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Production of single-walled carbon nanotube ropes under controlled gas flow conditions
12 October 2001
Chemical Physics Letters 346 (2001) 356±360
www.elsevier.com/locate/cplett
Production of single-walled carbon nanotube ropes under controlled gas ¯ow conditions Hisashi Kajiura a,*, Shigemitsu Tsutsui a, Houjin Huang a, Mitsuaki Miyakoshi a, Yoshiyuki Hirano b, Atsuo Yamada a, Masafumi Ata a a
Frontier Science Laboratories, Sony Corporation, 2-1-1 Shinsakuragaoka, Hodogaya-ku, Yokohama-shi, Kanagawa 240-0036, Japan b Technical Support Center, Sony Corporation, 4-16-1 Okata, Atsugi-shi, Kanagawa 243-0021, Japan Received 25 June 2001; in ®nal form 22 August 2001
Abstract Single-walled carbon nanotubes (SWNTs) were produced with a high content using a newly designed chamber. The feature of the chamber is that the DC arc-generation section is connected horizontally to the heater section with the ®ltering zone. Soot with a high content of SWNTs is selectively trapped on the sample collector located in the heater section, utilizing the ¯ight distance of arc-produced components in the ®ltering zone under controlled gas ¯ow. Further SWNT growth was demonstrated during in situ heat treatment in the heater section. The G=D value, which is obtained from Raman spectroscopy and is a good index of SWNT content, increased from 4.3 to 25.9. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Since the discovery in 1993 of single-walled carbon nanotubes (SWNTs) in arc-produced soot [1,2], much attention has been directed to the production of SWNTs by the arc-discharge and the laser ablation methods [3±5]. Although laser ablation produces a purer product, the production rate of SWNTs is far from satisfactory [6,7]. In contrast, the arc-discharge method is suitable for mass production of the soot. SWNTs are found in the soot along with the impurities such as amorphous carbon, graphite, and metal nano-particles. These impurities must be
*
Corresponding author. Fax: +81-45-353-6904. E-mail address: [email protected] (H. Kajiura).
eliminated to obtain high-purity SWNTs, and thus various puri®cation processes have been proposed [6,8]. These puri®cation processes, however, have proved to be applicable only to soot produced by particular processes [8,9], but nevertheless there has been damage and digestion of SWNTs [10]. Therefore, it is desirable to obtain soot with a high content of SWNTs to minimize and/or eliminate the need for a puri®cation process. In the present work, we designed a new chamber equipped with a ®ltering zone for eective in situ puri®cation. The original con®gurations of the chamber as well as the expected functions are presented in Section 2, followed by the characterization of the soot using transmission electron microscopy (TEM), thermogravimetry (TGA) and Raman spectroscopy. The eect of in situ heat treatment on the microstructure of the soot is also discussed.
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 7 9 - 4
H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
2. Experimental 2.1. Chamber con®guration A schematic diagram of the chamber is shown in Fig. 1. The chamber consists of a DC arcgeneration section and a heater section. To suppress conventional gas ¯ow in the arc-generation section, the arc plasma is generated under controlled gas ¯ow conditions in the small 50 mm inner diameter reaction tube. A cylindrical sample collector made of graphite is installed in the gas ¯ow in the heater section. The space between the arc-generation point and the sample collector acts as a ®ltering zone under the gas ¯ow conditions. The expected functions of the ®ltering zone are schematically shown in Fig. 2. In the ®ltering zone, arc-produced components would be deposited in order of density, and soot with a high content of SWNT would be ®nally trapped on the sample collector under appropriate gas ¯ow, since the density of SWNT is in the range 1.3±1.4 g/ cm3 , which is the smallest of the arc-produced
Fig. 1. Schematic diagram of horizontal chamber for SWNT synthesis. This chamber is composed of an arc-generation section and a heater section.
Fig. 2. Schematic presentation of the ®ltering zone functions under controlled gas ¯ow conditions.
