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Non-destructive purification of multi-walled carbon nanotubes produced by catalyzed CVD
December 2002
Materials Letters 57 (2002) 734 – 738 www.elsevier.com/locate/matlet
Non-destructive purification of multi-walled carbon nanotubes produced by catalyzed CVD X.H. Chen *, C.S. Chen, Q. Chen, F.Q. Cheng, G. Zhang, Z.Z. Chen College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, China Received 20 April 2002
Abstract A three-step purification process of multi-walled carbon nanotubes (MWNTs) produced by catalytic CVD method with Ni – Mg – O as catalysts is described. In the former two-step process, 3 M HNO3 and 5 M HCl treatment are effective to remove metal and metal oxide. With thermogravimetric analysis (TGA), the burning temperature of MWNTs in air was determined and 510 jC was chosen to be optimum temperature to eliminate non-nanotube carbon materials for the third step purification of MWNTs. By this way, larger than 96 wt.% purity of MWNTs is obtained without damage. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Multi-walled carbon nanotubes; Purity; CVD method
1. Introduction Carbon nanotubes (CNTs) [1] have unique mechanical and electrical properties that make them attractive systems for fundamental scientific studies as well as for a wide range of applications including electron field emission sources [2 – 4], nanoscale electronic devices [5,6], chemical filters and storage systems [7], and mechanical reinforcements [8,9]. Various synthesis methods have been developed for the production of nanotubes [1,10 – 12]. In addition to the laser evaporation and electric arc discharge techniques, the catalytic process is a very efficient method to produce
*
Corresponding author. Tel.: +86-731-882-1096; fax: +86-731882-4525. E-mail address: [email protected] (X.H. Chen).
multi-walled carbon nanotubes (MWNTs) [10,12]. However, all the raw materials produced by these methods contain impurities of amorphous carbon, graphite particles and metal catalysts. Purification of MWNTs is very important for applications using their excellent intrinsic properties. Recently, many purified methods have been proposed, such as the carbon dioxide oxidation, air oxidation, permanganate oxidation et al. [13 –17]. However, these methods are uncontrollable and unscalable. Especially when the air oxidation method was used to calcine the raw materials, different calcination temperatures were reported in different groups [13 –17]. An unsuitable calcination temperature would result in insufficient purification of MWNTs or damage of MWNTs. Here we describe a three-step purification method by which the raw materials can be purified completely without damage of MWNTs.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 6 3 - 7
X.H. Chen et al. / Materials Letters 57 (2002) 734–738
2. Experimental The MWNTs were prepared by catalytic decomposition of acetylene. The catalysts were produced by concentrating the nickel nitrate and magnesium nitrate solution to gelation and then grinding it to fine powder after sintering at 400 jC. Carbon deposit was obtained on the catalyst at 700 jC with the flow rate of acetylene 40 ml/min and nitrogen 400 ml/min. The crude was stirred in 3 M nitric acid and refluxed for 24 h at 60 jC, and then was suspended and refluxed in 5 M HCl solution for 6 h at 120 jC. After acid treatment, the samples was calcined in static air at 510 jC with the reaction time of about 60 min, and the pure MWNTs were obtained. The temperature of air oxidation of acid-treated MWNTs was determined by thermogravimetric analysis (TGA) using A Dupont 9900TGA meter with a rate of 10 jC/min from room temperature to 900 jC at an air flow rate of 70 sccm. Five grams acid-treated MWNTs
735
was oxidized by air at the temperature determined by TGA for 60 min. The samples were characterized by transmission electron microscopy (TEM) and XRD.
3. Results and discussion A low-magnification TEM image (Fig. 1a) shows that the raw materials contained carbon nanotubes and non-nanotube materials. From the XRD pattern for raw materials in Fig. 2a, we observe, apart from the graphite peak, the strong metal peaks and the nickel and magnesium oxide peaks, indicating that the raw materials contained much residual metal and metal oxide. After the first purification process by HNO3 treatment, most of the impurities were removed (Fig. 1b). Corresponding XRD pattern in Fig. 2b shows that the metal peaks and the oxide peaks are lower than those in Fig. 2a, suggesting that the metal particles and the oxide particles have been eliminated partially.
