  We study the electronic structure of a heterojunction made of two monolayers
of MoS2 and WS2. 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 MoS2 and WS2 single layers. We find
that 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 portrait the
MoS2-WS2 bilayer as a prototypical example for band-gap engineering of
atomically thin two-dimensional semiconducting heterostructures.
