Defect Scattering in Graphene

Irradiation of graphene on ${\mathrm{SiO}}_{2}$ by 500 eV Ne and He ions creates defects that cause intervalley scattering as is 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 4 times larger than for a similar concentration of singly charged impurities. The minimum conductivity decreases proportional to the mobility to values lower than $4{e}^{2}/\ensuremath{\pi}h$, 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 midgap states.