Evolution of Electrical, Chemical, and Structural Properties of Transparent and Conducting Chemically Derived Graphene Thin Films By Cecilia Mattevi,* Goki Eda, Stefano Agnoli, Steve Miller, K. Andre Mkhoyan, Ozgur Celik, Daniel Mastrogiovanni, Gaetano Granozzi, Eric Garfunkel, and Manish Chhowalla* 1. Introduction Graphene is a one-atom-thick layer of carbon with remarkable electronic properties that have focused the attention of scientists and engineers. In particular, graphene is a semiconductor with zero band gap and high carrier mobilities and concentrations, and shows nearly ballistic transport at room temperature. [1,2] A challenging aspect of graphene integration into electronic devices is the exfoliation of graphite into individual sheets in a controlled, scalable, and reproducible way. The reliable techni- que to produce single layer graphene of high quality is the micromechanical clea- vage. [3] However, this route is not practical for large-scale integration of graphene. Exfoliation of individual to few layered graphene sheets via agitation in a carefully chosen solvent and maintaining a stable suspension has been recently investi- gated. [4] However, the reported yield of single-layered graphene was found to be disappointingly small (<1 wt %). Growth of graphene on specific substrates (SiC and on transition metals) has also been demon- strated. [5–7] It is unclear if uniform deposition over large areas can be obtained using these methods and whether the grown graphene can be easily transferred onto more desirable substrates. Recent progress on transfer printing of CVD grown graphene appears to be promising for large area electronics. [8] The exfoliation of graphite oxide is efficient and results in high yields of single-layered graphene oxide (GO). [9] The individual graphene oxide sheets can then be readily deposited on virtually any substrate over large areas using solution based methods [10] and transfer printing. [11] GO forms over a range of O:C stoichiometries, with the oxygen bound to the carbon in the basal plane in the form of hydroxyl and epoxy functional groups (in variable ratios depending on the synthesis protocol), [12,13] and as carbonyl and carboxyl groups at the sheet edges. These functional groups make graphene oxide sheets strongly hydrophilic and decrease the interaction energy between the graphene layers (the interlayer distance increases from 0.35 nm in graphite to 0.7 nm in oxidized graphite). [9,14] Hence, graphite oxide can be readily exfoliated, forming a stable aqueous dispersion. A complete model to describe the exact ratio and spatial distribution of the functional groups that decorate the honeycomb carbon lattice has yet to be elucidated. [15–17] Graphene oxide is electrically insulating and must be reduced (using chemical and/or thermal treatment) to make it electrically active. [18] Although other methods have been reported, [19] the most FULL PAPER www.afm-journal.de [*] Dr. C. Mattevi, Prof. M. Chhowalla, G. Eda, and Dr. S. Miller Materials Science and Engineering Rutgers University, 607 Taylor Road Piscataway, New Jersey 08854 (USA) E-mail: cmattevi@rci.rutgers.edu; manish1@rci.rutgers.edu Dr. S. Agnoli, Prof. G. Granozzi Department of Chemical Science University of Padova, Via Marzolo 1, I-35131 Padova (Italy) Prof. K. A. Mkhoyan Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, Minnesota 55455 (USA) O. Celik, D. Mastrogiovanni, and Prof. E. Garfunkel Department of Chemistry and Chemical Biology Rutgers University, 607 Taylor Road Piscataway, New Jersey 08854 (USA) DOI: 10.1002/adfm.200900166 A detailed description of the electronic properties, chemical state, and structure of uniform single and few-layered graphene oxide (GO) thin films at different stages of reduction is reported. The residual oxygen content and structure of GO are monitored and these chemical and structural characteristics are correlated to electronic properties of the thin films at various stages of reduction. It is found that the electrical characteristics of reduced GO do not approach those of intrinsic graphene obtained by mechanical cleaving because the material remains significantly oxidized. The residual oxygen forms sp 3 bonds with carbon atoms in the basal plane such that the carbon sp 2 bonding fraction in fully reduced GO is 0.80. The minority sp 3 bonds disrupt the transport of carriers delocalized in the sp 2 network, limiting the mobility, and conductivity of reduced GO thin films. Extrapolation of electrical conductivity data as a function of oxygen content reveals that complete removal of oxygen should lead to properties that are comparable to graphene. Adv. Funct. Mater. 2009, 19, 2577–2583 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2577