Đang tải... (xem toàn văn)
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
Polymer-assisted synthesis of aligned amorphous silicon nanowires and their core/shell structures with Au nanoparticles Xing-bin Yan a,b , Tao Xu a , Shan Xu a,b , Gang Chen a,b , Qun-ji Xue a , Sheng-rong Yang a, * a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b Graduate School of the Chinese Academy of Sciences, Beijing, 100083, China Received 13 August 2004; in final form 23 August 2004 Available online 11 September 2004 Abstract Aligned amorphous Si nanowires (SiNWs) were synthesized directly from Si substrates with the assistance of a new carbon-based network polymer, poly(phenylcarbyne), during the heat-treatment in Ar atmosphere at 1120 °C. A core/shell structure of SiNWs wrapped with Au nanoparticles was simply fabricated as well. The analytic results of the morphology and microstructure confirmed the orientation and the amorphous nature of the SiNWs, and the high dispersion of Au nanoparticles on the surface of the SiNWs without any aggregation. The formation of the SiNWs was explained on the basis of the reaction of carbon with the native silica layer covering Si substrates. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Silicon-based nanoscale materials have attracted much attention in recent years for their valuable semi- conducting, mechanical, and optical properties, as well as their potential applications in mesoscopic research and nanodevices. They are, for example, considered as candidates for one-dimensional quantum transistor, composites, and light-emitting diodes [1,2] . Conse- quently, a great deal of effort has been made in fabricat- ing Si-based nanostructures, especially silicon nanowires (SiNWs). Upto now, several methods have been em- ployed to produce SiNWs, including chemical vapor deposition (CVD) [3], thermal evaporation of Si powder [4], metal catalyzed vapor–liquid–solid method [5], laser ablation [6], oxygen-assisted synthesis [7], heating SiO 2 – Si mixtures or pure SiO powders [8,9], and solution etch- ing [10]. Furthermore, it has been recently reported that enhanced yields of SiNWs are obtained by heating a Si substrate coated with carbon nanoparticles at 1050 °C under vacuum [11]. Wherein, the role of carbon is to react with the oxide probably producing a suboxide-type species. Although SiNWs can be mass-produced by the above methods, it is hard to increase the electrical conductivity of SiNWs during the growth process. A number of strat- egies to modify the electronic structure of SiNWs have been developed, including deposition Ag particles on SiNWs [12], or formation of metal silicide layers on SiNWs [13]. This new type of composite SiNWs is important for the potential applications in future micro- electronic and optoelectronic devices. In this communication, we report that aligned amor- phous SiNWs can be synthesized directly from Si sub- strates with the assistance of a new carbon-based network polymer, poly(phenylcarbyne). Moreover, a core/shell structure of SiNWs wrapped with highly dis- persed Au nanoparticles is simply fabricated as well. The methods significantly simplify the preparation of 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.08.099 * Corresponding author. Fax: +86 931 8277088. E-mail address: xbyan@lsl.ac.cn (S. Yang). www.elsevier.com/locate/cplett Chemical Physics Letters 397 (2004) 128–132 aligned SiNWs and the metal–SiNWs composite materi- als, and make the processes more cost-effective. 2. Experimental The poly(phenylcarbyne), PPC, was inexpensively synthesized by the procedure: the reduction of the appropriate PhCCl 3 monomer, with an ultrasonically generated emulsion of Na–K alloy and an ethereal sol- vent, tetrahydrofuran (THF), reported in detail else- where [14,15]. This polymer is composed of a randomly constructed network of tetrahedral hybridized phenylcarbyne units and can be converted into dia- mond-like carbon by pyrolysis [15]. Freshly prepared PPC powder was dissolved THF, followed by spin- coating onto the surfaces of different type single crystal silicon substrates (10 0) and (1 1 1), respectively, that sequentially cleaned with deionized water, ethanol, and acetone, to allow the formation of the polymer film with the thickness of 200 nm after the removal of the THF by evaporation at 60 °C. The polymer film on the silicon substrate was inserted into a quartz tube, heated in an