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Coaxial nanocables: Fe nanowires encapsulated in BN nanotubes with intermediate C layers
14 December 2001
Chemical Physics Letters 350 (2001) 1±5 www.elsevier.com/locate/cplett
Coaxial nanocables: Fe nanowires encapsulated in BN nanotubes with intermediate C layers Renzhi Ma *, Yoshio Bando, Tadao Sato National Institute for Materials Science, Advanced Materials Laboratory and Nanomaterials Laboratory, Namiki1-1, Tsukuba, Ibaraki 305-0044, Japan Received 18 September 2001; in ®nal form 18 October 2001
Abstract Fe-®lled BN nanocables with intermediate C layers were fabricated. Complete phase separation between C and BN was distinguished using electron energy loss spectroscopy (EELS) measurements and high-resolution transmission electron microscopy (HRTEM) images by the characteristics of well-ordered BN layers and turbostratic C ones. A twostep growth model is proposed for the formation of this remarkable structure. The idea of introducing intermediate C layer, overcoming the wetting restriction of BN surface, may be employed for fabrication of BN nanocables ®lled with other metal cores. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Since the discovery of carbon (C) nanotubes, much interest has been attracted to building nanotube-based devices [1±3]. Filling the inner cores of C nanotubes with metals or compounds is an eective route to exploit one-dimensional nanocables with various uses [4±7]. For example, nanowires constructed from magnetic materials (e.g., Fe, Co, Ni) are of particular interest, since they are likely to be used in high-density data storage devices. However, Fe nanowires are subjected to oxidation due to the ®ne dimension of Fe. Some authors have demonstrated the idea to synthesize Fe-®lled C nanotubes, whereby the pro-
*
Corresponding author. Fax: +81-298-51-6280. E-mail address: [email protected] (R. Ma).
tective carbon coating ensures that Fe is retained in a reduced state and magnetic coercivity may be enhanced [8,9]. BN nanocables, on the other hand, are also of potential use for nanoscale electronic devices and nanostructured ceramic materials because BN sheaths are insulating, chemically inert, oxidation- and corrosion-resistant [10±12]. Very recently, some success has been achieved in ®lling BN nanotubes with metallic species (Mo, Fe±Ni) cores via substitution and con®ned reaction from C nanotube templates [10,11]. There has also been a report on electrochemical deposition of Cu into BN nanotubes with relatively large diameters (300 nm) [12]. Nevertheless, signi®cant diculties still exist in directly fabricating BN nanocables with metal cores due to the poor wetting property of BN to metals. We report in this Letter that Fe-®lled BN nanocables could be achieved by introducing some
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 1 2 7 4 - X
2
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
intermediate C layers. The intermediate C layers ensure that the Fe nanowire is retained in a reduced state, whereas, outmost BN shielding makes the structure potential in a chemically aggressive environment or at elevated temperature. The revealed nanocable comprises multiple phase separation (nanoheterojunction) in radial direction, i.e., Fe core (metal)/intermediate C (semimetal)/ outmost BN (insulator). We also found that clear phase separation between C and BN could be easily distinguished by microscopic images due to the characteristics of well-ordered BN layers and turbostratic C ones. 2. Experimental The reaction experiment was carried out in an induction furnace with a graphite susceptor as a heating unit. The precursor B4 N3 O2 H [13,14] was charged into the central zone of the graphite susceptor and itself was placed in a quartz tube. A ¯ow of N2 gas current (1 l/min) was ®rst allowed to bubble through a 0.05 M aqueous solution of iron nitrate Fe NO3 3 to take up this solution into the ¯ow and pass carbon black species at 1400 °C to generate CO via C H2 O ! CO H2 ; it was then introduced into the susceptor after the temperature of the precursor zone reached 1700 °C. The boron oxide B2 O3 or B2 O2 vapor decomposed from the precursor was reduced by the N2 (CO) atmosphere into BN or B±C±N species in the
