**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 [21]. 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 *tw* is basically constant (Fig. 10(a)), while the current density is independent of the capacitor areas (Fig. 10 (b)). This indicates that the charging current around the coercive field is limited by domain switching instead of the series resistor in the measurement circuit. Thus, it can reflect directly the speed of domain switching. The relationship between switching current and the electric field across the film fits well to the creep model with μ=0.5 (Fig. 11). The exact dynamical exponent μ was found to be 0.5011, and resultant parameters di =2 and n=1, respectively. (An interface is characterized by its dimension di (di =1 for a line or 2 for a surface) and can move in n transverse directions.) The result implies that the two-dimensional domain walls propa‐ gate along one transverse direction.

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

New Properties and Applications of Polyvinylidene-Based Ferroelectric Polymer http://dx.doi.org/10.5772/60946 161

films under electric fields or on cooling demonstrate a behavior different from that of the polar nanoregions (PNRs) of the Pb(Mg1/3Nb2/3)O3 (PMN) system [19]. The weak intermolecular interactions impede the development of some *TTTG'* conformations into all-trans ones along the direction perpendicular to the molecular chains, so these *TTTG'* conformations can largely retain their dynamics. This may be responsible for both the relaxor nature of the P(VDF-TrFE-CFE) terpolymer and the difference between its *Pr* and *Ps* values, even in its ferroelectric state.

The temperature dependences of the ferroelectricity of P(VDF-TrFE-CFE) terpolymer films were systemically investigated. Both the polarization current (*∂*Pr/*∂*T) and the dielectric response derived from the *P-E* loop at zero field, suggesting that a ferroelectric phase transition occurs at 270 K. Distinct differences were observed in the P(VDF-TrFE-CFE) terpolymer films compared with perovskite relaxors, e.g., a broad peak in the *∂*Pr/*∂*T curve, deviation from Merz's law at high frequency, and a smaller activation field [20]. These differences are considered to be caused by the existence of the less-polar *TTTG* conformation in the ferroelec‐

**7. The creep process of the domain switching in P(VDF-TrFE) ferroelectric**

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 [21]. 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 *tw* is basically constant (Fig. 10(a)), while the current density is independent of the capacitor areas (Fig. 10 (b)). This indicates that the charging current around the coercive field is limited by domain switching instead of the series resistor in the measurement circuit. Thus, it can reflect directly the speed of domain switching. The relationship between switching current and the electric field across the film fits well to the creep model with μ=0.5 (Fig. 11). The exact dynamical

(di

in n transverse directions.) The result implies that the two-dimensional domain walls propa‐

for the polarization switching process in a crystalline lamella of the P(VDF-TrFE). Firstly, 180

 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

=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

=2 and n=1, respectively. (An

=1 for a line or 2 for a surface) and can move

=2 and n=1 in the present study, a model was proposed

exponent μ was found to be 0.5011, and resultant parameters di

interface is characterized by its dimension di

gate along one transverse direction.

Considering the derived parameters di

160 Ferroelectric Materials – Synthesis and Characterization

the domain switching in the lamellae

tric state.

**thin films**

o

with di

**Figure 10.** The variation of transient current with different capacitor areas under Va =40 V, RL=1 KΩ. (b) Jsw dependence of Vc with different capacitor areas for P(VDF-TrFE) film

**Figure 11.** Dependence of domain switching current on the reciprocal of the square root of Ec.

### **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 [22]. 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.

**Figure 12.** Pyroelectric voltage of the device under infrared radiation (a) before poling, (b) after poling at -1 V, (c) after poling at +1 V, and (d) 12 h after poling at -1 V

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.
