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Evaluation of aligned carbon nanotube thin film modified by an Argon-ion sputtering method
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Physics Procedia 14 (2011) 164–166
9th International Conference on Nano-Molecular Electronics
Evaluation of aligned carbon nanotube thin film modified by an Argon-ion sputtering method Fukunori Izumidaa,b,*, Rongbin Yea, Koji Ohtaa, Mamoru Babaa and Michiko Kusunokic a
Graduate School of Engineering, Iwate University, Morioka 020-8551, Japan Electronics Course, Iwate Industrial Technology Junior College, Yahaba, Iwate 028-3615, Japan c EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
b
Abstract In order to apply carbon nanotubes (CNTs) as the anode materials of a solid-state Li-ion secondary battery, the surface modifications of the CNT thin film formed by the SiC surface decomposition method were carried out by the Ar-ion sputtering method. Then, we observed the CNT film by field emission scanning electron and measured the Raman scattering spectra of the CNT film in order to evaluate the effect of Ar-ion bombardments on the CNT film. © 2010 Published by Elsevier B.V. Keywords: Carbon nanotube; SSD method; Li ion battery; Raman spectrum; SiC
1. Introduction The carbon nanotube (CNT) discovered by Iijima in 1991 [1] has a cylindrical structure of rolled-up the graphene sheets. The CNTs are about 1 - 50 nm in diameter and are known to show electric conduction as metals or semiconductors. The CNTs are expected to have various applications owing to their numerous unique properties. One of their applications is their use as the anode materials of a Li-ion secondary battery. In the Li-ion secondary battery, carbon is mainly used as the anode materials [2]. Graphite has a layered structure in which two-dimensional graphene sheets are stacked. When the battery is charged or discharged electrically, Li ions are intercalated or deintercalated between the graphene sheets [3]. Similarly to graphite, Li ions are expected to be inserted and deinserted inside the CNTs. Gao et al. reported that the Li-ion reversible capacity of single-wall carbon nanotubes (SWNTs) was Li1.6C6 (600 mAh/g) and the reversible capacity of ball-milled SWNTs reached Li2.7C6 (1000 mAh/g) [4]. The ideal Li-ion reversible capacity of graphite is LiC6 (372 mAh/g) [4]; hence, CNTs have a Li-ion reversible capacity of about twice or more than that of graphite. On the other hand, there are various methods of synthesizing CNTs, such as a chemical vapor deposition method [5] using a catalyst, an arc method and so on. Kusunoki et al. reported that aligned CNTs were formed on a SiC surface by heating single-crystal SiC at 1500 - 1700 qC in vacuum
* Corresponding author. Tel.: +81-19-697-9082; fax: +81-19-697-9089. E-mail address: [email protected].
1875-3892 © 2011 Published by Elsevier Ltd. doi:10.1016/j.phpro.2011.05.032
165
Fukunori Izumida et al. / Physics Procedia 14 (2011) 164–166
[6]. This method is called the SiC surface decomposition (SSD) method. It is expected that the CNT film fabricated by this method will function as a high-density anode material, because high-density CNTs grow vertical to the carbon face of a SiC wafer [7]. In order to apply CNT film as the anode materials of a solid-state Li-ion secondary battery, a smooth surface of the CNT film is required. Therefore, aiming to apply CNT film as the anode materials of a solid-state Li-ion secondary battery, we attempted to modify the CNT thin films surface by Ar-ion sputtering and evaluated it by field emission scanning electron microscopy (FE-SEM) and Raman scattering spectral measurements. 2. Experiment We formed aligned CNT thin films by heating a 6H-SiC wafer (SiXON Ltd.), 5×5×0.25 mm3 in size, at 1700 qC for 30 min in vacuum (1×10-4 Torr). Then, aluminum sheet with a hole was used as a mask to cover the SiC wafer, and Ar-ion sputtering (Anelva SPF-2108) at 2×10-2 Torr was carried out at three places on the wafer for 15, 30, and 60 s, respectively. The power of radio frequency was ~40 W. FE-SEM (JEOL Ltd. JSM-6701F) was used for the CNT region observations. The acceleration voltage of FE-SEM was 1 kV and the image resolution was 2.2 nm. The Raman scattering spectra of the CNT film were measured using a micro-Raman system (HORIBA Jobin Yvon LabRAM HR-800). A semiconductor laser (O=632.8 nm) was used as an excitation light source in this measurement. 