J. Phys. Chem. B 2004, 108, 10751-10753
10751
Random Networks of Single-Walled Carbon Nanotubes Zhenping Zhou, Lijie Ci, Li Song, Xiaoqin Yan, Dongfang Liu, Huajun Yuan, Yan Gao, Jianxiong Wang, Lifeng Liu, Weiya Zhou, Gang Wang, and Sishen Xie* Institute of Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: August 12, 2003; In Final Form: December 18, 2003
The self-assembled two-dimensional random networks composed of isolated SWNTs are obtained by a simple floating catalyst CVD method. A remarkable advantage of this technique is that the random networks can be formed at relatively low temperature with a wide density range, which is distinct from the previous reports. We also find that the deposited SWNTs constituting the networks are isolated rather than in bundles, and this result should be help for improving the preparation technique and exploring the formation mechanism of SWNTs.
Introduction During the past decade, carbon nanotubes, especially singlewalled carbon nanotubes (SWNTs), have attracted a great deal of attention due to their unique characteristics and many potential applications. And the electrical property of SWNTs (e.g., the high room-temperature mobility of semiconducting tubes1) may be one of the most intriguing and potential aspects, making nanotubes a promising class of materials for future functional devices and computing systems.2-5 In view of this, it inevitably involves fabricating building blocks of SWNTs and then integrating them into various practical devices. Up to now, two major strategies have been used to achieve the goal. The first is to spread SWNTs synthesized in advance in bulk quantity onto target substrates, followed by the fabrication of contact electrodes on them.4,5 However, the SWNTs produced in this way are popularly in the form of bundles, and the tough treatment applying the chemical or ultrasonic method to disperse them in solution is not very effective and generally induces some defects in the tubes. To overcome the limits, an alternative is to grow SWNTs directly from the catalyst particles prepatterned on target substrates.6-9 This method also provides a convenient route to connect the nanometer-scale SWNTs with the macroscopic electronic network.6 Unfortunately, this technique would inevitably involve the exposure of the fabricating process to high temperature, excluding a number of appropriate substrate materials. Therefore, the fabrication of carbon nanotube-based molecular electronics still remains a major challenge. Combination of the two strategies mentioned above may be a better solution. In this study, we developed a simple CVD method with which the preparation of isolated SWNTs and the assembly of them into the two-dimensional random networks could be achieved in one-step. A remarkable point for this approach is that it allows fabricating the SWNT networks onto the target substrates at quite low temperature (as low as 50 °C) and in a wide range of SWNT density. This point is very significant and distinct from the previous ones.6-9 Another attractive result is that the deposited SWNTs on Si substrates are isolated rather than in bundles, for which the detailed discussion will be presented in a later section. The random network architectures acquired this * Address correspondence to this author. E-mail:
[email protected].
Figure 1. Schematic diagram of the floating catalyst CVD apparatus for preparing random SWNT networks.
way are expected to have application in various fields, including electrical devices, sensors, membranes, templates, nanoelectromechanical systems, and biomimetic applications.8,9 Experimental Section Our CVD method for preparing SWNT networks was similar to the previous one.10 The difference is that the reactor employed in this study was a three-section quartz tube mounted in a dualfurnace system (Figure 1). It was found that the three-section reactor played a key role in fabricating the SWNT networks. Generally, ferrocene and sulfur (molar ratio 16:1), acting as the catalyst source, were mixed homogeneously and ground with mortar. Then the catalyst mixture was sublimed in the first furnace at a temperature of about 55 °C. A gas mixture of argon (300-2000 sccm) and acetylene (1 sccm) carried the sublimed catalyst through a narrow connection tube into the reaction zone in the second furnace (position B in Figure 1). The reaction temperature (position B) was 1100 °C and the system pressure was held at 1 atm. Under these circumstances, the temperature of region C-D could range between 50 and 300 °C. This process ultimately resulted in the formation of the SWNT networks on Si substrates that were horizontally preplaced in the outer-end section of the quartz tube reactor (position C-D). Results and Discussion A field-emission scanning electron microscope (FE-SEM) was used to observe the morphology of the as-prepared samples. A typical SEM image is shown in Figure 2. As is obvious in the picture, some line-like structures extend out from the white spots, intersecting each other and forming a continuous network. These lines vary from several to dozens of micrometers in length except for a few extending to more than 50 µm. These
10.1021/jp0363949 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/03/2004
10752 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Figure 2. Typical SEM image of the as-prepared network, which was deposited onto Si substrates at the mouth position of the second furnace (position C) for 15 min. Highly interconnected two-dimensional networks are clearly shown. The white spots are the catalyst particles.