357
components [11]. A radio-frequency induction coil (200 kHz) is installed in the heater section, which allows in situ heat treatment without exposure to air. 2.2. SWNTs synthesis procedure and characterization The cathode was a graphite rod /15 100 mm, while the anode was a carbon/metal composite rod /6 150 mm consisting of a mixture of catalysts (1.0 at.% Fe, 0.6 at.% Co, 2.4 at.% Ni), a growth promoter (0.4 at.% FeS) [12] and graphite powder. The arc plasma was generated for 3±5 min with a dc current of 200 A with 66.5 kPa He ¯owing at 5 l/min, and the distance between electrodes adjusted to ensure a voltage drop of 50 V. The soot on the sample collector was optionally in situ heated at 1140 °C for 10 min at 10 1 Pa. The soot was characterized using transmission electron microscopy (TEM, JEOL, JEM2000FXII), thermogravimetric analysis (TGA, PERKIN ELMER, Pylis 1 TGA), and Raman spectroscopy (CHROMEX, Raman2000). TEM observation was carried out at an acceleration voltage of 200 kV. For TGA measurements, after removing moisture from the soot at 105 °C in dry air, 1 mg of the sample was heated to 900 °C at a rate of 5 °C/min in a dry air ¯ow of 30 ml/min. Raman spectra were measured using an excitation wavelength of 532 nm from the second harmonic of a Nd:YAG laser. 3. Results and discussion After arc generation, the soot produced in the arc-generation section moved to the heater section through the ®ltering zone and was trapped on the sample collector. Several milligrams of soot could be trapped in 3 min of arc generation. The trapped soot formed a rope-shaped structure several millimeters long. Similar to the conventional arc-discharge method, web-like soot was also found to be deposited on the chamber wall in the arc-generation section. The soot on the sample collector tended to adhere strongly to the collector, whereas the web-like soot could be easily peeled from the
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H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
wall. Hereafter, we will represent the soot collected from the chamber wall in the arc-generation section as Sample A and the soot on the sample collector as Sample B. TEM observation was carried out to investigate the nanostructure of the soot and Figs. 3 and 4 show TEM images of Samples A and B, respectively. In both samples, four types of nanostructures were con®rmed, that is, SWNTs, graphite, amorphous carbon, and metal nano-particles. Several TEM photographs were taken under higher magni®cation (not shown here) and more than twenty tubes were selected randomly to measure the SWNT diameter. The SWNT diameters were in the range 1.2±3.0 nm with a mean diameter of 1:8 0:3 nm in both samples. The mean diameter is larger than that of SWNTs produced using conventional methods, which may be due to the sulfur-containing growth promoter [12,13]. In Sample A, the SWNTs were separated from each other and SWNT bundles were rarely found, while in Sample B the majority were bundles with diameters of 10±50 nm. The diameter of the bundles observed in Sample B was much larger than that of the few bundles in Sample A. The SWNTs produced by arc plasma are considered to
Fig. 3. TEM image of Sample A. This sample was collected from the chamber wall in the arc-generation section.
Fig. 4. TEM image of Sample B. This sample was collected on the sample collector.
align by van der Waals interactions during transportation in the ®ltering zone, consequently forming bundles. Most of the metal nano-particles were embedded in amorphous carbon in both Samples A and B. Determination of the residual metal content from the TEM images was dicult, so TGA measurements in air were used for this. The residual weight, after all the carbon species had been oxidized to form CO or CO2 , corresponds to the percentage weight of the residual metal in oxide form [8]. In addition to Samples A and B, the soot in situ heated at 1140 °C for 10 min in vacuum, which is represented as Sample C hereafter, was also measured. As a result, the residual percentage weight of Samples A, B and C at 900 °C was 45%, 32% and 35%, respectively. The residual percentage weight of sample B was about two-thirds of that of Sample A, which shows clearly that the metal content in soot can be reduced by transportation in the ®ltering zone under controlled gas ¯ow conditions. In Sample C, the metal content was almost the same as that in sample B, which implies that the metal nanoparticles did not evaporate during the in situ heat treatment.
H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
Fig. 5. Raman shift of Samples A, B, and C. Sample C underwent in situ heat treatment 1140 °C for 10 min at 10 1 Pa. The G=D value, which is the Raman intensity ratio of peaks at 1590 to 1340 cm 1 , is an index of the SWNT content in the samples.
Fig. 5 shows the results of Raman spectroscopy of Samples A, B, and C. Sample A showed a peak at 168 cm 1 and a shoulder at 186 cm 1 ; which are attributed to the radial breathing mode (RBM) of SWNTs. The frequency of the RBM is in inverse proportion to the diameter of the SWNTs [14], and is indicated by the following equation, xr 223:75=d, where xr is the RBM frequency in cm 1 and d is the SWNT diameter in nm [15]. According to this equation, the RBM frequencies of 168 and 186 cm 1 correspond to 1.39 and 1.26 nm, respectively. These values are in rough agreement with the TEM observation results shown in Fig. 3. Similar RBM peaks were also observed from Samples B and C and the frequencies of these peaks were the same as for Sample A. This suggests that the SWNT diameter does not alter either by being transported in the gas ¯ow or by in situ heat treatment.
