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Chemical modification of carbon nanotube for improvement of field emission property
Microelectronic Engineering 86 (2009) 2110–2113
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Accelarated Publication
Chemical modification of carbon nanotube for improvement of field emission property Sunwoo Lee a, Tetsuji Oda a, Paik-Kyun Shin b,*, Boong-Joo Lee c a
Electronic Engineering, The University of Tokyo, 113-8656 Hongo, Tokyo, Japan School of Electrical Engineering, Inha University, #253 Yonghyun-Dong, Nam-Gu, Incheon Metropolitan City 402-751, Republic of Korea c Electronic Engineering, Namseoul University, 21 Maeju-ri, Seounghwan-Eup, Cheonan City, Choongnam 330-707, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 17 November 2008 Received in revised form 31 December 2008 Accepted 17 February 2009 Available online 25 February 2009 Keywords: Chemical modification Carbon nanotube CNT Field emission Tunneling
a b s t r a c t In the present work, chemical modification of carbon nanotube was proposed for improvement of field emission property. Multi-wall carbon nanotubes (MWCNTs) were grown vertically on silicon substrate using catalytic chemical vapor deposition. Tips of grown MWCNTs were chemically modified using oxygen plasma, nitric acid, and hydrofluoric acid. Surface state and morphology of the chemically modified CNTs were investigated. CNT tips were opened and defects working as trap sites were generated on the CNT surface by the chemical modification process leading to improvement of field emission property. We suggest that two main factors determining the field enhancement factor are geometric factor and surface state of the CNT tips. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have attracted much attention because of their unique electrical properties and their potential applications [1,2]. Large aspect ratios of CNTs with high chemical stability, thermal conductivity, and high mechanical strength are advantageous for applications to the field emitter [3]. Since CNTs are grown directly on a substrate by CVD, the CNT emitter can be fabricated simply. Many researchers have devoted efforts to the artificial control of alignment, number density, and aspect ratio of CNTs [4–7]. Although it is essential for FED application to elucidate the correlation between the structural properties and field electron emission properties of CNTs, systematic experiments on the field emission property regarding the change of surface state of CNTs by chemical modification have not been carried out much. CNTs having strong covalent bonds are very stable against to chemical attacks. Breaking these strong covalent bonds and changing surface state would be expected to change the CNT’s physical property as well as chemical property [8,9]. As field emission behavior takes place at the tip of the CNT, one could control the field emission property by changing the structure and surface state of the CNT tips. In this study, the correlation between the field emission property and structural property or surface state of CNTs was investi-
* Corresponding author. Tel.: +82 32 860 7393; fax: +82 32 863 5822. E-mail address: [email protected] (P.-K. Shin). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.02.021
gated as a function of the chemical modification. Although the field emission properties of CNTs were improved with increasing the aspect ratio of the CNT, the field enhancement factor obtained from the Fowler–Nordheim plot was found to be much larger than that obtained from the geometric factors. These results suggest that the field emission from CNTs is strongly influenced by the surface states induced by surface defects and attached functional groups, rather than by their geometric factors.
2. Experimental In our experiment, the nickel catalyst films were prepared by sputtering method on silicon substrate using low power and long time (at 10 W for 1 h) to minimize size and distribution of the nickel catalyst particles. MWCNTs used in this work were grown in a thermal CVD system with C2H2 source gas and Ar carrier gas with a flow rate of 30/100 sccm at 700 °C on the nickel catalyst. The CNTs were chemically modified by oxygen plasma, nitric acid (HNO3), and hydrofluoric acid (HF). The modified samples were named as O2–CNT, HNO3–CNT, and HF–CNT, respectively. The oxygen plasma treatment was done with a gas flow rate of O2: Ar = 20: 200 sccm at 500 °C for 5 min. The HNO3 treatment was done in 20 vol% HNO3 solution at room temperature for 1 h, and the samples was subsequently rinsed in distilled water, and dried at room temperature for 1 h. The HF treatment was done in 20 vol% HF solution at room temperature for 1 h, and the sample was rinsed and dried.
