Unveiling the true band gap of fluorographene and its origins by teaming theory and experiment

Abstract

Fluorographene is a fully fluorinated derivative of graphene. Unlike graphene, fluorographene is a wide gap semiconductor/insulator that holds great potential for applications requiring two-dimensional dielectric nano materials. Despite growing interest and a well-defined structure, the basic questions of fluorographene's band gap nature and value remain a conundrum. Here, we resolve this long-standing issue, demonstrating a direct optical band gap at 5.75 eV by means of diffuse reflectance spectroscopy. The nature of the band gap and the factors contributing to earlier controversies are explained by combining spectroscopic methods, ab initio calculations based on the finite momentum Bethe-Salpeter equation, and structural characterization via x-ray diffraction and Raman scattering. Ab initio calculations complement the experimental results by showing an excitonic peak at 5.65 eV of a Frenkel exciton bound to a single atom. The calculations also reveal that the absorption bands at lower energies arise from the presence of fluorine vacancies in the material, which explains earlier controversies in the literature about the band gap of fluorographene.