359
On the other hand, dependences in the Raman shift were observed in the wave number region between 1000 and 2000 cm 1 as shown in Fig. 5. The broad peak called the D-band appeared at 1340 cm 1 , and is related to the disordered or defective graphitic structure. The intensity of the D-band decreases as the extent of the disordered carbon decreases [16]. The distinctive shoulder peak at 1568 cm 1 on the left shoulder of the main peak (called the G band) at 1590 cm 1 appeared in all samples. This shoulder peak originates from the splitting of the E2g mode of graphite and is one of the characteristic Raman peaks of SWNTs [17]. The ratio of the peak intensities at 1590 to 1340 cm 1 (called the G=D value) is a good index of the SWNT content, and the G=D value increases with increasing SWNT content in soot [16,18]. The G=D value of Samples A, B, and C were calculated to be 4.3, 18.1 and 25.9, respectively, as denoted in Fig. 5. Although the G=D value of Sample C is 1.4 times as high as that of Sample B, TEM observation revealed signi®cant dierence in the microstructure between the samples. In the light of similar tendencies reported in the literature [19,20], surface modi®cation and further growth of SWNTs may take place during in situ heat treatment. During synthesis using an arc-plasma, not only SWNTs but also by-products are produced, and most of these carbonaceous and metallic materials are deposited on the chamber wall together with SWNTs when a conventional arc-discharge chamber is used, bringing about a decrease of the SWNT content in the soot. For example, when we produced soot using a conventional He arc-discharge chamber at a static pressure of 66.5 kPa under the same conditions described here, the soot collected on the sidewall contained catalytic metal particles as high as 50±70 wt.%, and the G=D value was 3.5. While using our new chamber, arc-produced components are deposited in order of the density in the ®ltering zone and soot with a high content of SWNT can be ®nally trapped on the sample collector under the appropriate gas ¯ow conditions. SWNTs synthesis under controlled gas ¯ow conditions in a chamber with the ®ltering zone is one of the most promising techniques for production of SWNTs with a high content.
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H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
4. Conclusion SWNTs are produced with a high content using a horizontal small-diameter chamber equipped with a ®ltering zone. The chamber is characterized by that an arc-generation section is connected to a heater section with a ®ltering zone. After arc generation, soot with a high content of SWNT bundles was trapped on the sample collector located in the center of the heater section, utilizing the ¯ight distances of arc-produced components in the ®ltering zone under the controlled gas ¯ow. The SWNT content increased with in situ heat treatment at 1140 °C in the heater section. The G=D value, which is the ratio of the peak intensities at 1590 to 1340 cm 1 in Raman spectroscopy and is a good index of the SWNT content, increased from 4.3 to 25.9. The design concept of this chamber demonstrates a promising new way to produce SWNTs with a high content. References [1] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [2] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [3] C. Journet, P. Bernier, Appl. Phys. A 67 (1998) 1. [4] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y-H. Lee,, S-G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483.
