Effects of Al interlayer coating and thermal treatment on electron emission characteristics of carbon nanotubes deposited by electrophoretic method
© Kim et al.; licensee Springer. 2014
Received: 4 November 2013
Accepted: 16 April 2014
Published: 13 May 2014
The effects of aluminum (Al) interlayer coating and thermal post-treatment on the electron emission characteristics of carbon nanotubes (CNTs) were investigated. These CNTs were deposited on conical-shaped tungsten (W) substrates using an electrophoretic method. The Al interlayers were coated on the W substrates via magnetron sputtering prior to the deposition of CNTs. Compared with the as-deposited CNTs, the thermally treated CNTs revealed significantly improved electron emission characteristics, such as the decrease of turn-on electric fields and the increase of emission currents. The observations of Raman spectra confirmed that the improved emission characteristics of the thermally treated CNTs were ascribed to their enhanced crystal qualities. The coating of Al interlayers played a role in enhancing the long-term emission stabilities of the CNTs. The thermally treated CNTs with Al interlayers sustained stable emission currents without any significant degradation even after continuous operation of 20 h. The X-ray photoelectron spectroscopy (XPS) study suggested that the cohesive forces between the CNTs and the underlying substrates were strengthened by the coating of Al interlayers.
Recently, field emitters using carbon nanotubes (CNTs) have been utilized as cold electron sources for high-resolution X-ray apparatuses[1–3]. To use CNTs as electron sources, the turn-on electric field that triggers the field-driven electron emission must be low, and the generated emission current level must be high. Simultaneously, the stability of the emission current must be ensured during a long-term operation. Here, CNTs can be prepared on various types of substrates such as flat types and tip types either by direct[4–6] or indirect[7–10] methods. Practically, the indirect methods have certain advantages over the direct methods due to their simpler deposition systems, lower costs, lower processing temperatures, and easier scale-up. However, the indirect methods demonstrate weak adhesion often with the widely utilized metallic substrates[11, 12]. Under a prolonged emission condition, CNTs may be removed on substrates due to their weak adhesion. This makes it difficult to obtain uniform and consistent emission currents from the CNT emitter. For this reason, most of the indirect methods have employed flat-type substrates in preparing CNTs. The use of flat-type substrates, on the other hand, would be less desirable than the use of tip-type substrates for the application of CNTs as electron sources for micro-focus X-ray systems. Therefore, the combination of tip-type substrates and indirect deposition methods is recommended for such application of CNTs only if good adhesion and high levels of emitted currents are guaranteed. Regarding this issue, we have suggested the use of interlayer with hafnium (Hf) thin films between CNTs and tungsten (W) tips.
This study aims at fabricating tip-type CNT emitters that have good adhesion and illustrate high levels of emission currents. This has been achieved by depositing CNTs on conical-shaped tip-type W substrates via electrophoretic deposition, by coating interlayers with aluminum (Al) thin films and by performing thermal treatment. The effects of thermal treatment as well as Al interlayer coating on the electron emission behavior and long-term emission stability of CNTs have been investigated extensively.
Identification of the CNT emitters considered in this study
Results and discussion
For all of the CNTs, the changes in the surface morphologies due to thermal treatment and Al interlayer coating were monitored by using a field emission scanning electron microscope (FESEM; JSM-6330 F, JEOL, Tokyo, Japan). The FESEM images of the exterior shapes and the enlarged surfaces for the CNT-A (without Al interlayer) and CNT-C (with Al interlayer) emitters are compared in Figure 1. It seemed that no significant differences in their surface morphologies were observed. It was also observed in this study that thermal treatment hardly affected the surface morphologies of the CNTs, although their FESEM images are not displayed in Figure 1. This may indicate that neither the coating of Al interlayer nor the thermal treatment altered the structural aspect ratios of the CNTs. Also, this may be in good agreement with the results that the β values were similar for all of the CNTs.