presence of Fe NO3 3 . After the reaction, the product was collected from the graphite susceptor wall of a lower temperature region (1200 °C). The collected product was dispersed onto a carbon-coated copper grid after ultrasonic treatment in CCl4 solution. Microscopic characterization was performed by using a high-resolution transmission electron microscopy (HRTEM, JEM3000F, JEOL) with EELS spectrometer (Gatan PEELS 666). The electron energy loss spectroscopy (EELS) spectra were obtained by using a ®nely focused nanobeam of 1.0 nm diameter. 3. Results and discussion HRTEM and EELS analyses revealed a major fraction of multi-walled B±C±N and BN nanotubes in the resultant product. Some of these nanotubes appear to be ®lled. Fig. 1 shows a nanotube, 50 nm in external diameter, entirely ®lled with core ®lling in darker optical contrast over the length of 1 lm. The core ®lling is about 10 nm in diameter. Putting the nanobeam on the center of the ®lled nanotube, we obtained the EELS spectrum as shown in Fig. 2a. In addition to the K absorption edges of B, C and N, the presence of Fe was con®rmed by the metallic Fe-L edge at 708 eV. No oxygen (expected at 532 eV) was detected. The near-edge ®ne structure of Fe-L was shown as an enlarged inset. This suggests that the tubular layers
Fig. 1. TEM image of the ®lled nanotube with external diameter of 50 nm and the core ®lling of 10 nm.
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
Fig. 2. EELS measurements on the ®lled nanotube. (a) EELS spectrum identi®es that the ®lled nanotube exhibits B, C, N Kedges at 188, 284, and 401 eV, respectively, and metallic Fe-L edge at 708 eV. The inset shows the near-edge ®ne structure of Fe-L. (b) Evolution of element composition of the tubular layers from edge to center (without reaching the Fe core area), demonstrating the presence and increment of the C content in inner tubular layers.
may be B±C±N and the core ®lling is comprised of iron. We also recorded the EELS spectra, moving the nanobeam from the tube edge to the tube center (without reaching the Fe core area), to observe the evolution of the element composition of tubular layers. The spectra are compared in Fig. 2b (edge ! center). It was clear that only the B-K and N-K peaks were observed for the outmost tubular layers. The pro®le of C-K appeared and
Fig. 3. HRTEM image of the ®lled nanotube, exhibiting a-Fe core ! turbostratic C intermediate layers ! rhombohedral BN outmost layers (rectangular box). Speculated BN-rich island in the intermediate layers.
3
became higher when the recording was moved towards the center. In all spectra, the atomic ratio of B and N was always close to unity. This indicates that the outmost tubular layers are composed of pure BN while the intermediate ones are C-rich. Fig. 3 is the HRTEM image of the one-side part of the coaxial nanocables. As no oxygen was detected and the d110 0:203 nm spacing of a-Fe was well resolved in the core area, it appears that the nanowire is composed of a-Fe. The diraction pattern taken from the ®lling (not shown) also con®rms the body-centered cubic structure of a-Fe. On the other hand, the tubular layers around the nanowire are separated by an average interlayer spacing of 0.34 nm, which is characteristic of the (0 0 3) spacing of r-BN crystal or the (0 0 2) spacing of graphite. The outmost layers (30) are remarkably ordered, whereas the intermediate layers are turbostratic. Investigations on the BN nanotubes have indicated that they display unique morphology [15±18]. For example, BN nanotubes are found to exhibit remarkable three-dimensional ordering, contrary to turbostratic stacking in C nanotubes [16±18]. In Fig. 3, the rhombohedral BN stacking ordering (ABCABC. . .) with spacing d100 0:215 nm is dominant and apparent in the outmost layers. In combination with the EELS measurements in Fig. 2b, it seems that the metallic Fe nanowires are wrapped by outmost BN tubular layers with intermediate C layers, which may be doped by a small amount of B/N. We also speculate on the existence of BN-rich islands (rectangular box in Fig. 3) in the intermediate layers due to the ABC-stacked ordering, which is unlikely to occur in carbon tubular layer structure. However, this speculation has not been fully con®rmed at present. There have been some arguments on the atom distribution in B±C±N nanotubes. Complete phase separation providing pure C and BN layers [19±22] and consecutive layers composed of B/C/N species assembled within a single graphitic sheet [23±25] have both been investigated theoretically and experimentally. Our results shown here, which assume pure C and BN layers, give evidence for the phase separation in B±C±N system, possibly due to maximization of energetically favorable C±C and B±N bonds and minimization of frustrated B± C and N±C bonds [26]. Phase separation has so far
