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Synthesis and characterization of straight and stacked-sheet AlN nanowires with high purity
Journal of Alloys and Compounds 459 (2008) 338–342
Synthesis and characterization of straight and stacked-sheet AlN nanowires with high purity M. Lei a,b , H. Yang a , P.G. Li b , W.H. Tang b,∗ a
b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, China Received 13 March 2007; received in revised form 19 April 2007; accepted 20 April 2007 Available online 4 May 2007
Abstract Large-scale AlN nanowires with hexagonal crystal structure were synthesized by the direct nitridation method at high temperatures. The experimental results indicate that these single-crystalline AlN nanowires have high purity and consist of straight and stacked-sheet nanowires. It is found that straight AlN nanowire grows along [1, 1, −2, 0] direction, whereas the stacked-sheet nanowire with hexagonal cross section is along [0 0 0 1] direction. It is thought that vapor–solid (VS) mechanism should be responsible for the growth of AlN nanowires. © 2007 Elsevier B.V. All rights reserved. Keywords: AlN; Nanowires; Direct nitridation process
1. Introduction Solid wide-bandgap III–V nitride semiconductors, such as AlN, GaN and InN, have attracted increased attention due to their importance in both scientific research and technological applications [1–4]. Of these nitride semiconductors, aluminum nitride has higher thermal conductivity at low temperature, higher thermal stability, low thermal expansion coefficient, high dielectric breakdown strength, good mechanical strength, excellent chemical stability and nontoxicity [5,6]. Moreover, it is an attractive material for electronic substrates and can be also used as excellent filler for integrated circuits and rubber mixtures [7,8]. Recently, low-dimensional AlN nanomaterials, such as nanowires and nanotubes with small size and large area, are especially of interest because they exhibit some unique properties and promising applications in electronics and photonics devices. So far, low-dimensional AlN nanomaterials have been synthesized by some methods including CVD [9–13], plasma process [14], silica-assisted catalytic growth [15] and vapor–liquid–solid (VLS) growth process [16–19]. However, most of the methods have some disadvantages of low productivity and severe impurities from their employed precursors or incomplete con∗
Corresponding author. Tel.: +86 571 86843468; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.265
versions of the starting materials. In this paper, by a direct nitridation process, large-scale AlN nanowires with hexagonal crystal structure were synthesized by the reaction of aluminum and ammonia/nitrogen at high temperatures. The structural property of The AlN nanowires was characterized and investigated in detail. 2. Experimental The synthesis of AlN nanowires was carried out in a horizontal tubular furnace with two open-end straight alumina tube. One gram of rough aluminum was placed in an alumina boat. After flushed with ammonia/nitrogen (1:1) atmosphere to remove the remaining air in the alumina tube, the furnace temperature was heated to 900 ◦ C under ammonia/nitrogen flow at 80 sccm. Subsequently, the alumina boat was put in the center of the furnace. Then, the furnace was heated to 1350 ◦ C under ammonia/nitrogen flow at 60 sccm and was kept 1350 ◦ C for half an hour. After the furnace temperature was cooled to the room temperature in the flow of ammonia/nitrogen atmosphere, the gray white product was obtained from the surface of the alumina boat. Powder X-ray diffraction (XRD) data used for structural analysis was collected on PaNalytical X’Pert Pro MPD X-ray diffractometer with Cu K␣ radiation. The morphology of the product was examined by field-emission scanning electron microscope (FEI XL30 S-FEG). The X-ray photoelectron spectra (XPS) were recorded on a VGESCALAB MKII X-ray Photoelectron Spectrometer, using non-monochromatized Mg K␣ X-ray as the excitation source. The TEM image, electron diffraction (ED) patterns and high-resolution lattice fringe (HRTEM) of samples were collected on the JEOL 2010 transmission electron microscope. Raman measurement was performed on a JY-HR800 laser Raman spectrometer using a 532 nm solid-state laser as excitation source.
M. Lei et al. / Journal of Alloys and Compounds 459 (2008) 338–342
Fig. 1. XRD pattern of the as-prepared AlN nanowires.