The conical-type CNT-based field emitters were fabricated using the EPD method. Substantially, enhanced emission characteristics, such as lower turn-on voltage and higher emission currents, were obtained by thermally treating the CNTs. From the FESEM observations as well as from the electrical measurements of emission characteristics, the thermal treatment barely affected the CNTs' surface morphologies and field enhancement factors. The observations of the Raman spectra confirmed that the improved emission characteristics of the thermally treated CNTs were ascribed to their higher degrees of crystallinities. In addition, the long-term emission stabilities of the CNTs were significantly ameliorated by coating Al interlayers prior to the deposition of CNTs. The CNTs, when deposited on the Al interlayers and thermally treated, exhibited highly stable electron emission behaviors without any significant degradation of emission currents even after 20 h of operation. The XPS results indicated that the improved adhesion of CNT-D was ascribed to the increase of Al-O bonds and the creation of Al-C bonds by thermal treatment. This may diminish the possibility of electric arcing at the W tip and also enhance the W tip's robustness against melting, which may eventually lead to the improved long-term emission stability of the CNTs. It was also reported by our previous work that the emission stabilities of CNTs deposited on the W tips coated with Hf interlayer were improved only when the CNTs were thermally treated. This was due to the formation of carbide bonds (Hf-C) at elevated temperature. In this study, the CNTs using Al interlayers showed that the enhanced emission stabilities were observed not only for the thermally annealed CNTs but also for the as-deposited CNTs without thermal treatment. This was because oxide bonds (Al-O) already existed in the as-deposited CNTs, while carbide bonds (Al-C) were observed for the thermally annealed CNTs.
BJK is currently a Ph.D. student of Electronic Systems Engineering Department in Hanyang University. His research focuses on the application of carbon nanotube in X-ray system and transparent conductive films. JPK, Ph.D., is currently working in Health & Medical Equipment Business Team, Samsung Electronics. JSP, Ph.D., is a professor of Electronic Systems Engineering Department in Hanyang University. He is currently interested in flexible transparent displays and carbon and related nano-devices.
This research was supported by BK21PLUS program through the National Research Foundation of Korea funded by the Ministry of Education. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2012008365).
- Liu Z, Zhang J, Yang G, Cheng Y, Zhou O, Lu J: Development of a carbon nanotube based microfocus X-ray tube with single focusing electrode. Rev Sci Instrum 2006, 77: 054302–1-054302–6.Google Scholar
- Kawakita K, Hata K, Sato H, Saito Y: Development of microfocused X-ray source by using carbon nanotube field emitter. J Vac Sci Tech B Microelectron Nanometer Struct 2006, 24: 950–952. 10.1116/1.2183785View ArticleGoogle Scholar
- Musatov AL, Gulyaev YV, Izrael'Yants KR, Ormont AB, Chirkova EG, Maslennikov OY, Guzilov IA, Kiselev NA, Kukovitsky EF: Properties of field electron emitter based on carbon nanotubes installed in the small-sized X-ray tube. Fuller Nanotube Carbon Nanostruct 2011, 19: 69–74.View ArticleGoogle Scholar
- Koohsorkhi J, Abdi Y, Mohajerzadeh S, Hosseinzadegan H, Komijani Y, Soleimani EA: Fabrication of self-defined gated field emission devices on silicon substrates using PECVD-grown carbon nano-tubes. Carbon 2006, 44: 2797–2803. 10.1016/j.carbon.2006.03.038View ArticleGoogle Scholar
- Terrado E, Redrado M, Muñoz E, Maser WK, Benito AM, Martínez MT: Aligned carbon nanotubes grown on alumina and quartz substrates by a simple thermal CVD process. Diamond Relat Mater 2006, 15: 1059–1063. 10.1016/j.diamond.2005.10.071View ArticleGoogle Scholar
- Pandey A, Prasad A, Moscatello JP, Engelhard M, Wang C, Yap YK: Very stable electron field emission from strontium titanate coated carbon nanotube matrices with low emission thresholds. ACS Nano 2013, 7: 117–125. 10.1021/nn303351gView ArticleGoogle Scholar
