| I present studies of excitons in graphene quantum dots (GQDs), a class of electronically quasi-zero-dimensional materials with a two-dimensional sp2-hybridized carbon lattice. The weak screening associated with such a two-dimensional lattice of light atoms results in strong carrier interactions. Semiconductor quantum dots and low-dimensional carbon based materials have been developed and studied for decades and gradually applied in areas such as photovoltaics. One of the original motivations for our collaborators' synthesis of these particular GQDs is their potential as sensitizers for solar cells, and research on the electronic structure and the exciton behavior of GQDs will help to reveal the potential of this candidate material.;This thesis describes experimental investigations of biexcitons in GQDs. We use transient absorption spectroscopy to determine the biexciton binding energy. We find a value of ∼ 140 meV for a certain type of biexciton, which is in rough agreement with the theoretically calculated value. Compared with semiconductor quantum dots, GQDs display stronger biexciton binding, which highlights the importance of excitonic effects in explaining the optical and electronic properties of these systems. While we observe clear signatures of biexcitons, these states are short-lived. We observe biexciton Auger recombination times of ∼ 0.3 ps, which is comparable to the time scale of biexciton Auger recombination in single-wall carbon nanotubes with circumference comparable to the longest edge length of the GQDs studied here. Slower relaxation (a few ps and tens of ps) of excitons is believed to be related to cooling of the lattice. The strong interaction between carriers and rapid biexciton Auger recombination suggest that GQDs could be used for carrier multiplication and thus increase the efficiency of GQD-based solar cells. |
