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Synthesis of GaN nanospindles via a facile solid-state reaction route
ARTICLE IN PRESS
Journal of Crystal Growth 280 (2005) 341–345 www.elsevier.com/locate/jcrysgro
Synthesis of GaN nanospindles via a facile solid-state reaction route Xiaopeng Haoa,, Jie Zhana, Yongzhong Wua, Suwen Liub, Xiangang Xua, Minhua Jianga a State Key Lab of Crystal Materials, Shandong University, Jinan 250100, PR China Material Science and Engineering Department, Shangdong Institute of Light Industry, Jinan 250100, PR China
b
Received 22 December 2004; accepted 28 March 2005 Available online 10 May 2005 Communicated by M. Schieber
Abstract Gallium nitride (GaN) nanospindles have been synthesized via a solid-state reaction at a low-temperature condition. X-ray powder diffraction (XRD), Raman spectrum and high-resolution transmission electron microscopy (HRTEM) revealed that the synthesized GaN crystallized in a hexagonal structure and displaying spindly particles morphology has an average diameter of 100 nm and length of 400 nm X-ray photoelectron spectroscopy (XPS) of the sample gave the atomic ratio of Ga and N of 1.04:1. Room-temperature photoluminescence (PL) spectrum showed that the as-prepared product had a peak emission at 372 nm. The possible formation mechanism of the wurtzite GaN is briefly discussed. r 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+W; 61.66.Fn; 81.20.n Keywords: A1. X-ray diffraction; B1. Inorganic compounds; B1. Nanomaterials; B1. Nitrides
1. Introduction Hexagonal GaN, with a direct energy gap of 3.39 eV, is one potentially useful material for the manufacturing of blue light-emitting diodes and lasers. Furthermore, GaN-based semiconductors have made it possible to get light-emitting devices Corresponding author. Tel./fax: +86 53 1856 4260.
E-mail address: [email protected] (X. Hao).
over the entire color spectrum of the visible region of the electromagnetic spectrum [1–3]. One-dimensional (1D) GaN nanostructures are intrinsically attractive candidates for assembling active nanoscale devices for electronic and optical applications [4–7]. And there are various methods developed to grow 1D GaN nanostructures, such as laserinduced growth [8], template-assisted synthesis [4], catalyst-assisted synthesis [9] oxide-assisted method [10], the reaction of Ga and NH3 gas
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.03.072
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[11,12], pyrolysis [13], vapor-phase epitaxy (VPE) [14] and chemical vapor deposition [6,15]. However, some of the above-mentioned synthesis methods require critical reaction conditions and this limits the application fields. Because the applicability of GaN is sensitive to changes in the morphologies, further explorations of the synthesis of other nanostructures of GaN are a challenging subject. Herein, we report a novel facile route using NaN3 and Li3N as the nitrogen source reacting with Ga to synthesize GaN nanospindles at 400 1C and 5 atm. To the best of our knowledge, this distinct shape of GaN nanospindles has not been synthesized by the solid-state method.
2. Experimental procedure The experimental procedures is as follows: 1.0 g Ga, 1.0 g NaN3 and 0.8 g Li3N were placed in a BN crucible, and the BN crucible was placed in a stainless steel autoclave. The autoclave packing the BN crucible was vacuumized and then charged with N2 to 1 atm. The autoclave was heated to 400 1C and kept for 40 h. After the growth, the sample was cooled to room temperature. The product was filtered with ethanol and water to remove the by-products, separately. Finally, the GaN was obtained. The X-ray powder diffraction (XRD) pattern was recorded on a Rigaku D max-gA X-ray diffractometer with Ni-filtered Cuka radiation. The X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 2200-XL X-ray photoelectron spectrometer using Al and Mg Xray sources in the energy range from 1400 to 0 eV. The room-temperature Raman spectrum was taken on a Jasco Ventuno-21 Micro Raman Spectrophotometer. The wavelength was 532 nm and output power was 30 mW. High-resolution transmission electron microscopy (HRTEM) images were taken on a Philips Tecnai 20U-Twin HRTEM. The accelerating voltage was 200 KV. Photoluminescence (PL) spectra of GaN crystals were measured in an Edinburgh FLS920 fluorescent spectrophotometer with a Xe lamp at room temperature.
