Effect of Annealing Temperature on the Crystallinity, Morphology and Ferroelectric of Polyvinylidenefluoride-Trifluoroethylene (PVDF-TrFE)

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, WEE CHEN 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 250 nm 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). XRD measurement showed that, 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 known as ferroelectric crystal. However, thin film annealed at 160 ˚C (AN160) showed fibrous-like structure with appearance of nanoscale separations, which suggested 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) has received significant research interest in the past several years after the first finding by Kawai on piezoelectricity of PVDF in 1969 [1], followed by the ferroelecricity finding by Furukawa [2]. Ferroelectric thin films were largely contributed to the race for multifunctional device’s miniaturization in capacitors, transistors, electromechanicals and memory devices [3-6]. PVDF-TrFE copolymer is the most suitable polymeric material for ferroelectric thin film applications owing to its flexibility, ease in processing in comparison to the brittle and complicated processing of ferroelectric ceramic.

PVDF-TrFE with 50-80% of VDF content produced high dielectric and ferroelectric characteristic due to its switchable dipole moment from the β-phase dominant crystal structure [7]. 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 has the ability to achieve enhanced dielectric characteristic by treatments, such as annealing [8-12], stretching [13-15], fillers incorporations [16-18] and other advance methods [15, 19]. 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 contributed to an improved in crystallinity, hence an enhanced dielectrical properties in PVDF-TrFE [7]. This condition is apparently suitable as C-F molecules in PVDF-TrFE are able to orient its structure in polarized conformation. This will increase its dipole moment, hence increase in the dielectrical characteristic of PVDF-TrFE [13].

Advanced Materials Research Vol. 812 (2013) pp 60-65Online available since 2013/Sep/10 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.812.60

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In this experiment, a batch of spin coated PVDF-TrFE thin films were annealed at different temperatures ranging from the solvent evaporation temperature (Ts), Curie’s transition temperature (Tc) and up to its melting temperature (Tm). The crystallinity and ferroelectric of these thin films were successfully improved by increasing the annealing temperature to a certain extent. However, present of an apparent defect on the surface of the thin films (observed using Field Emission Scanning Electron Microscopy) upon annealing at 160°C contributed to the reduction of remnant polarization.

Experimental

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

Prior to fabrication of the thin films, PVDF-TrFE (70:30 mol%) pallets were dissolved in methyl ethyl ketone (MEK) (MERCK, UK) at concentration of 30 g.L-1 . The solution was stirred for 24 hours and then sonicated for half an hour in an 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) spectra (Netzsch DSC 200 F3 Maia

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

Table 1 Annealing temperatures 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 %) (DSC).

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

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 30 nm thick Aluminum acting as the top contact with area of 1x10-4 cm2 for each contact point (Fig. 1). Polarization hysteresis loops for the thin films were measured using ferroelectric tester (Radiant Tech. Inc., Precision LC) at 100 Hz testing frequency.

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

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Fig. 1 MIM Configuration consisted Aluminum-deposited bottom and top of the PVDF-TrFE spin

coated film.

Results and Discussion

Fig. 2 shows the XRD spectra for PVDF-TrFE thin films. The peaks of (200) and (110) planes at 2θ ~19.2˚ show gradual increment for both UN and annealed thin These were related to the increased in the crystallinity of PVDF-TrFE for annealed thin films, consistent with the observation by FESEM. These images showed an increase in crystallite size (as discussed below). It was observed that thin film annealed at 160˚C (AN160) produced the highest crystallinity amongst all other thin films prepared in this study. Moreover, the increased in the peak intensity of the XRD spectra indicated an improved of molecular structural orientation and of PVDF-TrFE thin films [20].

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 with FE-SEM at 100k magnification. Significant development of elongated crystallite structure was observed in AN100 (Fig. 3c) in comparison to the UN (Fig. 3a) and AN80 (Fig. 3b) in which undefined crystallite structures were observed. Fig. 3(c) to 3(e), clearly show that the crystallite sizes gradually increased as the annealing temperatures were increased. The formation of elongated crystallite structure enhanced the crystallinity properties of PVDF-TrFE thin films, consistent with the XRD measurement mentioned earlier. Previous research proved that the elongated crystallite structure in these annealed PVDF-TrFE thin films exhibited ferroelectric crystal structure [21]. However, dissimilar FESEM micrograph was observed for thin film annealed over Tm (AN160), in which fibrous-like structures with presence of ‘separation’ between the crystallites were observed on AN160 surface (Fig. 3(f)). This may be due to the re-crystallization of the AN160 thin film upon melting beyond Tm. During melting, thermal energy initiated the mobility and flow of the polymer molecules which damaged the crystal structure in PVDF-TrFE. Upon cooling, re-crystallization 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 be due to the fusion of neighboring crystallite which then formed long to fibrous-like crystallite structure.

Al-coated Glass

Al PVDF-TrFE Spin Coated Film

62 Progress in Polymer and Rubber Technology

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. 4 shows the hysteresis loops of UN and annealed thin films. It was observed that, the remnant polarization, Pr of thin films increased remarkably upon annealing. Thin film annealed at 120 °C (AN120) showed the highest Pr (~110 mC.m-2) compared to unannealed (UN) (~60 mC.m-2) and other annealed thin films. However, increased of annealing temperature to 140 °C (AN140) slightly reduced 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 ~80 mC.m-2. Moreover the Pr continued 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 concluded that although AN160 produced the highest crystallinity, the presence of crystallite separations on the surface cannot be compromise as they posed apparent defects such as leakage current and parasitic capacitance to the thin film. [22]. Moreover, the morphology of the PVDF-TrFE thin film produced large spontaneous polarization in AN120 as this thin film exhibited densely elongated crystallites compared to the other annealed films in this study.

-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

/m2)

E (MV/cm)

UN

AN80

AN100

AN120

AN140

AN160

Max. Pr

Fig. 4 Ferroelectric characteristic of PVDF-TrFE thin films at different annealing temperatures.

a b c d

e f

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Conclusion

The effect of annealed PVDF-TrFE thin films on the crystallinity, morphology and ferroelectricity were discussed. Thin film annealed at 120 °C (AN120) produced favourable morphology and optimized crystallinity with highest Pr (~110 mC.cm-2) in comparison to other annealed thin films observed in this study. The presence of “separations” on AN160 thin film caused a significant reduction in Pr , which was probably due to the observed defects observed in the film.

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.

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