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J. Phys. Chem. B 2007, 111, 4156-4160
Controlled Al-Doped Single-Crystalline Silicon Nitride Nanowires Synthesized via Pyrolysis of Polymer Precursors Weiyou Yang,*,† Huatao Wang,‡ Shuzhen Liu,† Zhipeng Xie,‡ and Linan An*,§ School of Mechanical Engineering, Ningbo UniVersity of Technology, Ningbo 315016, People’s Republic of China, State Key Lab of New Ceramics and Fine Processing, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and AdVanced Materials Processing and Analysis Center, UniVersity of Central Florida, Orlando, Florida 32816 ReceiVed: January 25, 2007; In Final Form: February 23, 2007
Al-doped single-crystalline Si3N4 nanowires were synthesized by catalyst-assisted pyrolysis of polymeric precursors. The doping levels can be controlled by tailoring the Al concentration in the precursors. It is found that the Al concentration has a significant effect on the shape, sizes, and phase compositions of the synthesized Si3N4 low-dimensional nanomaterials. The photoluminescence measurements revealed that the Al dopants have a profound effect on the emission behavior. The current study provides a simple way to realize the controlled doping in Si3N4 nanomaterials, which could be useful for applications in optoelectronic nanodevices.
1. Introduction Silicon nitride (Si3N4) is an important engineering material with excellent high-temperature properties and resistance to corrosion. The material is also a wide-band-gap semiconductor with a band gap of ∼5 eV.1 Similar to III-N compounds (such as GaN and AlN), Si3N4 is an excellent host material in which high dopant concentration can be incorporated.2,3 Recently, Munakata and co-workers reported the synthesis of millimeterscaled β-Si3N4 single crystals with a high concentration of Al doping and found that a band gap of 2.4 eV was introduced between impurity and conduction bands.2 This work suggested that Al doping could be an efficient method to tailor the optical and electronic properties of Si3N4, and thus enable its applications in optical/electronic devices for high-temperature and radiation environments. Inspired by recent research activities toward one-dimensional nanomaterials, Si3N4 nanowires, nanobelts, and nanotubes have received extensive attention because they exhibit superior properties as compared to their bulk counterparts, and thus promise many new applications.4-6 Previous studies in the field were focused on developing various synthesis technologies.7-23 It is surprising that less effort was devoted to synthesize doped nanostructured Si3N4, given the importance of doping in determining electric/optical properties of the material. This could possibly be due to the fact that it is difficult to form doped nanomaterials with the conventional doping techniques.24 In this study, we report the synthesis of single-crystalline Si3N4 nanowires with controlled Al doping. Unlike the conventional doping techniques in which the dopant ions were introduced by adding either corresponding metal powders or compoundscontainingdopingelementsinthestartingmaterials,25-32 the Al dopants are incorporated into Si3N4 nanowires via a direct pyrolysis of polymeric precursors containing aluminum. This new technique offers a simple method to realize Al-doped Si3N4 * Corresponding authors. E-mail:
[email protected] (W.Y.);
[email protected] (L.A.). † Ningbo University of Technology. ‡ Tsinghua University. § University of Central Florida.
nanomaterials in a controlled manner. The obtained nanowires could be useful for fabricating nanodevices and studying the growth habits of nanomaterials. 2. Experiments Al-doped Si3N4 nanowires were synthesized via the catalystassisted pyrolysis of polyaluminasilazane precursors. The precursors were obtained by reaction of commercially available polyureamethylvinylsilazane (Ceraset, Kion Corporation, U.S.A.) and aluminum isopropoxide (AIP, Beijing Bei Hua Fine Chemicals Company, Beijing, China) using the procedure reported previously.33 Three liquid-phased polyaluminasilazanes containing different amounts of aluminum were synthesized by tailoring the weight ratios of the two starting materials: Cerasetto-AIP ) 16:1, 8:1, and 4:1, referred to as PAS16-1, PAS8-1, and PAS4-1, respectively. The polyaluminasilazanes were then solidified by heat treatment at 260 °C for 0.5 h in N2. The obtained solids were crushed into fine powders by high-energy ball milling for 24 h, with 3 wt % FeCl2 powder (Beijing Bei Hua Fine Chemicals Company, Beijing, China) additives as the catalyst. The powder mixtures were then placed in a high-purity alumina crucible and pyrolyzed in a furnace under flowing ultrahigh-purity nitrogen of 0.1 MPa. The pyrolysis was carried out at 1300 °C for 2 h followed by furnace cooling to ambient temperature. The obtained samples were treated first before characterization, by separating the nanowires from unreacted powders and purifying the nanowires further by repeatedly washing them with ethanol. The obtained products were then characterized using field emission scanning electron microscopy (SEM, JSM-6301F, JEOL, Japan), X-ray diffraction (XRD, Automated D/Max-RB, Rigaku, Japan) with Cu Ka radiation (λ ) 1.541 78 Å), and high-resolution transmission electron microscopy (HRTEM, JEOL-2011, Japan) equipped with energy dispersive spectra (EDS) by using a Cu grid as the sample holder. Photoluminescence (PL) spectra of the nanowires were recorded using a UVlamp microzone Raman spectrometer under the excitation of a 325 nm HeCd laser at room temperature. The PL of the amorphous matrix has also been measured for comparison.
