ARTICLE IN PRESS Solid State Sciences xxx(2009)1-6 Contents lists available at science Direct Solid state sciences ELSEVIER journalhomepagewww.elsevier.com/locate/ssscie Ultra long SiC nanowires with fluctuating diameters synthesized in a polymer pyrolysis cvd route Gong-Yi Li, Xiao-Dong Li, Hao Wang, Lin Liu State Key Laboratory of Advanced Ceramic Fibers 8 Composites, College of Aerospace S Materials Engineering National University of Defense Technology, Changsha 410073, China ARTICLE INFO A BSTRACT rge areas of millimeters long B-SiC es with fluctuating diameters were sy mer pyrolysis CVD(PPCVD)route. Polycarbosilane was used as the raw material. The morphology structure of the nanowires were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the nanowires had non-periodically fluctu- vailable online xxx ating diameters in the range of 100-250 nm along the axial direction, and were composed of well talline B-Sic along the(111)direction. The vapor-solid(vs)mechanism was employed to interpret PACS he nanowires growth procedure, and the diameter fluctuation was resulted from the varying concen- 2009 Elsevier Masson SAS. All rights reserved. SiC nanowires Polycarbosilane 1. Introduction Wang prepared SiC nanowires with Eiffel-tower shape and spindle shape by varying the pressure of the source species [12]. Shen As a wide band gap semiconducting materials and a high synthesized Sic nanowires with numerous thin nanoplatelets temperature resistant ceramic, silicon carbide(sic)exhibits excel- perpendicular to the nanowires'core stem in a catalyst-assisted lent properties at high temperature, high power, high frequency. thermochemical process [14. Other kinds of Sic nanowires with high radiation environment and chemical harsh conditions [1. One special morphology such as needle-shaped nanowires [15, 16]. dimensional Sic nanostructures such as nanowires are superior to nanosprings[17]. nanoflowers [18 have also reported eir bulk counterpart, and have attracted considerable attention in Ultra long nanowires would be very important to study the recent years due to their importance in basic science and their mechanical, thermodynamic, and electric properties of nano otential application in panel displays [2, electronic nanodevices structures on a macroscopic scale compared with their short [ 3,4]. optoelectronic nanodevices [4, 5, nanocomposites [6-8. counterpart [19-21 Lieber's group have successfully prepared hotocatalysts 9, hydrophobic devices 10), etc. SiC nanowires with millimeters long Si nanowires, and integrated one single Si nano- special morphology, such as diameters fluctuation nanowires, will wire into one-dimensional arrays of field-effect transistors (22 find new applications in nanocomposites [11 and building blocks They consider that the ultra long nanowires may open up new for nanoelectro-mechanical systems(NEMS). For example, the opportunities for integrated nanoelectronics and could serve as ariations in nanowires diameters could be used as bench-marks for unique building blocks linking integrated structures from the precisely positioning them in device fabrication[12. Guo's group nanometer through macroscales. Additionally, ultra long nanowires synthesized beaded and periodically twinned sic nanowires by the can be easily spun into continuous yarns with high strength, similar carbothermal reduction of a carbonaceous silica xerogel [11, 13]. to carbon nanotubes yarns [23, resulting in them being more competitive for use in nanocomposites. Cai has reported millime- ters long Sic nanowires by pyrolysis of hexamethyldisilane [24). However, ultra long Sic nanowires with fluctuating diameters have neve 2558/s-see front matter o 2009 Elsevier Masson SAS. All rights reserved. 10. 1016 j.solidstatesciences 2009.09.003 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 2009.09.003
