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Fabrication of ZnO microtube arrays via vapor phase growth
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Materials Letters 62 (2008) 1528 – 1531 www.elsevier.com/locate/matlet
Fabrication of ZnO microtube arrays via vapor phase growth Tianjun Sun, Jieshan Qiu ⁎ Carbon Research Laboratory, Center for Nano Materials and Science, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, P. O. Box 49, Dalian 116012, China Received 8 June 2007; accepted 13 September 2007 Available 19 September 2007
Abstract Single-crystalline ZnO microtube arrays were fabricated from commercial Zn powder on a Si substrate coated with a composite of fluororesin and SiO2 powder by making use of a simple one-step thermal evaporation process. The as-prepared ZnO samples were examined by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. It has been found that these ZnO microtubes are single-crystalline wurtzite structures, and grow along [0002] direction. The ZnO tubes with an outer diameter of ca. 1–3 μm have a length of several tens micrometers, of which the wall thickness ranges from 0.5 μm to 1.5 μm. The results have evidenced that the composite coating with a thickness of 0.1–1 mm is critical to the growth of the ZnO microtubes. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Microstructure; Chemical vapor deposition; Composite materials
1. Introduction Zinc oxide with wide band gap of 3.37 eV and large excitation binding energy of 60 meV is an important semiconducting material, and has promising applications in a wide range of high-technology fields such as field-emission displays, optical electric devices, solar cells, gas sensors and catalysts [1–3]. Up to date, great efforts have been made on the controlled growth of various one-dimensional (1D) micro- or nano-sized ZnO materials including nanobelts [1], nanowires [2], nanotubes [3–9], and terapods [10]. Of these 1D micro- or nano-sized ZnO materials, the tubular material is of great importance for the fabrication of micro-/nano-sized devices because of the unique structure with high porosity and surface area. Generally, the low temperature solution route is used to make ZnO nano/microtubes due to its low growth temperature and simple procedure [7–9], but this is a time-consuming
⁎ Corresponding author. Fax: +86 411 88993991. E-mail address: [email protected] (J. Qiu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.015
process that normally lasts from 24 h to several days. Thermal evaporation methods [11–15] for making micro/nano-sized ZnO tubes have been reported, however, little success has been made in controlled growth of well-aligned ZnO tube arrays in large scale. Here, we report a simple gas-phase method for fabricating micro-sized ZnO tube arrays. With this method, single-crystalline ZnO microtubes with hexagonal shape and hollow core have been successfully prepared on resin-coated Si substrate in large scale. 2. Experimental ZnO microtubes were fabricated on Si substrates in a quartz tube reactor heated in a tube furnace via a modified chemical vapor deposition (CVD) process. Before the experiment, Si substrates (7 × 5 mm) were coated with a mixture of fluororesin, SiO2 powder and butyl acetate with a ratio of 25:5:70 in weight. The fluororesin with a molecular weight of ca. 6000–8000 was the quadripolymer of chlorotrifluoroethylene. The pre-coated Si substrate was dried in air, resulting in a composite film of fluororesin and SiO2 powder on the substrate, which leads to composite-coated Si substrates. The thickness of the composite film coated on the Si substrates ranges from several tens
T. Sun, J. Qiu / Materials Letters 62 (2008) 1528–1531
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3. Results and discussion
Fig. 1. a) SEM image of ZnO microtube array, inset showing the SEM image of a typical ZnO microtube; b) TEM image of an individual ZnO tube, inset is its SAED pattern; and c) HRTEM image of the tube shown in b.