S. Lee et al. / Microelectronic Engineering 86 (2009) 2110–2113
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3. Results and discussions
Fig. 1. Schematic drawing of the setup for measurement of the field emission current.
The field emission characteristics of the grown CNT film was measured by digital multimeter in a vacuum chamber with a base pressure of 1.5 108 Torr. A flat parallel diode type configuration was used in the setup as shown in Fig. 1. Both electrodes were glass plated with a conductive indium tin oxide (ITO) coating, and the cathode contained the grown CNT film. The distance between the anode and the CNT film surface was 100 lm as separated by spacers. The surface morphology and internal structure of the CNTs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
SEM images and TEM images (right side of each image) of the as-grown CNTs and the chemically modified CNTs are shown in Fig. 2. CNTs grown in this work are bamboo type multi-wall carbon nanotubes, which are vertically aligned to the substrate. The length of chemically modified CNTs is slightly shorter than that of asgrown CNTs due to the chemical etching during the chemical modification processes. In case of the HNO3–CNT, length was drastically reduced, because CNTs were partly delaminated and remained CNTs were fallen down during the chemical modification process. Tip of as-grown CNT is typically closed, while those of chemically modified CNTs are opened as shown in Fig. 2 (right side of each image). The most parts of CNT consist of stable hexagonal carbon structure, while the tip of CNT has pentagonal structure to close the tube end [10]. The pentagonal carbon structure is easily broken by the chemical attack relative to the hexagonal structure [11]. Relatively weak bonds at the CNT tip might be broken and opened by the chemical modification. Since the bond breaking might be started from the outer shell of the MWCNT used in this work and propagated into the inner shell, the shape of CNT tips became sharp. Furthermore, the chemical modification process might result in changing the surface state by the bond breaking as well as the structural change. In order to confirm the above mentioned surface state change, X-ray photoelectron spectroscopy (XPS) using the monochrome Al Ka X-ray was carried out. Wide scan spectra for as-grown and chemically modified CNTs are shown in Fig. 3. In all cases, carbon peak (C1s, 284.5 eV) and oxygen peak (O1s, 530 eV) are observed [12]. The oxygen peak stronger than that of the as-grown CNT film for the O2–CNT, the weak nitrogen peak for HNO3–CNT, and fluorine peak for HF–CNT are observed. This result corresponds to
Fig. 2. SEM (left) and TEM (right) images of as-grown and chemically modified CNTs.
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S. Lee et al. / Microelectronic Engineering 86 (2009) 2110–2113
Fig. 3. XPS wide scan spectra of the as-grown CNTs and the chemically modified CNTs. Since some parts of CNTs are delaminated during HNO3 chemical modification process as shown in Fig. 2c, strong oxygen and silicon signals are detected from the naturally oxidized Si substrate.
the previous TEM results that the chemical modification processes could change the surface states of the CNT tips. The chemical modification dependence on the field emission property was investigated. Fig. 4a shows emission current density as a function of applied electric field for the as-grown CNTs and the chemically modified CNTs. It is found that the chemically modified CNTs exhibit a better field emission property than that for the asgrown CNTs. If we define the threshold electric field (Eth) as the applied electric field that produces an emission current of 1 mA/cm2, it can be clearly seen from Fig. 4b that threshold electric field is chemical modification dependent. The Fowler–Nordheim (F–N) equation can be described as,
! 1:56 10 6ðbEÞ2 6:83 109 /3=2 ; J¼ exp / bE where J (A/cm2) is the emission current density, E (V/lm) is the applied electric field, b is the field enhancement factor, and / (eV) is the work function of the emitter [13]. The experimental value b can be estimated on the basis of the slope of the F–N plot as shown in Fig. 4c. Although there is no distinguishable difference in geometric factors such as diameter and length of each CNTs, the field emission property for chemically modified CNTs is better than that for as-grown CNTs. We estimated the field enhancement factors for each CNTs using geometric factors from SEM images and the FN plot of the experimental field emission data. The field enhance-
