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Journal of Nanomaterials
Volume 2012, Article ID 369763, 6 pages
http://dx.doi.org/10.1155/2012/369763
Research Article

Core-Shell Structure of a Silicon Nanorod/Carbon Nanotube Field Emission Cathode

1Graduate Institute of Electro-Optical Engineering and Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
2Department of Electronic Engineering, Feng Chia University, Taichung 407, Taiwan

Received 30 June 2011; Revised 23 August 2011; Accepted 24 August 2011

Academic Editor: Linbao Luo

Copyright © 2012 Bohr-Ran Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A novel core-shell structure of silicon nanorods/carbon nanotubes (SiNRs/CNTs) is developed for use in field emission cathodes. The CNTs were synthesized on SiNRs, using the Ag-assisted electroless etching technique to form the SiNRs/CNT core-shell structure. This resulting SiNRs/CNT field emission cathode demonstrated improved field emission properties including a lower turn-on electric field 𝐸 o n (1.3 V/μm, 1 μA/cm2), a lower threshold electric field 𝐸 t h (1.8 V/μm, 1 mA/cm2), and a higher enhancement factor 𝛽 (2347). These superior properties indicate that this core-shell structure of SiNRs/CNTs has good potential in field emission cathode applications.

1. Introduction

Carbon nanotubes (CNTs) have unique physical and chemical properties such as a high aspect ratio, small radius of curvature, exceptional chemical inertness, excellent environmental stability, and high mechanical strength [15]. Given their high field emission current density at low electric field and highly stable current, CNTs can be applied to field emission cathodes [6]. However, the field emission properties of densely packed CNTs are affected by the field-screening effect among neighboring nanotubes [7]. Patterning the alignment of CNTs has been found to improve field emission properties [811]. Combining CNTs and other nonplanar substrates has also been used to improve field emission properties, for example, on tungsten tips [12], nanocrystalline diamond films [13], and silicon nanostructures [1417]. CNTs have been grown on the top of silicon nanowires (SiNWs), performing the 𝐸 o n of 2 V/μm (10 μA/cm2) [15]. These structures could be used to effectively improve field emission properties. CNT field emission cathodes can be fabricated by various methods such as direct growth, electrophoresis, screen printing, the spray method, and composite plating [1822].

In this work, the CNTs were grown directly onto the SiNRs, forming the core-shell structure of a SiNRs/CNTs field emission cathode by thermal chemical vapor deposition. Investigation of the field emission properties of SiNRs, CNTs, and the SiNRs/CNTs field emission cathode showed that this core-shell structure of SiNRs/CNTs improves the cathode field emission properties.

2. Experimental

The SiNRs were synthesized on a p-type (1–10 Ωcm, B doped, 520 um) Cz silicon (100) wafer using the Ag-assisted electroless etching technique [23]. First, the wafer was cleaned ultrasonically for 20 min in both acetone and isopropyl alcohol. The cleaned silicon wafer was then immersed in a mixture of 5 mol/L aqueous hydrofluoric acid (HF) and 0.02 mol/L silver nitrate (AgNO3) solution for 3 hours at room temperature. Following the electroless etching process, the high-density tree-like dendritic structures of silver films were removed using 30 wt% HNO3 aqueous solution for 60 s. Finally, the samples were rinsed with deionized water and air-dried.

The as-prepared SiNRs and silicon substrate were deposited with iron (Fe) catalyst layers and immediately placed in a quartz tube furnace to grow CNTs. Furnace conditions were 700°C at atmospheric pressure with a mixed gas of N2 and C2H2 (10 : 3) using the thermal chemical vapor deposition method.

The surface morphologies of films were characterized by field emission scanning electron microscopy (FESEM, JEOL JSM-6700F, operated at 15 kV) and transmission electron microscopy (TEM, Philips Tecnai F20 G2 FEI-TEM). The bonding structures of CNTs were analyzed using micro-Raman spectroscopy with excitation of spectrum 514 nm. The field emission measurements were performed using parallel-plate geometry with a gap of 150 μm between the anode (copper) and the cathode (samples) in a vacuum chamber with a base pressure of about 1 × 10−6 Torr. A Keithley 237 was used to provide variable dc voltages and collect electric current across the specimen, and the emission measurement area was 10 × 10 mm2. Figure 1 schematically diagrams the procedures for fabricating the core-shell structure of the SiNRs/CNTs field emission cathode.

