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Photoluminescence and field-emission characteristics of ZnO nanowires synthesized by two-step method
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Vacuum 82 (2008) 30–34 www.elsevier.com/locate/vacuum
Photoluminescence and field-emission characteristics of ZnO nanowires synthesized by two-step method Qiuxiang Zhanga, Yongsheng Zhanga,b,, Ke Yua, Ziqiang Zhua a
Department of Electronic Engineering, East China Normal University, Shanghai 200062, PR China b Department of Computer, Luoyang Technology College, Luoyang 471003, PR China Received 8 June 2006; received in revised form 18 December 2006; accepted 19 December 2006
Abstract ZnO nanowires with excellent photoluminescence (PL) and field-emission properties were synthesized by a two-step method, and the ZnO nanowires grew along (0 0 2) direction. PL measurements showed that the ZnO nanowires have stronger ultraviolet emission properties at 376 nm and there is 3 nm blue shift after the nanowires were immersed in thiourea (TU) solution compared with those of without immersion. The immersed-ZnO nanowires show a turn-on field of 2.3 V/mm at a current density of 0.1 mA/cm and emission current density up to 1 mA/cm2 at an applied field of 6.8 V/mm, which demonstrate that the immersed-ZnO nanowires posses efficient field-emission properties in contrast with those not immersed. The ZnO nanowires may be ideal candidates for making luminescent devices and field-emission displays. r 2007 Elsevier Ltd. All rights reserved. PACS: 81.05.Dz; 78.55.m; 85.45.Db Keywords: ZnO nanowires; Photoluminescence; Field emission
1. Introduction Since carbon nanotubes (CNTS) were discovered in 1991 [1], other low-dimensional nanomaterials have drawn much attention. ZnO, a direct band gap (3.37 eV) semiconductor with exciton binding energy of 60 meV, is of much more importance in these nanomaterials as a result of its intriguing optical functions. Various ZnO nanostructures have been synthesized, such as nanobelts, nanocombs, nanorings, nanonails and nanowires [2–6]. The optical exploration of ZnO nanostructures usually focuses on the photoluminescence (PL) and field-emission characteristics. ZnO is considered a promising photonic material for the UV/blue devices such as short-wavelength light-emitting diodes and laser diodes. Several previous studies have shown that ZnO nanowires exhibit strong UV laser Corresponding author. Department of Electronic Engineering, East China Normal University, 3663 Zhongshan North Road, Shanghai 200062, PR China. Tel.: +86 21 62221912; fax: +86 21 62232517. E-mail address: [email protected] (Y. Zhang).
0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.12.016
emission, which can be used in luminescent device application [7–10]. A UV nanolaser working at room temperature has been fabricated using highly oriented ZnO nanowire arrays [11]. In addition, ZnO is more resistant to radiation and oxidation, which is essential to the application of field emission. The field-emission properties for both ZnO nanowire arrays [12–14] and single ZnO nanotips [15] have been studied. There have been many reports about obtaining high-quality crystalline ZnO nanowires with better optical properties and field-emission properties [7,13], but they need a catalyst or higher temperature. In this letter, we report a simple two-step method at lower temperature to synthesize ZnO nanowires with better optical and field-emission properties. The PL and fieldemission measurement results reveal that the ZnO nanowires immersed in thiourea (TU) solution possess stronger ultraviolet emission and efficient field emission. It is believed that the method has a great deal of potential applications for making luminescent devices and fieldemission displays.
