Control of Graphene Etching by Atomic Structures of the Supporting

Apr 11, 2011 - The surface structures of the supporting substrate are good templates for graphene processing. View: ACS ActiveView PDF | PDF ... Nanop...
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Control of Graphene Etching by Atomic Structures of the Supporting Substrate Surfaces Takahiro Tsukamoto* and Toshio Ogino Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan ABSTRACT: We attached single-layer graphene or few-layer graphene (FLG) on a sapphire (1-102) surface with well-ordered step/terrace structures and then etched them using catalytic nanoparticles. In the etching of FLG flakes, atomic steps can be utilized as guides or reflectors. In the case of single-layer graphene, the etching proceeds in a particular direction of a surface phase pattern on the terrace, and graphene nanoribbons are self-formed. The surface structures of the supporting substrate are good templates for graphene processing.

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raphene, a single layer of a hexagonal carbon network, is one of the most notable materials in the recent progress of nanomaterial science and technology owing to its remarkable properties,1 5 such as robust mechanical strength,6 high thermal conductivity,7 and extremely high mobility of charged carriers.8 Toward its device applications, control of the size, morphology, edge state, and shape is required because the electronic properties of graphene depend on those parameters.9 13 Several methods for processing of graphene have been proposed, including electron beam lithography,14 local anodic oxidation using atomic force microscopy,15,16 oxidation etching catalyzed by metal nanoparticles,17,18 and crystallographic etching using catalytic metal nanoparticles.19 22 The crystallographic etching can be applied to various self-patterning of graphene. Carbon atoms of graphene are removed through a reaction between graphene and hydrogen catalyzed by metal nanoparticles on the graphene flake during annealing in a hydrogen atmosphere. In this process, graphene is etched along particular directions of the graphene lattice. An advantage of the crystallographic etching is the atomic level controllability of the etched graphene edges. In the application of graphene to future integrated nanodevices, graphene should be fixed onto a substrate and then divided into individual device regions. Graphene on several kinds of substrates has been investigated.23 27 In our previous work, we proposed a technique for a graphene-on-insulator, in which graphene is tightly attached on an atomically controlled sapphire surface with a step/terrace structure.26 In this system, graphene exhibits an almost ideal height from the substrate. We observed atomic steps originating in the sapphire surface also on the graphene surface using atomic force microscopy (AFM). A sapphire surface with flat terraces and steep steps can also be used to control growth directions of carbon nanotubes.28 30 In this paper, we propose a new etching technique of single- or few-layer graphene (FLG) that is controlled by atomic structures of well-defined solid surfaces. We used Fe nanoparticles for the catalyst in the reaction between graphene and hydrogen gas and investigated effects of the substrate surface on etching patterns. r 2011 American Chemical Society

Single-stepped sapphire (1-102) substrates supplied by Namiki Precision Jewel Co. Ltd. were used. The sapphire surface was chemically cleaned using a H2SO4 and H2O2 mixed solution (H2SO4/H2O2 = 3:1) for 10 min, followed by ultrasonic washing in deionized water for 5 min. The treated sapphire substrates were heated at 1000 °C in air for 3 h to rearrange the atomic steps on the surface.31,32 The annealed surface was again cleaned using a H2SO4 and H2O2 mixed solution and rinsed in deionized water. Graphene flakes were deposited on the cleaned surface by mechanical exfoliation of graphite.1 To form Fe nanoparticles on the sapphire surface on which graphene flakes were attached, a solution of Fe(NO3)3 3 9H2O in isopropyl alcohol was spin-coated. The samples were then annealed at 900 °C in a hydrogen (320 sccm) and argon (600 sccm) mixed gas in a furnace for 10 or 45 min. The surface morphology and frictional force were observed by AFM, and the layer number of the graphene flake was determined by Raman spectroscopy. The crystal orientation of the sapphire (1-102) substrate was measured by an electron backscatter diffraction (EBSD) analysis. Figure 1 shows AFM images of the (a) topography and (b) frictional force of single-layer graphene on a sapphire (1-102) surface. After the air-annealing before graphene deposition, a step/ terrace structure appeared on the sapphire surface, as observed in the top-right area in Figure 1a. The height of a single step is about 0.24 nm, and the width of the terraces is about 52 nm. We can distinguish the graphene and the sapphire surfaces using a frictional force image in AFM. In Figure 1b, the frictional force on the brightly observed area is relatively large and that on the dark area relatively small. The frictional force observed in air is attributed to a meniscus force, which is related to the hydrophilicity of the surface. The large frictional force area corresponds to the sapphire surface and the small frictional force area the graphene surface because the sapphire surface is hydrophilic after Received: October 3, 2010 Revised: February 17, 2011 Published: April 11, 2011 8580

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Figure 1. AFM images of a graphene flake on a sapphire surface with a regularly ordered step/terrace structure: (a) topographic image and (b) frictional force image. The scale bars are 200 nm.

