Optical Properties Of Graphene Quantum Dots: The Role Of Chiral Symmetry

We analyse the electronic and optical properties of graphene quantum dots (GQD) using accurate ab initio many-body GW and Bethe-Salpeter calculations. We show that most pristine GQD, including structures with irregular shapes, are characterized by dark low energy singlet excitations that quench fluorescence. We rationalize this property by exploiting the chiral symmetry of the low energy electronic states in graphene. Edge sp(3) functionalization is shown to efficiently brighten these low lying excitations by distorting the sp(2) backbone planar symmetry. Such findings reveal an original indirect scenario for the influence of functionalization on the photoluminescence properties.