Electronic and Optical Properties of Edge-Functionalized Graphene Quantum Dots and the Underlying Mechanism

We systematically investigate the electronic structure and optical properties of edge-functionalized graphene quantum dots (GQDs) utilizing density functional and many-particle perturbation theories. A mechanism based on the competition and collaboration between frontier orbital hybridization and charge transfer is proposed. The frontier orbital hybridization of the GQD moiety and functional group reduces the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while the charge transfer from the GQD moiety to the functional group enlarges it. Contrarily, frontier orbital hybridization and charge transfer collaborate to shift down the energy of the first bright exciton, the former through activation of low-lying dark excitons and the latter via increased exciton binding energy. Functional groups containing a carbon oxygen double bond (C=0), namely, aldehyde (-CHO), ketone (-COCH3), and carboxyl (-COOH), are more favorable for tailoring the electronic and optical properties of pristine GQD among all the functional groups investigated here. The amino group (-NH2), although frequently employed in experiments, has a much weaker influence on electronic structure since the large charge transfer cancels out the effect of frontier orbital hybridization.