3. Results and discussion A typical XRD pattern of the as-prepared sample is shown in Fig. 1. All the peaks of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (2 0 1) and (2 0 2) reflections can be indexed ˚ and as hexagonal AlN with lattice constants of a = 3.114 A ˚ c = 4.981 A, agreeing well with the calculated diffraction pattern (ICDD-PDF No. 25-1133). No other impure peaks are detected, indicating that the sample is predominantly hexagonal AlN. The composition of the as-prepared sample can be also derived from XPS spectra (Fig. 2). Fig. 2a shows a typical survey spectrum of the sample, indicating the presence of elemental Al and N. The appearance of other peaks is due to the absorption of C contamination and O2 impurity on the surface of the sample. The
339
binding energies, centered at 74.6 eV for Al 2p and 398.7 eV for N 1s are in good agreement with the values of bulk AlN from the literature [17]. Quantification of the Al 2p and N 1s peaks gives an average Al/N atomic ratio of 1.021:1, which is close to the calculated atomic ratio. SEM images of the sample with different magnification are shown in Fig. 3. Fig. 3a gives an overall view of the sample, showing that there are large-scale AlN nanowires with high density. SEM images (Fig. 3b and c) with higher magnification display that the nanowires are composed of straight nanowires and a small quantity of stacked-sheet nanowires. The diameters and lengths of the nanowires are about 40–150 nm and 5–10 m, respectively. Fig. 3d shows the morphology of the stacked-sheet nanowire, exhibiting that the nanowire is composed of hexagonal sheet stacked along a direction. TEM is employed to further examine the microstructures of the AlN nanowires. Fig. 4a is a bright-field TEM image of an individual straight nanowire with diameter of about 100 nm. The selected area electron diffraction (SAED) pattern taken from the nanowire can be indexed based on the hexagonal AlN cell (ICDD-PDF No. 25-1133). The electron diffraction (Fig. 4b) along the [0 0 0 1] zone axis indicates that the nanowire grows perpendicular to (1, 1, −2, 0) crystal planes. Fig. 3c shows a high-resolution TEM image of the nanowire, showing exactly the (1, 1, −2, 0) lattice plane perpendicular to the wire axis, indicating the [1, 1, −2, 0] growth direction of the straight nanowire. The stacked-sheet nanowires are also detailedly characterized by TEM. Fig. 5a shows a typical TEM image of novel stacked-sheet nanowire with average diameter of about 80 nm. Two SAED patterns taken from the
Fig. 2. XPS spectra of the as-prepared AlN nanowires. (a) Survey spectrum; (b) N 1s region; (c) Al 2p region.
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M. Lei et al. / Journal of Alloys and Compounds 459 (2008) 338–342
Fig. 3. SEM images of the as-prepared AlN nanowires. (a)–(d) are SEM images with different magnification showing the configuration of the as-prepared AlN nanowires.
nanowire can be observed and be indexed based on the hexagonal AlN cell. These two electron diffraction patterns (Fig. 5b and c) along [2, −1, −1, 0] and [0, 1, −1, 0] zone axis, respectively, indicate that the growth direction of the nanowire is along [0 0 0 1]. The stacked-sheet growth morphology of AlN nanowires firstly reported here were also found in some hexagonal wurtzite nanowires including GaN, InN and ZnO [20,21],
which indicates the nanostructure may be a typical morphology for the hexagonal wurtzite isostructures. Fig. 6 shows the Raman scattering spectrum of the hexagonal AlN nanowires. Considering the Raman selection rules, six firstorder Raman active phonons, namely A1 (TO), A1 (LO), E1 (TO), E1 (LO), E2 (high) and E2 (low) modes can be observed in the hexagonal AlN [22]. In this study, five first-order Raman
Fig. 4. (a) Typical TEM image of a single straight AlN nanowire. (b) SAED pattern of the AlN nanowire. (c) HRTEM images of the AlN nanowire.
