  Magnesium hydroxide (Mg(OH)2) has a layered brucite-like structure in its
bulk form and was recently isolated as a new member of 2D monolayer materials.
We investigated the electronic and optical properties of monolayer crystals of
Mg(OH)2 and WS2 and their possible heterobilayer structure by means of first
principles calculations. It was found that both monolayers of Mg(OH)2 and WS 2
are direct-gap semiconductors and these two monolayers form a typical van der
Waals heterostructure with a weak interlayer interaction and a type-II band
alignment with a staggered gap that spatially seperates electrons and holes. We
also showed that an out-of-plane electric field induces a transition from a
staggered to a straddling type heterojunction. Moreover, by solving the
Bethe-Salpeter equation on top of single shot G0 W0 calculations, we show that
the oscillator strength of the intralayer excitons of the heterostructure is an
order of magnitude larger than the oscillator strength of the interlayer
excitons. Because of the staggered interfacial gap and the field- tunable
energy band structure, the Mg(OH)2 -WS2 heterobilayer can become an important
candidate for various optoelectronic device applications in nanoscale.
