Effects of Surface Chemistry of Substrates on Raman Spectra in

Jan 28, 2012 - (9) Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. J. Am. Chem. ... (14) Romero, H. E.; Shen, N.; Joshi, P.; Gutierrez, H. R...
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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 S Supporting Information *

ABSTRACT: We investigated the effects of surface chemistry of substrates on the Raman spectra of graphene flakes that come into contact with various insulating substrates, such as quartz and sapphire, under ambient conditions at room temperature. The Gpeak positions of graphene flakes on such substrates were investigated, and significant blue-shifts of the G-band were observed on a chemically single-phased sapphire (0001) substrate. On a phase-separated sapphire (0001) substrate with Al-terminated (hydrophilic) and O-terminated (hydrophobic) domains, the G-band of graphene flakes was composed of two peaks centered at 1587 cm−1 (G1-peak) and 1593 cm−1 (G2peak). The G1-peak originated from the O-terminated domain and the G2-peak from the Al-terminated one. Since the 2D-peak shifts were small, the Raman shifts in the G-band were attributed to chemical doping from environmental conditions, especially water layers at the graphene/substrate interface that cause hole-doping. The blue-shift in the G-band increased with the increase in the amount of water molecules subject to the surface chemistry of the substrate. Even though Raman spectroscopy is an excellent tool for characterizing the physical properties of graphene, this study indicates that preparation of the substrate surface is important for determining Raman spectroscopy of graphene because its peak positions are easily shifted due to the surface chemistry.

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the atomic-level structures on the surface can be designed and kept unchanged in air as well as in vacuum.21−28 For example, a phase-separated sapphire (0001) surface with Al-terminated (hydrophilic) and O-terminated (hydrophobic) domains can be obtained after annealing in vacuum.24,26−28 By using this phaseseparated substrate, the local density of water molecules at the graphene/substrate interface can be controlled, and the effect of the substrate’s surface chemistry on graphene properties can be examined. Raman spectroscopy is a key diagnostic tool for identifying the number of graphene layers and obtaining their physical properties.29−34 Chemical doping can also be detected using Raman spectroscopy because an increase in the charge density in graphene results in stiffening of the G-mode.35−38 We investigated the Raman spectra of graphene flakes attached to insulating substrates, such as SiO2/Si, quartz, and sapphire, under ambient conditions at room temperature to clarify the effects of the substrate materials, its surface chemistry, and morphology. Raman spectroscopy was carried out statistically because the lattice vibration is strongly affected by the charged impurities and Raman spectra are generally scattered.12

ince the discovery of a single sheet of graphene in 2004, this remarkable material has been the subject of many studies due to its unique mechanical, chemical, physical, and thermal properties.1−8 In particular, there has been much interest in chemical doping of graphene because tunable electrical properties of graphene can be obtained by incorporating the dopant atoms into potential graphene systems.9−11 However, graphene is also doped under environmental conditions such as adsorbed gases.12−15 To precisely control chemical doping of graphene, the effects of ambient conditions on unintentional doping should be elucidated. Water molecules adsorbed to the substrate behave as a dopant in graphene.14,16 It was reported that hole-doping in carbon nanotubes (CNTs) is induced by water molecules and increases with an increase in water molecules around the CNTs.17 This suggests that the doping level in graphene can be regulated by the surface chemistry of the substrate. Moreover, the effect of the substrate properties on graphene should be enhanced because graphene has a large contact area with the substrate due to its two-dimensional crystalline structure. However, the role of the substrate in unintentionally occurring doping has not been well-established. The surface chemistry of a substrate is determined by the termination groups.18−20 To investigate the effects of a substrate on graphene properties, well-defined surfaces are desirable. Single-crystalline oxide materials, such as quartz and sapphire, are suitable as substrates of graphene flakes because © 2012 American Chemical Society

Received: November 24, 2011 Revised: January 28, 2012 Published: January 28, 2012 4732

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Silicon covered with a 300 nm thick thermal SiO2 layer, quartz (0001), randomly stepped single-phased sapphire (0001), sapphire (11−20), and sapphire (1−102), and phaseseparated sapphire (0001) substrates were used to support graphene flakes. The fabrication method for the phaseseparated sapphire (0001) substrate was described by Isono.27 To investigate the effects of the surface morphology of the substrate, regularly ordered single-stepped and stepbunched sapphire (0001) substrates were prepared. The former was prepared by annealing the sample at 1000 °C for 3 h in air and the latter by annealing at 1400 °C for 3 h in vacuum. The substrates were cleaned by soaking into a mixture of sulfuric acid and hydrogen peroxide for 10 min at 90 °C. Graphene flakes were deposited on the cleaned surface by mechanical exfoliation of graphite. The samples were observed using the AFM contact mode, and Raman spectra were obtained with a Renishaw Raman spectrometer equipped with 532 nm excitation. The laser spot diameter was nearly 1 μm, and the laser power at the sample was below 0.4 mW to avoid the laser local heating.39−42 Figures 1a and 1c show atomic force microscopy (AFM) topographical images of graphene flakes attached to a randomly