micrometers to several millimeters, which could be controlled by repeated coatings. Then, the composite-coated Si substrate was put into the quartz tube in downstream, which was about 10–20 mm away from the quartz boat containing raw Zn powder (99.9%) that was located in the center of quartz tube reactor. For each run, the quartz tube reactor was flushed for 30 min using argon gas with a purity of 99.99% at a flow rate of 150 mL/min. Then, the flow rate of Ar gas was reduced to 95 mL/min, which functioned as carrier gas to carry the vaporized Zn powder into the reaction zone in downstream. Meanwhile, the reactant gas, oxygen with a purity of 99.99%, was introduced into the reactor at a flow rate of 5 mL/min. The outlet of the oxygen gas was above the Si substrate, which helps to avoid the oxidation of Zn powder before evaporation. The furnace was ramped to 650 °C at a rate of 50 °C/min, and was kept at 650 °C for 15 min before being cooled back to room temperature in flowing argon. Grayish ZnO products were formed on the substrate. For comparison, the Si substrates without any coating pretreatment or coated with carbon film were also tested under the identical conditions. The Si substrates coated with carbon film were obtained by heating the composite-coated Si substrates at 500 °C for 1 h in flowing argon. The morphologies and structures of the as-made ZnO samples were examined using X-ray powder diffraction (XRD, Shimadzu XRD-6000, CuKα radiation), scanning electron microscopy (SEM, JEOL JSM-5600LV), transmission electron microscopy, high-resolution TEM and selected area electron diffraction (TEM, HTRTEM and SAED, Tecnai-G2, 200 kV).
Fig. 1a shows a typical SEM image of the as-synthesized ZnO tube arrays grown on the composite-coated Si substrate in large scale. The aligned ZnO tubes are approximately perpendicular to the substrate, and have hollow cores. On average, these tubes are ca. 1–3 μm in diameter, and up to 10 μm in length. The inset in Fig. 1a is the highmagnification SEM image of a typical ZnO microtube with perfect hexagonal shape, of which the diameter is ca. 2.5 μm and the wall thickness is ca.1 μm. Fig. 1b is the TEM image of an individual ZnO microtube that is ca. 1 μm in diameter and 3 μm in length. This uniform ZnO tube has a small hollow core of ca. 250 nm in diameter, as shown in Fig. 1b. The SAED pattern (inset in Fig. 1b) reveals that this type of tube is singlecrystalline structure, and grows along [0002] direction. In Fig. 1c, it is clear that the d-spacing of the lattices is ca. 0.52 nm, which is consistent with the spacing of (0001) facet of ZnO crystal, further evidencing that the ZnO tube grows along [0002] direction. Fig. 2 is the typical XRD pattern of the micro-sized ZnO tube arrays grown on the composite-coated Si substrates, revealing that the asmade ZnO tube arrays are of the typical wurtzite structures with lattice constants of a = 0.325 nm and c = 0.521 nm (JCPDS card No.36-1451). It can be seen that the ZnO (002) peak is dominant because the ZnO microtubes have a larger wall thickness of 0.5–1.5 μm, evidencing that the ZnO tube arrays grow along the direction of (002) plane. Similar phenomena were also observed by other researchers [15]. The substrate effect on the formation of ZnO microtubes was addressed systematically. When bare substrates without any coating were used, ZnO hexagonal microrods would be obtained, as shown in Fig. 3a, while ZnO micronails would be formed on substrate coated with carbon film, as shown in Fig. 3b. It has been found that when the thickness of the composite coating is in a range of 0.1 mm to 1 mm, the formation of ZnO microtubes is favored. When the coating thickness on the Si substrate is less than 0.1 mm, a mixture of microrods and microtubes with small-size hollow core would be formed (Fig. 3c). When composite coating thickness is over 1 mm, tubular ZnO with poor alignment would be obtained (Fig. 3d). Obviously, the thickness of composite film coated on the substrates is critical to the growth of ZnO microtube arrays. The effect of the properties of composite coating and the process parameters on the formation of ZnO microstructures is currently under further investigation with an aim
Fig. 2. XRD pattern of as-synthesized ZnO microtube arrays.
