  Perturbations of the two dimensional carbon lattice of graphene, such as
grain boundaries, have significant influence on the charge transport and
mechanical properties of this material. Scanning tunneling microscopy
measurements presented here show that localized states near the Dirac point
dominate the local density of states of grain boundaries in graphene grown by
chemical vapor deposition. Such low energy states are not reproduced by
theoretical models which treat the grain boundaries as periodic
dislocation-cores composed of pentagonal-heptagonal carbon rings. Using ab
initio calculations, we have extended this model to include disorder, by
introducing vacancies into a grain boundary consisting of periodic
dislocation-cores. Within the framework of this model we were able to reproduce
the measured density of states features. We present evidence that grain
boundaries in graphene grown on copper incorporate a significant amount of
disorder in the form of two-coordinated carbon atoms.
