE) and relaxor ferroelectric P(VDF-TrFE-CFE) have been successfully fabricated by using the LB technique.
The P(VDF-TrFE-CFE) shows a typical relaxor from temperature dependences of the dielectric constant and dielectric loss versus frequency, as it is shown in Fig. 3.
A tunability of 80 % at 240 MV/m was obtained in P(VDF-TrFE-CFE) terpolymer LB films (shown in Fig. 4), which is due to the highly ordered molecules and the high breakdown electric filed.
What are the reasons for the large tunability in our terpolymer LB films? In our opinion, they should be associated with the special microstructure of the terpolymer LB films. It is known to us that LB polymers demonstrate some exceptional features such as good crystallinity and highly planar ordered and close parallel packing of the molecules.
4. Huge electrocaloric effect in LB ferroelectric polymer thin films
Recently, the huge electrocaloric effect (ECE) resulting from changes in the entropy and temperature of a material under an applied electric field has attracted the attention of researchers to ferroelectric materials [11,12]. The ECE occurs in both ferroelectric and paraelectric phases and is found to be larger in the paraelectric phase just above the ferroelectric–paraelectric phase transition .
In a working cycle-based ECE, the working material contacts the load and absorbs entropy from it. Then the material is isolated from the load and an electric field is applied. With an increase in the electric field, the polarization and temperature of the working material increase under adiabatic condition. The material is then placed in thermal contact with the heat sink and transfers the entropy absorbed from the load to the heat sink. Then the material is isolated from the heat sink. As the applied field is reduced, the temperature of the material decreases back to the temperature of the cooling load. Thus, the larger the ECE of the working material, the better the efficiency of cooling.
We present a detailed investigation of the ECE of P(VDF-TrFE) and P(VDF-TrFE-CFE) films grown by the LB technique on technologically desirable aluminized flexible polyimide substrates. Our results clearly show a large ECE, as the variations of adiabatic temperature ∆
Especially for P(VDF-TrFE-CFE) terpolymer films, the peak of ∆
5. Ferroelectric-like phase transition in P(VDF-TrFE-CFE) terpolymer ultrathin films
P(VDF-TrFE-CFE) terpolymer has attracted considerable attention for its fruitful properties and related potential application. Investigations have shown that the all-trans (
The temperature dependence of the complex permittivity for the terpolymer films measured at 1 kHz in the heating process is shown in Fig. 6. For the film with 10 nm, the temperature of permittivity maximum
The coercive voltage is 1.2 V, which is also in accord with the polarization versus electric field (
A hypothesis can be deduced from the above results that all-trans-like molecular conformations form in the ultrathin terpolymer films. As mentioned earlier, the dominant conformation in P(VDF-TrFE-CFE) terpolymer is the less polar
The ferroelectric-like phase transition is observed in the P(VDF-TrFE-CFE) terpolymer films as the thickness is lower than 3 nm. The ferroelectric-like features are considered to result from the induced electric field due to the mirror charges in the electrodes.
6. Abnormal polarization switching of relaxor terpolymer films at low temperature
The temperature dependences of the dielectric and ferroelectric properties of terpolymer films produced using the LB method were systematically investigated, with an emphasis on the nature of the ferroelectricity at low temperatures .
The change of remanent polarization (
A broad peak at ~270 K is displayed in the plot of
This suggests that some less-polar molecular conformations (TTTG') still affect the polarization switching. A deviation from Merz’s law was observed in the relationship between the coercive field and the frequency (Fig. 9). The deviation from Merz’s law at high frequency further evidences the presence of
The relaxor P(VDF-TrFE-CFE) terpolymer, the CFE monomer is introduced into the ferroelectric P(VDF-TrFE) copolymer as a defect, leading to all-trans polar conformations being converted into less-polar conformations, i.e.,
The temperature dependences of the ferroelectricity of P(VDF-TrFE-CFE) terpolymer films were systemically investigated. Both the polarization current (
7. The creep process of the domain switching in P(VDF-TrFE) ferroelectric thin films
The polarization switching behavior in poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) (70/30 mol%) thin films was investigated using a pulse transient current method . The dependence of the domain switching current on the coercive electric field was derived. The current in the plateau region increases with the capacitor areas, whereas
Considering the derived parameters di=2 and n=1 in the present study, a model was proposed for the polarization switching process in a crystalline lamella of the P(VDF-TrFE). Firstly, 180 o rotation of dipolar appears along a single-chain molecule; secondly, intermolecular expansion of chain rotations along external applied electric field with the switched molecular chains as the center because of the minimization of the depolarization energy; thirdly, domain walls with di=2 appeared at both sides of the switched dipolar plane. The domain wall, assumed to have a shape like a thin slab, propagates slowly, corresponding to n=1, till the completion of the domain switching in the lamellae
8. Self-polarization in ultrathin LB polymer films
Ultrathin copolymer films of P(VDF-TrFE) were deposited on Al-coated polyimide substrates, by the LB method. A top Al electrode was evaporated onto the polymer film to form an Al/polymer/Al structured infrared detector. The pyroelectric voltage response of the detector under various polarizing processes was characterized. The detector with only one transferred polymer layer exhibited a preferential polarization direction. This was considered to result from the self-polarization of the ultrathin polymer film . It was due to the preferred alignment of the dipoles on the Al substrates. This process can be applied for designing stable fast-response infrared detectors.