[5] C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M.L. Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fisher, Nature 388 (1997) 756. [6] A.G. Rinzer, J. Liu, H. Dai, H. Nikolaev, C.B. Human, F.J. Rodriguez-Macias, P.J. Boul, A.H. Lu, D. Heymann, D.T. Colbert, R.S. Lee, J.E. Fischer, A.M. Rao, P.C. Eklund, R.E. Smalley, Appl. Phys. A 67 (1998) 29. [7] Z. Shi, Y. Lian, F.H. Liao, A. Zhou, Z. Gu, Y. Zhang, S. Iijima, H. Li, K.T. Yue, S-L. Zhang, J. Phys. Chem. Solid 61 (2000) 1031. [8] A.C. Dillon, T. Gennett, K.M. Jones, J.L. Alleman, P.A. Parilla, M.J. Heben, Adv. Mater. 11 (1999) 1354. [9] M. Yudasaka, M. Zhang, C. Jabs, S. Iijima, Appl. Phys. A 71 (2000) 449. [10] K.B. Shelimov, R.O. Esenaliev, A.G. Rinzler, C.B. Human, R.E. Smalley, Chem. Phys. Lett. 282 (1998) 429. [11] G. Gao, T. Cagin, W.A. Goddard III, Nanotechnology 9 (1998) 184±191. [12] C. Liu, H.T. Cong, F. Li, P.H. Tan, H.M. Cheng, K. Lu, B.L. Zhou, Carbon 37 (1999) 1865. [13] C-H. Kiang, J. Phys. Chem. A 104 (2000) 2454. [14] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, 1998 (Chapter 10). [15] S. Bandow, S. Asaka, Y. Saito, A.M. Rao, L. Grigorian, E. Richter, P.C. Eklund, Phys. Rev. Lett. 80 (1998) 3779. [16] H. Kataura, Y. Kumazawa, Y. Maniwa, Y. Ohtsuka, R. Sen, S. Suzuki, Y. Achiba, Carbon 38 (2000) 1691. [17] A. Kasuya, Y. Sakaki, Y. Saito, K. Tohji, Y. Nishina, Phys. Rev. Lett. 78 (1997) 4434. [18] Y. Ando, X. Zhao, K. Hirahara, K. Suenaga, S. Bandow, S. Iijima, Chem. Phys. Lett. 323 (2000) 580. [19] T. Sugai, N. Tanaka, X. Zhao, S. Bandow, Y. Ando, H. Shinohara, in: The 19th Fullerene General Symposium 1A04, 2000, p. 6. [20] A. Bougrine, N. Dupont-Pavlovsky, A. Naji, J. Ghanbaja, J.F. Mareche, D. Billaud, Carbon 39 (2001) 685.
Chemical Physics Letters 346 (2001) 356±360
www.elsevier.com/locate/cplett
Production of single-walled carbon nanotube ropes under controlled gas ¯ow conditions Hisashi Kajiura a,*, Shigemitsu Tsutsui a, Houjin Huang a, Mitsuaki Miyakoshi a, Yoshiyuki Hirano b, Atsuo Yamada a, Masafumi Ata a a
Frontier Science Laboratories, Sony Corporation, 2-1-1 Shinsakuragaoka, Hodogaya-ku, Yokohama-shi, Kanagawa 240-0036, Japan b Technical Support Center, Sony Corporation, 4-16-1 Okata, Atsugi-shi, Kanagawa 243-0021, Japan Received 25 June 2001; in ®nal form 22 August 2001
Abstract Single-walled carbon nanotubes (SWNTs) were produced with a high content using a newly designed chamber. The feature of the chamber is that the DC arc-generation section is connected horizontally to the heater section with the ®ltering zone. Soot with a high content of SWNTs is selectively trapped on the sample collector located in the heater section, utilizing the ¯ight distance of arc-produced components in the ®ltering zone under controlled gas ¯ow. Further SWNT growth was demonstrated during in situ heat treatment in the heater section. The G=D value, which is obtained from Raman spectroscopy and is a good index of SWNT content, increased from 4.3 to 25.9. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Since the discovery in 1993 of single-walled carbon nanotubes (SWNTs) in arc-produced soot [1,2], much attention has been directed to the production of SWNTs by the arc-discharge and the laser ablation methods [3±5]. Although laser ablation produces a purer product, the production rate of SWNTs is far from satisfactory [6,7]. In contrast, the arc-discharge method is suitable for mass production of the soot. SWNTs are found in the soot along with the impurities such as amorphous carbon, graphite, and metal nano-particles. These impurities must be
*
Corresponding author. Fax: +81-45-353-6904. E-mail address: [email protected] (H. Kajiura).
eliminated to obtain high-purity SWNTs, and thus various puri®cation processes have been proposed [6,8]. These puri®cation processes, however, have proved to be applicable only to soot produced by particular processes [8,9], but nevertheless there has been damage and digestion of SWNTs [10]. Therefore, it is desirable to obtain soot with a high content of SWNTs to minimize and/or eliminate the need for a puri®cation process. In the present work, we designed a new chamber equipped with a ®ltering zone for eective in situ puri®cation. The original con®gurations of the chamber as well as the expected functions are presented in Section 2, followed by the characterization of the soot using transmission electron microscopy (TEM), thermogravimetry (TGA) and Raman spectroscopy. The eect of in situ heat treatment on the microstructure of the soot is also discussed.
