pubs.acs.org/Langmuir © 2010 American Chemical Society
Spatially Resolved Spontaneous Reactivity of Diazonium Salt on Edge and Basal Plane of Graphene without Surfactant and Its Doping Effect Hyunseob Lim,† Ji Sook Lee,§ Hyun-Joon Shin,‡ Hyeon Suk Shin,*,§ and Hee Cheul Choi*,† †
Department of Chemistry and Division of Advanced Materials Science, and ‡Pohang Accelerate Laboratory and Department of Physics, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Korea 790-784, and §Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Banyeon-ri 100, Ulsan, Korea 689-805 Received March 30, 2010. Revised Manuscript Received May 31, 2010 The site-dependent and spontaneous functionalization of 4-bromobenzene diazonium tetrafluoroborate (4-BBDT) and its doping effect on a mechanically exfoliated graphene (MEG) were investigated. The spatially resolved Raman spectra obtained from both edge and basal region of MEG revealed that 4-BBDT molecules were noncovalently functionalized on the basal region of MEG, while they were covalently bonded to the edge of MEG. The chemical doping effect induced by noncovalently functionalized 4-BBDT molecules on a basal plane region of MEG was successfully explicated by Raman spectroscopy. The position of Fermi level of MEG and the type of doping charge carrier induced by the noncovalently adsorbed 4-BBDT molecules were determined from systematic G band and 2D band changes. The successful spectroscopic elucidation of the different bonding characters of 4-BBDT depending on the site of graphene is beneficial for the fundamental studies about the charge transfer phenomena of graphene as well as for the potential applications, such as electronic devices, hybridized composite structures, etc.
Introduction Since its first experimental preparation by mechanical exfoliation,1 graphene has been highly attracted for its unique electronic properties2-4 such as high carrier mobility (200 000 cm2/(V s)),2 quantum hall effect,3 and ballistic transport.4 Besides the mechanical exfoliation method, graphene has been also directly obtained by epitaxial growth from SiC crystal,5 chemical vapor deposition process on various metal catalyst substrates,6 and solution approaches using dispersions of graphene or graphene *To whom correspondence should be addressed. E-mail: choihc@postech. edu (H.C.C.);
[email protected] (H.S.S.). (1) Novoselove, 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) (a) Morozov, S. V.; Novoselev, K. S.; Katsnelson, M. I.; Schedin, F.; Elias, D. C.; Jaszczak, J. A.; Geim, A. K. Phys. Rev. Lett. 2008, 100, 016602. (b) Bolotin, K.; Sikes, K. J.; Hone, J.; Stormer, H. L.; Kim, P. Phys. Rev. Lett. 2008, 101, 096802. (3) (a) Novoselove, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197–201. (b) Zhang, Y.; Tan, T.-W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201– 204. (c) Geim, A. K.; Novoselov, K. S. Nature Mater. 2007, 6, 183–191. (d) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2004, 315, 1379. (4) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Nature Nanotechnol. 2008, 3, 491–495. (5) (a) Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A. Y.; Feng, R.; Dai, Z.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. J. Phys. Chem. B 2004, 108, 19912–19916. (b) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayo, D.; Li, T. B.; Hass, J.; Marchenkon, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (6) (a) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Nature 2009, 457, 706–710. (b) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30–35. (c) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312–1314. (7) (a) Park, S.; Ruoff, R. S. Nature Nanotechnol. 2009, 4, 217–224. (b) Li, D.; M€uller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nature Nanotechnol. 2008, 3, 101–105. (c) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun'ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. Nature Nanotechnol. 2008, 3, 563–568. (d) Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Science 2008, 319, 1229–1232.
