Published: February 09, 2011 r2011 American Chemical Society 3313 dx.doi.org/10.1021/jp111637b | J. Phys. Chem. C 2011, 115, 3313–3317 ARTICLE pubs.acs.org/JPCC Electronic Properties of Cycloaddition-Functionalized Graphene Kelvin Suggs, Darkeyah Reuven, and Xiao-Qian Wang* Department of Physics and Center for Functional Nanoscale Materials, Clark Atlanta University, Atlanta, Georgia 30314, United States ABSTRACT: We have studied the electronic characteristics of covalently functionalized graphene by nitrene chemistry using first-principles density functional calculations. The perfluoro- phenylazide functionalization leads to a band-gap opening in graphene and transition from a semimetallic to a semiconduct- ing state. The [2 þ 1] cycloaddition-induced gap is shown to be attributed to the modification of the π conjugation that depends on the concentration of aziridine adducts. The implications of tailoring the band structure of functionalized graphene for fu- ture graphene-based device applications are discussed. ’ INTRODUCTION Graphene is a one-layer sheet of carbon arranged in a honey- comb lattice. Graphene has attracted a great deal of attention due to its remarkable properties and promising potential applications. 1-5 The effective application of graphene transistors, integrated circuits, and biosensors, requires an improved understanding and control of the structural and electronic properties of graphene. Because of the gapless character of the graphene band structure, the future of graphene electronics depends on developing routes to engineer a band gap. A gap can be formed in epitaxial graphene grown on a lattice- matched substrate. 6,7 Although the approach involving lattice- matched substrates is straightforward, combining it with electronic transport remains a challenging task. Another promising method for gap engineering relies on spatial confinement, such as patterning graphene into nanoribbons. 8,9 The gap obtained by such a method can be tuned by varying the spatial width of graphene ribbons. However, the approaches relying on spatial confinement are prone to rough edges and defects. Moreover, although graphene nanor- ibbon field-effect transistors have been shown to exhibit excellent properties, 8,10 mass production of graphene nanoribbon-based devices is beyond the capability of current lithography techno- logy. 6 Recently, there has also been a number of studies on gene- rating a band gap in the gapless bilayer graphene with a perpendi- cularly applied electric field. 11-14 In bilayer graphene, the Bernal stacking can be lifted by asymmetric chemical doping or electrical gating, 4 leading to a gap opening. On the other hand, a wealth of approaches has been developed for noncovalently and covalently functionalized graphene. 10,15-23 Graphene contains a paucity of functional moieties and limited dispersibility in solvents, seriously hindering the realization of its great potential. 16,21-23 As a result, developing chemical methods in order to tune the materials properties has become one of the most critical issues in exploring graphene technologies. Various chemical modification techniques have been shown to not only enhance its solubilities and processabilities but also render suitable properties for graphene-based nanoelectronic and nanophotonic devices. Modification of graphene's electronic properties has been carried out by well-established chemical functionalization tech- niques, in which groups, such as H, OH, or F, bind covalently to carbon atoms, transforming the trigonal sp 2 orbital to the tetra- gonal sp 3 state. 15,24-28 Such transformations drastically modify the local electronic properties. Recent experimental studies have demonstrated an efficient method to covalently functionalize pristine graphene with the use of nitrene chemistry, in which a perfluorophenylazide (PFPA) undergoes cycloaddition with C-C double bonds, forming an aziridine-ring linkage (see Figure 1). 23 A wide range of aryl azide derivatives are available and can be further functionalized with an array of polymeric functional groups. The aziridino-ring reaction can be carried out by thermal and photochemical activation, which results in graphene being soluble in organic solvents and water. The advancement of graphene-aryl-aziridine adduct nanocom- posites brings with it the need to understand their impact on the electrical properties of graphene. In lieu of the increasing amount of experimental and theoretical studies of chemically functiona- lized graphene, a better understanding of how covalent functio- nalization impacts the morphology and electron/hole transport in graphene becomes pivotal for its future application in nano- electronics. Experimental advances have motivated our study of electronic structure characteristics of PFPA-functionalized graphene. Here- in, we report on comprehensive results based on first-principles density functional calculations. PFPA-functionalized graphene perturbs the π conjugation of graphene, and the corresponding electronic properties change from metallic to semiconducting. We show that, with the increase of aziridine adducts, the resultant Received: December 7, 2010 Revised: January 11, 2011