Uniform Graphene Quantum Dots Patterned from Self-Assembled Silica Nanodots. 17,18 However, it still remains a critical challenge to control

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1 pubs.acs.org/nanolett Uniform Graphene Quantum Dots Patterned from Self-Assembled Silica Nanodots Jinsup Lee,,, Kyungho Kim,, Woon Ik Park, Bo-Hyun Kim,, Jong Hyun Park, Tae-Heon Kim, Sungyool Bong, Chul-Hong Kim, GeeSung Chae, Myungchul Jun, Yongkee Hwang, Yeon Sik Jung,*, and Seokwoo Jeon*,, Department of Material Science and Engineering, KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon , Republic of Korea Graphene Research Center (GRC), KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon , Republic of Korea LG Display R&D Center, 1007, Deogeun-ri, Wollong-myeon, Paju-si, Gyeonggi-do, , Republic of Korea *S Supporting Information ABSTRACT: Graphene dots precisely controlled in size are interesting in nanoelectronics due to their quantum optical and electrical properties. However, most graphene quantum dot (GQD) research so far has been performed based on flaketype graphene reduced from graphene oxides. Consequently, it is extremely difficult to isolate the size effect of GQDs from the measured optical properties. Here, we report the sizecontrolled fabrication of uniform GQDs using self-assembled block copolymer (BCP) as an etch mask on graphene films grown by chemical vapor deposition (CVD). Electron microscope images show that as-prepared GQDs are composed of mono- or bilayer graphene with diameters of 10 and 20 nm, corresponding to the size of BCP nanospheres. In the measured photoluminescence (PL) spectra, the emission peak of the GQDs on the SiO 2 substrate is shown to be at 395 nm. The fabrication of GQDs was supported by the analysis of the Raman spectra and the observation of PL spectra after each fabrication step. Additionally, oxygen content in the GQDs is rationally controlled by additional air plasma treatment, which reveals the effect of oxygen content to the PL property. KEYWORDS: Graphene quantum dots, block copolymer, chemical vapor deposition, photoluminescence Graphene, a single atomic layer of carbon network, has been highlighted as a novel two-dimensional (2D) material with their unique properties 1 4 and potential applications in electronics. The 2D sp 2 network of carbon in graphene gives peculiar electrical properties such as a zero band gap. However, the absence of a band gap in graphene imposes limitations in electronic applications; 5,6 even for optoelectronic applications, a discrete band gap is favorable to achieve sharp fluorescent peaks. To generate an electronic band gap in graphene, various methods such as lithography, nanoimprinting, and chemical techniques have been used to fabricate graphene nanoribbon (GNR), graphene nanomesh (GNM), and graphene quantum dots (GQDs). 7 9 These nanosized graphene structures can show tunable band gap characteristics depending on size, shape, and edge- or surface-functionalization Among the various graphene nanostructures, GQD is at an early developmental stage for practical fabrication. In previous research, GQDs were typically fabricated by mechanical or chemical subdividing methods from exfoliated graphene oxide (GO) or reduced GO These methods are simple to generate nanosized graphene, and the good solubility of GQDs in various solvents enabled wafer-scale deposition. 17,18 However, it still remains a critical challenge to control the size and structural disorder, along with a more profound understanding on the size-dependent quantum confinement effects. It is interesting to note that, in theoretical and experimental studies, GQDs with <10 nm in diameter could exhibit quantum confinement effects and consequently have the potential to be applied for carbon-based electronic and opto-electronic devices. Recently, one method to control the shape and size distribution for the formation of GQDs was reported by Liu and his collaborators. 19 They used hexa-peri-hexabenzocoronene and oxidative exfoliation to achieve a uniform size of GQDs. However, this process is limited to GQDs of 60 nm in diameter and has an issue of broad size distribution. Another method reported by Neubeck et al. used electron-beam lithography to form nanodot mask patterns and subsequently to perform plasma etching to create the desired graphene nanostructures. 20 This technique enabled a specific control to Received: July 8, 2012 Revised: November 10, 2012 Published: November 13, American Chemical Society 6078