369763.fig.001
Figure 1: Schematic diagram of the procedures for fabricating the core-shell structure of SiNRs/CNTs field emission cathodes.

3. Result and Discussion

Figure 2 shows the SEM images of the surface and cross-section morphology of the SiNRs, CNTs, and SiNRs/CNTs films. During the long etching time the silicon nanowires (SiNWs) take on a bundled structure, which is attributed to the strong SiNWs van der Waals attraction [24]. As a result, in this study, the SiNWs bundle structure is denominated the SiNRs. The SiNRs are separated from each other and are held perpendicular to the sample surface at an average height of ~17 μm. A large quantity of highly aligned CNTs film was deposited on the silicon substrate, as shown in Figure 2(b). Ten min growth produces a CNTs film with a thickness of ~26 μm. The physical deposition is limited to the nanoscale which causes discontinuous deposition of the Fe catalyst layer on the SiNRs. This suggests that the top of the SiNRs is covered by a thicker Fe layer than their sidewalls and bottom. Hence, the numerous entangled CNTs form the nest-shaped assemblages of CNTs on the top of each SiNRs, and the CNTs are also synthesized on the SiNRs side-walls to form a core-shell structure, as shown in Figure 2(c).

fig2
Figure 2: SEM images of surface and cross-section morphology of (a) SiNRs, (b) CNTs, and (c) SiNRs/CNTs.

Figures 3(a)3(c) show the TEM images of SiNRs, CNTs, and SiNRs/CNTs. Figure 3(b) shows a multiwall nanotube with a hollow structure and reveals the well-ordered lattice fringes of the nanotube. Figure 3(c) shows that the SiNRs are surrounded by CNTs. Figure 3(d) shows the magnified schematic diagram of the core-shell structure of SiNRs/CNTs.

fig3
Figure 3: TEM images of (a) SiNR, (b) CNT, and (c) core-shell structure of SiNRs/CNTs, and (d) schematic diagram of the SiNRs/CNTs.

The Raman spectrum of CNTs typically has two characteristic peaks at around 1350 and 1580 cm−1 [25]. The 1580 cm−1 peak can be identified as the G band of crystalline graphite arising from its zone-center 𝐸 2 g mode. For graphite-like materials with defects, the 1350 cm−1 peak is identified as the D band, which is activated due to defects in and disorder of the carbonaceous material. Thus, the smaller relative intensity ratio of the D and G bands ( 𝐼 D / 𝐼 G ) implies a better graphite structure and a higher degree of graphitization. In comparison with the localized electrons in 𝜎 bonds on CNTs, the delocalized electrons in the 𝜋 orbital of the sp2 bond have a higher degree of mobility and are more easily emitted from CNTs [26]. Therefore, greater sp2 bonding (i.e., lower 𝐼 D / 𝐼 G ratio) would improve the field emission properties [27]. Figure 4 shows the Raman spectra of the CNTs and the SiNRs/CNTs, with respective 𝐼 D / 𝐼 G ratios of about 0.59 and 0.55. The core-shell structure does not affect the graphitization of the CNTs, which makes them applicable to field emission devices.

369763.fig.004
Figure 4: Raman spectra of (a) CNTs and (b) SiNRs/CNTs.

