We employ scanning probe microscopy to reveal atomic structures and nanoscale morphology of graphene-based electronic devices (i.e., a graphene sheet supported by an insulating silicon dioxide substrate) for the first time. Atomic resolution scanning tunneling microscopy images reveal the presence of a strong spatially dependent perturbation, which breaks the hexagonal lattice symmetry of the graphitic lattice. Structural corrugations of the graphene sheet partially conform to the underlying silicon oxide substrate. These effects are obscured or modified on graphene devices processed with normal lithographic methods, as they are covered with a layer of photoresist residue. We enable our experiments by a novel cleaning process to produce atomically clean graphene sheets.

These authors contributed equally to this work. Since the experimental realization of graphene 1 , extensive theoretical work has focused on short-range disorder 2-5 , "ripples" 6, 7 , or charged impurities 2, 3, 8-13 to explain the conductivity as a function of carrier density σ(n)[1,14-18], and its minimum value σ min near twice the conductance quantum 4e 2 /h[14, 15, 19, 20]. Here we vary the density of charged impurities n imp on clean graphene 21 by deposition of potassium in ultra high vacuum. At non-zero carrier density, charged impurity scattering produces the ubiquitously observed 1, 14-18 linear σ(n) with the theoretically-predicted magnitude. The predicted asymmetry 11 for attractive vs. repulsive scattering of Dirac fermions is observed. σ min occurs not at the carrier density which neutralizes n imp , but rather the carrier density at which the average impurity potential is zero 10 . σ min decreases initially with n imp , reaching a minimum near 4e 2 /h at non-zero n imp , indicating that σ min in present experimental samples does not probe Dirac-point physics 14, 15, 19, 20 but rather carrier density inhomogeneity due to the impurity potential 3, 9, 10 .

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