Large Areas of Centimeters-Long SiC Nanowires Synthesized by

Sep 17, 2009 - Engineering, National UniVersity of Defense Technology, Changsha ... Defense Technology, Changsha 410073, China, and AdVanced ...
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J. Phys. Chem. C 2009, 113, 17655–17660

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Large Areas of Centimeters-Long SiC Nanowires Synthesized by Pyrolysis of a Polymer Precursor by a CVD Route Gong-yi Li,*,† Xiao-dong Li,*,† Zhong-dao Chen,‡ Jun Wang,† Hao Wang,† and Ren-chao Che§ State Key Laboratory of AdVanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National UniVersity of Defense Technology, Changsha 410073, China, Department of Materials Engineering & Applied Chemistry, College of Aerospace & Materials Engineering, National UniVersity of Defense Technology, Changsha 410073, China, and AdVanced Materials Laboratory, Fudan UniVersity, Shanghai 200433, China ReceiVed: May 7, 2009; ReVised Manuscript ReceiVed: August 30, 2009

Large areas of centimeters-long SiC nanowires have been prepared by pyrolysis of a polymer precursor with ferrocene as the catalyst by a CVD route. The nanowires, with lengths of several centimeters and diameters of 100-200 nm, were composed of single-crystal β-SiC along the 〈111〉 direction and were grown on ceramic substrates in areas of 11 cm × 4 cm. At high temperature, the silane fragments derived from decomposition of the polymer precursor, liquid polysilacarbosilane (l-PS), provided both the Si and C sources for the growth of the nanowires. The nanowires grew in a base-growth mode, which was governed by the Vapor-Liquid-Solid mechanism. The SiC nanowires showed good intense-current emitting properties when a pulsed high-voltage electric field was imposed. Introduction One-dimensional (1D) nanostructures have attracted considerable attention due to their importance in basic scientific research and their unique applications in nanodevices.1 Among the various 1D nanostructures, silicon carbide (SiC) nanowires have been extensively studied in recent years because of their unique electronic,2 field-emitting,3 optical,4 superhydrophobic,5 and mechanical properties6 and their potential applications in areas such as panel displays, electronic nanodevices, optoelectronic nanodevices, nanocomposites, photocatalysts, and hydrophobic devices.5 The field-emitting properties of oriented SiC nanowire arrays are excellent, with a lower turn-on field (0.7-1.5 V/µm), a lower threshold field (2.5-3.5 V/µm), and remarkable electron-emitting stability.3a The bending strength and Young’s modulus of SiC nanorods have been measured experimentally as 53.4 and 660 GPa, respectively, which are much higher than those of the bulk materials and approach the theoretical values.6b Several routes have been explored for the synthesis of SiC nanowires, such as template conversion,3a,7 an arc-discharge technique,8 thermal evaporation methods,9 carbothermal reduction,4b,10 a solvothermal route,11 a combustion method,12 a polymer pyrolysis route,13,14 chemical vapor deposition,15 and so forth. However, the SiC nanowires synthesized by these methods have typically had lengths in the micrometer range. Only Cai’s group prepared SiC nanowires of several millimeters in length.14 Centimeters-long SiC nanowires have some advantages compared to their shorter counterparts. First, they will be more convenient for manipulation for further applications in electronic devices. Second, they will be important in the research * To whom correspondence should be addressed. E-mail: nudtlgy@ gmail.com (G.-y.L.); [email protected] (X.-d.L.). † State Key Laboratory of Advanced Ceramic Fibers & Composites, College of Aerospace & Materials Engineering, National University of Defense Technology. ‡ Department of Materials Engineering & Applied Chemistry, College of Aerospace & Materials Engineering, National University of Defense Technology. § Fudan University.

of the physical properties of 1D nanostructures when considering macroscopic dimensions.16 Third, like carbon nanotubes,17 they may be easily spun into continuous nanoropes with high strength, resulting in them being more competitive for use in nanocomposites. The excellent field-emitting properties of SiC nanowires have usually been characterized in low-voltage fields.3a,b However high-voltage fields are often imposed, and intense-current emission is needed in vacuum microelectronics applications, such as high-power microwave tubes and betatrons.18 It has been reported that the application of high-voltage pulses between a cathode and an anode leads to intense-current electron beams.18 Therefore, it is very interesting to study the electron-emitting properties of SiC nanowires in high-voltage electric fields. In this paper, we report the successful synthesis of centimeterslong β-SiC nanowires in large areas by pyrolysis of liquid polysilacarbosilane (l-PS) at 1300 °C for 3 h by a chemical vapor deposition (CVD) route. l-PS is the decomposition (at around 300-400 °C) product of polydimethylsilane, which is an infusible and insoluble crystalline polymer.19 l-PS contains Si-Si and Si-C backbones and has been used as a precursor for SiC fibers20 and ceramic matrix composites.21 It can be decomposed at high temperature into small species such as cyclic silanes, silane fragments, H2, and CH4,22 and the silanes may serve as gaseous sources for SiC nanowires through the well-known Vapor-Liquid-Solid (VLS) growth mechanism.23 The compositions and the structure of the nanowires have been studied, and the growth mechanism has been interpreted in terms of the VLS mechanism. Finally, we have measured the intensecurrent emitting properties of the SiC nanowires in a singlepulsed high-voltage electric field. Experimental Section The l-PS used in this work was provided by the National University of Defense Technology (Changsha, China).19 Since l-PS is a liquid with a boiling point of about 200 °C, it would evaporate entirely before the temperature reached 1300 °C and