3. Results and discussion XRD was used to verify the crystal structures and the phase purity of the materials. A typical XRD pattern of the products is shown in Fig. 1. All the peaks of the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (0 0 4) and (2 0 2) reflections can be indexed to hexagonal wurtzite ( and GaN with lattice constants of a ¼ 3:188 A ( c ¼ 5:188 A, consistent with the reported values of hexagonal GaN [16]. It is noticeable that the relative intensity of the (0 0 2) reflections is higher compared with those of bulk GaN, which may indicate an anisotropic growth of the hexagonal GaN. No impurities such as Ga2O3 were detected by the XRD. The composition of the as-prepared wurtzite GaN could also be derived from XPS spectra (Fig. 2). Fig. 2 shows the N1s, Ga3p3 and Ga3d core level regions. The binding energy of N1s (Fig. 2a) with 397.2 eV of the observed GaN is very consistent with the reference value of 397.0 eV [17]. The Ga3p3 peak at 105.1 eV (Fig. 2b) and Ga3d peak at 19.6 eV (Fig. 2c) are also consistent with the reference values of GaN. The quantification of the peaks gives the atomic ratio of Ga to N of 1.04:1. In the Raman backscattering spectrum at room temperature (Fig. 3), the peaks at 533, 554, 569 and 734 agree with the phonon vibration frequency of the A1(TO), E1(TO), E2 and A1(LO) modes of the crystalline wurtzite GaN, within
Fig. 1. XRD pattern of the GaN powder sample.
ARTICLE IN PRESS X. Hao et al. / Journal of Crystal Growth 280 (2005) 341–345
400
500
A1(LO)
A1(TO)
Intensity (a.u.)
E1(TO)
E2
343
600
700
800
Raman Shift(cm-1)
Fig. 3. Raman spectrum of the GaN sample as in Fig. 1.
uniform spindly shape with an average diameter of 100 nm and length of 400 nm. The sample also contains the particles with loose aggregation. As can be seen from HRTEM images in Fig. 4(b), the space is about 0.27 nm between arrowheads and corresponds to the fringes distance of (1 0 0) planes. The zone axis is [0 0 1]. Furthermore, the clear lattice fringes in this image confirmed a single-crystal structure of the crystal. The inset of Fig. 4(b) shows a selected-area electron diffraction pattern of the crystal that can be indexed to the reflection of the hexagonal GaN crystals along [0 0 1] directions. Fig. 5 shows the room-temperature PL spectrum from the as-prepared GaN nanocrystal. The excitation wavelength was 264 nm and the filter wavelength was 310 nm. The spectrum exhibits a peak emission at 372 nm, which corresponds to the band-edge emission of GaN. According to the following equation l0 ¼ 1240=E 0 ðnmÞ, Fig. 2. XPS spectra of the GaN sample as in Fig. 1.
experimental errors. Our results agreed well with the reported data on GaN powder, GaN nanowires and GaN epitaxial layers [18–20]. Fig. 4(a) shows the TEM image for this sample indicates that the GaN are mostly composed of a
we can get that the energy band gap of the GaN nanospindles is about 3.33 eV. The energy gap of the spindles is less than that of the bulk crystals. This is because with the decrease in the particles, the inner stress will increase and cause the energy gap to decrease. In the present route, the growth of GaN nanospindles may undergo the following chemical
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Fig. 5. PL spectrum of the GaN sample as in Fig. 1.
Fig. 4. (a) TEM image and (b) HRTEM image of the GaN sample as in Fig. 1.
reactions: NaN3 ! Na þ N3 ,
(1)
Ga þ Li3 N ! GaN þ 3Li;
(2)
Ga þ N3 ! GaN þ N2 ,
(3)
N3 ! N2 .
