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Preparation and characterization of poly(vinylidene fluoride–trifluoroethylene) copolymer nanowires and nanotubes
Materials Letters 60 (2006) 2357 – 2361 www.elsevier.com/locate/matlet
Preparation and characterization of poly(vinylidene fluoride–trifluoroethylene) copolymer nanowires and nanotubes S.T. Lau ⁎, R.K. Zheng, H.L.W. Chan, C.L. Choy Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received 7 November 2005; accepted 4 January 2006 Available online 23 January 2006
Abstract Poly(vinylidene fluoride–trifluoroethylene) copolymer [(P(VDF–TrFE)] nanowires and nanotubes have been fabricated by a template technique. The morphology of the nanowires and nanotubes was studied by scanning electron microscopy, and the diameter of the nanowires and nanotubes was found to be in range of 55–360 nm. X-ray diffraction shows that the nanowires and nanotubes have the same crystal structure as that of P(VDF–TrFE) bulk sample. Relative permittivity measurements as a function of temperature reveal a ferroelectric-to-paraelectric phase transition, implying that ferroelectricity is still maintained in the nanowires and nanotubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Polymers; Ferroelectrics
1. Introduction Over the last decade, low-dimensional nanostructures have received considerable attention from the scientific and engineering communities because of their small size and large surface-to-volume ratios. Therefore, they can be promising candidates for realizing nanoscale electronic, optical and mechanical devices. One dimensional structures such as nanotubes [1,2] and nanowires [3,4] represent a particularly attractive class of nanostructures as they can function as both nanoscale device elements and interconnects while retaining unique properties due to size confinement in the radial direction [3,5,6]. To date, most research efforts have been directed toward synthesis and applications of metal and metal alloys, oxide ceramics, carbon, and semiconductor nanowires and nanotubes [1–4]. Experimental strategies applied to other classes of materials, i.e. polymers, are beginning to emerge [7–9]. Ferroelectric polymers, including polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride–trifluoroethylene) copolymer [P(VDF–TrFE)], have generated much interest in the past
⁎ Corresponding author. Tel.: +852 27665662; fax: +852 23337629. E-mail address: [email protected] (S.T. Lau). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.01.006
ten years because of their potential as functional materials for energy transduction and information recording. Current interest in P(VDF–TrFE) copolymer arises because these materials have high electromechanical properties and large ferroelectric polarization [10,11]. Other notable characteristics of this copolymer are low dielectric constant, low elastic stiffness and low density which result in a high voltage sensitivity (excellent sensor characteristics). Based on these features, P(VDF-TrFE) copolymer have been used as sensing and actuating devices in a wide range of applications [12,13]. With the miniaturization trend in device size, preparation of ferroelectric polymer nanowires and nanotubes has attracted much interest because of their potential applications as functional elements in sensing and microelectromechanical systems. However, there is limited report of preparing and characterizing P(VDF–TrFE) copolymer nanowires and nanotubes. In our previous work [14], we have prepared P(VDF–TrFE) nanotubes and nanowires using a vacuum infiltration method. In this paper, we describe a hot-press method for fabrication of the nanowires and nanotubes. This method enables more copolymer to infiltrate into the ordered porous template in comparison with the vacuum infiltration method, and thus high-dense nanowires can be fabricated. The morphology and structure of the nanowires and nanotubes are studied by a scanning electron microscope