ment factor estimated from the FN plot (b 1000 s) was two orders greater than that estimated from the geometric factors (b 10 s). This result implies that the field enhancement factor estimated from the F–N plot includes another factor for the improvement of field emission. Another factor affecting field emission more dominantly might be correlated with the surface state of the CNT tips. TEM results and XPS results strongly imply that defects working as trap sites might be on the CNT surfaces. As shown in Fig. 4c, there are two different kinds of tunneling mechanism from the slope of J/E2 vs. 1/E plots. The slope at low field regime is quite different from that at high field regime. Trap sites play a dominant role in tunneling mechanism at lower field than FN tunneling regime, so called trap assisted tunneling (TAT) [14]. Tunneling governed by TAT mechanism at low field regime affect the threshold electric field, and is related to trap sites on CNT tips. The tunneling model is based on a two-step tunneling process via traps on CNT surface which incorporates energy loss by phonon emission [15]. Fig. 4d shows the basic two-step process of an electron tunneling from a region with higher Fermi energy (the cathode) to a region with lower Fermi energy (the anode). Electrons could be emitted at relatively low electric field with an aid of trap sites. Finally, we suggest that two main factors determining the field enhancement factor are geometric factor and surface state. Therefore generation of trap sites on CNT surface is strongly required to improve the field emission property, as well as the geometric factor.
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Fig. 4. (a) J–E curves of the as-grown CNTs and the chemically modified CNTs. (b) Threshold electric field as a function of chemical modification. (c) J/E2–1/E curves of the asgrown CNTs and the chemically modified CNTs. (d) Field emission model considering trap sites on the surface of CNT tip.
4. Summary We have found that CNT tips were opened and defects working as trap sites were generated on the CNT surface by the chemical modification process leading to improvement of field emission property. Trap sites play a dominant role in tunneling mechanism at lower field than FN tunneling regime. We found that another factor affecting the field emission might be correlated with the surface state of the CNT tips. Therefore generation of trap sites on CNT surface is strongly required to improve the field emission property, as well as the geometric factor. References [1] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [2] B.I. Yakobson, R.E. Smalley, Am. Sci. 85 (1997) 324.
[3] T.W. Ebbesen, Carbon Nanotubes, CRC Press, Boca Raton, FL, 1997. [4] M. Chhowalla, K.B.K. Teo, C. Ducati, N.L. Rupesinghe, G.A.J. Amaratunga, A.C. Ferrari, D. Roy, J. Robertson, W.I. Milne, J. Appl. Phys. 90 (2001) 5308. [5] Y.Y. Wei, G. Eres, V.I. Merkulov, D.H. Lowndes, Appl. Phys. Lett. 78 (2001) 1394. [6] V.I. Merkulov, D.H. Lowndes, Y.Y. Wei, G. Eres, E. Voelkl, Appl. Phys. Lett. 76 (2000) 3555. [7] M. Katayama, K.-Y. Lee, S. Honda, T. Hirao, K. Oura, Jpn. J. Appl. Phys. 43 (2004) L774. [8] W.K. Hong, H.C. Shin, S.H. Tsai, et al., Jpn. J. Appl. Phys. 39 (2000) L925. [9] U.D. Weglikowska, J.M. Benoit, P.W. Chiu, et al., Curr. Appl. Phys. (2002) 2. [10] G.L. Martin, P.R. Schwoebel, Surf. Sci. 601 (2007) 1521. [11] X.Y. Zhu, S.M. Lee, Y.H. Lee, T. Frauenheim, Phys. Rev. Lett. 85 (2000) 2757. [12] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Minnesota, 1995. [13] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. Lond. Ser. (1928) A119. [14] M. Houssa, M. Tuominen, M. Naili, V. Afanas’ev, A. Stesmans, S. Haukka, M.M. Heyns, J. Appl. Phys. 87 (2000) 8615. [15] F. Jiménez-Molinos, A. Palma, F. Gámiz, J. Banqueri, J.A. Lopez-Villanueva, J. Appl. Phys. 90 (2001) 3396.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Accelarated Publication
Chemical modification of carbon nanotube for improvement of field emission property Sunwoo Lee a, Tetsuji Oda a, Paik-Kyun Shin b,*, Boong-Joo Lee c a
