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Linearly joined carbon nanotubes
Synthetic Metals 123 (2001) 381±383
Linearly joined carbon nanotubes Jung Sang Suh*, Jin Seung Lee, Hoseong Kim School of Chemistry & Molecular Engineering, Seoul National University, Seoul 151-747, South Korea Received 30 November 2000; received in revised form 1 December 2000; accepted 22 January 2001
Abstract Linearly joined carbon nanotubes (CNTs) have been fabricated on well-ordered porous anodic aluminum oxide (AAO) nanotemplates prepared by a multi-step anodization and pore widening process. The location and shape of the junctions are very uniform. The electronic properties of the tubes will be affected critically by the junctions. Therefore, one could design CNTs which have speci®c properties by changing the ratio of the two diameters of linearly joined CNTs. The linearly joined tubes could be used in fabrication of nanoscale electronic devices like ®eld-effect transistor (FET). # 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; Multi-step anodization; Field-effect transistor
1. Introduction There has been enormous interest in the area of carbon nanotubes (CNTs) since their discovery [1±6]. The strong interest in CNTs stems from their extraordinary mechanical [2,3] and electronic [4±6] properties. Single-wall carbon nanotubes (SWNT) are known to be either metallic or semiconducting, depending on their helicity [7]. It has been reported that semiconducting SWNTs exhibit ®eld effect transistor(FET)-like characteristics at room temperature [8,9]. Porous anodic aluminum oxide (AAO) templates prepared by a two-step anodization process [10] has been used for fabrication of highly ordered carbon nanotubes [11]. The CNTs were very uniform in diameter, highly ordered, and perfectly vertical with respect to the plane of the template. CNTs fabricated AAO templates are multi-walled tubes [12] which are known to be metallic or semiconductors, depending on the diameters and chiralities of the outer shell [13,14]. Treboux et al. have reported the calculations of the electronic properties of nanotubes joined linearly [15]. By their results, the pair of armchair and zigzag nanotubes joined linearly, is expected to exhibit rectifying behavior. A linear junction of two tubes with different diameters consists of ®ve- and seven-membered rings. These rings act as defects and decrease the conductivity of the joined
* Corresponding author. Fax: 82-2-889-0749. E-mail address: [email protected] (J.S. Suh).
tube. Therefore, by joining CNTs of different diameters, one can modify the electronic characters of CNTs. In this communication, we have reported a method to fabricate linearly joined CNTs using AAO templates and discussed a scheme to fabricate a ®eld-effect transistor (FET). 2. Experimental A high-purity aluminum sheet (99.999%, 0.5 mm thick) was used as a substrate material. It was electropolished in a solution of perchloric acid and ethanol to a mirror ®nish. Clean aluminum sheets were anodized in a 0.3 M oxalic acid solution at 178C at a constant applied voltage of 40 V for 24 h. The resultant aluminum oxide ®lm was subsequently removed by dipping the anodized sheet into an aqueous mixture of phosphoric acid (6 wt.%) and chromic acid (1.8 wt.%) at 608C. A second anodization was performed for 5 h under the same condition as the ®rst one. The pore depth was about 50 mm. The pores of the template were widened by dipping into an aqueous 0.1 M phosphoric acid solution for 40 or 60 min. After pore widening, a third anodization was performed for 5 h under the same conditions as the ®rst one. CNTs were fabricated in the pores by following a published method [11]. After placing the template in a tube furnace, acetylene was pyrolyzed to form CNTs by ¯owing a mixture of acetylene and nitrogen at 7008C for 20 min. Fabricated templates and CNTs were analyzed by using a scanning electron microscopy (SEM; Philips FEG XL, 30 kV).
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 3 1 6 - 2
382
J.S. Suh et al. / Synthetic Metals 123 (2001) 381±383
Fig. 1. An SEM image of the CNTs fabricated on the AAO template prepared by the second anodization, pore widening, and the third anodization processes after ion milling and then partially etching away the aluminum oxide by dipping in acid. Tubes are made in the pores of the template and exhibit a hexagonal pattern. The average diameter is about 75 nm.
