  We have mapped the Dirac point in exfoliated monolayer and bilayer graphene
using spatially resolved scanning tunneling spectroscopy (STS) 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.