Electronic Engineering, The University of Tokyo, 113-8656 Hongo, Tokyo, Japan School of Electrical Engineering, Inha University, #253 Yonghyun-Dong, Nam-Gu, Incheon Metropolitan City 402-751, Republic of Korea c Electronic Engineering, Namseoul University, 21 Maeju-ri, Seounghwan-Eup, Cheonan City, Choongnam 330-707, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 17 November 2008 Received in revised form 31 December 2008 Accepted 17 February 2009 Available online 25 February 2009 Keywords: Chemical modification Carbon nanotube CNT Field emission Tunneling
a b s t r a c t In the present work, chemical modification of carbon nanotube was proposed for improvement of field emission property. Multi-wall carbon nanotubes (MWCNTs) were grown vertically on silicon substrate using catalytic chemical vapor deposition. Tips of grown MWCNTs were chemically modified using oxygen plasma, nitric acid, and hydrofluoric acid. Surface state and morphology of the chemically modified CNTs were investigated. CNT tips were opened and defects working as trap sites were generated on the CNT surface by the chemical modification process leading to improvement of field emission property. We suggest that two main factors determining the field enhancement factor are geometric factor and surface state of the CNT tips. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have attracted much attention because of their unique electrical properties and their potential applications [1,2]. Large aspect ratios of CNTs with high chemical stability, thermal conductivity, and high mechanical strength are advantageous for applications to the field emitter [3]. Since CNTs are grown directly on a substrate by CVD, the CNT emitter can be fabricated simply. Many researchers have devoted efforts to the artificial control of alignment, number density, and aspect ratio of CNTs [4–7]. Although it is essential for FED application to elucidate the correlation between the structural properties and field electron emission properties of CNTs, systematic experiments on the field emission property regarding the change of surface state of CNTs by chemical modification have not been carried out much. CNTs having strong covalent bonds are very stable against to chemical attacks. Breaking these strong covalent bonds and changing surface state would be expected to change the CNT’s physical property as well as chemical property [8,9]. As field emission behavior takes place at the tip of the CNT, one could control the field emission property by changing the structure and surface state of the CNT tips. In this study, the correlation between the field emission property and structural property or surface state of CNTs was investi-
* Corresponding author. Tel.: +82 32 860 7393; fax: +82 32 863 5822. E-mail address: [email protected] (P.-K. Shin). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.02.021
gated as a function of the chemical modification. Although the field emission properties of CNTs were improved with increasing the aspect ratio of the CNT, the field enhancement factor obtained from the Fowler–Nordheim plot was found to be much larger than that obtained from the geometric factors. These results suggest that the field emission from CNTs is strongly influenced by the surface states induced by surface defects and attached functional groups, rather than by their geometric factors.
2. Experimental In our experiment, the nickel catalyst films were prepared by sputtering method on silicon substrate using low power and long time (at 10 W for 1 h) to minimize size and distribution of the nickel catalyst particles. MWCNTs used in this work were grown in a thermal CVD system with C2H2 source gas and Ar carrier gas with a flow rate of 30/100 sccm at 700 °C on the nickel catalyst. The CNTs were chemically modified by oxygen plasma, nitric acid (HNO3), and hydrofluoric acid (HF). The modified samples were named as O2–CNT, HNO3–CNT, and HF–CNT, respectively. The oxygen plasma treatment was done with a gas flow rate of O2: Ar = 20: 200 sccm at 500 °C for 5 min. The HNO3 treatment was done in 20 vol% HNO3 solution at room temperature for 1 h, and the samples was subsequently rinsed in distilled water, and dried at room temperature for 1 h. The HF treatment was done in 20 vol% HF solution at room temperature for 1 h, and the sample was rinsed and dried.
S. Lee et al. / Microelectronic Engineering 86 (2009) 2110–2113
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3. Results and discussions
Fig. 1. Schematic drawing of the setup for measurement of the field emission current.
