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Improved emission stability of HfC-coated carbon nanotubes field emitters
Solid State Communications 135 (2005) 390–393 www.elsevier.com/locate/ssc
Improved emission stability of HfC-coated carbon nanotubes field emitters Jun Jiang*, Jihua Zhang, Tao Feng, Bingyao Jiang, Yongjin Wang, Fumin Zhang, Lijuan Dai, Xi Wang, Xianghuai Liu, Shichang Zou The Research Center of Semiconductor Functional Film Engineering Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865, Changning Road, Shanghai 200050, China Received 22 December 2004; received in revised form 3 March 2005; accepted 3 May 2005 by A.K. Sood Available online 31 May 2005
Abstract Plasma-enhanced chemical vapor deposition (PECVD) method was employed to synthesize the Fe-catalyzed carbon nanotubes (CNTs). Hf films were deposited onto the synthesized CNTs, followed by heat treatment at 1200 8C which could form HfC. Field emission properties indicate that the HfC-coated CNTs have good emission current density due to low work function of HfC and also keep stable emission characteristics under poor vacuum owing to the chemical inertness of HfC. Consequently, field emission characteristics of the CNTs can be improved by the HfC-coated surface treatment compared with the synthesized CNTs. q 2005 Elsevier Ltd. All rights reserved. PACS: 79.70.Cq; 72.80.Rj Keywords: A. Carbon nanotubes; A. HfC; D. Chemical vapor deposition; D. Field emission
Since their discovery in 1991 [1], carbon nanotubes (CNTs) have aroused great attention all over the world due to their superior mechanical strength [2], varying electronic properties [3], large surface area for adsorption of hydrogen [4], and their chemical stability [5,6]. They are known for their field emission properties with electron emission at low macroscopic electric fields due to high aspect ratio. Therefore, CNTs can be applied to field emitters for field emission displays and vacuum microelectronic devices like microwave power amplifier tubes, nano-field effect transistors, nano-Schottky diodes [7–11]. However, CNTs have still several issues to be solved for field emission display. These issues involve their current densities, the uniformities of the emission and high reliabilities, and so on. In order to * Corresponding author. Tel.: C86 21 62511070; fax: C86 21 62513510. E-mail address: [email protected] (J. Jiang).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.05.018
improve emission characteristics of CNTs, several methods of surface treatment are employed, involving Ar plasma treatment [12], hydrogen plasma treatment [13], oxygen/ ozone oxidation process [14], and so on. It is well known that emission current strongly depends on the field enhancement factor and the work function according to the Fowler–Nordheim theory. The effective approaches for good emission current are to increase field enhancement factor and to lower work function. CNTs have good electron emission property due to their high field enhancement factor. While Shiraishi et al. reported that the work functions of multi- and single-wall carbon nanotubes were 4.95 and 5.05 eV [15], meaning that their work functions are very high. The transition metal carbides (e.g. TiC, ZrC, HfC, NbC, TaC, and so on) have extremely high melting point, good chemical inertness, good resistant to ion bombardment and low work function of 3.5 eV, about 1 eV lower than the commonly used field emission cathode
J. Jiang et al. / Solid State Communications 135 (2005) 390–393
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material tungsten [16]. They have been widely applied in Spindt field emitter arrays (FEAs). HfC has a lower work function by several tenths on an electron volt compared with ZrC [17]. In this paper, we employed plasma-enhanced chemical vapor deposition (PECVD) method to synthesize the Fe-catalyzed CNTs, then deposited Hf film onto the synthesized CNTs by ion beam assisted deposition method. By anneal at high temperature, the reactions between Hf and carbon atom formed HfC. Their field emission characteristics after and before the treatment were investigated. The 10 nm thick Fe film was only deposited onto p-type silicon substrate using electron-beam evaporation method. The CNTs were synthesized by Fe catalytic decomposition of acetylene in a PECVD system with a base pressure below 10K4 Pa. Three series samples were transferred to the growth chamber, which was pumped down to its base pressure, then were heated to 600 8C. Before growing the CNTs, hydrogen was introduced to pretreatment the Fe catalytic film for 10 min with a flow rate of 75 sccm. The CNTs were synthesized using a mixed flow of acetylene and hydrogen with a flow rate of 75 sccm via a separated mass flow controller. The C2H2:H2 ratio was kept constant at 15:60 sccm at a total pressure of 11.5 Pa. The deposition time was 15 min. After the deposition, the reactor was cooled down to room temperature. Hf was deposited onto the synthesized CNTs by ion beam assisted deposition method. The thickness of Hf film was about 5 nm. The samples coated with Hf were heated to 1200 8C and kept 2 h at a base pressure of 10K5 Pa. X-ray diffraction (XRD) was performed to identify the surface substances on D/max 2550V using Cu Ka radiation. XSAM800 X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state. Field emission characteristics of the CNTs were examined at room temperature in a vacuum below 10K5 Pa. The distance between cathode and anode was fixed 200 mm. Fig. 1 shows the typical XRD pattern of the Hf-coated CNTs after anneal at 1200 8C. The HfC peaks are observed,
