Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via

Feb 15, 2011 - We report the epitaxial formation of bilayer Bernal graphene on copper foil via chemical vapor deposition. The self-limit effect of gra...
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LETTER pubs.acs.org/NanoLett

Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition Kai Yan,† Hailin Peng,† Yu Zhou, Hui Li, and Zhongfan Liu* Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China

bS Supporting Information ABSTRACT: We report the epitaxial formation of bilayer Bernal graphene on copper foil via chemical vapor deposition. The self-limit effect of graphene growth on copper is broken through the introduction of a second growth process. The coverage of bilayer regions with Bernal stacking can be as high as 67% before further optimization. Facilitated with the transfer process to silicon/silicon oxide substrates, dual-gated graphene transistors of the as-grown bilayer Bernal graphene were fabricated, showing typical tunable transfer characteristics under varying gate voltages. The high-yield layer-bylayer epitaxy scheme will not only make this material easily accessible but reveal the fundamental mechanism of graphene growth on copper. KEYWORDS: Graphene, bilayer, Bernal stacking, CVD, epitaxy, band gap opening

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ernal-stacked bilayer graphene, where half of the carbon atoms in the second layer sit on top of the empty centers of hexagons in the first layer, is an interesting material because of its tunable bandgap of up to 250 mV by an external electric field.1-5 This unique property is attractive for fundamental research as well as applications where the ability to adjust the bandgap is desired, such as tunnel field-effect transistors6 and tunable laser diodes. Prototype bilayer Bernal graphene transistors with an on/off ratio exceeding 100 at room temperature7 have already been proposed with samples fabricated via mechanical exfoliation of graphite,8 which is a difficult technique to implement for scalable mass production. Recently, successful chemical vapor deposition (CVD) methods have been developed for the largearea growth of high quality graphene on transition metals.9-11 For example, CVD growth on Ni surface yields few-layered graphene films with various layer thicknesses and random stacking order.9,10 In addition, scalable growth of high-quality and uniform graphene films on Cu foil has been realized via lowpressure CVD (LPCVD), yielding predominantly monolayer graphene due to the self-limiting nature of the growth process.11,12 Most recently, Lee et al. reported a one-step CVD growth of bilayer graphene films on Cu foil.13 However, scalable production of high quality bilayer Bernal graphene is still in its infancy, especially lacking of reliable growth mechanisms. Here we propose a novel method to achieve the epitaxial growth of bilayer Bernal graphene using CVD on Cu foil. A second growth process is introduced to break the self-limit effect on copper. Coverage of technically qualified bilayer Bernal graphene can be as high as 67% with carrier mobility exceeding 500 cm2 V-1 s-1. This method may be beneficial for both potential applications r 2011 American Chemical Society

and more importantly, the fundamental understanding of the growth mechanism for graphene on Cu surface. Bilayer Bernal graphene is formed through the combination of an existing monolayer and another epitaxially deposited monolayer by a two-step growth scheme in a multizone, horizontal tube furnace (Figure 1a, also Figure S1 in Supporting Information). Uniform monolayer graphene used for this work was previously grown on a 25 μm thick copper foil strip at low growth pressure according to Li’s method11 and serves as the substrate placed downstream at 1000 °C for the following epitaxial growth. With growth pressure of 0.7 Torr, CH4 flow rate of 35 sccm, and H2 flow rate of 2 sccm, a piece of fresh copper foil was placed upstream where the temperature is relatively higher, as an efficient catalyst to continuously decompose CH4. At 1040 °C, sharp steps were observed to form on the surface of the copper catalyst (Figure S1 in Supporting Information), presumably preventing surface passivation (namely the selflimiting effect). Thus carbon radicals or small carbon fragments are transported downstream and deposited epitaxially onto the existing monolayer graphene film. Usually >30% coverage of bilayer region on monolayer graphene films can be achieved after 30 min of growth. Figure 1b,c shows scanning electron microscopy (SEM) images of the pristine monolayer and epitaxially grown bilayer graphene film transferred onto silicon substrates with 300 nm thick SiO2, respectively. Thicker graphene regions can seldom be found in the former (less than 5%) due to the self-limiting effect Received: November 15, 2010 Revised: February 7, 2011 Published: February 15, 2011 1106

