Mechanically exfoliated graphene layers deposited on ${\text{SiO}}_2$ substrate were irradiated with ${\text{Ar}}^{+}$ 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.

• Received 19 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.233407