indicating that during anneal the reactions between Hf and carbon atom formed HfC. XPS was used for further analysis of the Hf-coated CNTs after anneal at 1200 8C, as is shown in Fig. 2. The peak positions of the XPS spectra of C1s are 280.9 and 285.2 eV, respectively. The left C1s peak at 280.9 eV is consistent with the C1s peak at 280.8 eV in HfC given in XPS handbook. Consequently, we can conclude that carbon react with the coated Hf to form HfC at 1200 8C. The right C1s peak at 285.2 eV corresponds to the binding energy of CNTs [18]. For thermal treatment at 1600 8C, the structures of CNTs were not destroyed [19]. Therefore,
Fig. 1. The typical XRD pattern of the Hf-coated CNTs after anneal at 1200 8C.
Fig. 2. XPS spectra of the Hf-coated CNTs after anneal at 1200 8C for 2 h, (a) C1s, (b) Hf 4d, (c) O1s.
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CNTs have good thermal stability at the anneal temperature of 1200 8C. The Hf 4d peak positions in Fig. 2(b) are 213.4 and 224.2 eV, which is close to the Hf peak position 213.2 eV of Hf 4d in HfO2 given in XPS handbook. This confirms that Hf can react with O to form HfO2 during anneal at 1200 8C. Meanwhile, as shown in Fig. 2(c), the broad O1s peak ranging from 525 to 535 eV in the XPS spectra is well-fitted by two components that have two chemical states. The left O1s peak position is close to the peak position of O1s in HfO2, as is supported in Fig. 2(b). While the right O1s peak is identify as oxygen, showing that oxygen is adsorbed onto the surface of the HfC-coated CNTs emitters. Fig. 3 shows electron emission current density versus applied electric field curve from the CNTs and the corresponding Fowler–Nordheim (FN) plots. The emission current of the HfC-coated CNTs increases markedly after anneal at 1200 8C compared with the synthesized CNTs. Meanwhile, the emission current of the Hf-coated CNTs is lower than that of the synthesized CNTs films. It can be seen from FN plots in Fig. 3(b) that the slope of the FN plot is different in low and high electric field regions. Such a deviation from FN behavior may be attributed to the spacecharge effect, gas absorption, or the interaction between the neighbor CNTs tips [23]. The creation of the Hfcontaminated cluster could take part in the electron emission and lower electron current intensity as well as increase the
Fig. 3. Electron emission current density versus applied electric field curve from the CNTs and corresponding Fowler–Nordheim (FN) plots. (a) the synthesized CNTs, (b) the Hf-coated CNTs, (c) the Hf-coated CNTs, annealed at 1200 8C.
value of the starting threshold field, and also cause instability in emission process [20]. The open tip structure of carbon nanotubes plays a role of field emission current density without a low field [21]. The lower emission of the Hf-coated CNTs may be attributed to the coated Hf, which causes the creation of the Hf-contaminated cluster and its close tip structure. HfC is a refractory material with a melting point of about 2928 8C and has NaCl crystalline structure. It is electrically conducting and has a work function of about 3.6 eV [22]. Since the work functions of multi- and single-wall carbon nanotubes were 4.95 and 5.05 eV [15]. When the Hf-coated CNTs were heated to 1200 8C for 2 h, the reaction between Hf and carbon atom formed HfC which lower the work function. Meanwhile the surface of the HfC-coated CNTs gets cleaned after anneal and high current emissions density shows the intrinsic emission performance after the cleaning emitters by Jule heating. As a result the formed HfC can improve the electron emission characteristics. Fig. 4 shows that emission current of the Hf-coated CNTs after anneal at 1200 8C for 2 h as a function of pressure. The time of emission current stabilization was about 10 min during field emission measurement. Emission current density almost keeps constant at 10K2–10K5 Pa, indicating that they have good chemical inertness and emission reliability at high pressure. When the pressure increases further, the emission current increases greatly due to the discharge of the gas at high pressure. Emission currents of the conventional CNTs are not stable due to the adsorption of the residual gas and the ion bombardment to the tip. While the fluctuation of emission currents in the HfC-coated CNTs after anneal at 1200 8C under high pressure can be fully suppressed. In summary, the HfC-coated CNTs can be fabricated by PECVD method, followed by the deposition of Hf film onto the synthesized CNTs film and anneal at 1200 8C for 2 h at a base pressure of 10K5 Pa. The formed HfC can greatly improve emission current density of the CNTs field emitter
Fig. 4. Emission current of the Hf-coated CNTs after anneal at 1200 8C for 2 h as a function of pressure.
