Quantum capacitance and density of states of graphene

We report capacitance measurements in top-gated graphene sheets as a function of charge carrier density. A measurement method using an LC-circuit provides high sensitivity to small capacitance changes and hence allows the observation of the quantum part of the capacitance. The extracted density of states has a finite value of 1× 1017m−2 eV−1 in the vicinity of the Dirac point, which is in contrast to the theoretical prediction for ideal graphene. We attribute this discrepancy to fluctuations of the electrostatic potential with a typical amplitude of 100meV in our device. PACS numbers: 81.05.ue, 72.80.Vp, 84.37.+q (Some figures may appear in colour only in the online journal)