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2. Experimental In our experiment, ZnO nanowires were first obtained by the thermal-evaporation vapor-phase transport method. No catalyst was used in the growth process. The mixture of ZnO powders (purity 99.99%) and graphite powders (purity 99.9%), with a ratio of 1:1, was placed in a quartz boat, which was inserted into a horizontal tube furnace. The silicon substrate was placed downstream from the sources to collect the products. Then, the horizontal furnace was quickly heated to 650 1C at a flow rate of 150 sccm N2 (99.9%), and O2 was then added at a flow rate of 15 sccm for a further 1 h. After the system had cooled to room temperature, the silicon substrate was taken out from the furnace. It was found that a white wool-like material was deposited on the silicon substrate (sample A). Subsequently, the TU solution was prepared by dissolving 0.8 g TU in 50 ml distilled water. To change the morphology of the as-synthesized ZnO nanowires, the silicon substrate deposited with ZnO nanowires was directly immersed in TU solution and kept without shifting at room temperature for 20 h. Then, the substrate was taken out from the solution and washed several times with deionized water. Finally, the substrate was dried in an oven at 75 1C for 30 min (sample B). The morphology, composition and optic properties of the products were characterized using scanning electron microscopy (SEM, JEOLJSM-6700F), X-ray diffraction (XRD, D/max 2550 V) and PL spectrum. The field-emission measurements of the samples A and B were carried out with a diode structure in a vacuum chamber at a pressure of 5 105 Pa at room temperature. The sample (as a cathode) was separated from a phosphor/ITO/glass anode by two Teflon spacers with thickness of 200 mm. Current versus voltage curves were measured with standard electronic instruments. 3. Results and discussion Fig. 1 (a) and (b) show the XRD patterns of the sample A and B. The diffraction pattern indicates that the sample exhibits a hexagonal ZnO crystal structure with lattice constants of a ¼ 0.3248 nm and c ¼ 0.5199 nm, being consistent with the standard values of ZnO bulk crystal (JCPDS 36–1451). It is noticeable that the diffraction pattern of sample B shows a remarkable strong peak (0 0 2) plane compared with those of sample A. It is confirmed that the ZnO nanowires have grown with preferred orientation along the c-axis, and the trend of sample B is more evident than that of sample A. No diffraction peaks from other phases have been observed. So, we can conclude that the ZnO nanowires are single-crystalline wurtzite structure, and the quality of the ZnO nanowires can be improved after they were eroded by TU solution. Fig. 2 (a) is a SEM image of sample A. The inset is an enlarged-magnification SEM image. The ZnO nanowires with tetragonal structures were observed. The length and diameters of the nanowires are 10 mm and 300 nm,
Fig. 1. The XRD patterns of the ZnO nanowires: (a) sample A and (b) sample B.
respectively. To illustrate the effect of the TU solution on the morphology of the ZnO nanowires, SEM examination of sample B was also conducted, as shown in Fig. 2 (b), and the inset is an enlarged-magnification SEM image. It can be seen that the nanowires become thinner and longer after the sample A was immersed in TU solution for 20 h, and the length and diameters of the nanowires are 20 mm and 200 nm, respectively. The TU solution is acidic due to the slow release of H+ from TU. When the pure ZnO nanowires were dissolved in the TU solution, Zn2+ ions and O2 ions, which are from the surface of the pure ZnO nanowires, were released [16]. So the ZnO nanowires may become thinner and longer. It is believed that these thinner and longer ZnO nanowires as emitters have a high field enhancement factor, which could make it easy to emit electrons. However, the reason why the ZnO nanowires
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Fig. 3. Room-temperature photoluminescence spectra of the two samples.
Fig. 2. SEM images of the ZnO nanowires: (a) sample A and (b) sample B.
immersed in TU solution can become longer needs to be further studied. To examine the luminescence property of the ZnO nanowires, PL measurements were performed at room temperature using a He–Cd laser with an excitation wavelength of 325 nm. The PL spectra of sample A was compared with that of sample B, as shown in Fig. 3. The PL spectrum of sample A exhibits UV emission at 381 nm. The green emission around 520 nm was very weak. Vanheusden et al. [17] found that oxygen vacancies are responsible for the green luminescence in ZnO. Oxygen vacancies occur in three different charge states: the neutral oxygen vacancy (V 0o ), the singly ionized oxygen vacancy (V 0o ), and the doubly ionized oxygen vacancy (V 00o ). Only V 0o can act as the so-called luminescent centers [18]. Therefore, the weak green emission means that there is a low concentration of oxygen vacancies in the ZnO nanowires. For sample B, the UV emission occurs at 376 nm. The 5 nm blue shift may be the result of size confinement effect. In contrast with the spectrum of sample A, the UV emission of sample B becomes stronger and narrower and the green emission becomes much weaker. The reason may be as follows: in the TU solution, there is
some H+ released from the TU. On the one hand, it may erode the multicrystal grain on the surface of the ZnO nanowires and make the crystalline quality of ZnO nanowires become better. On the other hand, these H+ may combine with the V 0o and result in the decrease of oxygen vacancies. As a result, the green emission becomes much weaker. Fig. 4 illustrates the curves of electron-emission current density versus electric field from samples A and B. The inset shows the corresponding Fowler–Nordheim (F–N) plots, the F–N plots are approximately straight lines, indicating that field-emission process from the two samples is mainly from a barrier tunneling, quantum mechanical process [19–21]. The turn-on field, at which emission current density reaches 0.1 mA/cm2, is about 2.7 V/mm for sample A and is 2.3 for sample B. The threshold field, defined as the electric field required to produce a current density of 1 mA/cm2, is 7.9 and 6.8 V/mm for samples A and B, respectively. The FE properties are comparable with those of the reported ZnO nanowires, the MoO3 nanobelts and the CNTS [22–25]. Fig. 5 shows the electron-emission image of the samples. It can be seen that the size of emission sites is consistent with the sample microstructures. It is believed that this method is promising for making luminescent device and field-emission displays. 4. Conclusions ZnO nanowires with excellent PL and field-emission properties were synthesized using a two-step method. PL measurements showed that the ZnO nanowires immersed in TU have stronger ultraviolet emission properties and the crystalline quality is improved. The immersed ZnO nanowires show a lower turn-on field and threshold field, which demonstrate that the immersed ZnO nanowires
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Fig. 4. Emission current density versus electric field curves of the two samples. The inset is the corresponding F–N plot.
