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Zinc oxide nanowire based field emitters
Physica E 43 (2010) 285–288
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Zinc oxide nanowire based field emitters Sivakumar Ramanathan a,n,1, Yu-chun Chen b, Yonhua Tzeng b a b
Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA Department of Electrical Engineering, Auburn University, Auburn, Al 36849, USA
a r t i c l e in fo
abstract
Article history: Received 5 April 2010 Received in revised form 29 July 2010 Accepted 30 July 2010 Available online 22 August 2010
We report the field emission properties of 50 nm diameter ZnO nanowires that were electrochemically self-assembled within pores of an anodic alumina film. Current versus voltage measurements of the Fowler–Nordheim tunneling current indicate that the turn-on field is 15 V mm 1 and the field enhancement factor b calculated for two sets of samples turned out to be 670 and 1481, respectively (theoretical value for b in bulk ZnO is 205). The enhancement in b is most likely due to the large aspect ratio of the nanowires. Variation in the length of the nanowires causes the screening effect, which increases the turn-on field and reduces the emission current density. The current–voltage characteristics of the wires are non-linear and show a rectifying behavior. & 2010 Elsevier B.V. All rights reserved.
1. Introduction Semi-conducting and semi-metallic nanostructures such as nanorods, nanowires, nanotubes, etc. have recently attracted significant attention because of their potential applications in vacuum electronics [1–4]. High aspect ratio nanowire semiconductor materials are excellent candidates for field emitters because of the unique tip geometry and apex structures that can enhance the local field emission at the nanowire tips. By varying the radius of curvature of the tips, it is possible to increase the local electric field at the tip and thus drastically reduce the turn-on voltage while at the same time increase the field emission current. This has established nanowires as the primary choice for field emitters. Extensive research has been carried out on carbon nanotubes (CNT) based field emitters, which exhibit high field enhancement factor and low turn-on voltage [5,6]. Rare earth monosulfide (e.g. LaS) nanowires are also excellent field emitters because of the low work function of the monosulfides [7,8]. Recently, ZnO has become a popular material for field emitters owing to its negative electron affinity, high melting point and chemical stability in vacuum environment [9,10]. ZnO nanowires can be fabricated by different methods such as VLS technique, chemical vapor deposition technique, template assisted methods [11–13], etc. These fabrication techniques unfortunately have a major shortcoming that nanowires can bundle up with each other and it is very hard to control their diameter precisely. These shortcomings can be overcome if we adopt a unique fabrication technique using porous anodic alumina templates to grow highly ordered ZnO nanowires n
Corresponding author. E-mail address: [email protected] (S. Ramanathan). 1 Author is currently with Intel Corporation, Hillsboro, Oregon; USA.
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.07.072
within the pores using the DC electrochemical deposition technique. The aspect ratio of the nanowires is 19:1. The nanowire (active emitter) density achievable in this approach varies from 109 to 1011 /cm2 depending on the technique adopted to anodize aluminum. If the anodization is carried out in oxalic acid, the pore (and hence the nanowire) diameter will be 50 nm and the emitter density will be 109 /cm2. On the other hand, if the anodization is performed in sulfuric acid, the pore (and nanowire) diameter becomes 10 nm and the emitter density increases to 1011 /cm2 [14]. A very high density, however, may be counter-productive since there may be mutual screening among densely packed nanowires that reduces the emission efficiency. Here, we report the field emission results of the tapered ZnO nanowires produced in anodic films prepared by oxalic acid anodization. The wire diameter is 50 nm and the wire density is 109 /cm2. We have also reported their I–V characteristics.
