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Novel bismuth nanotube arrays synthesized by solvothermal method
Chemical Physics Letters 374 (2003) 348–352 www.elsevier.com/locate/cplett
Novel bismuth nanotube arrays synthesized by solvothermal method Xin-yuan Liu a, Jing-hui Zeng c, Shu-yuan Zhang b, Rong-bo Zheng a, Xian-ming Liu b, Yi-tai Qian a,b,* a
Department of Chemistry, 229-304 USTC, Hefei, Anhui 230026, PeopleÕs Republic of China Structure Research Laboratory, USTC, Hefei, Anhui 230026, PeopleÕs Republic of China c Nanoscience and Nanotechnology Initiative, NUS, Singapore
b
Received 17 March 2003; in final form 1 May 2003
Abstract Novel bismuth nanotube arrays have been successfully synthesized by a solvothermal method through the reduction of bismuth oxide (Bi2 O3 ) by ethylene glycol (EG). The productions were characterized by XRD, TEM and HRTEM. The diameter of the nanotubes is about 3–6 nm and length is up to 500 nm. The possible growth mechanism of Bi nanotubes was discussed. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The discovery of carbon nanotubes by Iijima [1] in 1991 is the milestone in the field of nanomaterials. Owing to unique physical properties and great potential applications of carbon nanotubes in electronic and optoelectronic devices, the onedimensional (1D) nanostructure materials, such as nanorods, nanowires and nanotubes, have attracted extensive attention in the past few years. Since then how to downscaling large quantity of materials to 1D nanoscopic structures has been a challenge for synthetic scientists [2]. A lot of
*
Corresponding author. Fax: +86-551-363-1760. E-mail addresses: [email protected] (X. Liu), [email protected] (Y. Qian).
methods have been established to prepare 1D nanostructures, for instance, laser-assisted catalytic growth (LCG) by a vapor–liquid–solid mechanism (VLS) [3], arc discharge [4], solution– liquid–solid method (SLS) [5], chemical vapor deposition (CVD) [6], template-assisted synthesis [7], solvothermal synthesis [8] and other approaches [15]. Up to now, many kinds of nanotubes have been successfully synthesized, such as Si [9], Au [10], Bi [11], Te [12], Ni [13], MoS2 [14], NbS2 , TaS2 [15], and ReS2 [16]. There is a more comprehensive introduction on many other nanotubes in RaoÕs recent review [17]. Elemental bismuth is a semimetal with the highest Hall effect and it is the most diamagnetic of any metal. It has unusual electronic properties, which result from its highly anisotropic Fermic surface, low carrier concentration, small effective
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00730-9
X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
mass (0.001m0 ), and long mean free path of carriers (0.4 mm at 4 K) [18]. Due to these unique qualities, Bi has been extensively studied for quantum confinement effects [19], transports properties [20], quantum conductance [21], magnetonresistance effect and finite size effect [22]. Precisely controlling the shape of nanomaterials and forming well-aligned arrays from these nanomaterials will benefit a lot for the investigation of their physical properties. However, the melting point of Bi is so low (271.3 °C) that it is not suitable to obtain it using those high temperature methods mentioned above. Figlarz and coworkers [23] have successfully prepared metal particles by polyol method. Based on it, a facile solvothermal approach has been developed in this Letter for single-walled aligned Bi nanotubes, using bismuth oxide (Bi2 O3 ) and ethylene glycol (EG) as the starting materials. Compared with the reported hydrothermal reduction synthesis of bismuth using aqueous hydrazine as the reductant [11], ours is more simple because EG serves as both reducing agent and solvent as that reported by Xia and Mayers [12], moreover, our method is more controllable because the reducibility of EG is relatively moderate. The as-prepared bismuth nanotubes typically with diameter of 3–6 nm and length up to 500 nm selfassembled into bundles. To the best of our knowledge, there are few publications relevant to single-walled aligned nanotubes using chemical methods.
349
The samples were characterized by powder X-ray diffraction (XRD) pattern at a scanning rate of in the 2h range from 10° to 70°, using Cu Ka ) on a Japan Rigaka Xradiation (h ¼ 1:54178 A ray diffractometer. The morphology of the samples was studied on Hitachi 800 transmission electron microscopy (TEM) using an accelerating voltage of 200 kV. And the further analysis about the crystallization of individual nanotube bundle was performed on a JEOL 2010 high-resolution transmission electron microscope (HRTEM) at 200 kV.
