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2008, 112, 1735-1738 Published on Web 11/28/2007
Visualization of Horizontally-Aligned Single-Walled Carbon Nanotube Growth with Isotopes
13C/12C
Hiroki Ago,*,†,‡,§ Naoki Ishigami,‡ Naoki Yoshihara,‡ Kenta Imamoto,‡ Seiji Akita,| Ken-ichi Ikeda,‡ Masaharu Tsuji,†,‡ Tatsuya Ikuta,⊥ and Koji Takahashi⊥ Institute for Materials Chemistry and Engineering, Graduate School of Engineering Sciences, Kyushu UniVersity, Kasuga, Fukuoka 816-8580, PRESTO, Japan Science and Technology Agency (JST), Department of Physics and Electronics, Osaka Prefecture UniVersity, Sakai, Osaka 599-8531, and Graduate School of Engineering, Kyushu UniVersity, Motooka, Fukuoka 819-0395, Japan ReceiVed: October 5, 2007; In Final Form: October 25, 2007
Horizontally aligned growth of single-walled carbon nanotubes (SWNTs) on single-crystal surfaces has attracted great interest in terms of nanoelectronic applications, but their growth mechanism is not fully understood. We report on the 13C/12C isotope-labeled growth of SWNTs on a sapphire surface to visualize their growth process. Switching carbon feedstock from 13CH4 to 12CH4 during SWNT growth induces a gradient distribution of the carbon isotopes along the tube axis. From the Raman mapping analysis, we succeeded to observe the gradual change in the isotope distribution of individual SWNTs. The results indicate the base-growth mode for the horizontally aligned SWNTs, which suggests that nanotube-sapphire interaction is essential to alignment. This method offers a unique technique to analyze the nanotube growth mechanism and kinetics.
Single-walled carbon nanotubes (SWNTs) have a unique onedimensional nanostructure and excellent electron transport properties, which make them attractive building blocks for future nanoelectronics.1 In-plane direction-controlled synthesis of SWNTs is important for efficient large-scale device integration.2-7 The nanotube alignment realized on single-crystal substrate surfaces, on which SWNTs grow either along step edges (steptemplated growth)4,5 or an anisotropic surface atomic arrangement (lattice-oriented growth),6-8 has opened up the possibility of assembling high-density nanotubes on large substrates. A combination of these two alignment mechanisms has the potential to achieve complex SWNT architectures through surface engineering.9 In the lattice-oriented growth, the SWNTs grow parallel to a specific crystalline direction with two possible interactions, nanotube-substrate and catalyst-substrate interactions, to guide the growth. These interactions are associated with the position of the metal catalyst during nanotube growth. The nanotube growth modes are labeled according to whether the catalyst is located at the base (base-growth) or tip (tip-growth) of the nanotube during its growth. These growth modes can be also essential for further control of the SWNT structure. In this letter, we utilized the carbon isotopes, 13C and 12C, to spatially label an individual SWNT. Switching the carbon feedstock from 13CH4 to 12CH4 during nanotube growth induces a gradual change in isotope distribution along the tube axis. The replacement of 12C-SWNT with the 13C isotope causes a * To whom corresponding should be addressed. E-mail: ago@ cm.kyushu-u.ac.jp. † Institute for Materials Chemistry and Engineering, Kyushu University. ‡ Graduate School of Engineering Sciences, Kyushu University. § PREST-JST. | Osaka Prefecture University. ⊥ Graduate School of Engineering, Kyushu University.
10.1021/jp709737q CCC: $40.75
downshift in Raman spectrum because of the modulation of phonon modes due to the heavier carbon atoms, as follows10
ν(13C) ) x12/13ν(12C)
(1)
where ν(13C) and ν(12C) are the Raman shifts of the nanotubes composed of pure 13C and pure 12C, respectively. For example, pure 13C-SWNT gives a G band at 1530 cm-1 because the G band of 12C-SWNT is at 1592 cm-1. Thus, the spatial Raman spectrum mapping is expected to reveal the distribution of the carbon isotopes. The 13C-labeled growth was applied to vertically aligned multiwalled carbon nanotubes (MWNTs),11,12 but the visualization of individual nanotubes has not been achieved. These previous papers measured the ensemble of MWNTs with certain intervals along their axis.11,12 More recently, by analyzing the bulk sample and using 13C isotopes, the termination of SWNT growth followed by amorphous carbon formation was observed.13 Figure 1a shows the timing of the introduction of carbon feedstock for chemical vapor deposition (CVD) growth of SWNTs. Initially, 13CH4 gas (13C ratio: 99%, Tokyo Gas Chemicals, Japan) was introduced for 1 min into a quartz tube heated at 900 °C, followed by a gradual switch to 12CH4 gas in 30 s, and then the reaction was continued for another 1 min. When a SWNT grows based on the base-growth mode, the initially formed 13C-rich nanotube part is pushed from the catalyst particle and it is gradually replaced with 12C-rich nanotube part (Figure 1b). This creates a SWNT that is 13C-rich at the tip and 12C-rich at the base, that is, near the catalyst pattern. The opposite tendency is expected for the tip-growth mode. The catalysts, Fe-Mo salts, and Fe nanoparticles were patterned on R-plane sapphire substrates using electron-beam lithography. Figure 1c and d shows scanning electron micro© 2008 American Chemical Society
1736 J. Phys. Chem. C, Vol. 112, No. 6, 2008
Letters
Figure 1. Timing of the carbon feedstock supply for 13C/12C-labeled SWNT growth (a) and the resulting nanotubes expected for the base- and tip-growth modes (b). (c and d) SEM images of the aligned SWNTs grown over the patterned catalyst on R-plane sapphire.
