Deposition of carbon nanotubes on si nanowires by chemical vapor deposition

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Deposition of carbon nanotubes on Si nanowires by chemical vapor deposition Y.F. Zhang, Y.H. Tang, Y. Zhang, C.S. Lee, I. Bello, S.T. Lee * Center of Super-Diamond & Advanced Films (COSDAF) and Department of Physics & Mater. Sci., City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China Received 19 July 2000 Abstract By using a hot ®lament chemical vapor deposition (HFCVD) method, deposition of carbon on Si nanowires (Si NWs) has been studied. Multi-walled carbon nanotubes (CNTs) were found to form on the surfaces of Si NWs at 900°C with a good surface coverage and adherence. However, as the temperature of deposition increased to 1000°C, Si cores tended to transform into b-SiC cores and the carbon layers grown on b-SiC cores were distorted. When the temperature of deposition was as high as 1100°C, the carbon layers bucked openly to form many feather-like carbon sheets sprouting from the surface of the nanowires. A mixture of large carbon sheets and b-SiC nanowires was formed when the temperature was over 1300°C. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Carbon nanotubes (CNTs) [1] and Si nano- wires (SiNWs) [2,3] have attracted considerable attention in recent years because they have demonstrated the potential to make a major con- tribution to a variety of nanotechnological applications [4±6]. It leads to speculations that the modi®cation and combination of these two kinds of nanomaterials, such as ®lling CNTs with silicon or coating Si NWs with CNTs, will be more ro- bust and result in an even more diverse range of applications. Up to date, a composite of CNTs with SiNWs in longitudinal has been synthesized [7]. However, a composite of these two materials in transversal has not been reported. Only CNTs sheathed on other materials including in situ growth in carbon gas mixtures and capillarity- driven ®lling of open nanotubes by liquid reagents have been reported as nanocables [8±11]. It should be noticed that the percentage of ®lled nanocables was practically either very low [9,11] or with very small length to diameter ratios [10,11]. More re- cently, we have reported a new oxide-assisted growth method by which high-purity SiNWs can be synthesized in large scale from a mixture of Si and SiO 2 powders or from pure SiO powder [12± 15]. This supplied a good as-grown nanoscale materials to synthesize a composite of CNTs and SiNWs in transversal. Here, we report the syn- thesis of this composite material by a hot ®lament chemical vapor deposition (HFCVD) method. The results provided a way to make CNTs coat on SiNWs or SiC nanowires for further applica- tions in both nanoscale electronic devices and composite materials. 3 November 2000 Chemical Physics Letters 330 (2000) 48±52 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +852-2784-4696. E-mail address: apannale@cityu.edu.hk (S.T. Lee). 0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2614(00)01084-8 2. Experimental The as-grown Si NWs used in the present ex- periment were produced by excimer pulsed laser ablation of a target made of a mixture of Si and SiO 2 powders under 9 Â 10 4 Pa Ar atmosphere at 1200°C. The experimental details were the same as those reported in our previous paper [3]. The experimental apparatus employed for coating the Si NWs by CNTs was a bell-jar shaped RF- plasma HFCVD system. One of the RF elec- trodes made of a Mo sheet parallel to the ®lament was used as the substrate holder. This sheet was insulated from the water-cooled stainless grounded supporter by putting it on a quartz plate. The other RF electrode was placed 4 cm above. The ®lament was placed between these two RF electrodes. The distance between the substrate and the ®lament could be adjusted. The as-grown SiNWs were put on a silicon sheet as a substrate. 100 sccm gas ¯ow rate with 8% of methane in hydrogen was fed from above the ®lament toward SiNWs. The electrodes were supplied with a 13.56 MHz RF power source through a L±C matching network. First, the amorphous silica outerlayer of the as-grown SiNWs was removed by RF plasma for 120 min with 300 W plasma power under 1X5 Â 10 2 Pa gas pressure at 300°C substrate temperature. Then, the pre-carbonized tungsten ®lament above the substrate surface was electri- cally heated to 2100°C. The temperature of the substrate was measured to be 900°C. The carbon deposition on SiNWs was carried out at 3 Â 10 3 Pa for 10 h. 3. Results and discussions The as-grown SiNWs used in this experiment for carbon coating were of high purity with a whitish yellow color. Transmission electron mi- croscope (TEM) image showed the nanowires to have primarily smooth and uniform wire-like structures (Fig. 1a). The average diameter was at 15 nm. Their lengths could extend up to a few millimeters with nearly the same diameter throughout the length. The structure of the nanowires was con®rmed to consist of a crystalline Si core and a silica outerlayer by using selected- area electron diraction (SAED) pattern (inset of Fig. 1a) and high resolution transmission electron microscope (HREM) (Fig. 1b). Fig. 1. (a) TEM image of the as-grown SiNWs. Inset is a SAED pattern; (b) HREM image of a typical SiNW. Y.F. Zhang et al. / Chemical Physics Letters 330 (2000) 48±52 49 After the deposition, the color of the nanowires changed to black. Fig. 2a showed the morphology of carbon coated on SiNWs as observed with scanning electron microscopy (SEM). It can be seen that the nanowires remained as a web on the Si substrate. Some carbon particles were also found to co-exist during the carbon coating pro- cesses. Fig. 2b showed a TEM image of the same sample and its corresponding SAED pattern. The diraction rings of crystalline cubic Si and b-SiC could be identi®ed. High resolution TEM images revealed that Si NW with carbon coating consisted of a single crystalline Si core and a sheath of carbon muti- layers