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and X-ray diffraction system. The relative permittivities of the nanowires and nanotubes are measured and compared with that of the bulk sample and nanowires prepared by vacuum infiltration method. 2. Experimental Two types of anodic aluminium oxide (AAO) templates were used in the fabrication of the copolymer nanowires and nanotubes. The first type, with the diameter 21 mm, thickness 60 μm and a pore size of 160–200 nm, was supplied by Whatman Co. Ltd. The second type, with the diameter 10 mm, thickness 20–40 μm and pore size 60–80 nm, was prepared using a two-step anodizing process. Fig. 1 shows the schematic steps of the process. High-purified aluminium (Al) foil was used as the starting material. Before anodization, the Al foil was degreased with acetone and then annealed at 450 °C for 2 h under nitrogen ambient to enhance the grain size in the metal and to obtain homogeneous conditions for pore growth over large areas. The Al foil was subsequently electropolished in a 1 : 9 by volume mixture of HClO4 : C2H5OH solution. Anodization was carried out under a constant voltage of 40 V in a 0.3 M oxalic acid (C2H2O4) solution for 8–10 h, and the temperature was kept constant at 5 °C. In the beginning of the first anodization process,
the pores were randomly distributed on the surface. During their growth into the bulk material they arranged in a hexagonal pattern due to a process of self-organization. This can be utilized to create surfaces with ordered pores. Then, the primarily created oxide layer is removed by immersing in a mixture of 6 wt.% phosphoric acid (H3PO4) and 1.8 wt.% chromic acid (H2CrO4). A second anodization process made with the same condition was carried out for 5–10 h using the pre-structured aluminium surface as template. To produce a free standing membrane through nano-channels, the remaining Al layer was first removed in a saturated HgCl2 solution for 2–3 h. To enlarge the nanochannels and remove the continuous oxide barrier, the template was immersed in H3PO4 solution for 20–30 min at 50 °C. The morphology of the two types of AAO templates was examined by a field-emission scanning electron microscope (FE-SEM). Copolymer of vinylidene fluoride and trifluoroethylene [P(VDF–TrFE)] with 70 mol% of VDF, supplied in pellet form by Piezotech Co., Saint Louis, France, was dissolved in methyl ethyl ketone (MEK) to form a solution which was then casted on a flat glass plate to form thin films of thickness 20–30 μm. The films were heated at 135 °C for 6 h to remove the residual MEK. The films were placed on the AAO template and heated to 270 °C, and then pressed at a pressure of 50 MPa for 15 min to enhance the infiltration of the copolymer into the pores of the
Fig. 1. Schematic diagram showing the process for fabricating porous AAO membrane.
S.T. Lau et al. / Materials Letters 60 (2006) 2357–2361
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AAO template. The sample was then cooled to room temperature at a rate of 10 °C/min. To achieve high crystallinity, the sample was annealed at 135 °C for 15 h. During the processing at 270 °C, while some of the copolymer had infiltrated into the pores of the AAO template, a layer of copolymer remained on the surface of AAO template. For FE-SEM measurements the AAO template was completely removed by immersing the sample in a 4 M sodium hydroxide (NaOH) solution, and then the sample was rinsed with distilled water. The structures of the copolymer nanowires and nanotubes were studied using a Bruker AXS D8 Advance X-ray diffractometer equipped with CuKα radiation. To study the electrical property of the copolymer nanowires and nanotubes (with the template as a support), circular shape chromium/gold electrode with diameter 3 mm was sputtered on the surfaces of the sample. The relative permittivity property of the sample was studied as a function of temperature using an HP 4194A impedance/gain-phase analyzer. 3. Results and discussion
Fig. 2. FE-SEM images of the AAO templates. Type 1: supplied by the Whatman Co. Ltd.; Type 2: prepared in our laboratory using a two-step anodizing process.
The FE-SEM images of the AAO templates used for the fabrication of the copolymer nanowires and nanotubes are presented in Fig. 2. The template prepared in our laboratory shows a clear hexagonal pore ordering in comparison with the one provided by Whatman Co. Ltd. After infiltration of the copolymer melt into the pores of the templates, the samples were annealed, and the copolymer nanowires and nanotubes were released by removing the templates in NaOH solution. Fig. 3 shows the morphology of the copolymer nanowires and
Fig. 3. FE-SEM images of the P(VDF–TrFE) copolymer nanowires and nanotubes.