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derivatives, such as graphene oxides.7 A variety of graphene electronic devices have been also fabricated to investigate its unprecedented electrical properties.2-4 Two fundamentally important issues in graphene electronics are (1) to understand the general influence of graphene structure on its electrical property, for example, electron transport influenced by the width and length of graphene8 as well as by the pathway of edge or basal plane on the conductance,9 and (2) how to modulate the intrinsic electrical property of graphene, such as its band structure and Fermi level.10,11 To the same degree of importance of the former issue regarding to the structure effect on the electrical transport property difference of graphene,12 there have been quite a few efforts to resolve the latter issue by developing potential chemical functionalization reactions that allow chemical coupling between various electronically active (electron-donating or electron-withdrawing) moieties and graphene, such as hydrogenation and € (8) (a) Han, M. Y.; Ozyilmaz, B.; Zhang, Y.; Kim, P. Phys. Rev. Lett. 2007, 98, 206805. (b) Li, Z.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Science 2008, 319, 1229– 1232. (9) (a) Areshkin, D. A.; Gunlycke, D.; White, C. T. Nano Lett. 2007, 7, 204–210. (b) Li, T. C.; Lu, S.-P. Phys. Rev. B 2008, 77, 085408. (c) Brey, L.; Fertig, H. A. Phys. Rev. B 2006, 74, 195417. (10) Boukhvalov, D. W.; Katsnelson, M. I. Phy. Rev. B 2008, 78, 04504. (11) (a) Das, B.; Voggu, R.; Rout, C. S.; Rao, C. N. R. Chem. Commun. 2008, 5155–5157. (b) Su, Q.; Pang, S.; Alijani, V.; Li, C.; Feng, X.; M€ullene, K. Adv. Mater. 2009, 21, 3191–3195. (c) Jung, N.; Kim., N.; Jockusch, S.; Turro, N. J.; Kim, P.; Brus., L. Nano Lett. 2009, 9, 4133–4137. (d) Benayad, A.; Shin, H.-J.; Park, H. K.; Yoon, S. M.; Kim, K. K.; Jin, M. H.; Jeong, H.-K.; Lee, J. C.; Choi, J.-Y.; Lee, Y. H. Chem. Phys. Lett. 2009, 475, 91–95. (e) Kong, B.-S.; Geng, J.; Jung, H.-T. Chem. Commun. 2009, 2174. (f ) Dong, X. C.; Fu, D. L.; Fang, W. J.; Shi, Y. M.; Chen, P.; Li, L. J. Small 2009, 5, 1422–1426. (12) (a) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347–349. (b) Cervantes-Sodi, F.; Csanyi, G.; Piscanes, S.; Ferrari, A. C. Phys. Rev. B 2008, 77, 165427. (c) Sharma, S.; Nair, N.; Strano, M. S. J. Phys. Chem. C 2009, 113, 14771– 14777. (d) Jia, X.; Hofmann, M.; Meunier, V.; Sumpter, B. G.; Compos-Delgado, J.; Romo-Herrera, J. M.; Son, H.; Hsieh, Y.-P.; Reina, A.; Kong, J.; Terrones, M.; Dresselhaus, M. S. Science 2009, 323, 1701–1705. (e) Girit, C-, O.; Meyer, J. C.; Erni, R.; Rossell, M. D.; Kisielowske, C.; Yang, L.; Park, C.-H.; Crommie, M. F.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 2009, 323, 1705–1708. (f ) Huang, J. Y.; Ding, F.; Yakobson, B. I.; Lu, P.; Qi, L.; Li, J. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10103– 10108.
