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
3.4.2 Effect of Hydrocarbon on Raman Signal
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PMMA film Hydrocarbon
5 µm Hydrocarbon irradiated (dosage: 9.3 x 1021cm-2)
Hydrocarbon irradiated (dosage: 16.5 x 1021cm-2) Before transferring
Lower line hydrocarbon (dosage: 16.5 x 1021cm-2) 20 µm Upper line hydrocarbon (dosage: 9.3 x 1021cm-2)
After transferring
b
c
d a
20 µm
Hydrocarbon -transferred Directly irradiated
Raman shift (cm-1) Raman shift (cm-1)
1300 1400 1500 1600 e
1300 1400
G
D
Figure 3.13 Hydrocarbon transferred onto SWNTs. (a) Scheme of the hydrocarbon transfer process.
(b) SEM image of the hydrocarbon before the transfer. (c) SEM image of the hydrocarbon after it was transferred onto SWNTs. SWNTs covered by the hydrocarbon are not directly exposed to the e-beam.
(d) Expanded SEM image of the area indicated by the blue rectangle in (c). (e) The Raman spectra was obtained from the black rectangular region.
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The electron beam was directly exposed to other area of the hydrocarbon-contacted nanotube.
Directly exposed nanotube area showed an increase in D-mode.
In contrast to the previous experiments, to prevent some concerns of the loss of hydrocarbon during transfer, the hydrocarbon was first placed on the empty wafer and nanotube was transferred on the hydrocarbon deposition (Figure 3.14). Although the nanotube had direct contact with the hydrocarbon without electron beam irradiation, there was no increase in Raman D-mode from the nanotube. And the nanotube was directly irradiated by electron beam, the nanotube showed the D- mode increases. In other words, the D-mode did not increase after the hydrocarbon was deposited on the nanotube.
I also found an electron beam-induced D-mode without hydrocarbon. Using 1% PMMA
Figure 3.14 Nanotube transferred onto the hydrocarbon. (a) Scheme of the SWNT transfer process.
(b) SEM image of hydrocarbon before and after the SWNT transfer. The left image shows the hydrocarbon (black rectangle) deposited (dosage: 3.1 ×1021 cm−2) onto an SiO2 wafer. The right image shows the SWNTs transferred onto the hydrocarbon. SWNTs in the red rectangle were directly irradiated by the e-beam (dosage: 3.1 × 1021 cm−2), whereas those in the black rectangle were not. (c) Raman spectra of SWNTs covering the hydrocarbon and irradiated by the e-beam; the D-mode increases after the direct e-beam irradiation (red). Although SWNTs were placed on top of the hydrocarbon, the D-mode did not increase in intensity (black), indicating that the main reason for the increase in D-mode intensity is not the hydrocarbon.
Raman shift (cm-1) Raman shift (cm-1)
1300 1400 1500 1600 1325 1375
Transferred onto hydrocarbon (dosage: 3.1 x 1021cm-2) Directly irradiated (dosage: 3.1 x 1021cm-2)
a
b
c
20 µm 20 µm
PMMA film with SWNTs
Before transferring After transferring
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solution, the nanotubes (13,6), where RBM of the nanotube is 185 cm-1 (Figure 3.15), synthesized by CVD technique was spin-coated to 50nm thickness as shown in Figure 3.17. When the PMMA film is too thick, the coated nanotubes images are not clear in SEM. To expose electron beam to selective nanotubes, the thickness of PMMA film should be low. And then, the PMMA film was baked at 170 °C for 5min and the electron beam with same condition of previous experiment was exposed to nanotubes. Although this method reduces the energy of e-beam delivered to nanotubes by PMMA film, the hydrocarbon generated by E-beam is deposited on the PMMA film. The electron beam-irradiated
nanotubes can avoid direct contact with hydrocarbon. As see the Raman Spectra, the pristine nanotube did not show D-mode, but the electron beam-induced D-mode occurred from the PMMA film-coated nanotube. When the PMMA film were removed with acetone, the hydrocarbon disappeared together on the PMMA film. The nanotube was free from the contact of hydrocarbon despite of irradiation of electron beam. However, the electron beam-induced D-mode was still seen even if the PMMA was removed and hydrocarbon was not on the irradiated nanotube.
Because PMMA was coated and Raman spectra is measured in this experiment, the effects of PMMA film on the Raman spectra should be checked. In Figure 3.16, the Raman spectra obtained from SiO2 Wafer is the same as the Raman spectra of SiO2 Wafer with coating the PMMA film.As see both the data, there is no signal difference between the two spectra signals. (100 cm-1 ~ 3000 cm-1) Hence, I conclude that the PMMA film does not affect the Raman spectra. Figure 3.17 shows the experimental scheme for removing hydrocarbon using PMMA film. When electron beam-irradiated PMMA film was removed using acetone, the crosslinked PMMA remained in the irradiated area of the electron beam. The crosslinked PMMA was completely removed with acetone. The same dosage of electron beam was irradiated on two samples with different PMMA thickness. As see the height profile of crosslinked PMMA (Figure 3.17), there is height for two samples. Although the thickness of
Raman shift (cm-1) 200 100
(13,6)
Figure 3.15 RBM of the pristine nanotube. The tube was assigned to (13,6) based on the RBM at 185 cm-1.
