APPLIED PHYSICS LETTERS 93, 263101 共2008兲

Matrix replacement route to vertically aligned nickel nanowire array/ polydimethylsiloxane nanocomposite film Xin Meng, Liang-Tian Zhou, Ji-Xiang Zhu, Jie Song, Xuan-Rui Wang, and Zheng-Ping Qiaoa兲 MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China

共Received 7 July 2008; accepted 20 November 2008; published online 29 December 2008兲 Vertically aligned magnetic anisotropic nickel 共Ni兲 nanowire 共NW兲 array/polydimethylsiloxane 共PDMS兲 film was prepared from 共Ni NW array兲/anodic aluminum oxide by a simple matrix replacement route. The main challenge is to preserve the parallelly aligned Ni NW during replacement. The diameter and thickness of the as-prepared Ni NW and the Ni NW array/PDMS film are 8 mm and 60 ␮m, respectively. The magnetic property measurement shows that the film has remarkably enhanced coercivity and remanence ratio compared to that of bulk nickel and exhibits perpendicular magnetic anisotropy. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3050456兴 As an important magnetic material, nickel has undoubtedly attracted much attention and has been the focus of intense research.1 Nickel nanowire 共Ni NW兲 array exhibiting uniaxial magnetic anisotropy is considered to be a good candidate for perpendicular magnetic recording media.2 Material based on the well-aligned Ni NW array is higher desirable than that based on Ni nanoparticles. However, NW are more difficult to align than particles since those particles touch each other at a point, whereas NW contact along a line.3,4 So nanocomposite that contained Ni particles has been well documented. A few references concerned about nanocomposite contained Ni NW array. One successful example for synthesis of well-aligned Ni NW array is electrodeposition in anodic aluminum oxide 共AAO兲 template.5–9 Ni NW are uniform, paralleling to each other, and standing vertically to the electrode substrate with magnetic anisotropy. However, AAO is too brittle to handle under certain conditions 共e.g., calcinated at an elevated temperature or treatment by NaOH solution兲,10 which limits the potential application of Ni NW arrays. Here we report a simple but effective method for preparing highly ordered vertically aligned 共Ni NW array兲/ polydimethylsiloxane 共noted as Ni NW array/PDMS兲 film. Ni NW array remains the parallel alignment as they embedded in AAO. The brittle AAO was replaced by PDMS, which own merits of transparency, chemical inertia, good flexibility, and biocompatibility.11 The strategy described in this paper was named as matrix replacement. Similar work is hard template replication, which has shown success in the formation of well ordered membranes and attracted considerable attention.12–20 However, no paper reported magnetic anisotropic film that can be handled by hand. It is noteworthy that the fabrication process does not depend on the chemical properties of components. So it is a versatile method that can be applied to prepare other nanocomposites contained vertically aligned NW array as long as the NW can be electrodeposited in AAO. a兲

Electronic mail: [email protected].

0003-6951/2008/93共26兲/263101/3/$23.00

AAO, PDMS prepolymer with curing agent, and Ni etchants were obtained from Whatman Co. 共Anodisc 13兲, Dow Corning Co. 共Sylgard 184兲, and Transene Co. 共TFG兲, respectively. All reagents were of analytic purity and were used without further purification. X-ray powder diffraction pattern of the product was obtained using a Bruker D8-Advance. The transmission electron microscope 共TEM兲 image and energy dispersive x-ray spectrometer 共EDS兲 were taken with a JEOL JEM-2010HR transmission electron microscope, using an accelerating voltage of 200 kV. Scanning electron microscopy 共SEM兲 was performed on a JEOL JSM-6330F scanning electron microscope operated at 20 kV. The magnetic measurements were carried out at room temperature using a magnetic property measurement system 共Quantum Design MPMS XL-7兲. Schematic illustration of the fabrication of the magnetic 共Ni NW array兲/PDMS is shown in Scheme 1. The synthesis of 共Ni NW兲/AAO was carried out by modifying the method in Refs. 8 and 9. Briefly, five steps were involved. First, alumina membranes, AAO with uniform pore size of 200 nm, was coated on one side with 500-nm-in-thick Cu film by e-beam evaporation. The Cu film acted as a work electrode. The open side of AAO was then immersed in the mixed solution of 300 g l−1NiSO4 · 6H2O, 45 g l−1NiCl2 · 6H2O,

SCHEME 1. Schematic illustrating fabrication of the magnetic 共Ni NW array兲/PDMS. 93, 263101-1

© 2008 American Institute of Physics

Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

263101-2

Meng et al.