Figure 2. AFM images of graphene flakes on sapphire surfaces after etching using the catalytic effect of Fe nanoparticles: (a) about 30 nm thick FLG flake, (b, c) about 6 nm thick FLG flake, and (d) about 3 nm thick FLG flake. The scale bars are 200 nm.

the present treatment using a mixture of H2SO4 and H2O2, whereas the graphene surface is extremely hydrophobic. Therefore, by taking a frictional force image, the graphene flake and the sapphire surfaces can be clearly distinguished. In Figure 1a, the graphene flake closely adheres to the sapphire surface and the buried step/terrace structure on the sapphire surface is clearly observed also on the graphene surface. The height of the graphene flake was measured to be approximately 0.36 nm on the sapphire surface, which is in good agreement with the height expected from the layer spacing in graphite, 0.34 nm. We investigated the effects of deformation of the graphene flake introduced by the atomic structure of the supporting substrate on graphene etching. In this experiment, we observed etching of the topmost layer of FLG to eliminate the effect of direct interaction between the sapphire surface and metal nanoparticles. The reaction time was 45 min for the FLG etching. Figure 2a shows an AFM image of an FLG flake with a thickness of about 30 nm on the sapphire (1-102) surface after etching

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using Fe nanoparticles at 900 °C in a hydrogen atmosphere. Because the FLG surface in Figure 2a is almost flat, distortion induced by the morphology of the substrate surface is almost relaxed on the graphene surface. In Figure 2a, a wide straight trench with a catalyst particle at the etching front, as indicated by the solid-line circle, is observed. Because this trench is straight and the edges are sharp, the crystallographic etching occurs, as previously reported on highly oriented pyrolytic graphite (HOPG) and FLG on SiO2 surfaces.19 22 In Figure 2a, a narrow trench accompanying a 120° turn is also observed around the middle of the image, where a small deformation is observed near the turning point, as indicated by the dashed-line circle, suggesting that relaxation of the graphene flake is not perfect even for a thickness of 30 nm. The turn of the trench, therefore, is believed to be attributed to the small deformation induced on the graphene flake near the turning point. Figure 2b shows an AFM image of an about 6 nm FLG on the sapphire surface after the etching. When the thickness of the FLG is smaller than about 20 nm, the buried step/terrace structure on the sapphire surface appears on the FLG surfaces, as shown in Figure 2b. In the middle area of Figure 2b, a trench confined in a single terrace, which is elongated roughly to the horizontal direction, is observed (trench A). The formation process of trench A can be interpreted by the movement of Fe nanoparticles. Apparently, they are reflected at the positions just above the steps of the sapphire surface and, therefore, trapped inside a particular terrace during the etching. As a result, the trench becomes zigzag because the Fe nanoparticle moves locally along the crystallographic orientations inside the terrace. Figure 2c shows another area of the same sample, where the graphene thickness is comparable with that of Figure 2b. In this area, the trench is almost straight along the sapphire step (trench C). We confirmed that any of the crystallographic orientations of the graphene lattice in Figure 2c is not coincident with the macroscopic direction of trench C. Trench C, therefore, is subject to the surface morphology of the substrate. Compared with trench A in Figure 2b, the width of trench C in Figure 2c is much narrower. Because the trench width is determined by the particle diameter, the present result suggests that the size of the Fe nanoparticles is an important factor for the etching direction. Figure 2d shows an AFM image of an about 3 nm thick FLG surface after the etching. In Figure 2d, two types of trenches are observed: a straight trench along the sapphire step (trench D) and a zigzag trench that is reflected at the steps several times (trench E). Reflection at the steps of the sapphire substrate in trench E seems to be periodic and does not always occur. One possible explanation for the selective reflection is a difference in the height of the steps. Because the sapphire surface becomes rough upon annealing in a hydrogen atmosphere, as shown in Figure 2d, local fluctuation of the step height is generated. Moreover, steps with different heights may be formed through step bunching during the hydrogen annealing. If this speculation is supported by the experimental results, we will be able to use the step-height difference for control of the movement of catalytic nanoparticles. We also used a sapphire (1-102) surface with wide terraces to investigate effects of atomic structures that form on a terrace surface shown in Figure 3a. The terrace width is approximately 530 nm, and the step height is approximately 0.23 nm. Figure 3b shows the surface morphology after the annealing in a hydrogen atmosphere at 900 °C for 10 min. On this surface, a stripe pattern is observed; a terrace consists of two different domains. We refer to this surface-phase pattern as a comb pattern.33 35 The elongated 8581

