We theoretically investigate the effects of long-range disorder and
electron-electron interactions on the optical properties of hexagonal armchair
graphene quantum dots consisting of up to 10806 atoms. The numerical
calculations are performed using a combination of tight-binding, mean-field
Hubbard and configuration interaction methods. Imperfections in the graphene
quantum dots are modelled as a long-range random potential landscape, giving
rise to electron-hole puddles. We show that, when the electron-hole puddles are
present, tight-binding method gives a poor description of the low-energy
absorption spectra compared to meanfield and configuration interaction
calculation results. As the size of the graphene quantum dot is increased, the
universal optical conductivity limit can be observed in the absorption
spectrum. When disorder is present, calculated absorption spectrum approaches
the experimental results for isolated monolayer of graphene sheet