3. Results and discussion The SEM images of the CNTs fabricated on the AAO templates prepared by the second anodization, pore widening, and then the third anodization processes are shown in Figs. 1 and 2. The pore widening time in preparation of the templates for Figs. 1 and 2 was 60 and 40 min, respectively. Fig. 1 is a top view of the AAO template fabricated CNTs after ion milling and then partially etching away aluminum oxide by dipping in acid. The surface of the AAO templates fabricated CNTs was covered with the graphitic carbon residue before ion milling. The CNTs are made in the pores
of the AAO template and the top of them is protruded by etching away the aluminum oxide partially. The tubes exhibit a hexagonal pattern and the average diameter is about 75 nm. The intertube distance is about 106 nm and the density of tubes is about 1 1010 /cm2. Fig. 2 is a cross-sectional view of CNTs made in the pores of the AAO template. For observation of it, the template, without ion milling, was bent and then partially etched away the aluminum oxide by dipping in acid. The second-layer tubes are also seen with the ®rst-layer ones. It is clearly seen that two tubes of different diameters are joined linearly. The shape of linearly joined tubes re¯ects that of the pores
Fig. 2. An SEM image of the linearly joined tubes fabricated on a well-ordered AAO template after bending the template and then partially etching away the aluminum oxide by dipping in acid. It is a cross-sectional view of CNTs made in the pores of the AAO template, which are almost perfectly vertical with respect to the template plane. The tubes of the first and second layers are seen together. The larger diameter is about 60 nm and the smaller one is 30 nm. Some tubes are slightly bent due to being etched away alumina surrounded the tubes.
J.S. Suh et al. / Synthetic Metals 123 (2001) 381±383
because the tubes are made in the pores of AAO template as the form of the pores. Linearly joined pores of two different diameters are made by the pore widening and third anodization processes. The larger diameter is about 60 nm and the smaller one is 30 nm. The location and shape of the junctions are very uniform and the curvature of them is very smooth. Some tubes of the smaller diameter are bent. This is due to being etched away alumina surrounded the tubes for observation of SEM images. The CNTs are made in the pores of AAO templates. Therefore, the diameter and length of the CNTs are determined by those of the pores. In a linearly joined CNT, the length of the larger and smaller diameter tubes is determined by the time of the second and third anodizations, respectively. The smaller diameter is determined by the anodization conditions, like the electrolyte, voltage, and temperature of anodization [16] and the larger diameter could be controlled within the interpore distance of the AAO template by changing the pore widening time [17]. Therefore, one could change the ratio of the two diameters in some range. The junction contains ®ve- and seven-membered carbon ring defects and the electronic character of linearly joined CNTs could be affected critically by the ratio of the two diameters. Therefore, one could fabricate the linearly joined CNTs which have speci®c properties by changing the ratio of the two diameters. This method could be applied in fabrication of linearly joined multi-walled CNTs. It is known that the pores of AAO templates prepared by the technique used in this study are very uniform in diameter and almost perfectly vertical with respect to the template plane, and have a well-ordered hexagonal pattern [11]. The CNTs are made in the pores formed in alumina. Therefore, each tube has six nearest neighboring tubes except those at edges and the tubes are surrounded alumina which is an insulator. The intertube distance is about 106 nm and the density of tubes is about 1 1010 /cm2. The separation between the tubes whose diameter is 60 nm is about 46 nm. Base on the SEM images in Figs. 1 and 2, one can con®gure the three-dimensional structure of the linearly joined tubes fabricated on AAO templates. A schematic of these tubes after milling the top and bottom of them is shown in Fig. 3. These well ordered CNTs could be used in fabrication of FET. As shown in Fig. 3, one end of a CNT could work as source and the other end as drain. One or some of the six neighbors could be used as the gate electrode. This is basically the same structure of insulated gate FET. As mentioned previously, the CNTs fabricated on AAO templates are well-ordered and almost perfectly vertical with respect to the template plane, and have a high density. Therefore, the linearly joined CNTs fabricated on AAO templates could be a promising material to be used in fabrication of nanoscale electronic devices like nano-FET. In conclusion, we have fabricated linearly jointed tubes using AAO templates. The electronic properties of linearly joined CNTs could be controlled by changing the ratio of the
383
Fig. 3. A schematic of the fabrication of FET using the linearly joined tubes fabricated on an AAO template.
