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Notes Self-Alignment of Shortened Multiwall Carbon Nanotubes on Polyelectrolyte Layers Bumsu Kim and Wolfgang M. Sigmund* Department of Materials Science & Engineering, University of Florida, 225 Rhines Hall, P.O. Box 116400, Gainesville, Florida 32611 Received October 10, 2002. In Final Form: March 12, 2003
multilayers by the microcontact have been investigated as templates for colloid arrays.16 In this study, sMWCNTs were produced by chemical oxidation. These sMWCNTs self-aligned on the polycationic poly(diallyldimethylammonium chloride) (PDAC) layer adsorbed on a silicon wafer. The carboxylate anion groups of sMWCNTs were bound on the opposite charged PDAC layer. No attachment of sMWCNTs was detected on the poly(sodium 4-styrenesulfonate) (PSS) layer due to electric repulsions between them. The sMWCNTs were deposited on the PDAC/PSS/PDAC multiplayer. The aim of this study is to suggest that patterned sMWCNT arrays can be fabricated by interactions between sMWCNTs and polyelectrolytes.
Introduction Carbon nanotubes have been increasingly recognized as a potential material for future nanodevices since Iijima’s discovery1 due to their outstanding physical properties:2-5 electrical, optical, mechanical, and structural properties. It is essential to fabricate well-aligned structures of nanotubes for various electric and optical applications. Some research groups have demonstrated well-organized nanotube arrays on patterned catalyst printed substrates.6,7 Recently, Liu et al. have reported that shortened single wall carbon nanotubes (sSWCNTs) can be aligned on gold8 and silver9 substrates by selfassembly. To organize sSWCNTs on the gold substrate, the carboxylic acid groups at the open end of nanotubes were thiol-functionalized to use Au-S chemical bonds. The electrical adsorption between carboxylate groups at the ends of sSWCNTs and the silver substrate was used to immobilize sSWCNTs on the silver substrate. sSWCNTs were also anchored on an Fe ion functionalized surface.10 The layer by layer assembly11 is a convenient method to manipulate the patterned surface by the sequential adsorption of polycations and polyanions. The layer by layer assembly is a versatile method with a variety of charged materials.12-15 Recently, patterned polyelectrolyte * To whom correspondence should be addressed. Telephone: +1-352-846-3343. Fax: +1-352-392-7219. E-mail: wsigm@ mse.ufl.edu. (1) Iijima, S. Nature 1991, 354, 56. (2) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (3) Xie, S. S.; Chang, B. H.; Li, W. Z.; Pan, Z. W.; Sun, L. F.; Mao, J. M.; Chen, X. H.; Qian, L. X.; Zhou, W. Y. Adv. Mater. 1999, 11, 1135. (4) Dai, L.; Mau, A. W. H. Adv. Mater. 2001, 13, 899. (5) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. ChemPhysChem 2001, 2, 78. (6) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105. (7) Fan, S. S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (8) Liu, Z. F.; Shen, Z. Y.; Zhu, T.; Hou, S. F.; Ying, L. Z.; Shi, Z. J.; Gu, Z. N. Lagnmuir 2000, 16, 3569. (9) Wu, B.; Zhang, J.; Wei, J.; Cai, S. M.; Liu, Z. F. J. Phys. Chem. B 2001, 105, 5075. (10) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2001, 123, 9451. (11) Decher, G. Science 1997, 277, 1232. (12) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117.
Experimental Section Multiwall carbon nanotubes (MWCNTs) were synthesized at Clemson University by the arc-discharge method. Si(100) wafers were obtained from Montco Silicon Technologies Inc. All the following materials were obtained from Aldrich and used as received: poly(diallyldimethylammonium chloride) (PDAC, 20 wt % in water, molecular weight 400 000-500 000), poly(sodium 4-styrenesulfonate) (PSS, molecular weight 70 000), sodium chloride (NaCl, 99+%), hydrochloric acid (HCl, 36%), sulfuric acid (H2SO4, 98%), and nitric acid (HNO3, 70%). MWCNT raw soot was heated in the air at 600 °C for 2 h and then soaked in hydrochloric acid for 24 h and centrifuged. The precipitate was rinsed with the deionized water three times and dried under the nitrogen gas. MWCNTs were chemically shortened by sonification in a mixture of sulfuric acid and nitric acid (3:1) for 8 h. The resulting sMWCNTs were washed with the deionized water and separated by centrifuging three times. After being dried in a nitrogen stream, sMWCNTs were dispersed in deionized water. PDAC and PSS were dissolved in deionized water at a concentration of 2.0 mg/2 mL, containing 0.01 M NaCl for layer by layer assembly. A Si(100) wafer was diced into 0.6 × 0.6 cm2 pieces and cleaned by sonification in acetone for 30 min and then rinsed with methanol and the deionized water. The experimental procedure is given in Figure 1. Briefly, a silicon substrate was coated with the prepared PDAC solution with 10 min waiting time to adsorb the polycationic layer. sMWCNTs were immobilized on the substrate by immersing substrates into the sMWCNT solution for several hours. To obtain a multilayer, PDAC modified substrates were dipcoated into a PSS solution, also allowing 10 min for adsorption. The modified substrates were rinsed with the deionized water throughout. These steps were repeated till desired layers were obtained. Finally, the sMWCNT aligned substrate was rinsed with the deionized water and dried under nitrogen atmosphere. sMWCNTs were observed with the field-emission scanning electron microscope (FE-SEM, 6335F, JEOL) and atomic force (13) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H.; Mikhailov, A.; Mtchedlishvily, B.; Morgunova, E.; Vainshtein, B. Langmuir 1994, 10, 4232. (14) Keller, S. W.; Kim, H. N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (15) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (16) Zheng, H. P.; Lee, I. S.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 569.
