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Synthesis and field-emission properties of the tungsten oxide nanowire arrays
ARTICLE IN PRESS
Physica E 39 (2007) 219–222 www.elsevier.com/locate/physe
Synthesis and field-emission properties of the tungsten oxide nanowire arrays Kai Huang, Qingtao Pan, Feng Yang, Shibi Ni, Deyan He School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, PR China Received 12 February 2007; received in revised form 21 April 2007; accepted 27 April 2007 Available online 8 May 2007
Abstract High-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized by a conventional thermal evaporation approach on indium tin oxide (ITO) coated glass substrates without any catalysts. The temperature of the substrate was 4502550 C. The tungsten oxide nanowires are single crystalline with growth direction of [0 1 0]. For commercial applications, field emission properties of tungsten oxide nanowires were studied under a poor vacuum at room temperature. The electron field-emission turn-on field ðE to Þ, defined as the macroscopic field required to produce a current density of 10 mA=cm2 , is about 3:6 V=mm. The performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application in fieldemission displays. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.b; 81.10.h; 79.70.þq Keywords: Tungsten oxide; Nanowire arrays; Field emission
1. Introduction One-dimensional (1D) nanomaterials have stimulated great attention due to their promising applications in nanodevices [1]. And field emission (FE) from various 1D nanostructures has been intensively investigated over the last several decades [2–8]. Especially, tungsten oxide 1D nanostructures have attracted more attention due to their high aspect ratio, low turn-on field, and highly stable emission [5,9,10]. Tungsten oxide is an n-type semiconductor with a work function in the range of 5.59–5.7 eV [11,12] which makes it attractive for the stated applications. Tungsten oxide has also been demonstrated to be suitable for various other applications such as electrochromic, optochromic and gasochromic coatings for smart windows, information display and various sensors [13,14].
Corresponding author. Tel./fax: +86 931 891 3554.
E-mail address: [email protected] (D.Y. He). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.04.007
Recently, various types of tungsten oxide nanostructures, including nanowires [15–19], nanotips [5], nanobelts [20], nanotubes [9], and 3D nanowire networks [10], have been synthesized. However, the tungsten oxide nanowires were grown at a relatively high temperature. For the practical application in devices, it is very necessary to synthesize tungsten oxide nanowire arrays at low temperature on alien substrate. Galle´a et al. and Li et al. have synthesized tungsten oxide nanorods on Si substrate at low temperature by heating tungsten coil [21,22]. But, most of them use tungsten as the source reacting with H2O or O2 to synthesize tungsten oxide nanostructures. In this paper, we have successfully synthesized 1D tungsten oxide nanowire arrays on indium tin oxide (ITO) coated glass substrates without external catalysts by a conventional thermal evaporation approach, using tungsten oxide powders as the thermal evaporation source. The temperature of the substrate was 4502550 C, and much lower than that having been reported by Zhou et al. [5,10]. For commercial applications, field-emission characteristics of these nanowire arrays have been measured under a poor vacuum condition.