10.1021/jp070642+ CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007
Al-Doped Single-Crystalline Si3N4 Nanowires
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Figure 1. (a) Low-magnification SEM image of the Si3N4 nanowires obtained from PAS16-1; (b) the typical tip of the Si3N4 nanowires synthesized from PAS16-1; (c-f) high-magnification SEM images of the Si3N4 nanowires, obtained from an undoped sample, PAS16-1, PAS8-1, and PAS4-1, respectively.
3. Results and Discussion The morphologies of the obtained Al-doped Si3N4 nanowires were first observed by SEM. Figure 1a shows a typical SEM image of the nanowires obtained from PAS16-1, showing relatively high-density nanowires grown homogeneously on the top of the powder matrix. Figure 1b is the corresponding typical tip of the nanowires without any droplets, suggesting that the nanowires are not grown via the vapor-liquid-solid (VLS) mechanism. The length of the nanowires is typically up to several millimeters with uniform diameters along the entire length. Figure 1c is the typical SEM image of the Si3N4 nanobelts synthesized without Al doping,34 implying that the Al dopants have a profound effect on the shape formation of the synthesized Si3N4 nanomaterials. Closer examinations under higher magnification (Figure 1d-f) reveal that the nanowires take a cylindrical shape with smooth surfaces, regardless of the precursors. The diameters of the nanowires were measured to be ∼70, ∼50, and ∼40 nm on average for the nanowires synthesized from PAS16-1, PAS8-1, and PAS4-1, respectively. These results suggest that aluminum can restrict the growth of nanowires along radial directions. Figure 2a shows XRD patterns of the nanowires synthesized from different precursors. The patterns reveal that, regardless of the precursors, all products contain two crystalline phases: R-Si3N4 and β-Si3N4 (JCPDS Card Nos. 41-0360 and 33-1160). This is different from our previous study,34 where only R-Si3N4 nanomaterials were obtained when pure Ceraset was used as the precursor. The amount of β-Si3N4 increases with Al concentration in the precursors: R-Si3N4 is dominant for the product synthesized from PAS16-1, while β-Si3N4 becomes dominant when PAS4-1 is used as the precursor. The results clearly demonstrate that Al can promote the formation of β-Si3N4. Closer examination of the XRD patterns (Figure 2b) reveals that the peaks of Al-doped R-Si3N4 nanowires shift to
Figure 2. (a) XRD patterns of the Si3N4 nanowires synthesized from different precursors as indicted in the figure; (b) closer XRD patterns of the samples synthesized from PAS16-1 and undoped Si3N4 nanobelts indicate the shifted diffraction angles.
a higher angle compared to that of pure R-Si3N4 nanobelts without doping,34 suggesting the Al was doped into the nanowires. On the other hand, the diffraction peaks of β-Si3N4 have no shift as compared with the standard data (JCPDS Card No. 33-1160), suggesting that no Al was doped into the β-Si3N4 nanowires. Table 1 summarizes the peak shifts and changes in the lattice parameters of the R-Si3N4 nanowires with different doping levels. It is seen that the calculated lattice parameter a is 7.7378, 7.7366, and 7.7042 Å for the nanowires synthesized from PAS16-1, PAS8-1, and PAS4-1, respectively, which corresponds to 0.21, 0.64, and 0.23% decreases compared to that of pure Si3N4. The lattice parameter c is 5.5641, 5.5300, and 5.5300 Å for the samples synthesized from PAS16-1, PAS81, and PAS4-1, respectively, corresponding to 1.02, 1.63, and
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Figure 3. (a) Typical TEM image of the Si3N4 nanowires; (b) corresponding SAED pattern of the nanowire; (c and d) HRTEM images of the nanowire.
