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498 CARBON 47 (2009) 493–499 graphene obtained has an electronic transport property suit- able for the application as conductive materials and elec- tronic devices. The electrical conductivity of individual single-layer graph- ene derived from AG was determined inside a HRTEM and the result is shown in Fig. 7. It is interesting to find that, similar to those prepared by micro-mechanical cleavage [2], our graph- ene offers a linear current-voltage (I–V) characteristic of typi- cal metallic material in the voltage range of 90 mV–90 mV with a conductivity of 1·103 S/cm (Fig. 7b), which is 3 orders of magnitude higher than that of individual GO reduced by hydrazine [22], and higher than that of graphene films ob- tained by hydrazine reduction or high temperature graphitiza- tion treatment [10,26–28]. Combining this with the above XPS results, we believe that this high conductivity is attributed to the good deoxygenation of GO to restore C–C and C@C bonds in the whole graphene layer after the reduction process [22]. 4. Conclusion A simple and effective strategy to tune the number of graph- ene layers by selecting suitable starting graphite is presented. Both the lateral size and crystallinity of the original graphite are found to play important roles in the number of graphene layers obtained by chemical exfoliation. The smaller the lat- eral size and the lower the crystallinity of the starting graph- ite, the fewer the number of layers in the graphene obtained. Graphite with a small lateral size and low crystallinity is appropriate to prepare single-layer graphene. Moreover, these graphenes exhibit high-quality with an electrical conductivity of 1 · 103 S/cm. These findings should enable further inves- tigation of the physical and chemical properties of graphene with different number of layers, and open up a wide spectrum of possibilities for technological applications including nano- electronics, sensors, composites, energy storage and absorp- tion media, etc. Acknowledgements We acknowledge support by National Science Foundation of China (No.50872136 and No. 90606008), MOST of China (No. 2006CB932701), Chinese Academy of Sciences (No. KJCX2- YW-M01), and the Knowledge Innovation Program of CAS. We thank Prof. P. Thrower for kind suggestions and correc- tions on our paper, Dr. F. Li and Dr. O. Lourie for valuable dis- cussion, and Mr. D. M. Tang for electric conductivity measurements. REFERENCES [1] Geim AK, Novoselov KS. The rise of graphene. Nature Mater 2007;6(3):183–91. [2] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306(5296):666–9. [3] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, et al. Two-dimensional atomic crystals. Proc Natl Acad Sci USA 2005;102(30):10451–3. [4] Zhang YB, Tan YW, Stormer HL, Kim P. Experimental observation of the quantum Hall effect and Berry’s phase in graphene. Nature 2005;438(7065):201–4. [5] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI, Grigorieva IV, et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005;438(7065):197–200. [6] Gilje S, Han S, Wang M, Wang KL, Kaner RB. A chemical route to graphene for device applications. Nano Lett 2007;7(11):3394–8. [7] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442(7100):282–6. [8] Schedin F, Geim AK, Morozov SV, Hill EW, Blake P, Katsnelson MI, et al. Detection of individual gas molecules adsorbed on graphene. Nature Mater 2007;6(9):652–5. [9] Liang X, Fu Z, Chou SY. 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