  We report on tip induced doping in local tunnel spectra of single layer
graphene (SLG) with tunable back-gate using room temperature scanning tunneling
microscopy and spectroscopy (STM/S). The SLG samples, prepared on silicon
dioxide surface by exfoliation method and verified by Raman spectra, show
atomically resolved honeycomb lattice. Local tunnel spectra show two minima
with a clear evolution in the position of both with doping by the back gate. A
similar variation in spectra is also observed spatially due to charge
inhomogeneity. With doping the two minima move in opposite directions with one
showing nearly a square root dependence and the other a linear dependence on
gate voltage. We explain these features as arising from the STM tip induced and
bias voltage dependent doping in SLG.