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Figure 3. AFM images of a sapphire (1-102) surface (a) before the annealing (scale bar is 1 μm) and (b) after the annealing in the hydrogen atmosphere (scale bar is 400 nm).

direction of the comb pattern is almost vertical to the steps. The height difference between the two domains in the comb pattern is about 0.2 nm, which is smaller than the step height. We used the surface shown in Figure 3a as a template of graphene etching. Figure 4 shows an AFM image of the graphene flake on the sapphire surface with the comb pattern after the etching catalyzed by Fe nanoparticles, where the graphene flake covers the whole area of the image. The etching time was 10 min to avoid excess etching. Some trenches perpendicular to the steps, which reach the sapphire surface, are observed. The etching proceeds in the step-up direction as well as the step-down direction. We confirmed that the gas flow direction during the etching does not affect the etching. In Figure 4, a nanoribbon fabricated by a parallel etching is observed, though its width is fluctuated. Because the width is determined by the initial positions of Fe nanoparticles along the graphene flake edge, the observed width of the graphene nanoribbon was scattered from 40 to 300 nm in our experiment. Figure 5 shows Raman spectra of the etched graphene flakes. The layer number of the graphene flake can be obtained from the 2D band, which is sensitive to the layer number.36 38 We confirmed that the graphene flake shown in Figure 4 is a single layer from the location of the 2D band. The D band was not observed before the graphene etching but appeared after the etching. The D band originated in the edge states of the etched graphene flake. The edge state can be characterized by the intensity ratio of the D band to the G band.39,40 The presence of the D band suggests that zigzag and armchair edges coexist in the etched graphene flake. Figure 5c shows a magnified spectrum of the G band in Figure 5a. The observed G band is split into two peaks located at 1575 and 1591 cm 1, respectively. It was reported that the G band of a graphene nanoribbon consists of the G1 (the E2g vibrational mode of a nanoribbon) and the G2 (the E2g vibrational mode of HOPG) peaks.41 Although the wavenumbers of the G1 and G2 peaks are slightly different owing to a difference in the excitation intensity, as pointed out in ref 41, we can conclude that the etched graphene is regarded as a graphene nanoribbon. We demonstrated that graphene or FLG etching can be controlled by atomic structures on the substrate surface. Factors that should be considered for the control of graphene etching are the effect of the graphene lattice, deformation of the graphene flake, and interaction between the metal nanoparticle and the supporting substrate surface. We discuss the effects of the supporting substrate surface on graphene or FLG etching. Figure 6 shows a schematic illustration of the phase diagram of the etching mode that depends on the number of graphene layers

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Figure 4. AFM image of a graphene flake after etching on a sapphire (1-102) surface annealed in a hydrogen atmosphere; the scale bar is 1 μm.

Figure 5. Raman spectra obtained from the etched graphene: (a) around the D and the G bands, (b) around the 2D band, and (c) a magnified spectrum of the G band. The intensity scales of (a) (c) are the same.

and diameter of the metal catalyst, and Figure 7 shows the schematic models of the graphene etching on a sapphire surface. Substrate-pattern-directed etching always occurs in the etching 8582

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The Journal of Physical Chemistry C of single-layer graphene. In the etching of FLG, three different modes appear: deformation-directed etching, direction-reflected etching, and crystallographic etching. The area of the deformation-directed etching in the phase diagram is much smaller than that of the direction-reflected etching. When the number of graphene layer increases, the effect of the supporting substrate is weakened, and finally, only the crystallographic etching occurs. In the case of thick FLG, the etching is subject to the crystallographic orientations of the graphene lattice, as shown in Figure 7a, which has already been demonstrated.19 22 In the case of FLG deformed by the atomic steps on the substrate surface, a deformation-directed etching as well as the crystallographic one occurs, as shown in Figure 7b. Fe nanoparticles are guided or reflected at the deformed sites of the FLG flake. The relationship between diameter of the metal nanoparticles and the strain introduced in the graphene flake is important for the directional control of the graphene etching. In Figure 2b, there are two trenches: trench A was produced by a relatively large nanoparticle and trench B by a relatively small one. Movement of the larger nanoparticle is influenced more strongly by the graphene deformation than that of the smaller nanoparticle. In these cases, the Fe nanoparticles move on the surface of the second graphene layer, because they do not penetrate into the deeper layers of the graphene flake. It was also reported that a metal nanoparticle moves in the direction parallel to another trench, and 10 nm wide graphene nanoribbons formed.21 This

Figure 6. Schematic illustration of the phase diagram in the etching of graphene, FLG, and graphite using Fe nanoparticles.

