Gate-tunable Graphene Flakes Probed by Scanning Tunneling Spectroscopy at Atomic Scale

The nanometer scale electronic properties of mechanically cleaved graphene flake devices having tunable back-gates are resolved using scanning tunneling microscopy and spectroscopy. We observe an energy gap feature in the graphene tunneling spectrum that is unexpectedly pinned to the Fermi level ($E_{F}$ ) for different gate-induced electron densities. The Dirac point, on the other hand, is shifted by the back-gate by an amount prescribed by the graphene linear band structure. This energy gap is found to arise from a suppression of elastic electronic tunneling to graphene states near $E_{F}$ and a significant enhancement of tunneling (seen as a more than factor of 10 increase in the conductance) at higher energies due to a phonon-mediated inelastic channel. This work reveals important new tunneling processes in gate-tunable graphitic layers.