Band gap tuning and excitonic effect in chloro‑fluorinated graphene

Abstract

We have studied the structural, electronic and optical properties of chloro‑fluorinated graphene (CF1-xClx) as a function of Cl concentration (x) by employing DFT and beyond DFT approaches. The increase in Cl concentration brings about an increase in lattice parameters while binding energy and electronic band gap are reduced. For all the properties studied, a non-linear behavior is observed as x is increased from 0.25 to 0.375. In particular, electronic band gap exhibits a sharp decline (>1.5 eV) for x = 0.25–0.375; while the reduction is relatively smooth for x ≤ 0.25 and x ≥ 0.375. Optical gap presents similar trend albeit with much smaller magnitude due to the presence of strong excitonic effect. We show that the observed non-linear behavior is associated with sharp reduction in the minimum distance between Cl adsorbates as x is increased from 0.25 to 0.375. As a consequence, new states caused by Cl-Cl interaction appear at an energy level higher than those caused by C-F and F-F, thus considerably reducing the band gap. Our result provides helpful insight for tailoring the band gap through hetero-functionalization of graphene by halides.