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2. Experimental The synthesis of tungsten oxide nanowire arrays in our experiment is based on thermal evaporation of tungsten trioxide powders (99.9%) under controlled conditions without any catalysts. The tungsten trioxide powders were placed at the center of an alumina tube that was inserted in a horizontal tube furnace. The ITO-coated glass ð10 mm 10 mmÞ as the substrates were placed at the end of the alumina tube. After evacuation to a pressure of about 2 103 Torr, the temperature in the center of the tube was elevated to 1100 C at a rate of 20 C min1 , and the temperature of the substrate region was 4502550 C. Under the gas flow of Ar and O2 at rate of 100 sccm (standard cubic centimeters per minute) for 2 h, the total pressure of the chamber was kept at 200 Torr. After the furnace slowly cooled down to room temperature, the substrates covered with the resulting products were collected. The as-deposited products were characterized and analyzed by X-ray diffraction (XRD) (Rigaku RINT2400 with Cu Ka radiation), scanning electron microscopy (SEM) (Hitachi S800), and high-resolution transmission electron microscopy (HRTEM) (JEM 2100, 200KV). 3. Results and discussion Fig. 1 shows typical SEM images of tungsten oxide nanowire arrays grown on ITO-coated glass substrate. Fig. 1a is a top view image, and the tungsten oxide nanowires were high-density, large-scale and well separated from each other. Fig. 1b shows a cross section SEM image of the tungsten oxides nanowires, revealing that those nanowires with an average length of 5 mm were grown perpendicular to the substrate, with a diameter of 1002150 nm. We suggest that the method in our experiment can be applied to manufacture large-scale, highdensity and uniformly distributed tungsten oxide nanowire arrays. The XRD spectrums of the tungsten oxide nanowire arrays and ITO-coated glass substrate are shown in Fig. 2. The diffraction peaks (in Fig. 2a) can be well indexed to a ˚ monoclinic W18O49 phase (cell constants: a ¼ 18:28 A, ˚ ˚ b ¼ 3:775 A, c ¼ 13:98 A, b ¼ 115:20 ; JCPDS 05-0392), and the [0 1 0] direction is the major growth direction of the nanostructure. It is noted that no peaks associated with the ITO substrate (in Fig. 2b) can be observed in the tungsten oxide nanowire arrays, probably because of the highdensity of the nanowires deposited. To further illuminate the detailed microstructures of the tungsten oxide nanowires, HRTEM images of a nanowire are given in Fig. 3. The fast Fourier transform (FFT) pattern (inset of Fig. 3a) is calculated from the squareenclosed areas in Fig. 3a. These streaks in the FFT possible arise from the presence of planar defects parallel to the growth direction. Fig. 3b presents the enlarged HRTEM image of the nanowire. The lattice spacing is 0.378 nm
Fig. 1. SEM images of the quasi-aligned tungsten oxide nanowire arrays on ITO-coated glass substrate. (a) Top view image, (b) cross section image.
correspond to (0 1 0) plane of monoclinic W18O49. FFT pattern and the spacing of the lattice plane measurement show that the monoclinic W18O49 (JCPDS 05-0392) nanowire is single crystalline with a growth direction of [0 1 0]. That is in good agreement with our XRD analysis. No catalyst was used in our growth process, and the vapor–solid (VS) growth process may be suitable for our case [1]. The tungsten oxide vapor can be evaporated at a higher temperature zone directly deposits on ITO-coated glass substrates at a lower temperature region and grows into 1D nanostructure. Field emission measurements were carried out in a chamber with a poor vacuum of 1:0 105 Torr at room temperature. A rod-like aluminum probe with 3 mm2 in cross section was used as an anode and the tungsten oxide nanowire arrays film served as a cathode. As with any emitter operated in commercial environments, the ability to withstand poor vacuum conditions can be crucial. Oxide
ARTICLE IN PRESS K. Huang et al. / Physica E 39 (2007) 219–222
Fig. 2. XRD patterns of the tungsten oxide nanowire arrays (a) and ITOcoated glass substrate (b).
materials have a relatively high melting temperature and have been observed to be stable in oxygen and poor vacuum conditions. Fig. 4a depicts the plot of the emission current density, J, as a function of applied field, E, measured at a vacuum gap of 200 mm. The electron fieldemission turn-on field ðE to Þ, defined as the macroscopic field required to produce a current density of 10 mA=cm2 , is about 3:6 V=mm. The turn-on field of the tungsten oxide nanowire arrays is not comparable with those reported for WO2.9 nanorods ð1:2 V=mmÞ, nanowire networks ð0:74 V=mmÞ or W18O49 nanotips ð2 V=mmÞ [5,23,24]. The possible reasons could be the adsorption of atoms in air which might have still remained on the emitter after turning to vacuum 105 Torr. However, it is lower than that of 3D tungsten oxide nanowire networks [10], the vertically aligned tungsten nanowires [25], indicating that the prepared tungsten oxide nanowire arrays is an alternative candidate suitable for field emission in poor vacuum conditions. The reasons maybe the nanowires are well separated from each other and vertically grown on the
221
Fig. 3. (a) HRTEM image of the tungsten oxide nanowire, and the growth direction is [0 1 0]. Inset is a FFT pattern from the square-enclosed area. (b) Enlarged HRTEM image.