Figure 5 shows the current density-electric field ( 𝐽 𝐸 ) plot and the corresponding Fowler-Nordheim (FN) plot. According to the FN equation, the emission current density 𝐽 = 𝐴 ( 𝛽 𝐸 ) 2 e x p ( 𝐵 𝜙 3 / 2 / 𝛽 𝐸 ) , where 𝐴 and 𝐵 are constants, 𝐸 is the applied electric field, 𝛽 is the enhancement factor, and 𝜙 is the work function of 5 and 4.15 eV for CNTs and SiNRs, respectively [15, 28]. The respective 𝐸 o n at a current density of 1 μA/cm2 for the SiNRs, CNTs, and SiNRs/CNTs field emission cathodes is 4.5, 2.4, and 1.3 V/μm. The respective 𝐸 t h at a current density of 1 mA/cm2 for CNTs and SiNRs/CNTs field emission cathodes is 4.2 and 1.8 V/μm. The respective enhancement factor 𝛽 of the SiNRs, CNTs, and SiNRs/CNTs field emission cathodes is estimated at about 1154, 1675, and 2347.

fig5
Figure 5: Field emission properties of the SiNRs, CNTs, and SiNRs/CNTs cathodes: (a) current density versus electric field ( 𝐽 𝐸 ) plots; (b) Fowler-Nordheim plots.

The field emission property with 𝐸 t h of the core-shell structure of SiNRs/CNTs is comparable to that found in previous reports. Li and Jiang. deposited CNTs on a silicon nanoporous pillar array prepared by hydrothermal etching technique [16], which provided good field emission properties with the 𝐸 t h about 1.9 V/μm. Qinke et al. synthesized CNTs on the top of a SiNW array prepared by Ag-assisted electroless etching [15], which showed the 𝐸 t h at about 3.3 V/μm. CNTs only synthesized on the top of the SiNW array, which failed to effectively enhance the field emission properties. The SiNRs/CNTs field emission cathode shows not only a high enhancement factor but also low 𝐸 o n and low 𝐸 t h . The screening effect of densely packed CNTs produced among neighboring nanotubes resulted in disappointing CNTs field emission properties [29]. The core-shell structure of the SiNRs/CNTs reduces the density of CNTs, thus further decreasing the screening effect and improving the field emission properties. In addition, we suggest that the SiNRs/CNTs structure could reduce the work function below 5 eV. Consistent with previous reports, the lower work function of SiNRs/CNTs field emission cathode also enhances field emission properties, resulting in a higher current density, lower turn-on electric field, and higher enhancement factor [30, 31]. However, excellent field emission properties were achieved in the CNTs field emission cathode with a core-shell structure using SiNRs. Further studies are needed to determine the optimum SiNRs diameter and CNTs film thickness for the core-shell structure of SiNRs/CNTs cathode to promote field emission properties.

4. Conclusion

We developed a novel core-shell structure of SiNRs/CNTs for field emission cathodes, effectively improving field emission properties including an 𝐸 o n of 1.3 V/μm, 𝐸 t h of 1.8 V/μm, and 𝛽 of 2347. These improvements were attributed to the SiNRs/CNTs core-shell structure reducing the screening effect and work function. These results indicate that the core-shell structure of SiNRs/CNTs might be suitable for improved cathode of field emission displays.

Acknowledgment

This work was supported by the National Science Council of Taiwan under Grant no. 99-2221-E-011-065.

