  The electric properties of mono- and multi-layer graphene films were
systematically studied with the layer number determined by their optical
contrast. The current modulation increased monotonically with a decrease in the
layer number due to the reduction of the interlayer scattering. Carrier
mobility in the monolayer was significantly greater than that in the multilayer
due to linear dispersion relation. On the other hand, in the monolayer, carrier
transport was extremely sensitive to charged impurity density due to the
reduction in screening effect, which causes larger mobility variation.
Reduction of the charged impurity density is thus key for high mobility.
