Evidence for Edge-State Photoluminescence in Graphene Quantum Dots

For a practical realization of graphene-based logic devices, the opening of a band gap in graphene is crucial and has proven challenging. To this end, several synthesis techniques, including unzipping of carbon nanotubes, chemical vapor deposition, and other bottom-up fabrication techniques have been pursued for the bulk production of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). However, only limited progress has been made towards a fundamental understanding of the origin of strong photoluminescence (PL) in GQDs. Here, it is experimentally shown that the PL is independent of the functionalization scheme of the GQDs. Following a series of annealing experiments designed to passivate the free edges, the PL in GQDs originates from edge-states, and an edge-passivation subsequent to synthesis quenches the PL. The results of PL studies of GNRs and carbon nano-onions are shown to be consistent with PL being generated at the edge sites of GQDs.