- Boccaccini AR, Cho J, Roether JA, Thomas BJC, Jane Minay E, Shaffer MSP: Electrophoretic deposition of carbon nanotubes. Carbon 2006, 44: 3149–3160. 10.1016/j.carbon.2006.06.021View ArticleGoogle Scholar
- Song YI, Kim GY, Choi HK, Jeong HJ, Kim KK, Yang CM, Lim SC, An KH, Jung KT, Lee YH: Fabrication of carbon nanotube field emitters using a dip-coating method. Chem Vap Depos 2006, 12: 375–379. 10.1002/cvde.200506442View ArticleGoogle Scholar
- Yu J, Chen J, Deng SZ, Xu NS: Field emission characteristics of screen-printed carbon nanotubes cold cathode by hydrogen plasma treatment. Appl Surf Sci 2011, 258: 738–742. 10.1016/j.apsusc.2011.08.027View ArticleGoogle Scholar
- Jung H, Van Quy N, Hoa ND, Kim D: Transparent field emission device from a spray coating of single-wall carbon nanotubes. J Electrochem Soc 2010, 157: J371-J375. 10.1149/1.3485039View ArticleGoogle Scholar
- Lim SC, Choi HK, Jeong HJ, Song YI, Kim GY, Jung KT, Lee YH: A strategy for forming robust adhesion with the substrate in a carbon-nanotube field-emission array. Carbon 2006, 44: 2809–2815. 10.1016/j.carbon.2006.03.030View ArticleGoogle Scholar
- Sung WY, Lee SM, Kim WJ, Ok JG, Lee HY, Kim YH: New approach to enhance adhesions between carbon nanotube emitters and substrate by the combination of electrophoresis and successive electroplating. Diamond Relat Mater 2008, 17: 1003–1007. 10.1016/j.diamond.2008.03.001View ArticleGoogle Scholar
- Ferrer D, Tanii T, Matsuya I, Zhong G, Okamoto S, Kawarada H, Shinada T, Ohdomari I: Enhancement of field emission characteristics of tungsten emitters by single-walled carbon nanotube modification. Appl Phys Lett 2006, 88: 1–3.View ArticleGoogle Scholar
- Kim JP, Chang HB, Kim BJ, Park JS: Emission stability enhancement of a tip-type carbon-nanotube-based field emitter via hafnium interlayer deposition and thermal treatment. Appl Phys Lett 2012, 100: 123103–1-123103–3.Google Scholar
- Park JS, Kim JP, Noh YR, Jo KC, Lee SY, Choi HY, Kim JU: X-ray images obtained from cold cathodes using carbon nanotubes coated with gallium-doped zinc oxide thin films. Thin Solid Films 2010, 519: 1743–1748. 10.1016/j.tsf.2010.08.154View ArticleGoogle Scholar
- Sun JP, Zhang ZX, Hou SM, Zhang GM, Gu ZN, Zhao XY, Liu WM, Xue ZQ: Work function of single-walled carbon nanotubes determined by field emission microscopy. Appl Phys Mater Sci Process 2002, 75: 479–483. 10.1007/s003390201403View ArticleGoogle Scholar
- Zhang YL, Zhang LL, Hou PX, Jiang H, Liu C, Cheng HM: Synthesis and field emission property of carbon nanotubes with sharp tips. Xinxing Tan Cailiao/New Carbon Mater 2011, 26: 52–56. 10.1016/S1872-5805(11)60065-6View ArticleGoogle Scholar
- Jung SI, Jo SH, Moon HS, Kim JM, Zang DS, Lee CJ: Improved crystallinity of double-walled carbon nanotubes after a high-temperature thermal annealing and their enhanced field emission properties. J Phys Chem C 2007, 111: 4175–4179. 10.1021/jp0676078View ArticleGoogle Scholar
- Okpalugo TIT, Papakonstantinou P, Murphy H, McLaughlin J, Brown NMD: High resolution XPS characterization of chemical functionalised MWCNTs and SWCNTs. Carbon 2005, 43: 153–161. 10.1016/j.carbon.2004.08.033View ArticleGoogle Scholar
- Lesiak B, Zemek J, Jiricek P, Stobinski L: Temperature modification of oxidized multiwall carbon nanotubes studied by electron spectroscopy methods. Phys Status Solidi B 2009, 246: 2645–2649. 10.1002/pssb.200982268View ArticleGoogle Scholar
- Choi HC, Bae SY, Jang WS, Park J, Song HJ, Shin HJ, Jung H, Ahn JP: Release of N2 from the carbon nanotubes via high-temperature annealing. J Phys Chem B 2005, 109: 1683–1688. 10.1021/jp046098bView ArticleGoogle Scholar
- Hinnen C, Imbert D, Siffre JM, Marcus P: An in situ XPS study of sputter-deposited aluminium thin films on graphite. Appl Surf Sci 1994, 78: 219–231. 10.1016/0169-4332(94)90009-4View ArticleGoogle Scholar
- Nilsson L, Groening O, Groening P, Schlapbach L: Collective emission degradation behavior of carbon nanotube thin-film electron emitters. Appl Phys Lett 2001, 79: 1036–1038. 10.1063/1.1392982View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.