ARTICLE IN PRESS G-Y. Li et al. Solid State Sciences xax(2009)1 diameter. Although the flowing rate of N2 is slow, the local silane fragments will be still supplied subsequently by the carrying gas. the lateral growth rate will increase, and thus the diameter becomes larger (Fig. 6c). With the periodically alternating concentration of local source species, the final diameter fluctuating iC nanowires are formed( fig. 6d). The flowing rate of the carrying gas plays important role during the growth process. When the wires with relatively homogeneous diameters were obtained as Several groups found some Sic nanowires when preparing porous SiC ceramics with polycarbosilane as adhesive agent 30 32: however, the length was only in micrometers scale. While in our case, the nanowires length was up to millimeters. As the chemicophysical properties of 1D nanostructures are highly size 11 28 SE diameter and aspect ratio) dependent, the high-aspect-ratio Sic nanowires will provide opportunities for both fundamental Fig. 7. SEM image of the Sic nanowires with relatively homogeneous diameter research and technical applications in nanodevices 33]. 4. Conclusions contained in the EDX pattern(Fig 3c), and it is very because of the diameters were synthesized by the decomposition of poly- rge areas of millimeters long B-SiC nanowires with fluctuating nevitable oxygen mixed in the carrying gases [4, 9]. Fig 5d is the carbosilane in a chemical vapor deposition route. The nanowires corresponding SAED pattern of the nanowire. The clear diffraction had the non-periodical fluctuating diameters in the range of 100- spots indicate a good single crystal nature of the nanowires And by 250 nm along the axial direction, and were composed of well dexing the pattern, we can see the nanowires grew along the crystalline B-Sic along the(111) direction. By the VS mechanism, we explained that the very slow flowing rate of the carrying gas was From the above characterization we demonstrated that the esponsible for the diameter fluctuation. It is believed that the white cotton-like products were B-SiC nanowires. Several mecha- nisms have been proposed for the growth of Sic nanowires, including vapor-liquid-solid(VLS)[13, 27]. vapor-solid (VS)(28]. nanocomposites, and electronic nanodevices solid-liquid-solid(SLS)[29 mechanisms. The Sic nanowire energy of the(111)plane. In the present work, we did not find any Acknowledgment droplets during the sEm observations; so the VLS and SlS mecha- nisms are not suitable for the as-prepared Sic nanowires growth re thankful for the financial support from the The as-prepared Sic nanowires were grown on a graphite wafer National Science Foundation of China( Grant No. 50702075) eyond the raw materials; the Si and C elements in the nanowires 9140C8202050804) Fund of State Key Laboratory of CFC(Grant No ported to the substrate by the flowing N2. Therefore, the ns nanowires may be grown by the vs mechanism in a CVD route References As for the Sic nanowires with non-uniform diameters growth, Hao suggested that the periodically alternating Si and C concen- asad, R.W. Johnson, Solid State Electron. 39(1996)1409 Z.W. Pan, H L Lai, CK Frederick, X.F. Duan, W.Y. Zhou, w.S. Shi, N. Wan trations in the catalytic droplets lead to the formation of the beaded CS. Lee, N.B. Wong. S.T. Lee, S.S. Xie, Adv Mater. 12(2000)1186. morphology [13 Wang prepared Sic nanowires with modulated 31 Co Jang. T.H. Kim, S.Y. Lee, D - Kim, S K Lee, Nanotechnology 19(2008) diameters via a VLS process, and proved that the pressure of the (4 H.K. Seong HJ. Choi, S.K. Lee, LL. Lee, D J Choi, Appl. Phys. Lett.85(2004) ource species was responsible for the special morphology 12. In the two cases the diameters of the sic nanowires were determined LG. ZI .Y Yang, J. Hua, ZH. Zheng. Z P. Xie, H.. Miao, L An, Appl Phys. by the volume of the catalytic droplets, which was varying due to [61 Y.F. Zhang. X.D. Han, K. Zheng, Z. Zhang x. Zhang. Fu, Y. i, Y. Hao, XY. Guo, the fluctuating contents of the dissolved Si and C elements ZL Wang, Adv Funct Mater. 