References

  1. S. Iijima, “Helical microtubules of graphitic carbon,” Nature, vol. 354, no. 6348, pp. 56–58, 1991. View at Google Scholar · View at Scopus
  2. B. I. Yakobson, C. J. Brabec, and J. Bernholc, “Nanomechanics of carbon tubes: instabilities beyond linear response,” Physical Review Letters, vol. 76, no. 14, pp. 2511–2514, 1996. View at Google Scholar
  3. K. H. An, S. Y. Jeong, H. R. Hwang, and Y. H. Lee, “Enhanced sensitivity of a gas sensor incorporating single-walled carbon nanotube-polypyrrole nanocomposites,” Advanced Materials, vol. 16, no. 12, pp. 1005–1009, 2004. View at Publisher · View at Google Scholar · View at Scopus
  4. H. Usui, H. Matsui, N. Tanabe, and S. Yanagida, “Improved dye-sensitized solar cells using ionic nanocomposite gel electrolytes,” Journal of Photochemistry and Photobiology A, vol. 164, no. 1-3, pp. 97–101, 2004. View at Publisher · View at Google Scholar · View at Scopus
  5. W. A. De Heer, A. Châtelain, and D. Ugarte, “A carbon nanotube field-emission electron source,” Science, vol. 270, no. 5239, pp. 1179–1180, 1995. View at Google Scholar · View at Scopus
  6. Y. Liu and S. Fan, “Field emission properties of carbon nanotubes grown on silicon nanowire arrays,” Solid State Communications, vol. 133, no. 2, pp. 131–134, 2005. View at Publisher · View at Google Scholar · View at Scopus
  7. L. Nilsson, O. Groening, C. Emmenegger et al., “Scanning field emission from patterned carbon nanotube films,” Applied Physics Letters, vol. 76, no. 15, pp. 2071–2073, 2000. View at Google Scholar · View at Scopus
  8. H. J. Jeong, S. C. Lim, K. S. Kim, and Y. H. Lee, “Edge effect on the field emission properties from vertically aligned carbon nanotube arrays,” Carbon, vol. 42, no. 14, pp. 3036–3039, 2004. View at Publisher · View at Google Scholar · View at Scopus
  9. J. S. Suh, K. S. Jeong, J. S. Lee, and I. Han, “Study of the field-screening effect of highly ordered carbon nanotube arrays,” Applied Physics Letters, vol. 80, no. 13, p. 2392, 2002. View at Publisher · View at Google Scholar · View at Scopus
  10. R. C. Smith and S. R. P. Silva, “Maximizing the electron field emission performance of carbon nanotube arrays,” Applied Physics Letters, vol. 94, no. 13, Article ID 133104, 3 pages, 2009. View at Publisher · View at Google Scholar
  11. S. H. Jo, Y. Tu, Z. P. Huang, D. L. Carnahan, D. Z. Wang, and Z. F. Ren, “Effect of length and spacing of vertically aligned carbon nanotubes on field emission properties,” Applied Physics Letters, vol. 82, no. 20, pp. 3520–3522, 2003. View at Publisher · View at Google Scholar · View at Scopus
  12. P. N. Hong, B. H. Thang, N. T. Hong, S. Lee, and P. N. Minh, “Electron field emission characteristics of carbon nanotube on tungsten tip,” Journal of Physics: Conference Series, vol. 187, no. 1, 5 pages, 2009. View at Publisher · View at Google Scholar
  13. K. J. Liao, W. L. Wang, C. Z. Cai, J. W. Lu, and C. G. Hu, “Investigation on field electron emission from carbon nanotubes on nanocrystalline diamond films,” Journal of Metastable and Nanocrystalline Materials, vol. 23, pp. 35–38, 2005. View at Google Scholar · View at Scopus
  14. K. Matsumoto, S. Kinosita, Y. Gotoh, T. Uchiyama, S. Manalis, and C. Quate, “Ultralow biased field emitter using single-wall carbon nanotube directly grown onto silicon tip by thermal chemical vapor deposition,” Applied Physics Letters, vol. 78, no. 4, pp. 539–540, 2001. View at Publisher · View at Google Scholar · View at Scopus
  15. S. Qinke, J. Wei, K. Wang et al., “Fabrication and field emission properties of multi-walled carbon nanotube/silicon nanowire array,” Journal of Physics and Chemistry of Solids, vol. 71, no. 4, pp. 708–711, 2010. View at Publisher · View at Google Scholar · View at Scopus
  16. X. J. Li and W. F. Jiang, “Enhanced field emission from a nest array of multi-walled carbon nanotubes grown on a silicon nanoporous pillar array,” Nanotechnology, vol. 18, no. 6, 5 pages, 2007. View at Publisher · View at Google Scholar