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T. Sun, J. Qiu / Materials Letters 62 (2008) 1528–1531
Fig. 3. SEM images of aligned ZnO structures grown under different conditions. a) ZnO nanorods, b) ZnO nanonails, c) low- and high-magnification (inset) images of typical ZnO microtubes with small hollow cores, d) low-magnification image of typical ZnO microtubes with large hollow cores, and inset showing an individual ZnO tube.
of making highly oriented ZnO microtubes, and tuning their diameters and wall thickness according to specific requirements. In the present work, the growth of ZnO microtubes proceeds via the vapor–solid (VS) scheme owing to the absence of catalyst [16]. It is known that fluororesin begins to decompose at 460 °C, in which some small yet reductive volatile species such as CO and CxHy are released, at the same time, a carbon film (Cn) is formed on the surface of the Si substrates. In the reductive environment resulting from the fluororesin pyrolysis, the ZnOx particles instead of ZnO particles would be formed and start to nucleate on the in situ formed carbon film. The surface
consisting of this kind of nucleus would be further oxidized to form ZnO o layer because the partial pressure of oxygen at the surface (P O2 ) is much i higher than at the inner part (PO2), whereas the ZnOx in the inner part would be reduced to Zn vapor due to the high reductive activity of the nascent carbon film, and is further vaporized at high temperature. Under the joint effect of the parameters discussed above, ZnO tubes sprout out from the ZnOx embryo, and continue to grow till the experiment is stopped. For clarity, the whole process is schematically shown in Fig. 4.
4. Conclusions In summary, oriented single-crystalline ZnO tubes have been fabricated in large area on the Si substrate coated with a composite film of fluororesin and SiO2 powder by a simple thermal evaporation process. These ZnO tubes with a wall thicknesses of 0.5–1.5 μm grow along [0002] direction up to several tens micrometers, and have outer diameters in a range of ca. 1–3 μm. It has been found that fluororesin is the most important factor for the growth of the ZnO microtubes, and the well-aligned ZnO tubes can be fabricated when the thickness of the composite coating is between 0.1 mm and 1 mm. This novel approach in which fluororesin is adopted to tune the growth of ZnO crystals may become one of the effective methods to control the shape of ZnO nano- /micro-sized structures. Acknowledgement Fig. 4. Schematic of growth process of ZnO microtubes. The tube along the caxis epitaxy sprouts out from the surface of Si substrate, and the inner part of the tube is eroded away gradually and continuously by reductive species.
This work was supported by the Program for New Century Excellent Talents in Universities of China (No. NCET-040274).
T. Sun, J. Qiu / Materials Letters 62 (2008) 1528–1531
References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, et al., Science 292 (2001) 1897. [3] J.J. Wu, S.C. Li, Appl. Phys. Lett. 81 (2002) 1312. [4] J.Q. Hu, Q. Li, X.M. Meng, C.S. Lee, S.T. Lee, Chem. Mater. 15 (2003) 305. [5] Y.J. Xing, Z.H. Xi, X.D. Zhang, J.H. Song, R.M. Wang, J. Xu, et al., Solid State Commun. 129 (2004) 671. [6] J. Zhang, L.D. Sun, C.S. Liao, C.H. Yan, Chem. Commun. (2002) 262. [7] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Chem. Mater. 13 (2001) 4395. [8] Z. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Langmuir 20 (2004) 3441.
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[9] H.D. Yu, Z.P. Zhang, M.Y. Han, X.T. Hao, F.R. Zhu, J. Am. Chem. Soc. 127 (2005) 2378. [10] H.Q. Yan, R.R. He, J. Pham, P. Yang, Adv. Mater. 15 (2003) 402. [11] D.S. King, R.M. Nix, J. Catal. 160 (1996) 76. [12] F. Quaranta, A. Valentini, F.R. Rizzi, G. Casamassima, J. Appl. Phys. 74 (1) (1993) 244. [13] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, et al., Appl. Phys. Lett. 70 (17) (1997) 2230. [14] K. Tennakone, G. Kumara, I. Kottegoda, V. Perera, Chem. Commun. (1999) 15. [15] J.S. Jeong, J.Y. Lee, J.H. Cho, H.J. Suh, C.J. Lee, Chem. Mater. 17 (2005) 2752. [16] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235.