4
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
been characterized only by sophisticated line-scan EELS [19±22] or element mapping [27]; we have now been able to distinguish phase separation by a combined use of general nanobeam EELS and HRTEM images. The Fe nanowires in our product are only encapsulated in B±C±N nanotubes. It seems very dicult to accommodate Fe nanowire in pure BN nanotubes, though we have occasionally observed some Fe nanoparticles in BN bamboos [28], re¯ecting the surface wetting restrictions of BN. Our results indicate that this restriction may be overcome by introducing some intermediate carbon layers. We believe that the two-step nucleation and growth mechanism is applicable for the formation of these peculiar coaxial nanocables (see Fig. 4). As the 0.05 M aqueous solution of Fe NO3 3 carried by ¯owing N2 gas current passed through carbon black species at 1400 °C, the growth of Fe-®lled multi-walled C nanotubes was generated, in a similar way to the process of pyrolyzing ferrocene and C60 [9]. A little B/N doping to the C nanotubes may also occur in this step by decomposition of the precursor B4 N3 O2 H and surface diusion of B and N species along the tube. BN-rich seed islands are thus formed on the C tube surface. Further BN crystallization proceeds in the second step and coats onto the previously Fe-®lled nanotubes. This two-step growth pattern is dierent from our previous studies where the same CVD method, without
Fig. 4. Schematic two-stage growth model for the formation of nanocables with phase separation in radial direction.
employing iron nitrate, mainly yielded pure hollow BN nanotubes. This dierence should be ascribed to the introduction of Fe species into the system. Excellent wetting and catalytic ability of Fe with C makes the growth of C nanotubes dominant in the ®rst step, leading to these selforganized coaxial nanocables.
4. Conclusions A new type of coaxial nanocables, Fe nanowires encapsulated in BN nanotubes with intermediate C layers, has been developed. Clear phase separation of the BN and C tubular layers has been characterized by EELS measurements and HRTEM images. This kind of self-organizing feature may be employed for fabrication of coaxial BN/C nanocables with other metal cores.
Acknowledgements R.M. acknowledges the funding from the Science and Technology Agency (STA) Fellowship carried out in the National Institute for Materials Science, Tsukuba.
References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] P.J.F. Harris, Carbon Nanotubes and Related Structures: New Materials for the Twenty-First Century, Cambridge University Press, Cambridge, 1999. [3] M.S. Dresselhaus, G. Dresselhaus, P. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic, San Diego, 1996. [4] C. Guerrer-Piecourt, Y. Le Bouar, A. Loiseau, H. Pascard, Nature (London) 372 (1994) 761. [5] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature (London) 372 (1994) 159. [6] P.M. Ajayan, O. Stephan, Ph. Redlich, C. Colliex, Nature (London) 375 (1995) 564. [7] D. Ugrate, A. Chatelain, W.A. Deheer, Science 274 (1996) 1897. [8] C.N.R. Rao, R. Sen, B.C. Satishkumar, A. Govindaraj, J. Chem. Soc. Chem. Commun. 15 (1998) 1525. [9] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, M. Terrones, H. Terrones, Ph. Redlich,
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
[10] [11] [12] [13] [14] [15] [16] [17] [18]
M. R uhle, R. Escudero, F. Morales, Appl. Phys. Lett. 75 (1999) 3363. D. Golberg, Y. Bando, K. Kurashima, T. Sato, J. Nanosci. Nanotech. 1 (2001) 49. Y. Bando, K. Ogawa, D. Golberg, Chem. Phys. Lett. 347 (2001) 349. K.B. Shelimov, M. Moskovits, Chem. Mater. 12 (2000) 250. R. Ma, Y. Bando, T. Sato, Chem. Phys. Lett. 337 (2001) 61. R. Ma, Y. Bando, T. Sato, K. Kurashima, Chem. Mater. 13 (2001) 2965. D. Golberg, W. Han, Y. Bando, L. Bourgeois, K. Kurashima, T. Sato, J. Appl. Phys. 86 (1999) 2364. L. Bourgeois, Y. Bando, T. Sato, J. Phys. D 33 (2000) 1902. D. Golberg, Y. Bando, L. Bourgeois, K. Kurashima, T. Sato, Appl. Phys. Lett. 77 (2000) 1979. D. Golberg, Y. Bando, Appl. Phys. Lett. 79 (2001) 415.
