Electronic Structure and Optical Absorption of Fluorographene
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Electronic Structure and Optical Absorption of Fluorographene Yufeng Liang and Li Yang Department of Physics, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA ABSTRACT A first-principles study on the quasiparticles energy and optical absorption spectrum of fluorographene is presented by employing the GW + Bethe-Salpeter Equation (BSE) method with many-electron effects included. The calculated band gap is increased from 3.0 eV to 7.3 eV by the GW approximation. Moreover, the optical absorption spectrum of fluorographene is dominated by enhanced excitonic effects. The prominent absorption peak is dictated by bright resonant excitons around 9.0 eV that exhibit a strong charge transfer character, shedding light on the exciton condensation and relevant optoelectronic applications. At the same time, the lowest-lying exciton at 3.8 eV with a binding energy of 3.5 eV is identified, which gives rise to explanation of the recent ultraviolet photoluminescence experiment. INTRODUCTION Graphene [1] has attracted intensive attention recently because of its extraordinary physical properties and broad applications. The linear dispersion relation near the Dirac point results in the relativistic nature of electrons [2] and leads to an ultrahigh mobility of charge carriers, rendering graphene a promising candidate for future high speed nano-electronic devices [3]. However, one known obstacle of putting graphene into realistic applications is the absence of a finite band gap that is essential for semiconductor devices, such as building bipolar junction structures. As a result, many efforts have been performed to generate a finite band gap in graphene or its derivatives. Among them, the chemical modification [4-7] has been regarded as a promising candidate because of its low cost and capability for large-scale productions. Through this chemical approach, the adsorbed atoms or chemical groups on graphene surface could strongly affect the band structure of ʌ-electrons. For example, the attached atoms may form covalence bonds with graphene and transform the sp2-hybridization of graphene into a sp3-hybridization. Consequently the relevant ʌ bands crossing at the Dirac point are perturbed, generating a finite band gap. To date two typical chemically functionalized graphene derivatives have been realized in experiments, i.e., hydrogenated [5] and fluorinated graphene [7]. They exhibit a pronounced band gap under appropriate conditions [5-9]. More importantly, their band gap can be tuned in a wide range (a few eV) [6, 7] by controlling the density of adsorbed atoms. This provides us a precious degree of freedom to tailor the electronic structure of graphene for specific applications [7, 9, 10]. In this work, we are especially interested in fluorinated graphene because of the extremely low
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electron affinity of fluorine, which is apt to make the fluorinated graphene an n-doped semiconductor and hence a better prototype material for electronic devices. Although there are a number of studies on the electronic
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