Ultra long SiC nanowires with fluctuating diameters synthesized in a polymer pyrolysis CVD route Gong-Yi Li*, Xiao-Dong Li*, Hao Wang, Lin Liu State Key Laboratory of Advanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, China article info Article history: Received 18 June 2009 Received in revised form 31 August 2009 Accepted 1 September 2009 Available online xxx PACS: 81.05.Hd 81.07.Bc 81.10.BK Keywords: SiC nanowires VS CVD Polycarbosilane Pyrolysis abstract Large areas of millimeters long b-SiC nanowires with fluctuating diameters were synthesized in a polymer pyrolysis CVD (PPCVD) route. Polycarbosilane was used as the raw material. The morphology and structure of the nanowires were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results showed that the nanowires had non-periodically 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. The vapor–solid (VS) mechanism was employed to interpret the nanowires growth procedure, and the diameter fluctuation was resulted from the varying concentration of the local silane fragments. 2009 Elsevier Masson SAS. All rights reserved. 1. Introduction As a wide band gap semiconducting materials and a high temperature resistant ceramic, silicon carbide (SiC) exhibits excellent properties at high temperature, high power, high frequency, high radiation environment and chemical harsh conditions [1]. One dimensional SiC nanostructures such as nanowires are superior to their bulk counterpart, and have attracted considerable attention in recent years due to their importance in basic science and their potential application in panel displays [2], electronic nanodevices [3,4], optoelectronic nanodevices [4,5], nanocomposites [6–8], photocatalysts [9], hydrophobic devices [10], etc. SiC nanowires with special morphology, such as diameters fluctuation nanowires, will find new applications in nanocomposites [11] and building blocks for nanoelectro-mechanical systems (NEMS). For example, the variations in nanowires diameters could be used as bench-marks for precisely positioning them in device fabrication [12]. Guo’s group synthesized beaded and periodically twinned SiC nanowires by the carbothermal reduction of a carbonaceous silica xerogel [11,13]. Wang prepared SiC nanowires with Eiffel-tower shape and spindle shape by varying the pressure of the source species [12]. Shen synthesized SiC nanowires with numerous thin nanoplatelets perpendicular to the nanowires’ core stem in a catalyst-assisted thermochemical process [14]. Other kinds of SiC nanowires with special morphology such as needle-shaped nanowires [15,16], nanosprings [17], nanoflowers [18] have also reported. Ultra long nanowires would be very important to study the mechanical, thermodynamic, and electric properties of nanostructures on a macroscopic scale, compared with their short counterpart [19–21]. Lieber’s group have successfully prepared millimeters long Si nanowires, and integrated one single Si nanowire into one-dimensional arrays of field-effect transistors [22]. They consider that the ultra long nanowires may open up new opportunities for integrated nanoelectronics and could serve as unique building blocks linking integrated structures from the nanometer through macroscales. Additionally, ultra long nanowires can be easily spun into continuous yarns with high strength, similar to carbon nanotubes yarns [23], resulting in them being more competitive for use in nanocomposites. Cai has reported millimeters long SiC nanowires by pyrolysis of hexamethyldisilane [24]. However, ultra long SiC nanowires with fluctuating diameters have never been reported. * Corresponding authors. Tel.: þ86 731 84576421; fax: þ86 731 84573165. E-mail addresses: nudtlgy@gmail.com (G.-Y. Li), xdli0153@sina.com (X.-D. Li). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie ARTICLE IN PRESS 1293-2558/$ – see front matter 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.09.003 Solid State Sciences xxx (2009) 1–6 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
ARTICLE IN PRESS G.-Y. Li et aL/ Solid State Sciences xxx(2009)1-6 N, inlet N, outlet Ceramic crucible Fig 1. Schematic of the apparatus for SiC nanowires preparation. In this communication, large areas of millimeters long single The synthesis apparatus was illustrated in Fig. 1. Polycarbosilane crystalline B-Sic nanowires with fluctuating diameters in the range (5 g was crushed into fine powders by milling in a mortar for 0.5 h, of 100-250 nm along the axial direction were prepared by pyrolysis and then was placed in a ceramic crucible. The crucible, together of polycarbosilane in a CVD route. The morphologies and crystal with a cleaned graphite substrate at the downstream, was pushed microstructure of the nanowires were investigated by scanning into a tube corundum furnace. Then the furnace was heated up to on microscopy(SEM), Energy Dispersive X-ray Spectroscopy 1200 Cat 10C/minin