Electron Microscopy Investigation at the Initial Growth Stage of Carbon Nanotubes

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1 Journal of the Korean Physical Society, Vol. 42, February 2003, pp. S727 S731 Electron Microscopy Investigation at the Initial Growth Stage of Carbon Nanotubes Sung-Jin Eum, Hee-Kwang Kang, Cheol-Woong Yang and Ji-Beom Yoo School of Metallurgical and Materials Eng. and Center for Nanotubes and Nanostructured Composities, Sungkyunkwan University, Suwon Hee-Jin Jung and Chong-Yun Park Department of Physics and Center for Nanotubes and Nanostructured Composities, Sungkyunkwan University, Suwon The growth mechanism and the geometry of carbon nanotubes (CNTs) are affected by several parameters such as formation process and catalytic materials. In order to understand the growth mechanism of CNT, it is essential to observe the CNTs at the initial growth stage. We synthesized CNTs directly on the Ni-deposited TEM Cu mesh grid using thermal CVD so as to preserve and investigate the original structure of CNTs from the initial to final growth state without any artifacts that might be introduced during TEM specimen preparation. The results obtained from the electron microscopy study show that the following growth modes of CNTs occur simultaneously at the initial stage: tip-growth, base-growth and intermediate-growth mode. The intermediate growth structure can be achieved by a mixture of base-growth mode and tip-growth mode, which occur sequentially. This is thought to be a probable mechanism for nanotube junctions. PACS numbers: c Keywords: Carbon nanotube(cnt), growth Mechanism, TEM I. INTRODUCTION There have been intensive research activities in the area of carbon nanotubes (CNTs) since their discovery in 1991 [1] not only because of their interesting structural features and novel properties, but also because of their potential technological applications. Synthesis of CNTs using various methods has also been intensively studied by many research groups [2 5]. Among them, Thermal CVD (chemical vapor deposition) is the most dominant way to produce CNTs with high purity, high yield and vertical alignment. In thermal CVD, the growth mechanism and the geometry of CNTs are affected by several parameters such as formation process and catalytic materials. There have been introduced two models for growth mechanism of CNTs using CVD, that is: tipgrowth model and base-growth model. These models are explained by the trace of catalyst particle used for triggering reaction to grow CNTs [6 11]. In the tip-growth model, the catalyst particle is located at the nanotube tip and has been carried away from the support surface, where the catalyst particle had been originally located, during the growth process. In the base-growth model, in contrast, the catalyst particle remains in contact with the support surface during the growth process and the CNT grows on the top of the catalyst particle. Baker [12] cwyang@skku.ac.kr -S727- has suggested that the strength of the metal-support interaction can play an important role in modifying the growth characteristics. Despite intensive studies on CNT, however, the growth mechanism has still not been completely understood. The lack of understanding of the growth mechanism arises mainly from an absence of experimental evidence. In order to understand the growth mechanism of CNTs, therefore, it is essential to observe CNTs at the initial growth stage. Due to the small size of the tubes and the relatively small quantities of material available, highresolution electron microscopy is the major technique to study growth mechanism of nanotubes. Especially, transmission electron microscopy (TEM) is the most appropriate technique to investigate CNTs because it provides us clear information of crystallography as well as morphology. For the TEM analysis, in general, a specimen is prepared by dispersing CNTs on a holey carbon TEM microgrid after separating from the substrate by ultrasonic treatment in acetone [13]. Although this conventional specimen preparation method is good enough to observe the fully developed structure of CNTs, it is not suitable for clarifying the growth characteristics of CNTs because it may alter the intrinsic nature of CNT growth. In this work, we present a new and simple technique to observe CNTs without any artifacts that might be introduced during TEM specimen preparation. We synthesized CNTs directly on a TEM grid (300 mesh) so

