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2009, 113, 9433–9435 Published on Web 03/27/2009
Covalent Functionalization of Epitaxial Graphene by Azidotrimethylsilane Junghun Choi,† Ki-jeong Kim,‡ Bongsoo Kim,‡ Hangil Lee,*,§ and Sehun Kim*,† Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea, Beamline Research DiVision, Pohang Accelerator Laboratory (PAL), Pohang, Kyungbuk 790-784, Republic of Korea, and Department of Chemistry, Sookmyung Women’s UniVersity, Seoul 140-742, Republic of Korea ReceiVed: February 4, 2009; ReVised Manuscript ReceiVed: February 27, 2009
Chemically modified epitaxial graphene (EG) by azidotrimethylsilane (ATS) was investigated using highresolution photoemission spectroscopy (HRPES). Through the spectral analysis, we clearly confirmed that EG is modified by thermally generated nitrene radicals and found that the bonding nature between the nitrene radicals and EG is covalent. As we observe bonding nature of N 1s peaks, we found that two distinct N peaks can be clearly distinguished in the spectra. Using a covalently bound stretched graphene (CSG) model, we elucidated that nitrene radicals adsorb on the graphene layer at two different adsorption sites. Moreover, we were able to control the band gap of EG using valence band spectra as we change the amount of the dosing of nitrene. Since the discovery of graphene, the molecular doping of this material has been of great importance to control its electronic structure.1 Some initial attempts have already been made to induce charge carriers to graphene like thin carbon film by means of the adsorption of various gas molecules including NH3, H2O, and NO2.2 The detection of such adsorbed molecules has allowed the successful application of the electronic devices as sensors.3 It is well-known that the reactivity of graphene is much lower than that of single walled carbon nanotubes (CNTs), because, in an ideally flat graphene structure, there is no local strain due to curvature-induced pyramidalization and misalignment of the π-orbital of the carbon atoms, which leads to the chemical inertness of graphene.4 Recently, the variation in electronic structure resulting from NO2 doping on graphene was investigated using theoretical and experimental methods.5 However, in that system, the moleculesubstrate interactions were found to be weaker than the molecule-molecule interactions. Given that the electrical properties of graphene are similar to those of isolated graphene despite graphene-substrate interactions, we could expect EG to play a crucial role for applications in electronic devices. Moreover, It is reported that the electronic structure of a EG can be controlled using a potassium (n-type) and gold (p-type).6,7 In other words, they manipulated the band gap closing and reopening of EG by changing the doping concentration. Chemical modification by aryl group was successful using diazonium salt.8 We also reported that electron-beam irradiation on an EG under ambient conditions can produce various oxygen-related features on that surface.9 However, this procedure is inadequate for electronic-device applications because the substrate morphology is seriously damaged by the electron beam. * To whom correspondence should be addressed. E-mail: (S.K.)
[email protected]; (H.L.)
[email protected]. † Korea Advanced Institute of Science and Technology. ‡ Pohang Accelerator Laboratory. § Sookmyung Women’s University.
10.1021/jp9010444 CCC: $40.75
Here, we report the covalent functionalization of EG and characterize the modified substrate using HRPES to observe the changes in electronic structure caused by molecular doping without damaging the graphene layer. For the covalent functionalization of the EG surface, we employed nitrene chemistry, which was used previously in the side-wall functionalization of CNTs.10-12 Figure 1a-e shows how the ATS molecules [panels a and b] adsorb, via nitrene radical [panels c and d] onto the graphene surface. After removing N2, nitrene reacts with graphene via an electrophilic [2 + 1] cycloaddition reaction [panels c-e] or a biradical pathway [panels d and e] after intersystem crossing (ISC). All experiments were performed in an ultrahigh vacuum chamber (base pressure ) 1.5 × 10-10 Torr) at the 8A2 photoemission spectroscopy (PES) beamline at the Pohang accelerator laboratory (PAL), which is equipped with an electron analyzer (SES100, Gamma Data Scienta). We used a nitrogendoped (ND ≈ 9 × 1017 cm-3), Si-terminated 6H-SiC(0001) substrate, purchased from Cree Research (USA), to fabricate a flat graphene layer. The substrate was first treated by hydrogen plasma, followed by annealing at around 900 °C under a Si flux (1 Å/min), and finally annealed at temperatures up to 1100 °C (for two minutes) to produce several layers of graphene. The annealing temperature was monitored with an infrared pyrometer (emissivity - 0.9). Azidotrimethylsilane (ATS) (Sigma Aldrich, purity - 95%) was purified through several freeze-pump-thaw cycles, using liquid nitrogen to remove all the dissolved gases before dosing it onto the graphene substrate. A direct doser, controlled by means of a variable leak valve, was used to dose ATS. The C 1s core-level spectra, N 1s core-level spectra, and valence spectra were obtained using photon energies of 320, 500, and 130 eV, respectively, to enhance the surface sensitivity. The binding energies of the core-level spectra were determined with respect to that of the clean Au valence band (Fermi energy) for the same photon energy. All spectra were recorded in the 2009 American Chemical Society
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Letters
Figure 1. (a and b) Resonance forms of azidotrimethylsilane (c and d) thermally generated nitrene radicals (e) nitrene adsorbed graphene (f) CSG model to describe (63 × 63) R30° interface layer of EG (black circles: carbon atoms at interface layer, red circles: Si atoms, gray circles: C atoms, yellow diamond box: (3 × 3) R30° unit cell).
