- Email: [email protected]
Effects of argon plasma treating on surface morphology and gas ionization property of carbon nanotubes
ARTICLE IN PRESS
Physica E 28 (2005) 88–92 www.elsevier.com/locate/physe
Effects of argon plasma treating on surface morphology and gas ionization property of carbon nanotubes Bingyong Yan, Kaiyou Qian, Yafei Zhang, Dong Xu Key Laboratory for thin film and Microfabrication of Ministry of Education, Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiaotong University, Shanghai 200030, PR China Received 2 February 2005; accepted 9 February 2005 Available online 18 April 2005
Abstract The effect of surface treatment by argon plasma on the surface morphology and gas ionization property of carbon nanotube film were studied. The surface of as-grown CNT films prepared by chemical vapor deposition was treated by argon plasma in reactive ion etcher (RIE). In order to compare the microstructure of untreated CNT film with treated CNT film, scanning electron microscopy was employed. The ionization properties of distinct gas based on the untreated and treated CNT film were measured and compared. The reasons for the observed effects were discussed. r 2005 Elsevier B.V. All rights reserved. PACS: 81.07.De; 79.70.+q; 51.50.+v; 52.77.Bn Keywords: Carbon nanotube; Surface treatment; Gas ionization; Argon plasma process
1. Introduction Since the discovery of carbon nanotube (CNT) by Iijima in 1991 [1], numerous novel applications of carbon nanotubes have been investigated, including gas sensing [2–12]. Recently, gas ionization sensors based on vertically grown multiwalled carbon nanotubes (MWNTs) have been successfully fabricated and tested [11,12]. This Corresponding author. Tel.: +86 21 62933294;
fax: +86 21 62683631. E-mail address: [email protected] (B. Yan).
kind of gas sensor operates owing to the different electrical ionization characteristics of various gases and gas mixtures at well vertically aligned carbon nanotube tips. The sharp tips of nanotubes generate very intense electric fields at relatively low voltages, lowering ionization voltages severalfold in comparison with traditional electrodes. Low gas ionization voltage is influenced by the surface morphology of MWNT. However, the homogeneity of the large-scaled MWNT film is hard to control in the process of MWNT growth. Therefore, in order to improve the gas ionization property of MWNT, it is necessary to develop
1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.02.004
ARTICLE IN PRESS B. Yan et al. / Physica E 28 (2005) 88–92
post-treatment techniques to obtain desired MWNT film. In this paper, we report a surface treatment method that would improve gas ionization property of carbon nanotube film by means of argon plasma (AP) treatment. The changes of morphology and ionization property of MWNT films were discussed.
2. Experimental The carbon nanotube samples were prepared by chemical vapor deposition (CVD) on Ni–Cr/Si substrates. The surface of as prepared MWNT films was treated in reactive ion etcher (Nextral 100, Alcatel Corp.). The argon plasma treatment was carried out for 2, 5, 10 min, respectively under the following conditions: argon flow rate of 10 sccm, operating pressure of 100 mTorr, RF power of 60 W, temperature of 291 K. The surface morphology of the carbon nanotube films was examined using scanning electron microscopy (SEM) (JSM 6700F, JEOL). The structure of the device used to measure the gas ionization property in the experiment is shown in Fig. 1, where the silicon wafer with MWNT film served as anode and aluminum plate as cathode. The two electrodes were separated and insulated by a glass insulator. This device was placed in a chamber, and the air could be pumped out of the chamber to establish a high vacuum. The ionization voltages and currents of distinct gas, such as helium, argon, nitrogen, oxygen, and air, were measured at room temperature (300 K) and at a chamber pressure of 760 Torr. During the measurement, the anode– cathode distance was kept as 150 mm. Al plate
A
Glass insulator V MWNT Si Fig. 1. Schematic diagram of the gas ionization device based on carbon nanotubes.