Fig. 5. Electron emission image of the samples: (a) sample A and (b) sample B.
possess efficient field emission in contrast with those not immersed. The samples have potential application in luminescent device and field-emission displays. Acknowledgments The authors acknowledge the financial support from the NSF of China (no. 60476004), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (no. 704022), Shanghai leader of the discipline projects, Specialized Research Fund for the Doctoral Program of Higher Education, the NSF of Henan Province (no. 0611023800) and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (no. WUT2004 Z01). References [1] Iijima S. Nature (London) 1991;354:56. [2] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947.
[3] Yang PD, Yan HQ, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323. [4] Kong XY, Ding Y, Yang RS, Wang ZL. Science 2004;303:1348. [5] Lao JY, Huang JY, Wang DZ, Ren ZF. Nano Lett 2003;3:235. [6] Kong YC, Yu DP, Zhang B, Fang W, Feng SQ. Appl Phys Lett 2001;78:407. [7] Leea JS, Kanga MI, Kima S, Leeb MS, Lee YK. J Cryst Growth 2003;249:201. [8] Park K, Lee JS, Sung MY, Kim S. Jpn J Appl Phys 1 2002;1:7317. [9] Tseng YK, Lin IN, Liu KS, Lin TS, Chen IC. J Mater Res 2003;18:714. [10] Ng HT, Chen B, Li J, Han J, Meyappan M, Wu J, et al. Appl Phys Lett 2003;82:2023. [11] Huang MH, Mao S, Feik H, Yan H, Wu Y, Kind H, et al. Science 2001;292:1897. [12] Kong YC, Yu DP, Zhang B, Fang W, Feng SQ. Appl Phys Lett 2001;78:407. [13] Lee CJ, Lee TJ, Lyu SC, Zhang Y, Ruh H, Lee H. J Appl Phys Lett 2002;81:3648. [14] Huang MH, Wu YY, Feick H, Tran N, Weber E, Yang PD. Adv Mater 2001;13:113. [15] Dong L, Jiao J, Tuggle DW, Petty JM, Elliff SA, Coulter M. Appl Phys Lett 2003;82:1096.
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[16] Shen GZ, Cho JH, Jung SI, Lee CJ. Chem Phys Lett 2005; 401:529–33. [17] Vanheusden K, Warren WL, Seager CH, Tallant DR, Voigt JA, Gnade BE. J Appl Phys 1996;79:7983. [18] Li W, Mao D, Zhang F, Wang X, Liu X, Zou S, et al. Nucl Instrum Meth Phys Res B 2005;169:53. [19] Rinzler AG, Hafner JH, Nikolaev P, Lou L, Kim SG, Tomnek D, et al. Science 1995;269:1550. [20] De Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179.
[21] Frederick CKAu, Wang KW, Tang YH, Zhang YF, Bello I, Lee ST. Appl Phys Lett 1999;75:1700. [22] Lee CJ, Lee TJ, Lyu SC, Zhang Y. Appl Phys Lett 2002;81:3148. [23] Jo SH, Lao JY, Ren ZF, Farrer RA, Baldacchini T, Fourkas JT. Appl Phys Lett 2003;83:4821. [24] Li YB, Bando Y, Golberg D, Kurashima K. Appl Phys Lett 2002;81:5148. [25] Jo SH, Tu Y, Huang ZP, Camahan DL, Wang DZ, Ren ZF. Appl Phys Lett 2003;82:3520.
