Spatial Mapping of the Dirac Point in Monolayer and Bilayer Graphene

We have mapped the Dirac point in exfoliated monolayer and bilayer graphene using spatially resolved scanning tunneling spectroscopy measurements at low temperature. The Dirac-point shifts in energy at different locations in graphene. However, a cross correlation with the topography shows no correlation indicating that topographic features, such as ripples are not the primary source of the variation. Rather, we attribute the shift of the Dirac point to random charged impurities located near the graphene. Our findings emphasize the need to advance exfoliated graphene sample preparation to minimize the effect of impurities.