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 7 9 - 4
H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
2. Experimental 2.1. Chamber con®guration A schematic diagram of the chamber is shown in Fig. 1. The chamber consists of a DC arcgeneration section and a heater section. To suppress conventional gas ¯ow in the arc-generation section, the arc plasma is generated under controlled gas ¯ow conditions in the small 50 mm inner diameter reaction tube. A cylindrical sample collector made of graphite is installed in the gas ¯ow in the heater section. The space between the arc-generation point and the sample collector acts as a ®ltering zone under the gas ¯ow conditions. The expected functions of the ®ltering zone are schematically shown in Fig. 2. In the ®ltering zone, arc-produced components would be deposited in order of density, and soot with a high content of SWNT would be ®nally trapped on the sample collector under appropriate gas ¯ow, since the density of SWNT is in the range 1.3±1.4 g/ cm3 , which is the smallest of the arc-produced
Fig. 1. Schematic diagram of horizontal chamber for SWNT synthesis. This chamber is composed of an arc-generation section and a heater section.
Fig. 2. Schematic presentation of the ®ltering zone functions under controlled gas ¯ow conditions.
357
components [11]. A radio-frequency induction coil (200 kHz) is installed in the heater section, which allows in situ heat treatment without exposure to air. 2.2. SWNTs synthesis procedure and characterization The cathode was a graphite rod /15 100 mm, while the anode was a carbon/metal composite rod /6 150 mm consisting of a mixture of catalysts (1.0 at.% Fe, 0.6 at.% Co, 2.4 at.% Ni), a growth promoter (0.4 at.% FeS) [12] and graphite powder. The arc plasma was generated for 3±5 min with a dc current of 200 A with 66.5 kPa He ¯owing at 5 l/min, and the distance between electrodes adjusted to ensure a voltage drop of 50 V. The soot on the sample collector was optionally in situ heated at 1140 °C for 10 min at 10 1 Pa. The soot was characterized using transmission electron microscopy (TEM, JEOL, JEM2000FXII), thermogravimetric analysis (TGA, PERKIN ELMER, Pylis 1 TGA), and Raman spectroscopy (CHROMEX, Raman2000). TEM observation was carried out at an acceleration voltage of 200 kV. For TGA measurements, after removing moisture from the soot at 105 °C in dry air, 1 mg of the sample was heated to 900 °C at a rate of 5 °C/min in a dry air ¯ow of 30 ml/min. Raman spectra were measured using an excitation wavelength of 532 nm from the second harmonic of a Nd:YAG laser. 3. Results and discussion After arc generation, the soot produced in the arc-generation section moved to the heater section through the ®ltering zone and was trapped on the sample collector. Several milligrams of soot could be trapped in 3 min of arc generation. The trapped soot formed a rope-shaped structure several millimeters long. Similar to the conventional arc-discharge method, web-like soot was also found to be deposited on the chamber wall in the arc-generation section. The soot on the sample collector tended to adhere strongly to the collector, whereas the web-like soot could be easily peeled from the
358
H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
wall. Hereafter, we will represent the soot collected from the chamber wall in the arc-generation section as Sample A and the soot on the sample collector as Sample B. TEM observation was carried out to investigate the nanostructure of the soot and Figs. 3 and 4 show TEM images of Samples A and B, respectively. In both samples, four types of nanostructures were con®rmed, that is, SWNTs, graphite, amorphous carbon, and metal nano-particles. Several TEM photographs were taken under higher magni®cation (not shown here) and more than twenty tubes were selected randomly to measure the SWNT diameter. The SWNT diameters were in the range 1.2±3.0 nm with a mean diameter of 1:8 0:3 nm in both samples. The mean diameter is larger than that of SWNTs produced using conventional methods, which may be due to the sulfur-containing growth promoter [12,13]. In Sample A, the SWNTs were separated from each other and SWNT bundles were rarely found, while in Sample B the majority were bundles with diameters of 10±50 nm. The diameter of the bundles observed in Sample B was much larger than that of the few bundles in Sample A. The SWNTs produced by arc plasma are considered to
Fig. 3. TEM image of Sample A. This sample was collected from the chamber wall in the arc-generation section.
Fig. 4. TEM image of Sample B. This sample was collected on the sample collector.
align by van der Waals interactions during transportation in the ®ltering zone, consequently forming bundles. Most of the metal nano-particles were embedded in amorphous carbon in both Samples A and B. Determination of the residual metal content from the TEM images was dicult, so TGA measurements in air were used for this. The residual weight, after all the carbon species had been oxidized to form CO or CO2 , corresponds to the percentage weight of the residual metal in oxide form [8]. In addition to Samples A and B, the soot in situ heated at 1140 °C for 10 min in vacuum, which is represented as Sample C hereafter, was also measured. As a result, the residual percentage weight of Samples A, B and C at 900 °C was 45%, 32% and 35%, respectively. The residual percentage weight of sample B was about two-thirds of that of Sample A, which shows clearly that the metal content in soot can be reduced by transportation in the ®ltering zone under controlled gas ¯ow conditions. In Sample C, the metal content was almost the same as that in sample B, which implies that the metal nanoparticles did not evaporate during the in situ heat treatment.
