  Mechanically exfoliated graphene layers deposited on SiO2 substrate were
irradiated with Ar+ ions in order to experimentally study the effect of atomic
scale defects and disorder on the low-energy electronic structure of graphene.
The irradiated samples were investigated by scanning tunneling microscopy and
spectroscopy measurements, which reveal that defect sites, besides acting as
scattering centers for electrons through local modification of the on-site
potential, also induce disorder in the hopping amplitudes. The most important
consequence of the induced disorder is the substantial reduction in the Fermi
velocity, revealed by bias-dependent imaging of electron-density oscillations
observed near defect sites.
