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Carbon nanotubes, nanofilaments and nanobeads by thermal chemical vapor deposition process
Materials Science and Engineering B96 (2002) 24 /28 www.elsevier.com/locate/mseb
Carbon nanotubes, nanofilaments and nanobeads by thermal chemical vapor deposition process Debabrata Pradhan, Maheshwar Sharon * Department of Chemistry, Indian Institute of Technology, Mumbai 400076, India Received 6 May 2002; accepted 21 June 2002
Abstract From commercial kerosene, straight and coiled nanotubes, nanofilaments and nanobeads are grown by suitably adjusting pyrolysis temperature and catalysts by the thermal chemical vapor deposition (CVD) technique. Nickel and iron facilitate the growth of straight and coiled nanotubes, respectively, at a temperature of 1000 8C. With an increase of pyrolysis temperature to 1100 8C, carbon nanobeads resulted in the presence of both the catalysts. Although, coiled nanotubes are formed at 900 8C in the presence of iron catalysts but nickel gives nanofilaments of diameter in the range 80 /100 nm at the same temperature. The crystallinity and thermal properties are studied. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanotubes; Nanobeads; Nanofilaments; Kerosene; CVD
1. Introduction With the discovery of carbon nanotubes [1], the science of nanotechnology takes a new shape and direction. Most of the nanomaterials synthesized in the last few decades has been tested for many applications and found superior than their micro- or macro-size counterparts. Carbon nanotubes is one such material having diameter in nanorange and length from micro- to millimeter. With this high aspect ratio, it has many attracting electronic properties depending upon diameter and chirality [2]. Due to its outstanding chemical, mechanical and electronic properties, this material has been found promising for the application like flat panel display [3,4], hydrogen storage [5,6], chemical force sensors [7,8], scanning probes [9,10] and many other nanoelectronic devices [11 /13]. Arc-vaporization [14], laser ablation [15] and thermal chemical vapor deposition (CVD) [16] methods are three techniques predominately used for the synthesis of these materials. Out of these three, thermal CVD method has proved suitable for the industrial production of any such carbon nano-
* Corresponding author. Tel.: /91-22-576-7174; fax: /91-22-5767152 E-mail address: [email protected] (M. Sharon).
or micro-material, because growth can be continuous as against the batch process [17]. It has also been observed that nature of carbon material varies with synthetic route, precursor and catalysts used in the growth process. Numerous studies have been reported for the production of carbon nanotubes. In all these cases, pure precursors like hydrocarbon (benzene, toluene, etc.) and gases (methane, acetylene, etc.) have been utilized for the production of nanotubes [18 /20]. There are not many reports available on the use of precursor like kerosene for the production of nanomaterials [21,22]. Keeping this in view, kerosene has been used as a precursor for this study. Kerosene, a crude petrochemical oil, contains mixture of many short- and long-chain aliphatic and aromatic organic hydrocarbons. This source has been chosen for this study because it is widely available and of low cost.
2. Experimental For the thermal CVD process, two 1 ft. long electric furnace and 1 m long quartz tube kept inside the furnace has been used (Fig. 1). Precursor and catalyst powder containing boat was kept in their respective position. An inert gas (nitrogen) was purged through the tube to replace air present inside. After 15/20 min of purging
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 0 9 - 4
D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
25
Fig. 1. Schematic diagram of experimental set-up for pyrolysis of kerosene (A */nitrogen gas; B */flow meter; C */water bubbler; F1 */vaporization furnace; F2 */pyrolysis furnace; T1, T2 */thermocouple).
gas, second furnace (F2) was heated to a desired set pyrolysis temperature. When the set temperature of this furnace was achieved, furnace containing kerosene (F1) was heated to 200 8C for its vaporization. The vaporized kerosene gas was carried by inert gas towards the high-temperature zone of the second furnace (F2) for its pyrolysis in the presence of catalysts. Nitrogen was used as an inert carrier gas. In each run, 4 /5 ml of kerosene was taken for vaporization. Iron particles (300 mesh) or nickel particles (100 mesh) were used as catalysts for this study in the temperature range 900 /1100 8C. Catalyst particles were spread over the boat and kept inside the quartz tube of pyrolysing furnace. After the pyrolysis of kerosene vapors in the presence of catalysts, powder deposited near catalyst particles were collected and characterized by transmission electron microscope (Philips CM 200) at an operating voltage of 200 kV, X-ray diffraction and thermal differential analysis.