5
[19] Y. Zhang, H. Gu, K. Suenaga, S. Iijima, Chem. Phys. Lett. 279 (1997) 264. [20] K. Suenaga, C. Colliex, N. Demoncy, A. Loiseau, H. Pascard, F. Willaime, Science 278 (1997) 653. [21] Y. Zhang, K. Suenaga, C. Colliex, S. Iijima, Science 281 (1998) 973. [22] Ph. Redlich, M. Terrones, C. Manteca-Diego, W.K. Hsu, H. Terrones, M. R uhle, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 310 (1999) 459. [23] Y. Miyamoto, A. Rubio, M.L. Cohen, S.G. Louie, Phys. Rev. B 50 (1994) 4976. [24] O. Stephan, P.M. Ajayan, C. Colliex, Ph. Redlich, J.M. Lambert, P. Bernier, P. Le®n, Science 266 (1995) 966. [25] M. Kawaguchi, Adv. Mater. 9 (1997) 615. [26] H. Nozaki, S. Itoh, J. Phys. Chem. Solids 57 (1996) 41. [27] Y. Bando, D. Golberg, M. Mitome, K. Kurashima, T. Sato, Chem. Phys. Lett. 346 (2001) 29. [28] R. Ma, Y. Bando, T. Sato, J. Electron Microsc. (submitted).
Chemical Physics Letters 350 (2001) 1±5 www.elsevier.com/locate/cplett
Coaxial nanocables: Fe nanowires encapsulated in BN nanotubes with intermediate C layers Renzhi Ma *, Yoshio Bando, Tadao Sato National Institute for Materials Science, Advanced Materials Laboratory and Nanomaterials Laboratory, Namiki1-1, Tsukuba, Ibaraki 305-0044, Japan Received 18 September 2001; in ®nal form 18 October 2001
Abstract Fe-®lled BN nanocables with intermediate C layers were fabricated. Complete phase separation between C and BN was distinguished using electron energy loss spectroscopy (EELS) measurements and high-resolution transmission electron microscopy (HRTEM) images by the characteristics of well-ordered BN layers and turbostratic C ones. A twostep growth model is proposed for the formation of this remarkable structure. The idea of introducing intermediate C layer, overcoming the wetting restriction of BN surface, may be employed for fabrication of BN nanocables ®lled with other metal cores. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Since the discovery of carbon (C) nanotubes, much interest has been attracted to building nanotube-based devices [1±3]. Filling the inner cores of C nanotubes with metals or compounds is an eective route to exploit one-dimensional nanocables with various uses [4±7]. For example, nanowires constructed from magnetic materials (e.g., Fe, Co, Ni) are of particular interest, since they are likely to be used in high-density data storage devices. However, Fe nanowires are subjected to oxidation due to the ®ne dimension of Fe. Some authors have demonstrated the idea to synthesize Fe-®lled C nanotubes, whereby the pro-
*
Corresponding author. Fax: +81-298-51-6280. E-mail address: [email protected] (R. Ma).
