Scanning tunneling microscopy and spectroscopy study of charge inhomogeneities in bilayer graphene

We report a room-temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on a SiO2/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contributions from regions of varying electron density, indicating significant charge inhomogeneity. Large-scale topographic images show ripple-like structures with a roughness of ∼1 nm, while the small-scale atomic resolution images show graphite-like triangular lattices. The local ( d I / d V – V ) tunnel spectra have an asymmetric V-shape with the minima location showing significant spatial variation, indicating inhomogeneity in electron density of order 1011 cm−2. The minimum in spectrum at a fixed location also shifts linearly with the gate voltage with a slope consistent with the field-induced carrier density.