Density functional theory study on optical and electronic properties of co-doped graphene quantum dots based on different nitrogen doping patterns

Heteroatom doping, especially co-doping, is an effective way to tailor electronic structures of graphene quantum dots (GQDs) with synergistic effects and desirable properties. However, due to different synthesis methods, the widespread use of GQDs co-doped with heteroatoms is hindered by the poor understanding of their optical properties and mechanisms. In this work, co-doped GQDs based on three N-doping configurations are chosen to reveal underlying mechanisms of optical properties using density functional theory and time-dependent density functional theory calculations. Based on different N-doping patterns, B, P and S atoms can endow GQDs with a wide spectrum of new optical properties and electronic structures. The HOMO-LUMO gaps of N-doped GQDs with graphitic N, pyrrolic N, and pyridinic N are 0.77, 0.25 and 2.69 eV, respectively. In the co-doped GQDs, B, P and S containing functional groups cause low absorptions in the range of 400 to 800 nm and multiple absorption peaks at about 400 and 600 nm, while the N atom affects the position and intensity of prominent absorption peak according to three different N-doping patterns. The B atom forms sp(2) hybridization in the graphene lattice, while the P and S atoms transform the sp(2) hybridized carbon into the sp(3) state. It is anticipated that this work will provide valuable insights for understanding absorption mechanisms and electronic properties of heteroatom co-doped GQDs as well as achieving new applications with well-defined and desired properties.