(4)
In the above process, approximately 360 1C (thermal decomposition of NaN3: 365 1C) is
essential to initiate the reactions. NaN3 decomposes into Na and N3 (reaction 1), the Ga reacts with N3 to form GaN (reaction 3), and some of the N3 of the system further decompose into N2 (reaction 4). Thus, the synthesis pressure was then increased accordingly with the temperature. Li3N remains stable at this temperature and it can dissolve in Na–Li–Ga melt. Thus, in this system, the Ga can continuously react with Li3N to synthesize GaN (reaction 2). The possible formation mechanism of the hexagonal GaN nanospindles may be as follows: according to the Bravais rule, the important faces governing the crystal morphology are those with highest reticular densities and the greatest interplanar distance. The morphology of a crystal depends on the growth rates of the different crystallographic faces. Some faces grow very fast and have little or no effect on the growth form; the ones having most influence are the slow-growing faces [21]. Furthermore, according to the linear kinetic model, which depends on the geometry of a particle through an anisotropic analogy of the Gibbs–Thomson equation and on the flux of solute to the interface [22], because of the anisotropy of the wurtzite GaN, the rate of growth
ARTICLE IN PRESS X. Hao et al. / Journal of Crystal Growth 280 (2005) 341–345
is different in different orientations. The GaN particles growth is observed to be faceted along the long sides and rough on the short ends, which tends to result in much faster attachment kinetics on the rough ends than on the faceted sides and gives rise to the spindle-like morphologies.
4. Conclusions In summary, GaN nanospindle can be synthesized from the reaction of Ga, NaN3 and Li3N at a low temperature. XRD patterns, Raman spectrum and HRTEM images revealed that the synthesized GaN crystallized in a hexagonal structure and displaying spindly particles morphology has an average diameter of 100 nm and length of 400 nm. XPS analysis gave the surface stoichiometric of Ga1.04N, and PL spectrum showed a peak emission at 372 nm of the as-prepared nanocrystalline GaN.
Acknowledgements This work was supported by the NSFC (Contact nos. 50302005, 90206042), the Fund for the Excellent Young Scientists of Shandong Province. References [1] S. Nakamura, M. Senoh, S. Nagahama, Appl. Phys. Lett. 72 (1998) 2014. [2] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687.
345
[3] J.L. Coffer, T.W. Zerda, R. Appel, R.L. Wells, J.F. Janik, Chem. Mater. 11 (1999) 20. [4] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287. [5] C.C. Chen, C.C. Yeh, Adv. Mater. 12 (2000) 738. [6] A. Wohlfart, A. Devi, E. Maile, R.A. Fischer, Chem. Commun. 9 (2002) 998. [7] R.F. Services, Science 293 (2001) 782. [8] X.F. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [9] C.C. Chen, C.C. Yeh, C.H. Chen, M.Y. Yu, H.L. Liu, J.J. Wu, K.H. Chen, L.C. Chen, J.Y. Peng, Y.F. Chen, J. Am. Chem. Soc. 123 (2001) 2791. [10] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Shu, Adv. Mater. 13 (2001) 591. [11] X.L. Chen, J.Y. Li, Y.G. Cao, Y.C. Lan, H. Li, M. He, C.Y. Wang, Z. Zhang, Z.Y. Qiao, Adv. Mater. 12 (2000) 1432. [12] M.Q. He, P.Z. Zhou, S.N. Mohammad, G.L. Harris, J.B. Halpern, R. Jacobs, W.L. Sarney, L. Salamanca-Riba, J. Crystal Growth 231 (2001) 357. [13] W.Q. Han, A. Zettl, Appl. Phys. Lett. 80 (2002) 303. [14] H.M. Kim, T.W. Kang, K.S. Chung, Adv. Mater. 15 (2003) 567. [15] H.Y. Peng, X.T. Zhou, N. Wang, Y.F. Zheng, L.S. Liao, W.S. Shi, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 327 (2000) 2630. [16] JCPDS No. 76-0703. [17] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin Elmer, USA, 1992, p. 227. [18] M.S. Kumar, J. Kumar, Mater. Chem. Phys. 77 (2002) 341. [19] G.S. Cheng, L.D. Zhang, Y. Zhu, G.T. Fei, L. Li, C.M. Mo, Y.Q. Mao, Appl. Phys. Lett. 75 (1999) 2455. [20] T. Azuhata, T. Sota, K. Suzuki, S. Nakamura, J. Phys.: Condens. Matter 7 (1995) L129. [21] J.W. Mullin, Crystallization, third ed., Butterworth Heinemann, London, 2000, p. 248. [22] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232.