2 -S728- Journal of the Korean Physical Society, Vol. 42, February 2003 that we can preserve and investigate the original structure of CNTs from the initial to final growth state without any artifacts. This method has provided us with a unique insight into the growth characteristics of CNT. II. EXPERIMENTAL Using a TEM Cu mesh grid (300 mesh) as a substrate, we synthesized CNTs in the form of a specimen ready for TEM observation without any further preparation. A transition metal (Ni) layer with thickness of 10 nm was thermally evaporated on a TEM Cu mesh grid in a vacuum of 10 6 torr. Then we pretreated Ni-deposited Cu mesh grid by PIPS (Precision Ion Polishing system, Gatan) to thin down the central part of the grid. This pretreatment is essential not only to remove the catalytic Ni layer deposited on the top surface of the grid so that we can grow CNTs on the side wall of the TEM grid selectively, but also to obtain a thin enough region to observe the morphology clearly at the initial stage of CNT growth by the TEM. The CNTs were synthesized by thermal CVD using acetylene(c 2 H 2 )gas (200 sccm) under a chamber pressure of 5.5 torr. Fig. 1 shows the schematic representation of the thermal CVD for production of CNTs on the TEM Cu mesh grid. A slice of Si wafer was placed on the mesh grid for preventing the thin mesh grid from deformation during CNT growth. Ambient Ar (1000 sccm) was used to prevent oxidation of the metal as temperature increases. The average heating rate was fixed at 45 C min 1. The catalytic Ni layer was pretreated using NH 3 gas with a flow rate of sccm for 5 15 min. We synthesized CNTs at various growth temperatures (ranging from 700 C to 800 C) and growth times (10 sec, 30 sec, 3 min, 5 min, 20 min). The scanning electron microscopy (SEM) and TEM images were obtained to investigate the growth characteristics of CNTs using Philips XL30-FEG ESEM and JEOL JEM-3011 TEM (with LaB 6 gun and operated at 300 kv), respectively. Fig. 1. Schematic diagram of Thermal CVD system for production of CNTs on a TEM Cu mesh grid. A slice of Si wafer was placed on the mesh grid for preventing deformation during CNT growth. bombardment during PIPS treatment could attack some of the catalyst deposited on the side-wall of the TEM grid as well as all the catalyst on top-surface. III. RESULTS AND DISCUSSION It is essential to survey the samples using the SEM prior to detailed structural analysis using the TEM in order to get the general information on the morphology. Fig. 2 is SEM micrographs of CNTs grown on a side-wall of a Ni deposited TEM grid at 700 C for 30 sec showing the effect of NH 3 treatment. It is noted that the CNTs are grown only on the side-wall of the TEM mesh grid as intended in the experiment. There was no CNT grown on the top-surface of the TEM mesh grid, on which Ni catalytic layer had been removed by PIPS treatment. The CNTs were not grown, however, on the whole part of the side-wall uniformly. It is thought that ion beam Fig. 2. SEM micrographs of CNTs grown on a side wall of the Ni deposited TEM grid at 700 C for 30 sec, showing the effect of NH 3 treatment. (a) NH 3 treatment for 5 min. (b) NH 3 treatment for 10 min. (c) NH 3 treatment for 15 min.

3 Electron Microscopy Investigation at the Initial Growth Stage Sung-Jin Eum et al. -S729- As shown in Fig. 2, the diameter of CNTs formed is decreased as the NH 3 treatment time increases. Fig. 2(a) shows samples that the catalytic Ni layer was pretreated using NH 3 gas with a flow rate of 30 sccm for 5 min. The diameter of Ni particles is mostly in the range from nm and the CNT was not formed. As the flow time of the NH 3 increases to min. (Fig. 2(b) and (c)), we can clearly observe the CNTs formed. Longer NH 3 pretreatment reduces the size of metal particles and increases their density. It is well known that the diameters of the outer dimension of the nanotubes are in the same range as the Ni particles, as seen in Fig. 2(b) and (c). The overall effect of the NH 3 treatment, producing finer metal particles therefore thinner CNTs, is consistent with the result of the previous study [14]. It is the initial growth state of nanotubes that we are most interested in to understand the growth mechanism of CNTs. Therefore, we observed the CNTs grown for 10 sec, 30 sec, and 5 min by using the TEM (Fig. 3). All the samples were prepared at the fixed temperature of 700 C with the 10 min-nh 3 pretreatment. As seen in Fig. 3(a), which may represent the initial formation step of CNT growth (10 sec), the catalytic Ni particles are surrounded by more likely amorphous carbon layer. During this initial stage, the C 2 H 2 molecules are presumably decomposed, bringing