3. Results and discussions XRD pattern revealed all of the reflection peaks can be readily indexed as rhombohedral phase Bi (JCPDS Card No. 5-519) [space group: R3 m (166)], as shown in Fig. 1. The lattice constants , calculated from this pattern (a ¼ 4:541 A ) are consistent with the reported c ¼ 11:855 A values. No other diffraction peaks belonging to oxide or other impurity were observed in this pattern. This XRD pattern indicates that under current synthetic conditions, the resulting product is highly crystallized elemental bismuth with high purity. The typical TEM images were presented in Fig. 2. The product consists of nanotubes with uniform diameter of 3–6 nm and lengths up to
2. Experiment In a typical process, 0.466 g Bi2 O3 (1 mmol) was put into 30 ml pure EG at room temperature. The solution was stirred strongly for 30 min and transferred into a stainless steel autoclave with a Teflon liner, which was then filled with EG to 80% of its capacity. The autoclave was sealed and maintained at 200 °C for 12 h, then cool to room temperature. The obtained black solids were filtered, washed with diluted HCl solution absolute methanol, distilled water for several times to remove any possible remaining bismuth oxide, and dried in a vacuum at 60 °C for 4 h.
Fig. 1. XRD pattern of the as-synthesized bismuth nanotubes (top) and standard XRD pattern for bulk bismuth (JCPDS 5519) (bottom).
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Fig. 2. Representative TEM images of the as-prepared Bi nanotubes arrays. (a)–(c) Note that most of them are aligned together with straight cylindrical morphology.
500 nm. It is interesting that all the tubes are straight and most of them are aligned together to form nanotube bundles. A bundle of well-crystallized Bi nanotubes is presented in Fig. 3a. Because of the low melting point, the bismuth nanotubes are very sensitive to electron beam radiation. When the nanotubes were irradiated with high-energy electron beam, the nanotubes were melting into liquid drops. The particles in Fig. 3a are the product from the re-crystalline of the Bi liquid drops and that is also demonstrated in ED patterns (inset in Fig. 3a), in which arcs of the diffraction spots turn into rings, suggesting that Bi nanotubes transformed into polycrystalline particles under the electron radiation. Fig. 3b shows a high-resolution TEM (HRTEM) image of the Bi nanotube bundle, which confirms that the nanotubes are single crystal with lattice fringes to be (0 1 2) direction. As far as we know, it is the first time to report the observed crystal lattices of Bi nanotubes. Energydispersive X-ray analysis (EDX) of the products in Fig. 3c shows that they are pure Bismuth, which is in agreement with the XRD result. According to the experimental experience and theoretical investigations, a predication can be made that in principle under suitable reaction conditions all layered materials should be transformed into tubular arrangements. Scroll formation will occur when layers of a lamellar material are being separated from each other by some
proper means and when the interaction of the individual layers with their coordination shells is weak enough [2]. Many already synthesized nanotubes have proved this theory, such as C, BN, MoS2 , WS2 , Bi2 S3 , Sb2 O3 , they all have layer structures and rolled to form a tubular structure [24]. In the structure of Bismuth, each atom is covalently bound to three others to form an indefinitely extended puckered sheet. Each atom therefore has three neighbors in is own sheet at a ), three more remote neighbors in distance (3.10 A ). the adjacent sheet at a greater distance (3.47 A Within the molecular sheets the intermolecular forces are involved in the covalent bonding and the forces between the sheets are primarily van der Waals bonds [25]. Due to the quasi-layered structure of elemental Bismuth, we speculate that the formation mechanism of Bi nanotubes might also be a rolling process. And the sheet-like structure in Fig. 2c can confirm the postulation as well. The homogeneous solution, modest intensive reducing agent and proper reaction temperature are crucial for the growth of bismuth nanotubes. When the reaction was carried out at 180 °C, very weak diffraction peaks from bismuth and other unknown peaks were measured by XRD. When the temperature was raised to 220 °C, spheres and rods morphologies were observed without nanotubes. Compared with normal reducing materials, like Zn powder, EG is a relatively moderate one. In this
X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
351
Fig. 3. (a) TEM of a bundle of Bi nanotubes. Inset shows an ED pattern of the nanotube bundle. (b) A high-resolution TEM image of the bundle shows that lattice fringe to be the (0 1 2) faces. (c) The EDX of the Bi nanotubes.
process, EG can coordinate with Bi3þ as the role of en reported by Chen and coworkers [26]. Thus, it is suggested a typical coordinative reduction course. Furthermore, it is easy for EG to form dimer or trimer at high temperature, and dimers or trimers may act as template for bismuth to form lamellar structures and help to align the nanotubes. Meanwhile, the increasing pressure affords a greater driving force for the reaction process [27]. Our understanding of the formation mechanism is still limited and further study is in progress.