The fresh unpolarized device shows an appreciable pyroelectric voltage response, suggesting a preferential polarization. Upon polarizing the device at -1 V, the voltage response increases by a factor of 2, compared with the fresh device. Upon polarizing at +1 V, the voltage response decreases, in comparison with the fresh device. The applied 1 V is higher than the coercive electric field of the P(VDF-TrFE) ultrathin film with only 1 ML, reported in our previous investigation. Thus, the pyroelectric voltage responses under different polarizing directions should exhibit a 180° phase difference, but no such phase difference is observed in Fig. 12. This may be due to the back switching of domains, after removal of the positive poling voltage. It also suggests that the preferential polarization of the fresh device is aligned from the bottom electrode to the surface of the P(VDF-TrFE) film. The unpolarized detector exhibited a preferential voltage response.
This was considered to result from the self-polarization of the ultrathin P(VDF-TrFE) polymer film, due to the preferred alignment of the dipoles on the Al substrates. This result can be used to fabricate fast-response room temperature infrared detectors.
9. Electronic transport property in ferroelectric polymer films
The leakage current mechanism of ferroelectric copolymer of P(VDF-TrFE) prepared by LB was investigated in the temperature range from 100 K to 350 K. The electron as the dominant injected carrier was observed in the ferroelectric copolymer films. The transport mechanisms in copolymer strongly depend on the temperature and applied voltage. From 100 K to 200 K, Schottky emission dominates the conduction. With the increase of temperature, the Frenkel–Poole emission instead of the Schottky emission conducts the carrier transport. When the temperature gets to 260 K, the leakage current becomes independent of temperature, and the space charge limited current conduction was observed .
The P(VDF-TrFE) film shows a saturated hysteresis loop with a remanent polarization (Pr) of ~6.8 μC/cm2 and saturation polarization (Ps) of ~11 μC/cm2, respectively, which indicates good ferroelectricity. It was previously reported that the electron affinity of β-P(VDF-TrFE) was about 4 eV, based on a density functional theory study . The work function value of Au and Al metals are 5.1 eV and 4.1 eV , respectively.
The conduction through the lowest unoccupied molecular level (LUMO) of P(VDF-TrFE) and the leakage current is controlled either by the interface energy barrier that exists between the Fermi level of the metal and the LUMO level of the polymer or by the bulk-controlled mechanics such as Frenkel–Poole emission and space-charge-limited current (SCLC) conduction [26,27]. The schematic diagram of the band structure of the P(VDF-TrFE) and the work functions of Au and Al are presented in the figure. The temperature dependence of the I-V behaviors from 100 K to 350 K was measured, and the temperature dependence of the current density under voltage 5 V (about 70 MV/m) is presented in Fig. 13. It can be seen that the current density increased with the temperature, increasing from 100 K to 260 K, but it is nearly independent of temperature as the temperature is higher than 260 K.
The electric conduction of the ferroelectric P(VDF-TrFE) copolymer films has been comprehensively investigated. It is found that the electrons are the dominant injected carriers in the P(VDF-TrFE) films, and the charge injection occurs either at the polymer/electrode interface or in the bulk polymer films. Various transport mechanisms are observed in the P(VDF-TrFE) films, which are influenced by both temperature and applied voltage.
Schottky emission and Frenkel–Poole emission are found to be the dominant transport mechanism in the temperature range from 100 K to 200 K and the range from 200 K to 260 K, respectively. Space-charge-limited current conduction is the main transport mechanism as the temperature is higher than 260 K.
10. Enhanced electric properties in the artificial polymer multilayers
Multilayers consisting of alternative ferroelectric P(VDF-TrFE) copolymer and relaxor P(VDF-TrFE-CFE) terpolymer with different periodicities in thickness were prepared. A superlattice-like structure is shown in the polymer multilayer as the periodic thickness is lower than a critical value. The dielectric constant of the multilayer with a small periodic thickness is two times higher than that of the P(VDF-TrFE) copolymer over a temperature range between 300 K and 350 K. The multilayer also shows a good ferroelectricity in the same temperature range. The enhanced electrical properties of the multilayers are due to the long-range ferroelectric coupling [28,29].