TABLE 1: Lattice Parameters of the r-Si3N4 Nanowires as a Function of Al-Doping Concentration lattice constant (Å) sample undoped PAS16-1 PAS8-1 PAS4-1
cryst plane 2θ (deg) (201) (210) (201) (210) (201) (210) (201) (210)
31.00 35.33 31.16 35.44 31.22 35.56 31.22 35.46
a
c
7.7541 7.7541 7.7378 7.7378 7.7042 7.7042 7.7366 7.7366
5.6217 5.6217 5.5641 5.5641 5.5300 5.5300 5.5300 5.5300
decrease ∆a (%) ∆c (%) 0 0 0.21 0.21 0.64 0.64 0.23 0.23
0 0 1.02 1.02 1.63 1.63 1.63 1.63
1.63% decreases as compared to pure Si3N4. There are two possible doping mechanisms: Si3N4
3AlN 98 3AlSi + 3NN + VN
(1)
or Si3N4
4AlN 98 3AlSi + Ali + 4NN
(2)
The decreases in lattice parameters suggest that reaction 1 should be the case, meaning the aluminum atoms replaced silicon atoms in the Si3N4 nanowires to form a substitutional solid solution accompanied by the formation of nitrogen vacancies. The increase in the lattice constants for the nanowires synthesized from PAS4-1 as compared to the nanowires synthesized from PAS8-1 suggests that some of the Al atoms prefer to form an interstitial solid solution when the Si3N4 is heavily doped. This is possibly due to the fact that the high lattice distortion energy prevented further formation of substitutional solid solution. Further characterization of the synthesized nanowires was carried out using TEM and HRTEM. Figure 3a shows a typical TEM image of the nanowires synthesized from PAS16-1, confirming the uniform structure along the nanowires. Figure 3b is a selected area electron diffraction (SAED) pattern recorded from nanowire A in Figure 3a, suggesting that the nanowire is R-Si3N4 and grew along the [001] direction. The SAED patterns are identical over the entire nanowire, indicating that the nanowire is a single crystal. Figure 3c is a typical HRTEM image recorded from nanowire A in Figure 3a, and Figure 3d is an enlarged HRTEM image. These images reveal
Figure 4. (a) EDS of the Si3N4 nanowires synthesized from different precursors as indicated in the figure; (b) the element maps of Si, N, and Al of nanowires, revealing the uniform distribution of Al dopants in the Si3N4 nanowires.
that the nanowire possesses a perfect crystal structure with few structural defects such as dislocations and stacking faults, and there is an amorphous layer of 1-2 nm in thickness on the outside of the nanowire. The lattice fringe spacing of 0.67 nm (Figure 3d) agrees well with the {100} planes of bulk R-Si3N4, where a ) 0.775 41 nm and c ) 0.562 17 nm (JCPDS Card No. 41-0360). The changes in lattice parameters observed from XRD patterns are not seen from fringe spacing measurement due to the changes in the lattice parameters being too small to be measured by HRTEM images. The HRTEM images also confirmed that the nanowires grew along the [001] direction. The compositions of the nanowires were measured using EDS under TEM. The typical EDS (Figure 4a) recorded from the nanowires obtained from three precursors reveal that the nanowires consist of Si, N, Al, Cu, and a small amount of O. The O may come from the amorphous layer on the surfaces of the nanowires, while the Cu comes from the copper grid used to support the TEM sample. It is seen that the Al concentrations increase with the amount of AIP used in the starting precursors: the Al concentrations are 2.22, 3.38, and 4.21 atom % in the Si3N4 nanowires synthesized from the 16:1, 8:1, and 4:1 precursors, respectively. Figure 4b shows the element maps of Si, N, and Al recorded from nanowires synthesized from PAS161, suggesting the uniform spatial distribution of Al dopants within the Si3N4 nanowires. These results suggest that the Aldoping level within Si3N4 nanowires can be controlled by varying the Al concentration in the precursors; thus, the current method offers a simple way to synthesize Si3N4 nanowires with controlled Al-doping concentration. As we discussed previously,35 the formation of the Al-doped Si3N4 nanowires via catalyst-assisted pyrolysis of polymeric precursors should follow the solid-liquid-gas-solid (SLGS)
Al-Doped Single-Crystalline Si3N4 Nanowires
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4159 4. Conclusions Al-doped single-crystalline R-Si3N4 nanowires were synthesized by pyrolysis of polymeric precursors using FeCl2 as the catalyst. The Al-doping levels can be tailored by varying the Al concentration in the polymeric precursors. The obtained nanowires are high quality with a smooth surface and up to several millimeters in length. The Al doping exhibited significant effects on the shape, size, and phase compositions of the obtained low-dimensional nanomaterials. The photoluminescence measurements revealed that Al-doping concentrations have a profound effect on the optical properties of the nanowires. The described synthesis technique, therefore, provides a simple way to control the doping levels and optical properties of Si3N4 nanomaterials. It is believed that the synthesized nanowires could have potential in many optical and electronic nanodevices.