4. Conclusions In summary, we have successfully synthesized single-walled aligned bismuth nanotubes under suitable reaction conditions. This single-walled bismuth nanotube bundles can serve as sacrifice template to prepare Bi2 E3 (E ¼ S, Se, Te, etc.) nanocrystals. In addition, they can be investigated for physical properties and have potential application in nanodevices. The convenient technique might be extended to synthesize the nanotubes of
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X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
materials with the analogous structure as Bi, such as As, Sb, Se, Te and so on.
Acknowledgements This work is supported by the National Natural Science Foundation of China. We gratefully thank Dr. J. Yang and Dr. Q. Yang for their beneficial discussions.
References [1] S. Iijima, Nature 354 (1991) 56. [2] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002) 2446. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] T.W. Ebbesen, P.M. Ajayan, Nature 358 (1992) 220. [5] X.F. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298. [6] M.H. Huang, S. Mao, H.N. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, R. Russo, P.D. Yang, Science 292 (2001) 1897. [7] J.M. Schnur, Science 262 (1993) 1669. [8] K.B. Tang, Y.T. Qian, J.H. Zeng, X.G. Yang, Adv. Mater. 15 (2003) 448. [9] H. Nakamura, Y. Matsui, J. Am. Chem. Soc. 117 (1995) 2651. [10] C.R. Martin, M. Nishizawa, K. Jirage, M. Kang, J. Phys. Chem. B 105 (2001) 1925.
[11] Y.D. Li, J.W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, P.D. Yang, J. Am. Chem. Soc. 123 (2001) 9904. [12] B. Mayers, Y.N. Xia, Adv. Mater. 14 (2002) 279. [13] J.C. Bao, C.Y. Tie, Z. Xu, Q.F. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631. [14] Y. Feldman, E. Wasserman, D.J. Srolovitz, R. Tenne, Science 267 (1995) 222. [15] M. Nath, C.N.R. Rao, J. Am. Chem. Soc. 123 (2001) 4841. [16] M. Brorson, T.W. Hansen, C.J.H. Jacobsen, J. Am. Chem. Soc. 124 (2002) 11582. [17] C.N.R. Rao, M. Nath, Dalton Trans. (2003) 1. [18] F.Y. Yang, K. Liu, K.M. Hong, D.H. Reich, P.C. Searson, C.L. Chien, Science 284 (1999) 1335. [19] X.F. Wang, J. Zhang, H.Z. Shi, Y.W. Wang, G.W. Meng, X.S. Peng, L.D. Zhang, J. Fang, J. Appl. Phys. 89 (2001) 3847. [20] Y.M. Lin, S.B. Cronin, J.Y. Ying, M.S. Dresselhaus, J.P. Heremans, Appl. Phys. Lett. 76 (2000) 3944. [21] J.G. Rodrigo, A. Garcia-Martin, J.J. Saenz, S. Vieira, Phys. Rev. Lett. 88 (2002) 246801. [22] F.Y. Yang, K. Liu, C.L. Chien, P.C. Searson, Phys. Rev. Lett. 82 (1999) 3328. [23] F. Fievet, J.P. Lagier, B. Blin, B. Beaudoin, M. Figlarz, Solid State Ionics 32/33 (1989) 198. [24] C.H. Ye, G.W. Meng, Z. Jiang, Y.H. Wang, G.Z. Wang, L.D. Zhang, J. Am. Chem. Soc. 124 (2002) 15180. [25] R.C. Evans, An Introduction to Crystal Chemistry, Cambridge University Press, Cambridge, MA, 1964. [26] Y.H. Gao, H.L. Niu, C. Zeng, Q.W. Chen, Chem. Phys. Lett. 367 (2003) 141. [27] P.R. Bonneau, R.F. Jarvis Jr., R.B. Kaner, Nature 349 (1991) 510.
