Tailoring the Electronic Band Gap of Graphyne Jahyun Koo, † Bing Huang, ‡ Hosik Lee, § Gunn Kim, ∥ Jaewook Nam, ⊥ Yongkyung Kwon, † and Hoonkyung Lee* ,† † School of Physics, Konkuk University, Seoul 143-701, Korea ‡ National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States § School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea ∥ Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Korea ⊥ School of Chemical Engineering, Sungkyunkwan University, Suwon 300, Korea ABSTRACT: We report a first-principles study on tuning the electronic band gap of graphyne, consisting of two-dimen- sional sp−sp 2 hybrid carbon atoms, by chemical functionaliza- tion. Halogen atoms form a sp 2 hybridization with sp-bonded carbon atoms. This is in sharp contrast to the adsorption of halogen atoms onto graphene: fluorine atoms on graphene form sp 3 bonds, while chlorine, bromine, and iodine atoms do not form any bond to graphene. The band gaps of graphyne increase by ∼3 eV as the halogen concentration varies, comparable to the ∼3.4 and ∼2.7 eV engineered band gaps of graphene by hydrogenation and fluorination, respectively. We also find that the mixture adsorption of hydrogen and halogen atoms is favorable compared with the segregation of the hydrogen-attached phase and the halogen-attached one and that the band gaps are tunable by ∼1.5 eV as the hydrogen−halogen concentration varies. We also consider sp 3 hybrid bonds by halogenation to sp-bonded carbon atoms. ■ INTRODUCTION Graphene, a two-dimensional (2D) atomic layer of sp 2 -bonded carbon atoms, has received a great deal of attention because of its unique electronic properties, such as the Dirac cone structure and high electron mobility. 1−3 Because of its metallicity, various methods have been devised for opening the band gap for applications in 2D device materials. For example, hydrogenation and fluorination of graphene were theoretically proposed to modify the band gap up to ∼3.4 and ∼2.7 eV, respectively. 4−6 Recent experimental studies con- firmed that the band gap of graphene can be opened by hydrogenation 7 or fluorination. 8 However, H or F atoms on graphene tend to aggregate with each other, thereby making it difficult to achieve band gap tuning (or engineering) of graphene by segregation. 9−13 Graphyne, a carbon allotrope of layered carbon structures consisting of sp−sp 2 hybrid carbon atoms, 14 was predicted to have intriguing electronic properties, such as the coexistence of symmetric and asymmetric Dirac cones. 15−17 The asymmetric Dirac cone can allow electrons to flow in a preferred direction. It was demonstrated theoretically that the energy band gap of graphyne can be opened by an AB sublattice symmetry breaking of the honeycomb lattice. 18 The γ-graphyne is known to have intrinsic band gaps by Kekule distortion. 18 This presents the possibility of applications for new 2D device materials. On the other hand, its porous structure and large surface area may allow for a variety of potential applications in energy storage, such as hydrogen storage and in lithium-ion batteries. 19−23 There have been experimental efforts to synthesize graphyne flakes 24−26 and graphdiyne films 27 and flakes, 28 providing evidence that the synthesis of graphyne and graphdiyne is possible. In a previous paper, 29 we studied the attractive geometrical and electrical properties of hydrogenated graphyne and found that no clustering of hydrogen atoms on graphyne took place. This implies that adjustment of the graphyne band gap can be achieved by adsorption of other elements without segregation to disturb the engineering, as in the case of graphene. In this paper, we propose that the electronic properties of graphyne are also adjustable by halogenation, which may be easier to handle. For example, XeF 2 is known to be usable for the fluorination of graphene and to adjust its electronic properties. 8 Herein, we investigate the band gap tunability of graphyne by halogenation. The halogen atoms preferentially adsorb on the sp-bonded carbon atoms to form sp 2 bonds, which is in sharp contrast to the adsorption on graphene, where only fluorine atoms attached to graphene form sp 3 hybridized bonds, while chlorine, bromine, and iodine atoms on graphene do not form any hybrid bonds. 30 We also investigate the band gaps of the halogenated graphyne as a function of the concentration of Received: September 1, 2013 Revised: January 1, 2014 Published: January 3, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 2463 dx.doi.org/10.1021/jp4087464 | J. Phys. Chem. C 2014, 118, 2463−2468