4368
J. Phys. Chem. B 2007, 111, 4368-4373
A Simple Route to Synthesize Scales of Aligned Single-crystalline SiC Nanowires Arrays with Very Small Diameter and Optical Properties Jun Jie Niu* and Jian Nong Wang School of Materials Science and Engineering, Shanghai Jiao Tong UniVersity Shanghai, 200030, People’s Republic of China ReceiVed: January 26, 2007; In Final Form: February 27, 2007
Scales of aligned single-crystalline SiC nanowires (SiCNWs) arrays with very small diameter were synthesized by a simple thermal evaporation of ZnS and carbon on silicon wafer. The as-received SiCNWs possess a uniform size distribution centered at ∼8.0 nm, even with a minimum of ∼3.0 nm. The highly oriented SiCNWs usually grew along [111] direction with a clean surface, very thin oxide shell, and small quantity of stacking faults. A crystalline tube-like SiC nanostructure is also obtained. The optical properties, including photoluminescence and Raman scattering spectra of the SiCNWs, were investigated, respectively. In the end, a growth model on basis of the experimental data is suggested.
Introduction One-dimensional semiconductor nanomaterials are being studied extensively because of the significant physical and chemical properties.1-3 The SiC nanowires (SiCNWs), as a wide band gap semiconductor, show potential applications in many harsh conditions, including high temperature, high power, and high frequency,4-6 because of high breakdown electric field, high thermal conductivity, high resistance, high hardness, and high strength.7-8 It has been reported that SiCNWs displayed a considerably excellent elasticity and strength compared to bulk SiC.9 E. W. Wong et al. measured the β-SiCNWs with a diameter of 21.5 nm and obtained a Young’s moduli (E) of 660 GPa.9 The outstanding mechanical properties make this special material a potential candidate for reinforcing additive in ceramic, metal, and other composites. Otherwise, well field-emission characteristics also have been investigated widely.10-12 S. Z. Deng et al. have found that SiCNWs with a small diameter possessed very low turn-on and threshold fields for electron emission.10 Up to now, a few methods have been used to synthesize SiCNWs, including carbon nanotubes confined reaction,13-17 carbothermal reduction,18-21 metal-assisted vapor-liquid-solid (VLS) mechanism,22-25 etc. Almost all the processes require a high system temperature over 1200 °C even up to 2200 °C.22 Furthermore, SiCNWs with diameters of several tens of nanometers normally were obtained and possessed a disordered orientation and thick oxide shell, and even high density of stacking faults. The random SiCNWs clearly will restrict the future applications in nanodevices. Thus, it is desirable to be able to fabricate oriented SiCNWs arrays. Promising nanoelectronic devices rely primarily on wellorganized building structures made of various semiconductor nanowires and/or nanotubes. Recently, a few efforts have been made with respect to assembling nanofunctional blocks.26-27 S. T. Lee et al. have demonstrated the synthesis of oriented SiCNWs (ranged from 10 to 40 nm) by reacting aligned carbon nanotubes as a template with SiO at 1400 °C.28 The oriented * Corresponding author. Tel: +86-21-62932050. E-Mail: jjniu@ sjtu.edu.cn.
