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Scanning Tunneling Microscopy and Spectroscopy study of charge inhomogeneities in bilayer Graphene

We report room temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on SiO$_2$/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contribution from regions of varying electron density indicating significant charge inhomogeneity. Large scale topographic images show ripple like structures with a roughness of $\sim$1nm while the small scale atomic resolution images show graphite-like triangular lattice. Local dI/dV-V tunnel spectra have an asymmetric V-shape with the minima location showing significant spatial variation indicating inhomogeneity in electron density of order 10$^{11}$ cm-2. The minima in spectra at a fixed location also shifts linearly with the gate voltage with a slope consistent with the field induced carrier density.