substrate, which will decrease the screen effect and increase the field enhancement factor as well. The emission characteristics were analyzed using Fowler–Nordheim (FN) theory: J ¼ ðAE 2 b2 =fÞ expðBf3=2 =EbÞ,
(1)
where A ¼ 1:543 106 AeV V2 , B ¼ 6:83 109 eV3=2 Vm1 , respectively. b is the field enhancement factor, and f is the work function of emitter material. By plotting lnðJ=E 2 Þ versus 1=E, a straight line was obtained (Fig. 4b). The linearity of these curves implies that the field emission from the nanowire arrays follows FN theory. The field enhancement factor was calculated from the slope ðBf3=2 b1 Þ of the FN plot assuming f as 5.7 eV similar to WO3 [11,12]. The calculated b value was 2700, which is high enough for various field emission applications. The above findings show the potential application of the tungsten oxide nanowire arrays as cold cathode electron emitters.
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commercial application in vacuum microelectronic devices, particularly flat panel displays. Acknowledgments The authors appreciate the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20040730029) and the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China. References
Fig. 4. (a) Field-emission current density versus electric field ðJ2EÞ plot of the tungsten oxide nanowire arrays. (b) The corresponding F–N plot.
4. Conclusion In summary, large-scale, high-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized on ITO-coated glass substrates by a conventional thermal evaporation approach. The nanowires are 5 mm in length and 100–150 nm in diameter. The growth direction of the tungsten oxide nanowires is in the [0 1 0] direction. Field emission property of tungsten oxide nanowire arrays was studied at a poor vacuum condition and the turn-on field ðE to Þ is about 3:6 V=mm. The remarkable performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for
[1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] Y.J. Chen, Q.H. Li, Y.X. Liang, T.H. Wang, Q. Zhao, D.P. Yu, Appl. Phys. Lett. 85 (2004) 5682. [3] S.Q. Li, Y.X. Liang, T.H. Wang, Appl. Phys. Lett. 87 (2005) 143104. [4] K.A. Dean, B.A. Chalamala, Appl. Phys. Lett. 76 (2000) 375. [5] J. Zhou, L. Gong, S.Z. Deng, J. Chen, J.C. She, N.S. Xu, R. Yang, Z.L. Wang, Appl. Phys. Lett. 87 (2005) 223108. [6] J. Chen, N.Y. Huang, S.Z. Deng, J.C. She, N.S. Xu, W. Zhang, X. Wen, S. Yang, Appl. Phys. Lett. 86 (2005) 151107. [7] Q. Zhao, J. Xu, X.Y. Xu, Z. Wang, D.P. Yu, Appl. Phys. Lett. 85 (2004) 5331. [8] Q.H. Li, Q. Wan, Y.J. Chen, T.H. Wang, H.B. Jia, D.P. Yu, Appl. Phys. Lett. 85 (2004) 636. [9] Y.B. Li, Y.S. Bando, D. Golberg, Adv. Mater. 15 (2003) 1294. [10] J. Zhou, Y. Ding, S.Z. Deng, L. Gong, N.S. Xu, Z.L. Wang, Adv. Mater. 17 (2005) 2107. [11] G. Vida, V.K. Josepovits, M. Gyor, P. De’ak, Microsc. Microanal. 