SiCNWs showed a large field emission density at low electric field. Z. Li et al. reported the fabrication of ordered SiCNWs with nanoporous alumina (AAO) template at 1230 °C by applying Si, SiO2, and C3H6.29 The as-obtained SiCNWs had a size distribution of 30-60 nm and possessed a high density of planar defects. To date, directly synthesizing oriented singlecrystalline SiCNWs with a small diameter even is a challenge. In this paper, scales of aligned single-crystalline SiCNWs arrays with a very small diameter were successfully synthesized by a simple thermal evaporation of ZnS and carbon on silicon wafer at 1100 °C. The majority of the as-received SiCNWs are well single-crystalline and have a uniform size distribution centered at ∼8.0 nm even with a minimum of ∼3.0 nm. The highly oriented SiCNWs generally formed along [111] direction with a clean surface, very thin oxide shell, and a few of stacking faults. Furthermore, a crystalline tube-like SiC nanostructure along [111] direction is also observed. Photoluminescence spectrum centered at ∼430 and ∼510 nm is referred to the SiCNWs and tube-like SiC nanostructures, respectively. Raman scattering spectrum with ∼787 and ∼925 cm-1 both show a red shift relative to the bulk SiC. Finally, a growth model on basis of the experimental data is suggested. Experimental Section The oriented SiCNWs were synthesized in a simple quartztube furnace. In a typical experiment, solid carbon materials were stuck on the surface of tube by sintering at a high temperature. ZnS powders covered with pieces of silicon wafers in a ceramic boat were sent to the middle of the chamber, and then heated to a temperature of 1100 °C. An argon gas flow was initiated at a rate of ∼16 l/h. After reaction for ∼2 h, the as-grown samples with gray-black color on silicon wafer were taken out for the following measurements. Morphology and crystal lattice of samples were observed by field emission scanning electric microscopy (FESEM, FEI Sirion) and highresolution field emission transmission electron microscopy (HRFETEM, JEM 2010F) at an acceleration voltage of 200 kV, respectively. The crystalline structure was analyzed by X-ray diffraction using Cu KR radiation (XRD, Rigaku, D/MAX 2550). The possible chemical composition of as-grown materials
10.1021/jp070682d CCC: $37.00 © 2007 American Chemical Society Published on Web 04/10/2007
Aligned Single-Cystalline SiC Nanowires
J. Phys. Chem. B, Vol. 111, No. 17, 2007 4369
Figure 1. FESEM images of the aligned SiCNWs arrays. (a) Low-magnification image of fan-shaped SiCNWs arrays. (b) A unique SiCNWs bundle. (c) Top view of a bundle surface. (d) High-magnification cross-section image of the bundle in panel c.
was investigated by using energy-dispersive X-ray spectroscopy (EDS) attached to the HR-FETEM. Photoluminescence (PL) spectral measurements were performed under a 325 nm HeCd laser line as the excitation source at room temperature. Raman scattering spectrum was investigated by a Raman microscope (Renishaw inVia plus) at room temperature. A 785.5 nm laser was used as the excitation source. Results and Discussion Figure 1 shows FESEM images of the highly oriented SiCNWs arrays with a length up to ∼7 µm. As can be seen from Figure 1a, scales of aligned SiCNWs with a small diameter formed many fan-shaped bundles with a width of ∼7 µm and length of tens of micrometers. From a unique bundle shown in Figure 1b, the amount of SiCNWs are parallel to each other and uniformly distributed. Figure 1c displays a top view of a bundle surface. It is observed that the top end of nanowire is clear and does not exist any metal catalyst particle as appearing in VLS mechanism. From the enlarged cross section image in Figure 1d, the isolated nanowires with a relatively smooth surface are orderly arranged with a space of ∼10 nm and have a high density of ∼1010-1011 cm-2. Obviously, the density of current SiCNWs arrays is improved compared to the SiCNWs arrays with a thicker diameter. TEM images of the SiCNWs are presented in Figure 2a,b,d. As can be seen from Figure 2a, a few of disordered SiCNWs with homogeneous size and smooth surface are uniformly distributed. In addition, some nuclear tips of deep-black color are clearly visible. In Figure 2b,d, SiCNWs remain well-oriented arrays even after drastic ultrasonic agitation. The enlarged image in Figure 2d displays a parallel SiCNWs array with a homogeneous diameter arrangement of ∼8 nm. The streaked lines, which correspond to high density of stacking faults, are seldom observed in the present TEM images. Diameter distribution is estimated after calculating
