Effects of Surface Chemistry of Substrates on Raman Spectra in ...
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Effects of Surface Chemistry of Substrates on Raman Spectra in Graphene *Takahiro Tsukamoto, Kenji Yamazaki, Hiroki Komurasaki and Toshio Ogino Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan AUTHOR EMAIL ADDRESS: [email protected]
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AFM observation of water layers at graphene/substrate interface. Figure S1a shows a topographical image of a few-layer graphene flake on a single-stepped sapphire (0001) substrate, S1b its frictional force image, S1c magnified image of the area indicated with square in Fig. S1a, and S1d a topographical image of a few-layer graphene fake on a step-bunched sapphire (0001) substrate. These AFM images were obtained using the AFM contact mode. Water layers confined at the graphene/substrate interface were observed on the graphene flake surface attached to the single-stepped or step-bunched sapphire (0001) substrate as previously demonstrated in graphene on mica [S1]. This is attributed to the atomic flatness of the sapphire surface. The height of the observed water layer was about 0.36 nm, which is consistent with the height of the water layer of 0.37 nm [S1]. On the singlestepped sapphire (0001) substrate, the water layers were divided by the sapphire steps in Fig. S1c. The visible water layers probably correspond to the fluidic water layers because similar water layers were not observed on sapphire substrates of the other orientation. These results suggest that a large amount of water molecules exist at the interface between graphene flakes and a sapphire (0001) surface. Details of the water layers between graphene and a sapphire substrate will be reported in a separate paper.
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Figure S1. (a) Topographical image of few-layer graphene flake on single-stepped sapphire (0001) substrate, (b) its frictional force image, (c) magnified image of area indicated with square in (a), and (d) topographical image of few-layer graphene flake on step-bunched sapphire (0001) substrate. Scale bars of (a) and (b) are 5 µm and those of (c) and (d) are 1 µm.
Model of graphene/substrate interface. Figure S2 shows a cross-sectional schematic of graphene on a phase-separated sapphire (0001) substrate. The difference in the heights of graphene flakes on a hydrophilic surface and a hydrophobic one can be observed in Fig. 4. This is believed due to the removal of the water layer from the graphene/substrate interface due to the affinity between graphene and the hydrophobic surface.
Figure S2. Cross-sectional schematic of graphene attached to hydrophilic and hydrophobic surface. 3
Effect of thermal treatment of support substrate on Raman spectra of graphene flakes To reveal the effect of a water layer at the interface between sapphire surfaces and graphene, we examined the Raman spectra using random-stepped rough sapphire (0001) surfaces whose hydrophilicity was controlled. These substrates were cleaned with an H2SO4-H2O2 solution at 90ºC for 10 min and sonicated in pure water for 5 min [S2]. After acid treatment, the sapphire surfaces were terminated with hydroxyl groups, which work as adsorption sites of water molecules. Hydroxyl groups can be desorbed by thermal treatment. Reduction of hydroxyl group density by annealing decreases the hydrophilicity on the annealed sapphire surfaces. We annealed the random-stepped sapphire (0001) surfaces at 800ºC for 1 h just before graphene deposition. We deposited graphene flakes by mechanical exfoliation of graphite. Figure S3 shows the measurement results of the G-peak and 2D-peak positions of Raman spectroscopy on the single layer graphene flake supported by the annealed sapphire surfaces. Both Gpeak and 2D-peak positions shifted to lower wave numbers than those on the hydrophilic sapphire (0001) substrate. We measured the shift at the same spot again after three and nine days. Figure S4 shows one of the results of the G-peak shift over time. We observed the peaks shifted to higher wave numbers on some spots. We could also observe the variation in the peak positions of the G-band and 2D-band under over time. It is well known that step/terrace structures of a sapphire substrate can be rearranged by high temperature annealing. However, the temperature was too low to affect the step arrangement in this case. Therefore, the peak shifts were induced by the amount of water molecules that existed at the interfaces between the sapphire surfaces and graphene flakes.
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Figure S3. Peak position of G-band and 2D-band of Raman spectra of graphene flakes on annealed random-stepped sapphire substrates.
Figure S4. Typical Raman shift of G-peak position on random-stepped sapphire surfaces.