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diazonium salt reactions occurring directly and spontaneously on a graphene surface.13-15 The spontaneous chemical functionalization of diazonium salt is one of the famous reactions to chemically dope graphitic carbon structures. The chemical functionalization of diazonium salt molecules has been first demonstrated to modify HOPG and glassy carbon surfaces by electrochemical reduction which is known as the Saevant method.16 This is the first example of covalent bond formation of diazonium salt molecules to graphitic carbons without preoxidation process. Similar reactions have been attempted on carbon nanotubes and graphene as well: Several research groups including Smalley and Shim groups have reported that metallic carbon nanotubes can be selectively and covalently functionalized with diazonium salts even without external energy source,17 owing to the significantly lowered activation energy due to the large π-orbital pyramidal angle originated from the high curvature of carbon nanotube surface.18 Although two-dimensional graphene has no such a geometrical advantage, similar chemical interactions between diazonium salts and chemically converted graphene (CCG)14 or epitaxial graphene (EG)15 have been also reported. More recently, the covalent reactivity of diazonium salt on mechanically exfoliated graphene (MEG) depending on the site (edge or basal plane) and the number of graphene layer has been reported by Strano’s group.19 However, an accurate reaction type, i.e., covalent or noncovalent coupling during the “pure reaction” of diazonium salt on MEG, and the consequent doping effect are still unclear because either surfactants (sodium dodecyl sulfonate (SDS)19 or sodium dodecylbenzenesulfonate (SDBS)14) or highly charged electrolytes (tetrabutylammonium hexafluorophosphate in acetonitrile15) have been always included for the successful covalent coupling, sometimes even at elevated temperature. These additives make further difficult to ascertain doping effects by diazonium salts due to the less functionalization activity with decreasing surfactant concentrations as well as to the effects of surfactant itself on doping.20 Meanwhile, Avouris’s group has demonstrated that a MEG field effect transistor (FET) device displays a prompt doping effect by diazonium salt treatment when a surfactant-free reaction condition is applied.21 Although their functionalization type has been predicted to be a noncovalent functionalization according to the insignificant decrease of the conductance after the treatment as opposed to the covalent bonding case,15 the high
population of defect in their starting graphene as judged from the D-band intensity of Raman spectrum still makes the direct correlation between the reactivity and doping effect of diazonium salts vague. Therefore, an accurate characterization about the reactivity of defect-free graphene to diazonium salts in the absence of surfactant or electrolyte is highly beneficial to clearly understand whether covalent or noncovalent functionalization occurs during the reaction and which type of functionalization is responsible for the doping effect. Herein, we report that diazonium salt molecules are noncovalently functionalized on the basal plane of defect-free graphene while covalent bonding is formed on the edge of the graphene. We also confirm that the noncovalent functionalization of diazonium salt on the basal plane of graphene readily induces doping to modulate Fermi level of the graphene as demonstrated by Raman spectroscopy studies. Note that we only focus on the Raman spectroscopic elucidation of the reaction characteristics because the electron transport change upon the similar treatment has been previously reported by the Avouris group. Raman spectroscopy is a powerful tool to characterize electronic and structural property changes in graphene.22,23 According to its well-established theory and experimental results, Raman G (∼1560 cm-1) and D bands (∼1360 cm-1) depict the E2g phonon mode at Brillouin zone center and the A10 mode at K point of Brillouin zone, respectively. The latter becomes Raman-active by defect-induced scattering. More importantly, 2D band (second order of the D band, also called the G0 band) appearing at around 2700 cm-1 signifies the specific number of graphene layer which is one of the critical factors determining the properties of graphene. Therefore, the aforementioned two different functionalization types of diazonium salts on graphene (covalent or noncovalent) can be identified by investigating the spectral shape and intensity changes of G and D bands. Meanwhile, a systematic modulation of doping level also can be confirmed from the correlated changes of the featured Raman G and 2D bands according to the degree of functionalization that is controlled by the concentration increase of diazonium salt. Up to date, such a specific correlation of doping effect on graphene to Raman spectral features has been established only for the cases of electrical gate doping on graphene-FET devices24,25 and for the chemical doping cases such as aromatic molecules and bromine gas.11
(13) (a) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Science 2009, 323, 610–613. (b) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Phys. Rev. B 2007, 75, 153401. (14) (a) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W.-F.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 16201–16206. (b) Sun, Z.; Kohama, S.-I.; Zhang, Z.; Lomeda, J. R.; Tour, J. M. Nano Res. 2010, 3, 117–125. (15) Beykarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C. J. Am. Chem. Soc. 2009, 131, 1336–1337. (16) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (b) Liu, Y. C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254–11259. (17) (a) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519–1522. (b) Wang, C.; Cao, Q.; Ozel, T.; Gaur, A.; Rogers, J. A.; Shim, M. J. Am. Chem. Soc. 2005, 127, 11460–11468. (c) Nguyen, K. T.; Shim, M. J. Am. Chem. Soc. 2009, 131, 7103–7106. (18) (a) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (b) Zimmerli, U.; Gonnet, P. G.; Walther, J. H.; Koumoutsakos, P. Nano Lett. 2005, 5, 1017–1022. (c) Gotovac, S.; Honda, H.; Hattori, Y.; Takahashi, K.; Kanoh, H.; Kaneko, K. Nano Lett. 2007, 7, 583– 587. (19) Sharma, R.; Baik, J. H.; Perera, C. J.; Strano, M. S. Nano Lett. 2010, 10, 398–405. (20) Moonoosawmy, K. R.; Kruse, P. J. Am. Chem. Soc. 2010, 132, 1572–1577. (21) (a) Farmer, D. B.; Golizadeh-Mojarad, R.; Perebeinos, V.; Lin, Y.-M.; Tulevski, G. S.; Tsang, J. C.; Avouris, P. Nano Lett. 2009, 9, 388–392. (b) Farmer, D. B.; Lin, Y.-M.; Afzali-Ardakani, A.; Avouris, P. Appl. Phys. Lett. 2009, 94, 213106.