The field emission characteristics of the grown CNT film was measured by digital multimeter in a vacuum chamber with a base pressure of 1.5 108 Torr. A flat parallel diode type configuration was used in the setup as shown in Fig. 1. Both electrodes were glass plated with a conductive indium tin oxide (ITO) coating, and the cathode contained the grown CNT film. The distance between the anode and the CNT film surface was 100 lm as separated by spacers. The surface morphology and internal structure of the CNTs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
SEM images and TEM images (right side of each image) of the as-grown CNTs and the chemically modified CNTs are shown in Fig. 2. CNTs grown in this work are bamboo type multi-wall carbon nanotubes, which are vertically aligned to the substrate. The length of chemically modified CNTs is slightly shorter than that of asgrown CNTs due to the chemical etching during the chemical modification processes. In case of the HNO3–CNT, length was drastically reduced, because CNTs were partly delaminated and remained CNTs were fallen down during the chemical modification process. Tip of as-grown CNT is typically closed, while those of chemically modified CNTs are opened as shown in Fig. 2 (right side of each image). The most parts of CNT consist of stable hexagonal carbon structure, while the tip of CNT has pentagonal structure to close the tube end [10]. The pentagonal carbon structure is easily broken by the chemical attack relative to the hexagonal structure [11]. Relatively weak bonds at the CNT tip might be broken and opened by the chemical modification. Since the bond breaking might be started from the outer shell of the MWCNT used in this work and propagated into the inner shell, the shape of CNT tips became sharp. Furthermore, the chemical modification process might result in changing the surface state by the bond breaking as well as the structural change. In order to confirm the above mentioned surface state change, X-ray photoelectron spectroscopy (XPS) using the monochrome Al Ka X-ray was carried out. Wide scan spectra for as-grown and chemically modified CNTs are shown in Fig. 3. In all cases, carbon peak (C1s, 284.5 eV) and oxygen peak (O1s, 530 eV) are observed [12]. The oxygen peak stronger than that of the as-grown CNT film for the O2–CNT, the weak nitrogen peak for HNO3–CNT, and fluorine peak for HF–CNT are observed. This result corresponds to
Fig. 2. SEM (left) and TEM (right) images of as-grown and chemically modified CNTs.
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S. Lee et al. / Microelectronic Engineering 86 (2009) 2110–2113
Fig. 3. XPS wide scan spectra of the as-grown CNTs and the chemically modified CNTs. Since some parts of CNTs are delaminated during HNO3 chemical modification process as shown in Fig. 2c, strong oxygen and silicon signals are detected from the naturally oxidized Si substrate.
the previous TEM results that the chemical modification processes could change the surface states of the CNT tips. The chemical modification dependence on the field emission property was investigated. Fig. 4a shows emission current density as a function of applied electric field for the as-grown CNTs and the chemically modified CNTs. It is found that the chemically modified CNTs exhibit a better field emission property than that for the asgrown CNTs. If we define the threshold electric field (Eth) as the applied electric field that produces an emission current of 1 mA/cm2, it can be clearly seen from Fig. 4b that threshold electric field is chemical modification dependent. The Fowler–Nordheim (F–N) equation can be described as,
! 1:56 10 6ðbEÞ2 6:83 109 /3=2 ; J¼ exp / bE where J (A/cm2) is the emission current density, E (V/lm) is the applied electric field, b is the field enhancement factor, and / (eV) is the work function of the emitter [13]. The experimental value b can be estimated on the basis of the slope of the F–N plot as shown in Fig. 4c. Although there is no distinguishable difference in geometric factors such as diameter and length of each CNTs, the field emission property for chemically modified CNTs is better than that for as-grown CNTs. We estimated the field enhancement factors for each CNTs using geometric factors from SEM images and the FN plot of the experimental field emission data. The field enhance-
ment factor estimated from the FN plot (b 1000 s) was two orders greater than that estimated from the geometric factors (b 10 s). This result implies that the field enhancement factor estimated from the F–N plot includes another factor for the improvement of field emission. Another factor affecting field emission more dominantly might be correlated with the surface state of the CNT tips. TEM results and XPS results strongly imply that defects working as trap sites might be on the CNT surfaces. As shown in Fig. 4c, there are two different kinds of tunneling mechanism from the slope of J/E2 vs. 1/E plots. The slope at low field regime is quite different from that at high field regime. Trap sites play a dominant role in tunneling mechanism at lower field than FN tunneling regime, so called trap assisted tunneling (TAT) [14]. Tunneling governed by TAT mechanism at low field regime affect the threshold electric field, and is related to trap sites on CNT tips. The tunneling model is based on a two-step tunneling process via traps on CNT surface which incorporates energy loss by phonon emission [15]. Fig. 4d shows the basic two-step process of an electron tunneling from a region with higher Fermi energy (the cathode) to a region with lower Fermi energy (the anode). Electrons could be emitted at relatively low electric field with an aid of trap sites. Finally, we suggest that two main factors determining the field enhancement factor are geometric factor and surface state. Therefore generation of trap sites on CNT surface is strongly required to improve the field emission property, as well as the geometric factor.