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Figure 1. (a) A schematic drawing of the bilayer Bernal graphene growth mechanism involving gas-phase carbon radicals and aromatic fragments transported and epitaxially grown on a monolayer graphene surface. A side view of the bilayer Bernal graphene shows AB (Bernal) stacking. (b,c) SEM images of CVD grown monolayer (1 L) and bilayer (2 L) graphene transferred onto silicon substrate with 300 nm thermally oxidized SiO2 layer, respectively. (d) Contrast enhanced optical image of high bilayer coverage sample with epitaxy time of 60 min. Regions thicker than 2 layers are spotted by arrows. Inset: Contrast enhanced photograph of the sample. (e) Coverage statistics of different thicknesses in (d) showing 67% area is covered by bilayer Bernal graphene.

(Figure 1b), consistent with recent studies by Li et al.11 On the other hand, the latter exhibits large bilayer hexagons on the uniform monolayer graphene film (Figure 1c). Remarkably, all these hexagons have identical orientations within the same underlying monolayer grain, suggesting the epitaxial nature of the additional layer growth. In contrast, control experiments without the Cu catalyst do not show analogous epitaxy phenomenon (Figure S2 in Supporting Information). Atomic force microscopy (AFM) was used to determine the thickness of the hexagons, yielding an average height of around 0.5 nm, which is in agreement with that of a graphene monolayer (Figure S3 in Supporting Information). As growth time increases, the bilayer hexagons became larger flakes with the lateral dimensions of several tens of micrometers. Additionally, the hexagons may deform into hexagrams or round disks, presumably owning to the perturbation of the Cu substrate surface steps and inhomogeneities such as wrinkles, boundaries, and defects of underlying graphene (Figure S4 in Supporting Information). To statistically evaluate the coverage of bilayer regions, we analyzed the green channel contrast14,15 in optical microscope images (Figure 1d inset). In certain areas, the coverage of bilayer graphene can be as high as 67% (Figure 1d,e). Since the self-limited growth of graphene was broken, regions other than one- and two-layers

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Figure 2. (a) Bright-field TEM image of as-grown graphene film. The monolayer and bilayer regions are indicated by the blue and red circles, respectively. (b) High-resolution TEM images showing the folded-edges of as-grown graphene films with monolayer and bilayer regions. (c,d) Typical normal incident SAED patterns of monolayer and bilayer regions, respectively. (e,f) Intensity profiles along arrows in (c,d), respectively.

can also be spotted. However, due to the highly anisotropic bonding nature of graphene, intralayer growth via dangling bonds is much faster than that of interlayer through van der Waals interactions. Consequently, the films can be predominantly bilayer graphene (>60%) with a small coverage ( 2 and full width at half-maximum (fwhm) of about 32 cm-1 (Figure 3b).11,18 In comparison with monolayer regions, the 2D band for bilayer regions is generally wider and more asymmetric with the I2D/IG ratio of approximately 3/4, in good agreement with the features of the reference exfoliated graphene sample. Additionally, the asymmetric 2D band consists of four sub-bands, corresponding to four permissible transition processes in the spectrum of the characteristic bilayer Bernal graphene.18,21,22 Note that 2D bands of both monolayer and bilayer regions are observed to be broader than those of exfoliated graphene. This broadening effect impedes the division of sub-bands in bilayer, resulting in the vanishing of the featuring lower shoulder peak. We ascribe this observation to the existence of disorder from the chemical transfer process and graphene-substrate interaction23 (See Figure S6 in Supporting Information). Micro-Raman maps with the corresponding