2. Tapered ZnO nanowire fabrication Electrochemical self-assembly of ZnO nanowires in porous anodic alumina films essentially consists of four steps: (1) anodization of an aluminum foil in 0.3 M oxalic acid with a dc voltage of 40 V to produce an alumina film containing a well-regimented array of 50 nm diameter pores, (2) removal of the barrier layer at the bottom of the pores using a reverse polarity etching process [15], which exposes the underlying aluminum at the pore bottom, (3) selective electrodeposition of Zn in the pores from solution of ZnSO4 using an ac voltage source, and (4) oxidation of Zn to ZnO by soaking in H2O2. Details of these steps can be found in Ref. [16]. After synthesis of the wires is complete, we etch the surface of the host alumina film slightly so that the nanowire tips are exposed and protrude over the surface. For this
286
S. Ramanathan et al. / Physica E 43 (2010) 285–288
purpose, the sample is etched in hot chromic/phosphoric acid for 3 min, followed by repeated rinsing in D.I water and air-drying. These samples are then imaged with an atomic force microscope (AFM). Fig. 1 is an AFM micrograph that clearly shows the exposed ZnO nanowire tips (conical in shape), which cause local field enhancement at the tip and reduce the turn-on voltage for field emission, while at the same time increasing the emission current. Cross-sectional imaging (Fig. 2) showed that the tips extend 30 nm above the surface. 3. Electrical characterization of tapered ZnO nanowires For measuring the current-voltage characteristics, electrical contacts are made at the top and bottom of the ZnO nanowires. Contact from the top is made by evaporating a gold film, and contact from the bottom is made by connecting to the aluminum substrate. I–V measurement is carried out using a Semiconductor Parameter Analyzer (HP 4156). We observe a non-linear rectifying characteristic behavior due to the metal/semiconductor/metal junctions. This is due to the two Schottky barrier junctions connected in series at the end of the nanowires. Fig. 3 shows the I–V characteristics of ZnO nanowires recorded at room temperature. Fig. 1. Topographical AFM image of a sample showing the tapered ZnO nanowires protruding from some of the pores.
4. Field emission characterization Field emission measurement was carried out in a vacuum chamber with an ambient pressure of 1.9 10 6 Torr at 300 K.
Fig. 2. Based on the sectional analysis we can assume the height of the exposed nanowires protruding above the surface of the host is 30 nm.
S. Ramanathan et al. / Physica E 43 (2010) 285–288
Fig. 3. Current–voltage characteristic showing non-linear rectifying behavior of ZnO nanowires.
The spacer has an opening to limit the area of emission and is replaced after each use in order to prevent any contamination from building up on the sidewalls. The opening was typically 2 mm 2 mm, and the gap distance ‘d’ between the nanowire tip and the scanning anode collector was maintained at 150 mm. A negative voltage was applied on the sample (cathode) while the collector was maintained at zero potential. The applied electric field was increased in terms of voltage with steps of 50 V, and the emission current was recorded using a pico-ammeter and digitally stored. Fig. 3 shows the field emission plot for two different samples. When the applied field reaches 15 V mm 1 or higher, the emission current increases rapidly, indicating that the turn-on field is 15 V mm 1. In some samples, no appreciable emission current was observed even when a high negative voltage was applied, possibly due to the incomplete removal of the barrier layer at the pore bottom or perhaps due to species absorbed on the surface. The insulating barrier layer is removed by a reverse polarity etching procedure [15], which may not always be successful. It is also important to note that when a large area of the sample is investigated during field emission measurement, the length of nanowires within such a large area can vary significantly (few hundred nm to few mm). This causes screening effects whereby electric field lines emanating from the shorter nanowires due to the applied voltage penetrate into the field lines of taller nanowire emitters, thereby reducing the local electric field strength and the emission current. This has the effect of increasing the threshold voltage for the onset of field emission [17]. Screening is likely the reason why we did not see higher emission current in our tapered ZnO nanowires. Field emitters emit electrons into the vacuum by the Fowler–Nordheim tunneling. The current–voltage relation obeys the Fowler–Nordheim (FN) equation. ! 3=2 aV 2 bdf J¼ 2 exp ð1Þ bV b d2 f where J is the current density (A/m2), V the applied voltage, f the work function of ZnO material 5.3 eV, b the geometric field enhancement factor due to the tapering of the emitter tips, a ¼1.56 10 10 A eV V 2 and b¼6.83 109 V eV 3/2m 1. By plotting ln (I/V2) versus (1/V), we can find b. Such a plot is called the Fowler–Nordheim plot. Fig. 4 shows the measured (I–V) characteristic in vacuum and Fig. 5 is the corresponding
287
Fig. 4. Field emission current–voltage characteristics of the tapered ZnO nanowires recorded for two sets of samples (red and black).
Fig. 5. Fowler–Nordheim plot of the field emission current. From the slope of the linear region, we can estimate the field enhancement factor. The data is shown for two sets of samples (red and black).