Figure 5. Photoluminescence spectra of the Al-doped Si3N4 nanowires and undoped pure Si3N4 under excitation of a 325 nm HeCd laser at room temperature.
growth process. In this process, the precursors were first thermally decomposed into amorphous SiAlCNs at ∼1000 °C.33 The SiAlCNs were further reacted with Fe to form liquid SiFe-C-Al alloy droplets at a temperature higher than the eutectic temperature of the quaternary system. Meanwhile, the nitrogen in the SiAlCN is released as N2 gas. Further reaction of the SiAlCN and the liquid droplets led to the formation of supersaturated alloys. This supersaturated liquid phase then reacted with N2 gas (at least part of the N2 was provided by the protection gas) on the liquid/gas interface to precipitate the Aldoped Si3N4 nanobelts. To study the effect of Al doping on the optical properties of the nanowires, the photoluminescence (PL) spectra of the nanowires were measured using the 325 nm line of the HeCd laser as the excitation source. The nanowires exhibited intense luminescence that can even be seen by the bare eye. The similar measurement on the amorphous SiCN particles fails to record any light emission, confirming that the PL spectra are from the Si3N4 nanowires. Figure 5 compares typical PL spectra of the samples with different Al-doping concentrations together with that of pure Si3N4 nanobelts.1,35 It is seen that the PL peaks of the Al-doped Si3N4 nanowires show red shifts compared to that of the undoped sample, suggesting that the Al doping caused the decrease in band energies, consistent with a previous study.2 While all the Al-doped Si3N4 nanowires show the same trend with strong and broad emissions ranging from 1.3 to 2.5 eV, the detailed peak structures are quite different. The emission of the nanowires with a relatively light doping level (synthesized from PAS16-1) shows two peaks centered at 1.76 and 2.04 eV and a shoulder at 1.46 eV. The emission of the nanowires with a middle doping level (synthesized from PAS8-1) shows one strong peak at 2.1 eV, but the intensity of the peak at 1.76 eV decreases compared to that of the nanowires from PAS16-1. On the other hand, the emission of the nanowires with a high doping level (synthesized from PAS4-1) shows one strong peak at 1.46 eV, but the intensities of the peaks at 1.76 and 2.06 eV significantly decrease compared to those of the PAS8-1 and PAS16-1 samples. While the detailed mechanisms that determined the emissions of these nanowires are not clear, current results suggest that the optical emissions of the Si3N4 nanowires can be significantly altered, leading to a possibility of manipulating the optical properties of nanostructured Si3N4 materials.
Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 50602025 and 50372031), Natural Science Foundation of Ningbo Municipal Government (Grant No. 2006A610059), and the two-base projects of NSFC (Grant No. 50540420104). References and Notes (1) Zhang, L.; Jin, H.; Yang, W.; Xie, Z.; Miao, H.; An, L. Appl. Phys. Lett. 2005, 86, 061908. (2) Munakata, F.; Matsuo, K.; Furuya, K.; Akimune, Y. J.; Ishikawa, I. Appl. Phys. Lett. 1999, 74, 3498. (3) Zanatta, A. R.; Nunes, L. A. O. Appl. Phys. Lett. 1998, 72, 3127. (4) Ziegler, G.; Heinrich, J.; Wo¨tting, C. J. Mater. Sci. 1987, 22, 3041. (5) Zhang, Y.; Wang, N.; He, R.; Zhang, Q.; Zhu, J.; Yan, Y. J. Mater. Res. 2000, 15, 1048. (6) Claussen, N.; Beyer, P.; Janssen, R.; May, M.; Selchert, T.; Yang, J. F.; Ohji, T.; Kanzaki, S.; Yamakawa, A. AdV. Eng. Mater. 2002, 4, 117. (7) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Gu, B. L.; Zhang, X. B.; Yu, D. P. Appl. Phys. Lett. 1997, 71, 2271. (8) Shen, G. Z.; Bando, Y.; Liu, B. D.; Tang, C. C.; Huang, Q.; Golberg, D. Chem.sEur. J. 2006, 12, 2987. (9) Xie, T.; Wu, G. S.; Geng, B. Y.; Jiang, Z.; Yuan, X. Y.; Lin, Y.; Wang, G. Z.; Zhang, L. D. Appl. Phys. A 2005, 80, 1057. (10) Wu, X. C.; Song, W. H.; Zhao, B.; Huang, W. D.; Pu, M. H.; Sun, Y. P.; Du, J. J. Solid State Commun. 2000, 115, 683. (11) Zhang, L. D.; Meng, G. W.; Phillipp, F. Mater. Sci. Eng., A 2000, 286, 34. (12) Zhang, Y. J.; Wang, N. L.; Gao, S. P.; He, R. R.; Miao, S.; Liu, J.; Zhu, J.; Zhang, X. Chem. Mater. 2002, 14, 3564. (13) Gao, Y. H.; Bando, Y.; Kurashima, K.; Sato, T. J. Appl. Phys. 2002, 91, 1515. (14) Tang, C. C.; Ding, X. X.; Huang, X. T.; Gan, Z. W.; Liu, W.; Qi, S. R.; Li, Y. X.; Qu, J. P.; Hu, L. Jpn. J. Appl. Phys., Part 2 2002, 41, L589. (15) Gundiah, G.; Madhav, G. V.; Govindaraj, A.; Seikh, M. M.; Rao, C. N. R. J. Mater. Chem. 2002, 12, 1606. (16) Farjas, J.; Rath, C.; Pinyol, A.; Roura, P.; Bertran, E. Appl. Phys. Lett. 2005, 87, 192114. (17) Cui, H.; Stoner, B. R. J. Mater. Res. 2001, 16, 3111. (18) Wang, F.; Jin, G. Q.; Guo, X. Y. Mater. Lett. 2006, 60, 330. (19) Chen, Y.; Guo, L.; Shaw, D. J. Cryst. Growth 2000, 210, 527. (20) Yin, L. W.; Bando, Y.; Zhu, Y. C.; Li, Y. B. Appl. Phys. Lett. 2003, 83, 3584. (21) Huo, K. F.; Ma, Y. W.; Hu, Y. M.; Fu, J. J.; Lu, B.; Lu, Y. N.; Hu, Z.; Chen, Y. Nanotechnology 2005, 16, 2282. (22) Hu, J. Q.; Bando, Y.; Sekiguchi, T.; Xu, F. F.; Zhan, J. H. Appl. Phys. Lett. 2004, 84, 804. (23) Hu, J. Q.; Bando, Y.; Liu, Z. W.; Xu, F. F.; Sekiguchi, T.; Zhan, J. H. Chem.sEur. J. 2004, 10, 554. (24) Yang, C.; Zhong, Z. H.; Lieber, C. M. Science 2005, 310, 1304. (25) Chi, B.; Victorio, E. S.; Jin, T. Nanotechnology 2006, 17, 2234. (26) Yuhas, B. D.; Zitoun, D. O.; Pauzauskie, P. J.; He, R. R.; Yang, P. D. Angew. Chem., Int. Ed. 2006, 45, 420. (27) Liu, J. J.; Yu, M. H.; Zhou, W. L. Appl. Phys. Lett. 2005, 87, 172505. (28) Radovanovic, P. V.; Barrelet, C. J.; Gradecak, S.; Qian, F.; Lieber, C. M. Nano Lett. 2005, 5, 1407. (29) Xu, C.; Kim, M.; Chun, J.; Kim, D. Appl. Phys. Lett. 2005, 86, 133107.
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