NANO LETTERS
Magnetic Manipulation of Copper−Tin Nanowires Capped with Nickel Ends
2004 Vol. 4, No. 3 487-490
Anne K. Bentley,† Jeremy S. Trethewey,‡ Arthur B. Ellis,† and Wendy C. Crone*,‡ Department of Chemistry and Department of Engineering Physics, UniVersity of Wisconsin-Madison, Madison, Wisconsin 53706 Received November 26, 2003; Revised Manuscript Received January 5, 2004
ABSTRACT CuSn alloy nanowires capped with Ni were prepared using sequential electrodeposition of nickel and a copper/tin bronze alloy into nanoporous alumina templates. The liberated nanowires were suspended in solvents and their behavior monitored using optical microscopy. The nanowires were oriented and spun in circles as “nano stirbars” using applied magnetic fields. These segmented nanowires were also trapped between magnetized Ni stripes, demonstrating that nonmagnetic metallic nanowires can be manipulated and positioned by capping them with magnetic ends.
A wide variety of one-dimensional nanostructures displaying useful properties has been fabricated in recent years.1 Carbon nanotubes, semiconductor nanowires, and metallic nanowires are of interest for their optical and electronic properties. Applications explored to date include nanoelectronic circuits,2,3 ultraviolet lasers,4 chemical sensors,5 and optical switches.6 To incorporate nanowires into devices, it is critically important to be able to control their motion and position. Work has begun in this area using a variety of alignment techniques, including microfluidic channels,7 liquid crystal templates,8 and patterned surfaces.9 Electric fields have been used to control the position of semiconductor10 and metallic nanowires.11 Recently, the alignment of nickel nanowires has been controlled using magnetic fields.12 In this paper we report on a general method for the manipulation of nonmagnetic metal nanowires that are prepared by electrodeposition in nanoporous membranes. Specifically, we show that by preparing nonmagnetic CuSn alloy nanowires so that they are capped with Ni ends, magnetic fields may be used to orient and spin the nanowires. This method of nanowire control can be applied in principle to any template-synthesized nanowire system containing conductive material. The synthetic technique employed in this work is the template method in which nanowires are made by filling the nanoscale pores of alumina or polycarbonate membranes.13,14 This is an attractive preparative route for two reasons: First, approximately one billion nanowires that are relatively uniform in length and diameter can be made simultaneously. * Corresponding author. Email:
[email protected] † Department of Chemistry. ‡ Department of Engineering Physics. 10.1021/nl035086j CCC: $27.50 Published on Web 01/22/2004
© 2004 American Chemical Society
Second, subsequent dissolution of the template easily liberates the nanowires for study and application in nanoscale devices. The technique has been used to make nanowires from a variety of materials, including pure metals,15 semiconductors,16 multilayer GMR structures,17,18 superconductors,19,20 conductive polymers,21 hydrogels,22 and magnetic metals.23,24 Moreover, the template method allows for precise control over the composition of the nanowires, as composition can be varied along the length and diameter of the wire.22,25 When electrodeposition is used to fill the pores, metallic stripes can be formed by using a solution containing two or more metal ions and varying the applied potential,17 or by exchanging different electrolyte solutions sequentially.26 Such segmented nanowires have demonstrated remarkable abilities as GMR materials,17,18 bio-labels,26 and self-assemblers.27 In the work reported here the template method is used to make segmented nanowires comprising a central bronze alloy capped by nickel ends. In a typical synthesis, Ni-capped bronze nanowires were formed by electrodeposition into an alumina membrane with pores 200 nm in diameter (Anodisc, Whatman, Inc.). One side of the alumina membrane was first sputtered with a 250nm thick layer of silver to form a working electrode. Nickel was then electrodeposited into the pores using a PAR model 173 potentiostat and a commercial nickel plating solution (Technic Techni Nickel S) at -1.0 V vs a saturated calomel electrode; a platinum counter-electrode was used. A bronze alloy (Cu41Sn11, the CuSn δ phase) was electrodeposited using a combination of two commercial plating baths (Technic Ready to Use Cu U2 and Technic Matte Sn 89) combined in a 6:1 Sn/Cu solution ratio. The deposition potential was alternated between -0.2 V and -0.9 V using a PAR model 175 universal programmer. At -0.2 V, only
Figure 1. SEM image of Ni-CuSn-Ni nanowires after treatment in NaOH, showing the oxidized center segments (rough texture) and the non-oxidized Ni ends (smooth). Branching at one end of each wire is a result of the porous alumina structure. The center bronze segment of this sample was grown to be 5 µm long.
