Mapping the Dirac point in gated bilayer graphene

We have performed low temperature scanning tunneling spectroscopy measurements on exfoliated bilayer graphene on SiO(2). By varying the back gate voltage we observed a linear shift of the Dirac point and an opening of a bandgap due to the perpendicular electric field. In addition to observing a shift in the Dirac point, we also measured its spatial dependence using spatially resolved scanning tunneling spectroscopy. The spatial variation of the Dirac point was not correlated with topographic features and therefore we attribute its shift to random charged impurities.