Functionalization of 4-BBDT. The graphene samples were
Experimental Methods
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prepared by a mechanical exfoliation method known as the “Scotch tape method” using Kish graphite (Toshiba Ceramics) on SiO2/Si substrates (300 nm thermally grown SiO2 oxide layer (22) (a) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 187401. (b) Graf., D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Nano Lett. 2007, 7, 238–242. (c) Calizon, I.; Balandin, A. A.; Bao, W.; Miao, F.; Lau, C. N. Nano Lett. 2007, 7, 2645–2649. (d) Yoon, D.; Moon, H.; Son, Y.-W.; Samsonidze, G.; Park, B. H.; Kim, J. B.; Lee., Y. P.; Cheong, H. Nano Lett. 2008, 7, 4270–4274. (23) (a) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic: San Diego, CA, 1996. (b) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1999, 275, 187–191. (c) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409, 47–99. (d) Lim, H.; Shin, H. S.; Song, H. J.; Choi, H. C. Nanotechnology 2009, 20, 145601. (e) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep. 2009, 473, 51–87. (f ) Ferrari, A. C. Solid State Commun. 2007, 143, 47–57. (24) Pisna, S.; Lazzeri, M.; Casiraghi, C.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Mauri, F. Nature Mater. 2007, 6, 198–201. (25) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; Sood, A. K. Nature Nanotechnol. 2008, 3, 210–215.
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Figure 1. (a). AFM image and height profile of MEG on a SiO2 substrate. (b) Raman spectra taken at the positions marked as 1, 2, and 3 in (a). (c) Expanded view of normalized 2D bands (dashed box in (b)) corresponding to single, double, and triple layers of graphene. The bands are fitted with the Lorentzian function. on p-type Si (100) wafer). Single-layer graphenes were identified by optical microscopy, AFM, and Raman spectroscopy. The various concentration solutions of 4-BBDT (96%, Aldrich) were prepared by dissolving 4-BBDT in water/methanol 1:1 mixture solvent at room temperature. The graphene-containing SiO2/Si substrates were immersed in the solutions for 30 min at room temperature and rinsed thoroughly with methanol and water to remove residual chemicals before further characterizations. Raman Spectroscopy Measurement. The location of single layer graphene was identified by observing the contrast difference in an optical microscope image, after which Raman spectra were acquired at room temperature using Alpha 300s (WITEC, Gemany). A diode laser of 532 nm was used as an excitation source at the power below ∼2 mW. The spatial resolution of the spectrometer was ∼250 nm, and Raman images were mapped by the integrated intensities of G, D, and 2D bands from 1530 to 1630 cm-1, from 1300 to 1400 cm-1, and from 2640 to 2740 cm-1, respectively.
Scanning Photoelectron Microscopy (SPEM, Spatially Resolved XPS). The elemental and chemical state information was obtained by SPEM at the Pohang Light Source. A spaceresolved XPS spectrum was obtained by measuring the photoelectrons generated from the X-ray focused area, and an elemental or chemical state image was obtained by mapping the specific photoelectron yield as scanning the sample. A Fresnel zone plate was used to focus the incident X-rays, and the space resolution of the SPEM was about 500 nm at the photon energy of 630 eV. The base pressure of the SPEM was 2.0 10-10 Torr. An SPEM image using C 1s photoelectron (284 eV) was first obtained to identify the location and geometry of graphene. Then, XPS spectra in the region of Br 3d photoelectron (66-71 eV) and N 1s photoelectron (394-406 eV) were obtained at the positions of single, multilayer graphene and bare SiO2 substrate. The spectra were deconvoluted and fitted as Lorentzian function.