scope (SEM) images of SWNTs grown over a 10-µm-wide catalyst line. A much higher degree of SWNT alignment was observed in SWNTs grown on the patterned substrate compared with those grown on a substrates with randomly dispersed catalyst6,8 because a clean sapphire surface is maintained in the unpatterned area. This result is consistent with a previous report on single-crystal quartz.14 The catalyst patterning also improved the nanotube length; the mean SWNT length exceeded 40 µm. This long and highly aligned SWNT array was suitable for the Raman mapping analysis. The resonant Raman spectrum was collected with an excitation wavelength of 514.5 nm and a spot size of ca. 1 µm. The sample stage was moved with a controller for the mapping measurement. Figure 2 shows the spatial distribution of the G-band peaks. Four SWNTs with lengths longer than 10 µm were imaged. We think that these four SWNTs are individual nanotubes because the G-band images are well apart from each other (full scale of the y axis of Figure 2 is 9.5 µm) and SWNTs are highly aligned so that they hardly formed a bundle (see Figure 1d). A gradual change in the 13C/12C isotope ratio was observed for three SWNTs, labeled A, B, and D, while nearly 13C-rich SWNT was also observed for one SWNT, labeled C. This, especially nanotube C, suggests that the SWNTs grew mainly in the period when the gases switched from 13CH4 to 12CH or just after this change. The growth rate estimated from 4 the Raman spectra and SEM images is ca. 0.5-3 µm/s, which is comparable to that of super-long SWNTs grown by a lamellar flow (5 µm/s).15 Nanotubes A, B, and D showed a similar tendency; 13C concentration is higher at the tip and lower at the base. In particular, nanotube B showed an intense G band at 1550 cm-1 at its tip, shifting to 1580-1590 cm-1 toward its base. This tendency can be explained by the base-growth mode, as illustrated in Figure 1b. It is also suggested that nanotube B grew even after changing the CH4 gases. Other mapping images are displayed in the Supporting Information (S-1). The present reaction condition, however, may suffer from the turbulence that occurs in the quartz tube. The gradual change of the gas composition from 13CH4 to 12CH4 might be disturbed because of heat flux. To exclude this possibility, we modified the timing of the gas supply, as shown in Figure 3a. In this condition, the G band appearing near 1530 cm-1 originates in nanotubes grown in the initial 1.5 min because it cannot be obtained in the last 1 min when the two gases are mixed. The
Figure 2. Spatial distribution of Raman G-band of the SWNTs grown by switching 13CH4 to 12CH4 during their growth. The SWNTs grew from the catalyst pattern (left to right).
Raman mapping image of the G-band peak is shown in Figure 3b. We observed a clear change in the carbon isotope distribution in an individual SWNT with a length of 68 µm, which also suggests the base-growth mode. The same tendency was observed for the radial breathing mode (RBM) peak as indicated in Figure 3c, but with a slight shift, ∼5 cm-1, resulting from the low frequency (see eq 1).
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J. Phys. Chem. C, Vol. 112, No. 6, 2008 1737
Figure 4. SEM image of SWNTs grown on the patterned SiO2/sapphire substrate, and the schematic illustration of the SWNT. The different SEM contrast is due to the difference in the conductivity of the SiO2 and the sapphire substrate.
Figure 3. Timing of the gas supply (a) and the Raman mapping images of G-band (b) and RBM (c) peaks. (d) G-band spectra measured at the positions shown in b.