M. Lei et al. / Journal of Alloys and Compounds 459 (2008) 338–342
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Fig. 5. (a) Typical TEM image of a single stacked-sheet AlN nanowire. (b) and (c) different SAED pattern of the stacked-sheet AlN nanowire.
active phonons A1 (TO), E2 (high), E1 (TO), A1 (LO) and E1 (LO) at 611.3, 657.6, 670.7, 893.8 and 910.4 cm−1 , respectively, are observed. These five Raman active phonons are in good agreement with that of the hexagonal AlN crystal [22]. E2 (low) mode is not observed here, which is permitted by P63 mc space group in the first-order Raman measurement at the zone center. Various experimental methods and the corresponding growth mechanisms have been reported for the syntheses of AlN. However, most of the methods have the disadvantages of low productivity and severe impurities from their employed catalyst or precursor. By the direct nitridation approach, Tian et al. have successfully prepared large-scale AlN nanowires using fine Al nanoparticles as precursor. However, the Al nanoparticles are more expensive, incurring additional costs. In this work, without any catalyst or assisted agent, we successfully synthesize large-scale AlN nanowires with high purity using rough Al powder instead of Al nanoparticles as starting material. It is thought that vapor–solid (VS) mechanism should be responsible for the growth of AlN nanowires. Firstly, Al powder is converted into Al vapor. Subsequently, the Al vapor will react with N2 /NH3 to form crystal seed for the growth of AlN nanowires. With increasing the temperature, more Al vapor is produced and the high yield AlN nanowires are formed. This explanation can be further confirmed by experimental observation. If the reaction temperature is not high enough (such as 1150 ◦ C), a small quantity of AlN nanowires can be found. If the temperature is increased to 1350 ◦ C, large-scale AlN nanowires with high purity are formed.
4. Conclusions In summary, large-scale hexagonal AlN nanowires were successfully synthesized by direct nitridation process. The experimental results indicate that these single-crystalline AlN nanowires have high purity and consist of straight and a small quantity of stacked-sheet nanowires. The straight nanowires show the smooth surface and grow along [1, 1, −2, 0] direction, whereas the stacked-sheet nanowires with hexagonal cross section grow along [0 0 0 1] direction. The growth mechanism can be ascribed to vapor–solid (VS) process. This novel method may provide a new route fabricate large-scale AlN nanowires with high purity. Acknowledgements This work was supported by the key project of the Zhejiang Provincial Natural Science Foundation (Z605131) and the National Natural Science Foundation of China (60571029, 50672088). References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] Fig. 6. Raman scattering spectrum of hexagonal AlN nanowires.
Y. Cui, C.M. Lieber, Science 291 (2001) 851–853. Y. Cui, Q.Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289–1292. V.N. Tondare, Nanotechnology 15 (2004) 1388–1389. S. Vaddiraju, A. Mohite, A. Chin, M. Meyyappan, G. Sumanasekera, B.W. Alphenaar, M.K. Sunkara, Nano Lett. 5 (2005) 1625–1631. G.A. Slack, R.A. Tanzilli, R.O. Pohl, J.W. Vandersande, J. Phys. Chem. Solids 48 (1987) 641–647. A.W. Weimer, G.A. Cochran, G.A. Eisman, J.P. Henley, B.D. Hook, L.K. Mills, T.A. Guiton, A.K. Knudsen, N.R. Nicholas, J.E. Volmering, W.G. Moore, J. Am. Ceram. Soc. 77 (1994) 3–18. O. Yamada, K. Hirao, M. Kozumi, Y. Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1735–1738. W.J. Meng, in: J.H. Edgar (Ed.), Properties of Group Nitrides, The Institution of Electrical Engineers, London, 1994, p. 22. K. Komeya, J. Ceram. Soc. Jpn. 101 (1993) 1319–1323. M. Radwan, M. Bahgat, J. Mater. Proc. Technol. 181 (2007) 99–105. I.C. Huseby, J. Am. Ceram. Soc. 66 (1983) 217–220. R. Riedel, K.U. Gaudl, J. Am. Ceram. Soc. 74 (1991) 1331–1334. T. Watari, T. Akizuki, H. Ikeda, T. Torikai, O. Matsuda, J. Ceram. Soc. Jpn., Int. Ed. 97 (1989) 851–853. Y.J. Tian, Y.P. Jia, Y.J. Bao, Y.F. Chen, Diamond Relat. Mater. 16 (2007) 302–305. C.C. Tang, S.S. Fan, M.L. de la Chapelle, P. Li, Chem. Phys. Lett. 333 (2001) 12–15.