stepped sapphire (0001) substrate and a step-bunched one, respectively, and Figures 1b and 1d are their respective frictional force images. These two substrates differ in step density. The step density of the randomly stepped sapphire (0001) substrate in Figure 1a was much higher than that of the step-bunched substrate in Figure 1c because the substrate in Figure 1c has only bunched steps originating from the miscut of the substrate, whereas that in Figure 1a has randomly distributed steps originating in the rough surface. Graphene flakes can clearly be distinguished from the substrate in the frictional force images shown in Figures 1b and 1d, and we can confirm that the graphene flakes uniformly contact the surface reflecting the substrate morphology. Figures 1e and 1f show cross-sectional profiles of the graphene flakes in Figures 1a and 1c. The height of the step-bunches in Figure 1c was about 1.24 nm. The height of the graphene flake on the randomly stepped sapphire (0001) substrate in Figure 1a was about 0.63 nm, and that on the step-bunched one in Figure 1c was about 0.38 nm. The height of single-layer graphene flakes depends on the step arrangement, and the effect of the steps cannot be ignored on the randomly stepped sapphire (0001) substrate due to the high density of steps. We also refer to the randomly stepped sapphire (0001) surface as a single-phased surface because this surface was found to be composed of only hydrophilic areas from AFM frictional force images. Figure 2a shows typical Raman spectra of graphene flakes on insulating substrates, and Figures 2b and 2c show the magnified

Figure 1. (a) Topographical image of graphene flake on randomly stepped sapphire (0001) substrate referred to as single-phased surface, (b) its frictional force image, (c) topographical image of graphene flake on step-bunched sapphire (0001) substrate, (d) its frictional force image, and (e) and (f) cross-sectional profiles of (a) and (c), respectively. Scale bars are 1 μm.

Figure 2. (a) Typical Raman spectra of graphene flakes on insulating substrates and (b) and (c) magnified spectra of G- and 2D-band, respectively.

spectra of the G- and 2D-band, respectively. The G-band of a graphite (multilayered graphene) flake on the SiO2/Si substrate 4733

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at 1580 cm−1 was a control sample, which is a usual value for the G-band in bulk graphite.43 The G- and 2D-band of a graphene flake on the SiO2/Si substrate appeared at 1586 and 2676 cm−1, those on the quartz (0001) substrate appeared at 1590 and 2679 cm−1, and those on the single-phased sapphire (0001) substrate appeared at 1595 and 2681 cm−1, respectively. The average peak positions of the G- and 2D-band are summarized in Table S1 (see Supporting Information). The Gand 2D-band of a graphene flake on the quartz (0001) substrate appeared at higher wavenumbers than those on the SiO2/Si substrate, and a significant blue-shift of the G-band was observed on the single-phased sapphire (0001) substrate. Statistical analysis on Raman spectra was carried out because the peak positions are scattered by charged impurities.12 Figure 3 summarizes the shifts of the G- and 2D-band of graphene (a)

sapphire (11−20) and sapphire (1−102) substrates than on the single-phased sapphire (0001) substrate. Figure 4 shows AFM images of a graphene flake on the phase-separated sapphire (0001) substrate: (a) topographical

Figure 4. (a) Topographical image of graphene flake on phaseseparated sapphire (0001) surface, (b) its frictional force image, and (c), (d), and (e) cross-sectional profiles along lines shown in (a). Profile symbols of (c), (d), and (e) correspond to those in (a). Scale bars are 1 μm.

image and (b) its frictional force image. The two domains on this surface are characterized by hydrophilic or hydrophobic surface chemistry, which originate from the topmost atomic structures.27 The hydrophobic domain is attributed to the Otermination and the hydrophilic surface situated outside the hydrophobic domains to the Al-termination. In Figure 4a, graphene uniformly came into contact with the hydrophilic surface, whereas some particles were observed on the hydrophobic domains covered with the graphene flakes. Figures 4c, 4d, and 4e are cross-sectional profiles along the lines shown in Figure 4a. The particles on the hydrophobic domains may be monolayer water droplets because the height is about 0.24 nm, which is similar to the size of a water molecule. The apparent height of a graphene flake on the hydrophilic surface is about 0.34 nm, and that on the hydrophobic domains is almost zero. This is believed to be attributed to the exclusion of the water layers from the graphene/substrate interface due to the affinity of interaction between graphene and a hydrophobic surface and the apparent zero thickness is sometimes observed in other samples. Figure 5 shows a Raman spectrum of a graphene flake on the phase-separated sapphire (0001) surface. The inset is the magnified spectrum of the G-band. Though the G-band of the