10.1021/jp904277f CCC: $40.75  2009 American Chemical Society Published on Web 09/17/2009

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Figure 1. Schematic image of the preparation apparatus.

would, therefore, reduce the productivity. We employed activated carbon as an absorbant to postpone the evaporation. The activated carbon (with a BET surface area of 390 m2/g) was first ground to a fine powder (100 mesh), ultrasonically washed with distilled water for 1 h, and dried in a box furnace (110 °C). To prepare SiC nanowires, a slurry solution was first formed by introducing l-PS (4.0 g) and some ferrocene (0.04 g) into the activated carbon powder (5.0 g). The slurry solution was placed in a common ceramic boat (30 mm × 60 mm, denoted as A), which was then pushed into the center of a tubular corundum furnace (Φ 41 mm, 1200 mm in length), with another two empty ceramic boats (30 mm × 60 mm, denoted as B1 and B2) placed downstream of boat A. The furnace was heated to 1300 at 10 °C/min in a flowing ultra-high-purity Ar atmosphere at a very low rate, maintained under these conditions for 3 h, and then allowed to cool to room temperature naturally. Finally, a large amount of a light-green cotton-like product was found covering the ceramic boats B1 and B2 (Figure 1). The cotton-like product was collected and characterized with an X-ray diffractometer (XRD, Siemens D500, λ ) 0.1541 nm), a scanning electron microscope (SEM, Hitachi S-4800) equipped with an energy-dispersive X-ray (EDX) unit, a high-resolution transmission electron microscope (HRTEM, JEOL JEM-3010) equipped with a selected-area electron diffraction (SAED) unit, a Raman spectrometer (Bruker SENTERRA), and a Fouriertransform infrared spectrometer (FT-IR, Nicolet-360). The single-pulsed high-voltage electron emission experiment was performed in a diode powered by a pulse-forming network generator under a vacuum of 1 × 10-3 Pa, which was similar

Li et al. to that used in Chen’s work.24 A polished stainless iron wafer (Φ ) 25 mm) coated with the as-synthesized SiC nanowires served as the cathode. The anode-cathode gaps were 5, 10, and 26 mm. The applied voltage and the emitted current were recorded by a circuit with a parallel-connected oscillograph. For comparison, experiments on various cathodes based on a bare stainless iron wafer, a graphite wafer, or a velvet film were also conducted. Results and Discussion After the furnace had cooled to room temperature, we were amazed to find that the ceramic boats were fully wrapped with a large amount of a light-green cotton-like product (Figure 2). Some of the light-green fibers were standing vertically in relation to the inner walls and thus formed aligned arrays (Figure 2a). The height of the arrays was more than 1 cm. Further lightgreen product was spread all over the outside walls and backs of the boats (Figure 2b), and the total area of the product was about 11 cm × 4 cm. By reeling fibers from the products, we obtained a fiber bundle of length up to several centimeters (Figure 2c). Scanning electron microscopy observation (Figure 2d, e) indicated that the product consisted of aligned flexible nanowires with slightly fluctuating diameters of 100-200 nm. The nanowires were composed of the elements Si and C, with a small amount of O, as is evident from the EDX pattern (inset in Figure 2d). The peak at 2.05 KeV can be assigned to Pt, which was introduced during the specimen preparation. During the SEM observation, we did not find any droplets or impurities on the nanowires. However, by observing the morphology of the inner walls of the ceramic boats by SEM after removing the nanowires, we found many residual nanowires with droplets at their tips, as shown in Figure 3. The EDX pattern of the droplets revealed that, besides Si and C, some Fe was also present, indicating that the well-known VLS mechanism,23 which is characterized by droplets at the tips of the nanowires, was responsible for their growth. As can be seen, the upper parts of the nanowires show an absence of droplets, but their roots, which

Figure 2. (a) Top view of the ceramic boat B1; light-green fibers of about 1 cm long were grown vertically to the ceramic walls; (b) light-green cotton-like products covering the outside backs of the ceramic boats B1 and B2; (c) A bundle of fibers with a length up to several centimeters was reeled from the products. The unit of the ruler in (a), (b), and (c) is centimeters. (d, e) SEM images of the products at different magnifications. The inset of (d) is the typical EDX pattern of the nanowires, showing Si, C, and O elements; the Pt peak was introduced during the SEM specimen preparation.

