  We report variation of the work function for single and bi-layer graphene
devices measured by scanning Kelvin probe microscopy (SKPM). Using the electric
field effect, the work function of graphene can be adjusted as the gate voltage
tunes the Fermi level across the charge neutrality point. Upon biasing the
device, the surface potential map obtained by SKPM provides a reliable way to
measure the contact resistance of individual electrodes contacting graphene.
