Physical properties

The composite thin film P(VDF-TrFE)/ TiOPc contained 0 wt%, 0.5 wt%, 2.5 wt%, 5 wt%, 10 wt% and 20 wt% TiOPc particles in the P(VDV-TrFE) solution were designated as Samples 1 to 6, respectively. The surface morphologies of sample 1~4 and TiOPc powders were analyzed by SEM images (Figure 4-10). The P(VDF-TrFE) copolymer grew along the TiOPc particles when the solvent DMF evaporated. From a morphology point of view, it was expected that the piezoelectric properties of the P(VDF-TrFE)-based composite thin film would be strongly governed by the TiOPc content. These particles without piezoelectric properties result in a weaker piezoelectric effect of the P(VDF-TrFE) film.

(a) (b)

(c) (d)

(e)

Figure 4-10. SEM images of (a) P(VDF-TrFE) and (b)-(d) P(VDF-TrFE)/ 0.5 wt%, 2.5 wt%, 5 wt% TiOPc powder surface morphology. (e) TiPOc powder.

A FTIR transmission spectrum was used to demonstrate the chemical bonding between the TiOPc and P(VDF-TrFE) copolymer (Figure 4-11). The peak values of the P(VDF-TrFE) and TiOPc are listed in (Table 4-1). Spectra shows that the TiOPc particles were simply blended into the P(VDF-TrFE) copolymer without chemical bonding. It was evident that the amount of TiOPc increased, the TiOPc and P(VDF-TrFE) peak intensities also increased and decreased, respectively [136~ 139].

Figure 4-11. FTIR transmission spectra of P(VDF-TrFE) and P(VDF-TrFE)/ TiOPc composite film. The symbol “★” marks the peaks of the TiOPc particles. (Note: νs and νa

represent symmetric and anti-symmetric stretching modes; ω, δ, and γ represent wagging, bending and rocking modes).

Table 4-1. Infrared active modes of P(VDF-TrFE) and TiOPc.

P(VDF-TrFE)

Peak Values (cm^-1) Attribution TiOPc

Peak Values (cm^-1) Attribution

3011 ν_a(CH2) 1333 Pyrrole stretch

The piezoelectric properties of P(VDF-TrFE) were closely related to its crystallinity and phase distribution. A β-phase P(VDF-TrFE) is known for its high piezoelectric efficiency. It was evident from the XRD pattern that the crystallization of the β-phase can be greatly improved with annealing (Figure 4-12a). After annealing, the β -phase peak at 2θ = 20.4° moved slightly to 19.7° with increased intensity and the amorphous region disappeared. As for the composite films, the β -phase peak of P(VDF-TrFE) was still the dominant peak (Figure 4-12b). The peaks at 13.2°, 26.2° and 27.1° were from the TiOPc particles. These particles has negligible effect of P(VDF-TrFE) crystallization. This is understandable since TiOPc particles, which are about the size of a few hundred nanometers, are too large to interfere with the microstructure of the crystalline P(VDF-TrFE) lamella (possessing only a few angstroms spacing) during annealing.

(a) (b)

Figure 4-12. XRD patterns of (a) P(VDF-TrFE) film before and after annealing. (b) P(VDF-TrFE) and P(VDF-TrFE)/ TiOPc composite film.

Polarization hysteresis of P(VDF-TrFE)/ TiOPc provides direct information on how much polarization can be induced and retained by the material during electrical poling.

The measured hysteresis loops show that the surface charge (P) and coercive field (Ec) are highly dependent on the TiOPc content (Figure 4-13). Both P and Ec increased with increased TiOPc concentration. The movement of the P(VDF-TrFE) molecules and their crystalline lamellae were inhibited by the presence of the TiOPc particles. This phenomenon means that a larger electrical field is required to re-orientate (i.e. switch) the crystalline dipole structure, which can increase Ec with increased TiOPc content.

Moreover, the simultaneous increase in surface charges is due to the semi-conducting characteristics of the TiOPc after charged with an electrical field. The polarization hysteresis loops also indicate there is current leakage at high electric fields for samples with high TiOPc content. In our study, the composite films with TiOPc percentages higher than 30 wt% showed electrical breakdown before reaching the maximum (e.g. saturation)

electric field. It was expected that the optimal TiOPc percentage would be less than 10 wt% to ensure a stable poling behavior and efficient piezoelectric characteristics.

Figure 4-13. Hysteresis loops of P(VDF-TrFE) and P(VDF-TrFE)/ TiOPc composite film.

The piezoelectric coefficient, d33 (C/N), represents the mechanical and electrical transferring ability of a “piezoelectric material” under its maximal poling voltage. In the

“composite P(VDF-TrFE)/ TiOPc films”, these non-piezoelectric TiOPc powders inhibit P(VDF-TrFE) achieving its highest d33 value. It means, the maximal poling voltage (PVM) for the composites must be smaller than for pure P(VDF-TrFE) film [111]. And the mechanical/ electrical transferability of the composite should be redefined as quasi-d33, d33(q)

(Table 4-2). These constants are smaller than d33 values in the pure P(VDF-TrFE) film.