2 Nano s Figure 1. (a) Chemical structure of the PS-PDMS BCP. (b) Schematic illustration of the fabrication of GQDs including the spin-coating of BCP, formation of silica dots, and etching process by O 2 plasma. Figure 2. (a), (b) SEM image of self-assembled 10 and 20 nm sized silica nanodots on graphene, respectively. (c) Size histograms of silica nanodots on graphene. (d) AFM image of the 10 nm sized GQDs on a mica substrate. The inset in d shows high resolution AFM image of 20-nm-sized GQD after removing silica dots. (e) Height profile of a monolayer GQDs along the line in d (white). achieve a narrow size distribution of GQDs. However, GQDs based on e-beam lithography and other top-down approaches face critical challenges such as low throughput and poor properties. Nevertheless, the synergic combination of CVD growth of graphene and high-throughput patterning methods that can precisely control the size of GQDs may offer an excellent opportunity to study the optical properties of GQDs. Herein, we propose a new route for the fabrication of uniform-sized GQDs, in which a BCP self-assembly process is adopted to control the size of GQDs as small as 10 and 20 nm. In our previous studies, uniform and robust nanoscale silica dot patterns were formed from Si-containing polystyrene-bpolydimethylsiloxane (PS-PDMS) block copolymers (BCPs). BCP self-assembly has been investigated as a way of highly scalable nanofabrication with ultrahigh resolution Uniform BCP self-assembly was realized on PDMS-functionalized graphene, and the patterned silica nanodots of PS-PDMS BCP were used as an etch mask for the fabrication of GQDs. The variable diameter of silica nanodots, depending on the molecular weight of the BCP, allowed the precise control of the size of the GQDs. Both monolayer GQDs formed on an Si substrate and dispersed GQDs in an aqueous solution show blue emissions at 395 nm under excitation with a 325 nm He Cd laser; the adjusted PL spectra also exhibited wavelength 6079

3 Nano s values between 385 and 460 nm with a xenon lamp. The relative insensitivity of PL position depending on GQD size and the effect of oxidation or defect states on PL intensity will provide solid ground for the understanding of graphene photonics. Figure 1 illustrates the chemical structure of the PS-PDMS BCP and a schematic of the overall process to obtain GQDs on an SiO 2 /Si substrate. A detailed description of the self-assembly process is given in the experimental section and Supporting Information (SI). PS-PDMS BCP can be self-assembled into well-ordered nanostructures due to its large Flory Huggins interaction parameter and the existence of Si in the PDMS block, as shown in Figure 1a, which provides a large etch selectivity under oxygen plasma. Therefore, a sphere-type silica dot array formed by PS-PDMS BCPs can be useful as an etch mask to generate GQDs. The first step of the fabrication process is the self-assembly of a PS-PDMS BCP film on a graphene/si/sio 2 substrate, as shown in Figure 1b. The singlelayer graphene on the Si/SiO 2 substrate was prepared using CVD and then treated with a thin (3 4 nm) hydroxyterminated PDMS homopolymer brush. Since BCP films cannot be directly self-assembled onto graphene, the prior process of brush coating served as an appropriate method to improve the uniformity of the self-assembled PS-PDMS pattern. The PS-PDMS BCP was dissolved in toluene and spin-coated onto the substrate. After spinning, thermal annealing and washing processes followed, leading to the creation of well-aligned silica spheres in a PS matrix. Depending on the molecular weight of the polymer blocks, it was possible to change the size of silica particles from 10 to 20 nm. Two-step CF 4 and oxygen plasma etching processes produced