Electronic properties of Janus MXY/graphene (M = Mo, W; X ≠ Y = S, Se) van der Waals structures: a first-principles study

Based on the first-principles calculations, we studied the intrinsic dipole moment and electronic properties of Janus MXY (M = Mo, W; X ≠ Y = S, Se) monolayers, bilayers and heterostructures with graphene, and the possibility of MXY encapsulating graphene. The results show that Janus MXY monolayer has an intrinsic dipole moment and a direct band gap. However, for MXY bilayers strong interlayer coupling will cause direct to indirect band gap transition, and the existence of the dipole moment leads to a significantly large interlayer band offset, being the driving force for the formation of interlayer excitons. In MXY/graphene heterostructures, changes in the direction of intrinsic dipole moment will cause a change in Schottky barrier height and even the transition between p- and n-type Schottky contacts. Independent of the interface atomic layer of Janus MXY, on one hand, the Dirac cone still exists in graphene, proving that MXY is an ideal coating material. On the other hand, the type-II band alignment will disappear as the intrinsic dipole moment disappears, confirming that the intrinsic dipole moment plays a vital role in the formation of a large band offset. Our results provide guidance for the study of interlayer excitonic states, the experimental construction of atomically thin p-n junctions and the encapsulation of graphene.