© Copyright 2000 American Chemical Society
APRIL 18, 2000 VOLUME 16, NUMBER 8
Letters Organizing Single-Walled Carbon Nanotubes on Gold Using a Wet Chemical Self-Assembling Technique Zhongfan Liu,* Ziyong Shen, Tao Zhu, Shifeng Hou, and Lizhen Ying Center for Nanoscale Science & Technology (CNST) and College of Chemistry & Molecular Engineering (CCME), Peking University, Beijing 100871, China
Zujin Shi and Zhennan Gu* Institute of Inorganic Chemistry, College of Chemistry & Molecular Engineering (CCME), Peking University, Beijing 100871, China Received October 26, 1999. In Final Form: February 8, 2000 We provide a wet chemical approach for organizing randomly tangled single-walled carbon nanotubes (SWCNTs) on gold surfaces. The as-grown nanotubes were first chemically cut into short pipes and thiolderivatized at the open ends. The ordered assembly of SWCNTs was made by their spontaneous chemical adsorption to gold via Au-S bonds. Tapping mode atomic force microscopy (AFM) images clearly show that the nanotubes have been organized on gold, forming a self-assembled monolayer structure with a perpendicular orientation. The adsorption kinetics of the nanotubes was very slow in comparison to conventional alkanethiols. The adsorption rate varied inversely with tube length. The nanotubes tend to form bundles as the adsorption propagates, following a “nucleation adsorption mechanism”. This work demonstrates that “giant” carbon nanotubes can be assembled on Au surfaces using wet chemistry similar to that exploited for “small” organic self-assembling species. We believe that assembled nanotube arrays will provide wide possibilities for applications.
Since their discovery in 1991,1 carbon nanotubes have attracted increasing attention because of their unique structural, mechanical, and electronic properties. Numerous novel applications of multiwalled and single-walled carbon nanotubes (MWCNTs and SWCNTs) have been investigated, including their use as field emitters,2,3 nanoelectronic devices,4-8 nanotube actuators,9 batteries,10 * To whom correspondence should be addressed. Z. Liu: tel & fax, 86-10-6275-7157; e-mail,
[email protected]. Z. Gu: tel, 86-10-62751495; fax, 86-10-62751708. (1) Iijima, S. Nature 1991, 354, 56. (2) De Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179. (3) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512. (4) Collins, P. G.; Zettl, A.; Bando, H.; Thess, A.; Smalley, R. E. Science 1997, 278, 100.
probe tips for scanning probe microscopy,11,12 nanotubereinforced materials,13 etc. For most of these applications, well-ordered arrays of nanotubes are highly desirable. (5) Frank, S.; Poncharal, P.; Wang, Z. L.; De Heer, W. A. Science 1998, 280, 1744. (6) Tans, S. J.; Verschueren, A. R. M.; Dekker: C. Nature 1998, 393, 49. (7) White, C. T.; Todorov, T. N. Nature 393, 240. (8) Menon, M.; Srivastava, D. Phys. Rev. Lett. 1997, 79, 4453. (9) Baughman, R. H.; Cui, C. X.; Zakhidov, A. A.; Lqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340. (10) Che, G. L.; Lakschmi, B. B.; Fisher, E. R.; Martin, C. R. Nature 1998, 393, 346. (11) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52. (12) Wong, S. S.; Woolley, A. T.; Joselevich, E.; Cheung, C. L.; Lieber, C. M. J. Am. Chem. Soc. 1998, 120, 8557. (13) Dresselhaus, M. S. Nature 1992, 358, 195.