Fig. 1. TEM images of: (a) as-produced crude materials, (b) material that was refluxed for 24 h in 3 M HNO3, (c) material produced by treating the HNO3-treated sample in 5 M HCl for 6 h, (d) purified MWNTs produced by oxiding the acid-treated sample for 60 min in air at 510 jC.
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X.H. Chen et al. / Materials Letters 57 (2002) 734–738
Fig. 2. XRD pattern of: (a) as-produced crude materials, (b) material that was refluxed for 24 h in 3 M HNO3, (c) material produced by treating the HNO3-treated sample in 5 M HCl for 6 h, (d) purified MWNTs produced by oxiding the acid-treated sample for 60 min in air at 510 jC.
The fact that the graphite peak is stronger reveals that the amorphous carbon coating the surface of nanotubes was consumed by oxidation reaction. After the second step of purification by HCl treatment, relatively pure MWNTs were obtained, as shown in Fig. 1c. Comparative with the Fig. 1b, Fig. 1c shows that little of black non-nanotube materials exists. XRD pattern of sample after HCl treatment exhibited detailed features in Fig. 2c. The peaks of MgNiO2 have disappeared, only very weak Ni peak. The results indicate that the two-step acid treatment is effective. It is well known that 3 M HNO3 reflux is very effective in dissolving metal particles. Also, because nitrate acid is a strong oxidizer, amorphous carbon can be removed by oxidation. The selection of 5 M HCl is to dissolve metal oxide. In this work, the Mg – Ni– O catalysts were produced by sintering gelation state of Mg (NO3)2 and Ni (NO3)2 at 400 jC at final step, which can be dissolved by 5 M HCl easily, but not by H2SO4 or HNO3 solution. When we used H2SO4 or HNO3 solution to treat the sample for three times, we found that the MgNiO2 always existed. Thus, 5 M HCl selected here is more effective in removing the metal oxide than H2SO4 or HNO3 solution, which is confirmed by XRD pattern in Fig. 2c. Furthermore, MWNTs need sufficient and nondestructive purification for applications and investiga-
tion of properties. The air oxidation is a necessary step in purification process. The aim of burning acid-treated materials is to purify MWNTs based on the difference of oxidation temperature between CNTs and non-nanotubes. Because atom sheets in CNT are formed by sp2.6 bond, the oxidation temperature is higher than that of amorphous carbon. However, there is no common burning temperature of CNTs since it is not only related to graphitization degree, but also to pre-treatment process. Therefore, to purify MWNTs with high quality and no damage, a suitable burning temperature in air must be determined. TGA is an effective method for determining the burning temperature in air. Fig. 3a is the TGA graph of the sample after acid treatment. It can be found that weight was lost slowly from 20 to 410 jC, corresponding to the loss of little water and a few amorphous carbon. However, some slight increase in weight was observed in the magnified TGA graph, as shown in Fig. 3b. The increase occurred at 27, 170 and 230 jC, respectively, and the increasing amount was estimated to be 0.5– 0.6 wt.%, which might be due to the oxidation of the Mg and Ni. From 20 to 410 jC, there are two stepwise weight loss, which can be assigned to amorphous carbon and carbon particles. At the temperature range from 510 to 645 jC, the weight decreased sharply to 3.92 wt.%, indicating that the combustion of the MWNTs started at 510 jC. Also a thing to note is that the curve slope maintained almost the same value from 510 to 645 jC. It is illustrated that the MWNTs were combusted at a constant speed, suggesting that the MWNTs reached to a high purity at 510 jC. After 645 jC, the weight of the sample remains unchanged. The remainders may be assigned to the metal or metal oxide which are inside the inner of tubes before combustion. Therefore, 510 jC is an optimum temperature to burn out non-nanotube carbon materials. The MWNTs combusted in air at 510 jC for 1 h are typically shown in Fig. 1d, in which high-purity MWNTs can be found. The XRD pattern of the combusted MWNTs (Fig. 2d) shows that the metal peaks and metal oxide peaks have disappeared completely, consistent with the result shown in Fig. 1d. The 002 peak is two times stronger than that of noncombusted MWNTs. This feature can be explained by consumption of amorphous carbon and increase of graphitization degree of the sample during the combustion in air.
