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Wet Chemical Needlelike Assemblies of Single-Walled Carbon Nanotubes on a Silicon Surface Xing-Jiu Huang, Seong-Wan Ryu, Hyung-Soon Im, and Yang-Kyu Choi* Nano-Bio-Electronic Lab, Department of Electrical Engineering and Computer Science, Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, South Korea ReceiVed October 27, 2006. In Final Form: December 22, 2006 Single-walled carbon nanotubes (SWCNT) have been organized covalently to form a uniform needlelike structure on a silicon surface using a wet chemical assembly technique. In this work, we successfully combine silanization with the condensation reaction of the carboxylic group with the amino group.
Single-walled carbon nanotubes (SWCNTs) are of great interest because of their unique electronic1 and mechanical2 properties combined with their chemical stability. It has been demonstrated that carbon nanotubes with shortened length might provide connectors and components for molecular electronic devices.3 Research on the self-assembly of SWCNTs has also become a hot topic. However, most of the attention has been focused on metal substrates such as Au,4 Ag,5 and Pt.6 Furthermore, chemically assembled SWCNTs on gold surfaces has been widely used in nanosensors7 and nanodevices.8 On the basis of our previous work,9 we develop here a wet chemical assembly technique that perpendicularly aligns SWCNTs on a nonmetal substrate (i.e., a silicon surface) by combining silanization10 with the condensation reaction of the carboxylic group with the amino group. Figure 1 depicts the method of assembling the SWCNT monolayer on a silicon wafer. Silicon wafers were cleaned and hydroxylated in piranha solution (7:3 v/v 98% H2SO4/30% H2O2) under sonication at 50 °C for 1 h. After being rinsed with doubly deionized water (DDW) and blown dry in ultrapure nitrogen, the amino-terminated monolayer was obtained by placing freshly * To whom correspondence should be addressed. E-mail: ykchoi@ ee.kaist.ac.kr. Phone: 82-42-869-3477. Fax: 82-505-869-3477. (1) Tans, S. J.; Devoret, M. H.; Dai, H. J.; Thess, A.; Smalley, R. E.; Geerligs, L. J.; Dekker, C. Nature 1997, 386, 474. (2) (a) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (b) Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 324. (3) 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.; Rodriguez-Macias, F.; Shon, Y. S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (4) (a) Liu, Z. F.; Shen, Z. Y.; Zhu, T.; Hou, S. F.; Ying, L. Z. Langmuir 2000, 16, 8, 3569. (b) Diao, P.; Liu, Z. F.; Wu, B.; Nan, X. L.; Zhang J.; Wei, Z. ChemPhysChem 2002, 3, 898. (c) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2001, 123, 9451. (d) Nan, X. L.; Gu, Z. N.; Liu, Z. F. J. Colloid Interface Sci. 2002, 245, 311. (5) Wu, B.; Zhang, J.; Wei, Z.; Cai, S. M.; Liu, Z. F. J. Phys. Chem. B 2001, 105, 5075. (6) Rosario-Castro, B. I.; Conte´s, E. J.; Pe´rez-Davis, M. E.; Cabrera, C. R. ReV. AdV. Mater. Sci. 2005, 10, 381. (7) (a) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113. (b) Gooding, J. J.; Wibowo, R.; Liu, J. Q.; Yang, W. R.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006. (c) Yu, X.; Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F.; Rusling, J. F. Electrochem. Commun. 2003, 5, 408. (8) (a) Sheeney-Haj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78. (b) Lee, O. J.; Lee, K. H. Appl. Phys. Lett. 2003, 82, 3770. (9) (a) Huang, X. J.; Im, H. S.; Yarimaga, O.; Kim, J. H.; Jang, D. Y.; Lee, D. H.; Kim, H. S.; Choi, Y. K. J. Electroanal. Chem. 2006, 594, 27. (b) Huang, X. J.; Li, Y.; Im, H. S.; Yarimaga, O.; Kim, J. H.; Jang, D. Y.; Cho, S. O.; Cai, W. P.; Choi, Y. K. Nanotechnology 2006, 17, 2988. (10) (a) Song, S. Y.; Ren, S. L.; Wang, J. Q.; Yang, S. R.; Zhang, J. Y. Langmuir 2006, 22, 6010. (b) Siqueira Petri, D. F.; Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520. (c) Silberzan, P.; Le´ger, L.; Ausserre´, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (d) Han, Y.; Mayer, D.; Offenha¨usser, A.; Ingebrandt, S. Thin Solid Films 2006, 510, 175.