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nanotubes. It can be seen that the copolymer nanowires are arranged roughly parallel to one another, and the diameter ranges from 55 to 65 nm when a template with a pore size of ∼60 nm was used (Fig. 3(a) and (b)). To fabricate copolymer nanotubes, templates with a larger pore size (about 200 nm) were used. The resulting nanotubes have an outer diameter of ∼360 nm and a wall thickness of ∼45 nm (Fig. 3(c) and (d)). The results imply that whether nanowires or nanotubes are formed depends on the pore size of the AAO template. Larger pore size favours the formation of nanotubes. The length of the nanowires and nanotubes is in the range of 30–55 μm, which is dependent on the processing temperature, infiltration time and thickness of the template. For X-ray diffraction measurements on the copolymer nanowires and nanotubes, the copolymer layer on the surface of the AAO template was removed by mechanical polishing and then the sample was cleaned with ethanol. For comparison, the X-ray diffraction patterns of the AAO template and a bulk copolymer sample were also measured. As shown in Fig. 4(a), no diffraction peak is observed for the AAO template, indicating that the template is amorphous. For the bulk copolymer sample, an intense peak at 2θ = 19.7° is observed, which corresponds to the composite (110, 200) reflection of the β phase (ferroelectric phase) composed of all trans chains [15]. The XRD
Fig. 5. Temperature dependence of the relative permittivity εr (at 100 kHz) of the (a) bulk copolymer sample, (b) copolymer nanowires, and (c) copolymer nanotubes.
patterns of the copolymer nanowires and nanotubes are presented in Fig. 4(b) and (c). It can be seen that the copolymer nanowires and nanotubes exhibit a peak at 2θ = 19.7° and 2θ = 19.6°, respectively,
Fig. 4. X-ray diffraction patterns of the (a) bulk copolymer sample and AAO template, (b) copolymer nanowires, and (c) copolymer nanotubes.
Fig. 6. Temperature dependence of the relative permittivity εr (at 100 kHz) of the copolymer nanowires prepared by vacuum infiltration method.
S.T. Lau et al. / Materials Letters 60 (2006) 2357–2361
which is close to that of the bulk sample. This implies that the nanowires and nanotubes have the same crystal structure as the bulk sample. The temperature dependence of the relative permittivity εr at 100 kHz for the bulk copolymer sample, nanowires and nanotubes is presented in Fig. 5. Both the nanowires and nanotubes exhibit a peak at ∼110 °C upon heating and another peak at ∼70 °C upon cooling, which can be attributed to the ferroelectric-to-paraelectric transition of the copolymer as observed in the bulk sample (Fig. 5(a)). These results indicate that both nanowires and nanotubes are ferroelectric at room temperature and are consistent with the XRD measurements. As compared with the nanowires prepared by vacuum infiltration method (Fig. 6), the εr of the nanowires prepared by hot-press method is increased obviously. This implies that the hot-press method enables more copolymer infiltrate into the pores of the AAO template, and favours to produce high-dense nanowires.
4. Conclusions P(VDF–TrFE) copolymer nanowires and nanotubes with diameters ranging from 55 to 360 nm have been fabricated using a template technique. It was found that larger pore size in the template favours the formation of nanotubes. XRD measurements show that the crystal structure of the nanowires and nanotubes is the same as that of the bulk sample. When the relative permittivity property is measured as a function of temperature, a ferroelectric-to-paraelectric phase transition is observed for both the nanowires and nanotubes, implying that ferroelectricity is still maintained in the nanowires and nanotubes. Besides, the εr of the nanowires prepared by hot-press method is increased comparing to those by vacuum infiltration method. This indicates that more copolymer can infiltrate into the porous template and produce high-dense nanowires.