Results and Discussion The starting graphene samples were prepared by mechanically exfoliating Kish graphite on a highly p-doped silicon wafer on which a 300 nm thick SiO2 layer was thermally deposited. Because the as-exfoliated graphene sample was expected to have random numbers of layers, we first attempted to evaluate the exact number of graphene layer by atomic force microscopy (AFM) and Raman spectroscopy. As shown in Figure 1a, three graphene regions (spots 1, 2, and 3) were identified by AFM, and the number of graphene layer was predicted to be single, double, and triple, respectively (Figure 1a). To determine the number of layer more precisely, the same region was scanned using a Raman spectrometer equipped with a laser having an excitation wavelength of 532 nm (power: ∼2 mW). The Raman spectra of which 2D bands were deconvoluted and fitted with a Lorentzian function (colored solid lines in Figure 1c) agreed the AFM 12280 DOI: 10.1021/la101254k
prediction: a single Lorentzian curve was well fitted for a single layer graphene, while four and two deconvoluted Lorentzian bands were fitted for double and triple layers of graphene, respectively. Note that all the bands were normalized in amplitude and offset vertically, and these results agree well with the previously reported ones.22 Although the center region of the MEG is almost defect-free, the edges must contain nongraphitic carbonaceous defects. In order to evaluate the general G and D band characteristics of graphene from edges to centers, G and D band Raman images were mapped by integrating the intensities of G and D bands from 1530 to 1630 cm-1 (Figure 2a) and from 1300 to 1400 cm-1 (Figure 2e), respectively. These images clearly depict that most of the basal plane region of the graphene contains high population of sp2 carbons (i.e., defectfree), while defects exist only at around the edges. To examine the spontaneous reactivity of diazonium salt on the edge and basal plane of MEG under the general reaction condition (no surfactant and short reaction time at room temperature), the MEG sample was immersed in a 100 μM 4-bromobenzene diazonium tetrafluoroborate (4-BBDT), a representative diazonium salt solution with water and methanol mixed solvents (1:1 by volume), and kept for 30 min at room temperature. The fully rinsed and dried sample was examined by collecting both Raman images mapped with G and D band and spatially resolved Raman spectra. When the basal plane region (spot 1) of the MEG was examined, there was no big change in D, G, and 2D bands before and after the reaction, except a slight change in G/2D ratio (Figure 2a-d). Because the diameter of laser spot is smaller than 500 nm, the data obtained from the spot 1 purely represents the chemical bonding information on the defect-free graphene, implying that 4-BBDT molecules are functionalized noncovalently on a defect-free basal region of MEG. Compared to the basal plane region, there was an obvious change in D band intensity from the graphene edges after the reaction (Figure 2: e, before; g, after), which implies that 4-BBDT molecules covalently couple to the edge of MEG (see Figure S1 and explanation in Supporting Information for more detailed discussion about the origins of the D band). More specific confirmation about the D band increase relative to G band (D/G ratio) was acquired from the statistical average of D/G ratio changes before and after the reaction. The average D/G ratio changes were calculated from 12 Raman spectra extracted from the points on the edge line indicated with green arrows in Figure 2e,g (see Supporting Information for detailed statistical analysis). The result shows an obvious increase of D/G ratio from 0.282 to 0.420 after the reaction. Two representative Raman spectra obtained before and after the reaction from the edge positions marked as A are also shown in Figures 2f and 2h, respectively. Langmuir 2010, 26(14), 12278–12284
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Figure 2. Reactivity difference on the edges and center region of MEG. (a-d) Bulk plane of graphene. Raman images mapped with the integrated intensities of G bands (a: before the reaction; c: after the reaction) and spectra ((b) before the reaction and (d) after the reaction) profiled from the spot 1 indicate that no covalent bond functionalization occurs on the center region of MEG during the 4-BBDT treatment. (e-h) Edge of graphene. Raman images mapped with the integrated intensities of D bands ((e) before the reaction and (g) after the reaction) and spectra ((f ) before the reaction and (h) after the reaction) profiled from the blue spots marked as A clearly depict the substantial increase of D peak intensity, which indicates that covalent bond formation readily occurs on the edges of MEG. We should note that the Raman images in (a) and (e) are identical to the ones shown in Figure 1d,e for clear description about the experiments.
