  We performed detailed studies of the current-voltage characteristics in
graphene/MoS2/metal vertical field-effect transistors. Owing to its low density
of states, the Fermi level in graphene is very sensitive to its carrier density
and thus the external electric field. Under the application of a bias voltage
VB between graphene and the metal layer in the graphene/MoS2/metal
heterostructure for driving current through the van der Waals interface, the
electric field across the MoS2 dielectric induces a shift in the Fermi level of
graphene. When the Fermi level of graphene coincides with the Dirac point, a
significant nonlinearity appears in the measured I-V curve, thus enabling us to
perform spectroscopy of the Dirac point. By detecting the Dirac point for
different back-gate voltages, we revealed that the capacitance of the
nanometer-thick MoS2 layer can be determined from a simple DC transport
measurement.