catalyst, respectively. Nanotubes formed at 1000 8C in presence of iron catalyst is exactly similar to those formed at 900 8C except their average diameter in-
3. Results and discussion 3.1. Transmission electron microscopic studies Fig. 2a, and b and c shows the products formed by pyrolysing kerosene vapors at 900 8C in the presence of iron and nickel catalyst particles, respectively, in nitrogen atmosphere. Although, iron gives coiled nanotubes having many joints at regular interval but nickel yields large quantity of nanofilaments. Around 1.5 /2 g of carbon nanofilaments were found in the boat out of 4/5 ml of kerosene in all experiments. There were some depositions in the inner wall of reactor quartz tube as well. The diameters of these nanotubes (Fig. 2a) are in the range 40/60 nm. It was difficult to measure the average length of these tubes due to their coiling nature. For the same reason, the length of nanofilaments of diameter 80/100 nm (Fig. 2b and c) could also not be measured. These nanofilaments are highly conducting appears to be made of interconnected plate-like structure. Fig. 3a and b shows coiled and straight nanotubes formed at 1000 8C in the presence of iron and nickel
Fig. 2. TEM images of (a) coiled nanotubes in the presence of iron particles (b, c) nanofilaments in the presence of nickel particles at 900 8C.
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D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
Fig. 3. TEM images of carbon nanotubes at pyrolysis temperature 1000 8C in presence of (a) iron particles and (b) nickel particles.
creased to 100 /125 nm. The outer and inner diameters of these tubes are 100/125 and 10 /20 nm, respectively, which indicates very thick wall having small inner diameter. Contrast to this, nickel catalyst gives very straight and well-defined tubes of outer and inner diameter in the range 80 /100 and 60/80 nm, respectively. Most of these nanotubes are open-ended with very wide diameter. Fig. 4a/c shows image of open-ended tube, bent Vshaped buckled tube and an unusual tube-like growth inside a nanotube. All these tubes were formed at 1000 8C and in the presence of nickel catalyst. These multi-walled open-ended nanotubes may be suitable for field emission studies. With increase of pyrolysis temperature to 1100 8C, both catalysts yield spherical nanobeads of size 600 /900 nm (Fig. 5). These nanobeads are rarely reported in literature [23,24] grown by CVD method. These are highly conducting and have very good thermal stability.
Fig. 4. TEM images of (a) open-ended (b) bent, V-shaped (c) inner novel growth of carbon nanotubes at 1000 8C in the presence of nickel particles.
3.2. X-ray diffraction studies X-ray diffraction pattern of straight CNTs, coiled CNTs and carbon beads are shown in Fig. 6. The sharp highest intensity peak is observed at an angle of 25.268 ˚ larger by 0.1 A ˚ than the ideal having d0 0 2 /3.4 A graphite crystal [25]. This wide interlayer spacing is due to the defect and some disorderliness in CNTs like
Fig. 5. TEM image of carbon beads.
D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
27
Fig. 7. Differential thermal analysis of (a) straight CNTs, (b) coiled CNTs and (c) carbon microbeads.
4. Conclusions
Fig. 6. XRD spectra of (a) straight CNTs, (b) coiled CNTs and (c) carbon microbeads.
carbon fibers with turbostratic structure [26]. In addition to sharp and symmetric 0 0 l peaks, slightly asymmetric d1 0 0 and d1 0 1 reflections are observed, those are associated with the two-dimensional periodicity of the layers that normally found in CNTs [25,27]. In case of carbon beads, a broad peak indicates its partial crystallinity or particles of very small size. 3.3. Thermal analysis TG and DT analyses were performed with an air flow rate of 30 ml min 1 and a constant heating rate of 10 8C min 1. The corresponding oxidation onset points of straight CNTs, coiled CNTs and carbon beads are at around 580, 600 and 740 8C, respectively (Fig. 7). The oxidation temperatures of CNTs are significantly lower than that of CNTs obtained by arc-discharge [28] but higher than those obtained by Sui et al. [29] in thermal CVD process. The higher onset oxidation temperature of coiled CNTs may be due to its thicker wall. Carbon beads show a much higher onset temperature of oxidation indicating that it consists of numerous carbon nanoparticles that can be stable in air at temperature higher than 800 8C [28].