Ar atmosphere at 10 °C/min to 1120 °C, and held at 1120 °C for 2 h. After this treatment, the targe t amor- phous SiNWs were obtained. The freshly prepared SiNWs were peeled off from Si substrate and dispersed in a solution of HAuCl 4 (10 mL, 0.1 M) with the aid of ultrasonication to give a sus- pension. The suspension was supersonicated for 30 min, aged for 1 h, and was gravi ty-filtered and air-dried at room temperature, respectively. Finally, the SiNWs wrapped highly dispersed Au nanoparticles were ob- tained by heating them upto 300 °C at a rate of 5 °C/ min and keeping them at that temperature for 1 h under Ar atmosphere in a seal pyrolysis quartz tube. The morphology and structure of nanowires (were examined by scanning electron microscopy (SEM, JSM-5600LV), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM, JEM - 1200EX), selected area electron diffraction (SAED), micro-Raman backscattering spectroscopy (Raman, Jo- bin Yvon T64000), and X-ray photoelectron spectros- copy (XPS, Perkin–Elmer PHI-5702). 3. Results and discussion After pyrolysis of the polymer/Si, a blue tinted gray colored wool-like film was all deposited on the different type Si sub strates, which indica tes that the type of Si substrate does not affect the formation of the SiNWs. The SEM images in Fig. 1a reveal that the nanowires (NWs) are highly aligned perpendicular to the Si sub- strate. As is seen form the cross-sectional view along the edge of the scratched film, the orientation of the NWs is widespread over the whole substrate. The homo- geneous thickne ss of the NWs film is easily obtained to be about 500 lm. Thus, the growth rate of the NWs is estimated to be about 70 nm/s. The magnified SEM im- age in the right inset shows that the NWs appear dense and parallel to each other. More interestingly, the SEM images in Fig. 1b reveal some dandelion-like wires. As is seen form the top view of the NWs film, the relatively straight nanowire spli ts into several curly sub-branche s to different orientation. The magnified image in the right inset shows these sub-branches are similar to wire-like spherical particle agglomerates with the width range from nanoscale to micron-scale. The EDX spectrum of aligned NWs on the cross- section of the SiNWs film, shown in Fig. 2a, reveals these NWs are mainly composed Si. The remaining oxygen peak comes from the surfa ce oxidation of the nanowires and the atomic rate for Si:O in these nano- wires is 7:1 on average. While, the ED X spectrum of particle-linked wires on the tip of SiNWs film, shown in Fig. 2b, reveals that these wires consist of Si, C and O, and further quantitative analysis shows that the atomic ratio for Si:C:O is ca. 1.7:7.3:1, indicating that these particles are mainly composed carbon, which can provide evidence for the growth mechanism of the SiNWs. The possibi lity of the formation amorphous sil- icon oxide nanowires can be excluded by the following discussions: first, PPC is a non-oxygenous carbon-based Fig. 1. SEM images of the SiNWs homogeneously grow on a large area of the Si substrate: (a) cross-sectional view; (b) top view. X. Yan et al. / Chemical Physics Letters 397 (2004) 128–132 129 network polymer; second, the pyrolysis quartz tube had been vacuumized before inputting Ar atmosphere; third, though the Si substrate have thin native oxide layer, it is too thin (usually less than 1 nm, measured by an ellips- ometer) to grow silicon oxide nanowires with the thick- ness of 500 lm. The low magnification TEM image in Fig. 3a shows the general morphology of the SiNWs. The diameters are in the range from several tens to hundreds of nanom- eters. Most of them are straight and have a smooth sur- face. Fig. 3b shows a TEM image of individual curly wire with the average diameter of 400 nm, revealing the typical structure of wire-like spherical particle agglomerates at the end of the wire. Fig. 3c shows a magnified TEM image of a part of individual relatively straight SiNW with the same diameter. The correspond- ing highly dispersed selected area electron diffraction (SAED) pattern reveals its amorphous nature (inset). Fig. 3d,e show TEM images of the core/shell structure of the SiNWs wrapped Au nanoparticles. The average particle size of the wrapped Au nanoparticles in the composite SiNWs is 5 ± 1.5 nm. It is noted that the Au nanoparticles are well dispersed on the surface of the SiNWs without any aggregation. The SAED pattern (a) Si O Intensity (a.u.) Intensity (a.u.) