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
3.4.3 Effect of Amorphous Carbon on Raman Signal
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Raman map of an electron beam to the substrates and nanotubes (Figure 3.23 ~ 3.25), the intensity of increasing D-mode was 31.54 ± 26.22 and was 42.67 ± 32.96. The reason for big standard deviation was that the amorphous carbon was not uniform to the substrate and nanotubes. The spectra of amorphous carbon and the spectra of the nanotubes is in Figure 3.22. So, there is even more doubt that electron beam-induced D-mode originated from amorphous carbon.
So, I assumed that D-mode would not increase if there were no amorphous carbon. So, in Figure 3.26, I synthesized CVD nanotubes and irradiated e-beam in the same way as before. In the Raman spectra, I found the electron beam-induced D-mode that was not present at pristine nanotube.
Raman spectra showed that increased D-mode disappeared during annealing process at high temperature. At this time, the condition of annealing process was at 1030 oC for 3 hours with flowing the N2 gas. To remove the hydrocarbon, short time of annealing was enough (~ 30 min). It took more time to remove of the amorphous carbon (~ 3hr). When the electron beam was irradiated again in the same area where D-mode was initially increased by electron beam, the electron beam-induced D- mode does not increase after removing amorphous carbon. When I checked the zoomed-in Raman spectra, there was some D-mode because the amorphous carbon was not completely removed. In fact, it is not important that the D-mode disappears completely, but it is important that the D-mode is less than before, even though e-beam was irradiated in exactly the same area. If the D-mode increased by electron beam came actually from the defect of the nanotube, the D-mode should be increased by the
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Figure 3.21 E-beam-induced D-mode of amorphous carbon. (a) SEM images of amorphous carbon before and after the e-beam irradiation. Blue rectangles indicate the same region. (b) Corresponding Raman spectra of amorphous carbon before (from black circle in (a)) and after (from red circle in (a)) the e-beam irradiation. When amorphous carbon is irradiated by e-beam, the intensity of its D-mode increases.
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same electron beam after the high temperature annealing process. Rather, the electron beam-induced D-mode may increase further. However, the tendency of D-mode to decrease by electron beam after the high temperature annealing process suggests that defect by electron beam is not from nanotubes.
The result applied to graphene same. For graphene, annealing time for one hour at ultra-high vacuum condition (1.0 x 10-10 Torr) at 500 oC. The reason why the conditions were different from those of nanotubes was that graphene was not tolerable at the high temperature annealing. When I investigated the intensity of the d-mode that was increased by e-beam again after annealing, I confirmed that the D-mode decreased sharply. When the amorphous carbon was removed through the annealing process, the D-mode by electron beam was found to be reduced.
Finally, I checked how RBM-mode is affected by with the presence of an amorphous carbon.
A nanotube synthesized by CVD technique as usual was selected, and the Raman signal was obtained from the area where the RBM-mode is included. All peaks were normalized in G-mode to compare the RBM/G ratio accurately. The RBM/G ratio of the pristine nanotube was 0.318. the RBM-mode of 193 cm-1 was selected as (15,2) using 532nm Raman spectroscopy (Figure 3.27). When the pristine nanotube was exposed to the electron beam, the RBM/G ratio was reduced by 22% to 0.248. In this Raman spectra, electron beam-induced D-mode increased. When the annealing process at high temperature was performed on a nanotube irradiated by an electron beam, the electron beam-induced D-mode disappeared and the RBM/G slightly raised to 0.278. When the electron beam re-irradiated in same area as before, the RBM/G ratio was 0.270. The RBM/G ratio was almost same as since annealing. Although amorphous carbon was removed in annealing process, the electron beam-induced
Figure 3.22 Effect of irradiated amorphous carbon on the D-mode of a nanotube. (a) The histograms indicated that the extent of the increase in the intensity of the D-mode is similar in both samples. (b) Raman spectra of amorphous carbon (left) and SWNT (right) after prolonged irradiation with an e-beam; both cases show enhanced D-modes.
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D-mode increased due to secondary irradiation of electron beam. However, the electron beam-induced D-mode was reduced compared to D-mode increased by irradiating the first electric beam. In other words, removing the amorphous carbon during the annealing process also reduces the electron beam-
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Figure 3.23 E-beam-irradiated amorphous carbon and SWNTs. (a) SEM images before (left) and after (right) the irradiation. SWNTs and amorphous carbon next to a SWNT were simultaneously irradiated by the e-beam. Hydrocarbon (dark rectangles) was observed on the surface after the irradiation. The exposed area is indicated by the green and purple rectangles for the amorphous carbon and SWNT, respectively, in the SEM images of samples in both the pristine and irradiated conditions. (b) Raman G-map corresponding to the image in (a) before and after the irradiation. The areas indicated by the green and purple rectangles in the Raman G-maps are the same as those in the SEM images. Each of the pixels in the green and purple rectangles are designated as A1–H7 (amorphous carbon) and #1 to
#18 (SWNT). The Raman G-map consists of 16 × 22 pixels with 500 nm intervals, summing the Raman spectra count between 1500 and 1700 cm−1.
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induced D-mode, and the signal from the RBM-mode is not significantly affected by the electric beam.
In summary, D-mode, which is increased by a low-energy electron beam, does not imply the defect of nanotube and graphene. The D-mode was increased by electron beam from the amorphous carbon, and this amorphous carbon was on the nanotube and graphene surfaces. When electron beam was irradiated to nanotube and graphene, it seemed as if the D-mode was increased in nanotube and graphene. When the amorphous carbon is removed, the D-mode will no longer be increased even if the e-beam is irradiated.
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Figure 3.24 Similarity between the e-beam-induced D-modes originated from amorphous carbon and SWNTs. Raman spectra before and after the irradiation corresponding to Fig. 34. Pristine and irradiated Raman spectra are shown purple (nanotubes) rectangles. Each spectra are designated as
#1 to #18 (SWNT). The intensity of the D-mode of the nanotube increases after the e-beam irradiation, and the increase in the background of the Raman spectra of the irradiated samples is due to hydrocarbon.
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Figure 3.25 Similarity between the e-beam-induced D-modes originated from amorphous carbon and SWNTs. (a) Raman spectra before and after the irradiation corresponding to Fig. 34.
Pristine and irradiated Raman spectra are shown in the green (amorphous carbon). Each spectra is designated as A1–H7 (amorphous carbon). The intensity of the D-mode of the amorphous carbon and the increase in the background of the Raman spectra of the irradiated samples is due to hydrocarbon.
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Figure 3.26 Effect of irradiated amorphous carbon on the D-mode of a nanotube(left) and graphene(right). (a), (c) SEM images and (b), (d) corresponding Raman spectra of a nanotube subjected to various treatments. After the amorphous carbon has been removed by annealing (blue), e- beam irradiation no longer increases the D-mode intensity (green). This conclusion applies well to the graphene. (right)
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Figure 3.27 Effect of annealing treatment to the RBM and D-mode of a SWNT. The SWNT with RBM at 193 cm-1 is assigned to (15,2). The experiment in Figure 4 was repeated while collecting both RBM and D-mode. The RBM decreased by 22% upon irradiation (#1), partially recovered by annealing, and then decreased by only 2.9% upon the irradiation #2, which is consistent with the results in Figure 4. The D-mode also followed a similar trend except that the peak slightly increased upon irradiation (#2) even after the annealing treatment, presumably due to insufficient removal of the amorphous carbon.
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