Novel bismuth nanotube arrays synthesized by solvothermal method Xin-yuan Liu a, Jing-hui Zeng c, Shu-yuan Zhang b, Rong-bo Zheng a, Xian-ming Liu b, Yi-tai Qian a,b,* a
Department of Chemistry, 229-304 USTC, Hefei, Anhui 230026, PeopleÕs Republic of China Structure Research Laboratory, USTC, Hefei, Anhui 230026, PeopleÕs Republic of China c Nanoscience and Nanotechnology Initiative, NUS, Singapore
b
Received 17 March 2003; in final form 1 May 2003
Abstract Novel bismuth nanotube arrays have been successfully synthesized by a solvothermal method through the reduction of bismuth oxide (Bi2 O3 ) by ethylene glycol (EG). The productions were characterized by XRD, TEM and HRTEM. The diameter of the nanotubes is about 3–6 nm and length is up to 500 nm. The possible growth mechanism of Bi nanotubes was discussed. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The discovery of carbon nanotubes by Iijima [1] in 1991 is the milestone in the field of nanomaterials. Owing to unique physical properties and great potential applications of carbon nanotubes in electronic and optoelectronic devices, the onedimensional (1D) nanostructure materials, such as nanorods, nanowires and nanotubes, have attracted extensive attention in the past few years. Since then how to downscaling large quantity of materials to 1D nanoscopic structures has been a challenge for synthetic scientists [2]. A lot of
*
Corresponding author. Fax: +86-551-363-1760. E-mail addresses: [email protected] (X. Liu), [email protected] (Y. Qian).
methods have been established to prepare 1D nanostructures, for instance, laser-assisted catalytic growth (LCG) by a vapor–liquid–solid mechanism (VLS) [3], arc discharge [4], solution– liquid–solid method (SLS) [5], chemical vapor deposition (CVD) [6], template-assisted synthesis [7], solvothermal synthesis [8] and other approaches [15]. Up to now, many kinds of nanotubes have been successfully synthesized, such as Si [9], Au [10], Bi [11], Te [12], Ni [13], MoS2 [14], NbS2 , TaS2 [15], and ReS2 [16]. There is a more comprehensive introduction on many other nanotubes in RaoÕs recent review [17]. Elemental bismuth is a semimetal with the highest Hall effect and it is the most diamagnetic of any metal. It has unusual electronic properties, which result from its highly anisotropic Fermic surface, low carrier concentration, small effective
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00730-9
X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
mass (0.001m0 ), and long mean free path of carriers (0.4 mm at 4 K) [18]. Due to these unique qualities, Bi has been extensively studied for quantum confinement effects [19], transports properties [20], quantum conductance [21], magnetonresistance effect and finite size effect [22]. Precisely controlling the shape of nanomaterials and forming well-aligned arrays from these nanomaterials will benefit a lot for the investigation of their physical properties. However, the melting point of Bi is so low (271.3 °C) that it is not suitable to obtain it using those high temperature methods mentioned above. Figlarz and coworkers [23] have successfully prepared metal particles by polyol method. Based on it, a facile solvothermal approach has been developed in this Letter for single-walled aligned Bi nanotubes, using bismuth oxide (Bi2 O3 ) and ethylene glycol (EG) as the starting materials. Compared with the reported hydrothermal reduction synthesis of bismuth using aqueous hydrazine as the reductant [11], ours is more simple because EG serves as both reducing agent and solvent as that reported by Xia and Mayers [12], moreover, our method is more controllable because the reducibility of EG is relatively moderate. The as-prepared bismuth nanotubes typically with diameter of 3–6 nm and length up to 500 nm selfassembled into bundles. To the best of our knowledge, there are few publications relevant to single-walled aligned nanotubes using chemical methods.
349
The samples were characterized by powder X-ray diffraction (XRD) pattern at a scanning rate of in the 2h range from 10° to 70°, using Cu Ka ) on a Japan Rigaka Xradiation (h ¼ 1:54178 A ray diffractometer. The morphology of the samples was studied on Hitachi 800 transmission electron microscopy (TEM) using an accelerating voltage of 200 kV. And the further analysis about the crystallization of individual nanotube bundle was performed on a JEOL 2010 high-resolution transmission electron microscope (HRTEM) at 200 kV.