wellordered silica dot arrays with hexagonal symmetry, which were used as a masking layer for the patterning of GQDs. Although adjacent silica dots are very closely packed, GQDs are overetched depending on the condition of oxygen plasma treatment (Figure S1). From scanning electron microscope (SEM) images, as shown in Figure 2a,b, silica dot assemblies with two different sizes are observed on the CVD-grown graphene. Silica dots with diameters of 10 nm (Figure 2a) and 20 nm (Figure 2b) were self-assembled into a hexagonal array. The insets in Figure 2a,b are high-resolution SEM images, showing a narrow size distribution. The comparison of the silica dots before (Figure 2a,b) and after (Figure S3) O 2 plasma treatment suggests that the sizes of the fabricated silica dots remain mostly unchanged. The uniformity of the pattered silica dot arrays was confirmed on a large area ( 10 μm 10 μm) surface (Figure S3a,b, SI). The histograms for the size distribution of the GQDs in Figure 2c show that GQDs fabricated by 10 nm silica dots were mainly in the range of 7 13 nm (gray bar, average 10 nm) and that by the 20 nm silica dots are in the range of nm (red bar, average 19 nm). The GQDs have a much narrower size distribution compared to other GQDs prepared using GO. 24,25 More than 50% of the GQDs consist of samples with an average size. The atomic force microscopy (AFM) image measured on mica substrate (Figure 2d) shows a topographic image of 10 nm sized GQDs after removing silica dots. The inset in Figure 2d is a high-resolution AFM image showing the nearly perfect hexagonal structures of the GQDs. It also clearly shows the uniform arrangements and size distributions of the as-fabricated 20-nm-sized GQDs after removing the silica dot masks. The heights of GQDs are between 0.4 and 0.7 nm, corresponding to single or bilayered graphene (Figure 2e). The transmission electron microscopy (TEM) image shown in Figure 3a was taken from the as-prepared GQDs (red Figure 3. (a) TEM image of GQDs in solution after removing of SiO 2 substrate. (b) HRTEM image of an individual GQD in the red arrow of part a. (c) Raman spectra of GQDs following the fabrication process. arrow). The small particles (white arrow) appear to be residues or GQDs located at different depths because their lattice fringes are either absent or dim when observed at high magnifications. Figure 3b is a high-resolution TEM image of a representative GQD. This shows the crystallinity with a lattice parameter of nm corresponding to the (1100) lattice fringes of graphene, 26 providing that our GQDs were fabricated from graphene. Figure 3c presents the Raman spectra of the GQDs tracing the steps of the fabrication process. As the patterning process proceeds, the G and 2D peaks in the Raman spectra decreased compared to those of pristine graphene. This change shows strong evidence that the etching process leads to a decrease in the coverage of graphene. After the PDMS brush treatment, the D peak related to the defects of graphene started to appear, indicating that the graphene has been damaged by chemically anchoring the PDMS brush on the surface. However, the defect caused by the PDMS brush treatment (red line) and block copolymer self-assembly (blue line) is negligible compared to that observed from the graphene after O 2 plasma treatment. The sharp decrease of the G and 2D peaks and the increase of the D peak after O 2 plasma treatment indicate the increase of edge carbons in the graphene as it changes from a continuous sheet into GQDs. Furthermore, the increase of edge carbons is supported by the splitting of G peak called the D -band appear at 1625 cm 1 related to the small size and vacancy-like defect at the edge of graphene. 27,28 Figure 4a presents the corresponding PL spectra of the 10- nm-sized GQDs shown in Figure 3b. The PL emission peaks remain at 395 nm regardless of the silica particles on the GQDs or the sizes of the GQDs (Figure S4, SI). The experimentally shown PL center of the 10 nm GQDs is 600 nm. 29 We note that no PL peak was observed from graphene after the PDMS brush treatment and also from the control 6080