17(2007)3435. Although the as-prepared Sic nanowires in our work were not [7] W Yang. H Araki. CC Tang. $. Thaveethavon, A.K. Hiroshi, T. Noda, Adv. governed by the VLS mechanism, the concept of the pressure or the (8)EW.Wong PE. Sheehan, C.M. Lieber, Science 277(1997)1971. growth process can be illustrated in Fig. 6. At high temperature silane fragments from the slow decom- 故A Niu, J.N. Wang. Q F. Xu, Wang, D Xu, Q Wang. Y.J. Hao, G Q Jin, X.Y. Guo, K N. Tu, Nanotechnology position of polycarbosilane will serve as the direct source species for the nanowires growth. At certain time, the local partial pressure [12] H.T. Wang, Z.P. Xie, W.Y. Yang, J.Y. Fang, LN. An, Cryst. Growth Des. 8(2008) [131 Y. ]. Hao, J.B. Wagner, DS. Su, GQ Jin, X.Y. Guo, Nanotechnology 17(2006) because of the depletion in the CVD reaction(Fig 6a and b). The [14] G.Z. Shen, Y Bando, D Golberg Cryst. Growth Des. 7(2007) esh silane fragments cannot be provided timely by the carrying N [15] Z.S. Wu, S.Z. Deng. N.S. Xu. ] Chen. ]. Zhou, J. Chen, Appl. Phys. Lett. 80(2002) gas, because the flowing rate of N2 is too slow(about 5 sccm). Therefore, the deposition rate and the crystal growth rate including [16 6] R.B. Wu, Y Pan, G.Y. Yang, MX. Gao, LL Wu, JJ. Chen, R. Zhai, ]. Lin, ]. Phys. hem.C11(2007)6233 the lateral growth rate decrease, followed by the decrease of the [17] H. Zhang C Wang, L Wang Nano Lett. 2(2002)941 Please cite this article in press as: G - Y. Li, et al., Ultra long Sic nanowires with fluctuating diameters synthesized in a polymer pyrolysis CVD route, Solid State Sci. (2009). doi: 10.1016j-solidstatesciences 2009.09.003contained in the EDX pattern (Fig. 3c), and it is very common in SiC nanowires synthesis in many published reports because of the inevitable oxygen mixed in the carrying gases [4,9]. Fig. 5d is the corresponding SAED pattern of the nanowire. The clear diffraction spots indicate a good single crystal nature of the nanowires. And by indexing the pattern, we can see the nanowires grew along the C111D direction. From the above characterization, we demonstrated that the white cotton-like products were b-SiC nanowires. Several mecha￾nisms have been proposed for the growth of SiC nanowires, including vapor–liquid–solid (VLS) [13,27], vapor–solid (VS) [28], solid–liquid–solid (SLS) [29] mechanisms. The SiC nanowires generally grow along the C111D direction due to the lowest surface energy of the (111) plane. In the present work, we did not find any droplets during the SEM observations; so the VLS and SLS mecha￾nisms are not suitable for the as-prepared SiC nanowires growth. The as-prepared SiC nanowires were grown on a graphite wafer beyond the raw materials; the Si and C elements in the nanowires must come from the pyrolysis of polycarbosilane, and was trans￾ported to the substrate by the flowing N2. Therefore, the SiC nanowires may be grown by the VS mechanism in a CVD route. As for the SiC nanowires with non-uniform diameters growth, Hao suggested that the periodically alternating Si and C concen￾trations in the catalytic droplets lead to the formation of the beaded morphology [13]. Wang prepared SiC nanowires with modulated diameters via a VLS process, and proved that the pressure of the source species was responsible for the special morphology [12]. In the two cases, the diameters of the SiC nanowires were determined by the volume of the catalytic droplets, which was varying due to the fluctuating contents of the dissolved Si and C elements. Although the as-prepared SiC nanowires in our work were not governed by the VLS mechanism, the concept of the pressure or the concentration will be also suitable for our case. The proposed growth process can be illustrated in Fig. 6. At high temperature, silane fragments from the slow decom￾position of polycarbosilane will serve as the direct source species for the nanowires growth. At certain time, the local partial pressure of the silane fragments around the SiC nucleus is decreasing because of the depletion in the CVD reaction (Fig. 6a and b). The fresh silane fragments cannot be provided timely by the carrying N2 gas, because the flowing rate of N2 is too slow (about 5 sccm). Therefore, the deposition