J. Jiang et al. / Solid State Communications 135 (2005) 390–393
due to its low work function, and also keep stable emission characteristics at below pressure of 10K2 Pa owing to its chemical inertness. Consequently, the HfC-coated CNTs field emitter with good emission characteristics and reliability at poor vacuum is a very promising application in field emission display.
Acknowledgements This work was financially supported by the grant from the National Natural Science Foundation of China (No. 59972039), the grant form the 973-Programme of the Ministry of Science and Technology of China (No. G2000067207-2), and the grant from the Science and Technology Committee of Shanghai (No. 0452 nm048).
References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature (London) 381 (1996) 687. [3] N. Hamada, S. Sawada, A. Oshiyama, Phys. Rev. Lett. 68 (1992) 1579. [4] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature (London) 386 (1997) 377. [5] W.A. de Heer, A. Chatelain, D. Vagaret, Science 270 (1995) 1179. [6] S.S. Fan, M.G. Chaplind, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Science 283 (1999) 512.
393
[7] J.C. She, N.S. Xu, S.Z. Deng, J. Chen, H. Bishop, S.E. Huq, L. Wang, D.Y. Zhong, E.G. Wang, Appl. Phys. Lett. 83 (2003) 2671. [8] C. Bower, W. Zhu, D. Shalom, D. lopez, L.H. Chen, P.L. Gammel, S. Jin, Appl. Phys. Lett. 80 (2002) 3820. [9] J. Guo, M. Lundstrom, S. Datta, Appl. Phys. Lett. 80 (2002) 3192. [10] S.J. Wind, J. Appenzeller, R. Martel, V. Deryche, Ph. Avouris, Appl. Phys. Lett. 80 (2002) 3817. [11] S.H. Jo, Y. Tu, Z.P. Huang, D.L. Carnahan, J.Y. Huang, D.Z. Wang, Z.F. Ren, Appl. Phys. Lett. 84 (2004) 413. [12] Y. Kanazawa, T. Oyama, L. Murakami, M. Takai, J. Vac. Sci. Technol. B 22 (2004) 1342. [13] C.Y. Zhi, X.D. Bai, E.G. Wang, Appl. Phys. Lett. 81 (2002) 1690. [14] S.C. Kung, K.C. Hwang, I.N. Lin, Appl. Phys. Lett. 80 (2002) 4819. [15] M. Shiraishi, M. Ata, Carbon 39 (2001) 1913. [16] W.A. Mackie, R.L. Hartman, M.A. Anderson, P.R. Davis, J. Vac. Sci. Technol. B 12 (1994) 722. [17] W.A. Mackie, T.B. Xie, J.E. Blackwood, S.C. Williams, P.R. Davis, J. Vac. Sci. Technol. B 16 (1998) 1215. [18] T.I.T. Okalugo, P. Papakonstantinou, H. Murphy, J. Mclaughlin, N.M.D. Brown, Carbon 43 (2005) 153. [19] K. Metenier, S. Bonnamy, F. Beguin, C. Journet, P. Bernier, M.L. Chapelle, O. Chauvet, S. Lefrant, Carbon 40 (2002) 1765. [20] E. Kowalska, E. Czerwosz, P.A. Dluzewski, M. Kozlowski, J. Radomska, Diamond Relat. Mater. 13 (2004) 1008. [21] J.I. Sohn, S. Lee, Y.H. Song, S.Y. Choi, K.L. Cho, K.S. Nam, Appl. Phys. Lett. 78 (2001) 901. [22] M.L. Yu, B.W. Hussey, E. Kratschmer, T.H.P. Chang, W.A. Mackie, J. Vac. Sci. Technol. B 13 (1995) 2436. [23] P.G. Collins, Z. Zettl, Phys. Rev. B 55 (1997) 9391.