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Acknowledgement This work was supported by the Centre for Smart Materials (CSM) of the Hong Kong Polytechnic University and the Hong Kong Polytechnic University Postdoctoral Fellowship Scheme (Project No.: G-YX16). References [1] M. Bockrath, W. Liang, D. Bozovic, J.H. Hafner, C.M. Lieber, M. Tinkham, H.K. Park, Science 291 (2001) 283. [2] W. Liang, M. Bockrath, D. Bozovic, J.H. Hafner, M. Tinkham, H. Park, Nature 411 (2001) 665. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [5] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [6] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [7] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [8] F.D. Morrison, Y. Luo, I. Szafraniak, V. Nagarajan, R.B. Wehrspohn, M. Steinhart, J.H. Wendorff, N.D. Zakharov, E.D. Mishina, K.A. Vorotilov, A.S. Sigov, S. Nakabayashi, M. Alexe, R. Ramesh, J.F. Scott, Rev. Adv. Mater. Sci. 4 (2003) 114. [9] J. Joo, K.T. Park, B.H. Kim, M.S. Kim, S.Y. Lee, C.K. Jeong, J.K. Lee, D.H. Park, W.K. Yi, S.H. Lee, K.S. Ryu, Synth. Met. 135 (2003) 7. [10] K. Koga, H. Ohigashi, J. Appl. Phys. 59 (1986) 2142. [11] T. Furukawa, Adv. Colloid Interface Sci. 71 (1997) 183. [12] H. Ohigashi, K. Koga, M. Suzuki, T. Nakanishi, K. Kimura, N. Hashimoto, Ferroelectrics 60 (1984) 263. [13] H.L.W. Chan, A.H. Ramelan, I.G. Guy, D.C. Price, Rev. Sci. Instrum. 62 (1991) 203. [14] R.Z. Zheng, Y. Yang, Y. Wang, J. Wang, H.L.W. Chan, C.L. Choy, C.G. Jin, X.G. Li, Chem. Commun. (2005) 1447. [15] A.J. Lovinger, G.T. Davis, T. Furukawa, M.G. Broadhurst, Macromolecules 15 (1982) 323.
Preparation and characterization of poly(vinylidene fluoride–trifluoroethylene) copolymer nanowires and nanotubes S.T. Lau ⁎, R.K. Zheng, H.L.W. Chan, C.L. Choy Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Received 7 November 2005; accepted 4 January 2006 Available online 23 January 2006
Abstract Poly(vinylidene fluoride–trifluoroethylene) copolymer [(P(VDF–TrFE)] nanowires and nanotubes have been fabricated by a template technique. The morphology of the nanowires and nanotubes was studied by scanning electron microscopy, and the diameter of the nanowires and nanotubes was found to be in range of 55–360 nm. X-ray diffraction shows that the nanowires and nanotubes have the same crystal structure as that of P(VDF–TrFE) bulk sample. Relative permittivity measurements as a function of temperature reveal a ferroelectric-to-paraelectric phase transition, implying that ferroelectricity is still maintained in the nanowires and nanotubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Polymers; Ferroelectrics
1. Introduction Over the last decade, low-dimensional nanostructures have received considerable attention from the scientific and engineering communities because of their small size and large surface-to-volume ratios. Therefore, they can be promising candidates for realizing nanoscale electronic, optical and mechanical devices. One dimensional structures such as nanotubes [1,2] and nanowires [3,4] represent a particularly attractive class of nanostructures as they can function as both nanoscale device elements and interconnects while retaining unique properties due to size confinement in the radial direction [3,5,6]. To date, most research efforts have been directed toward synthesis and applications of metal and metal alloys, oxide ceramics, carbon, and semiconductor nanowires and nanotubes [1–4]. Experimental strategies applied to other classes of materials, i.e. polymers, are beginning to emerge [7–9]. Ferroelectric polymers, including polyvinylidene fluoride (PVDF) and poly(vinylidene fluoride–trifluoroethylene) copolymer [P(VDF–TrFE)], have generated much interest in the past
⁎ Corresponding author. Tel.: +852 27665662; fax: +852 23337629. E-mail address: [email protected] (S.T. Lau). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.01.006