hundreds of nanowires. The calculated data show that the average diameter of SiCNWs is centered at ∼8 nm (Figure 2c). The minimal size even decreases to be ∼3 nm, while few f nanowires over 20 nm are observed. A representative XRD pattern of the aligned SiCNWs arrays is expressed in Figure 2e. The major diffraction lines of (111), (220), and (222) are well-consistent with the standard cubic β-SiC (a ) 0.4358, JCPDS card No. 29-1129). The peak of (111) face shows a significantly highest intensity than the other peaks. The crystal face of (220) shows a very low-intensity relative to the (111) face. These data strongly indicate that the resulting products are mainly single-crystalline of (111) face with a weak polycrystalline structure. Furthermore, the width of diffraction lines is relatively broadened, which is believed to be the result of decreasing size. It is interesting that we do not find any shoulder peak on the left of (111) face as reported previously.29-30 This indicates that the present SiCNWs samples possess free of stacking faults or with low quantity. A typical HR-FETEM image shown in Figure 3a depicts a well single-crystalline structure of a SiCNW. The lattice fringes show the imaging characteristics of a fccβ-SiC crystal in which a d-spacing of 0.25 nm (indicated by the parallel lines in Figure 3a) corresponds to the (111) plane spacing. It further displays that the lattice fringes are perpendicular to the wire axe, suggesting that the nanowire grew along [111] direction as well as the XRD results. It also can be seen the SiCNW has a uniform structure, clean surface, and almost no concentration of stacking faults. The similar results also can be obtained from analysis of the right-neighboring nanowire in Figure 3a. The fast Fourier transform (FFT, Figure 3b) indicates that the nanowire only possesses crystal orientation. EDS spectra recorded from a single nanowire demonstrate the chemical composition of Si, C, and O (Figure 3c). Quantitative analysis shows that the atomic ratio of Si and C is nearly 1:1, confirming a chemical
4370 J. Phys. Chem. B, Vol. 111, No. 17, 2007
Niu and Wang
Figure 2. Analysis of SiC nanomaterials. (a) TEM image of random SiCNWs. (b) Low-magnification TEM image of oriented SiCNWs array. (c) Diameter distribution of SiCNWs. (d) Enlarged TEM image of oriented SiCNWs array. (e) XRD pattern of the as-received samples.
Figure 3. Characteristics of a single SiCNW. (a) HR-FETEM image of a SiCNW. (b) A fast Fourier transform (FFT) of the corresponding SiCNW. (c) EDS spectra related to the SiCNW. (d) HR-FETEM image of a SiCNW tip. (e) EDS spectra related to the SiCNW tip. The Cu peak in EDS spectra comes from copper grid during TEM measurements.
composition of SiC. A very small amount of O comes from the survived thin oxide shell and remained oxygen in system. HRFETEM crystal lattice image of a tip of nanowire is analyzed
as well in Figure 3d. It clearly shows that the spacing of lattice fringes is 0.25 nm, indicating a crystal direction of [111], as well as the nanowire. It is also observed that the tip is wrapped
Aligned Single-Cystalline SiC Nanowires
J. Phys. Chem. B, Vol. 111, No. 17, 2007 4371
Figure 4. Analysis of tube-like SiC nanostructures. (a) FESEM images of tube-like SiC nanostructure array. (b) Enlarged TEM image of a zigzag structure. (c) TEM image of tube-like SiC nanostructures. The inset is the corresponding EDS analysis. (d) HR-FETEM image of a tube-like SiC nanostructure corresponding to the circle zone of panel c.