According to these results, the reactivity of MEG to diazonium salt is quite different from those of carbon nanotube or other graphenes, such as CCG or EG. Such an abnormal reactivity seems to be originated from the initial population of defect. The CCG and EG have high population of defect on a bulk plane as well as edge regions of graphene whereas MEG has negligible initial defects on its basal plane. Actually, it is well-known that the initial degree of defect affects the reactivity of carbon nanotube to diazonium salt molecules.26 Another reason for the unfavorable covalent reactivity of diazonium salt on the basal plane of MEG is the instability of the resulting covalent bond. In the case of MEG, syn-addition of diazonium salt molecule is the only possible way for the covalent bond formation because of the presence of SiO2 substrate underneath an MEG, while anti-addition can occur on CNT or CCG.13 Because syn-addition induces a high strain to sp3 C-C bonds, covalently bonded graphene may be very unstable. Consequently, as exemplified from the hydrogenation of graphene, syn-addition induces less reactivity than anti-addition does.13 Therefore, diazonium salt molecules prefer to be noncovalently functionalized on a defect-free graphene plane but may require extra additives for the covalent functionalization, such as external energy sources applied for HOPG16 or surfactants for CCG14 and EG.15 In order to confirm the effect of the number of graphene layer on the reactivity, Raman spectra were taken from the basal planes of double- and triple-layer graphenes as marked spots 2 and 3 in Figure 1a after the diazonium salt treatments. Similarly to the single-layer graphene, no D band was observed after the treatments (Figure S2a,b). The noncovalently functionalized 4-BBDT on MEG was evidenced by synchrotron scanning photoelectron microscopy (SPEM). The C 1s intensity distribution or C 1s SPEM image was first obtained to identify the location of a single (26) Abdula, D.; Nguyen, K. T.; Shim, M. J. Phys. Chem. C 2007, 111, 17755– 17760.
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layer graphene after the 4-BBDT treatment. (Figure 3b). Then, the space-resolved XPS spectra in the region of Br 3d photoelectron and N 1s photoelectron were obtained from single-, multilayer graphene and bare SiO2 substrate at spots 1, 2, and 3 in Figure 3b, respectively. As shown in Figure 3c,d, both Br 3d photoelectron and N 1s photoelectron were detected from the single-layer graphene, while negligible intensities were observed from the bare SiO2 substrate (Figure S3a,b). From the multilayer graphene, both photoelectrons are observed (Figure S3c,d), which implies that 4-BBDT molecules are quite selectively adsorbed on graphene regardless of the number of graphene. The direct evidence for the noncovalent functionalization of 4-BBDT on graphene is the fact that two kinds of nitrogen atoms having different oxidation state are found at 400.5 and 398.6 eV, which are assigned as R and β nitrogen atoms of 4-BBDT, respectively. If 4-BBDT were covalently bonded, the photoelectrons from nitrogen should not be detected because the N2 group in 4-BBDT is supposed to be eliminated as nitrogen gas. The noncovalently functionalized 4-BBDT on a basal plane of pristine graphene induced notable changes of the phonon modes in Raman spectra regardless of the number of graphene layer. The position and full width at half-maximum (fwhm) of G and 2D bands taken before and after the reaction are summarized in Table 1. The G and 2D band changes imply that the electronic structure of graphene is modulated. We evaluated the doping effect of 4-BBDT upon its noncovalent functionalizations on a basal plane of MEG at systematically controlled concentrations by Raman spectroscopy. For this, a new fresh MEG sample was prepared. The optical microscopy and Raman images (G, D, and 2D bands) confirmed that the new MEG had a single layer of graphene (Figure 4a-d) as similar to the ones shown in Figure 1. The fresh MEG sample was then immersed in 4-BBDT solutions prepared at various concentrations in mixed solvents of water and methanol (1:1 by volume) and kept for 30 min at room DOI: 10.1021/la101254k
