- Email: [email protected]
Structural and ferroelectric properties of P(VDF-TrFE) thin films depending on the annealing temperature
Materials Letters 238 (2019) 294–297
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Structural and ferroelectric properties of P(VDF-TrFE) thin films depending on the annealing temperature Jeongdae Seo a,b, Jong Yeog Son b,⇑, Woo-Hee Kim a,⇑ a b
Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, Republic of Korea Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin 17104, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t Organic P(VDF-TrFE) 70/30 copolymer thin films were prepared on ITO/glass substrates at 110–150 °C by spin coating. All the P(VDF-TrFE) thin films exhibited ferroelectric b-phase crystallinity in X-ray diffraction analysis. The polarization-electric field hysteresis analysis revealed that the P(VDF-TrFE) thin film annealed at 140 °C has superior ferroelectric properties with a high remanent polarization of 8.02 lC/cm2. From atomic force microscopic characterization, we observed lamellar and needle-like structures in the P(VDF-TrFE) thin film annealed at 140 °C. Further, piezoelectric force microscopic measurements confirmed a much higher piezoelectric property of the P(VDF-TrFE) thin film annealed at 140 °C than those of the films annealed at other temperatures. Ó 2018 Elsevier B.V. All rights reserved.
Article history: Received 6 November 2018 Received in revised form 23 November 2018 Accepted 28 November 2018 Available online 4 December 2018 Keywords: P(VDF-TrFE) thin film Piezoelectricity Ferroelectricity Crystallization temperature Needle-like structure
1. Introduction Poly(vinylidene fluoride) (P(VDF)) has been studied for flexible ferroelectric non-volatile memory devices, wearable piezoelectric generators, and artificial organs due to their ferroelectricity, piezoelectricity, flexibility, and non-toxicity [1–3]. P(VDF) exhibits four types of crystalline phases (a, b, c, and d), where all except the a phase are polar [4]. The spontaneous polarization of the c and d phases was reported to be approximately half that of the b phase [4,5]. On the other hand, poly(vinylidene fluorideco-trifluoroethylene) (P(VDF-TrFE)) can easily form the b phase with the addition of a small amount of TrFE [6]. Thus, the P (VDF-TrFE) copolymer can be expected to be more crystalline than P(VDF) [7]. P(VDF-TrFE) has lamellar crystals consisting of chain molecules [6,8]. The lamellar crystal has an edge-on (out-of-plane lamellar) structure that causes strong ferroelectricity and a face-on (inplane lamellar) structure that causes weak ferroelectricity [9]. The edge-on lamellar structure is observed at a temperature window between the Curie transition temperature (Tc) and the melting temperature (Tm), and the face-on lamellar structure is observed below the Tc or above the Tm [9]. In top-surface view, the edge-on lamellar structure appears needle shaped [8,9]. This ⇑ Corresponding authors. E-mail addresses: (W.-H. Kim).
[email protected]
(J.Y.
Son),
https://doi.org/10.1016/j.matlet.2018.11.156 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
[email protected]
needle-like structure gives rise to a diffraction peak at 20° in Xray diffraction (XRD) analysis, which reveals ferroelectric b-phase crystallinity [10]. In this study, we investigated the ferroelectric b-phase crystallinity of P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The needle-like lamellar structure of the P(VDF-TrFE) thin films was analyzed by XRD and atomic force microscopy (AFM). From the piezoelectric force microscopy (PFM) measurement, both the ferroelectricity and piezoelectricity of the P(VDF-TrFE) thin films were observed. Further, the transmittance result showed that the P(VDF-TrFE) thin films on ITO/glass substrates have possible applications in transparent piezoelectric nanogenerators. 2. Experimental procedure A 2 wt% dilute solution of P(VDF-TrFE) 70/30 copolymer in methylethylketone solvent was deposited on an ITO/glass substrate by spin coating at a spin rate of 2000 rpm for 10 s. The P (VDF-TrFE) thin films were dried at 70 °C for 10 min, and then annealed at 110, 120, 130, 140, and 150 °C for 2 h under air to improve the crystallization. To fabricate a ferroelectric capacitor, upper metal electrodes of Al were deposited on the P(VDF-TrFE) thin films by thermal evaporation. The XRD profiles of the P(VDF-TrFE) thin films were measured by a SmartLab system (Rigaku) with CuKa radiation. The polarization-electric field (P-E) hysteresis loops of the ferroelectric
J. Seo et al. / Materials Letters 238 (2019) 294–297