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[16] Q. Wu, Z. Hu, X.Z. Wang, Y.N. Lu, S.Z. Deng, N.S. Xu, B. Shen, R. Zhang, Y. Chen, J. Mater. Chem. 13 (2003) 2024–2027. [17] Q. Wu, Z. Hu, X.Z. Wang, Y.M. Hu, Y.J. Tian, Y. Chen, Diamond Relat. Mater. 13 (2004) 38–41. [18] J.H. He, R.S. Yang, Y.L. Chueh, L.J. Chou, L.J. Chen, Z.L. Wang, Adv. Mater. 18 (2006) 650–654. [19] J.H. Duan, S.G. Yang, H.W. Liu, J.F. Gong, H.B. Huang, X.N. Zhao, J.L. Tang, R. Zhang, Y.W. Du, J. Cryst. Growth 283 (2005) 291–296.
[20] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829–R858. [21] S.D. Luo, W.Y. Zhou, Z.X. Zhang, L.F. Liu, X.Y. Dou, J.X. Wang, X.W. Zhao, D.F. Liu, Y. Gao, L. Song, Y.J. Xiang, J.J. Zhou, S.S. Xie, Small 1 (2005) 1004–1009. [22] V.Y. Davydov, Y.E. Kitaev, I.N. Goncharuk, A.N. Smirnov, J. Graul, O. Semchinova, D. Uffmann, M.B. Smirnov, A.P. Mirgorodsky, R.A. Evarestov, Phys. Rev. B 58 (1998) 12899–12907.
Synthesis and characterization of straight and stacked-sheet AlN nanowires with high purity M. Lei a,b , H. Yang a , P.G. Li b , W.H. Tang b,∗ a
b
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, China Received 13 March 2007; received in revised form 19 April 2007; accepted 20 April 2007 Available online 4 May 2007
Abstract Large-scale AlN nanowires with hexagonal crystal structure were synthesized by the direct nitridation method at high temperatures. The experimental results indicate that these single-crystalline AlN nanowires have high purity and consist of straight and stacked-sheet nanowires. It is found that straight AlN nanowire grows along [1, 1, −2, 0] direction, whereas the stacked-sheet nanowire with hexagonal cross section is along [0 0 0 1] direction. It is thought that vapor–solid (VS) mechanism should be responsible for the growth of AlN nanowires. © 2007 Elsevier B.V. All rights reserved. Keywords: AlN; Nanowires; Direct nitridation process
1. Introduction Solid wide-bandgap III–V nitride semiconductors, such as AlN, GaN and InN, have attracted increased attention due to their importance in both scientific research and technological applications [1–4]. Of these nitride semiconductors, aluminum nitride has higher thermal conductivity at low temperature, higher thermal stability, low thermal expansion coefficient, high dielectric breakdown strength, good mechanical strength, excellent chemical stability and nontoxicity [5,6]. Moreover, it is an attractive material for electronic substrates and can be also used as excellent filler for integrated circuits and rubber mixtures [7,8]. Recently, low-dimensional AlN nanomaterials, such as nanowires and nanotubes with small size and large area, are especially of interest because they exhibit some unique properties and promising applications in electronics and photonics devices. So far, low-dimensional AlN nanomaterials have been synthesized by some methods including CVD [9–13], plasma process [14], silica-assisted catalytic growth [15] and vapor–liquid–solid (VLS) growth process [16–19]. However, most of the methods have some disadvantages of low productivity and severe impurities from their employed precursors or incomplete con∗
Corresponding author. Tel.: +86 571 86843468; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.265
versions of the starting materials. In this paper, by a direct nitridation process, large-scale AlN nanowires with hexagonal crystal structure were synthesized by the reaction of aluminum and ammonia/nitrogen at high temperatures. The structural property of The AlN nanowires was characterized and investigated in detail. 