3. Results and discussions XRD pattern revealed all of the reflection peaks can be readily indexed as rhombohedral phase Bi (JCPDS Card No. 5-519) [space group: R3 m (166)], as shown in Fig. 1. The lattice constants , calculated from this pattern (a ¼ 4:541 A ) are consistent with the reported c ¼ 11:855 A values. No other diffraction peaks belonging to oxide or other impurity were observed in this pattern. This XRD pattern indicates that under current synthetic conditions, the resulting product is highly crystallized elemental bismuth with high purity. The typical TEM images were presented in Fig. 2. The product consists of nanotubes with uniform diameter of 3–6 nm and lengths up to
2. Experiment In a typical process, 0.466 g Bi2 O3 (1 mmol) was put into 30 ml pure EG at room temperature. The solution was stirred strongly for 30 min and transferred into a stainless steel autoclave with a Teflon liner, which was then filled with EG to 80% of its capacity. The autoclave was sealed and maintained at 200 °C for 12 h, then cool to room temperature. The obtained black solids were filtered, washed with diluted HCl solution absolute methanol, distilled water for several times to remove any possible remaining bismuth oxide, and dried in a vacuum at 60 °C for 4 h.
Fig. 1. XRD pattern of the as-synthesized bismuth nanotubes (top) and standard XRD pattern for bulk bismuth (JCPDS 5519) (bottom).
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X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
Fig. 2. Representative TEM images of the as-prepared Bi nanotubes arrays. (a)–(c) Note that most of them are aligned together with straight cylindrical morphology.
500 nm. It is interesting that all the tubes are straight and most of them are aligned together to form nanotube bundles. A bundle of well-crystallized Bi nanotubes is presented in Fig. 3a. Because of the low melting point, the bismuth nanotubes are very sensitive to electron beam radiation. When the nanotubes were irradiated with high-energy electron beam, the nanotubes were melting into liquid drops. The particles in Fig. 3a are the product from the re-crystalline of the Bi liquid drops and that is also demonstrated in ED patterns (inset in Fig. 3a), in which arcs of the diffraction spots turn into rings, suggesting that Bi nanotubes transformed into polycrystalline particles under the electron radiation. Fig. 3b shows a high-resolution TEM (HRTEM) image of the Bi nanotube bundle, which confirms that the nanotubes are single crystal with lattice fringes to be (0 1 2) direction. As far as we know, it is the first time to report the observed crystal lattices of Bi nanotubes. Energydispersive X-ray analysis (EDX) of the products in Fig. 3c shows that they are pure Bismuth, which is in agreement with the XRD result. According to the experimental experience and theoretical investigations, a predication can be made that in principle under suitable reaction conditions all layered materials should be transformed into tubular arrangements. Scroll formation will occur when layers of a lamellar material are being separated from each other by some
proper means and when the interaction of the individual layers with their coordination shells is weak enough [2]. Many already synthesized nanotubes have proved this theory, such as C, BN, MoS2 , WS2 , Bi2 S3 , Sb2 O3 , they all have layer structures and rolled to form a tubular structure [24]. In the structure of Bismuth, each atom is covalently bound to three others to form an indefinitely extended puckered sheet. Each atom therefore has three neighbors in is own sheet at a ), three more remote neighbors in distance (3.10 A ). the adjacent sheet at a greater distance (3.47 A Within the molecular sheets the intermolecular forces are involved in the covalent bonding and the forces between the sheets are primarily van der Waals bonds [25]. Due to the quasi-layered structure of elemental Bismuth, we speculate that the formation mechanism of Bi nanotubes might also be a rolling process. And the sheet-like structure in Fig. 2c can confirm the postulation as well. The homogeneous solution, modest intensive reducing agent and proper reaction temperature are crucial for the growth of bismuth nanotubes. When the reaction was carried out at 180 °C, very weak diffraction peaks from bismuth and other unknown peaks were measured by XRD. When the temperature was raised to 220 °C, spheres and rods morphologies were observed without nanotubes. Compared with normal reducing materials, like Zn powder, EG is a relatively moderate one. In this
X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
351
Fig. 3. (a) TEM of a bundle of Bi nanotubes. Inset shows an ED pattern of the nanotube bundle. (b) A high-resolution TEM image of the bundle shows that lattice fringe to be the (0 1 2) faces. (c) The EDX of the Bi nanotubes.
process, EG can coordinate with Bi3þ as the role of en reported by Chen and coworkers [26]. Thus, it is suggested a typical coordinative reduction course. Furthermore, it is easy for EG to form dimer or trimer at high temperature, and dimers or trimers may act as template for bismuth to form lamellar structures and help to align the nanotubes. Meanwhile, the increasing pressure affords a greater driving force for the reaction process [27]. Our understanding of the formation mechanism is still limited and further study is in progress.