optical and SEM images were collected to further qualify the uniformity of our epitaxially grown graphene. Monolayer and bilayer regions can be identified through contrast differentiation in both SEM and optical images (Figure 3c,d). Accordingly, the Raman maps of the intensity of G band, position and fwhm of 2D band, as well as I2D/IG show qualitative difference between the monolayer and bilayer regions (Figure 3e-h). Remarkably, the maps of bilayer regions show uniformly distributed color, indicating that the high homogeneity of bilayer Bernal graphene domains. Transport measurements were performed to evaluate the electronic property of our epitaxially grown graphene, in particular to verify the tunability of the bandgap. Standard electron beam lithography was used to fabricate dual-gate bilayer graphene field-effect transistors after graphene samples were transferred onto silicon substrates with 300 nm thick silicon oxide dielectric. Device structure illustration and representative optical image of a dual-gate bilayer device are shown in Figure 4a,b, respectively, where Cr/Au (5 nm Cr and 50 nm Au) was used as the contact metal and a 30 nm thick aluminum strip was deposited cross the channel as top gate.24 After exposure in air for hours, natural alumina formed beneath the strip and acted as reliable top gate dielectric. To exclude the doping effect of H 2 O and O 2 at ambient conditions, the measurement was carried out in a vacuum chamber heated to 200 °C. We swept the voltage of top gate (VTG) from -1.5 to 1.5 V at different back gate voltages (VBG) applied on the silicon substrate. The transfer characteristic varies significantly with VBG (Figure 4c,d), confirming its tunability with the external electric field.3,4,25 The resistance ridge increases in both directions of back gate voltage, ruling out the possibility of random stacked bilayer and trilayer graphene. It was believed that disorder-induced localized states in gapped bilayer Bernal graphene can lead to energy band tails extending into the bandgap,26-28 thus lowering the resistance of off-states. The relatively higher temperature for compelling 1108

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Nano Letters dopants also increases the off-state current. Nevertheless, the carrier mobility extracted from the fabricated devices were between 350 and 550 cm2 V-1 s-1, slightly dependent upon VBG, which is only one order lower than the best mechanically exfoliated bilayer graphene.29,30 The electrical measurement results suggest that our CVD bilayer samples are of reasonable quality. Finally, we briefly discuss the growth mechanism of epitaxially grown bilayer Bernal graphene. Under low growth pressure, multilayer graphene with tiny domains and coverage lower than 5% was found,11 owing to the self-limiting effect of Cu surface. These multilayer domains are believed to arise from the excessive decomposition of methane,31 which is quite limited. On the other hand ambient pressure favors the growth of massive nonuniform multilayer graphene on copper surface due to the lowered mass transport rate of the carbon source.32 In the case with low growth pressure, we noticed that carbon radicals or fragments generated from methane can transport over a certain distance in the reactor, serve as additional carbon source, and deposit downstream, providing the possibility of breaking down the self-limiting effect (see Figure S7 in Supporting Information). Physical or chemical inhomogeneities of underlying graphene, such as boundaries and impurities, could serve as nucleation sites for the epitaxy of additional layers.33 After nucleation, the growth process continues with a high anisotropic nature. Compared with the inert surface of graphene nuclei bonded by weak interlayer van der Waals interaction, edges with dangling bonds are much more active to bind covalently with incoming atoms or fragments. Consequently, lateral growth rate can be much faster than the vertical one. The growth anisotropy can be several tens of thousands estimated by the ratio of the lateral to vertical dimension of few-layer graphene domains. Without spatial restriction, incoming carbon radicals are likely to arrange into thermodynamically stable Bernal stacking structure during epitaxy. As a result, growth of dense bilayer Bernal graphene domains as large as 50 μm was realized under low growth pressure, which can be scaled up. Further investigation on the detailed growth mechanism is still desired. In conclusion, we have proposed a novel vapor-phase epitaxy to grow bilayer Bernal graphene. The thickness and stacking order of bilayer regions were confirmed by AFM, TEM, and Raman spectroscpoy. The coverage of bilayer regions was as high as 67% with single bilayer domain size as large as 50 μm. Transport measurements show the bilayer Bernal graphene has a typical tunable transport band gap. The vapor-phase epitaxy method can lead to a new technology for producing designed structures of 2D layered materials 33 with van der Waals gaps including bilayer Bernal graphene, paving the way for nanoelectronics, photoelectronics, and other possible practical applications.