Vacuum 82 (2008) 30–34 www.elsevier.com/locate/vacuum
Photoluminescence and field-emission characteristics of ZnO nanowires synthesized by two-step method Qiuxiang Zhanga, Yongsheng Zhanga,b,, Ke Yua, Ziqiang Zhua a
Department of Electronic Engineering, East China Normal University, Shanghai 200062, PR China b Department of Computer, Luoyang Technology College, Luoyang 471003, PR China Received 8 June 2006; received in revised form 18 December 2006; accepted 19 December 2006
Abstract ZnO nanowires with excellent photoluminescence (PL) and field-emission properties were synthesized by a two-step method, and the ZnO nanowires grew along (0 0 2) direction. PL measurements showed that the ZnO nanowires have stronger ultraviolet emission properties at 376 nm and there is 3 nm blue shift after the nanowires were immersed in thiourea (TU) solution compared with those of without immersion. The immersed-ZnO nanowires show a turn-on field of 2.3 V/mm at a current density of 0.1 mA/cm and emission current density up to 1 mA/cm2 at an applied field of 6.8 V/mm, which demonstrate that the immersed-ZnO nanowires posses efficient field-emission properties in contrast with those not immersed. The ZnO nanowires may be ideal candidates for making luminescent devices and field-emission displays. r 2007 Elsevier Ltd. All rights reserved. PACS: 81.05.Dz; 78.55.m; 85.45.Db Keywords: ZnO nanowires; Photoluminescence; Field emission
1. Introduction Since carbon nanotubes (CNTS) were discovered in 1991 [1], other low-dimensional nanomaterials have drawn much attention. ZnO, a direct band gap (3.37 eV) semiconductor with exciton binding energy of 60 meV, is of much more importance in these nanomaterials as a result of its intriguing optical functions. Various ZnO nanostructures have been synthesized, such as nanobelts, nanocombs, nanorings, nanonails and nanowires [2–6]. The optical exploration of ZnO nanostructures usually focuses on the photoluminescence (PL) and field-emission characteristics. ZnO is considered a promising photonic material for the UV/blue devices such as short-wavelength light-emitting diodes and laser diodes. Several previous studies have shown that ZnO nanowires exhibit strong UV laser Corresponding author. Department of Electronic Engineering, East China Normal University, 3663 Zhongshan North Road, Shanghai 200062, PR China. Tel.: +86 21 62221912; fax: +86 21 62232517. E-mail address: [email protected] (Y. Zhang).
0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.12.016
emission, which can be used in luminescent device application [7–10]. A UV nanolaser working at room temperature has been fabricated using highly oriented ZnO nanowire arrays [11]. In addition, ZnO is more resistant to radiation and oxidation, which is essential to the application of field emission. The field-emission properties for both ZnO nanowire arrays [12–14] and single ZnO nanotips [15] have been studied. There have been many reports about obtaining high-quality crystalline ZnO nanowires with better optical properties and field-emission properties [7,13], but they need a catalyst or higher temperature. In this letter, we report a simple two-step method at lower temperature to synthesize ZnO nanowires with better optical and field-emission properties. The PL and fieldemission measurement results reveal that the ZnO nanowires immersed in thiourea (TU) solution possess stronger ultraviolet emission and efficient field emission. It is believed that the method has a great deal of potential applications for making luminescent devices and fieldemission displays.