Materials Letters 62 (2008) 1528 – 1531 www.elsevier.com/locate/matlet
Fabrication of ZnO microtube arrays via vapor phase growth Tianjun Sun, Jieshan Qiu ⁎ Carbon Research Laboratory, Center for Nano Materials and Science, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road, P. O. Box 49, Dalian 116012, China Received 8 June 2007; accepted 13 September 2007 Available 19 September 2007
Abstract Single-crystalline ZnO microtube arrays were fabricated from commercial Zn powder on a Si substrate coated with a composite of fluororesin and SiO2 powder by making use of a simple one-step thermal evaporation process. The as-prepared ZnO samples were examined by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. It has been found that these ZnO microtubes are single-crystalline wurtzite structures, and grow along [0002] direction. The ZnO tubes with an outer diameter of ca. 1–3 μm have a length of several tens micrometers, of which the wall thickness ranges from 0.5 μm to 1.5 μm. The results have evidenced that the composite coating with a thickness of 0.1–1 mm is critical to the growth of the ZnO microtubes. © 2007 Elsevier B.V. All rights reserved. Keywords: ZnO; Microstructure; Chemical vapor deposition; Composite materials
1. Introduction Zinc oxide with wide band gap of 3.37 eV and large excitation binding energy of 60 meV is an important semiconducting material, and has promising applications in a wide range of high-technology fields such as field-emission displays, optical electric devices, solar cells, gas sensors and catalysts [1–3]. Up to date, great efforts have been made on the controlled growth of various one-dimensional (1D) micro- or nano-sized ZnO materials including nanobelts [1], nanowires [2], nanotubes [3–9], and terapods [10]. Of these 1D micro- or nano-sized ZnO materials, the tubular material is of great importance for the fabrication of micro-/nano-sized devices because of the unique structure with high porosity and surface area. Generally, the low temperature solution route is used to make ZnO nano/microtubes due to its low growth temperature and simple procedure [7–9], but this is a time-consuming
⁎ Corresponding author. Fax: +86 411 88993991. E-mail address: [email protected] (J. Qiu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.09.015
process that normally lasts from 24 h to several days. Thermal evaporation methods [11–15] for making micro/nano-sized ZnO tubes have been reported, however, little success has been made in controlled growth of well-aligned ZnO tube arrays in large scale. Here, we report a simple gas-phase method for fabricating micro-sized ZnO tube arrays. With this method, single-crystalline ZnO microtubes with hexagonal shape and hollow core have been successfully prepared on resin-coated Si substrate in large scale. 2. Experimental ZnO microtubes were fabricated on Si substrates in a quartz tube reactor heated in a tube furnace via a modified chemical vapor deposition (CVD) process. Before the experiment, Si substrates (7 × 5 mm) were coated with a mixture of fluororesin, SiO2 powder and butyl acetate with a ratio of 25:5:70 in weight. The fluororesin with a molecular weight of ca. 6000–8000 was the quadripolymer of chlorotrifluoroethylene. The pre-coated Si substrate was dried in air, resulting in a composite film of fluororesin and SiO2 powder on the substrate, which leads to composite-coated Si substrates. The thickness of the composite film coated on the Si substrates ranges from several tens
T. Sun, J. Qiu / Materials Letters 62 (2008) 1528–1531
1529
3. Results and discussion
Fig. 1. a) SEM image of ZnO microtube array, inset showing the SEM image of a typical ZnO microtube; b) TEM image of an individual ZnO tube, inset is its SAED pattern; and c) HRTEM image of the tube shown in b.