A precursor (kerosene) containing mixture of many small- and long-chain aliphatic and aromatic compounds has been used successfully for the production of carbon nanomaterials. Coiled and open-ended nanotubes are formed in the presence of iron and nickel catalysts, respectively. The inner diameters of coiled nanotubes are narrow but wide in case of straight nanotubes. X-rays and thermal studies revealed the crystallinity and stability of these carbon nanomaterials.
Acknowledgements D.P. is thankful to Indian Institute of Technology, Mumbai, for providing him scholarship to carry out this work and Prof. I. Samajdar for his help in TEM characterization.
References [1] S. Iijima, Nature 354 (1991) 56. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes(Chapter 4), Imperial College Press, London, 1999. [3] P.G. Collins, A. Zettl, Phys. Rev. B 55 (1997) 9391. [4] X.P. Xu, G.R. Brandes, Appl. Phys. Lett. 74 (1999) 2549. [5] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [6] H.M. Cheng, Q.H. Yong, C. Liu, Carbon 39 (2001) 1447. [7] S. Wong, E. Joselevich, A. Wooley, C. Cheung, C. Lieber, Nature 394 (1998) 52.
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D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
[8] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622. [9] H. Dai, N. Franklin, J. Han, Appl. Phys. Lett. 73 (1998) 1508. [10] J.H. Hafner, Chin-Li Cheung, T.H. Oosterkamp, C.M. Lieber, J. Phys. Chem. B 105 (2001) 743. [11] E. Frackowiak, K. Metenier, V. Bertagna, F. Beguin, Appl. Phys. Lett. 77 (2000) 2421. [12] R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P.H. Avouris, Appl. Phys. Lett. 73 (1998) 2447. [13] S. Tans, A. Verschueren, C. Dekker, Nature 393 (1998) 49. [14] D.S. Bethune, C.H. Kiang, M. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [15] A. Thess, R. Lee, P. Nickolaev, H.J. Dai, P. Petit, J. Robert, C.H. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fisher, R.E. Smalley, Science 273 (1996) 483. [16] H.J. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 260 (1996) 471. [17] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Quian, X. Fan, E.C. Dickey, J. Chen, Chem. Phys. Lett. 303 (1999) 467. [18] D. Bernaerts, X.B. Zhang, X.F. Zhang, S. Amelinckx, G.V. Tendeloo, J.V. Landuyt, V. Ivanov, J.B. Nagy, Phil. Mag. A 71 (1995) 605.
[19] M. Okai, T. Muneyoshi, T. Yaguchi, S. Sasaki, Appl. Phys. Lett. 77 (21) (2000) 3468. [20] K. Mukhopadhyay, A. Koshio, T. Sugai, N. Tanaka, H. Shinohara, Z. Konya, J.B. Nagy, Chem. Phys. Lett. 303 (1999) 117. [21] M. Sharon, M. Kumar, P.D. Kichambare, Y. Ando, X. Zhao, Diamond Films Technol. 8 (1998) 143. [22] M. Kumar, P.D. Kichambare, M. Sharon, Y. Ando, X. Zhao, Mater. Res. Bull. 34 (1999) 791. [23] R.L. Jacobsen, M. Monthious, Nature 385 (1997) 211. [24] M. Sharon, K. Mukhopadhyay, K. Yase, S. Iijima, Y. Ando, X. Zhao, Carbon 36 (1998) 507. [25] Y. Saito, T. Yoshikawa, S. Babdow, M. Tomita, T. Hayashi, Phys. Rev. B 48 (1993) 1907. [26] J.S. Speck, M. Endo, M.S. Dresselhaus, J. Cryst. Growth 94 (1989) 834. [27] D. Reznik, C.H. Olk, D.A. Neumann, J.R.D. Copley, Phys. Rev. B 52 (1995) 116. [28] P.M. Ajayan, T.W. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Nature 362 (1993) 522. [29] Y.C. Sui, D.R. Acosta, J.A. Gonzalez-Leon, A. Bermudez, J. Feuchtwanger, B.Z. Cui, J.O. Flores, J.M. Saniger, J. Phys. Chem. B 105 (2001) 1523.
