Author name used in this publication: Kwo Ho LamAuthor name used in this publication: Cheng-Liang SunAuthor name used in this publication: Helen Lai Wa Chan2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Chao, C

Guo, S

Lam, KH

Lau, ST

Sun, C

Zhao, X

Applied Physics Letters

Author name used in this publication: Kwo Ho LamAuthor name used in this publication: Cheng-Liang SunAuthor name used in this publication: Helen Lai Wa Chan2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Fabrication and characterization of Ni/P„VDF-TrFE… nanoscaledcoaxial cablesCheng-Liang Sun,a Kwo Ho Lam, Chen Chao, Sien Ting Lau, and Helen Lai Wa ChanDepartment of Applied Physics and Materials Research Centre, The Hong Kong PolytechnicUniversity, Hunghom, Kowloon, Hong Kong SAR, ChinaShishang Guo and Xingzhong ZhaoDepartment of Physics, Wuhan University, Wuhan 430072, ChinaReceived 20 March 2007; accepted 25 May 2007; published online 19 June 2007The authors have prepared polyvinylidenefluoride-trifluoroethylene Ni/PVDF-TrFE nanoscaledcoaxial cables with Ni nanowires as the cores and PVDF-TrFE nanotubes as the shells by atwo-step process. Scanning electron microscope and x-ray diffraction measurements revealed themicrostructure and crystallinity of the nanocables. Electrical measurements indicated that thenanocables possessed a large specific capacitance 10.84 pC/mm2 and ferroelectric remnantpolarization 10 C/cm2 due to the composite structure. The distribution of electrical field in thenanostructure was also analyzed. © 2007 American Institute of Physics. DOI: 10.1063/1.2750391In recent years, the study of ferroelectric and piezoelec-tric nanostructures has attracted strong research interest dueto the potential applications of these materials in nanoscaledsensors, actuators, and energy harvesting devices.1–6 Amongmany well-studied ferroelectric and piezoelectric materials,the polyvinylidenefluoride-trifluoroethylene PVDF-TrFE copolymer is a very unique candidate for nanodevicesbecause it has a good sensitivity, high electromechanicalproperties, low dielectric constant, and low loss. It is a poly-mer so that its shape and size can be easily modified byconventional polymeric processing. Moreover, the ferroelec-tric and piezoelectric properties can be modified to a largeextent by thermal treatment, irradiation, or by varying themolar ratio of PVDF and TrFE. While thin films and com-posites of PVDF-TrFE have been extensively studied in thepast decades,7–13 recently there is an increasing interest todevelop nanowires and nanotubes of this material.14 In theeffort to fabricate sensors using such nanowires and nano-tubes, we have encountered much difficulty in handling themmainly because PVDF-TrFE is soft and flexible. To solvethis problem, we managed to introduce metallic nanowiresin the PVDF-TrFE nanotubes aiming to provide additionalmechanical support. In this letter we demonstrate one ofsuch structures that we have recently developed—Ni/PVDF-TrFE nanoscaled coaxial cables in which Ninanowires are the cores and PVDF-TrFE nanotubes are theshells. As to be demonstrated later in this letter, the introduc-tion of Ni nanowires has led to significant enhancement inthe dielectric and ferroelectric properties of the PVDF-TrFE. The nanocable structure studied in this work consistsof a functional organic material although many types of in-organic material-based nanocables have been reported inliterature.15–21The nanocables were fabricated via a two-step process,as schematically shown in Fig. 1. Firstly, PVDF-TrFEnanotubes were fabricated by hot pressing using an aluminaoxide AAO membrane as dual mould.14 The AAO mem-brane Whatman, Anodisc, Ref. No. 6809 7023 is 60 mthick and has through holes with a diameter of 450 nm.The PVDF-TrFE copolymer used in this work has a VDF/TrFE molar ratio of 70/30 and is known to possess goodpiezoelectric property. The hot pressing was conducted at270 °C and with the application of a pressure of 50 MPafor 15 min. By the capillary effect, PVDF-TrFE copolymerwas squeezed into the template. After cooling to room tem-perature, PVDF-TrFE nanotubes were formed. Post-hot-pressing treatments were conducted. For example, the re-sidual polymer on the major surfaces of the template wasremoved by using plasma etching, and a thin layer of goldwas coated on one of the major surfaces by magnetron sput-tering. Secondly, Ni nanowires were developed by cathodicelectrodeposition