Cu deposits; at -0.9 V, both Cu and Sn are deposited, but due to the low concentration of Cu in the solution, the deposit is primarily Sn. Pulsing the deposition potentials between -0.2 V for 60 ms and -0.9 V for 30 ms yielded nanowires of δ-CuSn: As the very thin layers of copper (4 nm) and tin (1 nm) are deposited, they interdiffuse to form the bronze alloy. This was confirmed using X-ray diffraction. To create segmented nanowires, sufficient nickel was initially electrodeposited to make a 3-µm-long segment. After rinsing the surface of the membrane with deionized water, the membrane was immersed in the combined Cu and Sn electrolyte solution and the stepped-potential sequence was applied to produce a 7-µm-long δ-CuSn segment. After a second rinse, Ni was again deposited to form a 3-µm-long Ni capping layer. These segmented nanowires will be referred to as Ni-CuSn-Ni nanowires. Following their electrosynthesis, the Ni-CuSn-Ni nanowires were removed from the template by first dissolving the silver layer with 6 M HNO3 and then dissolving the alumina membrane in 6 M NaOH. Repeated centrifuging, decanting of the supernatant, and rinsing with water removed excess NaOH. The liberated nanowires could then be suspended in different solvents. The segmented composition of the nanowires was confirmed using a number of methods. X-ray diffraction of the nanowires embedded in the alumina membrane at each stage of electrodeposition showed Ni, then δ-CuSn, then both δ-CuSn and Ni. After the third deposition step, the Ni peaks were more intense than the δ-CuSn peaks, as Ni was the topmost layer deposited in the alumina membrane. Individual nanowires were examined using energy-dispersive X-ray spectroscopy (EDS), which showed the composition varied from Ni to Cu/Sn to Ni along the length of the nanowires. In addition, leaving the nanowires in the NaOH solution for 1 h or more resulted in the oxidation of the center CuSn alloy segment but not the Ni ends. Evidence of the oxidation was obtained using scanning electron microscopy, as shown in Figure 1. Chemical evidence for the oxidation was found from X-ray powder diffraction patterns of the nanowires, which showed 488
both Cu2O and δ-CuSn in the rough-looking central segments. (X-ray diffraction patterns, EDS data, and an analysis of the size distribution of the Ni-CuSn-Ni nanowires are included in the Supporting Information.) Prior to releasing the wires from the alumina membrane, magnetization studies performed on a vibrating sample magnetometer at room temperature showed that the NiCuSn-Ni nanowires were ferromagnetic. To examine the responsiveness of the Ni-capped nanowires to a magnetic field after liberation from the alumina membrane, a suspension of the nanowires in ethylene glycol was dropped onto a glass surface and the nanowires’ movement within the solvent was monitored using a Nikon Eclipse ME600 optical microscope. The nanowires are 200 nm in diameter but appear larger because their diameter is smaller than the diffraction limit under the conditions used. Images of the magnified rods were acquired with a Diagnostic Instruments SpotRT camera. The alignment of the nanowires was controlled using magnetic fields applied using rectangular bar magnets. Field strengths ranged from 65 to 350 G. When a field was applied across the drop by placing attracting magnets on the left and right sides, the wires aligned horizontally, parallel to the magnetic field lines. Fields created by two repelling magnets caused the nanowires to adopt positions perpendicular to the horizontal alignment. Since the nanowires can be oriented in any direction by simply changing the applied magnetic field, a rotating field can be used to cause the wires to spin as “nano-stir bars.” This behavior is demonstrated in Figure 2. (Movies of the aligning and spinning nanowires are available in the Supporting Information.) A control sample of bronze alloy nanowires without nickel caps did not respond to applied magnetic fields. To fix the position of the nanowires for further study, nickel stripes were fabricated on a silicon substrate. First, a 100 nm-thick film of nickel was deposited on a silicon wafer using a sputter coater. Next, a striped photoresist pattern was applied to protect selected areas of nickel. Exposed nickel was etched using 6 M HCl and the photoresist was dissolved in acetone. The nickel stripes remaining on the Si wafer were 20 µm wide and were separated by gaps ranging from 10 µm to 40 µm. The nickel stripes were magnetized in a 350 G field and placed between two rectangular magnets oriented to apply a field of 65 G perpendicular to the long axis of the stripes. The nanowires were suspended in 1 mM aqueous 2-mercaptoethanesulfonic acid solution (approximately 105 nanowires/mL) by sonication for 10 s, and a drop of the suspension was immediately placed on the Ni stripe pattern and allowed to dry. The acid surfactant prevented nanowire agglomeration in the solution. As the water evaporated, the small assisting magnetic field helped to orient the nanowires horizontally across the stripes. An image of a Ni-CuSn-Ni nanowire aligned between two nickel stripes is shown in Figure 3. In a representative experiment, it was found that 66% of Ni-CuSn-Ni nanowires deposited in this manner aligned within 10 degrees of the horizontal, with almost all of those nanowires aligning in contact with a Ni stripe. The majority Nano Lett., Vol. 4, No. 3, 2004
Figure 2. Sequential optical microscope images of Ni-CuSn-Ni nanowires suspended in ethylene glycol as they are spun clockwise using a rotating magnetic field. The scale bar shown in frame (a) is 50 µm.
orientation can be controlled and the nanowires can be aligned between nickel contacts. This method may be of general use for positioning a wide variety of nanowires for mechanical testing and incorporation into nanoscale devices.
