High-Throughput Synthesis of Graphene by IntercalationExfoliation of Graphite Oxide and Study of Ionic Screening in Graphene Transistor Priscilla Kailian Ang, †,§ Shuai Wang, † Qiaoliang Bao, † John T. L. Thong, ‡ and Kian Ping Loh †, * † Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, ‡ Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, and § NUS Graduate School for Integrative Sciences and Engineering, Singapore 117597 G raphene-based nanoelectronic de- vices are of great interest because the active channel can be scaled lit- erally down to a single crystalline sheet of sp 2 -bonded carbon. Such devices can ex- hibit ultrahigh carrier mobility and long- range ballistic transport. 1,2 Currently, the “Scotch tape” peeling method used for pro- ducing graphene layers from graphite is not compatible with industrial production. The search is on for a high-yield production route toward high-purity, large-sized graphene sheets which can be deposited as a uniform film on substrate. Reported production methods are quite varied and range from the chemical and thermal reduc- tion of graphite oxide, 3-5 liquid-phase inter- calation and exfoliation of graphite, 6-10 and epitaxial growth 11,12 to chemical vapor deposition. 13 Of these, solution-processed graphene sheets offer low-cost and high- throughput for printable device fabrication on flexible substrates. The most commonly used Hummer’s method produces aqueous solution of graphene oxide sheets, which are used as precursors to generate mildly conducting graphene films. However, the harsh oxidation method results in small- sized insulating graphene oxide sheets with widths in the submicrometer range. 3 For ease of device fabrication, it is desirable to synthesize graphene sheets with a lateral size larger than 20 m. 5,14 Several research- ers have demonstrated exfoliation and in- tercalation of graphite to produce mono- layer graphene sheets. 6-10 However, these methods suffer from low yield (1%) of monolayer sheets and sometimes involve the use of hazardous exfoliating or reduc- ing agents such as oleum 6 and hydrazine. 4,5 Hence, a solution-phase method which can achieve high monolayer yield of large-sized conductive graphene sheets under mild conditions is highly desirable. In this report, we present an efficient and highly reproducible one-step intercala- tion and exfoliation method to produce large-sized, conductive graphene sheets without the use of surfactants. By remov- ing the ultrasonication step completely, we are able to obtain large-sized exfoliated graphene sheets (with lateral dimension 20 m) without sacrificing the high yield of monolayer sheets. The principle of the method is based on the rich intercalation chemistry of graphite oxide (GO). 15,16 Large amounts of GO sediments are formed after a brief oxidation of natural graphite by apply- ing the modified Hummer’s method (see Methods). These sediments consist of weakly oxidized graphite which cannot be dispersed well in aqueous solution due to their hydro- phobic nature and large size. Our hypothesis is that these large-sized GO aggregates con- sist of multilayer graphite flakes which are oxidized on the outer layers, while the inner *Address correspondence to chmlohkp@nus.edu.sg. Received for review August 28, 2009 and accepted September 23, 2009. Published online September 29, 2009. 10.1021/nn901111s CCC: $40.75 © 2009 American Chemical Society ABSTRACT We report a high-throughput method of generating graphene monolayer (>90% yield) from weakly oxidized, poorly dispersed graphite oxide (GO) aggregates. These large-sized GO aggregates consist of multilayer graphite flakes which are oxidized on the outer layers, while the inner layers consist of pristine or mildly oxidized graphene sheets. Intercalationexfoliation of these GO aggregates by tetrabutylammonium cations yields large-sized conductive graphene sheets (mean sheet area of 330  10 m 2 ) with high monolayer yield. Thin-film field-effect transistors made from these graphene sheets exhibit high mobility upon nullifying Coulomb scattering by ionic screening. Ionic screening versus chemical doping effects of different ions such as chloride and fluoride on these graphene films were investigated with a combination of in situ Raman spectroscopy and transport measurement. KEYWORDS: graphene · intercalation · exfoliation · ionic screening ARTICLE www.acsnano.org VOL. 3 ▪ NO. 11 ▪ 3587–3594 ▪ 2009 3587