cleaned hydroxyl silicon wafers into a 1 wt % solution of 3-aminopropyltrimethoxysilane (Sigma-Aldrich) and APS (ammonium persulfate, Sigma-Aldrich) in toluene for 24 h at room temperature. The functionalized substrates then were washed sequentially with toluene, ethanol, and DDW to eliminate any possible physically absorbed impurities. SWCNTs (1.1 nm in diameter, 0.5-100 µm in length) were obtained from SigmaAldrich. The long SWCNTs were chemically shortened by oxidation in a mixture of concentrated sulfuric acid and nitric acid (3:1 v/v, 98%, and 70%, respectively) and were subjected to sonication for 8 h in a water bath at 50 °C. The morphology of the SWCNTs changed from that of highly entangled strands to flexible rodlike ones that includes two, three, or four individual tubes after etching (Figure S1 in Supporting Information). This procedure introduces carboxylic acid functionalities at the ends of the CNTs as well as some carboxylic acid units at the CNT sidewalls.3,4a,7a,9 It is worth pointing out here that the cutting time and temperature have a significant effect on the morphology of shortened tubes, especially on the tube length. An investigation of different cutting times and temperatures was conducted, as shown in Figures S2 and S3 in Supporting Information. We found that the tube length was shortened with increasing cutting time at a given temperature of 50 °C (Figure S2). After a cutting time of 3 h, amorphous carbon, metal catalyst particles, and other nanotubes in the raw sample can be removed; the tube length is about 5-10 µm, and the length can reach 0.5-1.5 µm 7 h later. Similar results can be found for different cutting temperatures (i.e., at the given time, a higher temperature is beneficial to producing shortened tubes). At 20 °C, the tube length is about 10 µm after cutting for 6 h, whereas the length is about 1-2 µm at 50 °C (Figure S3). The reaction mixture was then diluted with DDW and filtered through a 0.1-µm-diameter pore size PTFE (polytetrafluoroethylene, Carrigtwohill, Ireland) membrane under vacuum and then washed until neutral pH was achieved. The resulting SWCNT suspension was further stabilized by sonication for 30 min in DMF (dimethylformamide, Merck KGaA, Germany) as the surfactant. To fabricate assembled singlewalled carbon nanotubes on a silicon surface, the aminoterminated silicon wafer was placed into a carboxyl-terminated SWCNT/DMF suspension with the aid of a condensation agent, DCC (dicyclohexylcarbodiimide, Aldrich), for the given time at room temperature. To obtain information on surface characteristics such as the uniformity, roughness, grain distributions, and defect formation, we observed the surface morphology of the prepared films by using AFM (atomic force microscopy, Nanoscope, Digital Instruments, Veeco Metrology, LLC, CA). The tapping mode
10.1021/la063144l CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007
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Figure 1. Schematic diagram of the surface assembly chemistry and structure of single-walled carbon nanotubes on silicon.