a relatively thick amorphous oxide shell. The EDS data collected from the tip further confirm the presence of SiC (Figure 3e). As can be seen from the figure, the size of tip is about 12 nm, while the nanowire is decreased to be ∼8 nm. This phenomenon is similar with VLS growth but the nuclear here is formed only with SiC but metal. Except for aligned SiCNWs arrays, a quantity of tube-like or sheet-like SiC nanostructures are synchronously found in our experiments. Shown in Figure 4a is a typical FESEM image of the tube-like SiC nanostructure arrays. It can be seen that plenty of tube-like SiC nanostructures with irregular zigzag are orderly arranged. The width of this special structure is about 40-50 nm and length of hundreds of nanometers. A clear highmagnification TEM image of a single tube-like structure can be observed from Figure 4b. Figure 4c displays a TEM image of quasi-tube SiC nanostructures. The corresponding composition analysis (Figure 4c inset) proves that the tube-like structure is composed of SiC. HR-FETEM image of the circle zone of Figure 4c shows the tube-like SiC structure is well crystalline along [111] direction and with a low density of amorphous oxide (Figure 4d). Figure 5A shows a photoluminescence (PL) spectrum taken from the as-received SiC nanostructures at room temperature. A He-Cd continuous wave laser (325 nm) was used as excitation source. As can be seen from the figure, two obvious PL emission bands centered at ∼430 nm (2.89 eV photo energy) and ∼510 nm (2.43 eV photo energy) are observed. The strong green emission band centered at ∼510 nm is similar to the previously reported irregular SiC tubes or SiC film.31 It may be ascribed to the OH group adsorbed on surface or some localized states.32 The emission line at ∼430 nm is consistent with the published data and normally is attributed to the blue
emission luminescence from SiC.14,33 Compared with other results, the PL bands in present SiC nanostructures are also blueshifted relative to the bulk SiC. It may be attributed to the quantum confinement effect because of the small size.34-35 The detailed explaining for photoluminescence in SiC nanostructures is yet unknown, and further research is necessary to be continued. A typical Raman scattering spectrum of the samples is shown in Figure 5B. Two obvious bands of ∼787 and ∼925 cm-1 are clearly observed. The band of ∼787 cm-1 shows a good agreement with the reported data.29-30,36-37 It is attributed to the TO phonon at the Γ point of cubic SiC. Although the both bands display a red shift compared to the bulk SiC,38 the peak of ∼925 cm-1 corresponding to the LO phonon shows a wider shift. Generally, the shift of TO band can mainly contribute to the phonon confinement, while the large frequency shift for LO band cannot solely be attributed to the phonon confinement. This may be explained by the strain variation because of the residual defects and the Raman selection rules in sample.38 The calculated data based on the optical phonon modes and the related effective density of states for the red shift of TO (∼790 cm-1) and LO (∼936 cm-1) phonons in SiC-NWs showed a well agreement with the experimental results.37 Furthermore, the Raman scattering spectrum has confirmed the as-obtained aligned SiC nanostructures are well crystalline. The further research on the Raman scattering of SiC-NWs is still under processing. Clearly, no any metal was employed during the whole procedure. Thus, the growth mechanism may not follow the previously reported vapor-liquid-solid (VLS) model. On the basis of experimental results, we suggest a possible growth model for current aligned SiCNWs and tube-like nanostructures.
4372 J. Phys. Chem. B, Vol. 111, No. 17, 2007
Niu and Wang
Figure 5. (A) Photoluminescence spectrum of the oriented SiC nanostructures. (B) Raman scattering spectrum of the oriented SiC nanostructures. (C) Schematic illustration of a growth model of SiC nanostructures: (a) Oriented nucleus are produced with a wide spacing. (b) The aligned SiCNWs array forms. (c) The produced nucleus contact closely. (d) The irregular tube-like SiC nanostructure generates.
The chemical reaction equations during the process can be followed as below:
2C + O2 f 2COv
(1)
2Si + xO2 f 2SiOx(0∼300 °C)
(4)
S + Si ) SiS (> ∼900 °C)
(5)
2SiSv ) Si + SiS2 (>∼1050 °C)
(6)
SiS2 + 2H2O ) SiO2 + 2H2Sv
(7)
and