using an electroplating system Top Wardpotentiostat with a dual-tracking dc power supply 6303D.This electroplating also led to the formation of a thin layer ofNi on the Au film. The quality of Ni nanowires was found tobe very sensitive to processing conditions, including theelectrolyte and its concentration, cathodic current, depositiontime, and deposition temperature. Some of the as-depositedsamples were further treated by soaking in a 4M sodiumhydroxide NaOH solution to remove the AAO template,followed by cleaning in a de-ionized water bath.The structure of the samples was characterized under ascanning electron microscopy SEM. Figure 2a shows thetop view of the sample after NaOH etching. It is observedthat the tubes are round in shape with a diameter of450 nm and exhibit a roughly parallel arrangement. Ninanowires are not observed in this figure because they areshorter than the PVDF-TrFE nanotubes. After the treatmentin oxygen plasma, however, Ni nanowires can be clearlyobserved due to the partial removal of PVDF-TrFE, asaElectronic mail: chengliangsun@hotmail.comFIG. 1. Schematic process for the fabrication of Ni/PVDF-TrFEnanocables.APPLIED PHYSICS LETTERS 90, 253107 20070003-6951/2007/9025/253107/3/$23.00 © 2007 American Institute of Physics90, 253107-1Downloaded 10 Feb 2009 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspshown in Fig. 2b. The diameter of the Ni nanowire is250 nm.The crystal structures of PVDF-TrFE and Ni in thesamples were characterized by x-ray diffraction XRD. Fig-ure 3a shows the XRD measurement result. In the scanningrange of 10°–60°, three major peaks located at 19.6°, 44.4°,and 51.6°, respectively, are observed. The first peak wasidentified to represent PVDF-TrFE, while the other twowere found to belong to the Ni 111 and 200 reflections,respectively. The sharpness and intensity of the last twopeaks indicate that Ni was well crystallized. To determine thecrystallinity Xc of the PVDF-TrFE in the nanostructure,the first XRD peak 13°225°  was further analyzed.This peak is rather broad and asymmetric, suggesting that itis a combination of an amorphous and a crystallized phase ofPVDF-TrFE. Our data treatment was conducted first by re-moving the background noise and then by decomposing theremaining peak into two symmetric peaks using Gaussianand least-squares fitting. As shown in Fig. 3b, two symmet-ric peaks were obtained, one located at 18.2° and another at19.5°, which represent the amorphous and crystalline phasesof PVDF-TrFE, respectively. In general, Xc is defined bythe formula Xc=Acry / Acry+Aam, where Acry is the area un-der the crystalline peak and Aam is the area under the amor-phous peak.22 By calculation we found that Xc=60%. Thisvalue suggests that the PVDF-TrFE sample has a goodcrystallinity.Dielectric and ferroelectric properties of the PVDF-TrFE in the nanocables were determined. The capacitanceof the sample was measured using an impedance analyzerAgilent 4294A at 1 kHz. The nominal specific capacitancedefined as capacitance over electrode area of the samplewas found to be 10.8 pF/mm2. This value is about fivetimes larger than the specific capacitance 1.9 pF/mm2 ofPVDF-TrFE thin films 50 m thick, dielectric constant is11 that we prepared and tested under the same conditions.The ferroelectric properties were determined by using a stan-dard Sawyer-Tower circuit. As shown in Fig. 4, well-definedferroelectric hysteresis loops were obtained if the appliedelectric field is strong enough. Tested under a field with amaximum strength of 100 V/m, for example, the samplewas found to exhibit a saturated hysteresis loop with a rem-nant polarization Pr of 10 C/cm2 and coercive fieldEc of 50 V/m. This remnant polarization is three timeslarger than that of PVDF-TrFE thin films 3.1 C/cm2.8The enhancement of the dielectric and ferroelectric prop-erties of PVDF-TrFE in the samples could be roughly ex-plained as a result of the increase of the contact area betweenthe metal and the polymer due to the existence of the Ninanowires. To fully understand the ferroelectric behavior ofthe nanocables, it is also important to analyze the distributionof electrical field inside the dielectrics. We have conducted acalculation to simulate such distribution and the result isschematically shown in Fig. 5. The electric potential con-tours and electric field line distribution in the nanocoaxialcables were simulated by the finite element modeling usingthe commercial program ANSYS. Because of the periodicityof the nanocoaxial cables, only four metal nanowires wereinvolved in the two-dimensional simulation. Quite obviously,the direction and strength of electrical field are dependent onlocation: i around the ends of Ni nanowires the electricalfield is very strong, ii in the air columns on top of Ninanowires the electric field is even stronger, and iii inFIG. 