AFM height images of bare silicon, H2SO4/H2O2 pretreated silicon, and the silanized silicon surface are shown in Figure S4 in Supporting Information. From Figure S4b, after the introduction of hydroxyl onto the silicon surface, the morphology is characterized by a little roughness (Rq ) 0.233 nm, Ra ) 0.186 nm) by comparing it with bare silicon (Rq ) 0.132 nm, Ra ) 0.105 nm, Figure S4a). Here, Ra is the mean roughness defined as the mean value of the surface height relative to the center plane, which is a calculated plane parallel to the mean plane such that the defined volumes above and below are equal11a 1
Ra ) 9 8 LL x y
∫0L ∫0L |f(x, y)| dx dy y
x
where f (x, y) is the surface relative to the center plane and Lx and Ly are the dimensions of the scan area. Rq represents the root-mean-square roughness of height deviations Zi taken from the mean data plane;11b it is defined by the following equation:
Rq )
x
N
N
Z2(xi, yj) ∑ ∑ i)1 j)1 N2
Z is the height deviation taken from the mean image data plan; N is the number of data points in one scan direction. Meanwhile, this kind of silicon wafer minimizes the water contact angle (CA) on the surface, indicating a hydrophilic surface with a large number of hydroxyl groups. The measurement shows that the CA of the surface decreased from 37 to 18°. As illustrated in Figure S4c, the image is markedly different from those that were taken for bare and H2SO4/H2O2-pretreated silicon surfaces. It can be seen that the silanized silicon surface is characterized by regular grains distributed on the surface (Rq ) 0.641 nm, Ra ) 0.527 nm), which might be caused by the introduction of 3-aminoproyltrimethoxysilane via the silanization reaction. Nevertheless, the silanized surface is still rather smooth on the micrometer scale, which is in agreement with previous reports.10 The fine structures of bare silicon, H2SO4/H2O2-pretreated silicon, and the silanized silicon surface can also be demonstrated by the cross-sectional analysis (Figure S5 in Supporting Information). To obtain more insight into the structure of the silanized silicon surface, the tapping mode AFM phase image was collected as shown in Figure 2a, corresponding to Figure S4c. Phase imaging (11) (a) Tian, F.; Wang, C.; Lin, Z.; Li, J. W.; Bai, C. L. Appl. Phys. A 1998, 66, S591. (b) Nanoscope Command Reference Manual for Software, version 5.12; Digital Instruments/Veeco Metrology Group, Inc., 2001.
Figure 2. Typical tapping mode AFM phase image of the silicon surface after silanization (a) and tapping mode height images (b and c) of single-walled carbon nanotubes covalently linked to an aminoterminated monolayer associated with the silicon wafer via a wet chemical approach at different assembly times: (b) 1 and (c) 6 h.
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Figure 3. XPS spectra of a silicon surface sequentially treated using H2SO4/H2O2 solution (green), silanization (red), and single-walled carbon nanotube assembly (black).
is a powerful extension of tapping mode AFM that provides nanometer-scale information about the surface structure including mapping of different components in composite materials.12 The resolution of phase imaging is comparable to the full resolution of tapping mode AFM. It provides a clearer observation of fine features, such as grain edges, that can be obscured by rough topography. From the phase image, we clearly observe uniformly distributed protrusions suggesting a significant difference in material properties at these spots as compared to those at other locations. This geometry is believed to be the result of the 3-aminoproyltrimethoxysilane-decorated silicon surface. The AFM images of the SWCNT-immobilized silicon surface for different time intervals are shown in Figure 2b,c. Obviously and most importantly, we observe that SWCNTs stand perpendicularly on the silicon surface and longer coupling times lead to higher surface coverage of the SWCNTs. As shown in Figure 2b, isolated needlelike protrusions are clearly seen on the silicon surface after 60 min of surface modification (Rq ) 2.365 nm, Ra ) 1.848 nm). The surface density increases gradually upon increasing the coupling time, hence a needlelike pattern of standing SWCNTs is obtained after 6 h of coupling (Rq ) 4.323 nm, Ra ) 3.520 nm, see Figure 2c). These observations are highly reproducible with different sites of the substrate surface and with different baths of assembly experiments. Just like SWCNT self-assembly on a gold surface,4 the immobilization of SWCNTs on the silicon surface exhibits good stability, indicating that the carboxylated SWCNTs have been successfully assembled on the silicon surface via covalent bonding. We could not find by AFM measurements SWCNTs that lie on the surface. This case can be explained by considering (i) that the number of carboxy groups at each end of the 1.1-nm-diameter SWCNT4a,7a may cause some amide bonds to be created between each nanotube and the amino-terminated silicon surface, thus leading to a preferred standing conformation of the SWCNTs on the surface, (12) (a) Chan, S. S. F.; Green, J. B. D. Langmuir 2006, 22, 6701. (b) Dong, R.; Yu, L. Y. EnViron. Sci. Technol. 2003, 37, 2813.
