The ability to understand and control the electronic properties of individual molecules in a
device environment is crucial for developing future technologies at the nanometer scale and
below. Achieving this, however, requires the creation of three-terminal devices that allow
single molecules to be both gated and imaged at the atomic scale, a difficult challenge. We
have accomplished this by integrating a field effect transistor (FET) with a scanning
tunneling microscope (STM), thus enabling isolated molecules on a graphene surface to be
electrostatically gated and spectroscopically interrogated. Using this technique we
demonstrate gate-controlled switching of the charge state of individual tetrafluoro-
tetracyanoquinodimethane (F4TCNQ) molecules at the surface of a graphene FET. We
observe a non-rigid shift in the F4TCNQ lowest unoccupied molecular orbital (LUMO)
energy relative to the Dirac point as a function of gate voltage. This can be explained by
gate-tunable graphene polarization effects that renormalize the molecular quasiparticle
energies. Our results show that electron-electron interactions play an important role in
how molecular energy levels align to the graphene Dirac point, and may significantly
influence charge transport through individual molecules incorporated in graphene-based
nanodevices.