Fowler–Nordheim plot for the tapered ZnO nanowires. In the linear region of this plot, the slope is given by 3=2
slope ¼
bf
b
d
ð2Þ
where d is the inter-electrode distance, b¼6.83 109 V eV 3/2m 1 and f the work function of ZnO material (5.3 eV) [18,19]. From the measured slope, we estimated the enhancement factor b for two sets of samples and found them to be 670 and 1481, respectively. Ref. [9] reports an empirical relation between the field enhancement factor b, the inter-electrode distance d, the tip’s radius of curvature r and the screening factor s (ranging 0–1). It is given by b ffi 1þ s d=r ð3Þ If we make a reasonable assumption that the radius of curvature of the tip is 25 nm (which is the radius of the wire) and take the value of b to be 1075 (which is the average of the two values calculated from the slope of the F–N plot), we obtain that
288
S. Ramanathan et al. / Physica E 43 (2010) 285–288
the screening factor s¼0.18. This indicates significant screening due to varying length of the nanowires (the smaller the value of s, the greater the screening).
5. Conclusion In conclusion, we report the field emission measurement of the tapered ZnO nanowires arrays fabricated by the electrochemical self-assembly technique. Based on our results we conclude that screening effect caused by major length variation in nanowires has direct impact on the emission current. It reduces the field enhancement factor and thus also reduces the emission current density. The situation might improve if we use dc electrodeposition of Zn within the pores instead of ac electrodeposition since the former often produces more uniform wire length than the latter.
Acknowledgement The work at Virginia Commonwealth University was supported by the US National Science Foundation under grant ECCS-0523966. I would like to thank my professor Supriyo Bandyopadhyay for his guidance in this project and Chris Moore for his help with AFM imaging.
Reference [1] Zhong Lin Wang, J. Phys.: Condens. Matter 16 (2004) R829. [2] S. Ramanathan, S. Patibandla, S. Bandyopadhyay, J. Anderson, J.D. Edwards, Nanotechnology 19 (2008) 195601. [3] M. Cahay, et al., J. Vac. Sci. Technol. B. 25 (2007) 594. [4] J.C. Liao, D.F. Li, C. Wang, C.S. Liu, Q. Liu, Fu, L.X. Fan, Nanotechnolgy 16 (2005) 985. [5] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1197. [6] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [7] M. Cahay, K. Garre, J.W. Fraser, D.J. Lockwood, V. Semet, B.-T. Binh, B. Kanchibotla, S. Bandyopadhyay, L. Grazulis, B. Das, J. Vac. Sci. Technol. B. 26 (2008) 885. [8] V. Semet, V.T. Binh, M. Cahay, K. Garre, J.W. Fraser, D.J. Lockwood, S. Pramanik, B. Kanchibotla, S. Bandyopadhyay, S. Fairchild L. Grazhulis, J. Nanomat. (2008) Article ID: 682920. [9] Q. Zhao, H.Z. Zhang, Y.W. Zhu, S.Q. Feng, X.C. Sun, J. Xu, D.P. Yu, Appl. Phys. Lett. 86 (2005) 203115. [10] Debasish Banerjee, Sung Ho Jo, Zhi Feng Ren, Adv. Mat. 16 (2004) 2028. [11] M. Senthil Kumar, T.Y. Kim, J.Y. Kim, E.-K. Suh, K.S. Nahm, Phys. Stat. Sol. (c) 1 (2004) 2554. [12] Sang-Woo Kima, Shizuo Fujita, Shigeo Fujita, Appl. Phys. Lett. 86 (2005) 153119. [13] Hong Jin Fan, Woo Lee, Robert Hauschild, Marin Alexe, Gwena Le Rhun, Roland Scholz, Armin Dadgar, Kornelius Nielsch, Heinz Kalt, Alois Krost, Margit Zacharias, Ulrich Gc- sele, Small 2 (2006) 561. [14] S. Bandyopadhyay, et al., Nanotechnology 7 (1996) 360. [15] O. Rabin, P.R. Herz, S.B. Cronin, Y.-M. Lin, A.I. Akinwande, M. Dresselhaus, Mat. Res. Soc. Symp. 636 (2001) D. 4.7.1. [16] S. Ramanathan, S. Patibandla, S. Bandyopadhyay, J.D. Edwards, J. Anderson, J. Mater. Sci.: Mater. Electron 17 (2006) 651. [17] Yung-Chiang Lan, MingXun Yan, Wen-Jen Liu, J. Vac. Sci. Technol. B. 25 (2007) 497. [18] X.D. Bai, E.G. Wang, P.X. Gao, Z.L. Wang, Nano. Lett. 3 (2003) 1147. [19] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (2002) 3648.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Zinc oxide nanowire based field emitters Sivakumar Ramanathan a,n,1, Yu-chun Chen b, Yonhua Tzeng b a b
Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284, USA Department of Electrical Engineering, Auburn University, Auburn, Al 36849, USA
a r t i c l e in fo
abstract
Article history: Received 5 April 2010 Received in revised form 29 July 2010 Accepted 30 July 2010 Available online 22 August 2010
We report the field emission properties of 50 nm diameter ZnO nanowires that were electrochemically self-assembled within pores of an anodic alumina film. Current versus voltage measurements of the Fowler–Nordheim tunneling current indicate that the turn-on field is 15 V mm 1 and the field enhancement factor b calculated for two sets of samples turned out to be 670 and 1481, respectively (theoretical value for b in bulk ZnO is 205). The enhancement in b is most likely due to the large aspect ratio of the nanowires. Variation in the length of the nanowires causes the screening effect, which increases the turn-on field and reduces the emission current density. The current–voltage characteristics of the wires are non-linear and show a rectifying behavior. & 2010 Elsevier B.V. All rights reserved.