Organic ferroelectric polymers have recently attracted much attention for their potential applications in flexible electronic devices, such as display, solar cell, information storage, and so on [30–32]. However, compared with its inorganic counterpart, organic ferroelectric polymer has some drawbacks, e.g., the lower dielectric constant and electric polarization, which is an obstacle for their practical applications. It is well known that many artificial superlattices (SL) and multilayers (ML) based on perovskite ferroelectrics show some amazing properties [33–35].
High-quality ultrathin films of both ferroelectric P(VDF-TrFE) and relaxor ferroelectric P(VDF-TrFE-CFE) have been successfully fabricated using the LB technique, which provide precise control of the film thickness in molecular scale. In the present study, periodic multilayers composed of alternating ferroelectric P(VDF-TrFE, 70/30) layer and relaxor ferroelectric P(VDF-TrFE-CFE, 56.2/36.3/7.5) layer were fabricated.
Figure 14 shows the XRD patterns of the multilayers with different periodicities. Two diffraction peaks were observed in the multilayer structure of (15/15)2, i.e., 2θ = 18.57 and 19.97, which are assigned to the typical (110,200) reflection of the P(VDF-TrFE-CFE) and (110,200) reflection of P(VDF-TrFE), respectively. Compared with the pure P(VDF-TrFE-CFE) and P(VDF-TrFE), there is no shift in the (110,200) peaks for the (15/15)2 multilayer, suggesting that both the terpolymer and copolymer components in the multilayer keep their original phase structure. When the periodic thickness is decreased to five transfer layers, a shift is observed in both the diffraction peaks. Simultaneously, the intensity of the diffraction peak associated with the pure terpolymers becomes weaker.
The temperature dependence of the dielectric constant of the multilayer structures on the heating process is shown in Fig. 15. Dielectric constant calculated with a series capacitor model of the individual copolymer and terpolymer is also shown in Fig. 15. It can be seen that the temperature dependence of the dielectric constant for the multilayer with the transfer number m larger than 10 is analogous to that of the series model, which shows a platform between 300 K and 360 K. This suggests that these multilayers with thick interlayer are just a combination of individual copolymer and terpolymer components.
The piezoelectric properties of the copolymer P(VDF-TrFE) and terpolymer P(VDF-TrFE-CFE) multilayers are presented in Fig. 17.
The piezoelectricity of the samples was measured by piezoresponse force microscopy (PFM). To increase the degree of accuracy and allow meaningful comparison of the piezoresponses of the samples, measurements were made at ten different locations for all samples and then averaged. The piezoelectric coefficient
In summary, the multilayers composed of alternating P(VDF-TrFE) copolymer and P(VDF-TrFE-CFE) terpolymer layers have been prepared, and their crystal structure and dielectric and ferroelectric properties have been studied. The multilayers with a periodic thickness of ~3 nm shows a superlattice-like crystal structure, high dielectric constant, good ferroelectricity, and piezoelectricity over a wide temperature range from 300 K to 350 K. The long-range ferroelectric coupling is considered to be dominant for multilayers with a smaller periodic interlayer.
11. Polarization switching properties of PVDF homopolymer films
PVDF homopolymer thin films have been prepared by the Langmuir–Blodgett technique, and their electrical properties have comprehensively been studied . The PVDF homopolymer films show better ferroelectricity with higher polarization and higher breakdown electric field than that of P(VDF-TrFE) copolymer films.
The phase image of the piezoresponse shows a polarization switching in the PVDF homopolymer films, suggesting a typical ferroelectric feature. The ferroelectric
In this study, the film thickness dependence of th
PVDF-based ferroelectric polymers have been studied for many years, and widely used in many electronic devices, for example, transducers, actuators, switches, and infrared sensors. Nonetheless, there are lots of novel properties that need to be explored. In this chapter, the LB method has been used for preparing ultrathin films of PVDF-based films. In addition, many special characteristics of these films have been generalized. These special properties include high tunability, huge electrocaloric effect, polarization switching, self-polarization, and enhanced electric properties in the artificial polymer multilayers. Besides these properties and potential applications, ferroelectric polymers possess many other advantages for applications. The advantages of this type of ferroelectric polymers include low cost, ease and flexibility of fabrication in different kinds of thin film forms, and resistance to degradation caused by strain. PVDF-based polymers are also more readily altered to conform to complex device requirements imposed by the environment, size, shape, physical flexibility, reliability, durability, and other constraints. PVDF-based films can be easily patterned for integrated electronic applications.