9 (2003) 337. [12] M. Gillet, R. Delamare, E. Gillet, Eur. Phys. J. D 34 (2005) 291. [13] H. Qiu, Y.F. Lu, Z.H. Mai, J. Appl. Phys. 91 (2002) 440. [14] J.L. Solis, S. Saukko, L. Kish, C.G. Granqvist, V. Lantto, Thin Solid Films 391 (2001) 255. [15] G. Gu, B. Zheng, W.Q. Han, S. Roth, J. Liu, Nano Lett. 2 (2002) 849. [16] K. Q. Hong, W. C. Yiu, H. S. Wu, J. G. M. H. Xie, Nanotechnology 16 (2005) 1608. [17] C.H. Chen, S.J. Wang, R.M. Ko, Y.C. Kuo, K.M. Uang, T.M. Chen, B.W. Liou, H.Y. Tsai, Nanotechnology 17 (2006) 217. [18] L.F. Chi, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Nanotechnology 17 (2006) 5590. [19] Y.Z. Jin, Y.Q. Zhu, R.L.D. Whitby, N. Yao, R. Ma, P.C.P. Watts, H.W. Kroto, D.R.M. Walton, J. Phys. Chem. B 1081 (2004) 5572. [20] Y.B. Li, Y.S. Bando, D. Golberg, K. Kurashima, Chem. Phys. Lett. 367 (2003) 214. [21] J. Liu, Y. Zhao, Z.J. Zhang, J. Phys.: Condens. Matter 15 (2003) L453. [22] F. Galle´a, Z.C. Li, Z.J. Zhang, Appl. Phys. Lett. 89 (2006) 193111. [23] J.G. Liu, Z.J. Zhang, Y. Zhao, X. Su, S. Liu, E.G. Wang, Small 1 (2005) 310. [24] Y.M. Zhao, Y.H. Li, I. Ahmad, D.G. McCartney, Y.Q. Zhua, W.B. Hu, Appl. Phys. Lett. 89 (2006) 133116. [25] Y.H. Lee, C.H. Choi, Y.T. Jang, E.K. Kim, B.K. Ju, Appl. Phys. Lett. 81 (2002) 745.
Physica E 39 (2007) 219–222 www.elsevier.com/locate/physe
Synthesis and field-emission properties of the tungsten oxide nanowire arrays Kai Huang, Qingtao Pan, Feng Yang, Shibi Ni, Deyan He School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, PR China Received 12 February 2007; received in revised form 21 April 2007; accepted 27 April 2007 Available online 8 May 2007
Abstract High-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized by a conventional thermal evaporation approach on indium tin oxide (ITO) coated glass substrates without any catalysts. The temperature of the substrate was 4502550 C. The tungsten oxide nanowires are single crystalline with growth direction of [0 1 0]. For commercial applications, field emission properties of tungsten oxide nanowires were studied under a poor vacuum at room temperature. The electron field-emission turn-on field ðE to Þ, defined as the macroscopic field required to produce a current density of 10 mA=cm2 , is about 3:6 V=mm. The performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application in fieldemission displays. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.b; 81.10.h; 79.70.þq Keywords: Tungsten oxide; Nanowire arrays; Field emission
1. Introduction One-dimensional (1D) nanomaterials have stimulated great attention due to their promising applications in nanodevices [1]. And field emission (FE) from various 1D nanostructures has been intensively investigated over the last several decades [2–8]. Especially, tungsten oxide 1D nanostructures have attracted more attention due to their high aspect ratio, low turn-on field, and highly stable emission [5,9,10]. Tungsten oxide is an n-type semiconductor with a work function in the range of 5.59–5.7 eV [11,12] which makes it attractive for the stated applications. Tungsten oxide has also been demonstrated to be suitable for various other applications such as electrochromic, optochromic and gasochromic coatings for smart windows, information display and various sensors [13,14].
Corresponding author. Tel./fax: +86 931 891 3554.
