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.

• Received 10 December 2012

DOI:https://doi.org/10.1103/PhysRevB.87.075451