4 Nano s Figure 4. PL spectra of GQDs (a) after each fabrication step and (b) at different excitation wavelengths. Figure 5. Variation of oxygen content in GQDs: (a) Raman spectra of GQDs treated in air plasma from 10 to 40 s. (b) PL spectra of GQDs for O 2 surface functionalization effect in a. (c) Raman spectra of GQDs treated with hydrazine to reduce oxygen content. (d) PL spectra of GQDs according to hydrazine treatment time. sample treated with air plasma without BCP patterning (Figure S5). However, previous studies indicate that the location center of the PL depends on the size of the sp 2 clusters isolated by oxidation or defects, 30,31 even though theoretical studies predict that 3-nm-sized GQDs can yield PL emission at 390 nm. 32,33 In our results, the PL emission peaks were observed to remain at around 395 nm with and without silica particles on the GQDs. It is worthy to mention that no PL emission peak was observed from graphene after the PDMS brush treatment and from the control sample treated with air plasma without BCP patterning (Figure S5). Raman spectra shown in Figure 3b and the PL spectra in Figure 4a demonstrate that the oxidation by the O 2 plasma during the fabrication of GQDs results in the quantum confinement effect of the GQDs. Similar to most near-blue emission at a wavelength of around 390 nm in GObased GQDs (size ranges from 3 to 60 nm), our PL properties are comparable to other previous reports. 19,24,29,30,34 This is a significant observation and is helpful to show that the emission wavelength of GQDs results more from oxidation and defects in the sp 2 carbon structure than from the influence of size effects. Although the GQDs in this work were 10 nm and 20 nm in size, the PL properties must have come from sp 2 clusters of various sizes created by the O 2 plasma treatment, where the dominant domain size of the clusters formed inside the uniform GQDs were 3 nm. 29,35 To further study the optical properties of 10-nm-sized GQDs, specific PL spectra measurements were carried out at different excitation wavelengths. Figure 4b shows the PL spectra from the 10-nm-sized GQDs as a function of the excitation wavelength. For this experiment, we prepared a solution containing the fabricated GQDs by etching SiO 2 substrates and collecting GQDs. A detailed description for the preparation of the GQD solution is given in the experimental section. As the PL excitation wavelength increases from 310 to 390 nm, the emission peak shifts to a longer 6081

5 Nano s wavelength, from 373 nm (black) to 456 nm (pink). A similar tendency also appeared for the GQDs fabricated by GO. 30,34 In both sets of results shown in Figures 3 and 4, it is recognized that oxygen defects can be formed on GQDs, which formation causes the variation of the Raman spectra and the PL emission. To prove the effect of oxygen at the surface of the GQDs, controlled oxidation and reduction were performed on the GQDs by air plasma and hydrazine (Figure 5). From the study of the Raman spectroscopy of the GQDs treated additionally with plasma (Figure 5a), it was found that, with increasing treatment time, the whole peak intensity decreased. A detailed composition analysis after air plasma treatment (15 s, 65% carbon ratio) and reduction treatment (10 min, 78% carbon ratio) was carried out using X-ray photoelectron spectroscopy (XPS) measurements (Figure S7 in SI). The ratio of I D /I G measured from all peaks, which is known to be related to the defect length of graphene, showed its greatest value between 10 and 20 s. 36 Since the formed GQDs were 10 nm in diameter, if the domain size of the GQDs is estimated to be less than 5 nm, it implies that the graphene structures are broken and torn by the air plasma treatment. The effect of air plasma surface treatment on GQDs shows no peak-shift, whereas the PL intensity declined after additional air plasma treatment for 10 s (Figure 5b). This shows that the component ratio of oxygen in GQDs can be optimized for the strongest PL emission. Moreover, excessive etching leads to additional defects and an increase of oxygen to carbon ratio in GQDs, resulting in the collapse of