89
3. Results and discussion The surface images of the MWNT film before and after argon plasma treatment at different times are shown in Fig. 2. The differences of the surface morphology between treated and untreated samples were evident. These SEM images showed that after argon plasma treatment for 2, 5, 10 min (Fig. 2c, e and g), the density of vertical nanotubes decreased and the spacing of tubes became larger compared with that of the untreated sample (Fig. 2a). It has also been shown that the carbon nanotubes on the surface of original MWNT arrays are randomly oriented (Fig. 2b). After the plasma treatment for 2 and 5 min, the end stems of CNTs were found to be straightened, as shown in Fig. 2d and f. Furthermore, after plasma treatment for 2 and 5 min, there still remained tubes with the tips protruding out of the film surface (Fig. 2d and f). However, after treatment for 10 min, as shown in the Fig. 2h, the tubes with protruding tips became fewer. Fig. 3 shows the gas ionization properties of MWNT samples before and after argon plasma treatment for different time in helium, argon, nitrogen, oxygen, and air. During the measurement, the anode–cathode distance was kept as150 mm. Fig. 3a reveals that after argon plasma treatment for 2, 5 min, the ionization voltage of He decreased from 129 to 121, 125 V, respectively, but after treatment for 10 min, the voltage increased to 142 V. The other test gases, argon, nitrogen, oxygen, and air, also show similar characteristics (Fig. 3b). The ionization voltage became lower for the samples after treatment for 2 and 5 min, but became higher for the samples after treatment for 10 min. Gas ionization was generated due to the sharply nonuniform electric fields near MWNT tips. Because small nanoscale radii of curvature of the tips are much smaller than the interelectrode gap distance (hundred micrometer level), nonuniform electric field was created near the tips when voltage is applied between the electrodes [13]. When the intensity of the electric field increased to a certain value, the gas surrounding the tips of the tubes were ionized locally and corona discharge occured, which caused a discharge current jump to about
ARTICLE IN PRESS 90
B. Yan et al. / Physica E 28 (2005) 88–92
Fig. 2. SEM images of carbon nanotube film before and after plasma treatment for different time: (a) and (b) before treatment; (c) and (d) after 2 min treatment; (e) and (f) after 5 min treatment; (g) and (h) after 10 min treatment.
ARTICLE IN PRESS B. Yan et al. / Physica E 28 (2005) 88–92 4.5
A B C D
4.0
Current (µ A)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 115
120
(a)
125 130 135 Appllied Voltalge (V)
140
145
350 A B C D
Ionization voltage (V)
300
250
200
150
100 He
(b)
Ar
N2
Air
O2
Different kinds of gases
Fig. 3. Comparison of gas ionization property of MWNTs before and after argon plasma treatment: (a) Ionization current/voltage curves of He gas for samples after treatment for different time. (b) Helium, argon, nitrogen, oxygen, and air for samples after treatment for different time. A, B, C, D are untreated, 2, 5, 10 min treatment for MWNT samples, respectively.
10 6 A at relatively low voltages. The corona discharge belonged to the group of self-sustaining discharge [13,14]. It is proposed that the low–ionization voltage of the treated films mainly originates from the geometrical enhancement effect. Gas ionization strongly depends on the local electric field intensity at the tips of MWNTs. Field enhancement factor b is defined as the ratio of the local electric field at the tips of MWNT (Elocal) to the macroscopic electric field (Emac) between anode and cathode. It has been reported in the research on CNT field emission property that field enhancement factor b
91
is influenced by spacing [15–19] between MWNTs and erection of tubes [20]. It has been shown that the carbon nanotubes on the surface of original CNT arrays are just disorderly tangled (Fig. 2b). After the plasma treatment with proper conditions, the end stems of all MWNTs with complex bent structures were found to be straightened, as shown in Fig. 2d and f and the spacing of tubes was enlarged, which increased the field enhancement factor b and decreased the electrostatic screen effect. Thus, higher local electric field at CNT tips was obtained at the given macroscopic electric field after 2 and 5 min argon plasma treatment, which were indicated as lower gas ionization voltage (Fig. 3). However, when the plasma exposure time was extended to 10 min, MWNTs were significantly damaged and almost all protruding CNTs were cleared away (Fig. 2g and h), and the ionization voltage was accordingly increased. Therefore, it is deduced that after argon plasma treatment, the surface morphology was changed, which caused the gas ionization voltage to be lowered (for 2 and 5 min treatment) and increased (for 10 min treatment). The reason is that argon plasma treatment changed the surface morphology of the nanotube film, increased the spacing of tubes, and straightened the top stem of MWNTs, resulting in the increase of the field enhancement factor b.