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indicates that the strain introduced by the preformed trench influences the movement of another metal nanoparticle. In the etching of FLG on a sapphire surface, deformation is introduced into FLG from the substrate surface. To perfectly control graphene etching, the relationship among the size of the metal nanoparticles, step height, number of etched graphene layers, and amplitude of the deformation should be elucidated. It is expected that the controllability of the graphene etching can be improved by using multistepped sapphire substrates because the multistep enhances the deformation effect. The etching direction of graphene is affected by defects of the graphene lattice.20 However, we found that the etching direction can be controlled by the deformation of graphene. It is difficult to precisely control the direction of graphene etching on an irregularly roughened surface because random deformation is introduced from the substrate. The step height of the sapphire surface used in this study is approximately 0.24 nm. Unexpected etching can occur if the height of the deformation is larger than 0.24 nm. For a high controllability, a flat surface is required. The interface between the graphene flake and the sapphire surface is very flat, and deformation on the graphene surface is generated only just above the steps of the sapphire substrate. Therefore, we can obtain a graphene flake that is deformed only by well-ordered atomic step arrays on the sapphire surface. In the case of single-layer graphene on a sapphire (1-102) surface, graphene is etched in a direction perpendicular to the sapphire steps. Figure 7c shows the schematic model of this etching mode. The graphene etching on a SiO2/Si substrate is controlled by the graphene lattice,21 whereas that on this sapphire surface proceeds in a direction perpendicular to the sapphire steps. Because this etching mode occurs for a single layer of graphene, an interaction between the metal nanoparticle and the supporting substrate surface plays a crucial role. In the present experiment, the graphene lattice orientation and the sapphire atomic row have no correlation because graphene flakes exfoliated from graphite flakes were randomly attached on the substrate surfaces. When singlelayer graphene flakes were etched on a sapphire r-surface, they were always etched in a particular direction determined by the substrate. This indicates that the graphene lattice is not effective for the movement of metal nanoparticles in the etching of single-layer graphene, but the supporting substrate strongly affects the etching. When the metal nanoparticles contact with the substrate surface,

Figure 7. Model of graphene etching on a sapphire surface with well-ordered atomic steps: (a) crystallographic etching of a thick FLG flake, (b) etching of a FLG flake controlled by atomic steps on the substrate surface, and (c) etching of graphene directed in a “comb pattern” on the sapphire surface. 8583