Figure 2. (a-c) C 1s core-level spectra obtained at photon energy of 320 eV. (d-f) N 1s core-level spectra obtained at photon energy of 500 eV. (g-i) Valence band spectra obtained at photon energy of 130 eV (a), (d), and (g) 0.5 ML of graphene, (b), (e), and (h) ATS deposition (7200 L) at 100 °C, (c), (f), and (i) 36 000 L at 100 °C. The inset shows the zoom-in valence-band spectra of the Fermi level.
normal-emission mode. The photoemission spectra were carefully analyzed using a standard nonlinear least-squares fitting procedure with Voigt functions.13 First, we carried out core-level HRPES experiments to characterize the nitrene-adsorbed graphene. Figure 2a shows the C 1s core-level spectrum of a graphene layer grown on 6HSiC(0001) obtained after annealing at 1100 °C. At this annealing temperature, the thickness of the graphene layer is known to be 0.5 ML (monolayer), as confirmed by PES (attenuation of Si 2p signal, data not shown) and low-energy electron diffraction experiments.14 As a consequence, we can speculate that onehalf of the substrate is a (63 × 63)R30° interface layer (hereafter we call this interface layer) and the other half-is covered by a graphene layer on a interface layer. As shown in this Figure 2a, three distinct peaks emerge at 285.5, 284.8, and 283.6 eV, which can be assigned, respectively, to sp2 carbon atoms weakly interacting with the substrate in the interface layer (two atoms around blue circles Figure 1f, marked as S2), carbon atoms located on top of subsurface Si atom in the interface layer (marked as S1), and SiC (marked as SiC).10,11 Figure 2, panels b and c, show the C 1s core-level spectra after the adsorption of nitrene (7200 and 36000 L, respectively) on a graphene layer while maintaining the substrate temperature at 100 °C to enhance the dissociation of N2 from the ATS
molecules. The large amount of exposed ATS indicates the low sticking coefficient on graphene due to the inertness of the substrate layer, with its two-dimensional π character, and the low efficiency of nitrene-radical generation. As shown in Figure 2, panels b and c, the intensity of the graphene-induced C 1s peak (S2) gradually decreases due to increased nitrene adsorption, while the other C 1s peak (S1) decrease only slightly. Therefore, we can speculate that the adsorption of nitrene predominantly occurs on the interface layer and that the small amounts of nitrene species react with graphene. In both cases, we can explain that nitrene does not affect other layers such as the subsurface SiC bilayer. Therefore, we could clearly observe the difference in the surface electronic structure between the as-grown graphene and nitrene adsorbed graphene. Eventually, our focus was to confirm the role of nitrogen contained in nitrene. Hence, we concurrently obtained N 1s corelevel spectra with increasing the exposure of ATS. We first checked the cleanness of the graphene layer [Figure 2d], and then took N 1s core-level spectra [Figure 2, panels e and f] after ATS deposition at 100 °C. Two distinct N peaks with binding energies of 398.5 eV (marked as N1) and 399.7 eV (marked as N2) can be clearly distinguished in the spectra. This means that nitrogen adsorbs on the graphene layer at two different adsorption sites. As already discussed, we proposed that nitrene primarily reacts with the sp2 carbon atoms in the interface layer. To simplify the interface layer, we introduced the covalently bound stretched graphene (CSG) model15 in Figure 1f. In this way, we can explain our results shown in Figure 2, panels e and f, in which an intensity ratio N1:N2 ) 2.81:1 (of about 3:1) is measured. As can be seen in Figure 1f, there are four possible reaction sites in the unit cell. In the three sites represented by blue circles in the unit cell, all six equivalent carbon atoms weakly interact with the substrate whereas the other site (marked as a green circle) contains two inequivalent carbon atoms. One of them strongly interacts with the underlying Si-terminated substrate and the other does not. Therefore, two different N peaks can be detected (with a ratio of 3:1). We assign the N1 peak to the adsorption at weakly interacting carbon atoms with the substrate and N2 peak to the adsorption at strongly interacting C atom.15 We exclude nondissociative adsorption of ATS because the central nitrogen peak in the azide (NN*N) is located at 403 eV, while the terminal nitrogen peak from azide (N*NN*) is located at 398.5 eV.16 Next, we acquired angle integrated valence-band spectra to clarify the molecular-doping effect caused by nitrene adsorption. In Figure 2g, we could find five distinct hybridized features