Figure 3d shows Raman G-band spectra at the positions marked in Figure 3b. A continuous shift of the G band from 1533 to 1573 cm-1 was observed from the tip to the base, indicating that the successive G-band peaks come from one SWNT and that the C isotope changes gradually along its long axis. We found that the G band at the middle is broader and weaker than that at the tip; that is, a nearly pure 13C-SWNT at the tip gives a narrower G band than mixed 13C-12C in the middle (see the Supporting Information, S-3). This broadening is accounted for by the inhomogeneous broadening of the G band, resulting from the microscopic random distribution of 13C/12C isotopes in the laser spot. We think that this is the first example of synthesis of an isotopically modified SWNT whose thermal and electrical conducting properties are interesting. The base-growth mode was mainly observed under the present CVD condition. A few SWNTs that might be assigned to the tip-growth mode were also observed, but the change in isotope distribution along their axis was not so clear, partly because these SWNTs grew in a very short period of time. The basegrowth mode has been widely observed for the SWNTs grown over metal-supported catalysts16 and SiO2/Si flat substrates.17,18
These studies used a transmission electron microscope (TEM) or an atomic force microscope (AFM) to observe the catalyst directly, but it is difficult to apply these methods to the present SWNT array. This is because TEM needs a very thin film and AFM height profile sometimes cannot resolve catalyst particles on sapphire. To confirm that the horizontally aligned SWNTs grew based on the base-growth mode, we studied the SWNT growth on a sapphire substrate with a SiO2 stripe pattern. We deposited a SiO2 stripe pattern on a R-plane sapphire surface and placed the Fe-Mo catalyst on this SiO2 pattern using poly(dimethylsiloxane) (PDMS), followed by CVD. As shown in Figure 4, we found that SWNTs grew randomly on SiO2 toward the sapphire surface because deposited SiO2 is amorphous. On the sapphire surface, SWNTs were aligned parallel to the [11h01h] direction and climbed up the neighboring SiO2 surface. The Supporting Information (S-5) shows that the SWNTs are not suspended between the SiO2 stripes. Because SWNTs are known to grow with the base-growth mode on SiO2, the present result demonstrates that the base-growth mode can provide aligned SWNTs, thus suggesting that nanotube-sapphire interaction contributes mainly to the aligned growth. In the base-growth model, the continuous growth of SWNTs requires a movement of initially grown SWNTs on the substrate surface, which seems to be difficult under such a strong SWNT-sapphire interaction, especially for long SWNTs. Therefore, it is surprising that the base growth resulted in long SWNTs with lengths more than 50 µm (see Figure 3). The van der Waals force between a SWNT and a substrate is expressed by the following equation19
U)-
ALxR 12x2D3/2
(2)
where A is the Hamaker constant, R and L are the diameter and length of a SWNT, and D is the distance between the SWNT and substrate. The sliding force that originates in the van der Waals force is given by
F)-
dU AxR ) dL 12x2D3/2
(3)
which indicates that the sliding force is independent of L. The sliding force is constant regardless of the nanotube length, once
1738 J. Phys. Chem. C, Vol. 112, No. 6, 2008 it starts to grow. This may explain why long SWNTs can grow with the base-growth mode. Because we used sapphire substrates with smooth surfaces and an inclined angle less than 0.3°,9 van der Waals force is considered to be essential for the latticeoriented growth of SWNTs. Actually, atomic steps were clearly observed for the R-plane after the CVD, suggesting that the sapphire surface has an atomically smooth surface.6 In summary, we visualized the growth process of horizontally aligned SWNTs on a sapphire R-plane by using 13C/12C isotopelabeled methane feedstock. The change in the carbon isotope shifted the G band and RBM peak positions and their spatial distributions revealed the growth mode and growth period of aligned SWNTs. Mainly, the base-growth mode is proposed for the present horizontally aligned SWNT array on sapphire. The growth of long SWNTs can be explained by van der Waals interaction between a nanotube and a substrate. The present 13C/12C isotope-labeling method in combination with the spatial Raman mapping technique provides a powerful tool for analyzing the nanotube growth mechanisms, such as re-growth,20 and intramolecular junction formation in a nanotube.15 Acknowledgment. This work is supported by a Grant-inAid for Scientific Research from the MEXT (no. 16710087), the Industrial Technology Research Program from NEDO, CREST-JST, and a joint project of the Chemical Synthesis Core Research Institutions.
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Supporting Information Available: Experimental details, additional Raman mapping results, and SEM images. This material is available free of charge via the Internet at http:// pubs.acs.org.
(18) He, M.; Duan, X.; Wang, X.; Zhang, J.; Liu, Z.; Robinson, C. J. Phys. Chem. B 2004, 108, 12665.
References and Notes
(19) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1991.
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(17) Ishida, M.; Hongo, H.; Nihey, F.; Ochiai, Y. Jpn. J. Appl. Phys. 2004, 43, L1356.