2. Experimental The synthesis of AlN nanowires was carried out in a horizontal tubular furnace with two open-end straight alumina tube. One gram of rough aluminum was placed in an alumina boat. After flushed with ammonia/nitrogen (1:1) atmosphere to remove the remaining air in the alumina tube, the furnace temperature was heated to 900 ◦ C under ammonia/nitrogen flow at 80 sccm. Subsequently, the alumina boat was put in the center of the furnace. Then, the furnace was heated to 1350 ◦ C under ammonia/nitrogen flow at 60 sccm and was kept 1350 ◦ C for half an hour. After the furnace temperature was cooled to the room temperature in the flow of ammonia/nitrogen atmosphere, the gray white product was obtained from the surface of the alumina boat. Powder X-ray diffraction (XRD) data used for structural analysis was collected on PaNalytical X’Pert Pro MPD X-ray diffractometer with Cu K␣ radiation. The morphology of the product was examined by field-emission scanning electron microscope (FEI XL30 S-FEG). The X-ray photoelectron spectra (XPS) were recorded on a VGESCALAB MKII X-ray Photoelectron Spectrometer, using non-monochromatized Mg K␣ X-ray as the excitation source. The TEM image, electron diffraction (ED) patterns and high-resolution lattice fringe (HRTEM) of samples were collected on the JEOL 2010 transmission electron microscope. Raman measurement was performed on a JY-HR800 laser Raman spectrometer using a 532 nm solid-state laser as excitation source.
M. Lei et al. / Journal of Alloys and Compounds 459 (2008) 338–342
Fig. 1. XRD pattern of the as-prepared AlN nanowires.
3. Results and discussion A typical XRD pattern of the as-prepared sample is shown in Fig. 1. All the peaks of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (2 0 1) and (2 0 2) reflections can be indexed ˚ and as hexagonal AlN with lattice constants of a = 3.114 A ˚ c = 4.981 A, agreeing well with the calculated diffraction pattern (ICDD-PDF No. 25-1133). No other impure peaks are detected, indicating that the sample is predominantly hexagonal AlN. The composition of the as-prepared sample can be also derived from XPS spectra (Fig. 2). Fig. 2a shows a typical survey spectrum of the sample, indicating the presence of elemental Al and N. The appearance of other peaks is due to the absorption of C contamination and O2 impurity on the surface of the sample. The
339
binding energies, centered at 74.6 eV for Al 2p and 398.7 eV for N 1s are in good agreement with the values of bulk AlN from the literature [17]. Quantification of the Al 2p and N 1s peaks gives an average Al/N atomic ratio of 1.021:1, which is close to the calculated atomic ratio. SEM images of the sample with different magnification are shown in Fig. 3. Fig. 3a gives an overall view of the sample, showing that there are large-scale AlN nanowires with high density. SEM images (Fig. 3b and c) with higher magnification display that the nanowires are composed of straight nanowires and a small quantity of stacked-sheet nanowires. The diameters and lengths of the nanowires are about 40–150 nm and 5–10 m, respectively. Fig. 3d shows the morphology of the stacked-sheet nanowire, exhibiting that the nanowire is composed of hexagonal sheet stacked along a direction. TEM is employed to further examine the microstructures of the AlN nanowires. Fig. 4a is a bright-field TEM image of an individual straight nanowire with diameter of about 100 nm. The selected area electron diffraction (SAED) pattern taken from the nanowire can be indexed based on the hexagonal AlN cell (ICDD-PDF No. 25-1133). The electron diffraction (Fig. 4b) along the [0 0 0 1] zone axis indicates that the nanowire grows perpendicular to (1, 1, −2, 0) crystal planes. Fig. 3c shows a high-resolution TEM image of the nanowire, showing exactly the (1, 1, −2, 0) lattice plane perpendicular to the wire axis, indicating the [1, 1, −2, 0] growth direction of the straight nanowire. The stacked-sheet nanowires are also detailedly characterized by TEM. Fig. 5a shows a typical TEM image of novel stacked-sheet nanowire with average diameter of about 80 nm. Two SAED patterns taken from the
Fig. 2. XPS spectra of the as-prepared AlN nanowires. (a) Survey spectrum; (b) N 1s region; (c) Al 2p region.