1. Introduction Semi-conducting and semi-metallic nanostructures such as nanorods, nanowires, nanotubes, etc. have recently attracted significant attention because of their potential applications in vacuum electronics [1–4]. High aspect ratio nanowire semiconductor materials are excellent candidates for field emitters because of the unique tip geometry and apex structures that can enhance the local field emission at the nanowire tips. By varying the radius of curvature of the tips, it is possible to increase the local electric field at the tip and thus drastically reduce the turn-on voltage while at the same time increase the field emission current. This has established nanowires as the primary choice for field emitters. Extensive research has been carried out on carbon nanotubes (CNT) based field emitters, which exhibit high field enhancement factor and low turn-on voltage [5,6]. Rare earth monosulfide (e.g. LaS) nanowires are also excellent field emitters because of the low work function of the monosulfides [7,8]. Recently, ZnO has become a popular material for field emitters owing to its negative electron affinity, high melting point and chemical stability in vacuum environment [9,10]. ZnO nanowires can be fabricated by different methods such as VLS technique, chemical vapor deposition technique, template assisted methods [11–13], etc. These fabrication techniques unfortunately have a major shortcoming that nanowires can bundle up with each other and it is very hard to control their diameter precisely. These shortcomings can be overcome if we adopt a unique fabrication technique using porous anodic alumina templates to grow highly ordered ZnO nanowires n
Corresponding author. E-mail address: [email protected] (S. Ramanathan). 1 Author is currently with Intel Corporation, Hillsboro, Oregon; USA.
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.07.072
within the pores using the DC electrochemical deposition technique. The aspect ratio of the nanowires is 19:1. The nanowire (active emitter) density achievable in this approach varies from 109 to 1011 /cm2 depending on the technique adopted to anodize aluminum. If the anodization is carried out in oxalic acid, the pore (and hence the nanowire) diameter will be 50 nm and the emitter density will be 109 /cm2. On the other hand, if the anodization is performed in sulfuric acid, the pore (and nanowire) diameter becomes 10 nm and the emitter density increases to 1011 /cm2 [14]. A very high density, however, may be counter-productive since there may be mutual screening among densely packed nanowires that reduces the emission efficiency. Here, we report the field emission results of the tapered ZnO nanowires produced in anodic films prepared by oxalic acid anodization. The wire diameter is 50 nm and the wire density is 109 /cm2. We have also reported their I–V characteristics.