Carbon nanotubes, nanofilaments and nanobeads by thermal chemical vapor deposition process Debabrata Pradhan, Maheshwar Sharon * Department of Chemistry, Indian Institute of Technology, Mumbai 400076, India Received 6 May 2002; accepted 21 June 2002
Abstract From commercial kerosene, straight and coiled nanotubes, nanofilaments and nanobeads are grown by suitably adjusting pyrolysis temperature and catalysts by the thermal chemical vapor deposition (CVD) technique. Nickel and iron facilitate the growth of straight and coiled nanotubes, respectively, at a temperature of 1000 8C. With an increase of pyrolysis temperature to 1100 8C, carbon nanobeads resulted in the presence of both the catalysts. Although, coiled nanotubes are formed at 900 8C in the presence of iron catalysts but nickel gives nanofilaments of diameter in the range 80 /100 nm at the same temperature. The crystallinity and thermal properties are studied. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanotubes; Nanobeads; Nanofilaments; Kerosene; CVD
1. Introduction With the discovery of carbon nanotubes [1], the science of nanotechnology takes a new shape and direction. Most of the nanomaterials synthesized in the last few decades has been tested for many applications and found superior than their micro- or macro-size counterparts. Carbon nanotubes is one such material having diameter in nanorange and length from micro- to millimeter. With this high aspect ratio, it has many attracting electronic properties depending upon diameter and chirality [2]. Due to its outstanding chemical, mechanical and electronic properties, this material has been found promising for the application like flat panel display [3,4], hydrogen storage [5,6], chemical force sensors [7,8], scanning probes [9,10] and many other nanoelectronic devices [11 /13]. Arc-vaporization [14], laser ablation [15] and thermal chemical vapor deposition (CVD) [16] methods are three techniques predominately used for the synthesis of these materials. Out of these three, thermal CVD method has proved suitable for the industrial production of any such carbon nano-
* Corresponding author. Tel.: /91-22-576-7174; fax: /91-22-5767152 E-mail address: [email protected] (M. Sharon).
or micro-material, because growth can be continuous as against the batch process [17]. It has also been observed that nature of carbon material varies with synthetic route, precursor and catalysts used in the growth process. Numerous studies have been reported for the production of carbon nanotubes. In all these cases, pure precursors like hydrocarbon (benzene, toluene, etc.) and gases (methane, acetylene, etc.) have been utilized for the production of nanotubes [18 /20]. There are not many reports available on the use of precursor like kerosene for the production of nanomaterials [21,22]. Keeping this in view, kerosene has been used as a precursor for this study. Kerosene, a crude petrochemical oil, contains mixture of many short- and long-chain aliphatic and aromatic organic hydrocarbons. This source has been chosen for this study because it is widely available and of low cost.
2. Experimental For the thermal CVD process, two 1 ft. long electric furnace and 1 m long quartz tube kept inside the furnace has been used (Fig. 1). Precursor and catalyst powder containing boat was kept in their respective position. An inert gas (nitrogen) was purged through the tube to replace air present inside. After 15/20 min of purging
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 0 9 - 4
D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
25
Fig. 1. Schematic diagram of experimental set-up for pyrolysis of kerosene (A */nitrogen gas; B */flow meter; C */water bubbler; F1 */vaporization furnace; F2 */pyrolysis furnace; T1, T2 */thermocouple).
gas, second furnace (F2) was heated to a desired set pyrolysis temperature. When the set temperature of this furnace was achieved, furnace containing kerosene (F1) was heated to 200 8C for its vaporization. The vaporized kerosene gas was carried by inert gas towards the high-temperature zone of the second furnace (F2) for its pyrolysis in the presence of catalysts. Nitrogen was used as an inert carrier gas. In each run, 4 /5 ml of kerosene was taken for vaporization. Iron particles (300 mesh) or nickel particles (100 mesh) were used as catalysts for this study in the temperature range 900 /1100 8C. Catalyst particles were spread over the boat and kept inside the quartz tube of pyrolysing furnace. After the pyrolysis of kerosene vapors in the presence of catalysts, powder deposited near catalyst particles were collected and characterized by transmission electron microscope (Philips CM 200) at an operating voltage of 200 kV, X-ray diffraction and thermal differential analysis.