the 10-nm-sized sp 2 carbon structure in GQDs. 30,37,38 Similar results can also be observed for larger GQDs (Figure S4). Furthermore, we investigated the influence of substrate functionalization on the PL emission characteristics of GQDs. As shown in Figure S6, slight variations ( 10 nm) of PL peak positions depending on self-assembled monolayer (SAM) molecules were observed. This suggests that the surface treatment of substrates can provide a tool to tune the emission spectra of GQDs. Likewise, an interesting phenomenon was observed in hydrazine-treated GQDs (Figure 5c). In the Raman spectra and XPS measurement, deoxidized GQDs showed improved quality with increasing hydrazine treatment time due to the enhancement of the I G /I 2D ratio. Nevertheless, the hydrazinetreated GQDs release weaker luminescence at the near-blue wavelength compared to nontreated GQDs. The corresponding PL spectra of the GQDs after each incremental reduction treatment (from 10 min up to 1 h) are shown in Figure 5d. The PL intensity was very weak due to the fabrication process that used only single layer graphene with low areal density of GQDs. The highest tendency of the luminescent spectrum is an emissive steady ratio of oxygen on the Si/SiO 2 substrate, which result is similar to a previous report by Eda et al. 30 In this study, however, due to GQDs composed of single layer graphene, it has received a lot of reduction effect through hydrazine treatments in a short time. Since we performed plasma and hydrazine treatment from GQDs, we expect that proper oxygen contents can be controlled to the intensity of the PL, along with the quenching effect. In summary, we demonstrated the first PL study of uniform GQDs on Si/SiO 2 substrate and aqueous solution. We described a controllable and scalable fabrication method for 10 and 20 nm GQDs from single-layer CVD grown graphene using self-assembled BCP. GQDs were fabricated on the CVD grown graphene using silica nanodots created from selfassembled PS-PDMS block-copolymer as an etching mask The fabricated GQDs show a hexagonal arrangement with a narrow size distribution; this arrangement is superior to those presented in previous studies. GQDs shows PL emission at 395 nm on the SiO 2 substrate, and this is comparable to other GQDs fabricated from GO. In the case of 10-nm-sized GQDs, we experimentally showed that the actual PL spectra correspond to near-blue emission instead of red emission. The observation of significant oxidation and defect value in GQDs will be important for the control and enhancement of optoelectronic properties. Finally, we believe our research provides a noteworthy contribution to enhancing the properties of GQDs and to the understanding of the effects of size and functionalization on GQDs. ASSOCIATED CONTENT *S Supporting Information Experimental details such as graphene growth and transfer methods, fabrication of GQDs, and characterization tools. Additional GQD characterization data including Raman spectra, SEM image, and XPS results. This material is available free of charge via the Internet at AUTHOR INFORMATION Corresponding Author * ysjung@kaist.ac.kr; jeon39@kaist.ac.kr. Author Contributions These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011K000623). It was also partially supported by the Future-based Technology Development Program (Nano Fields) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology ( ) and through a grant ( ) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. This research was also supported by the GRC project of KAIST Institute for the NanoCentury. REFERENCES (1) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, (3) Geim, A. K. Science 2009, 324, (4) Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Science 2008, 320, (5) 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, (6) Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. B 1996, 54, (7) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Science 2008, 319, (8) Bai, J. W.; Zhong, X.; Jiang, S.; Huang, Y.; Duan, X. F. Nat. Nanotechnol. 2010, 5, (9) Tang, C. L.; Yan, W. H.; Zheng, Y. S.; Li, G. S.; Li, L. P. Nanotechnology 2008, 19.

