  We probe the local inhomogeneities in the electronic properties of exfoliated
graphene due to the presence of charged impurities in the SiO2 substrate using
a combined scanning tunneling and electrostatic force microscope. Contact
potential difference measurements using electrostatic force microscopy permit
us to obtain the average charge density but it does not provide enough
resolution to identify individual charges. We find that the tunneling current
decay constant, which is related to the local tunneling barrier height, enables
one to probe the electronic properties of graphene distorted at the nanometer
scale by individual charged impurities. We observe that such inhomogeneities do
not show long range ordering and their surface density obtained by direct
counting is consistent with the value obtained by macroscopic charge density
measurements. These microscopic perturbations of the carrier density
significantly alter the electronic properties of graphene, and their
characterization is essential for improving the performance of graphene based
devices.