  17. J. Li, W. Lei, X. Zhang, B. Wang, and L. Ba, “Field emission of vertically-aligned carbon nanotube arrays grown on porous silicon substrate,” Solid-State Electronics, vol. 48, no. 12, pp. 2147–2151, 2004. View at Publisher · View at Google Scholar · View at Scopus
  18. Z. W. Pan, S. S. Xie, B. H. Chang, L. F. Sun, W. Y. Zhou, and G. Wang, “Direct growth of aligned open carbon nanotubes by chemical vapor deposition,” Chemical Physics Letters, vol. 299, no. 1, pp. 97–102, 1999. View at Google Scholar · View at Scopus
  19. A. R. Boccaccini, J. Cho, J. A. Roether, B. J. C. Thomas, E. Jane Minay, and M. S. P. Shaffer, “Electrophoretic deposition of carbon nanotubes,” Carbon, vol. 44, no. 15, pp. 3149–3160, 2006. View at Publisher · View at Google Scholar · View at Scopus
  20. Y. S. Shi, C. C. Zhu, W. Qikun, and L. Xin, “Large area screen-printing cathode of CNT for FED,” Diamond and Related Materials, vol. 12, no. 9, pp. 1449–1452, 2003. View at Publisher · View at Google Scholar · View at Scopus
  21. Y. D. Lee, K. S. Lee, Y. H. Lee, and B. K. Ju, “Field emission properties of carbon nanotube film using a spray method,” Applied Surface Science, vol. 254, no. 2, pp. 513–516, 2007. View at Publisher · View at Google Scholar · View at Scopus
  22. B. R. Huang, T. C. Lin, Y. K. Yang, and S. D. Tzeng, “The stability of the CNT/Ni field emission cathode fabricated by the composite plating method,” Diamond and Related Materials, vol. 19, no. 2-3, pp. 158–161, 2010. View at Publisher · View at Google Scholar · View at Scopus
  23. T. Qiu and P. K. Chu, “Self-selective electroless plating: an approach for fabrication of functional 1D nanomaterials,” Materials Science and Engineering R, vol. 61, no. 1-6, pp. 59–77, 2008. View at Publisher · View at Google Scholar · View at Scopus
  24. K. Zhu, T. B. Vinzant, N. R. Neale, and A. J. Frank, “Removing structural disorder from oriented TiO2 nanotube arrays: reducing the dimensionality of transport and recombination in dye-sensitized solar cells,” Nano Letters, vol. 7, no. 12, pp. 3739–3746, 2007. View at Publisher · View at Google Scholar · View at Scopus
  25. M. S. Dresselhaus, G. Dresselhaus, and P. H. Avouris, Carbon Nanotubes: Synthesis, Properties and Applications, vol. 80 of Springer Series in Topics in Applied Physics, Springer, Berlin, Germany, 2001.
  26. S. Han and J. Ihm, “Role of the localized states in field emission of carbon nanotubes,” Physical Review B, vol. 61, no. 15, pp. 9986–9989, 2000. View at Google Scholar · View at Scopus
  27. K. F. Chen, K. C. Chen, Y. C. Jiang et al., “Field emission image uniformity improvement by laser treating carbon nanotube powders,” Applied Physics Letters, vol. 88, no. 19, Article ID 193127, 3 pages, 2006. View at Publisher · View at Google Scholar
  28. F. Zhao, D. D. Zhao, S. L. Wu, G. A. Cheng, and R. T. Zheng, “Fabrication and electron field emission of silicon nanowires synthesized by chemical etching,” Journal of the Korean Physical Society, vol. 55, no. 6, pp. 2681–2684, 2009. View at Publisher · View at Google Scholar · View at Scopus
  29. L. Nilsson, O. Groening, C. Emmenegger et al., “Scanning field emission from patterned carbon nanotube films,” Applied Physics Letters, vol. 76, no. 15, pp. 2071–2073, 2000. View at Google Scholar · View at Scopus
  30. Z. S. Hu, F. Y. Hung, S. J. Chang et al., “Nanostructural characteristics of oxide-cap GaN nanotips by iodine-gallium ions etching,” Journal of Alloys and Compounds, vol. 509, no. 5, pp. 2360–2363, 2011. View at Publisher · View at Google Scholar
  31. Y. F. Tzeng, Y. C. Lee, C. Y. Lee, H. T. Chiu, and I. N. Lin, “Electron field emission properties on UNCD coated Si-nanowires,” Diamond and Related Materials, vol. 17, no. 4-5, pp. 753–757, 2008. View at Publisher · View at Google Scholar · View at Scopus