ten years because of their potential as functional materials for energy transduction and information recording. Current interest in P(VDF–TrFE) copolymer arises because these materials have high electromechanical properties and large ferroelectric polarization [10,11]. Other notable characteristics of this copolymer are low dielectric constant, low elastic stiffness and low density which result in a high voltage sensitivity (excellent sensor characteristics). Based on these features, P(VDF-TrFE) copolymer have been used as sensing and actuating devices in a wide range of applications [12,13]. With the miniaturization trend in device size, preparation of ferroelectric polymer nanowires and nanotubes has attracted much interest because of their potential applications as functional elements in sensing and microelectromechanical systems. However, there is limited report of preparing and characterizing P(VDF–TrFE) copolymer nanowires and nanotubes. In our previous work [14], we have prepared P(VDF–TrFE) nanotubes and nanowires using a vacuum infiltration method. In this paper, we describe a hot-press method for fabrication of the nanowires and nanotubes. This method enables more copolymer to infiltrate into the ordered porous template in comparison with the vacuum infiltration method, and thus high-dense nanowires can be fabricated. The morphology and structure of the nanowires and nanotubes are studied by a scanning electron microscope
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and X-ray diffraction system. The relative permittivities of the nanowires and nanotubes are measured and compared with that of the bulk sample and nanowires prepared by vacuum infiltration method. 2. Experimental Two types of anodic aluminium oxide (AAO) templates were used in the fabrication of the copolymer nanowires and nanotubes. The first type, with the diameter 21 mm, thickness 60 μm and a pore size of 160–200 nm, was supplied by Whatman Co. Ltd. The second type, with the diameter 10 mm, thickness 20–40 μm and pore size 60–80 nm, was prepared using a two-step anodizing process. Fig. 1 shows the schematic steps of the process. High-purified aluminium (Al) foil was used as the starting material. Before anodization, the Al foil was degreased with acetone and then annealed at 450 °C for 2 h under nitrogen ambient to enhance the grain size in the metal and to obtain homogeneous conditions for pore growth over large areas. The Al foil was subsequently electropolished in a 1 : 9 by volume mixture of HClO4 : C2H5OH solution. Anodization was carried out under a constant voltage of 40 V in a 0.3 M oxalic acid (C2H2O4) solution for 8–10 h, and the temperature was kept constant at 5 °C. In the beginning of the first anodization process,
the pores were randomly distributed on the surface. During their growth into the bulk material they arranged in a hexagonal pattern due to a process of self-organization. This can be utilized to create surfaces with ordered pores. Then, the primarily created oxide layer is removed by immersing in a mixture of 6 wt.% phosphoric acid (H3PO4) and 1.8 wt.% chromic acid (H2CrO4). A second anodization process made with the same condition was carried out for 5–10 h using the pre-structured aluminium surface as template. To produce a free standing membrane through nano-channels, the remaining Al layer was first removed in a saturated HgCl2 solution for 2–3 h. To enlarge the nanochannels and remove the continuous oxide barrier, the template was immersed in H3PO4 solution for 20–30 min at 50 °C. The morphology of the two types of AAO templates was examined by a field-emission scanning electron microscope (FE-SEM). Copolymer of vinylidene fluoride and trifluoroethylene [P(VDF–TrFE)] with 70 mol% of VDF, supplied in pellet form by Piezotech Co., Saint Louis, France, was dissolved in methyl ethyl ketone (MEK) to form a solution which was then casted on a flat glass plate to form thin films of thickness 20–30 μm. The films were heated at 135 °C for 6 h to remove the residual MEK. The films were placed on the AAO template and heated to 270 °C, and then pressed at a pressure of 50 MPa for 15 min to enhance the infiltration of the copolymer into the pores of the
Fig. 1. Schematic diagram showing the process for fabricating porous AAO membrane.