a flowing ultra-high purity N2 atmosphere at (EDX), X-ray diffraction(XRD), high resolution transmission elec- a very slow rate and held there for 2 h. When the furnace was cooled tron microscopy(HRTEM), and selected area electron diffraction to room temperature naturally, a large quantity of white cotton-like (SAED). Finally, the Sic nanowires growth procedure was discussed product was obtained on the surface of the graphite substrate by the vapor-solid(vs)mechanis The cotton-like product was collected and characterized with a Siemens D500 X-ray Diffractometer (2=0.1541 nm). The 2. Experiments morphology and chemical composition were observed using a JEOL JSM-6360 SEM with EDX equipment. Since the nanowires have Polycarbosilane was prepared by thermal backbone rearrange- several millimeters long and are difficult to disperse in solvent, for ment of polydimethylsilane in inert atmosphere in temperature HRTEM and SAED observation, the long samples were first ground range 250-450oC and its structure can be ideally represented by into fine powders in an agate mortar with some alcohol addition. [Si(CH3hCH2SiH(CH3)CH2hn [251. Polycarbosilane (Mw=2000. The alcohol containing nanowires was then dropped on a copper 180C)in this work was provided by National University of grid, and naturally dried for test on JEM-3010 at 300 KV acceler- Defense Technology(Changsha, China)[26] ating voltage. 5.0mm d 25k tbb d,. sem amases of me prod ology of the achieved cotton-like products under optical microscopy. The fibres are clearly seen to be several millimeters long in the ellipse area; oducts at different magnifications, showing non-periodically fluctuating diameters along the growth direction. lease 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
In this communication, large areas of millimeters long single crystalline b-SiC nanowires with fluctuating diameters in the range of 100–250 nm along the axial direction were prepared by pyrolysis of polycarbosilane in a CVD route. The morphologies and crystal microstructure of the nanowires were investigated by scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Finally, the SiC nanowires growth procedure was discussed by the vapor–solid (VS) mechanism. 2. Experiments Polycarbosilane was prepared by thermal backbone rearrangement of polydimethylsilane in inert atmosphere in temperature range 250–450 C and its structure can be ideally represented by [Si(CH3)2CH2SiH(CH3)CH2]n [25]. Polycarbosilane (Mw ¼ 2000, Tm ¼ 180 C) in this work was provided by National University of Defense Technology (Changsha, China) [26]. The synthesis apparatus was illustrated in Fig. 1. Polycarbosilane (5 g) was crushed into fine powders by milling in a mortar for 0.5 h, and then was placed in a ceramic crucible. The crucible, together with a cleaned graphite substrate at the downstream, was pushed into a tube corundum furnace. Then the furnace was heated up to 1200 C at 10 C/min in a flowing ultra-high purity N2 atmosphere at a very slow rate and held there for 2 h. When the furnace was cooled to room temperature naturally, a large quantity of white cotton-like product was obtained on the surface of the graphite substrate. The cotton-like product was collected and characterized with a Siemens D500 X-ray Diffractometer (l ¼ 0.1541 nm). The morphology and chemical composition were observed using a JEOL JSM-6360 SEM with EDX equipment. Since the nanowires have several millimeters long and are difficult to disperse in solvent, for HRTEM and SAED observation, the long samples were first ground into fine powders in an agate mortar with some alcohol addition. The alcohol containing nanowires was then dropped on a copper grid, and naturally dried for test on JEM-3010 at 300 KV accelerating voltage. Heating component Ceramic crucible Graphite substrate N2 inlet N2 outlet Raw materials Quartz boat Thermocouple Corundum furnace Fig. 1. Schematic of the apparatus for SiC nanowires preparation. Fig. 2. (a). Macroscopic morphology of the achieved cotton-like products under optical microscopy. The fibres are clearly seen to be several millimeters long in the ellipse area; (b)–(d). SEM images of the products at different magnifications, showing non-periodically fluctuating diameters along the growth direction. 2 G.-Y. Li et al. / Solid State Sciences xxx (2009) 1–6 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