more reactive hydrocarbons in the catalytic particles. Therefore, the catalytic particle with nanometer size is encapsulated by an amorphous carbon layer. There has been a considerable debate over the chemical nature of the active catalyst; whether it is carbide or the metallic state. However, it is a widespread agreement that the reaction is triggered by an unstable surface carbide which subsequently decomposes into tubular carbon and metal [12]. With increasing time, the nanotubes have a unique structure. The CNTs have metal particles associated with them. Most of the tips of CNTs observed are decorated with the catalytic particles, but a few particles of catalytic metals stay in the middle of the nanotube (see Fig. 3(c)). It has been reported that metallic particles showed quasi-liquid behavior and fragments of Ni particles could be produced from the Ni tips during the CNT growth [15]. The diameter of CNTs grown is smaller than 20 nm. The CNTs grown for 5 min are not straight but crooked, and show well developed graphitic sheets with a high crystallinity. In terms of a trace of Ni particle, the three representative types of CNT growth observed at the initial stage are shown in Fig. 4: tip-growth mode in which metal particle is located at the tip of the CNT, intermediate-growth mode in which metal particle is located in the middle of the CNT, and base-growth mode in which metal particle is located at the bottom of the CNT and is in contact with the substrate. The CNTs were grown at 700 C for 30 sec with C 2 H 2 gas flow rate of 200 sccm. It is also found that the CNTs in the tipgrowth mode become dominant as growth time increases up to 20 min, although the CNTs start to grow in the various forms as described above at the initial growth Fig. 3. TEM micrographs of CNTs grown on the Nideposited TEM Cu mesh-grid at 700 C for various growth duration; (a) 10 sec, (b) 30 sec and (c) 5 min. All the grids have been NH 3 pretreated for 10 min. stage. In the tip-growth mode, carbon that is supplied by decomposition of hydrocarbon diffuses through the catalyst particle by surface and/or bulk diffusion and pushes it away from the substrate, forming graphitic walls of CNT behind. In the base-growth mode, in contrast, the catalyst particle remains in contact with the substrate due to the strong interaction between the catalyst and substrate. Carbon diffuses via surface and/or bulk of the metal particles, forming graphitic layers as a cap on the particle [10]. As the cap lift off, a sufficient supply of carbon atoms from the hydrocarbon source allows CNT to keep growing. One of the most intriguing observations in this study is the intermediate-growth mode which is neither purely tip-growth nor base-growth. There are several probable explanations we could suggest for this intermediategrowth mode shown in Fig. 4(b). First, it can be explained by the process similar to reversible reaction of growth and gasification of the carbon filaments associated with Ni particles in the presence of acetylene and hydrogen as proposed by Baker [12]. Once a CNT forms by the tip-growth mode, carbon diffusion could occur in the opposite direction and the deposited carbon is

4 -S730- Journal of the Korean Physical Society, Vol. 42, February 2003 Fig. 4. TEM images showing three representative types of CNT growth; (a) tip-growth mode, (b) intermediate-growth mode and (c) base-growth mode. CNTs were grown on the Ni-deposited TEM Cu mesh-grid at 700 C for 30 sec. Fig. 5. TEM images showing intermediate-growth mode. (a) is an image of CNT which has protrusions on the metal particle at the tip of CNT as marked by arrows. It is thought to be an initial stage of the intermediate-growth mode by the third scenario (see text for detail). (b) is an image showing branch or dendrite structure developed from the initial stage as shown in (a). subsequently converted to methane by interaction with adsorbed hydrogen at the front face of the particle. It seems less probable, however, because there is no sufficient supply of hydrogen involved in the system. Second, considering the quasi-liquid behavior of the catalytic metal, we can deduce the formation of the intermediate growth structure from the tip-growth mode. Once a CNT grows by means of the tip-growth mode, the quasi-liquid phase metal originally located at the tip of CNT can move toward the root and stay in the middle of CNT, which results in the intermediate growth structure. Third, more probably, the intermediate growth structure can be achieved by a mixture of base-growth mode and tip-growth mode, which