(ii) strong hydrophobic interactions between adjacent SWCNTs,4c and (iii) the length of shortened SWCNTs.4a,9a In our previous work, we found that some nanotubes that are longer than 250 nm are lying down on the gold surface during the needlelike SWCNT electrode fabrication process.9a It seems that the shorter tubes tend to stand on the surface but longer tubes lie down on the surface. This case can be supported by the length distributions (Figure S6 in Supporting Information) as measured directly from AFM images of the suspended carbon nanotubes for 1 and 6 h of assembly, respectively. From the Figure, it is found that the length of carbon nanotubes self-assembled on the silicon surface ranges from 0.5 to 23 nm. For 1 h of immobilization, the dominant length of nanotubes is less than 3 nm, whereas the length distribution obviously shifts to longer lengths with increasing immobilization time. It is possible that the self-assembly kinetics facilitate the assembly of shorter nanotubes on silicon from the colloidal suspension, which contains various lengths of nanotubes. However, it is also important to consider that SWCNTs are very flexible and some long tubes in suspension bend well and thus appear shorter in tapping mode AFM. In addition, XPS (X-ray electron spectroscopy, ESCA 2000) experiments for H2SO4/H2O2-pretreated silicon, silanized silicon, and SWCNT assembled silicon were carried out to confirm key information concerning the chemical state of the film surface. The wide-scan spectra and high-resolution O 1s, C 1s, and N 1s spectra at each modified stage are illustrated in Figure 3. From the survey scans, first we can observe the O 1s signal after H2SO4/ H2O2 pretreatment, which demonstrates that HO- groups have been successfully introduced onto the silicon surface (green line). After silanization and carbon nanotube immobilization, the O 1s intensity decreases gradually, demonstrating the result of 3-aminoproyltrimethoxysilane layer formation and the linking of a carbon nanotube with a silanization reagent by considering the detecting depth of XPS. Similarly, as the C 1s peak intensity increases gradually with silanization and carbon nanotube immobilization, it demonstrates that carbon chains were formed
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and carbon nanotubes were immobilized on the silicon surface. Furthermore, we can get additional information by observing the N 1s signals from the survey scan. The N 1s peak in the carbon nanotube-immobilized silicon sample is weaker than that in silanized silicon, indicating the immobilization of carbon nanotubes onto the silicon surface. Because the survey scans can provide only a qualitative understanding of the surface chemistry, high-resolution data for C 1s, O 1s, and N 1s are also collected, which is also presented in Figure 3. For O 1s spectra, HOgroups correspond to the peak component at 535.1 eV. After the formation of the Si-O bond, the peak shifts to 534.1 eV. However, according to the XPS features, we suggest that the peak is at about 533.9 eV as a result of the existence of carbonyl group. For C 1s spectra, the first peak at 287.6 eV comes from the silicon substrate, and the peak at 287.4 eV is due to the -CH2CH2- and C-NH2 groups after silanization, whereas the peak at about 286.4 eV might originate from the C atoms in the carbon nanotube. As can be seen from the N 1s spectra, the peak at about 402.3 eV shows the amino group, and the peak at 401.8 eV presents the amido group. The high-resolution Si 2p spectra at each modified step are also given in Supporting Information (Figure S7). In conclusion, perpendicularly aligned assemblies of shortened SWCNTs on a nonmetal substrate silicon surface have been demonstrated using a wet chemical organization process by
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combining silanization with the condensation reaction of the carboxylic group with the amino group. We believe that the full standing carbon nanotubes on the silicon surface will greatly improve the field-emission performances of electrons for flatpanel display applications.8a Acknowledgment. This work was supported by the Brain Korea 21 project, the School of Information Technology, Korea Advanced Institute of Science and Technology in 2006, and the National Research and Development Program (NRDP, 200501274) for biomedical function monitoring biosensor development sponsored by the Korea Ministry of Science and Technology (MOST). Supporting Information Available: The morphology of SWCNTs before and after chemical cutting and the morphology of shortened SWCNTs at different cutting times and temperatures. Typical tapping mode AFM height images of bare silicon, H2SO4/H2O2-pretreated silicon, and silanized silicon wafers. Typical cross section of bare silicon, H2SO4/H2O2-pretreated silicon, and silanizied silicon in AFM images. Histograms of the length distributions of SWCNTs assembled on a silicon surface at different assembly times. High-resolution Si 2p spectra at each assembly stage. This material is available free of charge via the Internet at http://pubs.acs.org. LA063144L