capacitors were measured by a precision LC analyzer (Radiant Technologies Inc.). The surface morphology of the P(VDF-TrFE) thin films was measured by AFM (XE-100, Park Systems). The piezoelectricity of the P(VDF-TrFE) thin films was measured by PFM. 3. Results and discussion Fig. 1(a) shows the XRD profiles of the P(VDF-TrFE) 70/30 copolymer thin films on ITO/glass substrates annealed at various crystallization temperatures. Because the ferroelectricparaelectric phase transition of P(VDF-TrFE) originates from its large lattice strains and dimensional changes, the analysis of its crystal structure can clarify the ferroelectric behavior of the P (VDF-TrFE) thin films, which show ferroelectric b-phase crystallinity near 20° [11,12]. All the P(VDF-TrFE) thin films annealed at 110–150 °C displayed a peak at 19.7°, and are therefore expected to show ferroelectric hysteresis [13]. However, there are some differences in intensity depending on the crystallization temperature, and thus, the ferroelectric hysteresis characteristic may differ for the films annealed at different crystallization temperatures [13]. Fig. 1(b) shows the P-E hysteresis loops of the Al/P(VDF-TrFE)/ ITO capacitors annealed at various crystallization temperatures. The loop frequency of the Al/P(VDF-TrFE)/ITO capacitors were 100 Hz. The remanent polarization of the Al/P(VDF-TrFE)/ITO capacitors annealed at 110, 120, 130, 140, and 150 °C were approximately 0.09, 0.13, 5.42, 8.02, and 7.71 lC/cm2, respectively. The ferroelectric remanent polarization values of the Al/P(VDF-TrFE)/ ITO capacitors annealed at 130, 140, and 150 °C were much larger than those of the capacitors annealed at 110 and 120 °C, which was ascribed to the b-phase crystallinity of the P(VDF-TrFE) thin films depending on the crystallization temperature [9]. As the annealing temperature increased, the ferroelectric remanent polarization of P (VDF-TrFE) became higher and reached a maximum at 8.02 lC/cm2 at 140 °C. The increase in ferroelectric remanent polarization of the P(VDF-TrFE) thin films with increasing crystallization temperature correspond well with the findings of previous reports [13]. Fig. 2(a)–(e) show the surface morphology of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The root mean square (RMS) surface roughness of the P(VDF-TrFE) thin films annealed at 110, 120, 130, 140, and 150 °C were approximately 23.94, 11.57, 14.55, 10.92, and 13.66 nm. Although all the P(VDF-TrFE) surface morphologies reveal round grains, the substructures of the round grains of the
295
P(VDF-TrFE) thin films were different depending on the crystallization temperature. The large round grains separated into substructures as the annealing temperature increased. The grain sizes of P(VDF-TrFE) thin films annealed at 110, 120, 130, 140, and 150 °C were about 700, 300, 150, 300, and 150 nm, respectively. The P(VDF-TrFE) thin film annealed at 140 °C showed distinct substructures, while substructures became indistinct after annealing at 150 °C. The substructures of the films annealed at 140 °C exhibit lamellar and needle-like shapes. Particularly, lamellar structures were observed with a length of 300 nm, width of 200 nm, and height of 30 nm, whereas needle-like structures were observed with a length of 300 nm, width of 30 nm, and height of 200 nm in the P(VDF-TrFE) thin film annealed at 140 °C. These results indicate that the needle-like structure, the so-called edge-on lamella, is an erected lamellar structure. The needle-like shape reveals the b-phase crystal structure, which is responsible for the ferroelectric property of a typical P(VDF-TrFE) thin film [10]. The needle-like structure observed in P(VDF-TrFE) thin films corresponded well with results of ferroelectric hysteresis measurement. Fig. 2(k)–(o) show the PFM phase images of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The phase signal was extracted from the amplitude signal by a lock-in amplifier. The bright regions represent upward polarization, and the dark regions represent downward polarization on the P(VDF-TrFE) thin films. The ferroelectric domain structures were formed along the surface morphology of the P(VDF-TrFE) thin films. The ferroelectric domain structure of P(VDF-TrFE) with a needle-like shape at 140 °C is distinct from those of the films annealed at other temperatures with a round shape. Fig. 2(f)–(j) show the amplitude images of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The amplitude image reveals the magnitude of the ferroelectric response, and thus the dark regions may include the domain walls [14]. Fig. 2(p) and (q) show the local ferroelectric switching behaviors of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures measured by PFM. The phase responses as a function of DC bias were obtained with hysteresis loops. The hysteresis loop of phase response indicates that the direction of polarization was switched by 180°, as shown in Fig. 2(p) [14]. The amplitude responses as a function of DC bias show butterfly loops in Fig. 2(q), which pertain to the morphological variations in P(VDF-TrFE) thin films [14]. The piezoelectric
Fig. 1. (a) XRD profiles of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (b) Polarization-electric field hysteresis loops of Al/P(VDFTrFE)/ITO capacitor at various crystallization temperatures.