Letters evolving near 0.5, 3.0, 4.7, 8.2, and 11.0 eV below EF, which agrees well with the results obtained by Johansson et al. at 1150 °C.17,18 The strong peak near 4.7 eV is thought to be due to the strong π band at the M point.15 Moreover, we also confirmed that the graphene layer shows a metallic-like behavior. We did this by checking the existence of intensity at Fermi edge (EF). Note that the interface layer is nonmetallic.15 As discussed above, one-half of our substrate is covered by a interface layer and the other half-by a graphene layer. Thus, our substrate shows a metallic behavior due to the additional graphene layer on the interface layer. Figure 2h shows a remarkable change in the electronic structure after deposition of ATS (7200 L). First, we observed that the peak marked by A shift to lower binding energy. Compared to the graphene layered surface, the binding energy value was shifted in this case by 0.66 eV, which might be due to nitrene induced molecular doping. Second, we found that the band gap opened and the graphene-induced peak disappeared. This phenomenon could be explained by rehybridization of graphene due to covalently adsorbed nitrene, which breaks the mirror symmetry of the π-π* band crossing. Since we assigned two nitrogen peaks to adsorbed nitrene on the interface layer, the band gap opening provides indirect evidence of the adsorption of nitrene on the graphene layer. Third, the intensity of the peak at 4.7 eV decreases with respect to those of the other peaks. This observation is the signature of the adsorption of nitrene, as we have assigned this band to be the π band of graphene at the M point. Moreover, we clearly observe that band gap opened when increased molecular doping of nitrene up to 36 000 L [Figure 2i]. We successfully achieved the formation of covalent bond between thermally generated nitrene and EG. As already discussed, the bonding nature between nitrene and graphene is certainly covalent because the adsorbed nitrene species are stable at temperatures as low as 200 °C (Supporting Information). By investigating the N 1s to C 1s intensity ratio (N/C ≈ 1/53; 0.019), we see that the amounts of adsorbed nitrene on graphene are very small. This analysis suggests that most parts of the graphene layer are still preserved after adsorption of nitrene in view of the geometric structure. Also, we have investigated the variations in the electronic structure (i.e., band gap opening) for nitrene-adsorbed epitaxial graphene layer grown on a 6HSiC(0001) substrate upon with increasing the dose of nitrene. We believe that this nitrene-doped graphene layer could be useful for applications in electronic devices.
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9435 Acknowledgment. This work was supported by the Korea Research Foundation (Grant No. KRF-2006-312-C00565). One of the authors (H.L.) was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund; KRF-2008-314C00169). The experiments at PAL were supported in part by the Korean Ministry of Science and Technology (MOST) and POSTECH. Supporting Information Available: Additional data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (2) 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. (3) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652–655. (4) Niyogi, S; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105–1113. (5) Wehling, T. O.; Novoselov, K. S.; Morozov, S. V.; Vdovin, E. E.; Katsnelson, M. I.; Geim, A. K.; Lichtenstein, A. I. Nano Lett. 2008, 8, 173–177. (6) (a) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Science 2006, 313, 951–954. (7) Gierz, I.; Riedl, C.; Starke, U.; R.; Ast, C.; Kern, K. Nano. Lett. 2008, 8, 4603–4607. (8) Bekyarova, 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. (9) Kim, K.-j.; Choi, J.; Lee, H.; Lee, H,-K; Kang, T.-H.; Han, Y.-H.; Lee, B.-C; Kim, S.; Kim, B. J. Phys. Chem. C 2008, 112, 13062–13064. (10) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105–1136. (11) Holzinger, M.; Abraham, J.; Whelan, P.; Graupner, R.; Ley, L.; Hennrich, F.; Kappes, M.; Hirsch, A. J. Am. Chem. Soc. 2003, 125, 8566– 8580. (12) Holzinger, M.; Steinmetz, J.; Samaille, D.; Glerup, M.; Paillet, M.; Bernier, P.; Ley, L.; Graupner, R. Carbon 2004, 42, 941–947. (13) Schreier, F. J. Quant. Spectrosc. Radiat. Transfer 1992, 48, 743. (14) Kim, K.-j.; Lee, H.; Choi, J.; Lee, H.-K.; Kang, T.-H.; Kim, B.; Kim, S. J. Phys.: Condens. Matter 2008, 20, 225017. (15) Emtsev, K. V.; Speck, F.; Seyller, Th.; Ley, L.; Riley, J. D. Phys. ReV. B 2008, 77, 155303. (16) Moulder, J. F.; Stickle, P. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (17) Johansson, L. I.; Owman, F.; Martensson, P. Phys. ReV. B 1996, 53, 13793–13802. (18) Johansson, L. I.; Owman, F.; Martensson, P.; Persson, C.; Lindefelt, U. Phys. ReV. B 1996, 53, 13803–13807.
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