S.T. Lau et al. / Materials Letters 60 (2006) 2357–2361
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AAO template. The sample was then cooled to room temperature at a rate of 10 °C/min. To achieve high crystallinity, the sample was annealed at 135 °C for 15 h. During the processing at 270 °C, while some of the copolymer had infiltrated into the pores of the AAO template, a layer of copolymer remained on the surface of AAO template. For FE-SEM measurements the AAO template was completely removed by immersing the sample in a 4 M sodium hydroxide (NaOH) solution, and then the sample was rinsed with distilled water. The structures of the copolymer nanowires and nanotubes were studied using a Bruker AXS D8 Advance X-ray diffractometer equipped with CuKα radiation. To study the electrical property of the copolymer nanowires and nanotubes (with the template as a support), circular shape chromium/gold electrode with diameter 3 mm was sputtered on the surfaces of the sample. The relative permittivity property of the sample was studied as a function of temperature using an HP 4194A impedance/gain-phase analyzer. 3. Results and discussion
Fig. 2. FE-SEM images of the AAO templates. Type 1: supplied by the Whatman Co. Ltd.; Type 2: prepared in our laboratory using a two-step anodizing process.
The FE-SEM images of the AAO templates used for the fabrication of the copolymer nanowires and nanotubes are presented in Fig. 2. The template prepared in our laboratory shows a clear hexagonal pore ordering in comparison with the one provided by Whatman Co. Ltd. After infiltration of the copolymer melt into the pores of the templates, the samples were annealed, and the copolymer nanowires and nanotubes were released by removing the templates in NaOH solution. Fig. 3 shows the morphology of the copolymer nanowires and
Fig. 3. FE-SEM images of the P(VDF–TrFE) copolymer nanowires and nanotubes.
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nanotubes. It can be seen that the copolymer nanowires are arranged roughly parallel to one another, and the diameter ranges from 55 to 65 nm when a template with a pore size of ∼60 nm was used (Fig. 3(a) and (b)). To fabricate copolymer nanotubes, templates with a larger pore size (about 200 nm) were used. The resulting nanotubes have an outer diameter of ∼360 nm and a wall thickness of ∼45 nm (Fig. 3(c) and (d)). The results imply that whether nanowires or nanotubes are formed depends on the pore size of the AAO template. Larger pore size favours the formation of nanotubes. The length of the nanowires and nanotubes is in the range of 30–55 μm, which is dependent on the processing temperature, infiltration time and thickness of the template. For X-ray diffraction measurements on the copolymer nanowires and nanotubes, the copolymer layer on the surface of the AAO template was removed by mechanical polishing and then the sample was cleaned with ethanol. For comparison, the X-ray diffraction patterns of the AAO template and a bulk copolymer sample were also measured. As shown in Fig. 4(a), no diffraction peak is observed for the AAO template, indicating that the template is amorphous. For the bulk copolymer sample, an intense peak at 2θ = 19.7° is observed, which corresponds to the composite (110, 200) reflection of the β phase (ferroelectric phase) composed of all trans chains [15]. The XRD
Fig. 5. Temperature dependence of the relative permittivity εr (at 100 kHz) of the (a) bulk copolymer sample, (b) copolymer nanowires, and (c) copolymer nanotubes.
patterns of the copolymer nanowires and nanotubes are presented in Fig. 4(b) and (c). It can be seen that the copolymer nanowires and nanotubes exhibit a peak at 2θ = 19.7° and 2θ = 19.6°, respectively,
Fig. 4. X-ray diffraction patterns of the (a) bulk copolymer sample and AAO template, (b) copolymer nanowires, and (c) copolymer nanotubes.
Fig. 6. Temperature dependence of the relative permittivity εr (at 100 kHz) of the copolymer nanowires prepared by vacuum infiltration method.