Improved emission stability of HfC-coated carbon nanotubes field emitters Jun Jiang*, Jihua Zhang, Tao Feng, Bingyao Jiang, Yongjin Wang, Fumin Zhang, Lijuan Dai, Xi Wang, Xianghuai Liu, Shichang Zou The Research Center of Semiconductor Functional Film Engineering Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865, Changning Road, Shanghai 200050, China Received 22 December 2004; received in revised form 3 March 2005; accepted 3 May 2005 by A.K. Sood Available online 31 May 2005
Abstract Plasma-enhanced chemical vapor deposition (PECVD) method was employed to synthesize the Fe-catalyzed carbon nanotubes (CNTs). Hf films were deposited onto the synthesized CNTs, followed by heat treatment at 1200 8C which could form HfC. Field emission properties indicate that the HfC-coated CNTs have good emission current density due to low work function of HfC and also keep stable emission characteristics under poor vacuum owing to the chemical inertness of HfC. Consequently, field emission characteristics of the CNTs can be improved by the HfC-coated surface treatment compared with the synthesized CNTs. q 2005 Elsevier Ltd. All rights reserved. PACS: 79.70.Cq; 72.80.Rj Keywords: A. Carbon nanotubes; A. HfC; D. Chemical vapor deposition; D. Field emission
Since their discovery in 1991 [1], carbon nanotubes (CNTs) have aroused great attention all over the world due to their superior mechanical strength [2], varying electronic properties [3], large surface area for adsorption of hydrogen [4], and their chemical stability [5,6]. They are known for their field emission properties with electron emission at low macroscopic electric fields due to high aspect ratio. Therefore, CNTs can be applied to field emitters for field emission displays and vacuum microelectronic devices like microwave power amplifier tubes, nano-field effect transistors, nano-Schottky diodes [7–11]. However, CNTs have still several issues to be solved for field emission display. These issues involve their current densities, the uniformities of the emission and high reliabilities, and so on. In order to * Corresponding author. Tel.: C86 21 62511070; fax: C86 21 62513510. E-mail address: [email protected] (J. Jiang).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.05.018
improve emission characteristics of CNTs, several methods of surface treatment are employed, involving Ar plasma treatment [12], hydrogen plasma treatment [13], oxygen/ ozone oxidation process [14], and so on. It is well known that emission current strongly depends on the field enhancement factor and the work function according to the Fowler–Nordheim theory. The effective approaches for good emission current are to increase field enhancement factor and to lower work function. CNTs have good electron emission property due to their high field enhancement factor. While Shiraishi et al. reported that the work functions of multi- and single-wall carbon nanotubes were 4.95 and 5.05 eV [15], meaning that their work functions are very high. The transition metal carbides (e.g. TiC, ZrC, HfC, NbC, TaC, and so on) have extremely high melting point, good chemical inertness, good resistant to ion bombardment and low work function of 3.5 eV, about 1 eV lower than the commonly used field emission cathode
J. Jiang et al. / Solid State Communications 135 (2005) 390–393
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material tungsten [16]. They have been widely applied in Spindt field emitter arrays (FEAs). HfC has a lower work function by several tenths on an electron volt compared with ZrC [17]. In this paper, we employed plasma-enhanced chemical vapor deposition (PECVD) method to synthesize the Fe-catalyzed CNTs, then deposited Hf film onto the synthesized CNTs by ion beam assisted deposition method. By anneal at high temperature, the reactions between Hf and carbon atom formed HfC. Their field emission characteristics after and before the treatment were investigated. The 10 nm thick Fe film was only deposited onto p-type silicon substrate using electron-beam evaporation method. The CNTs were synthesized by Fe catalytic decomposition of acetylene in a PECVD system with a base pressure below 10K4 Pa. Three series samples were transferred to the growth chamber, which was pumped down to its base pressure, then were heated to 600 8C. Before growing the CNTs, hydrogen was introduced to pretreatment the Fe catalytic film for 10 min with a flow rate of 75 sccm. The CNTs were synthesized using a mixed flow of acetylene and hydrogen with a flow rate of 75 sccm via a separated mass flow controller. The C2H2:H2 ratio was kept constant at 15:60 sccm at a total pressure of 11.5 Pa. The deposition time was 15 min. After the deposition, the reactor was cooled down to room temperature. Hf was deposited onto the synthesized CNTs by ion beam assisted deposition method. The thickness of Hf film was about 5 nm. The samples coated with Hf were heated to 1200 8C and kept 2 h at a base pressure of 10K5 Pa. X-ray diffraction (XRD) was performed to identify the surface substances on D/max 2550V using Cu Ka radiation. XSAM800 X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state. Field emission characteristics of the CNTs were examined at room temperature in a vacuum below 10K5 Pa. The distance between cathode and anode was fixed 200 mm. Fig. 1 shows the typical XRD pattern of the Hf-coated CNTs after anneal at 1200 8C. The HfC peaks are observed,
indicating that during anneal the reactions between Hf and carbon atom formed HfC. XPS was used for further analysis of the Hf-coated CNTs after anneal at 1200 8C, as is shown in Fig. 2. The peak positions of the XPS spectra of C1s are 280.9 and 285.2 eV, respectively. The left C1s peak at 280.9 eV is consistent with the C1s peak at 280.8 eV in HfC given in XPS handbook. Consequently, we can conclude that carbon react with the coated Hf to form HfC at 1200 8C. The right C1s peak at 285.2 eV corresponds to the binding energy of CNTs [18]. For thermal treatment at 1600 8C, the structures of CNTs were not destroyed [19]. Therefore,
Fig. 1. The typical XRD pattern of the Hf-coated CNTs after anneal at 1200 8C.