2. Tapered ZnO nanowire fabrication Electrochemical self-assembly of ZnO nanowires in porous anodic alumina films essentially consists of four steps: (1) anodization of an aluminum foil in 0.3 M oxalic acid with a dc voltage of 40 V to produce an alumina film containing a well-regimented array of 50 nm diameter pores, (2) removal of the barrier layer at the bottom of the pores using a reverse polarity etching process [15], which exposes the underlying aluminum at the pore bottom, (3) selective electrodeposition of Zn in the pores from solution of ZnSO4 using an ac voltage source, and (4) oxidation of Zn to ZnO by soaking in H2O2. Details of these steps can be found in Ref. [16]. After synthesis of the wires is complete, we etch the surface of the host alumina film slightly so that the nanowire tips are exposed and protrude over the surface. For this
286
S. Ramanathan et al. / Physica E 43 (2010) 285–288
purpose, the sample is etched in hot chromic/phosphoric acid for 3 min, followed by repeated rinsing in D.I water and air-drying. These samples are then imaged with an atomic force microscope (AFM). Fig. 1 is an AFM micrograph that clearly shows the exposed ZnO nanowire tips (conical in shape), which cause local field enhancement at the tip and reduce the turn-on voltage for field emission, while at the same time increasing the emission current. Cross-sectional imaging (Fig. 2) showed that the tips extend 30 nm above the surface. 3. Electrical characterization of tapered ZnO nanowires For measuring the current-voltage characteristics, electrical contacts are made at the top and bottom of the ZnO nanowires. Contact from the top is made by evaporating a gold film, and contact from the bottom is made by connecting to the aluminum substrate. I–V measurement is carried out using a Semiconductor Parameter Analyzer (HP 4156). We observe a non-linear rectifying characteristic behavior due to the metal/semiconductor/metal junctions. This is due to the two Schottky barrier junctions connected in series at the end of the nanowires. Fig. 3 shows the I–V characteristics of ZnO nanowires recorded at room temperature. Fig. 1. Topographical AFM image of a sample showing the tapered ZnO nanowires protruding from some of the pores.
4. Field emission characterization Field emission measurement was carried out in a vacuum chamber with an ambient pressure of 1.9 10 6 Torr at 300 K.
Fig. 2. Based on the sectional analysis we can assume the height of the exposed nanowires protruding above the surface of the host is 30 nm.
S. Ramanathan et al. / Physica E 43 (2010) 285–288
Fig. 3. Current–voltage characteristic showing non-linear rectifying behavior of ZnO nanowires.
The spacer has an opening to limit the area of emission and is replaced after each use in order to prevent any contamination from building up on the sidewalls. The opening was typically 2 mm 2 mm, and the gap distance ‘d’ between the nanowire tip and the scanning anode collector was maintained at 150 mm. A negative voltage was applied on the sample (cathode) while the collector was maintained at zero potential. The applied electric field was increased in terms of voltage with steps of 50 V, and the emission current was recorded using a pico-ammeter and digitally stored. Fig. 3 shows the field emission plot for two different samples. When the applied field reaches 15 V mm 1 or higher, the emission current increases rapidly, indicating that the turn-on field is 15 V mm 1. In some samples, no appreciable emission current was observed even when a high negative voltage was applied, possibly due to the incomplete removal of the barrier layer at the pore bottom or perhaps due to species absorbed on the surface. The insulating barrier layer is removed by a reverse polarity etching procedure [15], which may not always be successful. It is also important to note that when a large area of the sample is investigated during field emission measurement, the length of nanowires within such a large area can vary significantly (few hundred nm to few mm). This causes screening effects whereby electric field lines emanating from the shorter nanowires due to the applied voltage penetrate into the field lines of taller nanowire emitters, thereby reducing the local electric field strength and the emission current. This has the effect of increasing the threshold voltage for the onset of field emission [17]. Screening is likely the reason why we did not see higher emission current in our tapered ZnO nanowires. Field emitters emit electrons into the vacuum by the Fowler–Nordheim tunneling. The current–voltage relation obeys the Fowler–Nordheim (FN) equation. ! 3=2 aV 2 bdf J¼ 2 exp ð1Þ bV b d2 f where J is the current density (A/m2), V the applied voltage, f the work function of ZnO material 5.3 eV, b the geometric field enhancement factor due to the tapering of the emitter tips, a ¼1.56 10 10 A eV V 2 and b¼6.83 109 V eV 3/2m 1. By plotting ln (I/V2) versus (1/V), we can find b. Such a plot is called the Fowler–Nordheim plot. Fig. 4 shows the measured (I–V) characteristic in vacuum and Fig. 5 is the corresponding
287
Fig. 4. Field emission current–voltage characteristics of the tapered ZnO nanowires recorded for two sets of samples (red and black).
Fig. 5. Fowler–Nordheim plot of the field emission current. From the slope of the linear region, we can estimate the field enhancement factor. The data is shown for two sets of samples (red and black).
