The role of center-N-doping in non-radiative recombination loss of nitrogen-doped graphene quantum dots

Abstract

The C–N bonding environment reduces the integrity of the graphene lattice, induces defect states into nitrogen-doped graphene quantum dots (NGQDs), and significantly impacts the photoluminescence quantum yield of NGQDs. In the present work, we investigate non-radiative relaxation dynamics in pristine and N-doped GQDs based on Fermi's golden rule. Our results show that the N-doping perturbs the electron distribution of the frontier orbitals, which reduces the highest occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital (LUMO) gaps and results in a redshift of the optical spectra of NGQDs compared to pristine GQD. In particular, the graphitic and pyridinic N create the midgap states but exhibit different effects on non-radiative decay in NGQDs. For the graphitic NGQD, the negative induction effect of graphitic N reduces the π-electron densities, giving rise to exceptionally small electronic couplings, especially during the electron trapping step; consequently, it has a less efficient non-radiative decay than the pyridinic and pyrrolic NGQDs. In contrast, the large intramolecular and solvent reorganization energies associated with the electron trapping step in pyridinic NGQD create a very efficient electron–hole recombination channel through the hole trapping step, accelerating its non-radiative decay. Overall, this work advances our current understanding of the specific role of each N functionality in the non-radiative decay of NGQDs.