two diameters. These linearly joined tubes could be used in fabrication of nanoscale electronic devices like FET. Acknowledgements This work was supported by the BK21 program, the nanostructure technology project, and the National Program for Tera-level Nanodevices. References [1] S. Iijma, Nature 354 (1991) 56. [2] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. [3] D.A. Walters, L.M. Ericson, M.J. Casavant, J. Liu, D.T. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (1999) 3803. [4] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [5] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [6] S. Fan, M.G. Chapline, N.M. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [7] R. Saito, G. Dresselhaus, M. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [8] S. Tans, A. Verschueren, C. Dekker, Nature (London) 393 (1998) 49. [9] R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P. Avouris, Appl. Phys. Lett. 73 (1998) 2447. [10] H. Masuda, M. Sotoh, Jpn. J. Appl. Phys. 35 (1996) L126. [11] J.S. Suh, J.S. Lee, Appl. Phys. Lett. 75 (1999) 2047. [12] J. Li, M. Moskovits, T.L. Haslett, Chem. Mater. 10 (1998) 1963. [13] Y.H. Lee, D.H. Kim, H. Kim, B.K. Ju, J. Appl. Phys. 88 (2000) 4181. [14] A. Hassanien, M. Tokumoto, S. Ohshima, Y. Kuriki, F. Ikazaki, K. Uchida, M. Yumura, Appl. Phys. Lett. 75 (1999) 2755. [15] G. Trboux, P. Lapstun, K. Silverbrook, J. Phys. Chem. B 103 (1999) 1871. [16] J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. [17] D. AlMawlawi, N. Coombs, M. Moskovits, J. Appl. Phys. 70 (1991) 4421.
Linearly joined carbon nanotubes Jung Sang Suh*, Jin Seung Lee, Hoseong Kim School of Chemistry & Molecular Engineering, Seoul National University, Seoul 151-747, South Korea Received 30 November 2000; received in revised form 1 December 2000; accepted 22 January 2001
Abstract Linearly joined carbon nanotubes (CNTs) have been fabricated on well-ordered porous anodic aluminum oxide (AAO) nanotemplates prepared by a multi-step anodization and pore widening process. The location and shape of the junctions are very uniform. The electronic properties of the tubes will be affected critically by the junctions. Therefore, one could design CNTs which have speci®c properties by changing the ratio of the two diameters of linearly joined CNTs. The linearly joined tubes could be used in fabrication of nanoscale electronic devices like ®eld-effect transistor (FET). # 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; Multi-step anodization; Field-effect transistor
1. Introduction There has been enormous interest in the area of carbon nanotubes (CNTs) since their discovery [1±6]. The strong interest in CNTs stems from their extraordinary mechanical [2,3] and electronic [4±6] properties. Single-wall carbon nanotubes (SWNT) are known to be either metallic or semiconducting, depending on their helicity [7]. It has been reported that semiconducting SWNTs exhibit ®eld effect transistor(FET)-like characteristics at room temperature [8,9]. Porous anodic aluminum oxide (AAO) templates prepared by a two-step anodization process [10] has been used for fabrication of highly ordered carbon nanotubes [11]. The CNTs were very uniform in diameter, highly ordered, and perfectly vertical with respect to the plane of the template. CNTs fabricated AAO templates are multi-walled tubes [12] which are known to be metallic or semiconductors, depending on the diameters and chiralities of the outer shell [13,14]. Treboux et al. have reported the calculations of the electronic properties of nanotubes joined linearly [15]. By their results, the pair of armchair and zigzag nanotubes joined linearly, is expected to exhibit rectifying behavior. A linear junction of two tubes with different diameters consists of ®ve- and seven-membered rings. These rings act as defects and decrease the conductivity of the joined
* Corresponding author. Fax: 82-2-889-0749. E-mail address: [email protected] (J.S. Suh).