H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
Fig. 5. Raman shift of Samples A, B, and C. Sample C underwent in situ heat treatment 1140 °C for 10 min at 10 1 Pa. The G=D value, which is the Raman intensity ratio of peaks at 1590 to 1340 cm 1 , is an index of the SWNT content in the samples.
Fig. 5 shows the results of Raman spectroscopy of Samples A, B, and C. Sample A showed a peak at 168 cm 1 and a shoulder at 186 cm 1 ; which are attributed to the radial breathing mode (RBM) of SWNTs. The frequency of the RBM is in inverse proportion to the diameter of the SWNTs [14], and is indicated by the following equation, xr 223:75=d, where xr is the RBM frequency in cm 1 and d is the SWNT diameter in nm [15]. According to this equation, the RBM frequencies of 168 and 186 cm 1 correspond to 1.39 and 1.26 nm, respectively. These values are in rough agreement with the TEM observation results shown in Fig. 3. Similar RBM peaks were also observed from Samples B and C and the frequencies of these peaks were the same as for Sample A. This suggests that the SWNT diameter does not alter either by being transported in the gas ¯ow or by in situ heat treatment.
359
On the other hand, dependences in the Raman shift were observed in the wave number region between 1000 and 2000 cm 1 as shown in Fig. 5. The broad peak called the D-band appeared at 1340 cm 1 , and is related to the disordered or defective graphitic structure. The intensity of the D-band decreases as the extent of the disordered carbon decreases [16]. The distinctive shoulder peak at 1568 cm 1 on the left shoulder of the main peak (called the G band) at 1590 cm 1 appeared in all samples. This shoulder peak originates from the splitting of the E2g mode of graphite and is one of the characteristic Raman peaks of SWNTs [17]. The ratio of the peak intensities at 1590 to 1340 cm 1 (called the G=D value) is a good index of the SWNT content, and the G=D value increases with increasing SWNT content in soot [16,18]. The G=D value of Samples A, B, and C were calculated to be 4.3, 18.1 and 25.9, respectively, as denoted in Fig. 5. Although the G=D value of Sample C is 1.4 times as high as that of Sample B, TEM observation revealed signi®cant dierence in the microstructure between the samples. In the light of similar tendencies reported in the literature [19,20], surface modi®cation and further growth of SWNTs may take place during in situ heat treatment. During synthesis using an arc-plasma, not only SWNTs but also by-products are produced, and most of these carbonaceous and metallic materials are deposited on the chamber wall together with SWNTs when a conventional arc-discharge chamber is used, bringing about a decrease of the SWNT content in the soot. For example, when we produced soot using a conventional He arc-discharge chamber at a static pressure of 66.5 kPa under the same conditions described here, the soot collected on the sidewall contained catalytic metal particles as high as 50±70 wt.%, and the G=D value was 3.5. While using our new chamber, arc-produced components are deposited in order of the density in the ®ltering zone and soot with a high content of SWNT can be ®nally trapped on the sample collector under the appropriate gas ¯ow conditions. SWNTs synthesis under controlled gas ¯ow conditions in a chamber with the ®ltering zone is one of the most promising techniques for production of SWNTs with a high content.
360
H. Kajiura et al. / Chemical Physics Letters 346 (2001) 356±360
4. Conclusion SWNTs are produced with a high content using a horizontal small-diameter chamber equipped with a ®ltering zone. The chamber is characterized by that an arc-generation section is connected to a heater section with a ®ltering zone. After arc generation, soot with a high content of SWNT bundles was trapped on the sample collector located in the center of the heater section, utilizing the ¯ight distances of arc-produced components in the ®ltering zone under the controlled gas ¯ow. The SWNT content increased with in situ heat treatment at 1140 °C in the heater section. The G=D value, which is the ratio of the peak intensities at 1590 to 1340 cm 1 in Raman spectroscopy and is a good index of the SWNT content, increased from 4.3 to 25.9. The design concept of this chamber demonstrates a promising new way to produce SWNTs with a high content. References [1] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [2] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [3] C. Journet, P. Bernier, Appl. Phys. A 67 (1998) 1. [4] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y-H. Lee,, S-G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483.
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