  Transport measurements have revealed several exotic electronic properties of
graphene. The possibility to influence the electronic structure and hence
control the conductivity by adsorption or doping with adatoms is crucial in
view of electronics applications. Here, we show that in contrast to
expectation, the conductivity of graphene increases with increasing
concentration of vacancy defects, by more than one order of magnitude. We
obtain a pronounced enhancement of the conductivity after insertion of defects
by both quantum mechanical transport calculations as well as experimental
studies of carbon nano-sheets. Our finding is attributed to the defect induced
mid-gap states, which create a region exhibiting metallic behavior around the
vacancy defects. The modification of the conductivity of graphene by the
implementation of stable defects is crucial for the creation of electronic
junctions in graphene-based electronics devices.