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Figure 3. (a) SEM image of the inner wall surface of the ceramic boats B1 after the nanowires were removed. (b) Typical EDX pattern of the droplets at the nanowires’ tips, showing that the Fe element was involved. The Pt peak was introduced during the SEM specimen preparation.

Figure 4. XRD pattern of the centimeters-long SiC nanowires. The peaks and the intensities are in good agreement with those of the known value (JCPDS Card No. 73-1665), indicating that the nanowires were mainly composed of β-SiC.

contact the inner walls of the ceramic boats, contain droplets. From this, we can conclude that the nanowires grew from their roots rather than from their tops. XRD analysis was carried out to determine the composition of the nanowires on a large scale. As shown in Figure 4, the 2θ values and the intensities of the main peaks at 35.7, 41.5, 60.1, 71.8, and 75.5° are in good agreement with the established values (JCPDS Card No. 73-1665) and can be attributed to the diffraction of the β-SiC (111), (200), (220), (311), and (222) planes, respectively. The strong, sharp peaks indicate that the SiC nanowires had good crystallinity. Thus, β-SiC was confirmed as the main crystalline phase of the nanowires. HRTEM was used to reveal the crystal structure of the SiC nanowires (Figure 5). Figure 5a shows the TEM image of a typical nanowire, while Figure 5b shows the corresponding HRTEM image. An amorphous sheath of about 2 nm was found on the surface of the nanowire. The EDX pattern (Figure 5c,d) indicates the presence of Si, C, and a small amount of O. The peak at 0.5 KeV can be assigned to oxygen. When the electron beam was moved away from the shell of the nanowire (Figure 5c) and toward its core (Figure 5d), the amount of oxygen decreased. Thus, the amorphous sheath may have been an amorphous SiO2 layer. The oxygen may have come from the activated carbon or the flowing Ar gas. The lattice fringes can clearly be seen, with 0.25 nm of (111) plane spacing and 0.22 nm of (002) plane spacing of the β-SiC crystal. Figure 5e shows the corresponding indexed selected-area electron diffraction (SAED) pattern of the nanowire in Figure 5b. The clear

diffraction spots indicate that the nanowires were single crystals that grew along the 〈111〉 direction. Figure 5f shows the HRTEM image of another nanowire with many crystal defects. This was due to the polymorphism of SiC materials and is very common for β-SiC formation.25,26 Additionally, these defects can also be viewed as nanoscale poly-type inclusions within the pure β-SiC phase. Raman scattering and Fourier-transform infrared analyses were also carried out to confirm the structure of the SiC nanowires. Figure 6 shows a typical Raman spectrum of the nanowires measured at room temperature. The wavelength of the excitation laser was 532 nm, and the power was 10 mW. The sharp peaks at 791.7 and 966.9 cm-1 correspond to the modes of transverse optical (TO) and longitudinal optical (LO) phonons, respectively, at the Γ point of the cubic SiC.27 These peaks had red shifts of 4.3 and 5.1 cm-1 with respect to the TO and LO modes of bulk β-SiC, which can be ascribed to the stacking faults in the nanowires. This is in accordance with the defects seen in the HRTEM images, as shown in Figure 5d. A shoulder peak at 941.3 cm-1 was also observed by Brioude’s group27 and was ascribed to the axial optical mode of SiC poly types. The FT-IR transmittance spectrum of the SiC nanowires is shown in Figure 7. The strong peaks at 787.5, 906.1, and 941.3 cm-1 correspond to the stretching vibration of the Si-C bonds. The peak at 787.5 cm-1 shows a 27.5 cm-1 red shift compared with that of the bulk β-SiC, which may be ascribed to size confinement and surface effects.28 The Raman scattering and FT-IR spectra confirm the β-SiC nature of the nanowires.

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Figure 5. (a) TEM image of the SiC nanowires; (b) HRTEM image of the nanowires; (c) EDX pattern of the nanowire’s shell marked as “c” in (a); (d) EDX pattern of the nanowire’s core marked as “d” in (a) and the amount of the O element decreased compared to that of (c); (e) SAED pattern of the SiC nanowire in (b); (f) one nanowire with many crystal defects.