2. SEM images of Ni/PVDF-TrFE nanocables: a Top view and bcross-sectional view.FIG. 3. a XRD pattern of PVDF-TrFE nanocables and b decomposingthe first peak into two peaks.FIG. 4. Ferroelectric hysteresis loops of PVDF-TrFE in the nanocablestest frequency=10 Hz and the structure schematic diagram of thenanocables.FIG. 5. a Schematic diagram of the nanocables, b distribution of poten-tial contours, and c electric field in a nanocable under an electric field.253107-2 Sun et al. Appl. Phys. Lett. 90, 253107 2007Downloaded 10 Feb 2009 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspPVDF-TrFE nanotubes the field is weaker and the strengthdecreases as the site gets farther away from Ni ends. Due tothe nonuniform distribution of the electrical field, the ferro-electric polarization in PVDF-TrFE is also strongly loca-tion sensitive. It is reasonable to assume that under an exter-nal field, the polarization around the Ni ends should be largerthan the polarization in other sites. When the field is strongenough, the polarizations in the PVDF-TrFE around Niends will get saturated before the other sites. The overallresult is that the ferroelectric hysteresis loop is quite tiltedbut the remnant polarization is enhanced if compared withPVDFF-TrFE thin films.In summary, we have prepared Ni/PVDF-TrFE nanos-caled coaxial cables by a two-step process and characterizedthe structure and properties. The introduction of Ni nano-wires into PVDF-TrFE has significantly changed the di-electric and ferroelectric properties. Compared with conven-tional PVDF-TrFE thin films, our nanocables exhibited animproved dielectric property and enhanced ferroelectric po-larization, suggesting that they may be useful for makingsome microsensors and microactuators.This work was supported by the Centre for Smart Mate-rials of the Hong Kong Polytechnic University and a grantProject No. 1-BB95.1X. D. Wang, J. Zhou, J. H. Song, J. Liu, N. S. Xu, and Z. L. Wang, NanoLett. 6, 2768 2006.2J. Junquera and P. Ghosez, Nature London 422, 506 2003.3Y. Luo, I. Szafraniak, N. D. Zakharov, V. Nagarajan, M. Steinhart, R. B.Wehrspohn, J. H. Wendorff, R. Ramesh, and M. Alexe, Appl. Phys. Lett.83, 440 2003.4X. Y. Zhang, X. Zhao, C. W. Lai, J. Wang, X. G. Tang, and J. Y. Dai, Appl.Phys. Lett. 85, 4190 2004.5M. W. Chu, I. Szafraniak, R. Scholz, C. Harnagea, D. Hesse, M. Alexe,and U. Gosele, Nat. Mater. 3, 87 2004.6Z. L. Wang and J. H. Song, Science 312, 242 2006.7Q. M. Zhang, V. Bharti, and X. Zhao, Science 280, 2101 1998.8J. J. Li, Y. Neo, H. Mimura, K. Omote, and K. Yokoo, Appl. Phys. Lett.89, 222907 2006.9F. Xia, Z. Y. Cheng, and Q. M. Zhang, IEEE Trans. Ultrason. Ferroelectr.Freq. Control 50, 932 2003.10J. Y. Li, Phys. Rev. Lett. 90, 217601 2003.11V. Bharti and Q. M. Zhang, Phys. Rev. B 63, 184103 2001.12P. A. Jacobson, L. G. Rosa, C. M. Othon, K. L. Kraemer, A. V. Sorokin, S.Ducharme, and P. A. Dowben, Appl. Phys. Lett. 84, 88 2004.13J. W. Choi, P. A. Dowben, S. Pebley, A. V. Bune, S. Ducharme, V. M.Fridkin, S. P. Palto, and N. Petukhova, Phys. Rev. Lett. 80, 1328 1998.14R. K. Zheng, H. L. W. Chan, and C. L. Choy, Nanotechnology 16, 19282005.15O. Carny, D. E. Shalev, and E. Gazit, Nano Lett. 6, 1594 2006.16Q. T. Wang, G. Z. Wang, X. H. Han, X. P. Wang, and J. G. Hou, J. Phys.Chem. B 109, 23326 2005.17J. H. Zhan, Y. Bando, J. Q. Hu, Y. B. Li, and D. Golberg, Chem. Mater.16, 5158 2004.18Q. Li and C. R. Wang, Appl. Phys. Lett. 82, 1398 2003.19J. S. Jang, B. Lim, J. Lee, and T. Hyeon, Chem. Commun. Cambridge2001, 83.20Y. Xie, Z. P. Qiao, M. Chen, X. M. Liu, and Y. T. Qian, Adv. Mater.Weinheim, Ger. 11, 1512 1999.21Y. Zhang, K. Suenaga, C. Colliex, and S. Iijima, Science 281, 973 1998.22M. V. Fernandez, A. Suzuki, and A. Chiba, Macromolecules 20, 18061987.253107-3 Sun et al. Appl. Phys. Lett. 90, 253107 2007Downloaded 10 Feb 2009 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Fabrication and characterization of Ni/P(VDF-TrFE) nanoscaled coaxial cables

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