FERROELECTRIC PROPERTIES OF POLYVINYLIDENEFLUORIDE-

TRIFLUOROETHYLENE (PVDF-TRFE) ANNEALED THIN FILM

Mohamad Hafiz Mohd Wahid 1,a, Rozana Mohd Dahan1,b, Siti Zaleha Sa’ad1,c,

Adillah Nurashikin Arshad1,d, Muhamad Naiman Sarip1,e,

Mohamad Rusop Mahmood2, W.C. Gan3 and Wan Haliza Abd. Majid3

1Faculty of Applied Sciences, Universiti Teknologi MARA,40450 Shah Alam, Selangor, MALAYSIA.

2Nano-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA,

40450 Shah Alam, Selangor, MALAYSIA. 3Low Dimensional Research Centre, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur,

MALAYSIA. ahafizwahid87@gmail.com, brozanadahan@hotmail.com, csiti_leha@hotmail.com,

dadillaharshad@gmail.com, enaimansarip@gmail.com

Keywords: PVDF-TrFE, Annealing, Morphology, Ferroelectric.

Abstract.The annealing temperature for 250nm PVDF-TrFE (70:30 mol %) spin coated thin films were varied at solvent evaporation (Ts = 79˚C), Curie’s transition (Tc= 113˚C), till melting temperature (Tm = 154˚C). From the XRD measurement, there was an improvement in the crystallinity of the annealed films, consistent with the increased in the annealing temperatures. Morphological studies of the annealed PVDF-TrFE thin films as observed with Field Emission Scanning Electron Microscope (FESEM) (100k magnification), showed enhanced development of elongated crystallite structures better known as ferroelectric crystal. However, the AN160 thin film showed fibrous-like structure with appearance of nanoscale separations, which suggested to posed high possibility of defects. Ferroelectric characterization indicated an improvement in the remnant polarization of annealed PVDF-TrFE thin films with the exception to AN160 in which leakage of current was inevitable due to the presence of cracks on the film surface.

INTRODUCTION

Ferroelectric thin film utilizing Polyvinylidenefluoride-Trifluoroethylene (PVDF-TrFE) have received significant research interest in the past several years after Kawai firstly found the piezoelectricity of PVDF in 1969 [1], followed by Furukawa found its ferroelectricity [2]. Ferroelectric thin films were largely contributed to the races for multifunctional device’s miniaturization such as capacitors, transistors, electromechanicals and memory devices [3-12]. PVDF-TrFE copolymer is the most suitable material for ferroelectric thin film applications owing to its flexibility, ease in processing and most important, high dielectric and ferroelectric.

PVDF-TrFE with 50-80% of VDF content produced high dielectric and ferroelectric characteristic due to its switchable dipole moment which consist of β-phase crystallite structure [13]. Parallel packing of β-phase conformation chain structure induced the molecular dipole to align in one direction in order to produce large spontaneous polarization. Furthermore, PVDF-TrFE have the ability to achieve favourable dielectric characteristic by performing additional treatments, such as annealing [14-18], stretching [8, 19-20], fillers incorporations [21-23] and other advance methods [20, 24]. In the ferroelectric thin film fabrication, annealing process was used due to its simplicity. Previous study reported that, annealing process at slightly above the first Curie’s temperature (during heating) but below the melting temperature resulting in improved PVDF-TrFE crystallinity, hence its dielectrical properties [13]. Apparently, this condition is suitable as C-F molecules in PVDF-TrFE are loose and able to orient its structure in polarized conformation. This will increase its dipole moment, hence increase in the dielectrical characteristic of PVDF-TrFE [8].

