Y. Duan, B. Chorpening, C.D. Stinespring
DOE National Energy Technology Laboratory,
Keywords: F-doped graphene, band-gap opening, optical properties
Summary:Graphene-based materials possess different electronic and optical properties for wide sensor applications. To better understand the effects of low-level fluorine in graphene-based sensors, the density functional theory (DFT) with van der Waals dispersion interactions is applied to investigate the structure and impact of low-concentration of fluorine (F) defects on the electronic and optical properties of single and multiple layers of graphene. The obtained results show that both pure graphite-2H and graphene have zero band gaps. When fluorine bonds to a carbon atom, the carbon atom is pulled slightly above the graphene plane creating what is referred to as a C_F defect. The lowest binding energy state is found to correspond to two C_F defects on nearest neighbor sites with one fluorine above the carbon plane and the other below the plane. Overall, this has the effect of buckling the graphene. The results further show that the addition of fluorine to graphene leads to the formation of a valence band (VB_f) near the Fermi level contributed mainly from the 2p orbitals of fluorine with a small contribution from the p orbitals of the nearest carbon atoms. Depending on the fluorine binding sites, VB_f can serve as a VB or a conduction band (CB). Among the ten binding configurations studied, our results show that only two configurations, in which these two C_F of graphene are separated farthest, the VBF serves as a CB and opens a 0.24 eV band gap. Such results indicate that the band gap opening for graphene with low F-adsorption level strongly depends on the F-binding configurations, which is different from the fully or highly partial fluorinated graphene. The fluorine binding energy decreases with decreasing the fluorine concentration due to the interaction between neighboring F atoms. When the concentration is lower than 20%, the fluorine binding energy does not significantly change. Our DFT results also show that the C_F defect can alter the optical properties of graphene significantly.