Figure 6. Raman scattering spectrum of the centimeters-long SiC nanowires. The sharp peaks at 791.7 and 966.9 cm-1 correspond to the modes of TO and LO phonons at the Γ point of the β-SiC, respectively. The shoulder peak at 941.3 cm-1 is related to the axial optical mode of SiC poly types.

From the above characterization, we can conclude that the light-green cotton-like product consisted of β-SiC nanowires. The Si and C in the nanowires must come from the same l-PS precursor in the present work. The vapor for the growth of the SiC nanowires came from the decomposition of l-PS and was

Figure 7. FT-IR transmittance spectrum of the centimeters-long SiC nanowires. The strong peaks at 787.5, 906.1, and 941.3 cm-1 correspond to the Si-C bonds’ stretching vibration.

transported to the ceramic boats. From the SEM images (Figure 3), we found many droplets containing Fe, from which we concluded that the VLS mechanism must have been responsible for the growth of the nanowires. The Fe serving as the catalyst in the droplets must have come from the ferrocene used. Although the detailed growth mechanism of the centimeterslong SiC nanowires is not yet fully understood, on the basis of

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Figure 8. Schematic illustration of the SiC nanowire formation. (a) Silane fragments and Fe droplets are formed at high temperature. (b) Silane fragments are dissolved in the Fe droplets, and the SiC nuclei are precipitated. (c) Long SiC nanowires are grown along the 〈111〉 direction in a base-growth mode.

the above results, we propose a possible VLS mechanism, which is outlined in Figure 8. When the temperature was high enough, l-PS was decomposed into cyclic silanes, silane fragments, as well as H2.22 At the same time, the ferrocene vaporized and was reduced by the H2 to form Fe atoms, which agglomerated into Fe droplets. The surface of the Fe droplets had a large accommodation coefficient and was therefore a preferred site for gas deposition (Figure 8a). The cyclic silanes and silane fragments containing the elements Si and C were absorbed and dissolved by the Fe droplets, forming Fe-Si-C alloy droplets. A more stable phase, SiC, will be formed by the reaction of Si and C atoms in the droplets. Due to the low solubility of SiC in Fe, the SiC crystals diffuse and precipitate from the droplets, giving rise to SiC nuclei (Figure 8b). Once the SiC nuclei form, a liquid-solid interface is created at the surface of the liquid droplets. A consequence of anisotropy in the solid-liquid interfacial energy is unidirectional growth. The dissolved Si and C atoms preferentially diffuse out through the droplets and stack on the existing liquid-solid interface because this process consumes less energy than that needed to form a new nucleation site within the droplet. With the slow release of silane fragments from the gradual decomposition of l-PS, the silane fragments containing Si and C gradually dissolve in the droplets and precipitate, and as this process continues, the nanowires grow longer (Figure 8c). It is noteworthy that the base-growth mode is the dominant mode for the SiC nanowires growth in the present work (Figures 2 and 3), unlike the usual tip-growth mode for nanowires.23 The base-growth mode has been observed in the preparation of carbon nanotubes29 but not previously in the case of SiC nanowires. A likely explanation is that this was due to the strong interactions between the ceramic substrates and the catalytic droplets. The nanowires were often broken at the roots when we tried to peel them off of the ceramic substrates, which was indicative of these strong interactions. The synthesis temperature was 1300 °C, at which the glazes on the ceramic boats would be slightly molten. Therefore, it is reasonable to assume that the liquid Fe nanoclusters were adhered to the molten glazes more strongly than the Si substrates typically used in other studies. The precipitated SiC nuclei were not able to overcome the strong interactions and could not raise the droplets. Rather, they were precipitated on the top surfaces of the droplets, which resulted in the base-growth mode. The detailed mechanism still needs to be further investigated. The growth of the centimeters-long SiC nanowires can be attributed to the specific structure of l-PS. First, the very active Si-H bonds in the cyclic silanes and silane fragments can produce H2, which is helpful for maintaining the activity of the catalytic droplets during the growth process.29 On the other hand, the C/Si ratio in l-PS is around 1.6, which is suitable for SiC formation, considering the release of some carbon-rich off-gas, mostly CH4, along with H2 at high temperatures. Too high pf a C/Si ratio seems unfavorable for this purpose, although hex-

Figure 9. Typical I-V waveforms of the SiC nanowire cathode in a single-pulsed high-voltage electric field. Channels 1 and 2 are the cathode voltage waveform and the emission current waveform, respectively. The anode-cathode gap is 10 mm.