E-mail address: [email protected] (D.Y. He). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.04.007
Recently, various types of tungsten oxide nanostructures, including nanowires [15–19], nanotips [5], nanobelts [20], nanotubes [9], and 3D nanowire networks [10], have been synthesized. However, the tungsten oxide nanowires were grown at a relatively high temperature. For the practical application in devices, it is very necessary to synthesize tungsten oxide nanowire arrays at low temperature on alien substrate. Galle´a et al. and Li et al. have synthesized tungsten oxide nanorods on Si substrate at low temperature by heating tungsten coil [21,22]. But, most of them use tungsten as the source reacting with H2O or O2 to synthesize tungsten oxide nanostructures. In this paper, we have successfully synthesized 1D tungsten oxide nanowire arrays on indium tin oxide (ITO) coated glass substrates without external catalysts by a conventional thermal evaporation approach, using tungsten oxide powders as the thermal evaporation source. The temperature of the substrate was 4502550 C, and much lower than that having been reported by Zhou et al. [5,10]. For commercial applications, field-emission characteristics of these nanowire arrays have been measured under a poor vacuum condition.
ARTICLE IN PRESS 220
K. Huang et al. / Physica E 39 (2007) 219–222
2. Experimental The synthesis of tungsten oxide nanowire arrays in our experiment is based on thermal evaporation of tungsten trioxide powders (99.9%) under controlled conditions without any catalysts. The tungsten trioxide powders were placed at the center of an alumina tube that was inserted in a horizontal tube furnace. The ITO-coated glass ð10 mm 10 mmÞ as the substrates were placed at the end of the alumina tube. After evacuation to a pressure of about 2 103 Torr, the temperature in the center of the tube was elevated to 1100 C at a rate of 20 C min1 , and the temperature of the substrate region was 4502550 C. Under the gas flow of Ar and O2 at rate of 100 sccm (standard cubic centimeters per minute) for 2 h, the total pressure of the chamber was kept at 200 Torr. After the furnace slowly cooled down to room temperature, the substrates covered with the resulting products were collected. The as-deposited products were characterized and analyzed by X-ray diffraction (XRD) (Rigaku RINT2400 with Cu Ka radiation), scanning electron microscopy (SEM) (Hitachi S800), and high-resolution transmission electron microscopy (HRTEM) (JEM 2100, 200KV). 3. Results and discussion Fig. 1 shows typical SEM images of tungsten oxide nanowire arrays grown on ITO-coated glass substrate. Fig. 1a is a top view image, and the tungsten oxide nanowires were high-density, large-scale and well separated from each other. Fig. 1b shows a cross section SEM image of the tungsten oxides nanowires, revealing that those nanowires with an average length of 5 mm were grown perpendicular to the substrate, with a diameter of 1002150 nm. We suggest that the method in our experiment can be applied to manufacture large-scale, highdensity and uniformly distributed tungsten oxide nanowire arrays. The XRD spectrums of the tungsten oxide nanowire arrays and ITO-coated glass substrate are shown in Fig. 2. The diffraction peaks (in Fig. 2a) can be well indexed to a ˚ monoclinic W18O49 phase (cell constants: a ¼ 18:28 A, ˚ ˚ b ¼ 3:775 A, c ¼ 13:98 A, b ¼ 115:20 ; JCPDS 05-0392), and the [0 1 0] direction is the major growth direction of the nanostructure. It is noted that no peaks associated with the ITO substrate (in Fig. 2b) can be observed in the tungsten oxide nanowire arrays, probably because of the highdensity of the nanowires deposited. To further illuminate the detailed microstructures of the tungsten oxide nanowires, HRTEM images of a nanowire are given in Fig. 3. The fast Fourier transform (FFT) pattern (inset of Fig. 3a) is calculated from the squareenclosed areas in Fig. 3a. These streaks in the FFT possible arise from the presence of planar defects parallel to the growth direction. Fig. 3b presents the enlarged HRTEM image of the nanowire. The lattice spacing is 0.378 nm
Fig. 1. SEM images of the quasi-aligned tungsten oxide nanowire arrays on ITO-coated glass substrate. (a) Top view image, (b) cross section image.
