Unexpected screening of field-effect in graphene-$MoS_{2}$ van der Waals heterojunctions.

Van der Waals heterostructures play a key role in devices built out of two-dimensional materials, but their response to field effect can be rather non-trivial, due to their low density of states. Unfortunately, a direct investigation of these effects is often complex, since the layers forming the heterojunction typically cannot be probed independently. In my contribution, I will illustrate back-gated $MoS_{2}$ field-effect transistor (FET) architectures integrating multiple graphene contacts, where each contact can act as an additional FET. This allows an independent probing and correlation of the conducting properties of bare $MoS_{2}$ and of the graphene contact regions. Experiments reveal how a $MoS_{2}$ overlayer can significantly suppress the n-side field-effect in graphene, even in a configuration where it would not be expected to do so. I will show ---thanks to $ab initio$ calculations--- that this effect can be understood as caused by deep traps associated with sulfur vacancies, which counterintuitively impact the field effect. Perspectives for the investigation of generic heterojunctions are discussed.