amethyldisilane, with a C/Si ratio of 3, has been used to obtain SiC nanowires of lengths in the millimeter range.14b-d The intense-current emitting properties of the SiC nanowires were characterized in a single-pulsed high-voltage mode experiment. Figure 9 shows typical I-V waveforms of a SiC nanowire cathode in a single-pulsed electric field, with the anode-cathode gap at 10 mm. Channel 1 is the voltage waveform, and Channel 2 is the current waveform. The emission current approached 118.6 kA, and the current density was calculated to be 23.8 kA/cm2 when a pulsed high voltage of 109 kV, the electric field of which was calculated to be 10.9 V/µm, was applied. For comparison, parallel experiments with different cathodes based on a stainless iron wafer, a graphite wafer, and a velvet film were also conducted, and the emission current densities were 4.2, 20.7, and 14.7 kA/cm2, respectively. Thus, the SiC nanowire cathode gave the largest current density among the various cathodes tested. The pulsed electric field can be adjusted by varying the anode-cathode gaps, and the emission current densities based on SiC nanowires cathode are 13.2 and 32.6 kA/cm2 at electric fields of 5.3and 15.6 V/µm, respectively (see Figure 1 in Supporting Information). The emission current density increases with the electric field. One-dimensional nanostructures such as carbon nanotubes30 and ZnO nanorods31 have been examined with regard to their intense-current emitting properties. In this context, it was reported that the morphology and diameter were important factors determining the emission characteristics.32 Therefore, the reasons for the intense-current emission of the SiC nanowires can be interpreted as discussed in the following. The diameters of the as-prepared SiC nanowires were in the nanometer range, yielding a high field enhancement factor, which is favorable for the emission of electrons from the cathodes. Additionally, the large aspect ratio of the SiC nanowires is also suitable for field emission. Velvets are usually

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employed in the area of intense-current emission at present.33 However, their emission stabilities are poor because they will be melted and decomposed in a high-voltage electric field, which will decrease the emission current. Furthermore, the decomposition gases from the velvets would lead to vacuum deterioration. In the present work, SiC nanowires exhibited higher current density than velvet. In addition, SiC has a high melting point and is thermally and chemically stable at high temperature. Therefore, SiC nanowires are expected to have great potential applications in the areas of high-power microwave devices, betatrons, and so on. Further investigations of the intense-current emissions of SiC nanowire cathodes are currently underway. Conclusion In summary, centimeters-long SiC nanowires have been synthesized in large areas (11 cm × 4 cm) by pyrolysis of the polymer precursor l-PS at 1300 °C for 3 h by a CVD route. The nanowires, with lengths of several centimeters and diameters of 100-200 nm, are composed of single-crystal β-SiC along the 〈111〉 direction. The single raw precursor l-PS was the source of both the Si and C atoms for the formation of SiC nanowires, the formation process being catalyzed by Fe derived from the decomposition of ferrocene. The SiC nanowires grow in a basegrowth mode because of the strong interactions between the Fe droplets and the ceramic substrate. The growth process was dictated by the VLS mechanism. The SiC nanowires showed good intense-current emission properties in a pulsed high-voltage electric field. The synthesis approach used in this work is simple, and the centimeters-long SiC nanowires may represent a significant step toward applications in vacuum microelectronics, nanocomposites, nanodevices, integrated circuits, and so forth. Acknowledgment. The Authors are thankful for the help from Yang Yong, as well as the financial support from the National Natural Science Foundation of China (Grant No. 50702075) and the Research Fund of the State Key Laboratory of CFC (No. 9140C8202050804). Supporting Information Available: I-V waveforms of SiC nanowire cathodes. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99. (2) (a) Seong, H. K.; Choi, H. J.; Lee, S. K.; Lee, J. I.; Choi, D. J. Appl. Phys. Lett. 2004, 85, 1256. (b) Rogdakis, K.; Bescond, M.; Bano, E.; Zekentes, K. Nanotechnology 2007, 18, 475715. (3) (a) Pan, Z. W.; Lai, H. L.; Frederick, C. K.; Duan, X. F.; Zhou, W. Y.; Shi, W. S.; Wang, N.; Lee, C. S.; Wong, N. B.; Lee, S. T.; Xie, S. S. AdV. Mater. 2000, 12, 1186. (b) Kim, D. W.; Choi, Y. J.; Choi, K. J.; Park, J. G.; Park, J. H.; Pimenov, S. M.; Frolov, V. D.; Abanshin, N. P.; Gorfinkel, B. I.; Rossukanyi, N. M.; Rukovishnikov, A. I. Nanotechnology 2008, 19, 225706.

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