Impact of Graphene Quantum Dot Edge Morphologies on Their Optical Properties

The optoelectronic properties of functionalized graphene quantum dots (GQDs) have been explored by simulating electronic structure of three different shapes of GQDs containing exclusively zigzag or armchair edges in both pristine and functionalized forms. Absorption spectra and transition densities for the low-lying excited states are evaluated by using time-dependent density functional theory and compared for different functionalization species. The functionalization position dictates the optical properties of square GQDs, where isomers with CH2 in the intermediate positions (excluding corner and center positions) have higher electronic transition energies and exciton delocalization than other isomers. Rhombic GQDs with all armchair edges exhibit high steric flexibility, and their complete passivation results in the largest structural deformation from planarity and strongest red-shifts. A steady red shift in the absorption energy is observed following the order F, CH3, Cl, and Br substitutions. This suggests that the steric effects due to large van der Waals radii overcome electronegative effects.

Automated summary

The optoelectronic properties of functionalized graphene quantum dots were studied by simulating different shapes and types of functionalization. The results showed that the functionalization position and edge shape played important roles in the optical properties. The isomers with CH2 in the intermediate positions showed higher electronic transition energies and exciton delocalization. Complete passivation of rhombic GQDs resulted in the largest structural deformation and strongest red-shift. The substituents F, CH3, Cl, and Br caused a steady red-shift in the absorption energy, suggesting the dominance of steric effects.