Fig. 2. XPS spectra of the Hf-coated CNTs after anneal at 1200 8C for 2 h, (a) C1s, (b) Hf 4d, (c) O1s.
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CNTs have good thermal stability at the anneal temperature of 1200 8C. The Hf 4d peak positions in Fig. 2(b) are 213.4 and 224.2 eV, which is close to the Hf peak position 213.2 eV of Hf 4d in HfO2 given in XPS handbook. This confirms that Hf can react with O to form HfO2 during anneal at 1200 8C. Meanwhile, as shown in Fig. 2(c), the broad O1s peak ranging from 525 to 535 eV in the XPS spectra is well-fitted by two components that have two chemical states. The left O1s peak position is close to the peak position of O1s in HfO2, as is supported in Fig. 2(b). While the right O1s peak is identify as oxygen, showing that oxygen is adsorbed onto the surface of the HfC-coated CNTs emitters. Fig. 3 shows electron emission current density versus applied electric field curve from the CNTs and the corresponding Fowler–Nordheim (FN) plots. The emission current of the HfC-coated CNTs increases markedly after anneal at 1200 8C compared with the synthesized CNTs. Meanwhile, the emission current of the Hf-coated CNTs is lower than that of the synthesized CNTs films. It can be seen from FN plots in Fig. 3(b) that the slope of the FN plot is different in low and high electric field regions. Such a deviation from FN behavior may be attributed to the spacecharge effect, gas absorption, or the interaction between the neighbor CNTs tips [23]. The creation of the Hfcontaminated cluster could take part in the electron emission and lower electron current intensity as well as increase the
Fig. 3. Electron emission current density versus applied electric field curve from the CNTs and corresponding Fowler–Nordheim (FN) plots. (a) the synthesized CNTs, (b) the Hf-coated CNTs, (c) the Hf-coated CNTs, annealed at 1200 8C.
value of the starting threshold field, and also cause instability in emission process [20]. The open tip structure of carbon nanotubes plays a role of field emission current density without a low field [21]. The lower emission of the Hf-coated CNTs may be attributed to the coated Hf, which causes the creation of the Hf-contaminated cluster and its close tip structure. HfC is a refractory material with a melting point of about 2928 8C and has NaCl crystalline structure. It is electrically conducting and has a work function of about 3.6 eV [22]. Since the work functions of multi- and single-wall carbon nanotubes were 4.95 and 5.05 eV [15]. When the Hf-coated CNTs were heated to 1200 8C for 2 h, the reaction between Hf and carbon atom formed HfC which lower the work function. Meanwhile the surface of the HfC-coated CNTs gets cleaned after anneal and high current emissions density shows the intrinsic emission performance after the cleaning emitters by Jule heating. As a result the formed HfC can improve the electron emission characteristics. Fig. 4 shows that emission current of the Hf-coated CNTs after anneal at 1200 8C for 2 h as a function of pressure. The time of emission current stabilization was about 10 min during field emission measurement. Emission current density almost keeps constant at 10K2–10K5 Pa, indicating that they have good chemical inertness and emission reliability at high pressure. When the pressure increases further, the emission current increases greatly due to the discharge of the gas at high pressure. Emission currents of the conventional CNTs are not stable due to the adsorption of the residual gas and the ion bombardment to the tip. While the fluctuation of emission currents in the HfC-coated CNTs after anneal at 1200 8C under high pressure can be fully suppressed. In summary, the HfC-coated CNTs can be fabricated by PECVD method, followed by the deposition of Hf film onto the synthesized CNTs film and anneal at 1200 8C for 2 h at a base pressure of 10K5 Pa. The formed HfC can greatly improve emission current density of the CNTs field emitter
Fig. 4. Emission current of the Hf-coated CNTs after anneal at 1200 8C for 2 h as a function of pressure.