occur sequentially. For instance, a base-growth mode occurring subsequent to the tip-growth mode may result in the intermediate-growth. In other words, CNT grows by means of the base-growth mode from the surface of the metal particle at the tip of the CNT previously grown by the tip-growth mode. Reaction in the reverse order is also possible. This growth mode may provide us a clue to understand the growth mechanism of branch or dendrite structure. Fig. 5(a) is thought to be an image showing the initial stage of the intermediate growth by this scenario. It was observed in a sample where the CNT had been grown at 750 C. There are two protrusions (arrow marked in the figure) developed on the metal particle at the tip of the CNT. It is thought that this CNT may turn into Y-junction. Fig. 5(b) shows branch or dendrite structure developed from the initial stage as shown in Fig. 5(a). These are strong evidence of the third scenario of the intermediate growth mode. It is worth to note that these three different growth

5 Electron Microscopy Investigation at the Initial Growth Stage Sung-Jin Eum et al. -S731- modes described above occur simultaneously at the initial growth state with the same process parameters. It was also found that the CNTs in the tip-growth mode became dominant as growth time increased up to 20 min, although the CNTs started to grow in the various forms at the initial growth stage. It suggests that there is a possibility that an other growth mechanism works, even though only a certain growth mode is observed in the specimen prepared with the fully developed CNTs by the conventional method. For a certain condition, in fact, there exists not a unique growth mechanism but only the most dominant growth mechanism. It is expected that the results obtained from this fundamental study will not only provide the necessary ingredients for refining the growth mechanisms, but are also essential for determining the process design parameters for producing CNTs for a variety of applications. IV. SUMMARY It is presented a new and simple technique to obtain CNTs without any artifacts that might be introduced during TEM specimen preparation. We synthesized CNTs directly on the Ni-deposited TEM Cu mesh grid so as to preserve and investigate the original features of CNTs from the initial to final growth state. From the electron microscopy study, it is found that the amorphous carbon layer supplied by the decomposition of hydrocarbon source is firstly formed on the catalytic metal particles at the initial growth state. Then, graphitic layers of CNT wall are formed by three different growth modes; tip-growth, base-growth and intermediate-growth. They occur simultaneously at the initial growth state with the same process parameters. One of the most intriguing observations in this study is the intermediate-growth mode which is neither purely tip-growth nor base-growth. The intermediate growth structure can be achieved by a mixture of base-growth mode and tip-growth mode, which occur sequentially. This is thought to be a probable mechanism for nanotube junctions. ACKNOWLEDGMENTS This work was supported by KOSEF through the Center for Nanotubes and Nanostructured Composites at Sungkyunkwan University (Grant No. R ). REFERENCES [1] S. Iijima, Nature 354, 56 (1991). [2] M. Liu and J. M. Cowley, Carbon 33, 225 (1995). [3] Q. H. Wang, T. D. Corrigan, J. Y. Dai and R. P. H.Chang, Appl. Phys. Lett. 70, 3308 (1997). [4] P. G. Collins and A. Zettl, Phys. Rev. B 55, 9391 (1997). [5] J. H. Han, J. E. Yoo and C. J. Lee, J. Korean Phys. Soc. 39, S116 (2001). [6] Z. P. Huang, J. W. Xu, Z. F. Ren, H. H. Wang, M. P. Siegal and P. N. Provencio, Appl. Phys. Lett. 73, 3845 (1998). [7] Y. C. Choi, Y. M. Shin, Y. H. Lee, B. S. Lee, G.-S. Park, W. B. Choi, N. S. Lee and J. M. Kim, Appl. Phys. Lett. 76, 2367 (2000). [8] S. Fan, M. S. Chapline, N. R. Franklin, T. W. Tombler, A. M. Casell and H. Dai, Science 283, 512 (1999). [9] J. C. Charlier, A. De Vita, X. Blase and R. Car, Science 275, 646 (1997). [10] C. J. Lee and J. Park, Appl. Phys. Lett. 77, 3397 (2000). [11] S. B. Sinnott, R. Andrews, D. Qian, A. M. Rao, Z. Mao, E. C. Dickey and F. Derbyshire, Chem. Phys. Lett. 315, 25 (1999). [12] R. T. K. Baker, Carbon, 27, 315 (1989). [13] C. J. Lee, J. H. Han, J. E. Yoo, S. Y. Kang, J. H. Lee and K. I. Cho, J. Korean Phys. Soc. 37, 858 (2000). [14] K. S. Choi, Y. S. Cho, S. Y. Hong, J. B. Park, D. J. Kim and H. J. Kim, J. Korean Phys. Soc. 39, S7 (2001). [15] M. Okai, T. Muneyoshi, T. Yaguchi and S. Sasaki, Appl. Phys. Lett. 77, 3468 (2000).