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Figure 3. (a, b) Optical microscope image and SPEM image of 4-BBDT treated graphenes. (c, d) XPS spectra of Br 3d photoelectron (c) and N 1s photoelectron (d). Two peaks shown in (d) are assigned as photoelectrons from R and β N of 4-BBDT as shown in the inset image. Table 1. Peak Position and Full Width at Half-Maximum (fwhm) Values of G and 2D Bands of Single-, Double-, and Triple-Layer Graphenes before and after the Reactions of Basal Planes of MEG with 4-BBDT original MEG Raman features G
-1
position (cm ) fwhm (cm-1) 2D position (cm-1) fwhm (cm-1) I(2D)/I(G)
after reaction with 4-BBDT
1L
2L
3L
1L
2L
3L
1586 17 2672 25 3.18
1584 20 2677 47 1.06
1584 20 2688 55 0.62
1585 20 2691 29 4.42
1582 20.5 2696 55 1.28
1582 20 2700 65 0.89
temperature. To investigate the systematic doping effect as a function of 4-BBDT concentration without changing the sample, the reaction was first carried out in a low concentration solution and then repeated in higher concentration solutions. Throughout the reactions, we found that G and 2D bands were systematically changed as the reaction concentration was increased (Figure 4e). These two key Raman feature changes are critical because the doping effect (p-doping, n-doping, or no doping) can be determined by monitoring (1) the changes of the position and width of G and 2D bands and (2) the changes of the intensity ratio of 2D to G (I(2D)/I(G)).24,25 Note that all the spectra were taken at the same position marked as A in Figure 4b using a low-power laser (below 0.5 mW) to minimize the thermal effect of graphene and to prevent any possible pyrolysis of the functionalized 4-BBDT. The changes of Raman band position and width were investigated from the G and 2D bands fitted and normalized with a Lorentzian function (Figure 5). The G band position of the original MEG was 1586 cm-1, which was clearly shifted to lower energy (1585 cm-1) upon the reaction with 50 μM 4-BBDT solution (Figure 5a,c). The G band was then shifted back to higher energy (1587 cm-1) upon further reaction with 100 μM solution. Beyond this concentration, the G band was continuously shifted to higher energy. The bandwidth, determined by the fwhm, showed an exactly opposite trend as the largest fwhf value was 12282 DOI: 10.1021/la101254k
measured upon the reaction with 50 μm 4-BBDT solution. Meanwhile, the 2D band position was continuously shifted toward higher energies as the concentration of 4-BBDT was increased, while the 2D bandwidth was not much changed (Figure 5b,d). Such changes of G and 2D band positions and widths induced by the reaction with 4-BBDT, i.e., chemical doping, are comparable with the previously reported changes induced by the electrical doping through the gate voltage application:24,25 the G band displays its smallest energy value when the Fermi level of the graphene is located at the Dirac point. When the Fermi level is far away from the Dirac point due to either n-doping or p-doping, the position of the G band is supposed to be shifted to higher energies. The fwhm displays its highest value when the Fermi level is located at the Dirac point and supposed to be shifted toward smaller values upon either p- or n-doping. Note that such abnormal changes and stiffening of G band cannot be explained by general adiabatic Bohn-Oppenheimer approximation. Because the inverse of the G band pulsation is much smaller than the typical electron-momentum relaxation times, time-dependent lattice displacement has to be considered.24 The calculation considering dynamic effect of lattice displacement during the vibration motion can explain the trend of G band change well. In our chemical doping case, the G band in Figure 5c was positioned at the lowest energy after the reaction with 50 μM of 4-BBDT, after which the fwhm shows also the largest value. Consequently, the noncovalent adsorption of 4-BBDT successfully dope holes to the graphene, of which Fermi level becomes located near at the Dirac point by the reaction with 50 μM 4-BBDT. This also indicates that the original pristine graphene is slightly predoped with charged impurities. Actually, it is known that the Fermi level of a single layer graphene prepared by mechanical exfoliation is not exactly located at the Dirac point due to the doping effect by impurities.27 (27) (a) Casiraghi, C.; Pisana, S.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C. Appl. Phys. Lett. 2007, 91, 233108. (b) Hulman, M.; Haluska, M.; Scalia, G.; Obergfell, D.; Roth, S. Nano Lett. 2008, 8, 3594–3597.