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 (b) Si C O Energy (KeV) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Energy (KeV) Fig. 2. EDX spectra of the aligned nanowires (a) and the particle-linked wires (b). Fig. 3. (a) Low magnification TEM image of SiNWs; (b) TEM image of a single particle-linked wire; (c) TEM image of a single straight SiNWs and the corresponding highly dispersed selected area electron diffraction (SAED) pattern; (d) and (e) TEM images of SiNWs wrapped Au nanoparticles and the corresponding SAED pattern. 130 X. Yan et al. / Chemical Physics Letters 397 (2004) 128–132 in the inset of the TEM image of individual Au–SiNW shows diffraction rings, which is corresponded to Au crystal. Moreover, the content of the Au nanoparticles in the Au–SiNWs is 5 at.%, which is determined by EDS. Part of the SiNWs film was peeled off from Si sub- strate and used to measure the Raman. Two peaks around 301 and 519 cm À1 were observed, as is shown in Fig. 4. It is wel l known that those two Raman peaks are characteristic of a silicon structure, corresponding to the second-order transverse acoustic phonon mode (2TA), and the first-order transverse optical phonon mode (TO) of silicon, respectively. The Raman result confirms that the nanowires are composed of silicon. In order to study the growth mechanism of SiNWs in our synthesis system, two additional experiments were carried out under similar conditions: (1) in the absence of PPC film on Si substrate to verify whether carbon plays an important role in the formation of the SiNWs; (2) in the addition of a procedure that Si substrate was cleaned with aqueous solut ion of 5% HF for 10 min to remove the native oxide layer, to verify whether the native oxide layer plays another important role in the formation of the SiNWs. The above experiential results show that there is no nanowire existing on the surface of the Si substrate. It indicates that the polymer film and the native oxide layer on Si substrate are both necessary for fabri cating SiNWs. The PPC polymer has a hydrocarbon network mainly composed of tetrahedral hybridized carbon atoms, each bearing one phenyl. Under high temperature of heat- treatment, phenyl rings and an amount of hydrogen evolved from the polymer network, resulted in the con- version of diamond-like carbon phase from polymer phase [15]. TEM analysis of carbon films prepared by the heat-treatment of PPC above 600 °C confirmed that the carbon pha se was composed of small carbon nanopar- ticles [14]. Moreover, when the temperature is below 1050 °C, the product is amorphous carbon film coated on Si substrate; when the temperature is 1050–1100 °C, the surface of the carbon film changes very rough and there are some erodible taints on Si substrate, which indi- cates that chemical reaction may take place; when the temperature is above 1100 °C, aligned amorphous SiNWs film is main product. Thus, we think that the higher pyro- lysis temperature (1120 °C) will lead to a large amount of carbon nanoparticles having high chemical activity. The growth of SiNWs may start from the reaction of the active carbon nanoparticles with the native oxide layer on Si substrate [11,16,17]. This native oxide layer is reduced by carbon nanoparticles to yield silicon monoxide, and then the nucleation site of the Si nanostructures is formed by the decomposition of silicon monoxide. The above reactions are proposed as below: Si x O 2 ! Si x O þ CO ðx > 1Þð1Þ Si x O ! Si xÀ1 þ SiO ð2Þ 2SiO ! Si þ SiO 2 ð3Þ Finally, aligned SiNWs are grown perpendicular to the Si substrate at 1120 °C. Moreover, due to the carbon particles are superfluous compared with native oxide layer, remnant carbon particles will stay on the top of the SiNWs and form wire-like spherical particle agglom- erates, linking with SiNWs. Based on the above discus- sions, we propose that the formation of the SiNWs follows the conversion from PPC to carbon particles, the reduction from native oxide layer to silicon monox- ide, and the growth of SiNWs, which is schematically shown in Fig. 5. The SiNWs are in amorphous state instead of a crys- talline phase in our synthesis. It may be due to the much too growth rate (about 70 nm/s). We speculate the growth rate is so rapid that the Si atoms have no time to stack themselves into crystalline order. Furthermore, due to high density of the SiNWs