X.H. Chen et al. / Materials Letters 57 (2002) 734–738
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methods applied in different laboratories produce materials with carbon nanotubes, non-nanotube carbon and metal fractions that vary in type and configuration. Up to now, most of purification methods deal with the carbon nanotubes produced by discharge arc or laser vaporization method. Shi et al. [13] reported that the effective temperature to burn out non-nanotubes carbon is 350 jC for purifying single-walled carbon nanotubes prepared by dc arc-discharge method. Purification studies developed by Dillon et al. [17] indicate the raw materials of carbon nanotubes synthesized by laser vaporization method exhibited such a temperature of 785 jC by TGA. Colomer et al. [16] reported that the optimum reaction temperature in air chosen was 500 jC for carbon nanotubes produced by catalytic CVD. It is illustrated from these results that the raw materials of CNTs produced by different catalysts and preparation method are not only various in component, but also different in graphitization degree. Therefore, purification strategies developed by us may not be applicable to their materials. Similarly, others’ techniques are not suitable for our materials. However, it is sure that TGA is an indispensable step for purification of MWNTs produced by any method.
4. Conclusion
Fig. 3. (a) TGA of 5-mg materials after acid treatment ranged from 20 to 900 jC at 10 jC per minute under 70 sccm flowing air. (b) The magnified TGA curve of the sample ranged from 20 to 410 jC.
The three-step purification procedure for MWNTs produced by catalyzed CVD method described here appears to be a simpler and more effective purification process than any previously reported. After the oxidation purification in air at 510 jC for 1 h, TGA studies indicate the chemical purity of 96 wt.%, while TEM image reveals a purity of almost 98%. Presently, there is no good method by which CNTs’ purity may be readily evaluated. In this work, TGA was used to evaluate the purity of the MWNTs, which may be accurately determined on a weight percentage basis. Although many purification methods of MWNTs have been proposed by others [13 – 17], these methods are not currently available since different production
With Ni – Mg – O as catalyst, without reduction of H2, large-scale MWNTs were produced by the CVD method. A three-step purification process for crude materials was proposed. Two-step process of acid treatment is effective to remove metal or metal oxide. The air oxidation is a necessary step in purification process to obtain high-quality MWNTs. The optimum oxidation temperature in air was determined to be 510 by TGA. The purification method developed in this work produces high purity of MWNTs with >96 wt.% without damage and may be a reliable means of purification and evaluation for MWNTs.
Acknowledgements Xiaohua Chen thanks the National Science Foundation of China (No. 59972031) and the National Science Foundation of Hunan Province (No. 01JJY 2052) for financial support.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9]
S. Iijima, Nature 354 (1991) 56. W.A. Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. Q.H. Wang, Appl. Phys. Lett. 72 (1998) 2912. A.G. Rinzler, Science 269 (1995) 1550. P.G. Collins, A. Zell, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science 280 (1998) 1744. H.M. Chen, C. Liu, Y.Y. Fan, Science 286 (1999) 1127. M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, et al., Nature 381 (1996) 678. D.H. Robertson, D.W. Brenner, J.W. Mintmire, et al., Phys. Rev., B 45 (1992) 12592.
[10] T.W. Ebbesen, H. Hirua, J. Fujita, et al., Chem. Phys. Lett. 209 (1993) 83. [11] Y. Zhang, S. Minima, Appl. Phys. Lett. 75 (1999) 3087. [12] S. Amelinckx, X.B. Zhang, T. Ichihashi, et al., Science 265 (1994) 635. [13] Z. Shi, Y. Lian, F. Liao, X. Zhou, S. Iijima, Solid State Commun. 112 (1999) 35. [14] P.M. Ajayan, T.W. Ebbesen, T. Ichihashi, Nature 362 (1993) 522. [15] T.W. Ebbesen, P.M. Ajayan, Nature 367 (1994) 519. [16] J.F. Colomer, P. Piedigrosso, A. Fonseca, et al., Synth. Met. 103 (1999) 2482. [17] A.C. Dillon, G. Thomas, K.M. Jones, et al., Adv. Mater. 11 (1999) 1354.