All of the experiments were performed under a simple horizontal tube-shaped furnace without any vacuum equipment. Therefore, the survived oxygen in system will easily react with the solid carbon and generates a plenty of CO gas according to the eq 1. Likewise, a thin oxide film (SiOx) will form on the surface of silicon wafer with the eq 2. Once the downstream CO gas encounters the SiOx, reaction 3 happens and a SiC nuclear forms (Figure 5Ca). With the reaction continues, the SiC will grow up to a nanowire favorably along 〈111〉 direction under the lowest energy principle (Figures 5Cb and 3d). The existence of the ZnS will assist to produce more nuclear centers on the basis of reaction (eqs 4-7). The exact reactions with
ZnS and Si can be referred from our previous results.39 The generated Si atoms from eq 6 will easily to form oxide nanoparticles from the eq 2. Thus, the formed oxide nanoparticles will supply more chance to react with CO and produce more SiC according to the eq 3. Consequently, it assists the formation of SiC nuclear and induces the production of quantity of aligned SiCNWs. It indicated that SiCNWs also could form without ZnS. In fact, we have found that SiCNWs could be generated with free of ZnS in the experiments. Evidently, the as-formed nuclear is very small (around 12 nm, Figure 3d) and thus contributes a SiCNW with a smaller diameter (around 8 nm, Figure 3d and Figure 2). There are two cases which should be addressed here. For the first one, if the spacing between nucleus is wide (Figure 5Ca), the generated regular nucleus will promote the formation of aligned SiCNWs arrays (Figure 5Cb). In the other case, if the nucleus contact compactly each other (Figure 5C c), a tube-like SiC nanostructure will form, as well as the common film formation (Figure 5Cd). Conclusion In summary, scales of highly aligned SiCNWs arrays with well single-crystalline of [111] direction and tube-like SiC nanostructures have been synthesized by a simple thermal evaporation at a relatively low temperature of 1100 °C. The SiCNWs show a uniform morphology with a very small diameter of ∼8 nm, very thin oxide shell, and low density of stacking faults. Photoluminescence spectrum confirms the structure of SiCNWs and tube-like SiC architectures. The red shift in Raman scattering spectrum is mainly contributed to the
Aligned Single-Cystalline SiC Nanowires phonon confinement and strain variation effects. Finally, a possible growth model is suggested. We believe that the aligned crystalline SiCNWs arrays with a small diameter described herewith will express excellent prospects in fields of high mechanical strength materials, field emission, and other advanced blocks of nanodevices. Acknowledgment. This work was supported by the ShanghaiApplied Materials Research and Development Fund (No. 06SA06) and Youth Teacher Fund of Shanghai Jiao Tong University (A2306B). We would like to thank Instrumental Analysis Center of Shanghai Jiao Tong University for its great help in measurements. References and Notes (1) Shi, W. S.; Zheng, Y. F.; Wang, N.; Lee, C. S.; Lee, S. T. AdV. Mater 2001, 13, 591. (2) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (3) Sha, J.; Niu, J. J.; Ma, X. Y.; Xu, J.; Zhang, X. B.; Yang, Q.; Yang, D. R. AdV. Mater. 2002, 14, 1219. (4) Fisher, A.; Schroter, B.; Richter, W. Appl. Phys. Lett. 1995, 66, 3182. (5) Feng, Z. C.; Mascarenhas, A. J.; Choyke, W. J.; Powell, J. A. J. Appl. Phys. 1998, 64, 3176. (6) Li, Y.; Dorozhkin, P. S.; Bando, Y.; Golberg, D. AdV. Mater. 2005, 17, 545. (7) Powell, J. A.; Matus, L. G.; Kuczmarski, M. A. J. Electrochem. Soc. 1998, 134, 1558. (8) Larkin, D. J. J. Mater. Res. Soc. Bull. 1997, 22, 36. (9) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1995, 277, 1971. (10) Deng, S. Z.; Li, Z. B.; Wang, W. L.; Xu, N. S.; Zhou, J.; Zheng, X. G.; Xu, H. T.; Chen, J.; Lee, J. C. Appl. Phys. Lett. 2006, 89, 0231181. (11) Wu, Z. S.; Deng, S. Z.; Xu, N. S.; Chen, J.; Zhou, J.; Chen, J. Appl. Phys. Lett. 2002, 80, 3829. (12) Wong, K. W.; Zhou, X. T.; Au, F. C. K.; Lai, H. L.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1999, 75, 2918. (13) Dai, H. J.; Wong, E. W.; Lu, Y. Z.; Fan, S. S.; Lieber, C. M. Nature 1995, 375, 769. (14) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Liang, W. J.; Gu, B. L.; Yu, D. P. Chem. Phys. Lett.1997, 265, 374. (15) Kharlamov, A. I.; Loichenko, S. V.; Kirillova, N. V.; Fomenko, V. V.; Bondarenko, M. E.; Zaitseva, Z. A. Inorg. Mater. 2003, 39, 260.