3. Results and Discussion The matter of concern on the CNT film is the growth of projecting micro crystallites (hereafter called CNT needles) on the as-grown CNT film-like region (hereafter called CNT film), as shown in Figure 1(a). The CNT needle is supposed to be composed of a CNT, stands alone on the surface and reaches a length of ~190 nm. In the case of applying the CNT thin film as the anode material of a Li-ion battery, a significant observation indicating that the CNT needle on the surface of the CNT film electrically shorts the battery between anode and cathode electrodes is obtained. After the sputtering process, the CNT needle on the film seems to be almost removed, as shown Figure 1. The electric field for the Ar-ion plasma is concentrated at the tip of the needle, and as a result, one-dimensional CNT needles seem to be effectively bombarded and removed compared with the two-dimensional CNT film surface by Ar-ion sputtering. The as-grown CNT film is ~290 nm in thickness and the sputtered CNT film becomes thinner than the as-grown CNT film. As shown in Figure 1, the CNT film maintains the shape of column like bundles of CNTs under the sputtering process, although a large number of carbon atoms constituting CNTs were sputtered by Ar ions. In order to evaluate the crystalline quality of CNTs damaged by Ar ions, the Raman scattering spectra of the CNT film were measured. As shown in Figure 2, two scattering peaks at around 1332 and 1584 cm -1 were observed. The peak of 1332 cm-1 is the D-band. Since the peak of 1584 cm -1 consists of one peak and one shoulder, it was 190nm 290nm
250nm
220nm
190nm
Figure 1: FE-SEM images of CNT film section on 6H-SiC wafer: (a) as-grown and Ar-ion sputtered for (b) 15, (c) 30, and (d) 60 s.
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Fukunori Izumida et al. / Physics Procedia 14 (2011) 164–166
fitted with two Lorentzian functions. As a result, the peak of 1584 cm-1 was separated into two peaks at around 1584 cm-1 (G-band) and 1617 cm-1 (D’-band). It is generally considered that the G-band originates from sp2 carbon networks, and the D-band and the D’-band, from defects mostly located at the edges of a crystallite [8]. As shown in Figure 3(a), the intensity ratios of I D/IG and ID’/IG increased with Ar-ion sputtering, and then became constant after more than 30 s of Ar-ion sputtering. In addition, the full-width at half-maximum (FWHM) of each peak became roughly constant, as shown in Figure 3(b). Hence, it is suggested that only the surface region of the CNT film was damaged and the crystalline quality of CNTs below the surface remained intact. 2.5 -1
1332cm
Intensity (arb. unit)
G-band -1
1584cm
D'-band -1
1617cm
Intensity Ratio
D-band
6H-SiC
60s
15s as-grown
500
1000
1.5 1.0 ID'/IG
0.5
(b)
30s
0
ID/IG
0.0
-1
968cm
1500
-1
-1
-1
790cm
FWHM (cm )
-1
768cm
(a)
2.0
60
D-band G-band
40
D'-band
20
2000
Raman Shift (cm )
Figure 2: Raman scattering spectra of CNT film on 6H-SiC wafer.
0
0
10
20 30 40 50 Sputtering Time (s)
60
Figure 3: Characteristics of Raman peaks for various Ar-ion sputtering times. (a) Intensity ratios of D-band to G-band (ID/IG) and Dc-band to G-band (ID’/IG). (b) FWHMs of D-band, G-band and Dcband.
4. Conclusions The surface modifications of the aligned CNT films formed by the SSD method have been successfully realized by the Ar-ion sputtering method in order to use CNT thin films as the anode materials of a solid-state Li-ion secondary battery. Namely, the CNT thin film was effectively etched and the removal of the CNT needles growing on the CNT films was confirmed. It was also confirmed from the Raman scattering spectra that the crystalline quality of CNTs remained intact by the surface modification. References [1] S. Iijima: Nature 354 (1991) 56. [2] J. M. Tarascon, and M. Armand: Nature 414 (2001) 359. [3] M.S. Dresselhaus, and G. Dresselhaus: Adv. Phys. 51 (2002) 1. [4] B. Gao, A. Kleinhammes, X.P. Tang, C. Bower, L. Fleming, Y. Wu, and O. Zhou: Chem. Phys. Lett. 307 (1999) 153. [5] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, and P.N. Provencio: Science 282 (1998) 1105. [6] M. Kusunoki, J. Shibata, M. Rokkaku, and T. Hirayama: Jpn. J. Appl. Phys. 37 (1998) L605. [7] M. Kusunoki, T. Suzuki, C. Honjo, H. Usami, and H. Kato: J. Phys. D: Appl. Phys. 40 (2007) 6278. [8] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, and R. Saito: Phys. Chem. Chem. Phys. 9 (2007) 1276.