FIG. 1. 共Color online兲 共a兲 The photo of 共Ni NW array兲/PDMS, AAO. The diameters of 共Ni NW array兲/PDMS and AAO are 8 and 13 mm, respectively. 共b兲 SEM micrograph of 共Ni NW array兲/PDMS films.

and 45 g l−1H3BO3 · 6H2O. The pH value is around 4.0. The counterelectrode is platinum 共Pt兲. The reference electrode is a saturated calomel electrode. The work voltage is −2.0 V dc. The pore of AAO was almost full with Ni NW after electrodeposition for 0.5 h. Ni NW grew throughout AAO and formed Ni bulk layer on the top of AAO after extended deposition time to 3 h. This sample is named as Cu-共Ni NW array兲/AAO-Ni based on its structure. Second, the Cu–共Ni NW array兲/AAO-Ni was turned over, immersed Cu layer into the solution to electrodeposit second Ni bulk layer on the Cu film side. The time was selected as 2.5 h to warrant that the thickness of the two Ni bulk layers was the same. This sample is named as Ni–Cu–共Ni NW array兲/AAO-Ni. Third, the AAO template was dissolved in 4 mol dm−3 NaOH solution to form Ni–Cu–共Ni NW array兲–Ni structure. It was cleaned with acetone, ethanol, and de-ionized water subsequently. Fourth, PDMS prepolymer and curing agent were mixed as 10:1 共wt/wt兲. After being degassed by vacuum for 1 h, the mixture was poured carefully into Ni–Cu–共Ni NW array兲–Ni. The system was degassed by vacuum in ice bath overnight and then was cured at 80 ° C for 8 h. PDMS was filled into the interspace of Ni NW array and also was coated out of Ni–Cu–共Ni NW array兲–Ni. The new structure was formed as PDMS-Ni–Cu–共Ni NW array兲/PDMS-Ni–PDMS. Finally, PDMS that coated out of the Ni bulk layers was cut off. Ni bulk layers and Cu layer were removed at room temperature by immersing in the nickel enchants for 48 h 共refreshed nickel enchants once when it had been immersed for 24 h兲 and then ammonia 共13.3 mol dm−3兲 for 60 min, respectively. So the 共Ni NW array兲/PDMS was prepared. Shortly, AAO worked as template for electrodepositing of Ni NW. After the AAO template was removed, Ni NW array worked as a second order template for filling PDMS polymer. The result is that PDMS replaced the position of AAO. Therefore, this strategy is named as a matrix replacement route. Figure 1共a兲 shows the pictures of 共Ni NW array兲/PDMS and AAO. The diameter of 共Ni NW array兲/PDMS is 8 mm, which is smaller than AAO 共13 mm兲, but it is equal to the contact interface of electrolytic deposition. 共Ni NW array兲/ PDMS is of black film. Figure 1共b兲 shows the SEM image of 共Ni NW array兲/PDMS film, in which the Ni NW array was even dispersed in the PDMS matrix. The average diameter of Ni NW is about 200 nm and the NW are almost all the same in size. The length of Ni NW is 60 ␮m that was calculated according to the thickness of AAO. The aspect ratio of NW was over 300. The x-ray diffraction 共XRD兲 pattern of 共Ni NW array兲/ PDMS is shown in Fig. 2. All of the diffraction peaks could be indexed to 共111兲, 共200兲, and 共220兲 of the face centered

Appl. Phys. Lett. 93, 263101 共2008兲

FIG. 2. Typical XRD pattern of 共Ni NW array兲/PDMS. All of the diffraction peaks can be indexed as the fcc phase Ni 共Ref. 30兲. The amorphous phase is PDMS.