micrometers to several millimeters, which could be controlled by repeated coatings. Then, the composite-coated Si substrate was put into the quartz tube in downstream, which was about 10–20 mm away from the quartz boat containing raw Zn powder (99.9%) that was located in the center of quartz tube reactor. For each run, the quartz tube reactor was flushed for 30 min using argon gas with a purity of 99.99% at a flow rate of 150 mL/min. Then, the flow rate of Ar gas was reduced to 95 mL/min, which functioned as carrier gas to carry the vaporized Zn powder into the reaction zone in downstream. Meanwhile, the reactant gas, oxygen with a purity of 99.99%, was introduced into the reactor at a flow rate of 5 mL/min. The outlet of the oxygen gas was above the Si substrate, which helps to avoid the oxidation of Zn powder before evaporation. The furnace was ramped to 650 °C at a rate of 50 °C/min, and was kept at 650 °C for 15 min before being cooled back to room temperature in flowing argon. Grayish ZnO products were formed on the substrate. For comparison, the Si substrates without any coating pretreatment or coated with carbon film were also tested under the identical conditions. The Si substrates coated with carbon film were obtained by heating the composite-coated Si substrates at 500 °C for 1 h in flowing argon. The morphologies and structures of the as-made ZnO samples were examined using X-ray powder diffraction (XRD, Shimadzu XRD-6000, CuKα radiation), scanning electron microscopy (SEM, JEOL JSM-5600LV), transmission electron microscopy, high-resolution TEM and selected area electron diffraction (TEM, HTRTEM and SAED, Tecnai-G2, 200 kV).
Fig. 1a shows a typical SEM image of the as-synthesized ZnO tube arrays grown on the composite-coated Si substrate in large scale. The aligned ZnO tubes are approximately perpendicular to the substrate, and have hollow cores. On average, these tubes are ca. 1–3 μm in diameter, and up to 10 μm in length. The inset in Fig. 1a is the highmagnification SEM image of a typical ZnO microtube with perfect hexagonal shape, of which the diameter is ca. 2.5 μm and the wall thickness is ca.1 μm. Fig. 1b is the TEM image of an individual ZnO microtube that is ca. 1 μm in diameter and 3 μm in length. This uniform ZnO tube has a small hollow core of ca. 250 nm in diameter, as shown in Fig. 1b. The SAED pattern (inset in Fig. 1b) reveals that this type of tube is singlecrystalline structure, and grows along [0002] direction. In Fig. 1c, it is clear that the d-spacing of the lattices is ca. 0.52 nm, which is consistent with the spacing of (0001) facet of ZnO crystal, further evidencing that the ZnO tube grows along [0002] direction. Fig. 2 is the typical XRD pattern of the micro-sized ZnO tube arrays grown on the composite-coated Si substrates, revealing that the asmade ZnO tube arrays are of the typical wurtzite structures with lattice constants of a = 0.325 nm and c = 0.521 nm (JCPDS card No.36-1451). It can be seen that the ZnO (002) peak is dominant because the ZnO microtubes have a larger wall thickness of 0.5–1.5 μm, evidencing that the ZnO tube arrays grow along the direction of (002) plane. Similar phenomena were also observed by other researchers [15]. The substrate effect on the formation of ZnO microtubes was addressed systematically. When bare substrates without any coating were used, ZnO hexagonal microrods would be obtained, as shown in Fig. 3a, while ZnO micronails would be formed on substrate coated with carbon film, as shown in Fig. 3b. It has been found that when the thickness of the composite coating is in a range of 0.1 mm to 1 mm, the formation of ZnO microtubes is favored. When the coating thickness on the Si substrate is less than 0.1 mm, a mixture of microrods and microtubes with small-size hollow core would be formed (Fig. 3c). When composite coating thickness is over 1 mm, tubular ZnO with poor alignment would be obtained (Fig. 3d). Obviously, the thickness of composite film coated on the substrates is critical to the growth of ZnO microtube arrays. The effect of the properties of composite coating and the process parameters on the formation of ZnO microstructures is currently under further investigation with an aim
Fig. 2. XRD pattern of as-synthesized ZnO microtube arrays.