Materials Letters 57 (2002) 734 – 738 www.elsevier.com/locate/matlet
Non-destructive purification of multi-walled carbon nanotubes produced by catalyzed CVD X.H. Chen *, C.S. Chen, Q. Chen, F.Q. Cheng, G. Zhang, Z.Z. Chen College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, China Received 20 April 2002
Abstract A three-step purification process of multi-walled carbon nanotubes (MWNTs) produced by catalytic CVD method with Ni – Mg – O as catalysts is described. In the former two-step process, 3 M HNO3 and 5 M HCl treatment are effective to remove metal and metal oxide. With thermogravimetric analysis (TGA), the burning temperature of MWNTs in air was determined and 510 jC was chosen to be optimum temperature to eliminate non-nanotube carbon materials for the third step purification of MWNTs. By this way, larger than 96 wt.% purity of MWNTs is obtained without damage. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Multi-walled carbon nanotubes; Purity; CVD method
1. Introduction Carbon nanotubes (CNTs) [1] have unique mechanical and electrical properties that make them attractive systems for fundamental scientific studies as well as for a wide range of applications including electron field emission sources [2 – 4], nanoscale electronic devices [5,6], chemical filters and storage systems [7], and mechanical reinforcements [8,9]. Various synthesis methods have been developed for the production of nanotubes [1,10 – 12]. In addition to the laser evaporation and electric arc discharge techniques, the catalytic process is a very efficient method to produce
*
Corresponding author. Tel.: +86-731-882-1096; fax: +86-731882-4525. E-mail address: [email protected] (X.H. Chen).
multi-walled carbon nanotubes (MWNTs) [10,12]. However, all the raw materials produced by these methods contain impurities of amorphous carbon, graphite particles and metal catalysts. Purification of MWNTs is very important for applications using their excellent intrinsic properties. Recently, many purified methods have been proposed, such as the carbon dioxide oxidation, air oxidation, permanganate oxidation et al. [13 –17]. However, these methods are uncontrollable and unscalable. Especially when the air oxidation method was used to calcine the raw materials, different calcination temperatures were reported in different groups [13 –17]. An unsuitable calcination temperature would result in insufficient purification of MWNTs or damage of MWNTs. Here we describe a three-step purification method by which the raw materials can be purified completely without damage of MWNTs.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 6 3 - 7
X.H. Chen et al. / Materials Letters 57 (2002) 734–738
2. Experimental The MWNTs were prepared by catalytic decomposition of acetylene. The catalysts were produced by concentrating the nickel nitrate and magnesium nitrate solution to gelation and then grinding it to fine powder after sintering at 400 jC. Carbon deposit was obtained on the catalyst at 700 jC with the flow rate of acetylene 40 ml/min and nitrogen 400 ml/min. The crude was stirred in 3 M nitric acid and refluxed for 24 h at 60 jC, and then was suspended and refluxed in 5 M HCl solution for 6 h at 120 jC. After acid treatment, the samples was calcined in static air at 510 jC with the reaction time of about 60 min, and the pure MWNTs were obtained. The temperature of air oxidation of acid-treated MWNTs was determined by thermogravimetric analysis (TGA) using A Dupont 9900TGA meter with a rate of 10 jC/min from room temperature to 900 jC at an air flow rate of 70 sccm. Five grams acid-treated MWNTs
735
was oxidized by air at the temperature determined by TGA for 60 min. The samples were characterized by transmission electron microscopy (TEM) and XRD.
3. Results and discussion A low-magnification TEM image (Fig. 1a) shows that the raw materials contained carbon nanotubes and non-nanotube materials. From the XRD pattern for raw materials in Fig. 2a, we observe, apart from the graphite peak, the strong metal peaks and the nickel and magnesium oxide peaks, indicating that the raw materials contained much residual metal and metal oxide. After the first purification process by HNO3 treatment, most of the impurities were removed (Fig. 1b). Corresponding XRD pattern in Fig. 2b shows that the metal peaks and the oxide peaks are lower than those in Fig. 2a, suggesting that the metal particles and the oxide particles have been eliminated partially.