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In this experiment, a batch of spin coated PVDF-TrFE thin films were annealed at different temperatures starting from the solvent evaporation temperature (Ts), Curie’s transition temperature (Tc) and up to its melting temperature (Tm). The crystallinity of these thin films were successfully improved by increasing the annealing temperature to a certain extent. However, present an apparent defect on the surface of the thin films after annealed at high temperature contribute to the reduction of remnant polarization for the thin film. EXPERIMENTAL PVDF-TrFE thin film preparation

PVDF-TrFE (70:30mol%) manufactured by PiezoTech, France was used to fabricate approximately 250nm thin films using spin coating method. The thin films were prepared on Aluminium (Al) coated glass, which was prepared by thermal evaporation of Al (Edwards AUTO 306 Turbo).

Prior the thin films fabrication, PVDF-TrFE (70:30mol%) pallets were dissolved in methyl ethyl ketone (MEK) (MERCK, UK) with concentration of 30g/L. The solution was stirred for 24hours, then sonicated for half an hour using ultrasonic water bath (Hwasin Technology Powersonic 420, 40 kHz).

Annealing Process

Table 1 shows the sample denotations and annealing temperature for each thin film. These temperatures were selected based on the solvent evaporation (MEK, evaporate at ~79˚C) and transition temperatures (Table 2) obtained from differential scanning calorimetric (DSC) observation (Netzsch DSC 200 F3 Maia

®). All samples were heated to the selected temperature and held isothermally for two hours. All thin films were allowed to cool to ambient temperature before removal from the oven.

Table 1 Annealing process of the PVDF-TrFE thin films.

Sample Annealing

temperature Note

UN Un-annealed Without

treatment AN80 80°C Ts

AN100 100°C Below Tc AN120 120°C Over Tc AN140 140°C Before Tm AN160 160°C Over Tm

Table 2 Transition temperatures of PVDF-TrFE (70-30mol %) obtained from DSC Netzsch 200 F3 Maia

®. Measurements

The crystallinity of the thin films was detected by means of XRD (PANanalytical Powder X’Pert) with nickel filter (Kα=1.54Ȧ) at 45kV and 20mA.

The morphological study of the thin films were observed using Field Emission Scanning Electron Microscopy, (FE-SEM) (JOEL JSM-7600F).

In order to measure the ferroelectric properties of the thin films, metal-insulator-metal (MIM) capacitor configuration was fabricated by depositing 30nm thick Al acting as the top contact with area of 1x10-7cm2 for each contact point (Fig. 1). Polarization hysteresis loops for the thin

Phase Transition Temperature (˚C) Curie’s (TC) 113 Melting (Tm) 154

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films were measured using ferroelectric tester (Radiant Tech. Inc., Precision LC) at 100Hz testing frequency.

Fig. 1 MIM Configuration consist Al-deposited bottom and top of the PVDF-TrFE spin coated film. RESULTS AND DISCUSSIONS

Fig. 2 shows the XRD spectra for PVDF-TrFE thin films. The peak of (200) and (110) planes at 2θ ~19.2˚ shows gradually increment for UN and annealed thin films were observed. This can be relates with the rises in the crystallinity of PVDF-TrFE for annealed thin films, which contribute to the increase of crystallite size observed in FESEM images (will be discussed below). It can be observed that, AN160 produced the highest crystallinity among other thin films. Moreover, the increase in the peak intensity indicated an improvement of molecular structural orientation and of PVDF-TrFE thin films [25].

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

cps (

a.u

.)

o2Theta

UN

AN80

AN100

AN120

AN140

AN160

(200)+(110)

2θ = 19.2o

Fig. 2 XRD diffraction pattern for the PVDF-TrFE thin film samples annealed at different

temperature.