4. Conclusions In summary, we have successfully synthesized single-walled aligned bismuth nanotubes under suitable reaction conditions. This single-walled bismuth nanotube bundles can serve as sacrifice template to prepare Bi2 E3 (E ¼ S, Se, Te, etc.) nanocrystals. In addition, they can be investigated for physical properties and have potential application in nanodevices. The convenient technique might be extended to synthesize the nanotubes of
352
X. Liu et al. / Chemical Physics Letters 374 (2003) 348–352
materials with the analogous structure as Bi, such as As, Sb, Se, Te and so on.
Acknowledgements This work is supported by the National Natural Science Foundation of China. We gratefully thank Dr. J. Yang and Dr. Q. Yang for their beneficial discussions.
References [1] S. Iijima, Nature 354 (1991) 56. [2] G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41 (2002) 2446. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] T.W. Ebbesen, P.M. Ajayan, Nature 358 (1992) 220. [5] X.F. Duan, C.M. Lieber, Adv. Mater. 12 (2000) 298. [6] M.H. Huang, S. Mao, H.N. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, R. Russo, P.D. Yang, Science 292 (2001) 1897. [7] J.M. Schnur, Science 262 (1993) 1669. [8] K.B. Tang, Y.T. Qian, J.H. Zeng, X.G. Yang, Adv. Mater. 15 (2003) 448. [9] H. Nakamura, Y. Matsui, J. Am. Chem. Soc. 117 (1995) 2651. [10] C.R. Martin, M. Nishizawa, K. Jirage, M. Kang, J. Phys. Chem. B 105 (2001) 1925.
[11] Y.D. Li, J.W. Wang, Z.X. Deng, Y.Y. Wu, X.M. Sun, D.P. Yu, P.D. Yang, J. Am. Chem. Soc. 123 (2001) 9904. [12] B. Mayers, Y.N. Xia, Adv. Mater. 14 (2002) 279. [13] J.C. Bao, C.Y. Tie, Z. Xu, Q.F. Zhou, D. Shen, Q. Ma, Adv. Mater. 13 (2001) 1631. [14] Y. Feldman, E. Wasserman, D.J. Srolovitz, R. Tenne, Science 267 (1995) 222. [15] M. Nath, C.N.R. Rao, J. Am. Chem. Soc. 123 (2001) 4841. [16] M. Brorson, T.W. Hansen, C.J.H. Jacobsen, J. Am. Chem. Soc. 124 (2002) 11582. [17] C.N.R. Rao, M. Nath, Dalton Trans. (2003) 1. [18] F.Y. Yang, K. Liu, K.M. Hong, D.H. Reich, P.C. Searson, C.L. Chien, Science 284 (1999) 1335. [19] X.F. Wang, J. Zhang, H.Z. Shi, Y.W. Wang, G.W. Meng, X.S. Peng, L.D. Zhang, J. Fang, J. Appl. Phys. 89 (2001) 3847. [20] Y.M. Lin, S.B. Cronin, J.Y. Ying, M.S. Dresselhaus, J.P. Heremans, Appl. Phys. Lett. 76 (2000) 3944. [21] J.G. Rodrigo, A. Garcia-Martin, J.J. Saenz, S. Vieira, Phys. Rev. Lett. 88 (2002) 246801. [22] F.Y. Yang, K. Liu, C.L. Chien, P.C. Searson, Phys. Rev. Lett. 82 (1999) 3328. [23] F. Fievet, J.P. Lagier, B. Blin, B. Beaudoin, M. Figlarz, Solid State Ionics 32/33 (1989) 198. [24] C.H. Ye, G.W. Meng, Z. Jiang, Y.H. Wang, G.Z. Wang, L.D. Zhang, J. Am. Chem. Soc. 124 (2002) 15180. [25] R.C. Evans, An Introduction to Crystal Chemistry, Cambridge University Press, Cambridge, MA, 1964. [26] Y.H. Gao, H.L. Niu, C. Zeng, Q.W. Chen, Chem. Phys. Lett. 367 (2003) 141. [27] P.R. Bonneau, R.F. Jarvis Jr., R.B. Kaner, Nature 349 (1991) 510.