with an inter-layer spacing of 0.34 nm (Fig. 3a). The multi-walled CNTs showed good uniformity and adherence to the Si core. However, there were also some disturbed interface areas of CNTs and SiNWs. The original silicon dioxide layer on the Si core disappeared, presumably due to the etching eect of atomic hydrogen in the plasma. Fig. 3b showed a nanowire with the b-SiC core and a sheath of only a few carbon layers. It had been noticed that two kinds of nanowires shown in Fig. 3a, b were from the same specimen. This case was owing to the poor thermal contact between the nanowires themselves and also with the Si substrate. So the temperatures of individual nanowires were dicult to be kept uniform during the carbon deposition process. When the substrate temperature was increased to 1100°C, electron diraction revealed that the cores of the nanowires were transformed to b-SiC completely. It could be seen from the electron diraction pattern in the right bottom corner of Fig. 4a. The carbon layers could not self-organize into nanotubes completely and bucked to form many feather-like sheets on the nanowire surface, as shown in the upper image in Fig. 4a. A typical nanowire in high magni®cation was shown more clearly in the left bottom image in Fig. 4a. As the deposition temperature increased to 1300°C, the coated carbon grew to become large carbon sheets, as shown in the bottom image in Fig. 4b. Even the nanowires were completely carbonized and mixed into a lot of carbon sheets to form a mixture of Fig. 2. (a) SEM image of the web-like product; (b) corresponding TEM image and SAED pattern. 50 Y.F. Zhang et al. / Chemical Physics Letters 330 (2000) 48±52 b-SiC nanowires and graphitic carbon, as shown in the top image in Fig. 4b. The characteristics of carbon deposition on the crystalline Si NW can be envisaged to be con- trolled by both a template eect for the formation of tube-like carbon shell structure and a carbon- ization eect for the transformation of Si NWs to SiC nanowires. At a relatively low substrate tem- perature ($900°C), the deposition rate of carbon on the surface of Si NWs was slow and if a carbon nucleation happened on a nanowire surface, it was possible that a carbon tube-like network formed around the nanowire. As the diameters of the nanowires were very small, tube-like carbon structures were more stable than any other bulk carbon structures. However, if the substrate tem- perature was high enough ($1000°C) so that SiC can be formed by carbon atoms diusing into the Si NWs, CNTs could not be formed very well on the surface of nanowires because of both the volume change of the nanowires and breaking of the carbon network sheathed on the nanowires. When the substrate temperature was relatively high (b1100°C), a mixture of graphitic carbon and SiC nanowires was formed because of both the carbonization of the nanowires and the fast carbon deposition to form the carbon-like structure. 4. Conclusions In summary, CNTs encapsulated crystalline SiNWs were synthesized by the HFCVD method. At $900°C, carbon multilayers were formed on the surface of carbonized Si NWs. At a higher reaction temperature ($1000°C), the silicon cores reacted with carbon and transformed into b-SiC cores. Carbon shells formed on the SiC core were not as uniform as those on the Si core. As the substrate temperature increased further to 1100°C, Fig. 3. (a) HRTEM image of a nanowire consisting of a crystalline Si core and a sheath of multi-walled CNTs; (b) a nanowire with a part of the core transformed into b-SiC and coated with a sheath of CNTs. Y.F. Zhang et al. / Chemical Physics Letters 330 (2000) 48±52 51 feather-like carbon sheets sprouting from the sur- face of the nanowires were formed. At an even higher deposition temperature (1300°C), a mixture of carbon pieces with b-SiC nanowires was ob- tained. Acknowledgements The authors wish to thank Prof. F.H. Li and Prof. X.F. Duan for useful discussions. This work is supported by the Research Grants Council of Hong Kong (Project no. 9040365). References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [3] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835. [4] A.G. Rinzler, J.H. Hafner, P. Nikolaev, L. Lou, S.G. Kin, D. Tomanek, P. Nordlander, D.T. Colbert, R.E. Smalley, Science 276 (1995) 1550. [5] F.C.K. Au, K.W. Wong, Y.H. Tang, Y.F. Zhang, I. Bello, S.T. Lee, Appl. Phys. Lett. 75 (1999) 1700. [6] S.W. Chung, J.Y. Yu, J.R. Heath, Appl. Phys. Lett. 76 (2000) 2068. [7] J. Hu, M. Ouyang, P. Yang, C.M. Lieber, Nature (London) 399 (1999) 49. [8] C.H. Wang, J.S. Choi, T.T. Tran, A.D. Bacher, J. Phys. Chem. B 103 (1999) 7449. [9] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [10] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159. [11] J. Sloan, J. Hammer, M.Z. Sibbley, M.L.H. Green, Chem. Commun. 3 (1998) 347. [12] S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bull. 24 (1999) 36. [13] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, S.T. Lee, Phys. Rev. B 58 (1999) R16024. [14] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, I. Bello, S.T. Lee, Chem. Phys. Lett. 299 (1999) 237. [15] N. Wang, Y.F. Zhang, Y.H. Tang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 73 (1998) 3902. Fig. 4. (a) TEM image of the nanowires with carbon deposition at 1100°C, and a higher magni®cation image of a single nanowire in the same sample at the left bottom corner and the corresponding SEAD pattern at the right bottom corner; (b) TEM image of the nanowires with carbon deposition at $1300°C and the higher magni®cation image of a part of the sample in the upper image. 52 Y.F. Zhang et al. / Chemical Physics Letters 330 (2000) 48±52 . 2000 Abstract By using a hot ®lament chemical vapor deposition (HFCVD) method, deposition of carbon on Si nanowires (Si NWs) has been studied. Multi-walled carbon nanotubes. Deposition of carbon nanotubes on Si nanowires by chemical vapor deposition Y.F. Zhang, Y.H. Tang, Y. Zhang, C.S. Lee, I. Bello, S.T. Lee * Center of