J. Phys. Chem. B, Vol. 111, No. 17, 2007 4373 (16) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Han, H. AdV. Mater. 2003, 15, 353. (17) Tang, C. C.; Fan, S. S.; Dang, H. Y.; Zhao, J. H.; Zhang, Z.; Li, P.; Gu, Q. J. Cryst. Growth 2000, 210, 595. (18) Meng, G. W.; Zhang, L. D.; Mo, C. M.; Zhang, S. Y.; Qin, Y.; Feng, S. P.; Li, H. J. J. Mater. Res. 1998, 13, 2533. (19) Yang, Z. X.; Xia, Y. D.; Mokaya, R. Chem. Mater. 2004, 16, 3877. (20) Ye, H.; Titchenal, N.; Gogotsi, Y.; Ko, F. AdV. Mater. 2005, 17, 1531. (21) We, J.; Li, K. Z.; Li, H. J.; Fu, Q. G.; Zhang, L. Mater. Chem. Phys. 2006, 95, 140. (22) Zhou, X. T.; Wang, N.; Lai, H. L.; Peng, H. Y.; Bello, I.; Wong, N.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 3942. (23) Vyshnyakova, K. L.; Pereselentseva, L. N.; Cambaz, Z. G.; Yushin, G. N.; Gogotsi, Y. Br. Ceram. Trans. 2004, 103, 193. (24) Liang, C. H.; Meng, G. W.; Zhang, L. D.; Wu, Y. C.; Cui, Z. Chem. Phys. Lett. 2000, 329, 323. (25) Choi, H. J.; Seong, H. K.; Lee, J. C.; Sung, Y. M. J. Cryst. Growth 2004, 269, 472. (26) Zhang, X. Y.; Zhang, L. D.; Meng, G. W.; Li, G. H.; Phillipp, N. Y. J.; Phillipp, F. AdV. Mater. 2001, 13, 1238. (27) Niu, J. J.; Wang, J. N. Chem. Vapor. Deposition 2006, 12, 1. (28) Pan, Z.; Lai, H. L.; Au, F. C. K.; Duan, Z.; Zhou, W.; Shi, W.; Wang, N.; Lee, C. S.; Wong, N. B.; Lee, S. T.; Xie, S. AdV. Mater. 2000, 12, 1186. (29) Li, Z.; Zhang, J.; Meng, A.; Guo, J. J. Phys. Chem. B 2006, 110, 22382. (30) Shen, G.; Chen, D.; Tang, K.; Qian, Y.; Zhang, S. Chem. Phys. Lett. 2003, 375, 177. (31) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Golberg, D. Appl. Phys. Lett. 2004, 85, 2932. (32) Shim, H. W.; Kim, K. C.; Seo, Y. H.; Nahm, K. S.; Suh, E. K.; Lee, H. J.; Hwang, Y. G. Appl. Phys. Lett. 1997, 70, 1757. (33) Hu, J. Q.; Lu, Q. Y.; Tang, K. B.; Deng, B.; Jiang, R. R.; Qian, Y. T.; Yu, W. C.; Zhou, G. E.; Liu, X. M.; Wu, J. X. J. Phys. Chem. B 2000, 104, 5251. (34) Choi, H. J.; Johnson, J. J.; He, R.; Lee, S. K.; Kim, F.; Pauzaauski, P.; Goldgerger, J.; Saykally, R. J.; Yang, P. J. Phys. Chem. B 2003, 107, 8721. (35) Seong, H. K.; Choi, H. J.; Lee, S. K.; Lee, J. I.; Choi, D. J. Appl. Phys. Lett. 2004, 85, 1256. (36) Xi, G.; Peng, Y.; Wan, S.; Li, T.; Yu, W.; Qian, Y. J. Phys. Chem. B 2004, 108, 20102. (37) Zhang, S.; Zhu, B.; Huang, F.; Yan, Y.; Shang, E.; Fan, S.; Han, W. Solid Stat. Commun. 1999, 111, 647. (38) Olego, D.; Cardona, M. Phys. ReV. B 1982, 25, 3889. (39) Niu, J. J.; Sha, J.; Yang, D. Physica E 2004, 24, 178.