10.1021/la9914110 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/24/2000
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Scheme 1 . (a) Scheme for the Thiolization Reaction of Carboxyl-Terminated Carbon Nanotubes with Cysteamine (NH2CH2CH2SH) and (b) Schematic Diagram of the Assembling Structure of Single-Walled Carbon Nanotubes on Gold
However conventional methods such as arc discharge,14,15 laser ablation,16 and catalytic chemical vapor deposition17 only produce carbon nanotubes in randomly tangled states, though several groups3,18-23 have recently demonstrated the preparation of aligned nanotubes on surfaces. It remains a great challenge to find an effective way to organize and/or to manipulate as-grown tangled nanotubes into well-ordered arrays. Important progress along these lines was recently achieved by Liu and co-workers.24 In this previous work, long SWCNT ropes were cut into short lengths of open-ended and chemically functionalizable tubes by oxidation in concentrated sulfuric and nitric acids. Herein, we report the first chemical assembly of singlewalled carbon nanotubes fabricated by the wet self(14) Journet, C.; Maser, W. K.; Bernier, P.; Loisear, A.; de la Chapelle, M. L.; Lefrant, S.; Deniard, P.; Lee, R.; Fischer, J. E. Nature 1997, 388, 756. (15) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (16) Thesis, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, H. Y.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (17) Dai, H. J.; Rinzler, A. G.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1996, 260, 471. (18) De Heer, W. A.; Bacsa, W. S.; Chatelain, A.; Gerfin, T.; HumphreyBaker, R.; Forro, L.; Ugarte, D. Science 1995, 268, 845. (19) Li, W. Z.; Xie, S. S.; Qian, L. X.; Chang, B. H.; Zou, B. S.; Zhou, W. Y.; Zhao, R. A.; Wang, G. Science 1996, 274, 1701. (20) Terrones, M.; Grobert, N.; Olivares, J.; Zhang, J. P.; Terrones, H.; Kordatos, K.; Hsu, W. K.; Hare, J. P.; Townsend, P. D.; Prassides, K.; Cheetham, A. K.; Kroto, H. W.; Walton, D. R. M. Nature 1997, 388, 52. (21) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (22) Kosunoki, M.; Shibata, J.; Rokkaku, M.; Hirayama, T. Jpn. J. Appl. Phys. 1998, 37, L605. (23) Li, J.; Moskovits, M.; Haslett, T. L. Chem. Mater. 1998, 10, 1963. (24) Liu, J.; Rinzler, A. G.; Dai, H. J.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; RodriguezMacias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253.
assembled monolayer technique.25 We will demonstrate the formation of thiol-functionalized SWCNT monolayers on gold surface via Au-S chemical bonding. The SWCNTs were produced by the dc arc discharge method using Y-Ni catalyst26 and purified following a procedure described previously.27 Most of the SWCNTs produced have diameters of 1.3-1.4 nm, as determined by high-resolution electron microscopy (HRTEM).28 Similar to Liu’s approach,24 the purified SWCNTs were further cut into short pipes by chemical oxidation in a mixture of concentrated sulfuric and nitric acids (3:1, 98% and 70%, respectively) under ultrasonication for 8 h. The reaction mixture was then diluted with water and allowed to stand overnight for precipitation. The supernatant was decanted, and the remains were diluted with deionized water and filtered with a 1 µm diameter pore poly(tetrafluoroethylene) (PTFE) membrane (Gelman) under vacuum. The solid shortened SWCNT sample was obtained by washing the remains on the PTFE filter with deionized water until the filtrate pH became nearly neutral. The SWCNT pipes obtained were found to form stable colloidal suspensions in water, ethanol, acetone, and dimethylformamide (DMF). Suspensions were prepared by ultrasonication without using surfactants, suggesting that relatively short nanotubes have been made as compared with Liu’s work.24 The characteristic stretching band (νCdO) of carboxylic groups at 1710 cm-1 in the FT-IR spectrum strongly (25) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (26) Shi, Z. J.; Lian, Y. F.; Zhou, X. H.; Gou, Z. N.; Zhang, Y. G.; Iijima, S.; Zhou, L. X.; Yue, K. T.; Zhang, S. L. Carbon 1999, 37, 1449. (27) Shi, Z. J.; Lian, Y. F.; Liao, F. H.; Zhou, X. H.; Gu, Z. N.; Zhang, Y. G.; Iijima, S. Solid State Commun. 1999, 112, 35. (28) Zhang, Y.; Iijima, S.; Shi, Z.; Gu, Z. N. Philos. Mag. Lett. 1999, 79, 473.
