  Graphene-derived nanomaterials are emerging as ideal candidates for
postsilicon electronics. Elucidating the electronic interaction between an
insulating substrate and few-layer graphene (FLG) films is crucial for device
applications. Here, we report electrostatic force microscopy (EFM) measurements
revealing that the FLG surface potential increases with film thickness,
approaching a "bulk" value for samples with five or more graphene layers. This
behavior is in sharp contrast with that expected for conventional conducting or
semiconducting films, and derives from unique aspects of charge screening by
graphene's relativistic low energy carriers. EFM measurements resolve
previously unseen electronic perturbations extended along crystallographic
directions of structurally disordered FLGs, likely resulting from long-range
atomic defects. These results have important implications for graphene
nanoelectronics and provide a powerful framework by which key properties can be
further investigated.