Figure 3. SEM image of a Ni-CuSn-Ni nanowire aligned between two nickel stripes (lighter color) on silicon (darker color). Table 1: Alignment Data for Ni-CuSn-Ni and CuSn Nanowires on Ni Stripes
Ni-CuSn-Ni nanowires with assisting horizontal field applied Ni-CuSn-Ni nanowires without an assisting horizontal field CuSn nanowires with assisting horizontal field applied
nanowires aligned within 10° of horizontal
aligned nanowires bridging a pair of Ni stripes
66% (68 of 103)
13% (9 of 68)
21% (26 of 123)
15% (4 of 26)
8% (12 of 153)
17% (2 of 12)
of the aligned nanowires touched the outer edge of a stripe or formed cantilevers that extended beyond a stripe. A smaller percentage bridged the gap between a pair of stripes. When no assisting field was applied, the Ni-CuSn-Ni nanowires still aligned at a statistically significant rate: 21% were aligned, with the majority of these found on or touching a Ni stripe. This is probably a result of magnetic fields generated by remnant magnetization of the Ni stripes. Nonmagnetic δ-CuSn nanowires deposited on the stripes with the assisting magnetic field demonstrated only 8% horizontal alignment, which was equivalent to the vertically aligned percentage. This control experiment verifies that δ-CuSn nanowires do not respond to magnetic fields of this strength. The Ni caps on each end thus substantially enhance the ability to position the segmented nanowires for further mechanical testing and incorporation into devices by using a magnetic field. The experiments conducted are summarized in Table 1. In summary, we have introduced a unique method of nanowire manipulation that allows the alignment of nonmagnetic nanowires by the addition of Ni ends. The nanowire Nano Lett., Vol. 4, No. 3, 2004
Acknowledgment. This work was supported by the Department of Energy (award #DE-FC36-01G011055), the Air Force Office of Scientific Research (award #F4962001-1-0146), and a NSF Graduate Research Fellowship (A.K.B.). We thank Scott Dhuey at the UW-Madison Center for Nanotechnology and Richard Noll of the UW-Madison Materials Science Center. George Antoun, Janice Eddington, Robert Hamers, Kevin Metz, Gordon Shaw, and Nick Smith are thanked for fruitful discussions. Supporting Information Available: X-ray diffraction patterns demonstrating the growth of segmented nanowires and confirming the oxidation of the center segments, EDS data showing the variation in composition along the length of a nanowire, a histogram of the size distribution of NiCuSn-Ni nanowires, and videos of the Ni-tipped nanowires aligning and rotating as controlled by magnetic fields. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353-389. (2) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317-1320. (3) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313-1317. (4) Huang, M. H.; Mao, S.; Feick, H.; Yah, H.; We, Y.; Kind, H.; Weber, E.; Russon, R.; Yang, P. Science 2001, 292, 1897-1899. (5) Favier, F. W.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227-2231. (6) Kouklin, N.; Menon, L.; Wong, A. Z.; Thompson, D. W.; Woollam, J. A.; Williams, P. F.; Bandyopadhyay, S. Appl. Phys. Lett. 2001, 79, 4423-4425. (7) Messer, B.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2000, 122, 10232-10233. (8) Lynch, M. D.; Patrick, D. L. Nano Lett. 2002, 2, 1197-2001. (9) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125-129. (10) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66-69. (11) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 13991401. (12) Tanase, M.; Silevitch, D. M.; Hultgren, A.; Bauer, L. A.; Searson, P. C.; Meyer, G. J.; Reich, D. H. J. Appl. Phys. 2002, 91, 85498551. (13) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075-1087. (14) Al-Mawlawi, D.; Liu, C. Z.; Moskovits, M. J. Mater. Res. 1994, 9, 1014-1018. 489
(15) Tierney, M. J.; Martin, C. R. J. Phys. Chem. 1989, 93, 2878-2880. (16) Routkevitch, D.; Bigioni, T.; Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037-14047. (17) Piraux, L.; George, J. M.; Despres, J. F.; Leroy, C.; Ferain, E.; Legras, R.; Ounadjela, K.; Fert, A. Appl. Phys. Lett. 1994, 65, 2484-2486. (18) Blondel, A. M.; Meier, J. P.; Doudin, B.; Ansermet, J.-Ph. J. Appl. Phys. 1994, 65, 3019-3021. (19) Dubois, S.; Michel, A.; Eymery, J. P.; Duvail, J. L.; Piraux, L. J. Mater. Res. 1999, 14, 665-671. (20) Yi, G.; Schwarzacher, W. Appl. Phys. Lett. 1999, 74, 1746-1748. (21) Cai, Z.; Martin, C. R. J. Am. Chem. Soc. 1989, 111, 4138-4139. (22) Yang, Z.; Niu, Z. Chem. Commun. 2002, 17, 1972-1973.
490
(23) Al-Mawlawi, D.; Coombs, N.; Moskovits, M. J. Appl. Phys. 1991, 70, 4421-4425. (24) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316-1319. (25) Cao, H.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. AdV. Mater. 2001, 13, 121-123. (26) Nicewarner-Pen˜a, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pena, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137-141. (27) Love, J. C.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696-12697.
NL035086J
Nano Lett., Vol. 4, No. 3, 2004