Figure 3. Raman shifts of G- and 2D-band of graphene flakes: (a) on SiO2/Si (solid circles) and quartz (0001) (open circles) substrates and (b) on single-phased sapphire (0001) (squares), sapphire (11−20) (triangles), and sapphire (1−102) (crosses) substrates.

on the SiO2/Si substrate (solid circles) and quartz (0001) (open circles) substrate and (b) on the single-phased sapphire (0001) (square dots), sapphire (11−20) (triangles), and sapphire (1−102) (crosses) substrates. The dashed line at 1590 cm−1 in Figure 3b is a guide. The G- and 2D-peak positions of graphene flakes normally appeared at higher wavenumbers on the quartz substrates than on the SiO2/Si substrates. They also appeared at lower wavenumbers on the 4734

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graphene flakes on the single-phased sapphire (0001) substrate appeared at the highest wavenumbers in our experiments. Though the surface composition is almost the same,52 Raman spectra depend on the crystal orientation of the substrate. This is caused by a difference in the surface chemistry originating from the atomic structure of the substrate. Protons of the hydroxyl groups on the sapphire (0001) surface are not strongly bound to the oxygen.52 Since the protons are easily removed, the surface becomes negatively charged at low pH. Deprotonation promotes the aggregation of water molecules on the surface.53 A tightly bound water layer (referred to as structured water layer in some references) and a fluid water layer exist on the substrate surface.53,54 In our experiments, the fluid water layer confined at the graphene/substrate interface was observed via the graphene flake attached to the regularly ordered single-stepped sapphire (0001) substrate by using the AFM contact mode (see Supporting Information), and a water layer was not observed on the other planes.26,55 This suggests that a fluid water layer exists only on the sapphire (0001) substrate, though a tightly bound water layer always exists on every hydrophilic surface. This is also evidence of the large amount of water molecules at the interface between graphene flakes and the single-phased sapphire (0001) substrate. These results indicate that the chemical doping of graphene in our experiments originated from the water layer and that the doping level was determined by the hydrophilicity on the substrate surfaces. Previously, effects of the water layer bound to a graphene surface on its electronic properties were simulated.56−58 The adsorbed water molecules have very little effect on the electronic structure of graphene because their dipole moments tend to be oppositely directed and canceled on average. However, when the concentration of water molecules becomes high enough to form a structured water layer, which has been referred to as an ice layer,57 the water layers have a considerable effect on the electronic properties of graphene because the water molecules have less spatial freedom in their orientation and their dipole moments are aligned in the same direction. The magnitude of the dipole moment of the water layer depends on the regularity of the structured water layer because the restriction of the movement of individual water molecules yields a macroscopic dipole moment. Single-crystalline quartz (0001) substrate surfaces under ambient conditions induce a more ordered interfacial water structure than amorphous silica surfaces.53 Single-crystalline substrates can enhance the chemical doping of graphene. Dipole moments may be one of the main factors causing the difference in the Raman shifts observed in our experiments. We also investigated the effects of surface morphology of the substrates on the Raman spectra. The average G-peak position of the graphene flakes on the step-bunched sapphire (0001) substrate shown in Figure 1c was 1592 cm−1 (see Supporting Information), which was similar to that on the single-phased sapphire (0001) substrate shown in Figure 1a. Furthermore, we obtained polarized Raman spectra48,49 and confirmed that the G-peak position was not affected by the atomic steps on the step-bunched sapphire substrate, though sapphire steps can be used as a template for control of nanofabrication.59−64 No effect of the substrate morphology is believed to be due to the existence of water layers at the graphene/substrate interface because the water layer weakens the effect of the morphology. The G-band of graphene flakes on the phased-separated sapphire (0001) substrate was composed of two peaks referred

Figure 5. Raman spectrum of graphene flake on phase-separated sapphire (0001) surface. Inset is the magnified spectrum of G-band.

