**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 superlatticelike 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 diffrac‐ tion 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.

or in the bulk polymer films. Various transport mechanisms are observed in the P(VDF-TrFE)

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

**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 superlatticelike 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

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

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

Figure 14 shows the XRD patterns of the multilayers with different periodicities. Two diffrac‐ tion 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

films, which are influenced by both temperature and applied voltage.

temperature is higher than 260 K.

164 Ferroelectric Materials – Synthesis and Characterization

coupling [28,29].

properties [33–35].

terpolymers becomes weaker.

P(VDF-TrFE-CFE, 56.2/36.3/7.5) layer were fabricated.

**Figure 14.** The XRD patterns of the multilayers with various periodicities. The inset shows the XRD patterns of copoly‐ mer and terpolymer

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.

**Figure 15.** The dielectric constant of multilayers with different periodicities and the calculated dielectric constant using series model as a function of temperature

The *P-E* hysteresis loops for the multilayers as well as the pure copolymer and terpolymer films are shown in Fig. 16. The (1/1)30 multilayer structure displays a good ferroelectricity with a remanent polarization (*P*r) of ~ 5.2 μC/cm2 , which is just a little smaller than that of pure P(VDF-TrFE) thin films. The temperature dependence of the *P*<sup>r</sup> of the multilayers with various periodic thicknesses is summarized in the inset of Fig. 3.

**Figure 16.** *P-E* hysteresis loops of the multilayers with different periodicities, pure copolymer and terpolymer films at 1 kHz. The inset shows *Pr* of (1/1)30 multilayer as a function of temperature

The piezoelectric properties of the copolymer P(VDF-TrFE) and terpolymer P(VDF-TrFE-CFE) multilayers are presented in Fig. 17.

**Figure 17.** The piezoelectric response curves of multilayers with different periodicities. The inset compares the piezo‐ electric response curves of the (1/1)30 multilayer, P(VDF-TrFE) and P(VDF-TrFE-CFE) homogenous films

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 *d33* for various multilayer structures are listed in Fig. 19. The *d33* of the pure copolymer and terpolymer are 41 pC/N and 52 pC/N, respectively, which is close to the reported value for the copolymer (49 pC/N) and equal to that observed for the terpolymer. Interestingly, an enhanced piezoelectric response of 81.6 pC/N is observed for the (1/1)30 multilayers, which is nearly 57 % larger than the pure terpolymer. As the periodic thickness increases, the piezoelectric response of the multilayers decreases dramatically.

a remanent polarization (*P*r) of ~ 5.2 μC/cm2

166 Ferroelectric Materials – Synthesis and Characterization

periodic thicknesses is summarized in the inset of Fig. 3.

1 kHz. The inset shows *Pr* of (1/1)30 multilayer as a function of temperature

multilayers are presented in Fig. 17.

, which is just a little smaller than that of pure

P(VDF-TrFE) thin films. The temperature dependence of the *P*<sup>r</sup> of the multilayers with various

**Figure 16.** *P-E* hysteresis loops of the multilayers with different periodicities, pure copolymer and terpolymer films at

The piezoelectric properties of the copolymer P(VDF-TrFE) and terpolymer P(VDF-TrFE-CFE)

**Figure 17.** The piezoelectric response curves of multilayers with different periodicities. The inset compares the piezo‐

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

electric response curves of the (1/1)30 multilayer, P(VDF-TrFE) and P(VDF-TrFE-CFE) homogenous films

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.