tective carbon coating ensures that Fe is retained in a reduced state and magnetic coercivity may be enhanced [8,9]. BN nanocables, on the other hand, are also of potential use for nanoscale electronic devices and nanostructured ceramic materials because BN sheaths are insulating, chemically inert, oxidation- and corrosion-resistant [10±12]. Very recently, some success has been achieved in ®lling BN nanotubes with metallic species (Mo, Fe±Ni) cores via substitution and con®ned reaction from C nanotube templates [10,11]. There has also been a report on electrochemical deposition of Cu into BN nanotubes with relatively large diameters (300 nm) [12]. Nevertheless, signi®cant diculties still exist in directly fabricating BN nanocables with metal cores due to the poor wetting property of BN to metals. We report in this Letter that Fe-®lled BN nanocables could be achieved by introducing some
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 1 2 7 4 - X
2
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
intermediate C layers. The intermediate C layers ensure that the Fe nanowire is retained in a reduced state, whereas, outmost BN shielding makes the structure potential in a chemically aggressive environment or at elevated temperature. The revealed nanocable comprises multiple phase separation (nanoheterojunction) in radial direction, i.e., Fe core (metal)/intermediate C (semimetal)/ outmost BN (insulator). We also found that clear phase separation between C and BN could be easily distinguished by microscopic images due to the characteristics of well-ordered BN layers and turbostratic C ones. 2. Experimental The reaction experiment was carried out in an induction furnace with a graphite susceptor as a heating unit. The precursor B4 N3 O2 H [13,14] was charged into the central zone of the graphite susceptor and itself was placed in a quartz tube. A ¯ow of N2 gas current (1 l/min) was ®rst allowed to bubble through a 0.05 M aqueous solution of iron nitrate Fe NO3 3 to take up this solution into the ¯ow and pass carbon black species at 1400 °C to generate CO via C H2 O ! CO H2 ; it was then introduced into the susceptor after the temperature of the precursor zone reached 1700 °C. The boron oxide B2 O3 or B2 O2 vapor decomposed from the precursor was reduced by the N2 (CO) atmosphere into BN or B±C±N species in the
presence of Fe NO3 3 . After the reaction, the product was collected from the graphite susceptor wall of a lower temperature region (1200 °C). The collected product was dispersed onto a carbon-coated copper grid after ultrasonic treatment in CCl4 solution. Microscopic characterization was performed by using a high-resolution transmission electron microscopy (HRTEM, JEM3000F, JEOL) with EELS spectrometer (Gatan PEELS 666). The electron energy loss spectroscopy (EELS) spectra were obtained by using a ®nely focused nanobeam of 1.0 nm diameter. 3. Results and discussion HRTEM and EELS analyses revealed a major fraction of multi-walled B±C±N and BN nanotubes in the resultant product. Some of these nanotubes appear to be ®lled. Fig. 1 shows a nanotube, 50 nm in external diameter, entirely ®lled with core ®lling in darker optical contrast over the length of 1 lm. The core ®lling is about 10 nm in diameter. Putting the nanobeam on the center of the ®lled nanotube, we obtained the EELS spectrum as shown in Fig. 2a. In addition to the K absorption edges of B, C and N, the presence of Fe was con®rmed by the metallic Fe-L edge at 708 eV. No oxygen (expected at 532 eV) was detected. The near-edge ®ne structure of Fe-L was shown as an enlarged inset. This suggests that the tubular layers
Fig. 1. TEM image of the ®lled nanotube with external diameter of 50 nm and the core ®lling of 10 nm.
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
Fig. 2. EELS measurements on the ®lled nanotube. (a) EELS spectrum identi®es that the ®lled nanotube exhibits B, C, N Kedges at 188, 284, and 401 eV, respectively, and metallic Fe-L edge at 708 eV. The inset shows the near-edge ®ne structure of Fe-L. (b) Evolution of element composition of the tubular layers from edge to center (without reaching the Fe core area), demonstrating the presence and increment of the C content in inner tubular layers.
may be B±C±N and the core ®lling is comprised of iron. We also recorded the EELS spectra, moving the nanobeam from the tube edge to the tube center (without reaching the Fe core area), to observe the evolution of the element composition of tubular layers. The spectra are compared in Fig. 2b (edge ! center). It was clear that only the B-K and N-K peaks were observed for the outmost tubular layers. The pro®le of C-K appeared and
Fig. 3. HRTEM image of the ®lled nanotube, exhibiting a-Fe core ! turbostratic C intermediate layers ! rhombohedral BN outmost layers (rectangular box). Speculated BN-rich island in the intermediate layers.