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M. Lei et al. / Journal of Alloys and Compounds 459 (2008) 338–342
Fig. 3. SEM images of the as-prepared AlN nanowires. (a)–(d) are SEM images with different magnification showing the configuration of the as-prepared AlN nanowires.
nanowire can be observed and be indexed based on the hexagonal AlN cell. These two electron diffraction patterns (Fig. 5b and c) along [2, −1, −1, 0] and [0, 1, −1, 0] zone axis, respectively, indicate that the growth direction of the nanowire is along [0 0 0 1]. The stacked-sheet growth morphology of AlN nanowires firstly reported here were also found in some hexagonal wurtzite nanowires including GaN, InN and ZnO [20,21],
which indicates the nanostructure may be a typical morphology for the hexagonal wurtzite isostructures. Fig. 6 shows the Raman scattering spectrum of the hexagonal AlN nanowires. Considering the Raman selection rules, six firstorder Raman active phonons, namely A1 (TO), A1 (LO), E1 (TO), E1 (LO), E2 (high) and E2 (low) modes can be observed in the hexagonal AlN [22]. In this study, five first-order Raman
Fig. 4. (a) Typical TEM image of a single straight AlN nanowire. (b) SAED pattern of the AlN nanowire. (c) HRTEM images of the AlN nanowire.
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Fig. 5. (a) Typical TEM image of a single stacked-sheet AlN nanowire. (b) and (c) different SAED pattern of the stacked-sheet AlN nanowire.
active phonons A1 (TO), E2 (high), E1 (TO), A1 (LO) and E1 (LO) at 611.3, 657.6, 670.7, 893.8 and 910.4 cm−1 , respectively, are observed. These five Raman active phonons are in good agreement with that of the hexagonal AlN crystal [22]. E2 (low) mode is not observed here, which is permitted by P63 mc space group in the first-order Raman measurement at the zone center. Various experimental methods and the corresponding growth mechanisms have been reported for the syntheses of AlN. However, most of the methods have the disadvantages of low productivity and severe impurities from their employed catalyst or precursor. By the direct nitridation approach, Tian et al. have successfully prepared large-scale AlN nanowires using fine Al nanoparticles as precursor. However, the Al nanoparticles are more expensive, incurring additional costs. In this work, without any catalyst or assisted agent, we successfully synthesize large-scale AlN nanowires with high purity using rough Al powder instead of Al nanoparticles as starting material. It is thought that vapor–solid (VS) mechanism should be responsible for the growth of AlN nanowires. Firstly, Al powder is converted into Al vapor. Subsequently, the Al vapor will react with N2 /NH3 to form crystal seed for the growth of AlN nanowires. With increasing the temperature, more Al vapor is produced and the high yield AlN nanowires are formed. This explanation can be further confirmed by experimental observation. If the reaction temperature is not high enough (such as 1150 ◦ C), a small quantity of AlN nanowires can be found. If the temperature is increased to 1350 ◦ C, large-scale AlN nanowires with high purity are formed.
