Prat I. Effect of E-Beam on Raman Spectra of Carbon Nanotube and Graphene
Chapter 3. Effect of Low-Energy Electron Beam on Nanotubes and Graphene
3.4 A Low-Energy Electron Beam Does Not Damage Single-Walled Carbon Nanotubes and Graphene
3.4.1 Effect of Low-energy Electron Beam on Nanotube and Graphene
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3.4 A Low-Energy Electron Beam Does Not Damage Single-Walled Carbon
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0.000 8.750 17.50 26.25 35.00
d
Pristine Irradiated Annealed
Pristine Irradiated Annealed
Raman shift (cm-1) Raman shift (cm-1)
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b a
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Figure 3.8 SEM and Raman spectroscopic analysis for pristine, irradiated and annealed. (a) SEM images and (b) Raman D-maps show the presence of hydrocarbon in the irradiated rectangular area, which is removed after high-temperature annealing. (c) Raman G-maps. In the G-map, Hydrocarbon in the G-map does not look as clear as D-map. This is because the intensity of the G-mode is relatively greater than the intensity of the hydrocarbon. As you look at the irradiated and the annealed G-map, the intensity of the G-mode does not change after irradiation of electron beam or after annealing process. The Raman G-map consists of 25 × 30 pixels at 1 µm intervals. (d) SWNT Raman spectra collected from the circled areas in panel a showing the increased D-mode intensity upon irradiation, followed by elimination of the D- mode by annealing.
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In other words, the area where hydrocarbon was placed means the area where the e-beam was exposed. As see the Raman D-mode map, the irradiated area showed increased D-mode. The zoomed-in Raman spectra showed an increased D-mode from a nanotube exposed to e-beam. This increased D-mode has led many researchers to accept that low-energy electron beam created the defect in nanotubes. And this increased D-mode disappeared through the high temperature annealing process. As see the SEM images, the hydrocarbon, which was deposited by an electron beam, disappeared. And the increased D-mode disappeared as like before being irradiated by the electron beam, and the zoomed-in Raman spectra also showed same result. The removal of increased D-mode was attributed to disappearing defect in the nanotubes. However, when I looked at the SEM image after annealing process, I confirmed that the electron beam-induced hydrocarbon also disappeared.
When an annealing process was performed at higher temperatures than 1000 oC, it had a possibility that the amorphous carbon on the surface of the nanotube would be also removed. After recovering the defect in the nanotube, removal of irradiated amorphous carbon and burning out the hydrocarbon occurred simultaneously during high temperature annealing process, I obtained annealed Raman spectra. It is unknown what of the three phenomena caused the increased D-mode to disappear.
However, previous studies concluded that these annealing treatment simply restored the electron beam-induced defect in the nanotube with disappearing e-beam induced D-mode [134, 135, 142].
Three phenomena occurred simultaneously through the high temperature annealing process. I should find out what phenomenon is related with the increased D-mode to disappear.
Raman shift (cm-1)
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(16,5)
Figure 3.9 RBM of the SWNT from pristine. Based on the RBM at 161 cm-1, the nanotube was assigned to (16,5) SWNT.
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However, I found that the increased D-mode at room temperature disappears without high temperature annealing process. Figure 3.9 shows that aligned nanotube (16,5), where RBM of the nanotube is 161 cm-1 using 532 laser Raman spectra, is used as before with same method. The Raman D-mode map showed that there was no D-mode, and Raman spectra also showed no D-mode (Figure
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Figure 3.10 Removal of the e-beam-induced D-mode without heat treatment. (a) SEM images of a pristine SWNT, a SWNT after e-beam-irradiation, and a SWNT after application and removal of a PMMA coating. (b) Raman D-maps corresponding to the rectangular areas in panel a. The D-mode of the irradiated SWNT disappeared after the application and removal of the PMMA coating (dotted areas). (c) Raman G-map images. Because of the relatively high intensity of the G-mode in the Raman spectra of SWNTs and the long exposure time of the SWNTs to the laser, the e-beam- irradiated area is not clearly resolved in the G-map. The Raman G-map consists of 40 × 15 pixels with intervals of 0.5 µm and 1 µm, respectively. (d) The corresponding Raman spectra confirm that the e- beam induced D-mode of SWNT can be eliminated by chemical treatment at room temperature.
