  A charge-carrier reservoir is necessary for electrostatic control of the
carrier concentration in a solid. The source/drain electrodes serve as carrier
reservoirs in a field-effect transistor, but it is still unknown what serves as
a reservoir in a technique based on a polar self-assembled monolayer formed
underneath a solid to be controlled. Here, the carrier-doping level of isolated
single-layer graphene was found to be the same as that of the single-layer part
in a flake containing multilayer graphene, indicating that the multilayer part
is not a dominant carrier reservoir but adsorbates like oxygen and water serve
as a dominant reservoir.