cubic 共fcc兲 phase Ni 关space group is Fm3m 共225兲兴, which is in agreement with the reported date.30 To further investigate the microstructure of the Ni NW composite, 共Ni NW array兲/ PDMS was baked at 300 ° C for 10 h, grinded into powdered sample, and were ultrasonically dispersed in ethanol. Then a droplet of the suspension was placed on a copper grid and dried in air for TEM observation. The result was shown in Fig. 3. Figure 3共a兲 shows that the Ni NW was evenly coated with uniform PDMS after calcinated; the thickness of PDMS is 20⬃ 25 nm. An EDS pattern of the center part of 共Ni NW兲/PDMS 关Fig. 3共b兲兴 shows that a large amount of Ni and little Si coexist. An EDS study of the shell of 共Ni NW兲/ PDMS 关Fig. 3共c兲兴 shows the main component of the shell is elemental Si, C, and O come from PDMS, which confirmed the modification. The Cu in Figs. 3共b兲 and 3共c兲 and C, O in Fig. 3共b兲 come from Cu grid and contaminants, respectively. The magnetic hysteresis loops of 共Ni NW array兲/PDMS films were measured with a superconduncting quantum interference device magnetometer at room temperature. Figure 4 shows the hysteresis loops with the applied magnetic field perpendicular 共⬜兲 and parallel 共储兲 to the film of 共Ni NW array兲/PDMS, respectively. Hc共⬜兲 and Hc共储兲 are 220.24 and 84.79 Oe. The Hc and M r / M s of those perpendicular to the surface of film are larger than those of parallel to the film surface. It is evident that the easy magnetization axis is perpendicular 共⬜兲 to the surface of 共Ni NW array兲/PDMS films, i.e., along the axis of the NW. The data of perpendicular to the surface of films exhibit greatly enhanced magnetic coercivity. Their strong shape anisotropy and magnetic crystalline anisotropy of Ni may play some roles.21 This result in-

FIG. 3. 共Ni NW array兲/PDMS was baked at 300 ° C for 10 h, grinded into powdered sample. 共a兲 Typical TEM image, 共b兲 EDS of center of 共Ni NW兲/ PDMS, and 共c兲 EDS of shell of 共Ni NW兲/PDMS.

Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

263101-3

Appl. Phys. Lett. 93, 263101 共2008兲

Meng et al.

patible. In addition, this method can be expanded to prepare other NW array/polymer nanocomposites by simply changing the polymer or NW. We thank NSFC 共No. 20401016兲, the Science & Technology, Bureau of Guangdong Province 共No. 04009720兲, and Charles Lieber’s group in Harvard University. 1

FIG. 4. 共Color online兲 Magnetic hysteresis loops for 共Ni NW array兲/PDMS film. Line with dot and square are magnetic hysteresis loops that measured by applying magnetic field perpendicular 共⬜兲 and parallel 共储兲 to the surface of film, respectively.

dicated that the as-prepared 共Ni NW array兲/PDMS film exhibits perpendicular magnetic anisotropy. Highly ordered vertically aligned 共Ni NW array兲/ polystyrene nanocomposite film can also be prepared by copolymer route.22–28 Diblock copolymer of polystyrene and polymethylmethanacrylate worked as template for electrodepositing of Ni NW. However, to our knowledge the thickness of the copolymer is only nanoscale, which will severely restrict the thickness of the nanocomposite film. The procedure of matrix replacement route in this paper is so simple and effective that can be extended to industry. The only challenge is that the embedded arrays of NW with a high aspect ratio normally collapse into an entangled mess after the template was removed due to the surface tension force exerted on the NW.29 Here we introduce that Ni and Cu/Ni bulk layer worked as the supporting layers for Ni NW array after AAO was removed. NW are reasonably stuck between the supporters. In summary, the characterizations 共XRD, SEM, and TEM兲 indicated that the highly ordered vertically aligned magnetic anisotropic 共Ni NW array兲/PDMS film was prepared from 共Ni NW array兲/AAO by a simple matrix replacement route. AAO and Ni NW array worked as template successively. The critical challenge of preservation of the high order of NW array was resolved by simply “sticking” NW on Ni bulk supported layer by overdeposition. The two Ni bulk layers were simply selectively removed with Ni etchants by controlling the etching time. PDMS replaced AAO that has weak chemical inertia and brittleness. Magnetization measurement shows that 共Ni NW array兲/PDMS film exhibits perpendicular magnetic anisotropy. From a technology point of view, highly ordered 共Ni NW array兲/PDMS film in this study could probably be applied to magnetic biological devices, which can be implanted into organs since PDMS is biocom-