Journal of Crystal Growth 280 (2005) 341–345 www.elsevier.com/locate/jcrysgro
Synthesis of GaN nanospindles via a facile solid-state reaction route Xiaopeng Haoa,, Jie Zhana, Yongzhong Wua, Suwen Liub, Xiangang Xua, Minhua Jianga a State Key Lab of Crystal Materials, Shandong University, Jinan 250100, PR China Material Science and Engineering Department, Shangdong Institute of Light Industry, Jinan 250100, PR China
b
Received 22 December 2004; accepted 28 March 2005 Available online 10 May 2005 Communicated by M. Schieber
Abstract Gallium nitride (GaN) nanospindles have been synthesized via a solid-state reaction at a low-temperature condition. X-ray powder diffraction (XRD), Raman spectrum and high-resolution transmission electron microscopy (HRTEM) revealed that the synthesized GaN crystallized in a hexagonal structure and displaying spindly particles morphology has an average diameter of 100 nm and length of 400 nm X-ray photoelectron spectroscopy (XPS) of the sample gave the atomic ratio of Ga and N of 1.04:1. Room-temperature photoluminescence (PL) spectrum showed that the as-prepared product had a peak emission at 372 nm. The possible formation mechanism of the wurtzite GaN is briefly discussed. r 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+W; 61.66.Fn; 81.20.n Keywords: A1. X-ray diffraction; B1. Inorganic compounds; B1. Nanomaterials; B1. Nitrides
1. Introduction Hexagonal GaN, with a direct energy gap of 3.39 eV, is one potentially useful material for the manufacturing of blue light-emitting diodes and lasers. Furthermore, GaN-based semiconductors have made it possible to get light-emitting devices Corresponding author. Tel./fax: +86 53 1856 4260.
E-mail address: [email protected] (X. Hao).
over the entire color spectrum of the visible region of the electromagnetic spectrum [1–3]. One-dimensional (1D) GaN nanostructures are intrinsically attractive candidates for assembling active nanoscale devices for electronic and optical applications [4–7]. And there are various methods developed to grow 1D GaN nanostructures, such as laserinduced growth [8], template-assisted synthesis [4], catalyst-assisted synthesis [9] oxide-assisted method [10], the reaction of Ga and NH3 gas
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.03.072
ARTICLE IN PRESS 342
X. Hao et al. / Journal of Crystal Growth 280 (2005) 341–345
[11,12], pyrolysis [13], vapor-phase epitaxy (VPE) [14] and chemical vapor deposition [6,15]. However, some of the above-mentioned synthesis methods require critical reaction conditions and this limits the application fields. Because the applicability of GaN is sensitive to changes in the morphologies, further explorations of the synthesis of other nanostructures of GaN are a challenging subject. Herein, we report a novel facile route using NaN3 and Li3N as the nitrogen source reacting with Ga to synthesize GaN nanospindles at 400 1C and 5 atm. To the best of our knowledge, this distinct shape of GaN nanospindles has not been synthesized by the solid-state method.
2. Experimental procedure The experimental procedures is as follows: 1.0 g Ga, 1.0 g NaN3 and 0.8 g Li3N were placed in a BN crucible, and the BN crucible was placed in a stainless steel autoclave. The autoclave packing the BN crucible was vacuumized and then charged with N2 to 1 atm. The autoclave was heated to 400 1C and kept for 40 h. After the growth, the sample was cooled to room temperature. The product was filtered with ethanol and water to remove the by-products, separately. Finally, the GaN was obtained. The X-ray powder diffraction (XRD) pattern was recorded on a Rigaku D max-gA X-ray diffractometer with Ni-filtered Cuka radiation. The X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 2200-XL X-ray photoelectron spectrometer using Al and Mg Xray sources in the energy range from 1400 to 0 eV. The room-temperature Raman spectrum was taken on a Jasco Ventuno-21 Micro Raman Spectrophotometer. The wavelength was 532 nm and output power was 30 mW. High-resolution transmission electron microscopy (HRTEM) images were taken on a Philips Tecnai 20U-Twin HRTEM. The accelerating voltage was 200 KV. Photoluminescence (PL) spectra of GaN crystals were measured in an Edinburgh FLS920 fluorescent spectrophotometer with a Xe lamp at room temperature.