graphene flakes contacting the other substrates normally exhibited a single peak, that to the phase-separated sapphire substrate was composed of two peaks centered at 1587 cm−1 (G1-peak) and 1595 cm−1 (G2-peak). The 2D-band appeared at 2678 cm−1, which is typical in graphene flakes on substrates. The blue-shift of the G-band in the Raman spectra of graphene is generally interpreted by either a compressive strain44−50 or carrier doping.35−38 The present blue-shift is attributed to the charge transfer because the blue-shift of the 2D-band of the same graphene flakes is small.44 In CNTs, chemical doping is induced by water layers on their surfaces.17 Since our experiments were performed in atmospheric environments, chemical doping was induced by water layers on the graphene flakes or the substrate surfaces that came into contact with the graphene flakes. Since graphene is hydrophobic, the major contribution would be the water layer that remains at the substrate and graphene interface. Therefore, one possible origin for the observed blue-shift of the G-band is chemical doping by the water layer on or beneath the graphene flakes. If this is the case, the blue-shift should depend on the water layer thickness and thus on the hydrophilicity of the substrates. We analyzed our experimental results based on the hypothesis that chemical doping is induced by the interface water layer that depends on the hydrophilicity of the substrates and show that all the results can be interpreted with this model. The G-band of graphene flakes on the single-crystalline quartz (0001) substrate appeared at higher wavenumbers than that on the amorphous SiO2/Si substrate, as shown in Figure 2. Single-crystalline quartz (0001) substrate surfaces are terminated with stable OH groups due to lack of flexibility in the surface atomic structure. On the other hand, stable siloxane groups (≡Si−O−Si≡ bridges) form on the amorphous SiO2 surface after annealing above 400 °C51 due to the thermally induced condensation process. Thus, the density of OH groups on the SiO2/Si surfaces is lower than that on the quartz (0001) surfaces. This indicates that the density of water molecules adsorbed on the quartz (0001) substrate is higher than that on the SiO2/Si substrate. The difference in OH group density leads to the difference in the blue-shift of the G-band peak positions. The G-bands of graphene flakes on the sapphire (11−20) and (1−102) substrates appeared at lower wavenumbers than that on the single-phased (0001) substrate. The G-band of 4735

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on the Raman spectra of attached graphene under ambient conditions.

to as G1- and G2-peaks. The G2-peak originated from the hydrophilic domains because the average G2-peak position was similar to that on the single-phased sapphire (0001) substrate shown in Figure 1a, which normally appeared at 1593 cm−1. On the other hand, the G1-peak originated from the hydrophobic domains. The difference in the heights of graphene flakes on the hydrophilic and hydrophobic domains was ∼0.34 nm, which corresponds to the height of the water layer. Since graphene is strongly hydrophobic, it directly contacts hydrophobic surfaces. However, water molecules were observed via the graphene flake on the hydrophobic domains, as shown in Figure 4a. Though water molecules were not completely excluded from the graphene/hydrophobic domain interface, their density was at least lower than that at the graphene/ hydrophilic interface. The confinement of water molecules between graphene and a hydrophobic surface would restrict the movement of the water molecules and yield a dipole moment. The blue-shift of the G-peak of graphene flakes attached to the annealed substrates was reduced and the dispersion was narrow (see Supporting Information). This also suggests that the smaller blue-shift of the G1-peak can be explained by the reduced amount of water molecules at the graphene/substrate interface, and the variation of the Raman shifts in Figure 3 was caused by the nonuniformity of the water layer at the interface. The hole-doping level of graphene flakes on the singlephased sapphire (0001) substrate was estimated to be ∼6 × 1012/cm2 using the average G-peak position of 1593 cm−1.35 This level includes the effect of other ambient conditions such as adsorbed oxygen molecules. The intrinsic G-peak position is 1580 cm−1.43 The difference in the G-band wavenumbers between the highest position, 1598 cm−1, and the lowest one, 1582 cm−1, in Figure 3b was 16 cm−1, which is mostly attributed to the water layers because the other conditions were not changed.

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ASSOCIATED CONTENT

S Supporting Information *

AFM observation of water layers at a graphene/substrate interface, a model of the graphene/substrate interface, effect of thermal treatment of support substrate on Raman spectra, and summary of the G- and 2D-band of graphene flakes on insulating substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. The sapphire wafers were provided by Namiki Precision Jewel Co. Ltd.

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SUMMARY The effects of surface chemistry of insulating substrates on the Raman spectra of graphene flakes were investigated by attaching graphene flakes to the substrates under ambient conditions at room temperature. We demonstrated that Raman spectroscopy can be used to characterize the water layers at the graphene/substrate interface. It was found that water layers at the interface induce hole-doping in the graphene flakes and the G-bands are blue-shifted with an increase in the amount of water molecules. Our findings indicate that preparation of the substrate is important for determining the Raman spectroscopy of graphene because chemical doping of graphene depends on the surface chemistry of the substrate. This study also suggests that improvement in the electrical properties of graphene may be possible through inserting appropriate chemical dopants at the graphene/substrate interface instead of water molecules, as demonstrated by the formation of self-assembled monolayers (SAMs) on the substrate.11,65 From the point of view of the 2D-band shifts, an intrinsic effect of the substrate on graphene flakes was not observed due to the existence of water layers at the interface. To investigate such an intrinsic effect, samples after annealing in vacuum should be used.14 The direct interaction between graphene and substrates opens a band gap.66 The investigation of the intrinsic effect is a future work. Furthermore, Raman spectroscopy of carbon materials often works under ambient conditions. This work can support to understand the environmental effects of various contaminations. Our experiments demonstrated the effect of the substrate 4736

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