Fig. 1. TEM images of: (a) as-produced crude materials, (b) material that was refluxed for 24 h in 3 M HNO3, (c) material produced by treating the HNO3-treated sample in 5 M HCl for 6 h, (d) purified MWNTs produced by oxiding the acid-treated sample for 60 min in air at 510 jC.
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Fig. 2. XRD pattern of: (a) as-produced crude materials, (b) material that was refluxed for 24 h in 3 M HNO3, (c) material produced by treating the HNO3-treated sample in 5 M HCl for 6 h, (d) purified MWNTs produced by oxiding the acid-treated sample for 60 min in air at 510 jC.
The fact that the graphite peak is stronger reveals that the amorphous carbon coating the surface of nanotubes was consumed by oxidation reaction. After the second step of purification by HCl treatment, relatively pure MWNTs were obtained, as shown in Fig. 1c. Comparative with the Fig. 1b, Fig. 1c shows that little of black non-nanotube materials exists. XRD pattern of sample after HCl treatment exhibited detailed features in Fig. 2c. The peaks of MgNiO2 have disappeared, only very weak Ni peak. The results indicate that the two-step acid treatment is effective. It is well known that 3 M HNO3 reflux is very effective in dissolving metal particles. Also, because nitrate acid is a strong oxidizer, amorphous carbon can be removed by oxidation. The selection of 5 M HCl is to dissolve metal oxide. In this work, the Mg – Ni– O catalysts were produced by sintering gelation state of Mg (NO3)2 and Ni (NO3)2 at 400 jC at final step, which can be dissolved by 5 M HCl easily, but not by H2SO4 or HNO3 solution. When we used H2SO4 or HNO3 solution to treat the sample for three times, we found that the MgNiO2 always existed. Thus, 5 M HCl selected here is more effective in removing the metal oxide than H2SO4 or HNO3 solution, which is confirmed by XRD pattern in Fig. 2c. Furthermore, MWNTs need sufficient and nondestructive purification for applications and investiga-
tion of properties. The air oxidation is a necessary step in purification process. The aim of burning acid-treated materials is to purify MWNTs based on the difference of oxidation temperature between CNTs and non-nanotubes. Because atom sheets in CNT are formed by sp2.6 bond, the oxidation temperature is higher than that of amorphous carbon. However, there is no common burning temperature of CNTs since it is not only related to graphitization degree, but also to pre-treatment process. Therefore, to purify MWNTs with high quality and no damage, a suitable burning temperature in air must be determined. TGA is an effective method for determining the burning temperature in air. Fig. 3a is the TGA graph of the sample after acid treatment. It can be found that weight was lost slowly from 20 to 410 jC, corresponding to the loss of little water and a few amorphous carbon. However, some slight increase in weight was observed in the magnified TGA graph, as shown in Fig. 3b. The increase occurred at 27, 170 and 230 jC, respectively, and the increasing amount was estimated to be 0.5– 0.6 wt.%, which might be due to the oxidation of the Mg and Ni. From 20 to 410 jC, there are two stepwise weight loss, which can be assigned to amorphous carbon and carbon particles. At the temperature range from 510 to 645 jC, the weight decreased sharply to 3.92 wt.%, indicating that the combustion of the MWNTs started at 510 jC. Also a thing to note is that the curve slope maintained almost the same value from 510 to 645 jC. It is illustrated that the MWNTs were combusted at a constant speed, suggesting that the MWNTs reached to a high purity at 510 jC. After 645 jC, the weight of the sample remains unchanged. The remainders may be assigned to the metal or metal oxide which are inside the inner of tubes before combustion. Therefore, 510 jC is an optimum temperature to burn out non-nanotube carbon materials. The MWNTs combusted in air at 510 jC for 1 h are typically shown in Fig. 1d, in which high-purity MWNTs can be found. The XRD pattern of the combusted MWNTs (Fig. 2d) shows that the metal peaks and metal oxide peaks have disappeared completely, consistent with the result shown in Fig. 1d. The 002 peak is two times stronger than that of noncombusted MWNTs. This feature can be explained by consumption of amorphous carbon and increase of graphitization degree of the sample during the combustion in air.