correspond to (0 1 0) plane of monoclinic W18O49. FFT pattern and the spacing of the lattice plane measurement show that the monoclinic W18O49 (JCPDS 05-0392) nanowire is single crystalline with a growth direction of [0 1 0]. That is in good agreement with our XRD analysis. No catalyst was used in our growth process, and the vapor–solid (VS) growth process may be suitable for our case [1]. The tungsten oxide vapor can be evaporated at a higher temperature zone directly deposits on ITO-coated glass substrates at a lower temperature region and grows into 1D nanostructure. Field emission measurements were carried out in a chamber with a poor vacuum of 1:0 105 Torr at room temperature. A rod-like aluminum probe with 3 mm2 in cross section was used as an anode and the tungsten oxide nanowire arrays film served as a cathode. As with any emitter operated in commercial environments, the ability to withstand poor vacuum conditions can be crucial. Oxide
ARTICLE IN PRESS K. Huang et al. / Physica E 39 (2007) 219–222
Fig. 2. XRD patterns of the tungsten oxide nanowire arrays (a) and ITOcoated glass substrate (b).
materials have a relatively high melting temperature and have been observed to be stable in oxygen and poor vacuum conditions. Fig. 4a depicts the plot of the emission current density, J, as a function of applied field, E, measured at a vacuum gap of 200 mm. The electron fieldemission turn-on field ðE to Þ, defined as the macroscopic field required to produce a current density of 10 mA=cm2 , is about 3:6 V=mm. The turn-on field of the tungsten oxide nanowire arrays is not comparable with those reported for WO2.9 nanorods ð1:2 V=mmÞ, nanowire networks ð0:74 V=mmÞ or W18O49 nanotips ð2 V=mmÞ [5,23,24]. The possible reasons could be the adsorption of atoms in air which might have still remained on the emitter after turning to vacuum 105 Torr. However, it is lower than that of 3D tungsten oxide nanowire networks [10], the vertically aligned tungsten nanowires [25], indicating that the prepared tungsten oxide nanowire arrays is an alternative candidate suitable for field emission in poor vacuum conditions. The reasons maybe the nanowires are well separated from each other and vertically grown on the
221
Fig. 3. (a) HRTEM image of the tungsten oxide nanowire, and the growth direction is [0 1 0]. Inset is a FFT pattern from the square-enclosed area. (b) Enlarged HRTEM image.
substrate, which will decrease the screen effect and increase the field enhancement factor as well. The emission characteristics were analyzed using Fowler–Nordheim (FN) theory: J ¼ ðAE 2 b2 =fÞ expðBf3=2 =EbÞ,
(1)
where A ¼ 1:543 106 AeV V2 , B ¼ 6:83 109 eV3=2 Vm1 , respectively. b is the field enhancement factor, and f is the work function of emitter material. By plotting lnðJ=E 2 Þ versus 1=E, a straight line was obtained (Fig. 4b). The linearity of these curves implies that the field emission from the nanowire arrays follows FN theory. The field enhancement factor was calculated from the slope ðBf3=2 b1 Þ of the FN plot assuming f as 5.7 eV similar to WO3 [11,12]. The calculated b value was 2700, which is high enough for various field emission applications. The above findings show the potential application of the tungsten oxide nanowire arrays as cold cathode electron emitters.
ARTICLE IN PRESS 222
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commercial application in vacuum microelectronic devices, particularly flat panel displays. Acknowledgments The authors appreciate the financial support of the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20040730029) and the Teaching and Research Award Program for Outstanding Young Teachers in High Education Institutions of MOE, China. References
Fig. 4. (a) Field-emission current density versus electric field ðJ2EÞ plot of the tungsten oxide nanowire arrays. (b) The corresponding F–N plot.