’ ASSOCIATED CONTENT

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Supporting Information. Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: zfl[email protected].

Author Contributions †

These authors contributed equally to this work.

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’ ACKNOWLEDGMENT We thank Alan Y. Liu for helpful discussion, financial support by the National Science Foundation of China (No 20973007, 20973013, 51072004, 50821061, and 20833001) and the National Basic Research Program of China (nos. 2007CB936203 and 2011CB921904). ’ REFERENCES (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Science 2006, 313, 951. (3) Oostinga, J. B.; Heersche, H. B.; Liu, X.; Morpurgo, A. F.; Vandersypen, L. M. K. Nat. Mater. 2008, 7, 151. (4) Zhang, Y.; Tang, T.-T.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.; Shen, Y. R.; Wang, F. Nature 2009, 459, 820. (5) Castro, E. V.; Novoselov, K. S.; Morozov, S. V.; Peres, N. M. R.; dos Santos, J. M. B. L.; Nilsson, J.; Guinea, F.; Geim, A. K.; Neto, A. H. C. Phys. Rev. Lett. 2007, 99, No. 216802. (6) Fiori, G.; Iannaccone, G. IEEE Electron Device Lett. 2009, 30, 1096. (7) Xia, F.; Farmer, D. B.; Lin, Y.-m.; Avouris, P. Nano Lett. 2010, 10, 715. (8) 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. (9) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30. (10) 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. (11) 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. (12) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Ri Kim, H.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Nat. Nanotechnol. 2010, 5, 574. (13) Lee, S.; Lee, K.; Zhong, Z. Nano Lett. 2010, 10, 4702. (14) Blake, P.; Hill, E. W.; Neto, A. H. C.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Appl. Phys. Lett. 2007, 91, No. 063124. (15) Reina, A.; Thiele, S.; Jia, X.; Bhaviripudi, S.; Dresselhaus, M.; Schaefer, J.; Kong, J. Nano Res. 2009, 2, 509. (16) Suh, Y.; Park, S.; Kim, M. Microsc. Microanal. 2009, 15, 1168. (17) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. Nature 2007, 446, 60. (18) 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, No. 187401. (19) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Obergfell, D.; Roth, S.; Girit, C.; Zettl, A. Solid State Commun. 2007, 143, 101. (20) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep. 2009, 473, 51. (21) Malard, L. M.; Nilsson, J.; Elias, D. C.; Brant, J. C.; Plentz, F.; Alves, E. S.; Castro Neto, A. H.; Pimenta, M. A. Phys. Rev. B 2007, 76, No. 201401. (22) Hao, Y.; Wang, Y.; Wang, L.; Ni, Z.; Wang, Z.; Wang, R.; Koo, C. K.; Shen, Z.; Thong, J. T. L. Small 2010, 6, 195. (23) Berciaud, S. p.; Ryu, S.; Brus, L. E.; Heinz, T. F. Nano Lett. 2009, 9, 346. (24) Li, S.-L.; Miyazaki, H.; Kumatani, A.; Kanda, A.; Tsukagoshi, K. Nano Lett. 2010, 10, 2357. (25) Zhu, W.; Neumayer, D.; Perebeinos, V.; Avouris, P. Nano Lett. 2010, 3572. (26) Miyazaki, H.; Tsukagoshi, K.; Kanda, A.; Otani, M.; Okada, S. Nano Lett. 2010, 10, 3888. (27) Mkhitaryan, V. V.; Raikh, M. E. Phys. Rev. B 2008, 78, No. 195409. 1109

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