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the two PMMA films was different, the thickness of the crosslinked PMMA was similar. In other words, the same dosage of electron beam was irradiated and the crosslinked PMMA was formed with a similar thickness regardless of initial thickness of PMMA film. The crosslinked PMMA thickness was approximately ~10 nm from AFM measurement (Figure 3.18). When the PMMA film is thicker
PMMA
Hydrocarbon Amorphous carbon
Crosslinked PMMA a
Raman shift (cm-1) Raman shift (cm-1)
b
1300 1400 1500 1600 Pristine
PMMA-coated
& irradiated PMMA-removed
Pristine
PMMA-coated & irradiated PMMA-removed
Irradiated amorphous carbon
E-beam
1300 1400
G D
Figure 3.17 Detailed analysis of the effect of hydrocarbon on the increased D-mode intensity in the Raman spectra of a nanotube. (a) Schematic showing the e-beam-induced deposition of hydrocarbon onto a PMMA-coated nanotube and (b) the corresponding Raman spectra. Lifting off the hydrocarbon layer from the irradiated SWNT does not eliminate the D-mode from the spectra.
SiO2wafer with PMMA coating
Raman shift (cm-1) 3000
1000 2000
Count
3000 1000 2000
0 SiO2wafer
Raman shift (cm-1)
Count
3000 1000 2000
0 3000
1000 2000
Figure 3.16 Effect of the PMMA film on the Raman spectra. Although the SiO2 wafer was coated with a PMMA film, its Raman signals did not change, indicating that PMMA did not affect the Raman spectra in our study.
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than 50nm, unable to focus and could not continue to irradiate the beam in the same position. As see the optical and AFM images, crosslinked PMMA is clearly visible.
Examining the effect of the crosslinked PMMA on the Raman spectra results in an overall background increasing from 1000 cm-1 to 3000 cm-1 (Figure 3.19). but not by e-beam. However, it is not too high to distinguish electron beam-induced D-mode. The important thing here is that the electron beam-induced D-mode comes with PMMA film removed and crosslinked PMMA remaining.
0 10 20 30
μm
Thickness (nm)
5 10 15
0 20
0 10 20 30
μm
Thickness (nm)
5 10 15
0 20
b c
a
PMMA film
Substrate
E-beam
Hydrocarbon
Crosslinked PMMA E-beam
irradiation Acetone rinse
5 µm 5 µm
d e
Thickness: 9.4 nm Thickness: 13.4 nm
Figure 3.18 PMMA film over-exposed to the e-beam. (a) Scheme of PMMA over-exposure to the e- beam. When the PMMA film was over-exposed to the e-beam, the PMMA was crosslinked and could not be fully removed via acetone rinsing. (b) Height profile of the crosslinked PMMA. PMMA film (thickness: 27.9 nm) was irradiated by the e-beam (dosage: 3.1 × 1021 cm−2). After the acetone rinse, the PMMA film was only partially removed, leaving crosslinked PMMA. The thickness of the crosslinked PMMA was 9.4 nm. (c) Height profile of the crosslinked PMMA formed from a thicker PMMA film. The PMMA film (thickness: 45.4 nm) was irradiated using the same e-beam dosage (3.1
× 1021 cm−2). The thickness of the residual crosslinked PMMA was 13.4 nm after the acetone rinse.
The results in (b) and (c) verify that the hydrocarbon deposited on the PMMA film was fully removed after the acetone rinse. (d) Optical micrograph and (e) AFM image of the crosslinked PMMA (thickness: 30 nm) formed by over-exposing the PMMA film (thickness: 50 nm) to the e-beam.
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Obviously, the D-mode, which was increased in e-beam, introduced the phenomenon of elimination by PMMA Coating and Removing. The beam was first irradiated on the nanotube and coated with PMMA film, and the electron beam-induced D-mode disappear. However, when the crosslinked PMMA was remaining on the nanotube, the same coating and removing of PMMA film did not remove the electron beam-induced D-mode. This means that something on the surface of a nanotube was irradiated by an e-beam and shows the D-mode, which is not removed together by the crosslinked PMMA. With this assumption, I conducted an experiment on an amorphous carbon in the next chapter.
In summary, it is inevitable that the hydrocarbon would be deposited on the exposed part of the electron beam. When nanotubes were irradiated by electron beam, electron beam-induced D-mode would increase, whether these electron beam-induced D-mode came from nanotubes or hydrocarbon could not be distinguished. Hence, the hydrocarbon deposition was transferred onto and under the un- irradiated nanotube. It means that the nanotube didn’t expose to electron beam, but the nanotube has a direct contact with the hydrocarbon. The result was that D-mode did not come out even though the nanotube was contacted directly with the hydrocarbon. Although there is a crosslinked PMMA, the D- mode was found even without the hydrocarbon. In other words, the hydrocarbon is not the cause of the increasing D-mode of nanotubes exposed by e-beam.
Raman shift (cm-1)
2000 1500
1000
Crosslinked PMMA No crosslinked PMMA
Figure 3.19 Effect of the crosslinked PMMA on the Raman signal. When PMMA was over- exposed to the e-beam, PMMA was partially crosslinked and could not be removed completely by the acetone rinse, thereby affecting the Raman background signal. The residual and crosslinked PMMA only slightly raised the background Raman signal in the region from 1000 to 2000 cm−1, which can be distinguished from the D-mode.
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