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CARBON 47 (2009) 493–499 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon Synthesis of high-quality graphene with a pre-determined number of layers Zhong-Shuai Wu, Wencai Ren*, Libo Gao, Bilu Liu, Chuanbin Jiang, Hui-Ming Cheng* Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China ARTICLEINFO Article history: Received 27 June 2008 Accepted 20 October 2008 Available online 5 November 2008 ABSTRACT A simple and effective strategy is proposed to tune the number of graphene layers by selecting suitable starting graphite, using a chemical exfoliation method. It is found that both the lateral size and the crystallinity of the starting graphite play important roles in the number of graphene layers obtained. Using artificial graphite, flake graphite powder, Kish graphite, and natural flake graphite as starting materials, 80% of the final products are single-layer, single- and double-layer, double- and triple-layer, and few-layer (4–10 lay- ers) graphene, respectively, while a mixture of few-layer (4–10 layers) and thick graphene (>10 layers) is obtained when highly-oriented pyrolytic graphite is used. The smaller the lateral size and the lower the crystallinity of the starting graphite, the fewer the number of graphene layers obtained. Moreover, the graphenes obtained are of high-quality with an electrical conductivity of 1 · 103 S/cm. These findings open up the possibility for con- trolled production of high-quality graphene with a selected number of layers in a large quantity. 1. Introduction Graphene shows great importance for fundamental studies and technological applications due to its unique structure and a wide range of unusual properties [1]. Its electron trans- port is described by the relativistic-like Dirac equation, and this allows access to the rich and subtle physics of quantum electrodynamics in relatively simple condensed matter exper- iments [2–5]. The scalability of graphene devices to true nano- meter dimensions makes it a promising candidate for future electronics because of its ballistic transport at room tempera- ture combined with chemical and mechanical stability [6]. In addition, graphene also exhibits great promise for potential applications in many other technological fields [1,7–12] such as sensors, composites, supercapacitors, transparent conduc- tive films, solar cell and gas storage media. However, the properties of graphene strongly depend on their structures. Ó 2008 Elsevier Ltd. All rights reserved. For example, a variation of the number of graphene layers may result in a striking change of their electronic properties [1]. Accordingly, it is very important to explore the production of graphene with a selected number of layers in large quanti- ties for their further fundamental studies and extensive applications. Currently, several methods have been proposed to prepare graphene, such as micro-mechanical cleavage [2], epitaxial growth on SiC wafer [13] and chemical exfoliation [14–20]. For the first two methods, both the productivity and layer selectivity of graphene are poor. A chemical exfoliation strat- egy from bulk graphite has been suggested to be an effective general way to produce graphene in a large quantity and at low cost [15,16], in which a process of graphite oxidation and/or thermal expansion of graphite oxide (GO)/expandable graphite is involved. During oxidation, the introduction of oxygen-containing functional groups such as hydroxyl and * Corresponding authors: Fax: +86 24 2390 3126. E-mail addresses: wcren@imr.ac.cn (W.C. Ren), cheng@imr.ac.cn (H.-M. Cheng). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.10.031PDF Image | graphene with a pre-determined number of layers
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