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Fig. 2. AFM and PFM analysis of P(VDF-TrFE) thin films on ITO/glass substrates (10 lm 10 lm). (a)–(e) AFM topography of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (f)–(j) PFM amplitude images of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (k)–(o) PFM phase images of P(VDF-TrFE) thin films ITO/glass substrates at various crystallization temperatures. (p) Piezoelectric phase responses of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (q) Piezoelectric amplitude responses of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures.
properties of the P(VDF-TrFE) thin films annealed at 140 and 150 °C showed much higher values than those of the films annealed at 110, 120, and 130 °C. The piezoelectric coefficient (approximately 22.0 pm/V) was observed for the P(VDF-TrFE) thin film annealed at 140 °C. Fig. 3 shows the optical transmittance spectra of about 170-nmthick P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The average transmittance
of the ITO/glass substrate was around 80.7% in the visible spectral region of 400–800 nm. The average transmittance of P(VDF-TrFE) thin films on ITO/glass gradually increased with increasing annealing temperature, and a maximum transmittance of around 65.7% was observed for the film annealed at 140 °C due to its high crystallinity. It is worth noting that this result implies the potential applicability of P(VDF-TrFE) thin films to transparent piezoelectric nanogenerators.
J. Seo et al. / Materials Letters 238 (2019) 294–297
297
(NRF) funded by the Ministry of Education of South Korea (No. 2018R1A6A3A11045896) and a National Research Foundation of South Korea (NRF) grant funded by the South Korean Government (MSIP) (No. NRF-2017R1C1B5076821). This research is supported by ‘‘Rediscovery of the Past R&D Result’’ through the Ministry of Trade of South Korea, Industry and Energy (MOTIE) and the South Korea Institute for Advancement of Technology (KIAT) (Grant No.: P0004074). References
Fig. 3. Transmittances of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures.
4. Conclusions The ferroelectric and piezoelectric properties of P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures were comparatively investigated. The peak intensity at 19.7° indicating the ferroelectric b-phase crystallinity increased with increasing annealing temperature up to 140 °C. AFM analysis revealed the presence of lamellar and needle-like structures on the surface of P(VDF-TrFE) thin film annealed at 140 °C. Further, PFM measurements revealed that the piezoelectric property of the P(VDF-TrFE) thin film annealed at 140 °C is superior to those of the films annealed at other temperatures. The transmittance spectra of the P(VDF-TrFE) thin films on ITO/glass substrates show their potential applicability in transparent piezoelectric nanogenerators. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of South Korea
[1] S. Das, J. Appenzeller, FeTRAM.An organic ferroelectric material based novel random access memory cell, Nano Lett. 11 (2011) 4003–4007. [2] D.M. de Leeuw, High-performance solution-processed polymer ferroelectric field-effect transistors, Nat. Mater. 4 (2005) 243–248. [3] D. Wu, Flexible piezoelectric nanogenerator made of poly(vinylidenefluorideco-trifluoroethylene) (PVDF-TrFE) thin film, Nano Energy 7 (2014) 33–41. [4] N.C. Banik, Theory of structural phase transitions in poly(vinylidene fluoride), Phys. Rev. Lett. 43 (1979) 456–460. [5] T.C. Mike Chung, A. Petchsuk, Physical Science and Technology – Polymers, Associated Press America, 2001, pp. 659–674. [6] Q.M. Zhang, Vivek Bharti, X. Zhao, Giant electrostriction and relaxor ferroelectric behavior in electron-irradiated poly(vinylidene fluoricetrifluoroethylene copolymer, Science 280 (1998) 2101–2104. [7] S.