tube. Therefore, by joining CNTs of different diameters, one can modify the electronic characters of CNTs. In this communication, we have reported a method to fabricate linearly joined CNTs using AAO templates and discussed a scheme to fabricate a ®eld-effect transistor (FET). 2. Experimental A high-purity aluminum sheet (99.999%, 0.5 mm thick) was used as a substrate material. It was electropolished in a solution of perchloric acid and ethanol to a mirror ®nish. Clean aluminum sheets were anodized in a 0.3 M oxalic acid solution at 178C at a constant applied voltage of 40 V for 24 h. The resultant aluminum oxide ®lm was subsequently removed by dipping the anodized sheet into an aqueous mixture of phosphoric acid (6 wt.%) and chromic acid (1.8 wt.%) at 608C. A second anodization was performed for 5 h under the same condition as the ®rst one. The pore depth was about 50 mm. The pores of the template were widened by dipping into an aqueous 0.1 M phosphoric acid solution for 40 or 60 min. After pore widening, a third anodization was performed for 5 h under the same conditions as the ®rst one. CNTs were fabricated in the pores by following a published method [11]. After placing the template in a tube furnace, acetylene was pyrolyzed to form CNTs by ¯owing a mixture of acetylene and nitrogen at 7008C for 20 min. Fabricated templates and CNTs were analyzed by using a scanning electron microscopy (SEM; Philips FEG XL, 30 kV).
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 3 1 6 - 2
382
J.S. Suh et al. / Synthetic Metals 123 (2001) 381±383
Fig. 1. An SEM image of the CNTs fabricated on the AAO template prepared by the second anodization, pore widening, and the third anodization processes after ion milling and then partially etching away the aluminum oxide by dipping in acid. Tubes are made in the pores of the template and exhibit a hexagonal pattern. The average diameter is about 75 nm.
3. Results and discussion The SEM images of the CNTs fabricated on the AAO templates prepared by the second anodization, pore widening, and then the third anodization processes are shown in Figs. 1 and 2. The pore widening time in preparation of the templates for Figs. 1 and 2 was 60 and 40 min, respectively. Fig. 1 is a top view of the AAO template fabricated CNTs after ion milling and then partially etching away aluminum oxide by dipping in acid. The surface of the AAO templates fabricated CNTs was covered with the graphitic carbon residue before ion milling. The CNTs are made in the pores
of the AAO template and the top of them is protruded by etching away the aluminum oxide partially. The tubes exhibit a hexagonal pattern and the average diameter is about 75 nm. The intertube distance is about 106 nm and the density of tubes is about 1 1010 /cm2. Fig. 2 is a cross-sectional view of CNTs made in the pores of the AAO template. For observation of it, the template, without ion milling, was bent and then partially etched away the aluminum oxide by dipping in acid. The second-layer tubes are also seen with the ®rst-layer ones. It is clearly seen that two tubes of different diameters are joined linearly. The shape of linearly joined tubes re¯ects that of the pores
Fig. 2. An SEM image of the linearly joined tubes fabricated on a well-ordered AAO template after bending the template and then partially etching away the aluminum oxide by dipping in acid. It is a cross-sectional view of CNTs made in the pores of the AAO template, which are almost perfectly vertical with respect to the template plane. The tubes of the first and second layers are seen together. The larger diameter is about 60 nm and the smaller one is 30 nm. Some tubes are slightly bent due to being etched away alumina surrounded the tubes.