Fig. 3 shows the morphology of UN and annealed PVDF-TrFE (70:30 mol %) thin films observed by using FE-SEM at 100k magnification. Significant development of elongated crystallite structure was observed in AN100 (Fig. 3c) in comparison to the UN and AN80 with undefined crystallite structure. From Fig. 3(c) to 3(e), clearly shows that the crystallite size gradually increased in length and width as the annealing temperatures were increased. The formation of elongated crystallite structure assists the crystallinity properties of PVDF-TrFE thin films consistent with the XRD measurement earlier. From previous research, it also shows an evident that elongated crystallite structure these annealed PVDF-TrFE thin films exhibit the ferroelectric crystal structure [26]. However, different FESEM micrograph was observed for the thin film annealed over Tm (AN160). Fibrous-like structures with presence of ‘separation’ between the crystallites were observed on AN160 surface as shown in Fig. 4(f). This may be due to the re-crystallization of the AN160 thin film after melted over Tm. During melting, thermal energy initiated the mobility and flow of the polymer molecules which demolish the crystal structure in PVDF-TrFE. Upon cooling, recrystallization by the reorganization of crystallite and at the same time the formation of fibrous-like structure attributed to the contraction on the film surface, hence formation of cracks. Moreover, it can be observed that, the absence of crystallite at the area of the cracks may due to the fusion of neighboring crystallite which then forming long to fibrous-like crystallite structure.

Al-coated Glass

Al PVDF-TrFE Spin Coated Film

Advanced Materials Research Vol. 879 3

Fig. 3 FE-SEM micrograph at 100K magnification for sample of (a) UN, (b) AN80, (c) AN100, (d)

AN120, (e) AN140 and (f) AN160. Fig. 5 shows the hysteresis loops of UN and annealed thin films. It can be observed that, the Pr

of thin films increased remarkably upon annealing. Thin film annealed at 120°C (AN120) showed the highest Pr (~110mC/cm2) compared to UN (~60mC/cm2) and other annealed thin films. However, increase in the annealing temperature to 140°C (AN140) slightly reduces the Pr of the thin film, whilst the Pr value of AN140 is comparable with the thin film annealed at 100°C (AN100) which is ~80mC/cm2. Moreover, the Pr continue to decrease as the thin film was annealed to 160°C (AN160) due to the apparent defects from the ‘separation’ between the fibrous-like crystallites observed in FESEM images. It can be conclude that, although AN160 produced the highest crystallinity, the presence of crystallite separations on the surface will not be preferable as they may posed apparent defects such as leakage current and parasitic capacitance. [27]. Moreover, the morphology of the PVDF-TrFE thin film also one of the factor to produce large spontaneous polarization as the AN120 thin film exhibit the same shape of elongated crystallites but different in sizes compared to the other thin films.

-900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 900

-250

-225

-200

-175

-150

-125

-100

-75

-50

-25

0

25

50

75

100

125

150

175

200

225

250

P (

mC

/cm

2)

E (MV/cm)

UN

AN80

AN100

AN120

AN140

AN160

Max. Pr

Fig. 4 Ferroelectric characteristic of PVDF-TrFE thin films with different annealing temperature. CONCLUSION

The effect of annealed PVDF-TrFE thin films on the crystallinity, morphology and ferroelectricity were discussed. However, the presence of nanoscale cracks on AN160 suggesting high possibility to posed defects while in device applications. Thin film annealed at 120°C (AN120)

a b c d

e f

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produced the optimized morphology and crystallinity as it gives the highest Pr (~110mC/cm2) compared to other annealed thin films.

ACKNOWLEDGEMENT

This research was fully supported by Fundamental Research Grant Scheme from Ministry of Higher Education, Faculty of Applied Sciences, NanoSciTech, Institute of Science and Nano-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA, Malaysia. Our highest appreciation goes to Microwave Technology Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA for their assistance in sample preparation. Our special thanks goes to Fellowship Scheme, Universiti Teknologi MARA Malaysia for the funding this study REFERENCE [1] H. Kawai, "The Piezoelectricity of Poly (vinylidene Fluoride)," Japanese Journal of Applied

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Advanced Materials Conference (AMC 2012) 10.4028/www.scientific.net/AMR.879 Ferroelectric Properties of Polyvinylidenefluoride-Trifluoroethylene (PVDF-TrFE) Annealed Thin

Film 10.4028/www.scientific.net/AMR.879.1