-W. Jung, Low-voltage-operated top-gate polymer thin-film transistors with high-capacitance P(VDF-TrFE)/PVDF-blended dielectrics, Curr. Appl Phys. (2011). [8] S. Akama, Lamellar and bulk single crystals grown in annealed films of vinylidene and trifluoroethylene coloymers, Jpn. J. Appl. Phys. 27 (1988) 2144– 2150. [9] K.J. Kim, Annealing effect upon chain orientation, crystalline morphology, and polarizability of ultra-thin P(VDF-TrFE) film for nonvolatile polymer memory device, Polymer 51 (2010) 6319–6333. [10] J. Zhang, Annealing effect on poly(vinylidene fluoride/trifluoroethylene) (70/ 30) copolymer thin films above the melting point, J. Appl. Polym. Sci. 435938 (2013) 1–5. [11] T. Feng, D. Xie, Y. Zang, X. Wu, T. Ren, W. Pan, Temperature control of P(VDFTrFE) copolymer thin films, Integr. Ferroelectr. 141 (2013) 187–194. [12] S.-W. Jung, S.-M. Yoon, S.Y. Kang, B.-G. Yu, Properties of ferroelectric P(VDFTrFE) 70/30 copolymer films as a gate dielectric, Integr. Ferroelectr. 100 (2010) 198–205. [13] R.L. Withers, Effect of annealing temperature on the morphology and piezoresponse characterisation of poly(vinylidene fluoride-trifluoroethylene) films via scanning probe microscopy, Adv. Cond. Matter Phys. 2013 (2013) 1–5. [14] J.-H. Kim, D.-Y. Khang, High-performance needle-shaped crystals in thin and ultrathin P(VDF-TrFE) films formed by melt recrystallization, Eur. Polym. J. 59 (2014) 78–83.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Structural and ferroelectric properties of P(VDF-TrFE) thin films depending on the annealing temperature Jeongdae Seo a,b, Jong Yeog Son b,⇑, Woo-Hee Kim a,⇑ a b
Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, Republic of Korea Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin 17104, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t Organic P(VDF-TrFE) 70/30 copolymer thin films were prepared on ITO/glass substrates at 110–150 °C by spin coating. All the P(VDF-TrFE) thin films exhibited ferroelectric b-phase crystallinity in X-ray diffraction analysis. The polarization-electric field hysteresis analysis revealed that the P(VDF-TrFE) thin film annealed at 140 °C has superior ferroelectric properties with a high remanent polarization of 8.02 lC/cm2. From atomic force microscopic characterization, we observed lamellar and needle-like structures in the P(VDF-TrFE) thin film annealed at 140 °C. Further, piezoelectric force microscopic measurements confirmed a much higher piezoelectric property of the P(VDF-TrFE) thin film annealed at 140 °C than those of the films annealed at other temperatures. Ó 2018 Elsevier B.V. All rights reserved.
Article history: Received 6 November 2018 Received in revised form 23 November 2018 Accepted 28 November 2018 Available online 4 December 2018 Keywords: P(VDF-TrFE) thin film Piezoelectricity Ferroelectricity Crystallization temperature Needle-like structure
1. Introduction Poly(vinylidene fluoride) (P(VDF)) has been studied for flexible ferroelectric non-volatile memory devices, wearable piezoelectric generators, and artificial organs due to their ferroelectricity, piezoelectricity, flexibility, and non-toxicity [1–3]. P(VDF) exhibits four types of crystalline phases (a, b, c, and d), where all except the a phase are polar [4]. The spontaneous polarization of the c and d phases was reported to be approximately half that of the b phase [4,5]. On the other hand, poly(vinylidene fluorideco-trifluoroethylene) (P(VDF-TrFE)) can easily form the b phase with the addition of a small amount of TrFE [6]. Thus, the P (VDF-TrFE) copolymer can be expected to be more crystalline than P(VDF) [7]. P(VDF-TrFE) has lamellar crystals consisting of chain molecules [6,8]. The lamellar crystal has an edge-on (out-of-plane lamellar) structure that causes strong ferroelectricity and a face-on (inplane lamellar) structure that causes weak ferroelectricity [9]. The edge-on lamellar structure is observed at a temperature window between the Curie transition temperature (Tc) and the melting temperature (Tm), and the face-on lamellar structure is observed below the Tc or above the Tm [9]. In top-surface view, the edge-on lamellar structure appears needle shaped [8,9]. This ⇑ Corresponding authors. E-mail addresses: (W.-H. Kim).
[email protected]
(J.Y.