3. Results and discussion XRD was used to verify the crystal structures and the phase purity of the materials. A typical XRD pattern of the products is shown in Fig. 1. All the peaks of the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (0 0 4) and (2 0 2) reflections can be indexed to hexagonal wurtzite ( and GaN with lattice constants of a ¼ 3:188 A ( c ¼ 5:188 A, consistent with the reported values of hexagonal GaN [16]. It is noticeable that the relative intensity of the (0 0 2) reflections is higher compared with those of bulk GaN, which may indicate an anisotropic growth of the hexagonal GaN. No impurities such as Ga2O3 were detected by the XRD. The composition of the as-prepared wurtzite GaN could also be derived from XPS spectra (Fig. 2). Fig. 2 shows the N1s, Ga3p3 and Ga3d core level regions. The binding energy of N1s (Fig. 2a) with 397.2 eV of the observed GaN is very consistent with the reference value of 397.0 eV [17]. The Ga3p3 peak at 105.1 eV (Fig. 2b) and Ga3d peak at 19.6 eV (Fig. 2c) are also consistent with the reference values of GaN. The quantification of the peaks gives the atomic ratio of Ga to N of 1.04:1. In the Raman backscattering spectrum at room temperature (Fig. 3), the peaks at 533, 554, 569 and 734 agree with the phonon vibration frequency of the A1(TO), E1(TO), E2 and A1(LO) modes of the crystalline wurtzite GaN, within
Fig. 1. XRD pattern of the GaN powder sample.
ARTICLE IN PRESS X. Hao et al. / Journal of Crystal Growth 280 (2005) 341–345
400
500
A1(LO)
A1(TO)
Intensity (a.u.)
E1(TO)
E2
343
600
700
800
Raman Shift(cm-1)
Fig. 3. Raman spectrum of the GaN sample as in Fig. 1.
uniform spindly shape with an average diameter of 100 nm and length of 400 nm. The sample also contains the particles with loose aggregation. As can be seen from HRTEM images in Fig. 4(b), the space is about 0.27 nm between arrowheads and corresponds to the fringes distance of (1 0 0) planes. The zone axis is [0 0 1]. Furthermore, the clear lattice fringes in this image confirmed a single-crystal structure of the crystal. The inset of Fig. 4(b) shows a selected-area electron diffraction pattern of the crystal that can be indexed to the reflection of the hexagonal GaN crystals along [0 0 1] directions. Fig. 5 shows the room-temperature PL spectrum from the as-prepared GaN nanocrystal. The excitation wavelength was 264 nm and the filter wavelength was 310 nm. The spectrum exhibits a peak emission at 372 nm, which corresponds to the band-edge emission of GaN. According to the following equation l0 ¼ 1240=E 0 ðnmÞ, Fig. 2. XPS spectra of the GaN sample as in Fig. 1.
experimental errors. Our results agreed well with the reported data on GaN powder, GaN nanowires and GaN epitaxial layers [18–20]. Fig. 4(a) shows the TEM image for this sample indicates that the GaN are mostly composed of a
we can get that the energy band gap of the GaN nanospindles is about 3.33 eV. The energy gap of the spindles is less than that of the bulk crystals. This is because with the decrease in the particles, the inner stress will increase and cause the energy gap to decrease. In the present route, the growth of GaN nanospindles may undergo the following chemical
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X. Hao et al. / Journal of Crystal Growth 280 (2005) 341–345
Fig. 5. PL spectrum of the GaN sample as in Fig. 1.
Fig. 4. (a) TEM image and (b) HRTEM image of the GaN sample as in Fig. 1.
reactions: NaN3 ! Na þ N3 ,
(1)
Ga þ Li3 N ! GaN þ 3Li;
(2)
Ga þ N3 ! GaN þ N2 ,
(3)
N3 ! N2 .