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2. Experimental In our experiment, ZnO nanowires were first obtained by the thermal-evaporation vapor-phase transport method. No catalyst was used in the growth process. The mixture of ZnO powders (purity 99.99%) and graphite powders (purity 99.9%), with a ratio of 1:1, was placed in a quartz boat, which was inserted into a horizontal tube furnace. The silicon substrate was placed downstream from the sources to collect the products. Then, the horizontal furnace was quickly heated to 650 1C at a flow rate of 150 sccm N2 (99.9%), and O2 was then added at a flow rate of 15 sccm for a further 1 h. After the system had cooled to room temperature, the silicon substrate was taken out from the furnace. It was found that a white wool-like material was deposited on the silicon substrate (sample A). Subsequently, the TU solution was prepared by dissolving 0.8 g TU in 50 ml distilled water. To change the morphology of the as-synthesized ZnO nanowires, the silicon substrate deposited with ZnO nanowires was directly immersed in TU solution and kept without shifting at room temperature for 20 h. Then, the substrate was taken out from the solution and washed several times with deionized water. Finally, the substrate was dried in an oven at 75 1C for 30 min (sample B). The morphology, composition and optic properties of the products were characterized using scanning electron microscopy (SEM, JEOLJSM-6700F), X-ray diffraction (XRD, D/max 2550 V) and PL spectrum. The field-emission measurements of the samples A and B were carried out with a diode structure in a vacuum chamber at a pressure of 5 105 Pa at room temperature. The sample (as a cathode) was separated from a phosphor/ITO/glass anode by two Teflon spacers with thickness of 200 mm. Current versus voltage curves were measured with standard electronic instruments. 3. Results and discussion Fig. 1 (a) and (b) show the XRD patterns of the sample A and B. The diffraction pattern indicates that the sample exhibits a hexagonal ZnO crystal structure with lattice constants of a ¼ 0.3248 nm and c ¼ 0.5199 nm, being consistent with the standard values of ZnO bulk crystal (JCPDS 36–1451). It is noticeable that the diffraction pattern of sample B shows a remarkable strong peak (0 0 2) plane compared with those of sample A. It is confirmed that the ZnO nanowires have grown with preferred orientation along the c-axis, and the trend of sample B is more evident than that of sample A. No diffraction peaks from other phases have been observed. So, we can conclude that the ZnO nanowires are single-crystalline wurtzite structure, and the quality of the ZnO nanowires can be improved after they were eroded by TU solution. Fig. 2 (a) is a SEM image of sample A. The inset is an enlarged-magnification SEM image. The ZnO nanowires with tetragonal structures were observed. The length and diameters of the nanowires are 10 mm and 300 nm,
Fig. 1. The XRD patterns of the ZnO nanowires: (a) sample A and (b) sample B.
respectively. To illustrate the effect of the TU solution on the morphology of the ZnO nanowires, SEM examination of sample B was also conducted, as shown in Fig. 2 (b), and the inset is an enlarged-magnification SEM image. It can be seen that the nanowires become thinner and longer after the sample A was immersed in TU solution for 20 h, and the length and diameters of the nanowires are 20 mm and 200 nm, respectively. The TU solution is acidic due to the slow release of H+ from TU. When the pure ZnO nanowires were dissolved in the TU solution, Zn2+ ions and O2 ions, which are from the surface of the pure ZnO nanowires, were released [16]. So the ZnO nanowires may become thinner and longer. It is believed that these thinner and longer ZnO nanowires as emitters have a high field enhancement factor, which could make it easy to emit electrons. However, the reason why the ZnO nanowires
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Fig. 3. Room-temperature photoluminescence spectra of the two samples.
Fig. 2. SEM images of the ZnO nanowires: (a) sample A and (b) sample B.
immersed in TU solution can become longer needs to be further studied. To examine the luminescence property of the ZnO nanowires, PL measurements were performed at room temperature using a He–Cd laser with an excitation wavelength of 325 nm. The PL spectra of sample A was compared with that of sample B, as shown in Fig. 3. The PL spectrum of sample A exhibits UV emission at 381 nm. The green emission around 520 nm was very weak. Vanheusden et al. [17] found that oxygen vacancies are responsible for the green luminescence in ZnO. Oxygen vacancies occur in three different charge states: the neutral oxygen vacancy (V 0o ), the singly ionized oxygen vacancy (V 0o ), and the doubly ionized oxygen vacancy (V 00o ). Only V 0o can act as the so-called luminescent centers [18]. Therefore, the weak green emission means that there is a low concentration of oxygen vacancies in the ZnO nanowires. For sample B, the UV emission occurs at 376 nm. The 5 nm blue shift may be the result of size confinement effect. In contrast with the spectrum of sample A, the UV emission of sample B becomes stronger and narrower and the green emission becomes much weaker. The reason may be as follows: in the TU solution, there is
some H+ released from the TU. On the one hand, it may erode the multicrystal grain on the surface of the ZnO nanowires and make the crystalline quality of ZnO nanowires become better. On the other hand, these H+ may combine with the V 0o and result in the decrease of oxygen vacancies. As a result, the green emission becomes much weaker. Fig. 4 illustrates the curves of electron-emission current density versus electric field from samples A and B. The inset shows the corresponding Fowler–Nordheim (F–N) plots, the F–N plots are approximately straight lines, indicating that field-emission process from the two samples is mainly from a barrier tunneling, quantum mechanical process [19–21]. The turn-on field, at which emission current density reaches 0.1 mA/cm2, is about 2.7 V/mm for sample A and is 2.3 for sample B. The threshold field, defined as the electric field required to produce a current density of 1 mA/cm2, is 7.9 and 6.8 V/mm for samples A and B, respectively. The FE properties are comparable with those of the reported ZnO nanowires, the MoO3 nanobelts and the CNTS [22–25]. Fig. 5 shows the electron-emission image of the samples. It can be seen that the size of emission sites is consistent with the sample microstructures. It is believed that this method is promising for making luminescent device and field-emission displays. 4. Conclusions ZnO nanowires with excellent PL and field-emission properties were synthesized using a two-step method. PL measurements showed that the ZnO nanowires immersed in TU have stronger ultraviolet emission properties and the crystalline quality is improved. The immersed ZnO nanowires show a lower turn-on field and threshold field, which demonstrate that the immersed ZnO nanowires
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Fig. 4. Emission current density versus electric field curves of the two samples. The inset is the corresponding F–N plot.
