Electronic properties of the MoS2-WS2heterojunction

We study the electronic structure of a heterojunction made of two monolayers of MoS${}_{2}$ and WS${}_{2}$. Our first-principles density functional calculations show that, unlike in the homogeneous bilayers, the heterojunction has an optically active band gap, smaller than the ones of MoS${}_{2}$ and WS${}_{2}$ single layers. We find that the optically active states of the maximum valence and minimum conduction bands are localized on opposite monolayers, and thus the lowest energy electron-holes pairs are spatially separated. Our findings portray the MoS${}_{2}$-WS${}_{2}$ bilayer as a prototypical example for band-gap engineering of atomically thin two-dimensional semiconducting heterostructures.