Son),
https://doi.org/10.1016/j.matlet.2018.11.156 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
[email protected]
needle-like structure gives rise to a diffraction peak at 20° in Xray diffraction (XRD) analysis, which reveals ferroelectric b-phase crystallinity [10]. In this study, we investigated the ferroelectric b-phase crystallinity of P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The needle-like lamellar structure of the P(VDF-TrFE) thin films was analyzed by XRD and atomic force microscopy (AFM). From the piezoelectric force microscopy (PFM) measurement, both the ferroelectricity and piezoelectricity of the P(VDF-TrFE) thin films were observed. Further, the transmittance result showed that the P(VDF-TrFE) thin films on ITO/glass substrates have possible applications in transparent piezoelectric nanogenerators. 2. Experimental procedure A 2 wt% dilute solution of P(VDF-TrFE) 70/30 copolymer in methylethylketone solvent was deposited on an ITO/glass substrate by spin coating at a spin rate of 2000 rpm for 10 s. The P (VDF-TrFE) thin films were dried at 70 °C for 10 min, and then annealed at 110, 120, 130, 140, and 150 °C for 2 h under air to improve the crystallization. To fabricate a ferroelectric capacitor, upper metal electrodes of Al were deposited on the P(VDF-TrFE) thin films by thermal evaporation. The XRD profiles of the P(VDF-TrFE) thin films were measured by a SmartLab system (Rigaku) with CuKa radiation. The polarization-electric field (P-E) hysteresis loops of the ferroelectric
J. Seo et al. / Materials Letters 238 (2019) 294–297
capacitors were measured by a precision LC analyzer (Radiant Technologies Inc.). The surface morphology of the P(VDF-TrFE) thin films was measured by AFM (XE-100, Park Systems). The piezoelectricity of the P(VDF-TrFE) thin films was measured by PFM. 3. Results and discussion Fig. 1(a) shows the XRD profiles of the P(VDF-TrFE) 70/30 copolymer thin films on ITO/glass substrates annealed at various crystallization temperatures. Because the ferroelectricparaelectric phase transition of P(VDF-TrFE) originates from its large lattice strains and dimensional changes, the analysis of its crystal structure can clarify the ferroelectric behavior of the P (VDF-TrFE) thin films, which show ferroelectric b-phase crystallinity near 20° [11,12]. All the P(VDF-TrFE) thin films annealed at 110–150 °C displayed a peak at 19.7°, and are therefore expected to show ferroelectric hysteresis [13]. However, there are some differences in intensity depending on the crystallization temperature, and thus, the ferroelectric hysteresis characteristic may differ for the films annealed at different crystallization temperatures [13]. Fig. 1(b) shows the P-E hysteresis loops of the Al/P(VDF-TrFE)/ ITO capacitors annealed at various crystallization temperatures. The loop frequency of the Al/P(VDF-TrFE)/ITO capacitors were 100 Hz. The remanent polarization of the Al/P(VDF-TrFE)/ITO capacitors annealed at 110, 120, 130, 140, and 150 °C were approximately 0.09, 0.13, 5.42, 8.02, and 7.71 lC/cm2, respectively. The ferroelectric remanent polarization values of the Al/P(VDF-TrFE)/ ITO capacitors annealed at 130, 140, and 150 °C were much larger than those of the capacitors annealed at 110 and 120 °C, which was ascribed to the b-phase crystallinity of the P(VDF-TrFE) thin films depending on the crystallization temperature [9]. As the annealing temperature increased, the ferroelectric remanent polarization of P (VDF-TrFE) became higher and reached a maximum at 8.02 lC/cm2 at 140 °C. The increase in ferroelectric remanent polarization of the P(VDF-TrFE) thin films with increasing crystallization temperature correspond well with the findings of previous reports [13]. Fig. 2(a)–(e) show the surface morphology of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The root mean square (RMS) surface roughness of the P(VDF-TrFE) thin films annealed at 110, 120, 130, 140, and 150 °C were approximately 23.94, 11.57, 14.55, 10.92, and 13.66 nm. Although all the P(VDF-TrFE) surface morphologies reveal round grains, the substructures of the round grains of the
295
P(VDF-TrFE) thin films were different depending on the crystallization temperature. The large round grains separated into substructures as the annealing temperature increased. The grain sizes of P(VDF-TrFE) thin films annealed at 110, 120, 130, 140, and 150 °C were about 700, 300, 150, 300, and 150 nm, respectively. The P(VDF-TrFE) thin film annealed at 140 °C showed distinct substructures, while substructures became indistinct after annealing at 150 °C. The substructures of the films annealed at 140 °C exhibit lamellar and needle-like shapes. Particularly, lamellar structures were observed with a length of 300 nm, width of 200 nm, and height of 30 nm, whereas needle-like structures were observed with a length of 300 nm, width of 30 nm, and height of 200 nm in the P(VDF-TrFE) thin film annealed at 140 °C. These results indicate that the needle-like