rate and the crystal growth rate including the lateral growth rate decrease, followed by the decrease of the diameter. Although the flowing rate of N2 is slow, the local silane fragments will be still supplied subsequently by the carrying gas. Once its concentration reaches a high degree, the reaction rate and the lateral growth rate will increase, and thus the diameter becomes larger (Fig. 6c). With the periodically alternating concentration of local source species, the final diameter fluctuating SiC nanowires are formed (Fig. 6d). The flowing rate of the carrying gas plays important role during the growth process. When the flowing rate of N2 gas was increased to about 15 sccm, SiC nano￾wires with relatively homogeneous diameters were obtained as shown in Fig. 7. Several groups found some SiC nanowires when preparing porous SiC ceramics with polycarbosilane as adhesive agent [30– 32]; however, the length was only in micrometers scale. While in our case, the nanowires length was up to millimeters. As the chemicophysical properties of 1D nanostructures are highly size (diameter and aspect ratio) dependent, the high-aspect-ratio SiC nanowires will provide opportunities for both fundamental research and technical applications in nanodevices [33]. 4. Conclusions Large areas of millimeters long b-SiC nanowires with fluctuating diameters were synthesized by the decomposition of poly￾carbosilane in a chemical vapor deposition route. The nanowires had the non-periodical fluctuating diameters in the range of 100– 250 nm along the axial direction, and were composed of well crystalline b-SiC along the C111D direction. By the VS mechanism, we explained that the very slow flowing rate of the carrying gas was responsible for the diameter fluctuation. It is believed that the millimeters long SiC nanowires with fluctuating diameters would find applications in various areas such as building blocks in NEMS, nanocomposites, and electronic nanodevices. Acknowledgment The authors are thankful for the financial support from the National Natural Science Foundation of China (Grant No. 50702075) and the Research Fund of State Key Laboratory of CFC (Grant No. 9140C8202050804). References [1] J.B. Casady, R.W. Johnson, Solid State Electron. 39 (1996) 1409. [2] Z.W. Pan, H.L. Lai, C.K. Frederick, X.F. Duan, W.Y. Zhou, W.S. Shi, N. Wang, C.S. Lee, N.B. Wong, S.T. Lee, S.S. Xie, Adv. Mater. 12 (2000) 1186. [3] C.O. Jang, T.H. Kim, S.Y. Lee, D.J. Kim, S.K. Lee, Nanotechnology 19 (2008) 34520334. [4] H.K. Seong, H.J. Choi, S.K. Lee, J.I. Lee, D.J. Choi, Appl. Phys. Lett. 85 (2004) 1256. [5] L.G. Zhang, W.Y. Yang, J. Hua, Z.H. Zheng, Z.P. Xie, H.Z. Miao, L. An, Appl. Phys. Lett. 89 (2006) 143101. [6] Y.F. Zhang, X.D. Han, K. Zheng, Z. Zhang, X. Zhang, J. Fu, Y. Ji, Y.J. Hao, X.Y. Guo, Z.L. Wang, Adv. Funct. Mater. 17 (2007) 3435. [7] W. Yang, H. Araki, C.C. Tang, S. Thaveethavorn, A.K. Hiroshi, T. Noda, Adv. Mater. 17 (2005) 1519. [8] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [9] W.M. Zhou, L.J. Yan, Y. Wang, Y.F. Zhang, Appl. Phys. Lett. 89 (2006) 13105. [10] J.J. Niu, J.N. Wang, Q.F. Xu, Langmuir 24 (2008) 6918. [11] D.H. Wang, D. Xu, Q. Wang, Y.J. Hao, G.Q. Jin, X.Y. Guo, K.N. Tu, Nanotechnology 19 (2008) 215602. [12] H.T. Wang, Z.P. Xie, W.Y. Yang, J.Y. Fang, L.N. An, Cryst. Growth Des. 8 (2008) 3893. [13] Y.J. Hao, J.B. Wagner, D.S. Su, G.Q. Jin, X.Y. Guo, Nanotechnology 17 (2006) 2870. [14] G.Z. Shen, Y. Bando, D. Golberg, Cryst. Growth Des. 7 (2007) 35. [15] Z.S. Wu, S.Z. Deng, N.S. Xu, J. Chen, J. Zhou, J. Chen, Appl. Phys. Lett. 80 (2002) 3829. [16] R.B. Wu, Y. Pan, G.Y. Yang, M.X. Gao, L.L. Wu, J.J. Chen, R. Zhai, J. Lin, J. Phys. Chem. C 111 (2007) 6233. [17] H. Zhang, C. Wang, L. Wang, Nano Lett. 2 (2002) 941. Fig. 7. SEM image of the SiC nanowires with relatively homogeneous diameter. G.-Y. Li et al. / Solid State Sciences xxx (2009) 1–6 5 ARTICLE IN PRESS Please cite this article in press as: G.-Y. Li, et al., Ultra long SiC nanowires with fluctuating diameters synthesized in a polymer pyrolysis CVD route, Solid State Sci. (2009), doi:10.1016/j.solidstatesciences.2009.09.003