  Understanding the interactions of ambient molecules with graphene and
adjacent dielectrics is of fundamental importance for a range of graphene-based
devices, particularly sensors, where such interactions could influence the
operation of the device. It is well-known that water can be trapped underneath
graphene and its host substrate, however, the electrical effect of water
beneath graphene and the dynamics of how it changes with different ambient
conditions has not been quantified. Here, using a metal-oxide-graphene
variable-capacitor (varactor) structure, we show that graphene can be used to
capacitively sense the intercalation of water between graphene and HfO2 and
that this process is reversible on a fast time scale. Atomic force microscopy
is used to confirm the intercalation and quantify the displacement of graphene
as a function of humidity. Density functional theory simulations are used to
quantify the displacement of graphene induced by intercalated water and also
explain the observed Dirac point shifts as being due to the combined effect of
water and oxygen on the carrier concentration in the graphene. Finally,
molecular dynamics simulations indicate that a likely mechanism for the
intercalation involves adsorption and lateral diffusion of water molecules
beneath the graphene.
