Libo Gao ( 高力波 ), Wencai Ren ( 任文才 ), a) Jinping Zhao ( 赵金平 ), Lai-Peng Ma ( 马来鹏 ),
Zongping Chen ( 陈宗平 ), and Hui-Ming Cheng ( 成会明 ) a)
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P. R. China a) Electronic addresses: wcren@imr.ac.cn (W. C. Ren); cheng@imr.ac.cn (H. M. Cheng).
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Growth of graphene films on Cu foils: Graphene films were grown on 25  m thick Cu foils (Alfa
Aesar, item No. 13382) at 1000  C in an electric tube furnace (Lindberg Blue M, TF55030C) with a 22mm inner diameter fused silica tube under ambient pressure. Before growth, we used acetone and ethanol to clean the Cu foils, and then cut them into small strips with desired sizes. A typical growth procedure (Fig. S1) is: (1) load a piece of the Cu foil into the fused silica tube, and then heat the tube to
1000  C with the protection of 400 sccm of H
2
; (2) anneal the Cu foil at 1000  C for 20 min in the atmosphere of H
2
to reduce the surface of Cu foil and make its crystal grains bigger; (3) introduce Ar into the system while keeping a total flow rate of 300 sccm, and heat the Cu foil under such atmosphere for another 20 min; (4) initiate the reaction by introducing 1 sccm of CH
4
at 1000  C, and maintain the reaction for 10 min; (5) cool down the furnace to below 700  C and turn off the CH
4
.
Transfer of graphene films: Polydimethylsiloxane (PDMS) and scotch tape were used to transfer the graphene films from Cu foils. The surface of a graphene-on-Cu foil was first coated with PDMS or scotch tape. Then the sample was immersed into an aqueous solution of FeCl
3
at 40  C to etch away the
Cu foil. After that, the graphene-PDMS (graphene-tape) was lifted from the solution. When the PDMS or tape attached on graphene films were removed by organic solvents, the graphene films were unavoidably distorted, resulting in breaking or cracking of graphene films and consequently a great decrease in electrical conductivity. Therefore, in order to evaluate the real electrical property of graphene obtained, we directly measured the sheet resistance of graphene films on tape by a standard four-probe instrument (like on PMMA in Ref. 1&2). For optical, Raman, and transmittance measurements, the graphene-PDMS was further transferred to Si/280 nm thick SiO
2
and glass substrates.
In brief, it was first pressed on glass or Si/SiO
2
substrate for more than 1h. After the bubbles between
PDMS and substrate disappeared, the PDMS was peeled off from the substrate, with the graphene films left. This dry-transfer process turns out to be very useful in keeping the intrinsic properties of graphene films without organic contamination. However, similar to the wet-transfer by organic solvents, the
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transferred graphene films usually have some cracks. Optimization of the transfer process with substrate control can decrease the concentration of cracks in the graphene films transferred. Considering the large domain size of transferred graphene films (usually larger than 100  m in lateral size for an intact domain) and very small cracks (usually narrower than 0.5  m and shorter than 10  m), we believe that these cracks would not significantly influence the optical, Raman and transmittance measurements (similar to the process in Ref. 3).
Characterization of graphene films: The color images of graphene samples and substrate were taken using a Nikon LV100D optical microscope. The Raman spectra were measured and collected using
632.8 nm laser under ambient conditions with JY HR800, and a laser power below 0.1 mW was used to avoid laser-induced heating. The transmittance of graphene films on glass were measured by using ultraviolet-visible-near infrared spectrophotometer (Varian, Cary 5000). All the transmittance values were calibrated by clean glass. The structure evolution of graphene films with growth time were observed by scanning electron microscope (SEM, FEI Nova 430, operated at 15 keV).
Fig. S1. Time dependence of experimental parameters: temperature and flow rate.
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Fig. S2. SEM image of the graphene films prepared for 30 min at both H
2
and Ar flow rate of 150 sccm.
Note that there is no distinguishable difference from the surface with a growth period of 10 min (Fig. 4d in the main text).
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