4. Conclusion In summary, after the plasma treatment for 2 and 5 min, the spacing of tubes became larger and the end stems of CNTs were straightened, which caused the as ionization voltages to be decreased. The observed improvement of gas ionization property in comparison with that of the untreated MWNTs can be mainly attributed to the geometrical effect of the increased field enhancement factor by increasing the spacing of MWNTs and the straightening of the top stems of carbon nanotubes. References [1] S. Iijima, Nature 354 (1991) 56. [2] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622.
ARTICLE IN PRESS 92
B. Yan et al. / Physica E 28 (2005) 88–92
[3] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (2000) 1801. [4] M. Bioenfait, B. Asmussen, M. Jonson, P. Zeppenfeld, Surf. Sci. 460 (2000) 243. [5] O.K. Varghese, P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey, C.A. Grimes, Sens. Act. B 81 (2001) 32. [6] M. Kudeara, H.A.O. Hill, P.J. Dobson, P.A. Leign, W.S. McIntire, Sensors 1 (2001) 18. [7] K.G. Ong, K. Zong, C.A. Grimes, Sensors 1 (2001) 193. [8] R. Pati, Y.M. Zhang, S.K. Nayak, Appl. Phys. Lett. 81 (2002) 2638. [9] K.G. Ong, K. Zong, C.A. Grimes, IEEE Sens. J. 2 (2002) 82. [10] L. Valentini, L. Lozzi, C. Canatalini, I. Armentano, J.M. Kenny, L. Ottaviano, S. Santucci, Thin Solid Films 436 (2003) 95. [11] A. Modi, N. Koratkar, E. Lass, B.Q. Wei, P.M. Ajayan, Nature 424 (2003) 171. [12] D.J. Riley, M. Mann, D.A. MacLaren, P.C. Dastoor, W. Allison, Nano Lett. 3 (2003) 1455.
[13] Yuri P. Raizer, Gas Discharge Physics, Springer, Berlin, 1991, p. 345–352. [14] M. Goldman, A. Goldman, Corona discharge, in: Merle N. Hirsh, H.J. Oskam (Eds.), Gaseous Electronics I, Electrical Discharge, Academic Press, New York, 1978, pp. 219–290. [15] O. Gro¨ning, O.M. Ku¨ttel, Ch. Emmenegger, P. Gro¨ning, L. Schlapbach, J. Vac. Sci. Technol. B 18 (2000) 665. [16] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J-M. Bonard, K. Kern, Appl. Phys. Lett. 76 (2000) 2071. [17] M. Chhowalla, C. Ducati, N.L. Rupesinghe, K.B.K. Teo, G.A.J. Amaratunga, Appl. Phys. Lett. 79 (2001) 2079. [18] K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I. Milne, G. Pirio, P. Legagneux, F. Wyczisk, D. Pribat, D.G. Hasko, Appl. Phys. Lett. 80 (2002) 2011. [19] S.H. Jo, Y. Tu, Z.P. Huang, D.L. Carnahan, D.Z. Wang, Z.F. Ren, Appl. Phys. Lett. 82 (2003) 3520. [20] D.-H. Kim, C.-D. Kim, H.R. Lee, Carbon 42 (2004) 1807.