film, the van der Waals force between nanowires should be large. Thus, this interaction force plays an important role to keep the SiNWs grow upward and to be highly oriented [18]. For the formation mechanism of the core/shell struc- ture of SiNWs wrapped highly dispersed Au nanoparti- cles, we explain as followed: unde r the ultrasonication, some AuCl À 4 anions in the solution of HAuCl 4 are uni- formly absorbed on the surface of the SiNWs. Because of the high standard electrode potential of the Au 3+ /Au 0 couple, Au 3+ has high polarization and high chemical reactivity. Au 3+ could be easily reduced to Au 0 in air at 200 °C [19]. Thus, the Au–SiNWs can be obtained by heat-treatment of the SiNWs and HAuCl 4 Æ H 2 O at 300 °C in Ar atmosphere. It is believed that all chlorin have been evaporated during the thermal 200 300 400 500 600 700 800 900 2TA Intensity (a.u.) Raman shift (cm -1 ) TO Fig. 4. Raman spectrum of the SiNWs scratched off from the substrate. Two peaks at 302 and 520 cm À1 were observed which correspond to 2TA and TO modes of silicon, respectively. X. Yan et al. / Chemical Physics Letters 397 (2004) 128–132 131 processes, which is confirmed by EDS and XPS analysis of chlorin. 4. Conclusion In summary, we have success fully synthesized highly aligned amorphous SiNWs on a large-area Si substrate with the assistance of PPC polymer, and simply fabri- cated SiNWs wrapped highly dispersed Au nanoparti- cles. In our synthesis system, Si substrate is used as the source of SiNWs, and the native oxide layer and car- bon-based network polymer film are crucial for the growth of SiNWs. We expect that the present method would be a promising way for a mass -production of highly aligned SiNWs and their core/shell structures with metal nanoparticles for the future applications in microelectronic and optoelectronic dev ices. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 50172052, 50375151 and 50323007), 863 program (Grant No. 2003 AA305670) and ÔTop Hundred Talents ProgramÕ of Chinese Academy of Sciences for financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version at doi:10.1016/ j.cplett.2004.08.099. References [1] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [2] A.P. Alivisatos, Science 271 (1996) 933. [3] X.Y. Zhang, L.D. Zhang, G.W. Meng, G.H. Li, N.Y.J. Phillipp, F. Phillipp, Adv. Mater. 13 (2001) 1238. [4] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J. Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q. Feng, Appl. Phys. Lett. 72 (1998) 3458. [5] M.K. Sunkara, S. Sharma, R. Miranda, G. Lian, E.C. Dickey, Appl. Phys. Lett. 79 (2001) 1546. [6] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [7] R.Q. Zhang, Y. Lifshitz, S.T. Lee, Adv. Mater. 15 (2003) 635. [8] J.L. Gole, J.D. Stout, W.L. Rauch, Z.L. Wang, Appl. Phys. Lett. 76 (2000) 2346. [9] C.P. Li, C.S. Lee, X.L. Ma, N. Wang, R.Q. Zhang, S.T. Lee, Adv. Mater. 15 (2003) 607. [10] K.Q. Peng, Z.P. Huang, J. Zhu, Adv. Mater. 16 (2004) 73. [11] S. Botti, R. Gardi, R. Larciprete, A. Goldoni, L. Gregoratti, B. Kaulich, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 371 (2003) 394. [12] X.H. Sun, H.Y. Peng, Y.H. Tang, W.S. Shi, N.B. Wong, C.S. Lee, S.T. Lee, T.K. Sham, J. Appl. Phys. 89 (2000) 6396. [13] C.P. Li, N. Wang, S.P. Wong, C.S. Lee, S.T. Lee, Adv. Mater. 14 (2002) 218. [14] X.B. Yan, T. Xu, X.B. Wang, H.W. Liu, S.R. Yang, Carbon 42 (2004) 232. [15] G.T. Visscher, D.C. Nesting, J.V. Badding, P.A. Bianconi, Science 260 (1993) 1496. [16] C.N.R. Rao, G. Gundiah, F.L. Deepak, A. Govindaraj, A.K. Cheetham, J. Mater. Chem. 14 (2004) 440. [17] G. Gundiah, F.L. Deepak, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 381 (2003) 579. [18] X.H. Chen, Y.J. Xing, J. Xu, J. Xiang, D.P. Yu, Chem. Phys. Lett. 374 (2003) 626. [19] H. Kozuka, S. Sakka, Chem. Mater. 5 (1993) 222. PPC film Si x O 2 film Si substrate C film C particles Sizeable SiO SiNWs Diffusion of Si Particle-linked wires Fig. 5. A schematic diagram of the growth mechanism of amorphous SiNWs from Si substrate with the assistance of polymer-converted carbon film. 132 X. Yan et al. / Chemical Physics Letters 397 (2004) 128–132 . Polymer-assisted synthesis of aligned amorphous silicon nanowires and their core/ shell structures with Au nanoparticles Xing-bin. and the amorphous nature of the SiNWs, and the high dispersion of Au nanoparticles on the surface of the SiNWs without any aggregation. The formation of