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Summary of G-band and 2D-band of graphene flakes on insulating substrates. Table S1 lists the average G-peak and 2D-peak positions of graphene on various insulating substrates and their full width at half mazimum (FWHM).
Table S1. Average G-peak and 2D-peak positions and their FWHM for graphene/graphite on various insulating substrates. Substrate
G-band
Pos (2D)
Peak position
FWHM
Peak position
FWHM
SiO2/Si substrate
1587.6
8.8
2675.7
29
Quartz (0001) substrate
1590.3
8.0
2678.4
28
Sapphire (0001) substrate
1593.4
9.5
2681.7
30
1592.3
8.3
2681.9
27
Sapphire (11-20) substrate
1586.4
10
2677.6
27
Sapphire (1-102) substrate
1585.8
11
2677.1
29
Phase-separated sapphire (0001) substrate: G1
1587.3
11
2679.7
28
Phase-separated sapphire (0001) substrate: G2
1594.6
7
Bunched substrate
sapphire
(0001)
References
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[S1] Xu, K.; Cao, P.; Heath, J. R. Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. Science 2010, 329, 1188-1191. [S2] Isono, T.; Ikeda, T.; Aoki, R.; Yamazaki, K.; Ogino, T. Surf. Sci. 2010, 604, 2055-2063.
Complete author list [49] Mohiuddin, T. M. G.; Lombardo, A.; Nair, R. R.; Bonetti, A.; Savini, G.; Jalil, R.; Bonini, N.; Basko, D. M.; Galiotis, C.; Marzari, N.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C. Phys. Rev. B 2009, 79, 205433/1-205433/8. [54] Huang, C.; Wikfeldt, K. T.; Tokushima, T.; Nordlund, D.; Harada, Y.; Bergmann, U.; Niebuhr, M.; Weiss, T. M.; Horikawa, Y.; Leetmaa, M.; Ljungberg, M. P.; Takahashi, O.; Lenz, A.; Ojamäe, L.; Lyubartsev, A. P.; Shin, S.; Pettersson, L. G. M.; Nilsson, A. PNAS 2009, 106, 15214-15218.
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Effects of Surface Chemistry of Substrates on Raman Spectra in Graphene *Takahiro Tsukamoto, Kenji Yamazaki, Hiroki Komurasaki and Toshio Ogino Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan AUTHOR EMAIL ADDRESS: [email protected]
1
AFM observation of water layers at graphene/substrate interface. Figure S1a shows a topographical image of a few-layer graphene flake on a single-stepped sapphire (0001) substrate, S1b its frictional force image, S1c magnified image of the area indicated with square in Fig. S1a, and S1d a topographical image of a few-layer graphene fake on a step-bunched sapphire (0001) substrate. These AFM images were obtained using the AFM contact mode. Water layers confined at the graphene/substrate interface were observed on the graphene flake surface attached to the single-stepped or step-bunched sapphire (0001) substrate as previously demonstrated in graphene on mica [S1]. This is attributed to the atomic flatness of the sapphire surface. The height of the observed water layer was about 0.36 nm, which is consistent with the height of the water layer of 0.37 nm [S1]. On the singlestepped sapphire (0001) substrate, the water layers were divided by the sapphire steps in Fig. S1c. The visible water layers probably correspond to the fluidic water layers because similar water layers were not observed on sapphire substrates of the other orientation. These results suggest that a large amount of water molecules exist at the interface between graphene flakes and a sapphire (0001) surface. Details of the water layers between graphene and a sapphire substrate will be reported in a separate paper.
2
Figure S1. (a) Topographical image of few-layer graphene flake on single-stepped sapphire (0001) substrate, (b) its frictional force image, (c) magnified image of area indicated with square in (a), and (d) topographical image of few-layer graphene flake on step-bunched sapphire (0001) substrate. Scale bars of (a) and (b) are 5 µm and those of (c) and (d) are 1 µm.
Model of graphene/substrate interface. Figure S2 shows a cross-sectional schematic of graphene on a phase-separated sapphire (0001) substrate. The difference in the heights of graphene flakes on a hydrophilic surface and a hydrophobic one can be observed in Fig. 4. This is believed due to the removal of the water layer from the graphene/substrate interface due to the affinity between graphene and the hydrophobic surface.