ARTICLE IN PRESS G-Y. Li et al. Solid State Sciences xax(2009)1 0.5 4000 2000 1000 Fig. 3. (a).(b)TEM images of the Sic nanowires at different magnifications (c)The corresponding EDX pattern of the nanowires, indicating the nanowires are composed of Si, c and 3. Results and discussions B-SiC ( (220).(311)and(222)planes, respectively. Therefor crystalline phase of the nanowires is B-Sic The white cotton-like products were spreading over the whole ed to reveal the detailed crystal structures of the graphite substrate(5.5 cm x 3.3 cm), and were carefully pealed off SiC nanov 5a shows a typical nanowire with fluctuating for observation under an optical microscopy(as shown in Fig. 2a). The products are clean with little impurity, and fibres of several millimeters long are clearly seen in the ellipse area. The fibres exhibit flexible nanowires in the SEM images(Fig. 2b-d), and the surfaces are clean. The diameters of almost all the nanowires along the axial direction are non-periodically fluctuating from 100 nm t 2 ao an see more clearly in the TEM images at different 8 magnifications(Fig. 3a and b), there are many furrows on the 2 nanowires surface. The corresponding EDX pattern in Fig 3c shows 200 that the nanowires are composed of Si, C and little amount of o ements. The peak at 0.9 Kev is attributed to the Cu element from the Cu grid for TEM observation. XRD was carried out to find out the composition of the nano- wires on a large scale, as shown in Fig. 4. The strong and sharp peaks indicate that the nanowires are well crystallized The inten- sities and the 28 values of the peaks at 35.677.41 481 60.071. Fig 4. XRD pattern of the obtained product peaks and the int 71.848, 75.558 are in good agreement with the known value agreement with those of the known value (CPDS Card No. 73-166 ating that GCPDS Card No. 73-1665) and are assigned to the diffraction of the nanowires are mainly composed of p-sic. 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 2009.09.003
3. Results and discussions The white cotton-like products were spreading over the whole graphite substrate (5.5 cm 3.3 cm), and were carefully pealed off for observation under an optical microscopy (as shown in Fig. 2a). The products are clean with little impurity, and fibres of several millimeters long are clearly seen in the ellipse area. The fibres exhibit flexible nanowires in the SEM images (Fig. 2b–d), and the surfaces are clean. The diameters of almost all the nanowires along the axial direction are non-periodically fluctuating from 100 nm to 250 nm. As we can see more clearly in the TEM images at different magnifications (Fig. 3a and b), there are many furrows on the nanowires surface. The corresponding EDX pattern in Fig. 3c shows that the nanowires are composed of Si, C and little amount of O elements. The peak at 0.9 Kev is attributed to the Cu element from the Cu grid for TEM observation. XRD was carried out to find out the composition of the nanowires on a large scale, as shown in Fig. 4. The strong and sharp peaks indicate that the nanowires are well crystallized. The intensities and the 2q values of the peaks at 35.677 , 41.481 , 60.071 , 71.848, 75.558 are in good agreement with the known value (JCPDS Card No. 73-1665), and are assigned to the diffraction of b-SiC (111), (200), (220), (311) and (222) planes, respectively. Therefore the main crystalline phase of the nanowires is b-SiC. HRTEM was used to reveal the detailed crystal structures of the SiC nanowires. Fig. 5a shows a typical nanowire with fluctuating Fig. 3. (a), (b) TEM images of the SiC nanowires at different magnifications. (c) The corresponding EDX pattern of the nanowires, indicating the nanowires are composed of Si, C and little amount of O elements. The peak at 0.9 KeV is the Cu signal which was from the Cu grid for TEM observation. 30 40 50 60 70 80 0 200 400 600 (111) (200) (220) (311) (222) 2θ (°) Intensity (Counts) Fig. 4. XRD pattern of the obtained products. The peaks and the intensities are in good agreement with those of the known value (JCPDS Card No. 73-1665), indicating that the nanowires are mainly composed of b-SiC. G.-Y. Li et al. / Solid State Sciences xxx (2009) 1–6 3 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