catalyst, respectively. Nanotubes formed at 1000 8C in presence of iron catalyst is exactly similar to those formed at 900 8C except their average diameter in-
3. Results and discussion 3.1. Transmission electron microscopic studies Fig. 2a, and b and c shows the products formed by pyrolysing kerosene vapors at 900 8C in the presence of iron and nickel catalyst particles, respectively, in nitrogen atmosphere. Although, iron gives coiled nanotubes having many joints at regular interval but nickel yields large quantity of nanofilaments. Around 1.5 /2 g of carbon nanofilaments were found in the boat out of 4/5 ml of kerosene in all experiments. There were some depositions in the inner wall of reactor quartz tube as well. The diameters of these nanotubes (Fig. 2a) are in the range 40/60 nm. It was difficult to measure the average length of these tubes due to their coiling nature. For the same reason, the length of nanofilaments of diameter 80/100 nm (Fig. 2b and c) could also not be measured. These nanofilaments are highly conducting appears to be made of interconnected plate-like structure. Fig. 3a and b shows coiled and straight nanotubes formed at 1000 8C in the presence of iron and nickel
Fig. 2. TEM images of (a) coiled nanotubes in the presence of iron particles (b, c) nanofilaments in the presence of nickel particles at 900 8C.
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D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
Fig. 3. TEM images of carbon nanotubes at pyrolysis temperature 1000 8C in presence of (a) iron particles and (b) nickel particles.
creased to 100 /125 nm. The outer and inner diameters of these tubes are 100/125 and 10 /20 nm, respectively, which indicates very thick wall having small inner diameter. Contrast to this, nickel catalyst gives very straight and well-defined tubes of outer and inner diameter in the range 80 /100 and 60/80 nm, respectively. Most of these nanotubes are open-ended with very wide diameter. Fig. 4a/c shows image of open-ended tube, bent Vshaped buckled tube and an unusual tube-like growth inside a nanotube. All these tubes were formed at 1000 8C and in the presence of nickel catalyst. These multi-walled open-ended nanotubes may be suitable for field emission studies. With increase of pyrolysis temperature to 1100 8C, both catalysts yield spherical nanobeads of size 600 /900 nm (Fig. 5). These nanobeads are rarely reported in literature [23,24] grown by CVD method. These are highly conducting and have very good thermal stability.
Fig. 4. TEM images of (a) open-ended (b) bent, V-shaped (c) inner novel growth of carbon nanotubes at 1000 8C in the presence of nickel particles.
3.2. X-ray diffraction studies X-ray diffraction pattern of straight CNTs, coiled CNTs and carbon beads are shown in Fig. 6. The sharp highest intensity peak is observed at an angle of 25.268 ˚ larger by 0.1 A ˚ than the ideal having d0 0 2 /3.4 A graphite crystal [25]. This wide interlayer spacing is due to the defect and some disorderliness in CNTs like
Fig. 5. TEM image of carbon beads.
D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
27
Fig. 7. Differential thermal analysis of (a) straight CNTs, (b) coiled CNTs and (c) carbon microbeads.
4. Conclusions
Fig. 6. XRD spectra of (a) straight CNTs, (b) coiled CNTs and (c) carbon microbeads.
carbon fibers with turbostratic structure [26]. In addition to sharp and symmetric 0 0 l peaks, slightly asymmetric d1 0 0 and d1 0 1 reflections are observed, those are associated with the two-dimensional periodicity of the layers that normally found in CNTs [25,27]. In case of carbon beads, a broad peak indicates its partial crystallinity or particles of very small size. 3.3. Thermal analysis TG and DT analyses were performed with an air flow rate of 30 ml min 1 and a constant heating rate of 10 8C min 1. The corresponding oxidation onset points of straight CNTs, coiled CNTs and carbon beads are at around 580, 600 and 740 8C, respectively (Fig. 7). The oxidation temperatures of CNTs are significantly lower than that of CNTs obtained by arc-discharge [28] but higher than those obtained by Sui et al. [29] in thermal CVD process. The higher onset oxidation temperature of coiled CNTs may be due to its thicker wall. Carbon beads show a much higher onset temperature of oxidation indicating that it consists of numerous carbon nanoparticles that can be stable in air at temperature higher than 800 8C [28].