N. Cordente, M. Respaud, F. Senocq, M. J. Casanove, C. Amiens, and B. Chaudret, Nano Lett. 1, 565 共2001兲. 2 H. Q. Cao, Z. Xu, H. Sang, D. Sheng, and C. Y. Tie, Adv. Mater. 共Weinheim, Ger.兲 13, 121 共2001兲. 3 D. W. Schaefer and R. S. Justice, Macromolecules 40, 8501 共2007兲. 4 W. Chen, L. Qu, D. Chang, L. S. Dai, S. Ganguli, and A. Roy, Chem. Commun. 共Cambridge兲 2008, 163. 5 C. R. Martin, Science 266, 1961 共1994兲. 6 M. Lahav, E. A. Weiss, Q. Xu, and G. Whitesides, Nano Lett. 6, 2166 共2006兲. 7 D. H. Park, Y. B. Lee, M. Y. Cho, B. H. Kim, S. H. Lee, Y. K. Hong, J. Joo, H. C. Cheong, and S. R. Lee, Appl. Phys. Lett. 90, 093122 共2007兲. 8 K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, J. Phys. Chem. B 107, 668 共2003兲. 9 J. Qin, J. Nogués, M. Mikhaylova, A. Roig, J. S. Muñoz, and M. Muhammed, Chem. Mater. 17, 1829 共2005兲. 10 M. Kim, K. Kim, N. Y. Lee, K. Shin, and Y. S. Kim, Chem. Commun. 共Cambridge兲 2007, 2237. 11 J. C. McDonald and G. M. Whitesides, Acc. Chem. Res. 35, 491 共2002兲. 12 H. Masuda and K. Fukuda, Science 268, 1466 共1995兲. 13 D. H. Pearson and R. J. Tonucci, Science 270, 68 共1995兲. 14 S. A. Johnson, P. J. Ollivier, and T. E. Mallouk, Science 283, 963 共1999兲. 15 T. Yanagishita, K. Nishio, and H. Masuda, Jpn. J. Appl. Phys., Part 2 45, L804 共2006兲. 16 M. Choi and R. Ryoo, Nature Mater. 2, 473 共2003兲. 17 M. A. Ghanem, P. N. Bartlett, P. de Groot, and A. Zhukov, Electrochem. Commun. 6, 447 共2004兲. 18 A. A. Zhukov, E. T. Filby, A. V. Goncharov, M. A. Ghanema, P. N. Bartletta, R. Boardmanb, H. Fangohrb, V. V. Metlushkoc, V. Novosadd, G. Karapetrovd, and P. A. J. de Groot, J. Low Temp. Phys. 139, 339 共2005兲. 19 D. H. Kim, Z. Lin, H.-C. Kim, U. Jeong, and T. P. Russell, Adv. Mater. 共Weinheim, Ger.兲 15, 811 共2003兲. 20 G. Duan, W. Cai, Y. Luo, Z. Li, and Y. Lei, J. Phys. Chem. B 110, 15729 共2006兲. 21 K. Nielsch, R. B. Wehrspohn, J. Barthel, J. Kirschner, U. Gösele, S. F. Fischer, and H. Kronmüller, Appl. Phys. Lett. 79, 1360 共2001兲. 22 T. Thurn-Albrecht, J. Schotter, G. A. Kästle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C. T. Black, M. T. Tuominen, and T. P. Russell, Science 290, 2126 共2000兲. 23 T. Xu, H.-C. Kim, J. DeRouchey, C. Seney, C. Levesque, P. Martin, C. M. Stafford, and T. P. Russell, Polymer 42, 9091 共2001兲. 24 O. Seifarth, R. Krenek, I. Tokarev, Y. Burkov, A. Sidorenko, S. Minko, M. Stamm, and D. Schemißer, Thin Solid Films 515, 6552 共2007兲. 25 A. Sidorenko, I. Tokarev, S. Minko, and M. Stamm, J. Am. Chem. Soc. 125, 12211 共2003兲. 26 Y. Li, H. C. Maire, and T. Ito, Langmuir 23, 12771 共2007兲. 27 O.-H. Park, J. Y. Cheng, M. Hart, T. Topuria, P. M. Rice, L. E. Krupp, R. D. Miller, H. Ito, and H.-C. Kim, Adv. Mater. 共Weinheim, Ger.兲 20, 738 共2008兲. 28 S. O. Kim, B. H. Kim, D. Meng, D. O. Shin, C. M. Koo, H. H. Solak, and Q. Wang, Adv. Mater. 共Weinheim, Ger.兲 19, 3271 共2007兲. 29 Y. Liang, C. Zhen, D. Zou, and D. Xu, J. Am. Chem. Soc. 126, 16338 共2004兲. 30 JCPDS Card No. 04–0850.

Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp