  Local electrical imaging using microwave impedance microscope is performed on
graphene in different modalities, yielding a rich hierarchy of the local
conductivity. The low-conductivity graphite oxide and its derivatives show
significant electronic inhomogeneity. For the conductive chemical graphene, the
residual defects lead to a systematic reduction of the microwave signals. In
contrast, the signals on pristine graphene agree well with a lumped-element
circuit model. The local impedance information can also be used to verify the
electrical contact between overlapped graphene pieces.