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T. Sun, J. Qiu / Materials Letters 62 (2008) 1528–1531
Fig. 3. SEM images of aligned ZnO structures grown under different conditions. a) ZnO nanorods, b) ZnO nanonails, c) low- and high-magnification (inset) images of typical ZnO microtubes with small hollow cores, d) low-magnification image of typical ZnO microtubes with large hollow cores, and inset showing an individual ZnO tube.
of making highly oriented ZnO microtubes, and tuning their diameters and wall thickness according to specific requirements. In the present work, the growth of ZnO microtubes proceeds via the vapor–solid (VS) scheme owing to the absence of catalyst [16]. It is known that fluororesin begins to decompose at 460 °C, in which some small yet reductive volatile species such as CO and CxHy are released, at the same time, a carbon film (Cn) is formed on the surface of the Si substrates. In the reductive environment resulting from the fluororesin pyrolysis, the ZnOx particles instead of ZnO particles would be formed and start to nucleate on the in situ formed carbon film. The surface
consisting of this kind of nucleus would be further oxidized to form ZnO o layer because the partial pressure of oxygen at the surface (P O2 ) is much i higher than at the inner part (PO2), whereas the ZnOx in the inner part would be reduced to Zn vapor due to the high reductive activity of the nascent carbon film, and is further vaporized at high temperature. Under the joint effect of the parameters discussed above, ZnO tubes sprout out from the ZnOx embryo, and continue to grow till the experiment is stopped. For clarity, the whole process is schematically shown in Fig. 4.
4. Conclusions In summary, oriented single-crystalline ZnO tubes have been fabricated in large area on the Si substrate coated with a composite film of fluororesin and SiO2 powder by a simple thermal evaporation process. These ZnO tubes with a wall thicknesses of 0.5–1.5 μm grow along [0002] direction up to several tens micrometers, and have outer diameters in a range of ca. 1–3 μm. It has been found that fluororesin is the most important factor for the growth of the ZnO microtubes, and the well-aligned ZnO tubes can be fabricated when the thickness of the composite coating is between 0.1 mm and 1 mm. This novel approach in which fluororesin is adopted to tune the growth of ZnO crystals may become one of the effective methods to control the shape of ZnO nano- /micro-sized structures. Acknowledgement Fig. 4. Schematic of growth process of ZnO microtubes. The tube along the caxis epitaxy sprouts out from the surface of Si substrate, and the inner part of the tube is eroded away gradually and continuously by reductive species.
This work was supported by the Program for New Century Excellent Talents in Universities of China (No. NCET-040274).
T. Sun, J. Qiu / Materials Letters 62 (2008) 1528–1531
References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, et al., Science 292 (2001) 1897. [3] J.J. Wu, S.C. Li, Appl. Phys. Lett. 81 (2002) 1312. [4] J.Q. Hu, Q. Li, X.M. Meng, C.S. Lee, S.T. Lee, Chem. Mater. 15 (2003) 305. [5] Y.J. Xing, Z.H. Xi, X.D. Zhang, J.H. Song, R.M. Wang, J. Xu, et al., Solid State Commun. 129 (2004) 671. [6] J. Zhang, L.D. Sun, C.S. Liao, C.H. Yan, Chem. Commun. (2002) 262. [7] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Chem. Mater. 13 (2001) 4395. [8] Z. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Langmuir 20 (2004) 3441.
1531
[9] H.D. Yu, Z.P. Zhang, M.Y. Han, X.T. Hao, F.R. Zhu, J. Am. Chem. Soc. 127 (2005) 2378. [10] H.Q. Yan, R.R. He, J. Pham, P. Yang, Adv. Mater. 15 (2003) 402. [11] D.S. King, R.M. Nix, J. Catal. 160 (1996) 76. [12] F. Quaranta, A. Valentini, F.R. Rizzi, G. Casamassima, J. Appl. Phys. 74 (1) (1993) 244. [13] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, et al., Appl. Phys. Lett. 70 (17) (1997) 2230. [14] K. Tennakone, G. Kumara, I. Kottegoda, V. Perera, Chem. Commun. (1999) 15. [15] J.S. Jeong, J.Y. Lee, J.H. Cho, H.J. Suh, C.J. Lee, Chem. Mater. 17 (2005) 2752. [16] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 3 (2003) 235.