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The Journal of Physical Chemistry C the effect of the supporting substrate is larger than that of the graphene lattice. It has been reported that aligned single-walled carbon nanotubes (SWCNTs) grow on sapphire, quartz, or SiO2/Si surfaces with trenches or steps.28 30,42 45 The van der Waals force plays important roles for the growth of aligned SWCNTs. SWCNTs begin to extend in random directions from the catalyst when they grow on an amorphous SiO2/Si substrate. On a patterned surface, SWNTs follow the grooves or the buried atomic steps on the SiO2/ Si substrate.43 45 It is attributed to the enhanced van der Waals force. Because the elongation direction of the comb pattern shown in Figure 3b is coincident with the etching direction, a strong interaction between the metal nanoparticle and the comb pattern is the driving force of the controlled etching. In the etching of single-layer graphene, a direct interaction between the nanoparticle and the substrate is larger than the deformation effect caused by the sapphire steps, though the etching of FLG is strongly affected by the steps. In the etching of FLG, interlayer graphene exists between the metal catalysts and the substrate, and the interlayer graphene prevents direct contact of the nanoparticles with the substrate surface. Because the comb pattern is a stripe with a narrow width, the interlayer graphene cannot completely follow the surface at the atomic level and the attached graphene surface is made gentle compared with the case of the step. Therefore, the effect of the comb pattern on the nanoparticle movement is small, and the etching is affected by the deformation induced by the substrate steps. In the etching of single-layer graphene, the metal nanoparticles directly contact with the substrate. Therefore, their movement is strongly affected by the atomic structures on the substrate surface, such as the comb pattern. Note that there is no physical correlation between the direction of the comb pattern, which is crystallographically determined, and the step direction, which is artificially determined by the macroscopic miscut orientation. The comb pattern appears over the sapphire (1-102) surface after the annealing in a hydrogen atmosphere. Alerhand et al. predicted the formation of stress domains on a terrace based on a reduction of strain energy, resulting in splitting of a wide terrace into striped domains.33 It was also reported that comb-shaped step structures form on the highly B-doped or P-doped Si(001) surface after annealing in vacuum.34,35 Because a sapphire (1-102) surface is anisotropic, the observed comb pattern is related to a strain relaxation during or after the annealing. Details of the formation mechanism of the comb pattern on sapphire surfaces will be reported in a separate paper. Graphene on a sapphire (1-102) surface is etched in the same direction as that of the comb patterns. This suggests that the comb pattern enhances the van der Waals force, or any other interaction, between the sapphire surface and the metal nanoparticle due to its morphology or chemistry. The present results show that graphene patterning based on surface structure control of the substrate is very promising in future graphene-on-insulator devices. We have demonstrated that the direction of the nanoparticlecatalyzed etching of graphene or FLG can be controlled by the ordered atomic structures on the sapphire surface, such as atomic steps. In the etching of FLG, Fe nanoparticles move along the buried steps of the substrate or turn near the deformation introduced from nanostructures on the sapphire surface. This etching mode was obtained owing to the tight adhesion of the FLG flakes to the sapphire surface. The etching of a single layer of graphene proceeds in a direction perpendicular to the sapphire step, resulting in the formation of graphene nanoribbons. The

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present study suggests a possibility of controlling graphene shapes using atomic structures on the supporting substrate surface. By arranging nanostructures on the substrate, various nanographene can be fabricated in a wafer scale. This is the first report on the control of graphene processing using the atomic structure of the supporting substrate surface.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank Dr. Okayasu for his technical discussion in the EBSD experiment. This work was partly supported by a 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. ’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197–200. (3) Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201–204. (4) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (5) Miller, D. L.; Kubista, K. D.; Rutter, G. M.; Ruan, M.; de Heer, W. A.; First, P. N.; Stroscio, J. A. Science 2009, 324, 924–927. (6) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385–388. (7) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Nano Lett. 2008, 8, 902–907. (8) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Solid State Commun. 2008, 146, 351–355. (9) Yamashiro, A.; Shimoi, Y.; Harigaya, K.; Wakabayashi, K. Phys. Rev. B 2003, 68, 193410. (10) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347–349. (11) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Phys. Rev. Lett. 2006, 97, 216803. (12) Pisani, L.; Chan, J. A.; Montanari, B.; Harrison, N. M. Phys. Rev. B 2007, 75, 064418. (13) Kim, W. Y.; Kim, K. S. Nat. Nanotechnol. 2008, 3, 408–412. (14) Bell, D. C.; Lemme, M. C.; Stern, L. A.; Williams, J. R.; Marcus, C. M. Nanotechnology 2009, 20, 455301. (15) Weng, L.; Zhang, L.; Chen, Y. P.; Rokhinson, L. P. Appl. Phys. Lett. 2008, 93, 093107. (16) Masubuchi, S.; Ono, M.; Yoshida, K.; Hirakawa, K.; Machida, T. Appl. Phys. Lett. 2009, 94, 082107. (17) Severin, N.; Kirstein, S.; Sokolov, I. M.; Rabe, J. P. Nano Lett. 2009, 9, 457–461. (18) Bulut, L.; Hurt, R. H. Adv. Mater. 2009, 22, 337–341. (19) Datta, S. S.; Strachan, D. R.; Khamis, S. M.; Johnson, A. T. C. Nano Lett. 2008, 8, 1912–1915. (20) Ci, L.; Xu, Z.; Wang, L.; Gao, W.; Ding, F.; Kelly, K. F.; Yakobson, B. I.; Ajayan, P. M. Nano Res. 2008, 1, 116–122. (21) Campos, L. C.; Manfrinato, V. R.; Sanchez-Yamagishi, J. D.; Kong, J.; Jarillo-Herrero, P. Nano Lett. 2009, 9, 2600–2604. (22) Ci, L.; Song, L.; Jariwala, D.; Elías, A. L.; Gao, W.; Terrones, M.; Ajayan, P. M. Adv. Mater. 2009, 21, 4487–4491. 8584

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