  Irradiation of graphene on SiO2 by 500 eV Ne and He ions creates defects that
cause intervalley scattering as evident from a significant Raman D band
intensity. The defect scattering gives a conductivity proportional to charge
carrier density, with mobility decreasing as the inverse of the ion dose. The
mobility decrease is four times larger than for a similar concentration of
singly charged impurities. The minimum conductivity decreases proportional to
the mobility to values lower than 4e^2/(pi*h), the minimum theoretical value
for graphene free of intervalley scattering. Defected graphene shows a
diverging resistivity at low temperature, indicating insulating behavior. The
results are best explained by ion-induced formation of lattice defects that
result in mid-gap states.
