Effects of tip induced carrier density in local tunnel spectra of graphene

We report on tip induced carrier density changes in local tunnel spectra of single layer graphene (SLG) with backgate using room-temperature scanning tunneling microscopy (STM) and spectroscopy. The SLG samples, prepared by exfoliation method and verified by Raman spectra, show atomically resolved honeycomb lattice. Local tunnel spectra show two minima with the two moving in opposite directions along the bias axis. One minimum shows nearly a square-root dependence, and the other shows a linear dependence on gate voltage. We understand these features as arising from the STM tip induced and bias voltage dependent change in carrier density in SLG.