S. Lee et al. / Microelectronic Engineering 86 (2009) 2110–2113
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Fig. 4. (a) J–E curves of the as-grown CNTs and the chemically modified CNTs. (b) Threshold electric field as a function of chemical modification. (c) J/E2–1/E curves of the asgrown CNTs and the chemically modified CNTs. (d) Field emission model considering trap sites on the surface of CNT tip.
4. Summary We have found that CNT tips were opened and defects working as trap sites were generated on the CNT surface by the chemical modification process leading to improvement of field emission property. Trap sites play a dominant role in tunneling mechanism at lower field than FN tunneling regime. We found that another factor affecting the field emission might be correlated with the surface state of the CNT tips. Therefore generation of trap sites on CNT surface is strongly required to improve the field emission property, as well as the geometric factor. References [1] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [2] B.I. Yakobson, R.E. Smalley, Am. Sci. 85 (1997) 324.
[3] T.W. Ebbesen, Carbon Nanotubes, CRC Press, Boca Raton, FL, 1997. [4] M. Chhowalla, K.B.K. Teo, C. Ducati, N.L. Rupesinghe, G.A.J. Amaratunga, A.C. Ferrari, D. Roy, J. Robertson, W.I. Milne, J. Appl. Phys. 90 (2001) 5308. [5] Y.Y. Wei, G. Eres, V.I. Merkulov, D.H. Lowndes, Appl. Phys. Lett. 78 (2001) 1394. [6] V.I. Merkulov, D.H. Lowndes, Y.Y. Wei, G. Eres, E. Voelkl, Appl. Phys. Lett. 76 (2000) 3555. [7] M. Katayama, K.-Y. Lee, S. Honda, T. Hirao, K. Oura, Jpn. J. Appl. Phys. 43 (2004) L774. [8] W.K. Hong, H.C. Shin, S.H. Tsai, et al., Jpn. J. Appl. Phys. 39 (2000) L925. [9] U.D. Weglikowska, J.M. Benoit, P.W. Chiu, et al., Curr. Appl. Phys. (2002) 2. [10] G.L. Martin, P.R. Schwoebel, Surf. Sci. 601 (2007) 1521. [11] X.Y. Zhu, S.M. Lee, Y.H. Lee, T. Frauenheim, Phys. Rev. Lett. 85 (2000) 2757. [12] F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., Minnesota, 1995. [13] R.H. Fowler, L.W. Nordheim, Proc. R. Soc. Lond. Ser. (1928) A119. [14] M. Houssa, M. Tuominen, M. Naili, V. Afanas’ev, A. Stesmans, S. Haukka, M.M. Heyns, J. Appl. Phys. 87 (2000) 8615. [15] F. Jiménez-Molinos, A. Palma, F. Gámiz, J. Banqueri, J.A. Lopez-Villanueva, J. Appl. Phys. 90 (2001) 3396.