Ionic screening of charged-impurity scattering in graphene.

We have studied the ionic screening effect on the charge transport properties of graphene field effect transistors. By increasing the ionic concentration, we found dramatic increases in the carrier mobilities and systematic changes in the position and magnitude of minimum conductivity, as well as the width of minimum conductivity plateau, which supports the theory of long-range Coulomb scattering. We also observed clear conductivity saturation and systematic crossover from the linear to constant conductivity regimes.