▎ 摘　　要

Density functional theory (DFT) computations were performed to study the change in electronic and optical properties of free-standing graphene monolayer by the adsorption of PTCDI molecule. It is revealed that the K point symmetry of the graphene is broken due to this molecular adsorption. As a consequence, the semi-metallic graphene monolayer turns into a conventional semiconductor at room temperature with conduction and valence bands separated by a gap of similar to 0.21 eV. The LUMO state of PTCDI molecule acts as a localized state in the electronic structure of the graphene/PTCDI system and induces a bandgap in the semi-metallic band structure of graphene. From our calculations, it is also explored that the localized gap state induced conduction gap is created due to the charge transfer mediated molecular sites specific van der waals (vdW) interaction across the graphene/PTCDI interface. The most interesting findings in this system are that the electrons follow the rules of the ordinary two dimensional (2D) materials to participate in the charge conduction process. The effective masses for holes as well as electrons are found to marginally change due to the PTCDI adsorption. Furthermore, the study of their optical properties for both directions of polarization exposes that the optical response of the semiconducting graphene/PTCDI system is highly anisotropic in nature. It is exposed that the sample has birefringence characteristics also. Moreover, the adsorption of a PTCDI molecule leads to notable modifications in the dielectric constant, reflectivity, absorption coefficients, and electron energy loss spectra of pristine graphene. The findings in this study may pave the way towards the development of new graphene-based molecular electronic and optoelectronic devices.