3
became higher when the recording was moved towards the center. In all spectra, the atomic ratio of B and N was always close to unity. This indicates that the outmost tubular layers are composed of pure BN while the intermediate ones are C-rich. Fig. 3 is the HRTEM image of the one-side part of the coaxial nanocables. As no oxygen was detected and the d110 0:203 nm spacing of a-Fe was well resolved in the core area, it appears that the nanowire is composed of a-Fe. The diraction pattern taken from the ®lling (not shown) also con®rms the body-centered cubic structure of a-Fe. On the other hand, the tubular layers around the nanowire are separated by an average interlayer spacing of 0.34 nm, which is characteristic of the (0 0 3) spacing of r-BN crystal or the (0 0 2) spacing of graphite. The outmost layers (30) are remarkably ordered, whereas the intermediate layers are turbostratic. Investigations on the BN nanotubes have indicated that they display unique morphology [15±18]. For example, BN nanotubes are found to exhibit remarkable three-dimensional ordering, contrary to turbostratic stacking in C nanotubes [16±18]. In Fig. 3, the rhombohedral BN stacking ordering (ABCABC. . .) with spacing d100 0:215 nm is dominant and apparent in the outmost layers. In combination with the EELS measurements in Fig. 2b, it seems that the metallic Fe nanowires are wrapped by outmost BN tubular layers with intermediate C layers, which may be doped by a small amount of B/N. We also speculate on the existence of BN-rich islands (rectangular box in Fig. 3) in the intermediate layers due to the ABC-stacked ordering, which is unlikely to occur in carbon tubular layer structure. However, this speculation has not been fully con®rmed at present. There have been some arguments on the atom distribution in B±C±N nanotubes. Complete phase separation providing pure C and BN layers [19±22] and consecutive layers composed of B/C/N species assembled within a single graphitic sheet [23±25] have both been investigated theoretically and experimentally. Our results shown here, which assume pure C and BN layers, give evidence for the phase separation in B±C±N system, possibly due to maximization of energetically favorable C±C and B±N bonds and minimization of frustrated B± C and N±C bonds [26]. Phase separation has so far
4
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
been characterized only by sophisticated line-scan EELS [19±22] or element mapping [27]; we have now been able to distinguish phase separation by a combined use of general nanobeam EELS and HRTEM images. The Fe nanowires in our product are only encapsulated in B±C±N nanotubes. It seems very dicult to accommodate Fe nanowire in pure BN nanotubes, though we have occasionally observed some Fe nanoparticles in BN bamboos [28], re¯ecting the surface wetting restrictions of BN. Our results indicate that this restriction may be overcome by introducing some intermediate carbon layers. We believe that the two-step nucleation and growth mechanism is applicable for the formation of these peculiar coaxial nanocables (see Fig. 4). As the 0.05 M aqueous solution of Fe NO3 3 carried by ¯owing N2 gas current passed through carbon black species at 1400 °C, the growth of Fe-®lled multi-walled C nanotubes was generated, in a similar way to the process of pyrolyzing ferrocene and C60 [9]. A little B/N doping to the C nanotubes may also occur in this step by decomposition of the precursor B4 N3 O2 H and surface diusion of B and N species along the tube. BN-rich seed islands are thus formed on the C tube surface. Further BN crystallization proceeds in the second step and coats onto the previously Fe-®lled nanotubes. This two-step growth pattern is dierent from our previous studies where the same CVD method, without
Fig. 4. Schematic two-stage growth model for the formation of nanocables with phase separation in radial direction.
employing iron nitrate, mainly yielded pure hollow BN nanotubes. This dierence should be ascribed to the introduction of Fe species into the system. Excellent wetting and catalytic ability of Fe with C makes the growth of C nanotubes dominant in the ®rst step, leading to these selforganized coaxial nanocables.