J. Jiang et al. / Solid State Communications 135 (2005) 390–393
due to its low work function, and also keep stable emission characteristics at below pressure of 10K2 Pa owing to its chemical inertness. Consequently, the HfC-coated CNTs field emitter with good emission characteristics and reliability at poor vacuum is a very promising application in field emission display.
Acknowledgements This work was financially supported by the grant from the National Natural Science Foundation of China (No. 59972039), the grant form the 973-Programme of the Ministry of Science and Technology of China (No. G2000067207-2), and the grant from the Science and Technology Committee of Shanghai (No. 0452 nm048).
References [1] S. Iijima, Nature (London) 354 (1991) 56. [2] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature (London) 381 (1996) 687. [3] N. Hamada, S. Sawada, A. Oshiyama, Phys. Rev. Lett. 68 (1992) 1579. [4] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature (London) 386 (1997) 377. [5] W.A. de Heer, A. Chatelain, D. Vagaret, Science 270 (1995) 1179. [6] S.S. Fan, M.G. Chaplind, N.R. Franklin, T.W. Tombler, A.M. Cassell, H.J. Dai, Science 283 (1999) 512.
393
[7] J.C. She, N.S. Xu, S.Z. Deng, J. Chen, H. Bishop, S.E. Huq, L. Wang, D.Y. Zhong, E.G. Wang, Appl. Phys. Lett. 83 (2003) 2671. [8] C. Bower, W. Zhu, D. Shalom, D. lopez, L.H. Chen, P.L. Gammel, S. Jin, Appl. Phys. Lett. 80 (2002) 3820. [9] J. Guo, M. Lundstrom, S. Datta, Appl. Phys. Lett. 80 (2002) 3192. [10] S.J. Wind, J. Appenzeller, R. Martel, V. Deryche, Ph. Avouris, Appl. Phys. Lett. 80 (2002) 3817. [11] S.H. Jo, Y. Tu, Z.P. Huang, D.L. Carnahan, J.Y. Huang, D.Z. Wang, Z.F. Ren, Appl. Phys. Lett. 84 (2004) 413. [12] Y. Kanazawa, T. Oyama, L. Murakami, M. Takai, J. Vac. Sci. Technol. B 22 (2004) 1342. [13] C.Y. Zhi, X.D. Bai, E.G. Wang, Appl. Phys. Lett. 81 (2002) 1690. [14] S.C. Kung, K.C. Hwang, I.N. Lin, Appl. Phys. Lett. 80 (2002) 4819. [15] M. Shiraishi, M. Ata, Carbon 39 (2001) 1913. [16] W.A. Mackie, R.L. Hartman, M.A. Anderson, P.R. Davis, J. Vac. Sci. Technol. B 12 (1994) 722. [17] W.A. Mackie, T.B. Xie, J.E. Blackwood, S.C. Williams, P.R. Davis, J. Vac. Sci. Technol. B 16 (1998) 1215. [18] T.I.T. Okalugo, P. Papakonstantinou, H. Murphy, J. Mclaughlin, N.M.D. Brown, Carbon 43 (2005) 153. [19] K. Metenier, S. Bonnamy, F. Beguin, C. Journet, P. Bernier, M.L. Chapelle, O. Chauvet, S. Lefrant, Carbon 40 (2002) 1765. [20] E. Kowalska, E. Czerwosz, P.A. Dluzewski, M. Kozlowski, J. Radomska, Diamond Relat. Mater. 13 (2004) 1008. [21] J.I. Sohn, S. Lee, Y.H. Song, S.Y. Choi, K.L. Cho, K.S. Nam, Appl. Phys. Lett. 78 (2001) 901. [22] M.L. Yu, B.W. Hussey, E. Kratschmer, T.H.P. Chang, W.A. Mackie, J. Vac. Sci. Technol. B 13 (1995) 2436. [23] P.G. Collins, Z. Zettl, Phys. Rev. B 55 (1997) 9391.