Physica E 28 (2005) 88–92 www.elsevier.com/locate/physe
Effects of argon plasma treating on surface morphology and gas ionization property of carbon nanotubes Bingyong Yan, Kaiyou Qian, Yafei Zhang, Dong Xu Key Laboratory for thin film and Microfabrication of Ministry of Education, Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiaotong University, Shanghai 200030, PR China Received 2 February 2005; accepted 9 February 2005 Available online 18 April 2005
Abstract The effect of surface treatment by argon plasma on the surface morphology and gas ionization property of carbon nanotube film were studied. The surface of as-grown CNT films prepared by chemical vapor deposition was treated by argon plasma in reactive ion etcher (RIE). In order to compare the microstructure of untreated CNT film with treated CNT film, scanning electron microscopy was employed. The ionization properties of distinct gas based on the untreated and treated CNT film were measured and compared. The reasons for the observed effects were discussed. r 2005 Elsevier B.V. All rights reserved. PACS: 81.07.De; 79.70.+q; 51.50.+v; 52.77.Bn Keywords: Carbon nanotube; Surface treatment; Gas ionization; Argon plasma process
1. Introduction Since the discovery of carbon nanotube (CNT) by Iijima in 1991 [1], numerous novel applications of carbon nanotubes have been investigated, including gas sensing [2–12]. Recently, gas ionization sensors based on vertically grown multiwalled carbon nanotubes (MWNTs) have been successfully fabricated and tested [11,12]. This Corresponding author. Tel.: +86 21 62933294;
fax: +86 21 62683631. E-mail address: [email protected] (B. Yan).
kind of gas sensor operates owing to the different electrical ionization characteristics of various gases and gas mixtures at well vertically aligned carbon nanotube tips. The sharp tips of nanotubes generate very intense electric fields at relatively low voltages, lowering ionization voltages severalfold in comparison with traditional electrodes. Low gas ionization voltage is influenced by the surface morphology of MWNT. However, the homogeneity of the large-scaled MWNT film is hard to control in the process of MWNT growth. Therefore, in order to improve the gas ionization property of MWNT, it is necessary to develop
1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.02.004
ARTICLE IN PRESS B. Yan et al. / Physica E 28 (2005) 88–92
post-treatment techniques to obtain desired MWNT film. In this paper, we report a surface treatment method that would improve gas ionization property of carbon nanotube film by means of argon plasma (AP) treatment. The changes of morphology and ionization property of MWNT films were discussed.
2. Experimental The carbon nanotube samples were prepared by chemical vapor deposition (CVD) on Ni–Cr/Si substrates. The surface of as prepared MWNT films was treated in reactive ion etcher (Nextral 100, Alcatel Corp.). The argon plasma treatment was carried out for 2, 5, 10 min, respectively under the following conditions: argon flow rate of 10 sccm, operating pressure of 100 mTorr, RF power of 60 W, temperature of 291 K. The surface morphology of the carbon nanotube films was examined using scanning electron microscopy (SEM) (JSM 6700F, JEOL). The structure of the device used to measure the gas ionization property in the experiment is shown in Fig. 1, where the silicon wafer with MWNT film served as anode and aluminum plate as cathode. The two electrodes were separated and insulated by a glass insulator. This device was placed in a chamber, and the air could be pumped out of the chamber to establish a high vacuum. The ionization voltages and currents of distinct gas, such as helium, argon, nitrogen, oxygen, and air, were measured at room temperature (300 K) and at a chamber pressure of 760 Torr. During the measurement, the anode– cathode distance was kept as 150 mm. Al plate
A
Glass insulator V MWNT Si Fig. 1. Schematic diagram of the gas ionization device based on carbon nanotubes.