Fowler–Nordheim plot for the tapered ZnO nanowires. In the linear region of this plot, the slope is given by 3=2
slope ¼
bf
b
d
ð2Þ
where d is the inter-electrode distance, b¼6.83 109 V eV 3/2m 1 and f the work function of ZnO material (5.3 eV) [18,19]. From the measured slope, we estimated the enhancement factor b for two sets of samples and found them to be 670 and 1481, respectively. Ref. [9] reports an empirical relation between the field enhancement factor b, the inter-electrode distance d, the tip’s radius of curvature r and the screening factor s (ranging 0–1). It is given by b ffi 1þ s d=r ð3Þ If we make a reasonable assumption that the radius of curvature of the tip is 25 nm (which is the radius of the wire) and take the value of b to be 1075 (which is the average of the two values calculated from the slope of the F–N plot), we obtain that
288
S. Ramanathan et al. / Physica E 43 (2010) 285–288
the screening factor s¼0.18. This indicates significant screening due to varying length of the nanowires (the smaller the value of s, the greater the screening).
5. Conclusion In conclusion, we report the field emission measurement of the tapered ZnO nanowires arrays fabricated by the electrochemical self-assembly technique. Based on our results we conclude that screening effect caused by major length variation in nanowires has direct impact on the emission current. It reduces the field enhancement factor and thus also reduces the emission current density. The situation might improve if we use dc electrodeposition of Zn within the pores instead of ac electrodeposition since the former often produces more uniform wire length than the latter.
Acknowledgement The work at Virginia Commonwealth University was supported by the US National Science Foundation under grant ECCS-0523966. I would like to thank my professor Supriyo Bandyopadhyay for his guidance in this project and Chris Moore for his help with AFM imaging.
Reference [1] Zhong Lin Wang, J. Phys.: Condens. Matter 16 (2004) R829. [2] S. Ramanathan, S. Patibandla, S. Bandyopadhyay, J. Anderson, J.D. Edwards, Nanotechnology 19 (2008) 195601. [3] M. Cahay, et al., J. Vac. Sci. Technol. B. 25 (2007) 594. [4] J.C. Liao, D.F. Li, C. Wang, C.S. Liu, Q. Liu, Fu, L.X. Fan, Nanotechnolgy 16 (2005) 985. [5] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1197. [6] S. Fan, M.G. Chapline, N.R. Franklin, T.W. Tombler, A.M. Cassell, H. Dai, Science 283 (1999) 512. [7] M. Cahay, K. Garre, J.W. Fraser, D.J. Lockwood, V. Semet, B.-T. Binh, B. Kanchibotla, S. Bandyopadhyay, L. Grazulis, B. Das, J. Vac. Sci. Technol. B. 26 (2008) 885. [8] V. Semet, V.T. Binh, M. Cahay, K. Garre, J.W. Fraser, D.J. Lockwood, S. Pramanik, B. Kanchibotla, S. Bandyopadhyay, S. Fairchild L. Grazhulis, J. Nanomat. (2008) Article ID: 682920. [9] Q. Zhao, H.Z. Zhang, Y.W. Zhu, S.Q. Feng, X.C. Sun, J. Xu, D.P. Yu, Appl. Phys. Lett. 86 (2005) 203115. [10] Debasish Banerjee, Sung Ho Jo, Zhi Feng Ren, Adv. Mat. 16 (2004) 2028. [11] M. Senthil Kumar, T.Y. Kim, J.Y. Kim, E.-K. Suh, K.S. Nahm, Phys. Stat. Sol. (c) 1 (2004) 2554. [12] Sang-Woo Kima, Shizuo Fujita, Shigeo Fujita, Appl. Phys. Lett. 86 (2005) 153119. [13] Hong Jin Fan, Woo Lee, Robert Hauschild, Marin Alexe, Gwena Le Rhun, Roland Scholz, Armin Dadgar, Kornelius Nielsch, Heinz Kalt, Alois Krost, Margit Zacharias, Ulrich Gc- sele, Small 2 (2006) 561. [14] S. Bandyopadhyay, et al., Nanotechnology 7 (1996) 360. [15] O. Rabin, P.R. Herz, S.B. Cronin, Y.-M. Lin, A.I. Akinwande, M. Dresselhaus, Mat. Res. Soc. Symp. 636 (2001) D. 4.7.1. [16] S. Ramanathan, S. Patibandla, S. Bandyopadhyay, J.D. Edwards, J. Anderson, J. Mater. Sci.: Mater. Electron 17 (2006) 651. [17] Yung-Chiang Lan, MingXun Yan, Wen-Jen Liu, J. Vac. Sci. Technol. B. 25 (2007) 497. [18] X.D. Bai, E.G. Wang, P.X. Gao, Z.L. Wang, Nano. Lett. 3 (2003) 1147. [19] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (2002) 3648.