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3.10). When the electron beam was exposed to the nanotubes as before (dosage 2.3 x 1021 to 3.1 x 1021 cm-1), the hydrocarbon was deposited on the nanotube. The Raman D-mode map showed that more clearly increased D-mode appeared throughout nanotubes. The zoomed-in Raman spectra showed the increased D-mode. It is important that PMMA solution was coated on the irradiated nanotube and
removed with acetone without nay bake process, the increased D-mode disappeared. After coating and removing PMMA, checked the same nanotube using SEM, the hydrocarbon was still in the same location. However, the Raman D-mode map showed that the increased D-mode disappeared and zoomed-in Raman spectra also showed the increased D-mode disappeared. The electron beam- induced D-mode was removed without high temperature treatment. That was making one wonder if these increased D-mode originated from defect of nanotubes. The reason is that a recovering from defect nanotube requires a high temperature treatment to form a covalent bonding. Because this formation of the covalent bonding cannot be achieved at low temperatures, it is evidence that previously increased D-mode is not the defect of nanotube.
Previous studies showed that small-diameter nanotubes were vulnerable to electron beam- induced damage and large-diameter nanotubes had relatively tolerance to low-energy electron beam [134]. After exposing electron beam to aligned nanotubes, the ratio of D/G ratio was investigated according to diameter. The diameter of the nanotube was assigned using a known reference [143, 144], after measuring the RBM mode using the 532nm laser Raman spectroscopy. As see the Figure 3.11, it is the statistics for increasing D/G ratio after irradiation of electronic beam using 64 nanotubes. The
Metallic SWNTs0.84-1.15 nm Semiconducting SWNTs1.15-1.83 nm
Diameter (nm)
△(D/G)
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0.8 1.0 1.2 1.4 1.6 1.8
0.04 0.06 0.08 0.10
Figure 3.11 Correlation between the diameter of SWNTs and Δ(D/G). Our results indicate that the e-beam-induced damage has no direct correlation with the diameter of the SWNTs.
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diameter from 0.84 nm to 1.15 nm is metallic nanotube and from 1.15 nm to 1.83 nm is semiconducting nanotubes. Although the metallic nanotubes showed high D/G ratio after exposure to electron beam, all metallic nanotubes do not show the same phenomenon. In particular, when methane was turned off during nanotube synthesis and N2 flowed for long periods of time at the same temperature to clean the nanotube, the electron beam-induced D/G ratio was less. As see the semiconducting nanotube part, the nanotubes do not show a tendency for D/G ratio to increase with diameter. Several nanotubes were investigated simultaneously using the random network nanotube in previous studies, but to avoid these complicated situation, individual nanotubes were investigated in this study. After investigating the effect of electron beam in individual nanotubes, it was found to have no constant tendency for electron beam-induced D/G ratio depending on the diameter.
When see the Raman spectra which is normalized with a G-mode, (Figure 3.12), the RBM peak tends to be reduced by an electron beam. Figure 3.12 (a) shows that the D-mode was not increased by the electron beam, but the RBM/G ratio showed a 7.9% decrease from 0.140 to 0.129.
This means that the peak of the RBM is reduced by the electron beam regardless of whether there is an increase in D-mode. In particular, in case of nanotubes with increased D-mode after irradiation of the electron beam, the RBM mode tends to reduce the peak intensity further. Figure 3.12 (b) shows a 22% decrease in RBM/G ratio from 0.318 to 0.248 before and after an exposure of electron beam.
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