A precursor (kerosene) containing mixture of many small- and long-chain aliphatic and aromatic compounds has been used successfully for the production of carbon nanomaterials. Coiled and open-ended nanotubes are formed in the presence of iron and nickel catalysts, respectively. The inner diameters of coiled nanotubes are narrow but wide in case of straight nanotubes. X-rays and thermal studies revealed the crystallinity and stability of these carbon nanomaterials.
Acknowledgements D.P. is thankful to Indian Institute of Technology, Mumbai, for providing him scholarship to carry out this work and Prof. I. Samajdar for his help in TEM characterization.
References [1] S. Iijima, Nature 354 (1991) 56. [2] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes(Chapter 4), Imperial College Press, London, 1999. [3] P.G. Collins, A. Zettl, Phys. Rev. B 55 (1997) 9391. [4] X.P. Xu, G.R. Brandes, Appl. Phys. Lett. 74 (1999) 2549. [5] C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [6] H.M. Cheng, Q.H. Yong, C. Liu, Carbon 39 (2001) 1447. [7] S. Wong, E. Joselevich, A. Wooley, C. Cheung, C. Lieber, Nature 394 (1998) 52.
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D. Pradhan, M. Sharon / Materials Science and Engineering B96 (2002) 24 /28
[8] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622. [9] H. Dai, N. Franklin, J. Han, Appl. Phys. Lett. 73 (1998) 1508. [10] J.H. Hafner, Chin-Li Cheung, T.H. Oosterkamp, C.M. Lieber, J. Phys. Chem. B 105 (2001) 743. [11] E. Frackowiak, K. Metenier, V. Bertagna, F. Beguin, Appl. Phys. Lett. 77 (2000) 2421. [12] R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P.H. Avouris, Appl. Phys. Lett. 73 (1998) 2447. [13] S. Tans, A. Verschueren, C. Dekker, Nature 393 (1998) 49. [14] D.S. Bethune, C.H. Kiang, M. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [15] A. Thess, R. Lee, P. Nickolaev, H.J. Dai, P. Petit, J. Robert, C.H. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tomanek, J.E. Fisher, R.E. Smalley, Science 273 (1996) 483. [16] H.J. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 260 (1996) 471. [17] R. Andrews, D. Jacques, A.M. Rao, F. Derbyshire, D. Quian, X. Fan, E.C. Dickey, J. Chen, Chem. Phys. Lett. 303 (1999) 467. [18] D. Bernaerts, X.B. Zhang, X.F. Zhang, S. Amelinckx, G.V. Tendeloo, J.V. Landuyt, V. Ivanov, J.B. Nagy, Phil. Mag. A 71 (1995) 605.
[19] M. Okai, T. Muneyoshi, T. Yaguchi, S. Sasaki, Appl. Phys. Lett. 77 (21) (2000) 3468. [20] K. Mukhopadhyay, A. Koshio, T. Sugai, N. Tanaka, H. Shinohara, Z. Konya, J.B. Nagy, Chem. Phys. Lett. 303 (1999) 117. [21] M. Sharon, M. Kumar, P.D. Kichambare, Y. Ando, X. Zhao, Diamond Films Technol. 8 (1998) 143. [22] M. Kumar, P.D. Kichambare, M. Sharon, Y. Ando, X. Zhao, Mater. Res. Bull. 34 (1999) 791. [23] R.L. Jacobsen, M. Monthious, Nature 385 (1997) 211. [24] M. Sharon, K. Mukhopadhyay, K. Yase, S. Iijima, Y. Ando, X. Zhao, Carbon 36 (1998) 507. [25] Y. Saito, T. Yoshikawa, S. Babdow, M. Tomita, T. Hayashi, Phys. Rev. B 48 (1993) 1907. [26] J.S. Speck, M. Endo, M.S. Dresselhaus, J. Cryst. Growth 94 (1989) 834. [27] D. Reznik, C.H. Olk, D.A. Neumann, J.R.D. Copley, Phys. Rev. B 52 (1995) 116. [28] P.M. Ajayan, T.W. Ebbesen, T. Ichihashi, S. Iijima, K. Tanigaki, H. Hiura, Nature 362 (1993) 522. [29] Y.C. Sui, D.R. Acosta, J.A. Gonzalez-Leon, A. Bermudez, J. Feuchtwanger, B.Z. Cui, J.O. Flores, J.M. Saniger, J. Phys. Chem. B 105 (2001) 1523.