  Electric field screening plays an important role in the physical and chemical
properties of materials and their devices. Here, we use a compelling set of
theoretical and experimental techniques involving van der Waals (vdW) ab initio
density functional theory (DFT) simulations, quantum capacitance-based
classical model and electric force microscopy (EFM) to elucidate the intrinsic
dielectric screening properties of vdW heterostructures (vdWHs) formed by MoS2
and graphene layers. We experimentally observed an asymmetric electric response
in the MoS2/Graphene vdWHs under different directions of the external electric
field. That is, when the electric fields are shed towards graphene, a large
amount of polarized charges screen the fields, but as the sign of the field was
reversed, a strong depolarization field was present, and a partial screening
was detected. This effect is thickness-dependent, in particular on the number
of the MoS2 layers; whereas increased thickness of graphene showed a small
effect on their electrical and screening behavior. Our results indicate that
asymmetric dipolar contributions at the interface between graphene and MoS2 are
the main cause to the unusual field-effect screening in the vdWHs. This work
not only provides new insights on the screening properties of a vast amount of
heterojunction fabricated so far, but also uncovers the great potential of
controlling a fundamental property, such as screening, for device applications.