J.S. Suh et al. / Synthetic Metals 123 (2001) 381±383
because the tubes are made in the pores of AAO template as the form of the pores. Linearly joined pores of two different diameters are made by the pore widening and third anodization processes. The larger diameter is about 60 nm and the smaller one is 30 nm. The location and shape of the junctions are very uniform and the curvature of them is very smooth. Some tubes of the smaller diameter are bent. This is due to being etched away alumina surrounded the tubes for observation of SEM images. The CNTs are made in the pores of AAO templates. Therefore, the diameter and length of the CNTs are determined by those of the pores. In a linearly joined CNT, the length of the larger and smaller diameter tubes is determined by the time of the second and third anodizations, respectively. The smaller diameter is determined by the anodization conditions, like the electrolyte, voltage, and temperature of anodization [16] and the larger diameter could be controlled within the interpore distance of the AAO template by changing the pore widening time [17]. Therefore, one could change the ratio of the two diameters in some range. The junction contains ®ve- and seven-membered carbon ring defects and the electronic character of linearly joined CNTs could be affected critically by the ratio of the two diameters. Therefore, one could fabricate the linearly joined CNTs which have speci®c properties by changing the ratio of the two diameters. This method could be applied in fabrication of linearly joined multi-walled CNTs. It is known that the pores of AAO templates prepared by the technique used in this study are very uniform in diameter and almost perfectly vertical with respect to the template plane, and have a well-ordered hexagonal pattern [11]. The CNTs are made in the pores formed in alumina. Therefore, each tube has six nearest neighboring tubes except those at edges and the tubes are surrounded alumina which is an insulator. The intertube distance is about 106 nm and the density of tubes is about 1 1010 /cm2. The separation between the tubes whose diameter is 60 nm is about 46 nm. Base on the SEM images in Figs. 1 and 2, one can con®gure the three-dimensional structure of the linearly joined tubes fabricated on AAO templates. A schematic of these tubes after milling the top and bottom of them is shown in Fig. 3. These well ordered CNTs could be used in fabrication of FET. As shown in Fig. 3, one end of a CNT could work as source and the other end as drain. One or some of the six neighbors could be used as the gate electrode. This is basically the same structure of insulated gate FET. As mentioned previously, the CNTs fabricated on AAO templates are well-ordered and almost perfectly vertical with respect to the template plane, and have a high density. Therefore, the linearly joined CNTs fabricated on AAO templates could be a promising material to be used in fabrication of nanoscale electronic devices like nano-FET. In conclusion, we have fabricated linearly jointed tubes using AAO templates. The electronic properties of linearly joined CNTs could be controlled by changing the ratio of the
383
Fig. 3. A schematic of the fabrication of FET using the linearly joined tubes fabricated on an AAO template.
two diameters. These linearly joined tubes could be used in fabrication of nanoscale electronic devices like FET. Acknowledgements This work was supported by the BK21 program, the nanostructure technology project, and the National Program for Tera-level Nanodevices. References [1] S. Iijma, Nature 354 (1991) 56. [2] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. [3] D.A. Walters, L.M. Ericson, M.J. Casavant, J. Liu, D.T. Colbert, K.A. Smith, R.E. Smalley, Appl. Phys. Lett. 74 (1999) 3803. [4] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [5] W. Zhu, C. Bower, O. Zhou, G. Kochanski, S. Jin, Appl. Phys. Lett. 75 (1999) 873. [6] S. Fan, M.G. Chapline, N.M. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [7] R. Saito, G. Dresselhaus, M. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [8] S. Tans, A. Verschueren, C. Dekker, Nature (London) 393 (1998) 49. [9] R. Martel, T. Schmidt, H.R. Shea, T. Hertel, P. Avouris, Appl. Phys. Lett. 73 (1998) 2447. [10] H. Masuda, M. Sotoh, Jpn. J. Appl. Phys. 35 (1996) L126. [11] J.S. Suh, J.S. Lee, Appl. Phys. Lett. 75 (1999) 2047. [12] J. Li, M. Moskovits, T.L. Haslett, Chem. Mater. 10 (1998) 1963. [13] Y.H. Lee, D.H. Kim, H. Kim, B.K. Ju, J. Appl. Phys. 88 (2000) 4181. [14] A. Hassanien, M. Tokumoto, S. Ohshima, Y. Kuriki, F. Ikazaki, K. Uchida, M. Yumura, Appl. Phys. Lett. 75 (1999) 2755. [15] G. Trboux, P. Lapstun, K. Silverbrook, J. Phys. Chem. B 103 (1999) 1871. [16] J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969) 365. [17] D. AlMawlawi, N. Coombs, M. Moskovits, J. Appl. Phys. 70 (1991) 4421.