S.T. Lau et al. / Materials Letters 60 (2006) 2357–2361
which is close to that of the bulk sample. This implies that the nanowires and nanotubes have the same crystal structure as the bulk sample. The temperature dependence of the relative permittivity εr at 100 kHz for the bulk copolymer sample, nanowires and nanotubes is presented in Fig. 5. Both the nanowires and nanotubes exhibit a peak at ∼110 °C upon heating and another peak at ∼70 °C upon cooling, which can be attributed to the ferroelectric-to-paraelectric transition of the copolymer as observed in the bulk sample (Fig. 5(a)). These results indicate that both nanowires and nanotubes are ferroelectric at room temperature and are consistent with the XRD measurements. As compared with the nanowires prepared by vacuum infiltration method (Fig. 6), the εr of the nanowires prepared by hot-press method is increased obviously. This implies that the hot-press method enables more copolymer infiltrate into the pores of the AAO template, and favours to produce high-dense nanowires.
4. Conclusions P(VDF–TrFE) copolymer nanowires and nanotubes with diameters ranging from 55 to 360 nm have been fabricated using a template technique. It was found that larger pore size in the template favours the formation of nanotubes. XRD measurements show that the crystal structure of the nanowires and nanotubes is the same as that of the bulk sample. When the relative permittivity property is measured as a function of temperature, a ferroelectric-to-paraelectric phase transition is observed for both the nanowires and nanotubes, implying that ferroelectricity is still maintained in the nanowires and nanotubes. Besides, the εr of the nanowires prepared by hot-press method is increased comparing to those by vacuum infiltration method. This indicates that more copolymer can infiltrate into the porous template and produce high-dense nanowires.
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Acknowledgement This work was supported by the Centre for Smart Materials (CSM) of the Hong Kong Polytechnic University and the Hong Kong Polytechnic University Postdoctoral Fellowship Scheme (Project No.: G-YX16). References [1] M. Bockrath, W. Liang, D. Bozovic, J.H. Hafner, C.M. Lieber, M. Tinkham, H.K. Park, Science 291 (2001) 283. [2] W. Liang, M. Bockrath, D. Bozovic, J.H. Hafner, M. Tinkham, H. Park, Nature 411 (2001) 665. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] J.D. Holmes, K.P. Johnston, R.C. Doty, B.A. Korgel, Science 287 (2000) 1471. [5] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [6] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [7] V.F. Puntes, K.M. Krishnan, A.P. Alivisatos, Science 291 (2001) 2115. [8] F.D. Morrison, Y. Luo, I. Szafraniak, V. Nagarajan, R.B. Wehrspohn, M. Steinhart, J.H. Wendorff, N.D. Zakharov, E.D. Mishina, K.A. Vorotilov, A.S. Sigov, S. Nakabayashi, M. Alexe, R. Ramesh, J.F. Scott, Rev. Adv. Mater. Sci. 4 (2003) 114. [9] J. Joo, K.T. Park, B.H. Kim, M.S. Kim, S.Y. Lee, C.K. Jeong, J.K. Lee, D.H. Park, W.K. Yi, S.H. Lee, K.S. Ryu, Synth. Met. 135 (2003) 7. [10] K. Koga, H. Ohigashi, J. Appl. Phys. 59 (1986) 2142. [11] T. Furukawa, Adv. Colloid Interface Sci. 71 (1997) 183. [12] H. Ohigashi, K. Koga, M. Suzuki, T. Nakanishi, K. Kimura, N. Hashimoto, Ferroelectrics 60 (1984) 263. [13] H.L.W. Chan, A.H. Ramelan, I.G. Guy, D.C. Price, Rev. Sci. Instrum. 62 (1991) 203. [14] R.Z. Zheng, Y. Yang, Y. Wang, J. Wang, H.L.W. Chan, C.L. Choy, C.G. Jin, X.G. Li, Chem. Commun. (2005) 1447. [15] A.J. Lovinger, G.T. Davis, T. Furukawa, M.G. Broadhurst, Macromolecules 15 (1982) 323.