Figure 3. (a) Typical SEM image of a nanotube-deposited grid sample prepared by collecting the nanotubes onto the copper grid during the preparation process. (b) TEM of the grid sample, with an individual SWNT shown.
morphology results revealed that some carbon nanotubes might have been synthesized and deposited onto the Si substrate. The reason these thin nanotubes could be observed by FE-SEM is that local potential differences between nanotubes and Si substrate lead to the image contrast.11 To further characterize the networks, these line-like structures have been collected onto carbon-coated copper grids during the deposition process. Figure 3a shows a typical SEM image of the “grid” sample. It can be seen that the lines span the grid hole and interconnect or intercross each other just as is examined on Si substrates. Some of the lines have slouched in the holes due to their large aspect ratio. This has brought in difficulty in focusing for highresolution transition electron microscopy characterization (HRTEM) as a result of mechanical and thermal vibration. To minimize this effect, plenty of these lines had to be deposited to support each other before the HRTEM measurement. The HRTEM characterizations on the thus-prepared samples (as typically shown in Figure 3b) confirmed that these line-like structures are SWNTs and the white spots are catalyst particles. The dark gray part in the upper picture is the amorphous carbon coat. The SWNT shown here is 1.7 nm in diameter. To further investigate the obtained nanotube networks, a tapping-mode atomic force microscope (TM-AFM) has been employed. Figure 4 demonstrates a typical TM-AFM image. The height measurements on these samples through the crosssection method indicated that most of the nanotubes (>95%) have diameters ranging from 0.7 to 3.0 nm, which is consistent with what is determined by HRTEM. Hence, we can come to the conclusion that most of these lines appearing on the Si
Zhou et al.
Figure 4. Topography image of the nanotube network recorded by a tapping-mode AFM. About 95% of the nanotubes, determined by their height measurement, are below 3.0 nm in diameter.
substrate are individual or isolated SWNTs, though some small SWNT bundles or ropes inevitably exist. This conclusion is meaningful and will be discussed in detail in a later section. Several important features about our SWNT networks should be emphasized here when compared with the previous reports.6-9 First, the technique described here provides a simple and effective way to yield nanotube networks (or isolated SWNTs) as building blocks for future electronic applications. The whole production process contains only one step so that it eliminates the troublesome process of pre-preparing the definite catalyst on the target substrate before the nanotube synthesis.6-9 Furthermore, the SWNTs synthesized with this method are nearly free of defects such as kinks and twisting, with only gradual bends. Second, a noticeable feature for this method is that the deposition position of the nanotube network (region C to D in Figure 1) is far away from their formation zone (region B), even away from the reaction furnace (position D). This actually avoids the exposure of the target substrates as well as the nanotube networks to high temperature. As a rule, the substrate bears temperature of 50-300 °C depending on the deposition position of the nanotube networks (from C to D in Figure 1). This is promising because the point might be explored to deposit the nanotube networks on the substrate susceptible to high temperature. For future various applications, different substrates may be required. Third, the density of the SWNT networks on Si substrate can be readily adjusted in a wide range. The network density actually is determined primarily by the deposition position (from C to D) and process duration under the optimized conditions. That is to say, more nanotubes would be deposited and thus the density of the nanotube networks would increase significantly with prolonging the production process or reducing the space between the deposition position and the mouth of the second furnace (position C). Provided that the process duration is 15 min, the network density on the Si substrate will vary from about 2 tubes per µm2 (Figure 2) at position C to nearly none at the outlet position D (Figure 1). Finally, it is extremely convenient to characterize the properties of the as-prepared nanotubes with HRTEM. The HRTEM sample consequently can be prepared only by simultaneously placing the copper grid in the deposition position during the preparation process as demonstrated previously. On the basis of the method described above, SWNTs were also deposited onto Au-coated Si substrates and their surfaceenhanced resonant Raman spectroscopy (SERRS) was measured. Figure 5 (spots 1-3) illustrates the typical SERRS results of the SWNTs deposited onto 10-nm Au-coated Si substrates. The 632.8-nm line of a He-Ne laser with 5 mW power was used as the excitation source, and the focused spot was 2 µm in diameter. In spot 4, we also display the normal resonant Raman spectroscopy (NRRS) for comparison. The different RBM
Random Networks of Single-Walled Carbon Nanotubes
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10753 within our experimental limit (300-2000 sccm). However, for the over-narrow tube (