6 Nano s (10) Wang, Z. F.; Shi, Q. W.; Chen, J. J. Nanosci. Nanotechnol. 2009, 9, (11) Wang, L. J.; Li, H. O.; Tu, T.; Cao, G.; Zhou, C.; Hao, X. J.; Su, Z.; Xiao, M.; Guo, G. C.; Chang, A. M.; Guo, G. P. Appl. Phys. Lett. 2012, 100. (12) Gupta, V.; Chaudhary, N.; Srivastava, R.; Sharma, G. D.; Bhardwaj, R.; Chand, S. J. Am. Chem. Soc. 2011, 133, (13) Pereira, J. M.; Vasilopoulos, P.; Peeters, F. M. Nano Lett. 2007, 7, (14) Moriyama, S.; Tsuya, D.; Watanabe, E.; Uji, S.; Shimizu, M.; Mori, T.; Yamaguchi, T.; Ishibashi, K. Nano Lett. 2009, 9, (15) Ritter, K. A.; Lyding, J. W. Nat. Mater. 2009, 8, (16) Zhuo, S. J.; Shao, M. W.; Lee, S. T. ACS Nano 2012, 6, (17) Zhu, S. J.; Zhang, J. H.; Liu, X.; Li, B.; Wang, X. F.; Tang, S. J.; Meng, Q. N.; Li, Y. F.; Shi, C.; Hu, R.; Yang, B. Roy. Soc. Chem. Adv. 2012, 2, (18) Pan, D. Y.; Guo, L.; Zhang, J. C.; Xi, C.; Xue, Q.; Huang, H.; Li, J. H.; Zhang, Z. W.; Yu, W. J.; Chen, Z. W.; Li, Z.; Wu, M. H. J. Mater. Chem. 2012, 22, (19) Liu, R. L.; Wu, D. Q.; Feng, X. L.; Mullen, K. J. Am. Chem. Soc. 2011, 133, (20) Neubeck, S.; Ponomarenko, L. A.; Freitag, F.; Giesbers, A. J. M.; Zeitler, U.; Morozov, S. V.; Blake, P.; Geim, A. K.; Novoselov, K. S. Small 2010, 6, (21) Jung, Y. S.; Ross, C. A. Adv. Mater. 2009, 21, (22) Jung, Y. S.; Ross, C. A. Small 2009, 5, (23) Jeong, J. W.; Park, W. I.; Kim, M. J.; Ross, C. A.; Jung, Y. S. Nano Lett. 2011, 11, (24) Pan, D. Y.; Zhang, J. C.; Li, Z.; Wu, M. H. Adv. Mater. 2010, 22, (25) Shen, J. H.; Zhu, Y. H.; Yang, X. L.; Zong, J.; Zhang, J. M.; Li, C. Z. New J. Chem. 2012, 36, (26) Park, S.; An, J.; Piner, R. D.; Jung, I.; Yang, D.; Velamakanni, A.; Nguyen, S. T.; Ruoff, R. S. Chem. Mater. 2008, 20, (27) Eckmann, A.; Felten, A.; Mishchenko, A.; Britnell, L.; Krupke, R.; Novoselov, K. S.; Casiraghi, C. Nano Lett. 2012, 12, (28) Ryu, S.; Maultzsch, J.; Han, M. Y.; Kim, P.; Brus, L. E. ACS Nano 2011, 5, (29) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L. H.; Song, L.; Alemany, L. B.; Zhan, X. B.; Gao, G. H.; Vithayathil, S. A.; Kaipparettu, B. A.; Marti, A. A.; Hayashi, T.; Zhu, J. J.; Ajayan, P. M. Nano Lett. 2012, 12, (30) Eda, G.; Lin, Y. Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. A.; Chen, I. S.; Chen, C. W.; Chhowalla, M. Adv. Mater. 2010, 22, (31) Mattevi, C.; Eda, G.; Agnoli, S.; Miller, S.; Mkhoyan, K. A.; Celik, O.; Mostrogiovanni, D.; Granozzi, G.; Garfunkel, E.; Chhowalla, M. Adv. Funct. Mater. 2009, 19, (32) Rusli; Robertson, J.; Amaratunga, G. A. J. J. Appl. Phys. 1996, 80, (33) Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008, 130, (34) Shen, J. H.; Zhu, Y. H.; Chen, C.; Yang, X. L.; Li, C. Z. Chem. Commun. 2011, 47, (35) Sun, Y.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; Luo, P. G.; Yang, H.; Kose, M. E.; Chen, B.; Veca, L. M.; Xie, S. J. Am. Chem. Soc. 2006, 128, (36) Jorio, A.; Ferreira, E. H. M.; Moutinho, M. V. O.; Stavale, F.; Achete, C. A.; Capaz, R. B. Phys. Status Solidi B 2010, 247, (37) Gokus, T.; Nair, R. R.; Bonetti, A.; Bohmler, M.; Lombardo, A.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. ACS Nano 2009, 3, (38) Kim, D. C.; Jeon, D. Y.; Chung, H. J.; Woo, Y.; Shin, J. K.; Seo, S. Nanotechnology 2009,