Physics Procedia 14 (2011) 164–166
9th International Conference on Nano-Molecular Electronics
Evaluation of aligned carbon nanotube thin film modified by an Argon-ion sputtering method Fukunori Izumidaa,b,*, Rongbin Yea, Koji Ohtaa, Mamoru Babaa and Michiko Kusunokic a
Graduate School of Engineering, Iwate University, Morioka 020-8551, Japan Electronics Course, Iwate Industrial Technology Junior College, Yahaba, Iwate 028-3615, Japan c EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
b
Abstract In order to apply carbon nanotubes (CNTs) as the anode materials of a solid-state Li-ion secondary battery, the surface modifications of the CNT thin film formed by the SiC surface decomposition method were carried out by the Ar-ion sputtering method. Then, we observed the CNT film by field emission scanning electron and measured the Raman scattering spectra of the CNT film in order to evaluate the effect of Ar-ion bombardments on the CNT film. © 2010 Published by Elsevier B.V. Keywords: Carbon nanotube; SSD method; Li ion battery; Raman spectrum; SiC
1. Introduction The carbon nanotube (CNT) discovered by Iijima in 1991 [1] has a cylindrical structure of rolled-up the graphene sheets. The CNTs are about 1 - 50 nm in diameter and are known to show electric conduction as metals or semiconductors. The CNTs are expected to have various applications owing to their numerous unique properties. One of their applications is their use as the anode materials of a Li-ion secondary battery. In the Li-ion secondary battery, carbon is mainly used as the anode materials [2]. Graphite has a layered structure in which two-dimensional graphene sheets are stacked. When the battery is charged or discharged electrically, Li ions are intercalated or deintercalated between the graphene sheets [3]. Similarly to graphite, Li ions are expected to be inserted and deinserted inside the CNTs. Gao et al. reported that the Li-ion reversible capacity of single-wall carbon nanotubes (SWNTs) was Li1.6C6 (600 mAh/g) and the reversible capacity of ball-milled SWNTs reached Li2.7C6 (1000 mAh/g) [4]. The ideal Li-ion reversible capacity of graphite is LiC6 (372 mAh/g) [4]; hence, CNTs have a Li-ion reversible capacity of about twice or more than that of graphite. On the other hand, there are various methods of synthesizing CNTs, such as a chemical vapor deposition method [5] using a catalyst, an arc method and so on. Kusunoki et al. reported that aligned CNTs were formed on a SiC surface by heating single-crystal SiC at 1500 - 1700 qC in vacuum
* Corresponding author. Tel.: +81-19-697-9082; fax: +81-19-697-9089. E-mail address: [email protected].
1875-3892 © 2011 Published by Elsevier Ltd. doi:10.1016/j.phpro.2011.05.032
165
Fukunori Izumida et al. / Physics Procedia 14 (2011) 164–166
[6]. This method is called the SiC surface decomposition (SSD) method. It is expected that the CNT film fabricated by this method will function as a high-density anode material, because high-density CNTs grow vertical to the carbon face of a SiC wafer [7]. In order to apply CNT film as the anode materials of a solid-state Li-ion secondary battery, a smooth surface of the CNT film is required. Therefore, aiming to apply CNT film as the anode materials of a solid-state Li-ion secondary battery, we attempted to modify the CNT thin films surface by Ar-ion sputtering and evaluated it by field emission scanning electron microscopy (FE-SEM) and Raman scattering spectral measurements. 2. Experiment We formed aligned CNT thin films by heating a 6H-SiC wafer (SiXON Ltd.), 5×5×0.25 mm3 in size, at 1700 qC for 30 min in vacuum (1×10-4 Torr). Then, aluminum sheet with a hole was used as a mask to cover the SiC wafer, and Ar-ion sputtering (Anelva SPF-2108) at 2×10-2 Torr was carried out at three places on the wafer for 15, 30, and 60 s, respectively. The power of radio frequency was ~40 W. FE-SEM (JEOL Ltd. JSM-6701F) was used for the CNT region observations. The acceleration voltage of FE-SEM was 1 kV and the image resolution was 2.2 nm. The Raman scattering spectra of the CNT film were measured using a micro-Raman system (HORIBA Jobin Yvon LabRAM HR-800). A semiconductor laser (O=632.8 nm) was used as an excitation light source in this measurement. 