10.1021/la026679x CCC: $25.00 © 2003 American Chemical Society Published on Web 04/19/2003
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Figure 1. Schematic reaction procedure and the molecular structures of the polyelectrolytes. microscopy (AFM, DIMENSION 3100, Digital Instrument) by tapping mode measurement with silicon cantilevers in the ambient condition.
Results and Discussion sMWCNTs were produced by the chemical oxidation method.8,17 These sMWCNTs are well dispersed in water, ethanol, and other solvents without surfactants. Like a shortened single wall carton nanotube (sSWCNT) suspension,8 a homogeneous sMWCNT suspension was stabilized without precipitation for 24 h. The carboxylic groups at the ends of sMWCNTs were verified by FT-IR, giving stretching bands of carboxylic groups at 1700 cm-1. Figure 2a shows the FE-SEM images of sMWCNTs. Inlet images are magnified sMWCNTs. Most of the sMWCNTs have 10-15 nm in diameter and 20-170 nm in length shown in the graph of Figure 2b. The length distribution of sMWCNTs was measured from sMWCNTs by FE-SEM picture analysis of five samples with three pictures each. A total of 600 sMWCNTs were evaluated. These dimensions correspond with the size of sMWCNTs observed by AFM. The relations between polyions and functionalized groups on the surface were researched by using chemical force microscopy.18 Carboxylic groups had a strong adhesion force with amine groups in deionized water (pH 6.5). This reaction was used to adsorb PDAC in synthesizing multilayer templates.16 Carboxylic groups at the open ends of sSWCNTs were bound on the silver substrate via (17) 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. (18) Jiang, X. P.; Ortiz, C.; Hammond, P. T. Langmuir 2002, 18, 1131.
Figure 2. FE-SEM images of sMWCNTs on carbon tape (a) and the length distribution of sMWCNTs (b).
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Figure 3. AFM images of a PDAC layer (a) and of organized sMWCNTs on the PDAC layer with height profile after 18 h immersion in sMWCNT solution (b).
Coulombic forces.9 Using these principles, the carboxylate anion groups of sMWCNTs absorbed on the oppositely charged polycationic PDAC by Coulombic attractions. The AFM images of the PDAC layers before (a) and after (b) organizing sMWCNTs are shown in Figure 3. Before the adsorption of sMWCNTs, the substrate shows a flat surface. After the alignment of sMWCNTs for 18 h, perpendicularly standing sMWCNTs are observed. The height of the sMWCNT layer ranges from 10 to 80 nm. A range of height rationalizes that the orientation of sMWCNTs is perpendicular to the surface layer. It is believed that the lateral elastic deformation of the nanotubes and tilt angles on the surface result in the difference between the length of sMWCNTs by FE-SEM and the height of sMWCNTs on the surface.9,19 The range of lateral dimensions observed by AFM varies from several tens of nanometers to 250 nm. These results are attributed to the aggregation of sMWCNTs due to van der Waals interaction between sMWCNTs and, furthermore, the limits of convolution of the AFM tip (19) Walters, D. A.; Ericson, L. M.; Casavant, M. J.; Liu, J.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Appl. Phys. Lett. 1999, 74, 3803.
and the lateral elastic deformation, as has been shown for sSWCNTs self-assembled on gold and silver substrates.8,9,19 Figure 4a shows the surface of the PDAC/SPS bilayer after reaction with sMWCNTs. No changes on the surface are detected by AFM. No sMWCNTs are self-assembled on the surface due to electric repulsion between the carboxylate anion groups of sMWCNTs and the same charged polyanionic SPS layer. A (PDAC/PSS)2/PDAC multilayer has a PDAC layer as outer surface. As shown in Figure 4b, sMWCNTs are well organized on the surface of the multilayer after 24 h immersion in sMWCNT solution. sMWCNTs on the (PDAC/PSS)2/PDAC have almost the same height and lateral dimension range as sMWCNTs on the PDAC layer. By using this method, we believe that the prepatterned multilayer structure with PDAC and PSS16 can be successfully used as a template for patterned nanotube arrays. In summary, sMWCNTs are prepared by the chemical oxidation method. sMWCNTs form the self-assembled layer on the polycationic PDAC layer by electrostatic attraction. A SPS layer prevents the sMWCNTs from arraying on the surface due to the electric repulsion. These
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Figure 4. AFM images of the PDAC/PSS layer after reaction with sMWCNTs (a) and of the (PDAC/PSS)2/PDAC multilayer after arraying sMWCNT with height profile (b) for 24 h.
results imply that the patterns of carbon nanotubes can be manipulated by patterned multilayer templates.16
00-1-0002 through the center for materials in sensors and actuators (MINSA).
Acknowledgment. This work was supported by DARPA/Army Research Office under Grant No.DAAD19-
LA026679X