(4)
In the above process, approximately 360 1C (thermal decomposition of NaN3: 365 1C) is
essential to initiate the reactions. NaN3 decomposes into Na and N3 (reaction 1), the Ga reacts with N3 to form GaN (reaction 3), and some of the N3 of the system further decompose into N2 (reaction 4). Thus, the synthesis pressure was then increased accordingly with the temperature. Li3N remains stable at this temperature and it can dissolve in Na–Li–Ga melt. Thus, in this system, the Ga can continuously react with Li3N to synthesize GaN (reaction 2). The possible formation mechanism of the hexagonal GaN nanospindles may be as follows: according to the Bravais rule, the important faces governing the crystal morphology are those with highest reticular densities and the greatest interplanar distance. The morphology of a crystal depends on the growth rates of the different crystallographic faces. Some faces grow very fast and have little or no effect on the growth form; the ones having most influence are the slow-growing faces [21]. Furthermore, according to the linear kinetic model, which depends on the geometry of a particle through an anisotropic analogy of the Gibbs–Thomson equation and on the flux of solute to the interface [22], because of the anisotropy of the wurtzite GaN, the rate of growth
ARTICLE IN PRESS X. Hao et al. / Journal of Crystal Growth 280 (2005) 341–345
is different in different orientations. The GaN particles growth is observed to be faceted along the long sides and rough on the short ends, which tends to result in much faster attachment kinetics on the rough ends than on the faceted sides and gives rise to the spindle-like morphologies.
4. Conclusions In summary, GaN nanospindle can be synthesized from the reaction of Ga, NaN3 and Li3N at a low temperature. XRD patterns, Raman spectrum and HRTEM images revealed that the synthesized GaN crystallized in a hexagonal structure and displaying spindly particles morphology has an average diameter of 100 nm and length of 400 nm. XPS analysis gave the surface stoichiometric of Ga1.04N, and PL spectrum showed a peak emission at 372 nm of the as-prepared nanocrystalline GaN.
Acknowledgements This work was supported by the NSFC (Contact nos. 50302005, 90206042), the Fund for the Excellent Young Scientists of Shandong Province. References [1] S. Nakamura, M. Senoh, S. Nagahama, Appl. Phys. Lett. 72 (1998) 2014. [2] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687.
345
[3] J.L. Coffer, T.W. Zerda, R. Appel, R.L. Wells, J.F. Janik, Chem. Mater. 11 (1999) 20. [4] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287. [5] C.C. Chen, C.C. Yeh, Adv. Mater. 12 (2000) 738. [6] A. Wohlfart, A. Devi, E. Maile, R.A. Fischer, Chem. Commun. 9 (2002) 998. [7] R.F. Services, Science 293 (2001) 782. [8] X.F. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188. [9] C.C. Chen, C.C. Yeh, C.H. Chen, M.Y. Yu, H.L. Liu, J.J. Wu, K.H. Chen, L.C. Chen, J.Y. Peng, Y.F. Chen, J. Am. Chem. Soc. 123 (2001) 2791. [10] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Shu, Adv. Mater. 13 (2001) 591. [11] X.L. Chen, J.Y. Li, Y.G. Cao, Y.C. Lan, H. Li, M. He, C.Y. Wang, Z. Zhang, Z.Y. Qiao, Adv. Mater. 12 (2000) 1432. [12] M.Q. He, P.Z. Zhou, S.N. Mohammad, G.L. Harris, J.B. Halpern, R. Jacobs, W.L. Sarney, L. Salamanca-Riba, J. Crystal Growth 231 (2001) 357. [13] W.Q. Han, A. Zettl, Appl. Phys. Lett. 80 (2002) 303. [14] H.M. Kim, T.W. Kang, K.S. Chung, Adv. Mater. 15 (2003) 567. [15] H.Y. Peng, X.T. Zhou, N. Wang, Y.F. Zheng, L.S. Liao, W.S. Shi, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 327 (2000) 2630. [16] JCPDS No. 76-0703. [17] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin Elmer, USA, 1992, p. 227. [18] M.S. Kumar, J. Kumar, Mater. Chem. Phys. 77 (2002) 341. [19] G.S. Cheng, L.D. Zhang, Y. Zhu, G.T. Fei, L. Li, C.M. Mo, Y.Q. Mao, Appl. Phys. Lett. 75 (1999) 2455. [20] T. Azuhata, T. Sota, K. Suzuki, S. Nakamura, J. Phys.: Condens. Matter 7 (1995) L129. [21] J.W. Mullin, Crystallization, third ed., Butterworth Heinemann, London, 2000, p. 248. [22] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232.