4. Conclusions A new type of coaxial nanocables, Fe nanowires encapsulated in BN nanotubes with intermediate C layers, has been developed. Clear phase separation of the BN and C tubular layers has been characterized by EELS measurements and HRTEM images. This kind of self-organizing feature may be employed for fabrication of coaxial BN/C nanocables with other metal cores.
Acknowledgements R.M. acknowledges the funding from the Science and Technology Agency (STA) Fellowship carried out in the National Institute for Materials Science, Tsukuba.
References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] P.J.F. Harris, Carbon Nanotubes and Related Structures: New Materials for the Twenty-First Century, Cambridge University Press, Cambridge, 1999. [3] M.S. Dresselhaus, G. Dresselhaus, P. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic, San Diego, 1996. [4] C. Guerrer-Piecourt, Y. Le Bouar, A. Loiseau, H. Pascard, Nature (London) 372 (1994) 761. [5] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature (London) 372 (1994) 159. [6] P.M. Ajayan, O. Stephan, Ph. Redlich, C. Colliex, Nature (London) 375 (1995) 564. [7] D. Ugrate, A. Chatelain, W.A. Deheer, Science 274 (1996) 1897. [8] C.N.R. Rao, R. Sen, B.C. Satishkumar, A. Govindaraj, J. Chem. Soc. Chem. Commun. 15 (1998) 1525. [9] N. Grobert, W.K. Hsu, Y.Q. Zhu, J.P. Hare, H.W. Kroto, D.R.M. Walton, M. Terrones, H. Terrones, Ph. Redlich,
R. Ma et al. / Chemical Physics Letters 350 (2001) 1±5
[10] [11] [12] [13] [14] [15] [16] [17] [18]
M. R uhle, R. Escudero, F. Morales, Appl. Phys. Lett. 75 (1999) 3363. D. Golberg, Y. Bando, K. Kurashima, T. Sato, J. Nanosci. Nanotech. 1 (2001) 49. Y. Bando, K. Ogawa, D. Golberg, Chem. Phys. Lett. 347 (2001) 349. K.B. Shelimov, M. Moskovits, Chem. Mater. 12 (2000) 250. R. Ma, Y. Bando, T. Sato, Chem. Phys. Lett. 337 (2001) 61. R. Ma, Y. Bando, T. Sato, K. Kurashima, Chem. Mater. 13 (2001) 2965. D. Golberg, W. Han, Y. Bando, L. Bourgeois, K. Kurashima, T. Sato, J. Appl. Phys. 86 (1999) 2364. L. Bourgeois, Y. Bando, T. Sato, J. Phys. D 33 (2000) 1902. D. Golberg, Y. Bando, L. Bourgeois, K. Kurashima, T. Sato, Appl. Phys. Lett. 77 (2000) 1979. D. Golberg, Y. Bando, Appl. Phys. Lett. 79 (2001) 415.
5
[19] Y. Zhang, H. Gu, K. Suenaga, S. Iijima, Chem. Phys. Lett. 279 (1997) 264. [20] K. Suenaga, C. Colliex, N. Demoncy, A. Loiseau, H. Pascard, F. Willaime, Science 278 (1997) 653. [21] Y. Zhang, K. Suenaga, C. Colliex, S. Iijima, Science 281 (1998) 973. [22] Ph. Redlich, M. Terrones, C. Manteca-Diego, W.K. Hsu, H. Terrones, M. R uhle, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 310 (1999) 459. [23] Y. Miyamoto, A. Rubio, M.L. Cohen, S.G. Louie, Phys. Rev. B 50 (1994) 4976. [24] O. Stephan, P.M. Ajayan, C. Colliex, Ph. Redlich, J.M. Lambert, P. Bernier, P. Le®n, Science 266 (1995) 966. [25] M. Kawaguchi, Adv. Mater. 9 (1997) 615. [26] H. Nozaki, S. Itoh, J. Phys. Chem. Solids 57 (1996) 41. [27] Y. Bando, D. Golberg, M. Mitome, K. Kurashima, T. Sato, Chem. Phys. Lett. 346 (2001) 29. [28] R. Ma, Y. Bando, T. Sato, J. Electron Microsc. (submitted).