4. Conclusion In summary, large-scale, high-density, uniformly distributed and quasi-aligned tungsten oxide nanowire arrays have been synthesized on ITO-coated glass substrates by a conventional thermal evaporation approach. The nanowires are 5 mm in length and 100–150 nm in diameter. The growth direction of the tungsten oxide nanowires is in the [0 1 0] direction. Field emission property of tungsten oxide nanowire arrays was studied at a poor vacuum condition and the turn-on field ðE to Þ is about 3:6 V=mm. The remarkable performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for
[1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] Y.J. Chen, Q.H. Li, Y.X. Liang, T.H. Wang, Q. Zhao, D.P. Yu, Appl. Phys. Lett. 85 (2004) 5682. [3] S.Q. Li, Y.X. Liang, T.H. Wang, Appl. Phys. Lett. 87 (2005) 143104. [4] K.A. Dean, B.A. Chalamala, Appl. Phys. Lett. 76 (2000) 375. [5] J. Zhou, L. Gong, S.Z. Deng, J. Chen, J.C. She, N.S. Xu, R. Yang, Z.L. Wang, Appl. Phys. Lett. 87 (2005) 223108. [6] J. Chen, N.Y. Huang, S.Z. Deng, J.C. She, N.S. Xu, W. Zhang, X. Wen, S. Yang, Appl. Phys. Lett. 86 (2005) 151107. [7] Q. Zhao, J. Xu, X.Y. Xu, Z. Wang, D.P. Yu, Appl. Phys. Lett. 85 (2004) 5331. [8] Q.H. Li, Q. Wan, Y.J. Chen, T.H. Wang, H.B. Jia, D.P. Yu, Appl. Phys. Lett. 85 (2004) 636. [9] Y.B. Li, Y.S. Bando, D. Golberg, Adv. Mater. 15 (2003) 1294. [10] J. Zhou, Y. Ding, S.Z. Deng, L. Gong, N.S. Xu, Z.L. Wang, Adv. Mater. 17 (2005) 2107. [11] G. Vida, V.K. Josepovits, M. Gyor, P. De’ak, Microsc. Microanal. 9 (2003) 337. [12] M. Gillet, R. Delamare, E. Gillet, Eur. Phys. J. D 34 (2005) 291. [13] H. Qiu, Y.F. Lu, Z.H. Mai, J. Appl. Phys. 91 (2002) 440. [14] J.L. Solis, S. Saukko, L. Kish, C.G. Granqvist, V. Lantto, Thin Solid Films 391 (2001) 255. [15] G. Gu, B. Zheng, W.Q. Han, S. Roth, J. Liu, Nano Lett. 2 (2002) 849. [16] K. Q. Hong, W. C. Yiu, H. S. Wu, J. G. M. H. Xie, Nanotechnology 16 (2005) 1608. [17] C.H. Chen, S.J. Wang, R.M. Ko, Y.C. Kuo, K.M. Uang, T.M. Chen, B.W. Liou, H.Y. Tsai, Nanotechnology 17 (2006) 217. [18] L.F. Chi, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Nanotechnology 17 (2006) 5590. [19] Y.Z. Jin, Y.Q. Zhu, R.L.D. Whitby, N. Yao, R. Ma, P.C.P. Watts, H.W. Kroto, D.R.M. Walton, J. Phys. Chem. B 1081 (2004) 5572. [20] Y.B. Li, Y.S. Bando, D. Golberg, K. Kurashima, Chem. Phys. Lett. 367 (2003) 214. [21] J. Liu, Y. Zhao, Z.J. Zhang, J. Phys.: Condens. Matter 15 (2003) L453. [22] F. Galle´a, Z.C. Li, Z.J. Zhang, Appl. Phys. Lett. 89 (2006) 193111. [23] J.G. Liu, Z.J. Zhang, Y. Zhao, X. Su, S. Liu, E.G. Wang, Small 1 (2005) 310. [24] Y.M. Zhao, Y.H. Li, I. Ahmad, D.G. McCartney, Y.Q. Zhua, W.B. Hu, Appl. Phys. Lett. 89 (2006) 133116. [25] Y.H. Lee, C.H. Choi, Y.T. Jang, E.K. Kim, B.K. Ju, Appl. Phys. Lett. 81 (2002) 745.