structure, the so-called edge-on lamella, is an erected lamellar structure. The needle-like shape reveals the b-phase crystal structure, which is responsible for the ferroelectric property of a typical P(VDF-TrFE) thin film [10]. The needle-like structure observed in P(VDF-TrFE) thin films corresponded well with results of ferroelectric hysteresis measurement. Fig. 2(k)–(o) show the PFM phase images of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The phase signal was extracted from the amplitude signal by a lock-in amplifier. The bright regions represent upward polarization, and the dark regions represent downward polarization on the P(VDF-TrFE) thin films. The ferroelectric domain structures were formed along the surface morphology of the P(VDF-TrFE) thin films. The ferroelectric domain structure of P(VDF-TrFE) with a needle-like shape at 140 °C is distinct from those of the films annealed at other temperatures with a round shape. Fig. 2(f)–(j) show the amplitude images of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The amplitude image reveals the magnitude of the ferroelectric response, and thus the dark regions may include the domain walls [14]. Fig. 2(p) and (q) show the local ferroelectric switching behaviors of the P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures measured by PFM. The phase responses as a function of DC bias were obtained with hysteresis loops. The hysteresis loop of phase response indicates that the direction of polarization was switched by 180°, as shown in Fig. 2(p) [14]. The amplitude responses as a function of DC bias show butterfly loops in Fig. 2(q), which pertain to the morphological variations in P(VDF-TrFE) thin films [14]. The piezoelectric
Fig. 1. (a) XRD profiles of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (b) Polarization-electric field hysteresis loops of Al/P(VDFTrFE)/ITO capacitor at various crystallization temperatures.
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Fig. 2. AFM and PFM analysis of P(VDF-TrFE) thin films on ITO/glass substrates (10 lm 10 lm). (a)–(e) AFM topography of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (f)–(j) PFM amplitude images of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (k)–(o) PFM phase images of P(VDF-TrFE) thin films ITO/glass substrates at various crystallization temperatures. (p) Piezoelectric phase responses of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures. (q) Piezoelectric amplitude responses of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures.
properties of the P(VDF-TrFE) thin films annealed at 140 and 150 °C showed much higher values than those of the films annealed at 110, 120, and 130 °C. The piezoelectric coefficient (approximately 22.0 pm/V) was observed for the P(VDF-TrFE) thin film annealed at 140 °C. Fig. 3 shows the optical transmittance spectra of about 170-nmthick P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures. The average transmittance
of the ITO/glass substrate was around 80.7% in the visible spectral region of 400–800 nm. The average transmittance of P(VDF-TrFE) thin films on ITO/glass gradually increased with increasing annealing temperature, and a maximum transmittance of around 65.7% was observed for the film annealed at 140 °C due to its high crystallinity. It is worth noting that this result implies the potential applicability of P(VDF-TrFE) thin films to transparent piezoelectric nanogenerators.
J. Seo et al. / Materials Letters 238 (2019) 294–297
297
(NRF) funded by the Ministry of Education of South Korea (No. 2018R1A6A3A11045896) and a National Research Foundation of South Korea (NRF) grant funded by the South Korean Government (MSIP) (No. NRF-2017R1C1B5076821). This research is supported by ‘‘Rediscovery of the Past R&D Result’’ through the Ministry of Trade of South Korea, Industry and Energy (MOTIE) and the South Korea Institute for Advancement of Technology (KIAT) (Grant No.: P0004074). References
Fig. 3. Transmittances of P(VDF-TrFE) thin films on ITO/glass substrates at various crystallization temperatures.
4. Conclusions The ferroelectric and piezoelectric properties of P(VDF-TrFE) thin films on ITO/glass substrates annealed at various crystallization temperatures were comparatively investigated. The peak intensity at 19.7° indicating the ferroelectric b-phase crystallinity increased with increasing annealing temperature up to 140 °C. AFM analysis revealed the presence of lamellar and needle-like structures on the surface of P(VDF-TrFE) thin film annealed at 140 °C. Further, PFM measurements revealed that the piezoelectric property of the P(VDF-TrFE) thin film annealed at 140 °C is superior to those of the films annealed at other temperatures. The transmittance spectra of the P(VDF-TrFE) thin films on ITO/glass substrates show their potential applicability in transparent piezoelectric nanogenerators. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of South Korea
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