4. Conclusions In summary, large-scale hexagonal AlN nanowires were successfully synthesized by direct nitridation process. The experimental results indicate that these single-crystalline AlN nanowires have high purity and consist of straight and a small quantity of stacked-sheet nanowires. The straight nanowires show the smooth surface and grow along [1, 1, −2, 0] direction, whereas the stacked-sheet nanowires with hexagonal cross section grow along [0 0 0 1] direction. The growth mechanism can be ascribed to vapor–solid (VS) process. This novel method may provide a new route fabricate large-scale AlN nanowires with high purity. Acknowledgements This work was supported by the key project of the Zhejiang Provincial Natural Science Foundation (Z605131) and the National Natural Science Foundation of China (60571029, 50672088). References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] Fig. 6. Raman scattering spectrum of hexagonal AlN nanowires.
Y. Cui, C.M. Lieber, Science 291 (2001) 851–853. Y. Cui, Q.Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289–1292. V.N. Tondare, Nanotechnology 15 (2004) 1388–1389. S. Vaddiraju, A. Mohite, A. Chin, M. Meyyappan, G. Sumanasekera, B.W. Alphenaar, M.K. Sunkara, Nano Lett. 5 (2005) 1625–1631. G.A. Slack, R.A. Tanzilli, R.O. Pohl, J.W. Vandersande, J. Phys. Chem. Solids 48 (1987) 641–647. A.W. Weimer, G.A. Cochran, G.A. Eisman, J.P. Henley, B.D. Hook, L.K. Mills, T.A. Guiton, A.K. Knudsen, N.R. Nicholas, J.E. Volmering, W.G. Moore, J. Am. Ceram. Soc. 77 (1994) 3–18. O. Yamada, K. Hirao, M. Kozumi, Y. Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1735–1738. W.J. Meng, in: J.H. Edgar (Ed.), Properties of Group Nitrides, The Institution of Electrical Engineers, London, 1994, p. 22. K. Komeya, J. Ceram. Soc. Jpn. 101 (1993) 1319–1323. M. Radwan, M. Bahgat, J. Mater. Proc. Technol. 181 (2007) 99–105. I.C. Huseby, J. Am. Ceram. Soc. 66 (1983) 217–220. R. Riedel, K.U. Gaudl, J. Am. Ceram. Soc. 74 (1991) 1331–1334. T. Watari, T. Akizuki, H. Ikeda, T. Torikai, O. Matsuda, J. Ceram. Soc. Jpn., Int. Ed. 97 (1989) 851–853. Y.J. Tian, Y.P. Jia, Y.J. Bao, Y.F. Chen, Diamond Relat. Mater. 16 (2007) 302–305. C.C. Tang, S.S. Fan, M.L. de la Chapelle, P. Li, Chem. Phys. Lett. 333 (2001) 12–15.
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[16] Q. Wu, Z. Hu, X.Z. Wang, Y.N. Lu, S.Z. Deng, N.S. Xu, B. Shen, R. Zhang, Y. Chen, J. Mater. Chem. 13 (2003) 2024–2027. [17] Q. Wu, Z. Hu, X.Z. Wang, Y.M. Hu, Y.J. Tian, Y. Chen, Diamond Relat. Mater. 13 (2004) 38–41. [18] J.H. He, R.S. Yang, Y.L. Chueh, L.J. Chou, L.J. Chen, Z.L. Wang, Adv. Mater. 18 (2006) 650–654. [19] J.H. Duan, S.G. Yang, H.W. Liu, J.F. Gong, H.B. Huang, X.N. Zhao, J.L. Tang, R. Zhang, Y.W. Du, J. Cryst. Growth 283 (2005) 291–296.
[20] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829–R858. [21] S.D. Luo, W.Y. Zhou, Z.X. Zhang, L.F. Liu, X.Y. Dou, J.X. Wang, X.W. Zhao, D.F. Liu, Y. Gao, L. Song, Y.J. Xiang, J.J. Zhou, S.S. Xie, Small 1 (2005) 1004–1009. [22] V.Y. Davydov, Y.E. Kitaev, I.N. Goncharuk, A.N. Smirnov, J. Graul, O. Semchinova, D. Uffmann, M.B. Smirnov, A.P. Mirgorodsky, R.A. Evarestov, Phys. Rev. B 58 (1998) 12899–12907.