And this constant decreased with the TiOPc increment. This phenomenon also confirms the SEM images which show that too much TiOPc in the composite can possibly destroy

the crystallization of the P(VDF-TrFE). Thus, it can be expected that a high piezoelectric efficiency can be obtained when the TiOPc percentage is less than 10 wt%.

Table 4-2. Relationship between piezoelectric efficiency d33 values and corona charging voltage of P(VDF-TrFE) and P(VDF-TrFE)/ TiOPc composite film.

Sample Photo PVM (kV) d33(q) (pC/N)

1 17.5 31.7

2 17.5 26.0

3 17.5 21.4

4 16.0 20.4

5 15.0 18.6

6 15.0 9.2

A visible light spectra can provide the absorbance band of the composite P(VDF-TrFE)/ TiOPc films (Figure 4-14). Pure P(VDF-TrFE) films have a weak ability to absorb visible light. For the composite P(VDF-TrFE)/ TiOPc, the absorbance ability in the range between visible light and infrared. We found that the absorbance intensity of Samples 4, 5, 6 were similar which confirms that the absorbance reached saturation when the TiOPc content was higher than 5 wt%. Based on the spectra results, we can conclude that our composite material possesses a high and broad sensitivity to visible light.

Figure 4-14. UV-Vis spectra of P(VDF-TrFE) and P(VDF-TrFE)/ TiOPc composite film.

For piezoelectric materials, the electrical displacement (D) and strain (S) tensor have a direct relationship, e.g. D = eS. The piezoelectric stress/ charge tensor, e, which is closely related to the crystalline structure, is a constant. For an optopiezoelectric sensor film, the strain on the specimen can be calculated from the electrical displacement (D), which changes due to the light illumination pattern. We found that the material impedance values are closely related to TiOPc content. (Figure 4-15a) shows the impedance variation of the P(VDF-TrFE)/ TiOPc composite films. The variation, (Z-Z0)/ Z0, decreased more for films with higher TiOPc content since more excited e^-/ e⁺ pairs traveling inside (Figure 4-15b). Furthermore, the variation was more obvious at low frequencies but tended to be constant after 5 kHz. Our experimental results confirm that the composite P(VDF-TrFE)/

TiOPc material works well at low frequencies.

(a) (b

Figure 4-15. (a) Impedance variation of P(VDF-TrFE)/ TiOPc composite film. (b) Schematic of the stimulated electrons cause the impedance variation.

The capacitance and resistance changes are shown in (Figure 4-16). The capacitance increment was clearly smaller than the decreased resistance after light illumination. At a 20 wt% of the composite film, the capacitance variation increased 25 % while the resistance dropped 80 %. This result confirms that the impedance variation decreases at higher frequencies when a capacity effect dominates the equivalent circuit.

(a) (b)

(c) (d)

Figure 4-16. The (a) resistance and (c) capacitance values of Sample 3 and Sample 6, as well as (b, d) their variation before and after illumination.

A further optoelectrical effect with 450 nm and 750 nm light illumination was discussed because their absorbance are similar. The same white light mercury lamp (Apex Fiber Illuminator 70531, 200W Hg, Newport) was filtered to 450 nm and 750 nm. The impedance changes of a P(VDF-TrFE)/ 10% TiOPc film was measured before and after five minutes at 0.025 W/cm² intensity light illumination (Figure 4-17). The electrical response at 450 nm wavelength illumination was larger than that at 750 nm wavelength illumination. For example at 100 Hz, the impedance variation as follows: -15.58 % at 450 nm and -12.58 % at 750 nm light illumination. It might because the e^-/ e⁺ pairs have more kinetic energy after they absorb 450 nm wavelength. Thus the impedance decrement is more obvious with 450 nm than 750 nm light illumination.

Based on the optoelectrical response, the equivalent circuit of our composite was established to be a resistor parallel to a capacitor (Figure 4-18a). Both the resistor and capacitor were light adjustable. The impedance variation at 750 nm was simulated based

on the experimental results shown in (Figure 4-18b).

Figure 4-17. Impedance of P(VDF-TrFE)/ 10% TiOPc at 750nm and 450nm.

(a)

(b)

Figure 4-18. (a) The equivalent circuit of the composite. (b) Simulation and experimental results of the impedance before and after 750nm illumination.

Above all, the optical response of this optopiezoelectric sensor, P(VDF-TrFE)/

TiOPc, was coated to form a thin film using solution processing and which makes it possible to integrate with MEMS processing. The characteristics of P(VDF-TrFE)/ TiOPc composite appears that it is a good potential material for dynamic light-spatial controlling applications.