89
3. Results and discussion The surface images of the MWNT film before and after argon plasma treatment at different times are shown in Fig. 2. The differences of the surface morphology between treated and untreated samples were evident. These SEM images showed that after argon plasma treatment for 2, 5, 10 min (Fig. 2c, e and g), the density of vertical nanotubes decreased and the spacing of tubes became larger compared with that of the untreated sample (Fig. 2a). It has also been shown that the carbon nanotubes on the surface of original MWNT arrays are randomly oriented (Fig. 2b). After the plasma treatment for 2 and 5 min, the end stems of CNTs were found to be straightened, as shown in Fig. 2d and f. Furthermore, after plasma treatment for 2 and 5 min, there still remained tubes with the tips protruding out of the film surface (Fig. 2d and f). However, after treatment for 10 min, as shown in the Fig. 2h, the tubes with protruding tips became fewer. Fig. 3 shows the gas ionization properties of MWNT samples before and after argon plasma treatment for different time in helium, argon, nitrogen, oxygen, and air. During the measurement, the anode–cathode distance was kept as150 mm. Fig. 3a reveals that after argon plasma treatment for 2, 5 min, the ionization voltage of He decreased from 129 to 121, 125 V, respectively, but after treatment for 10 min, the voltage increased to 142 V. The other test gases, argon, nitrogen, oxygen, and air, also show similar characteristics (Fig. 3b). The ionization voltage became lower for the samples after treatment for 2 and 5 min, but became higher for the samples after treatment for 10 min. Gas ionization was generated due to the sharply nonuniform electric fields near MWNT tips. Because small nanoscale radii of curvature of the tips are much smaller than the interelectrode gap distance (hundred micrometer level), nonuniform electric field was created near the tips when voltage is applied between the electrodes [13]. When the intensity of the electric field increased to a certain value, the gas surrounding the tips of the tubes were ionized locally and corona discharge occured, which caused a discharge current jump to about
ARTICLE IN PRESS 90
B. Yan et al. / Physica E 28 (2005) 88–92
Fig. 2. SEM images of carbon nanotube film before and after plasma treatment for different time: (a) and (b) before treatment; (c) and (d) after 2 min treatment; (e) and (f) after 5 min treatment; (g) and (h) after 10 min treatment.
ARTICLE IN PRESS B. Yan et al. / Physica E 28 (2005) 88–92 4.5
A B C D
4.0
Current (µ A)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 115
120
(a)
125 130 135 Appllied Voltalge (V)
140
145
350 A B C D
Ionization voltage (V)
300
250
200
150
100 He
(b)
Ar
N2
Air
O2
Different kinds of gases
Fig. 3. Comparison of gas ionization property of MWNTs before and after argon plasma treatment: (a) Ionization current/voltage curves of He gas for samples after treatment for different time. (b) Helium, argon, nitrogen, oxygen, and air for samples after treatment for different time. A, B, C, D are untreated, 2, 5, 10 min treatment for MWNT samples, respectively.
10 6 A at relatively low voltages. The corona discharge belonged to the group of self-sustaining discharge [13,14]. It is proposed that the low–ionization voltage of the treated films mainly originates from the geometrical enhancement effect. Gas ionization strongly depends on the local electric field intensity at the tips of MWNTs. Field enhancement factor b is defined as the ratio of the local electric field at the tips of MWNT (Elocal) to the macroscopic electric field (Emac) between anode and cathode. It has been reported in the research on CNT field emission property that field enhancement factor b
91
is influenced by spacing [15–19] between MWNTs and erection of tubes [20]. It has been shown that the carbon nanotubes on the surface of original CNT arrays are just disorderly tangled (Fig. 2b). After the plasma treatment with proper conditions, the end stems of all MWNTs with complex bent structures were found to be straightened, as shown in Fig. 2d and f and the spacing of tubes was enlarged, which increased the field enhancement factor b and decreased the electrostatic screen effect. Thus, higher local electric field at CNT tips was obtained at the given macroscopic electric field after 2 and 5 min argon plasma treatment, which were indicated as lower gas ionization voltage (Fig. 3). However, when the plasma exposure time was extended to 10 min, MWNTs were significantly damaged and almost all protruding CNTs were cleared away (Fig. 2g and h), and the ionization voltage was accordingly increased. Therefore, it is deduced that after argon plasma treatment, the surface morphology was changed, which caused the gas ionization voltage to be lowered (for 2 and 5 min treatment) and increased (for 10 min treatment). The reason is that argon plasma treatment changed the surface morphology of the nanotube film, increased the spacing of tubes, and straightened the top stem of MWNTs, resulting in the increase of the field enhancement factor b.
