DFT calculation in design of near-infrared absorbing nitrogen-doped graphene quantum dots

The near-infrared light (NIR) absorption of nitrogen-doped graphene quantum dots (NGQDs) containing different N-doping sites is systematically investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations with Perdew-Burke-Ernzerhof (PBE) functionals. The results show that the ultra-small HOMO-LUMO gaps (0.3-1.0 eV) of various N-doping structures (graphitic, amino, and pyridinic at center, and graphitic at edge) are attributed to the spin-polarization of the energy states, which effectively enhances the NIR absorption for NGQDs. Overall, the graphitic N-doping structure exhibits the best NIR absorption. Moreover, the electron attraction effect of the different N-sites is found to be crucial for the LUMO level, where stronger electron attraction lowers the LUMO energy. This work provides critical insight in further design of NGQDs for NIR absorption.