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suggests the formation of open-ended and carboxylterminated SWCNTs after oxidation treatment. For fabricating assembled monolayers on gold surface via Au-S chemical bonding, the carboxyl-terminated nanotubes were further thiol-derivatized by reacting with NH2(CH2)2-SH (TCI, Japan) in ethanol suspension with the aid of a condensation agent, DCC (dicyclohexylcarbodiimide, Aldrich, USA), for 24 h at room temperature(see Scheme 1a). The condensation between the carboxylic termini of the SWCNTs and the amino group of the thiols is evidenced by the appearance of an amide I band around 1600 cm-1 in the FT-IR spectrum. The self-assembled monolayer of thiol-functionalized nanotubes was prepared by dipping a Au(111) ball into the ethanol suspension for a given time, followed by ultrasonicating in absolute ethanol and drying in a stream of high-purity nitrogen. The single-crystal gold balls were prepared by crystallization at the end of 0.5 mm diameter gold wires in a hydrogen-oxygen flame according to the well-established method.29 Scheme 1b illustrates the expected structure of SWCNT monolayer assembly on a gold surface. We exploited tapping mode atomic force microscopy (AFM, Nanoscope IIIa, Digital Instruments) to characterize the formed structures of the thiol-derivatized carbon nanotubes on gold. Figure 1 shows the typical AFM images obtained, where (a) through (c) are corresponding to different assembling times. Before adsorption of nanotubes, the substrate shows a series of atomically flat triangle terraces, characteristic of Au(111) facets. After adsorption, needlelike protrusions are clearly seen on the gold surface, the density of which becomes gradually increased with increasing the adsorption times (see, panels a to c of Figure 1). These observations are highly reproducible with different sites of the substrate surface and with different batches of adsorption experiments. The adsorption seems likely chemisorbed, as evidenced by the stability of the films when ultrasonicated. From these observations, we believe that the thiol-derivatized singlewalled carbon nanotubes have been successfully immobilized on gold via Au-S chemical bonding, with the nanotubes being perpendicularly standing on the substrate surface (see Scheme 1b). The possibility of forming Au-S chemical bonds has also been demonstrated by Liu and his colleagues,24 who found that thiol-functionalized nanotubes can strongly adsorb gold nanoparticles at their ends. We could not find nanotubes which are flatly lying on gold surface throughout AFM measurements though both ends of the nanotubes are likely to be thiol derivatized. Multiple Au-S bonds likely form at the end of each surfaceadsorbed tube. The number of thiol groups at each end of a 1.3 nm diameter single-walled nanotube of (16,0) zigzag structure could be as many as 8. Therefore in the most stable adsorption state, 8 Au-S bonds can be created between each nanotube and gold surface. This multiple pinning makes it possible to create a stable self-assembled monolayer of the “giant” nanotube pipes on gold using the conventional wet chemistry approach. To obtain more information about the nanotube adsorbates and the adsorption kinetics, we analyzed the size distribution of adsorbed SWCNTs using AFM. Figure 2 shows the histograms of nanotube lengths (parts a and b) and diameters (parts c and d) at 3 and 24 h adsorption, respectively. We found that the length of carbon nanotubes self-assembled on gold falls into a range of 5-25 nm. For short time adsorption (3 h, Figure 2a), nanotubes with a length of less than 10 nm are dominant (>94.5%) while (29) Sawaguchi, T.; Yamada, T.; Okinaka, Y.; Itaya, K. J. Phys. Chem. 1995, 99, 14149.
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Figure 1. Typical AFM images of single-walled carbon nanotubes self-assembled on Au(111) surface via a wet chemistry approach at different assembling times: (a) 0.5 h; (b) 3 h; (c) 24 h.
for long time adsorption (24 h, Figure 2b), the length distribution remarkably shifts to greater lengths. In these films, nanotubes having a length of longer than 10 nm comprise 33.4% of the total. The SWCNTs here are significantly shorter than those cited in Liu’s report,24 where nanotubes prepared by similar oxidative cutting typically had lengths of hundreds of nanometers. One possible reason for the observed length differences is that the nanotubes synthesized in this work are truly shorter than those prepared by Liu et al., originating from possible
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Figure 2. Histograms of the length (a, 3 h; b, 24 h) and width (c, 3 h; d, 24 h) distributions of single-walled carbon nanotubes assembled on gold obtained from AFM images at different adsorption times.