Figure S2. Cross-sectional schematic of graphene attached to hydrophilic and hydrophobic surface. 3
Effect of thermal treatment of support substrate on Raman spectra of graphene flakes To reveal the effect of a water layer at the interface between sapphire surfaces and graphene, we examined the Raman spectra using random-stepped rough sapphire (0001) surfaces whose hydrophilicity was controlled. These substrates were cleaned with an H2SO4-H2O2 solution at 90ºC for 10 min and sonicated in pure water for 5 min [S2]. After acid treatment, the sapphire surfaces were terminated with hydroxyl groups, which work as adsorption sites of water molecules. Hydroxyl groups can be desorbed by thermal treatment. Reduction of hydroxyl group density by annealing decreases the hydrophilicity on the annealed sapphire surfaces. We annealed the random-stepped sapphire (0001) surfaces at 800ºC for 1 h just before graphene deposition. We deposited graphene flakes by mechanical exfoliation of graphite. Figure S3 shows the measurement results of the G-peak and 2D-peak positions of Raman spectroscopy on the single layer graphene flake supported by the annealed sapphire surfaces. Both Gpeak and 2D-peak positions shifted to lower wave numbers than those on the hydrophilic sapphire (0001) substrate. We measured the shift at the same spot again after three and nine days. Figure S4 shows one of the results of the G-peak shift over time. We observed the peaks shifted to higher wave numbers on some spots. We could also observe the variation in the peak positions of the G-band and 2D-band under over time. It is well known that step/terrace structures of a sapphire substrate can be rearranged by high temperature annealing. However, the temperature was too low to affect the step arrangement in this case. Therefore, the peak shifts were induced by the amount of water molecules that existed at the interfaces between the sapphire surfaces and graphene flakes.
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Figure S3. Peak position of G-band and 2D-band of Raman spectra of graphene flakes on annealed random-stepped sapphire substrates.
Figure S4. Typical Raman shift of G-peak position on random-stepped sapphire surfaces.
5
Summary of G-band and 2D-band of graphene flakes on insulating substrates. Table S1 lists the average G-peak and 2D-peak positions of graphene on various insulating substrates and their full width at half mazimum (FWHM).
Table S1. Average G-peak and 2D-peak positions and their FWHM for graphene/graphite on various insulating substrates. Substrate
G-band
Pos (2D)
Peak position
FWHM
Peak position
FWHM
SiO2/Si substrate
1587.6
8.8
2675.7
29
Quartz (0001) substrate
1590.3
8.0
2678.4
28
Sapphire (0001) substrate
1593.4
9.5
2681.7
30
1592.3
8.3
2681.9
27
Sapphire (11-20) substrate
1586.4
10
2677.6
27
Sapphire (1-102) substrate
1585.8
11
2677.1
29
Phase-separated sapphire (0001) substrate: G1
1587.3
11
2679.7
28
Phase-separated sapphire (0001) substrate: G2
1594.6
7
Bunched substrate
sapphire
(0001)
References
6
[S1] Xu, K.; Cao, P.; Heath, J. R. Graphene Visualizes the First Water Adlayers on Mica at Ambient Conditions. Science 2010, 329, 1188-1191. [S2] Isono, T.; Ikeda, T.; Aoki, R.; Yamazaki, K.; Ogino, T. Surf. Sci. 2010, 604, 2055-2063.
Complete author list [49] Mohiuddin, T. M. G.; Lombardo, A.; Nair, R. R.; Bonetti, A.; Savini, G.; Jalil, R.; Bonini, N.; Basko, D. M.; Galiotis, C.; Marzari, N.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C. Phys. Rev. B 2009, 79, 205433/1-205433/8. [54] Huang, C.; Wikfeldt, K. T.; Tokushima, T.; Nordlund, D.; Harada, Y.; Bergmann, U.; Niebuhr, M.; Weiss, T. M.; Horikawa, Y.; Leetmaa, M.; Ljungberg, M. P.; Takahashi, O.; Lenz, A.; Ojamäe, L.; Lyubartsev, A. P.; Shin, S.; Pettersson, L. G. M.; Nilsson, A. PNAS 2009, 106, 15214-15218.
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