3. Results and Discussion The matter of concern on the CNT film is the growth of projecting micro crystallites (hereafter called CNT needles) on the as-grown CNT film-like region (hereafter called CNT film), as shown in Figure 1(a). The CNT needle is supposed to be composed of a CNT, stands alone on the surface and reaches a length of ~190 nm. In the case of applying the CNT thin film as the anode material of a Li-ion battery, a significant observation indicating that the CNT needle on the surface of the CNT film electrically shorts the battery between anode and cathode electrodes is obtained. After the sputtering process, the CNT needle on the film seems to be almost removed, as shown Figure 1. The electric field for the Ar-ion plasma is concentrated at the tip of the needle, and as a result, one-dimensional CNT needles seem to be effectively bombarded and removed compared with the two-dimensional CNT film surface by Ar-ion sputtering. The as-grown CNT film is ~290 nm in thickness and the sputtered CNT film becomes thinner than the as-grown CNT film. As shown in Figure 1, the CNT film maintains the shape of column like bundles of CNTs under the sputtering process, although a large number of carbon atoms constituting CNTs were sputtered by Ar ions. In order to evaluate the crystalline quality of CNTs damaged by Ar ions, the Raman scattering spectra of the CNT film were measured. As shown in Figure 2, two scattering peaks at around 1332 and 1584 cm -1 were observed. The peak of 1332 cm-1 is the D-band. Since the peak of 1584 cm -1 consists of one peak and one shoulder, it was 190nm 290nm
250nm
220nm
190nm
Figure 1: FE-SEM images of CNT film section on 6H-SiC wafer: (a) as-grown and Ar-ion sputtered for (b) 15, (c) 30, and (d) 60 s.
166
Fukunori Izumida et al. / Physics Procedia 14 (2011) 164–166
fitted with two Lorentzian functions. As a result, the peak of 1584 cm-1 was separated into two peaks at around 1584 cm-1 (G-band) and 1617 cm-1 (D’-band). It is generally considered that the G-band originates from sp2 carbon networks, and the D-band and the D’-band, from defects mostly located at the edges of a crystallite [8]. As shown in Figure 3(a), the intensity ratios of I D/IG and ID’/IG increased with Ar-ion sputtering, and then became constant after more than 30 s of Ar-ion sputtering. In addition, the full-width at half-maximum (FWHM) of each peak became roughly constant, as shown in Figure 3(b). Hence, it is suggested that only the surface region of the CNT film was damaged and the crystalline quality of CNTs below the surface remained intact. 2.5 -1
1332cm
Intensity (arb. unit)
G-band -1
1584cm
D'-band -1
1617cm
Intensity Ratio
D-band
6H-SiC
60s
15s as-grown
500
1000
1.5 1.0 ID'/IG
0.5
(b)
30s
0
ID/IG
0.0
-1
968cm
1500
-1
-1
-1
790cm
FWHM (cm )
-1
768cm
(a)
2.0
60
D-band G-band
40
D'-band
20
2000
Raman Shift (cm )
Figure 2: Raman scattering spectra of CNT film on 6H-SiC wafer.
0
0
10
20 30 40 50 Sputtering Time (s)
60
Figure 3: Characteristics of Raman peaks for various Ar-ion sputtering times. (a) Intensity ratios of D-band to G-band (ID/IG) and Dc-band to G-band (ID’/IG). (b) FWHMs of D-band, G-band and Dcband.
4. Conclusions The surface modifications of the aligned CNT films formed by the SSD method have been successfully realized by the Ar-ion sputtering method in order to use CNT thin films as the anode materials of a solid-state Li-ion secondary battery. Namely, the CNT thin film was effectively etched and the removal of the CNT needles growing on the CNT films was confirmed. It was also confirmed from the Raman scattering spectra that the crystalline quality of CNTs remained intact by the surface modification. References [1] S. Iijima: Nature 354 (1991) 56. [2] J. M. Tarascon, and M. Armand: Nature 414 (2001) 359. [3] M.S. Dresselhaus, and G. Dresselhaus: Adv. Phys. 51 (2002) 1. [4] B. Gao, A. Kleinhammes, X.P. Tang, C. Bower, L. Fleming, Y. Wu, and O. Zhou: Chem. Phys. Lett. 307 (1999) 153. [5] Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P. Siegal, and P.N. Provencio: Science 282 (1998) 1105. [6] M. Kusunoki, J. Shibata, M. Rokkaku, and T. Hirayama: Jpn. J. Appl. Phys. 37 (1998) L605. [7] M. Kusunoki, T. Suzuki, C. Honjo, H. Usami, and H. Kato: J. Phys. D: Appl. Phys. 40 (2007) 6278. [8] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, and R. Saito: Phys. Chem. Chem. Phys. 9 (2007) 1276.