differences in experimental parameters. The increased solubility of the nanotubes prepared here is strong evidence that they are much shorter. Again, no surfactant was needed to make a colloidal suspension as mentioned above. It is also possible that the adsorption kinetics facilitate assembly of shorter nanotubes on gold from the colloidal suspension, which contains various lengths of nanotubes. The adsorption of the “giant” carbon nanotubes was observed to be considerably slower than that for “small” alkanethiols. The latter typically needs only minutes to form a nearly complete monolayer on gold.30 It is reasonable to believe that the short tubes more rapidly adsorb to the surface than the longer ones. The time dependence of nanotube length observed in this study supports this explanation. At present, however, we do not know which possibility contributes more to the observed results. The diameter of the SWCNTs directly measured from AFM images falls in the range of 20-60 nm (see parts c and d of Figure 2), significantly larger than the result obtained from HRTEM images.28 Considering the extremely sharp geometry of the SWCNTs, the measured widths are likely broadened by convolution with the AFM tip.31 The AFM tip used in this work has a typical curvature (30) Zhao, J. W.; Yu, H. Z.; Wang, Y. Q.; Tang, M.; Cai, S. M.; Liu, Z. F. Acta Phys.-Chim. Sin. 1996, 12, 581. (31) Keller, D. Surf. Sci. 1991, 253, 353.
radius of ca. 10 nm. Deconvoluting the AFM data based on a simple geometric considerations, we can roughly estimate the true lateral dimensions of the SWCNTs. For the case of the smallest lateral size we observed from AFM images (20 nm), the deconvolution gives a true tube diameter of ca. 1-2 nm, comparable with the results of HRTEM studies. These small tubes comprised 10.7% of the population in surfaces prepared by the chemical assembly of nanotubes at 3 h adsorption (see Figure 2c) but decreased to 5.8% at 24 h adsorption (see Figure 2d). On the other hand, the largest nanotubes from AFM data (60 nm widths) give a deconvoluted diameter of ca. 40 nm. We believe, these wider features are due to aggregates of carbon nanotubes. As is clearly seen in parts c and d of Figure 2, the nanotubes tend to form bundles in the chemical assembly, and the bundle size increases with increasing adsorption time. As noted above, the larger nanotubes adsorb more slowly than smaller ones. Therefore, it is believed the bundles observed were likely assembled during the adsorption process. A model for the chemical assembling of nanotubes on gold may be as follows: At the initial stage, relatively short nanotubes are randomly adsorbed on gold surface; because of the strong hydrophobic interactions between nanotubes, the following adsorption would occur more easily nearby the preadsorbed nanotubes, forming bundles of nanotubes, where the preadsorbed nanotubes function as nucleation
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centers. With the increase of adsorption time, the bundling proceeds. Simultaneously, longer tubes also get adsorbed on surface, leading to the increase of bundle size and nanotube length, consistent with the experimental observation. We noted that the surface density of carbon nanotubes on gold is not high enough to form a complete monolayer though it increases by prolonging the adsorption time. We believe that the final surface coverage achieved may be reduced by competing adsorption of the coexisting NH2(CH2)2-SH molecules, which were not separated after thiol-functionalization reaction. We are now making efforts to purify the thiol-functionalized nanotubes for fabricating true self-assembled monolayers of singlewalled carbon nanotubes on gold. In summary, the present studies demonstrate that “giant” single-walled carbon nanotubes can be chemically assembled on surfaces using a similar wet chemistry as exploited for “small” organic self-assembling species. This kind of chemical manipulation has a great versatility and is not limited to the present system. One can design the terminal functionality of nanotubes and assemble them on various substrates via a predesigned bonding nature. We have succeeded in assembling carboxylic acid terminated SWCNTs on an amino-terminated silicon (111) surface via electrostatic interaction.32 Nanotube assemblies provide wide possibilities for nanotube applica(32) Unpublished results.
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tions. We believe that the full standing nanotubes on an electrode surface will greatly improve the field-emission performances of electrons for flat-panel display applications,2,3 increasing the current density, lowering the operation voltage, and miniaturizing the device dimensions. The chemical assembling technique also provides a more effective and convenient way to fabricate singlewalled nanotube probes for scanning probe microscopy studies.11,12 Well-controlled aligning of nanotubes is of particular importance for creating nanotube-based nanoelectronic and molecular electronic devices.4-8 Considering the accessibility of the inner cavities of nanotubes,33-35 the present study may also open new avenues for fabricating nanotube-based chemical sensors and for performing various nanochemistry involving a limited number of reactive species. Acknowledgment. Financial support from the Ministry of Science and Technology, The Ministry of Education, and the National Natural Science Foundation of China (NSFC) are gratefully acknowledged. LA9914110 (33) Sloan, J.; Hammer, J.; Zwiefka-Sibley, M.; Green, L. H. Chem. Commun. 1998, 347. (34) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (35) Ugarte, D.; Chatelain, A.; de Heer, W. A. Science 1996, 274, 1897.