X.H. Chen et al. / Materials Letters 57 (2002) 734–738
737
methods applied in different laboratories produce materials with carbon nanotubes, non-nanotube carbon and metal fractions that vary in type and configuration. Up to now, most of purification methods deal with the carbon nanotubes produced by discharge arc or laser vaporization method. Shi et al. [13] reported that the effective temperature to burn out non-nanotubes carbon is 350 jC for purifying single-walled carbon nanotubes prepared by dc arc-discharge method. Purification studies developed by Dillon et al. [17] indicate the raw materials of carbon nanotubes synthesized by laser vaporization method exhibited such a temperature of 785 jC by TGA. Colomer et al. [16] reported that the optimum reaction temperature in air chosen was 500 jC for carbon nanotubes produced by catalytic CVD. It is illustrated from these results that the raw materials of CNTs produced by different catalysts and preparation method are not only various in component, but also different in graphitization degree. Therefore, purification strategies developed by us may not be applicable to their materials. Similarly, others’ techniques are not suitable for our materials. However, it is sure that TGA is an indispensable step for purification of MWNTs produced by any method.
4. Conclusion
Fig. 3. (a) TGA of 5-mg materials after acid treatment ranged from 20 to 900 jC at 10 jC per minute under 70 sccm flowing air. (b) The magnified TGA curve of the sample ranged from 20 to 410 jC.
The three-step purification procedure for MWNTs produced by catalyzed CVD method described here appears to be a simpler and more effective purification process than any previously reported. After the oxidation purification in air at 510 jC for 1 h, TGA studies indicate the chemical purity of 96 wt.%, while TEM image reveals a purity of almost 98%. Presently, there is no good method by which CNTs’ purity may be readily evaluated. In this work, TGA was used to evaluate the purity of the MWNTs, which may be accurately determined on a weight percentage basis. Although many purification methods of MWNTs have been proposed by others [13 – 17], these methods are not currently available since different production
With Ni – Mg – O as catalyst, without reduction of H2, large-scale MWNTs were produced by the CVD method. A three-step purification process for crude materials was proposed. Two-step process of acid treatment is effective to remove metal or metal oxide. The air oxidation is a necessary step in purification process to obtain high-quality MWNTs. The optimum oxidation temperature in air was determined to be 510 by TGA. The purification method developed in this work produces high purity of MWNTs with >96 wt.% without damage and may be a reliable means of purification and evaluation for MWNTs.
Acknowledgements Xiaohua Chen thanks the National Science Foundation of China (No. 59972031) and the National Science Foundation of Hunan Province (No. 01JJY 2052) for financial support.
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X.H. Chen et al. / Materials Letters 57 (2002) 734–738
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
S. Iijima, Nature 354 (1991) 56. W.A. Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. Q.H. Wang, Appl. Phys. Lett. 72 (1998) 2912. A.G. Rinzler, Science 269 (1995) 1550. P.G. Collins, A. Zell, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science 280 (1998) 1744. H.M. Chen, C. Liu, Y.Y. Fan, Science 286 (1999) 1127. M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, et al., Nature 381 (1996) 678. D.H. Robertson, D.W. Brenner, J.W. Mintmire, et al., Phys. Rev., B 45 (1992) 12592.
[10] T.W. Ebbesen, H. Hirua, J. Fujita, et al., Chem. Phys. Lett. 209 (1993) 83. [11] Y. Zhang, S. Minima, Appl. Phys. Lett. 75 (1999) 3087. [12] S. Amelinckx, X.B. Zhang, T. Ichihashi, et al., Science 265 (1994) 635. [13] Z. Shi, Y. Lian, F. Liao, X. Zhou, S. Iijima, Solid State Commun. 112 (1999) 35. [14] P.M. Ajayan, T.W. Ebbesen, T. Ichihashi, Nature 362 (1993) 522. [15] T.W. Ebbesen, P.M. Ajayan, Nature 367 (1994) 519. [16] J.F. Colomer, P. Piedigrosso, A. Fonseca, et al., Synth. Met. 103 (1999) 2482. [17] A.C. Dillon, G. Thomas, K.M. Jones, et al., Adv. Mater. 11 (1999) 1354.