4. Conclusion In summary, after the plasma treatment for 2 and 5 min, the spacing of tubes became larger and the end stems of CNTs were straightened, which caused the as ionization voltages to be decreased. The observed improvement of gas ionization property in comparison with that of the untreated MWNTs can be mainly attributed to the geometrical effect of the increased field enhancement factor by increasing the spacing of MWNTs and the straightening of the top stems of carbon nanotubes. References [1] S. Iijima, Nature 354 (1991) 56. [2] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science 287 (2000) 622.
ARTICLE IN PRESS 92
B. Yan et al. / Physica E 28 (2005) 88–92
[3] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Science 287 (2000) 1801. [4] M. Bioenfait, B. Asmussen, M. Jonson, P. Zeppenfeld, Surf. Sci. 460 (2000) 243. [5] O.K. Varghese, P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey, C.A. Grimes, Sens. Act. B 81 (2001) 32. [6] M. Kudeara, H.A.O. Hill, P.J. Dobson, P.A. Leign, W.S. McIntire, Sensors 1 (2001) 18. [7] K.G. Ong, K. Zong, C.A. Grimes, Sensors 1 (2001) 193. [8] R. Pati, Y.M. Zhang, S.K. Nayak, Appl. Phys. Lett. 81 (2002) 2638. [9] K.G. Ong, K. Zong, C.A. Grimes, IEEE Sens. J. 2 (2002) 82. [10] L. Valentini, L. Lozzi, C. Canatalini, I. Armentano, J.M. Kenny, L. Ottaviano, S. Santucci, Thin Solid Films 436 (2003) 95. [11] A. Modi, N. Koratkar, E. Lass, B.Q. Wei, P.M. Ajayan, Nature 424 (2003) 171. [12] D.J. Riley, M. Mann, D.A. MacLaren, P.C. Dastoor, W. Allison, Nano Lett. 3 (2003) 1455.
[13] Yuri P. Raizer, Gas Discharge Physics, Springer, Berlin, 1991, p. 345–352. [14] M. Goldman, A. Goldman, Corona discharge, in: Merle N. Hirsh, H.J. Oskam (Eds.), Gaseous Electronics I, Electrical Discharge, Academic Press, New York, 1978, pp. 219–290. [15] O. Gro¨ning, O.M. Ku¨ttel, Ch. Emmenegger, P. Gro¨ning, L. Schlapbach, J. Vac. Sci. Technol. B 18 (2000) 665. [16] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J-M. Bonard, K. Kern, Appl. Phys. Lett. 76 (2000) 2071. [17] M. Chhowalla, C. Ducati, N.L. Rupesinghe, K.B.K. Teo, G.A.J. Amaratunga, Appl. Phys. Lett. 79 (2001) 2079. [18] K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, W.I. Milne, G. Pirio, P. Legagneux, F. Wyczisk, D. Pribat, D.G. Hasko, Appl. Phys. Lett. 80 (2002) 2011. [19] S.H. Jo, Y. Tu, Z.P. Huang, D.L. Carnahan, D.Z. Wang, Z.F. Ren, Appl. Phys. Lett. 82 (2003) 3520. [20] D.-H. Kim, C.-D. Kim, H.R. Lee, Carbon 42 (2004) 1807.