Fig. 5. Electron emission image of the samples: (a) sample A and (b) sample B.
possess efficient field emission in contrast with those not immersed. The samples have potential application in luminescent device and field-emission displays. Acknowledgments The authors acknowledge the financial support from the NSF of China (no. 60476004), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (no. 704022), Shanghai leader of the discipline projects, Specialized Research Fund for the Doctoral Program of Higher Education, the NSF of Henan Province (no. 0611023800) and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (no. WUT2004 Z01). References [1] Iijima S. Nature (London) 1991;354:56. [2] Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947.
[3] Yang PD, Yan HQ, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater 2002;12:323. [4] Kong XY, Ding Y, Yang RS, Wang ZL. Science 2004;303:1348. [5] Lao JY, Huang JY, Wang DZ, Ren ZF. Nano Lett 2003;3:235. [6] Kong YC, Yu DP, Zhang B, Fang W, Feng SQ. Appl Phys Lett 2001;78:407. [7] Leea JS, Kanga MI, Kima S, Leeb MS, Lee YK. J Cryst Growth 2003;249:201. [8] Park K, Lee JS, Sung MY, Kim S. Jpn J Appl Phys 1 2002;1:7317. [9] Tseng YK, Lin IN, Liu KS, Lin TS, Chen IC. J Mater Res 2003;18:714. [10] Ng HT, Chen B, Li J, Han J, Meyappan M, Wu J, et al. Appl Phys Lett 2003;82:2023. [11] Huang MH, Mao S, Feik H, Yan H, Wu Y, Kind H, et al. Science 2001;292:1897. [12] Kong YC, Yu DP, Zhang B, Fang W, Feng SQ. Appl Phys Lett 2001;78:407. [13] Lee CJ, Lee TJ, Lyu SC, Zhang Y, Ruh H, Lee H. J Appl Phys Lett 2002;81:3648. [14] Huang MH, Wu YY, Feick H, Tran N, Weber E, Yang PD. Adv Mater 2001;13:113. [15] Dong L, Jiao J, Tuggle DW, Petty JM, Elliff SA, Coulter M. Appl Phys Lett 2003;82:1096.
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[16] Shen GZ, Cho JH, Jung SI, Lee CJ. Chem Phys Lett 2005; 401:529–33. [17] Vanheusden K, Warren WL, Seager CH, Tallant DR, Voigt JA, Gnade BE. J Appl Phys 1996;79:7983. [18] Li W, Mao D, Zhang F, Wang X, Liu X, Zou S, et al. Nucl Instrum Meth Phys Res B 2005;169:53. [19] Rinzler AG, Hafner JH, Nikolaev P, Lou L, Kim SG, Tomnek D, et al. Science 1995;269:1550. [20] De Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179.
[21] Frederick CKAu, Wang KW, Tang YH, Zhang YF, Bello I, Lee ST. Appl Phys Lett 1999;75:1700. [22] Lee CJ, Lee TJ, Lyu SC, Zhang Y. Appl Phys Lett 2002;81:3148. [23] Jo SH, Lao JY, Ren ZF, Farrer RA, Baldacchini T, Fourkas JT. Appl Phys Lett 2003;83:4821. [24] Li YB, Bando Y, Golberg D, Kurashima K. Appl Phys Lett 2002;81:5148. [25] Jo SH, Tu Y, Huang ZP, Camahan DL, Wang DZ, Ren ZF. Appl Phys Lett 2003;82:3520.