ARTICLE IN PRESS G.-Y. Li et aL/ Solid State Sciences xxx(2009)1-6 c d 5 nmm B-Sic [112 (bc) HRTEM images of the square areas marked as"b respectively; the planar space is 0.25 nm, which is coincident with the plane space of B-SiC (CPDS Card No 73-1665): (d)The corresponding indexed SAED pattern of the nanowire, indicating the nanowire is grown along the [lll direction. diameter, and furrows are found on the surface. The furrows may be the thin stem and the knot of the nanowire. Surprisingly, the crystal some crystal facets which are common in the Sic nanowires growth in the region is so good that we did not find any stacking faults like [11, 14 The square region marked as""b"is at the part with thin others'work [11, 13]. And this is consistent with the XRD pattern diameter, and the correspond HrTEM image is shown in Fig 5b. The (Fig 4). Usually stacking faults in the Sic nanowires would result in crystal lattice fringe spacing is measured to be 0. 25 nm, which small peak at 33.5 of 20 values in the XRD pattern, which is not consistent with the(111)plane space of B-SiC (CPDS Card No 73- observed in Fig 4. In Fig 5b and c, ar thous phase of about 665 ). Fig 5c is the corresponding HRTEM image of the square 0.5 nm thickness is found on the surface of the nanowires, which region marked as"c"in Fig 5a, which is at the joint part between may be some amorphous Sio2 according to the o element Silane fragments ' SiC nucleus b Fig. 6. Schematic illustration of the proposed formation process for the nanowires diameter fluctuation. (a) Sic nucleus is formed; ( b)The diameter of the growing nanowire he local silane fragments concentration; ( c) The diameter of the nanowire increases followed by the increase of the silane fragments conce ation;(d) Finally the diameter fluctuating SiC nanowires are form lease 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
diameter, and furrows are found on the surface. The furrows may be some crystal facets which are common in the SiC nanowires growth [11,14]. The square region marked as ‘‘b’’ is at the part with thin diameter, and the correspond HRTEM image is shown in Fig. 5b. The crystal lattice fringe spacing is measured to be 0.25 nm, which is consistent with the (111) plane space of b-SiC (JCPDS Card No. 73- 1665). Fig. 5c is the corresponding HRTEM image of the square region marked as ‘‘c’’ in Fig. 5a, which is at the joint part between the thin stem and the knot of the nanowire. Surprisingly, the crystal in the region is so good that we did not find any stacking faults like others’ work [11,13]. And this is consistent with the XRD pattern (Fig. 4). Usually stacking faults in the SiC nanowires would result in small peak at 33.5 of 2q values in the XRD pattern, which is not observed in Fig. 4. In Fig. 5b and c, an amorphous phase of about 0.5 nm thickness is found on the surface of the nanowires, which may be some amorphous SiO2 according to the O element Fig. 5. (a) TEM image of a typical nanowire; (b)–(c) HRTEM images of the square areas marked as ‘‘b’’ and ‘‘c’’, respectively; the planar space is 0.25 nm, which is coincident with the (111) plane space of b-SiC (JCPDS Card No. 73-1665); (d) The corresponding indexed SAED pattern of the nanowire, indicating the nanowire is grown along the [111] direction. SiC nucleus Silane fragments abc d Fig. 6. Schematic illustration of the proposed formation process for the nanowires diameter fluctuation. (a) SiC nucleus is formed; (b) The diameter of the growing nanowire decreases due to the reduction of the local silane fragments concentration; (c) The diameter of the nanowire increases followed by the increase of the silane fragments concentration; (d) Finally the diameter fluctuating SiC nanowires are formed. 4 G.-Y. Li et al. / Solid State Sciences xxx (2009) 1–6 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
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 2009.09.003
contained 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 mechanisms 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 mechanisms 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 transported 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 concentrations 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 decomposition 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 nanowires 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 polycarbosilane 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. 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