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Figure 4. Effect of concentration of 4-BBDT on the Raman spectral changes of MEG. (a) Optical image of MEG used for the functionalization with 4-BBDT. (b-d) Raman images of the MEG mapped with the integrated intensities of G, D, and 2D bands. (e) Raman spectra obtained before and after the reactions at various concentrations of 4-BBDT.
Figure 5. Doping effect on MEG upon the functionalization of 4-BBDT at various concentrations. (a, b) Normalized and fitted G and 2D bands from the Raman spectra obtained in Figure 4e, respectively. Gray dotted curves track the band position change trends. (c, d) The band position (black squares) and fwhm (gray triangles) of G and 2D bands are plotted as a function of the reaction concentration, respectively.
Despite the successful determination of chemical doping phenomena, the exact doping charge carreir type (either n-doping or p-doping) is still unresolved yet. Since the 2D band in a doped graphene shows a different tendency from the G band due to its negligible influence of dynamic effects (considered in G phonon mode), the change of 2D band position allows tracking of the doping type. For example, the position of 2D band is supposed to be shifted to higher energy when the Fermi level is decreased (p-doped) and to lower energy when the Fermi level is increased (n-doped).25 In our case, the 2D band was gradually shifted toward a higher energy Langmuir 2010, 26(14), 12278–12284
direction upon the functionalization with 4-BBDT (Figure 5b), which indicates that the functionalization of graphene with 4-BBDT induces p-doping. This result agrees well with the previously reported one demonstrated by electron transport measurement.21 Lastly, the intensity ratio change of 2D to G bands (I(2D)/ I(G)) as a function of reaction concentration was plotted to confirm the degree of doping on graphene and the corresponding Fermi level change. As shown in Figure 6, the I(2D)/I(G) value shows its maximum at 50 μM, at which the Fermi level matches to Dirac point of the graphene. This trend also agrees well with the DOI: 10.1021/la101254k
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Figure 6. Evaluation of the Fermi level changes of MEG upon the 4-BBDT functionalization. I(2D)/I(G) value changes as a function of the concentration of 4-BBDT. The inset images depict the positions of Fermi level of graphene before (I) and after the reaction with 50 μM (II) and 100 μM (III) of 4-BBDT solutions.
aforementioned results obtained from the G band position change and the trend of fwhm change of the G band (Figure 5a,c). The modulation of Fermi level position with respect to the Dirac point is also depicted according to the degree of functionalization by 4-BBDT (inset in Figure 6). Although the trend of I(2D)/I(G) change resembles to that of fwhm change of G band, more obvious change is observed from the I(2D)/I(G) graph.
Conclusion In summary, we demonstrated that 4-BBDT molecules were spontaneously functionalized noncovalently on a defect-free
12284 DOI: 10.1021/la101254k
basal plane of MEG in a surfactant-free reaction environment at room temperature. The noncovalently functionalized 4-BBDT on the basal plane of MEG induced Raman G and 2D band changes in position as well as bandwidth, from which p-doping of MEG was confirmed. Throughout the characterizations, we successfully established the correlation between the chemical doping effect and the Raman spectral changes, such as the position of the Dirac point, the changes of Fermi level, and I(2D)/I(G) for the diazonium salt functionalization on graphene. Considering that the diazonium salt reaction is based on the spontaneous electron transfer phenomenon, and that such an electron transfer in carbon-based nanostructures including graphene is one of the critically important pathways for various physical and chemical applications, we believe that the Raman spectral correlation to chemical doping will boost up in-depth studies about various charge transfer-based phenomena, especially occurring between graphene and organic/inorganic molecules, by which new multifunctional hybridized systems can be obtained. Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF) grant funded by MEST (2008-04306, 2007-8-1158, 2005-01325), KOSEF through EPB center (R11-2008-052-02000), and Ministry of Health, Welfare & Family Affairs (A0900062), Korean Research Foundation (MOEHRD, KRF-2005-005-J13103). H.C.C. and H.S.S. thank the World Class University (WCU) program (R31-2008-00010059-0, R31-2008-000-20012-0). Supporting Information Available: Method for Raman imaging, statistical analysis, and spectroscopic results (Raman, XPS, and SPEM) on multilayer graphene. This material is available free of charge via the Internet at http:// pubs.acs.org.
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