Magnesium hydroxide $\left[\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\right]$ has a layered brucitelike structure in its bulk form and was recently isolated as a new member of two-dimensional monolayer materials. We investigated the electronic and optical properties of monolayer crystals of $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ and ${\mathrm{WS}}_2$ and their possible heterobilayer structure by means of first-principles calculations. It was found that both monolayers of $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ and ${\mathrm{WS}}_2$ 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 separates 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 ${\mathrm{G}}_0{\mathrm{W}}_0$ calculations, we show that the low-energy spectrum of the heterobilayer is dominated by the intralyer excitons of the ${\mathrm{WS}}_2$ monolayer. Because of the staggered interfacial gap and the field-tunable energy-band structure, the $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2-{\mathrm{WS}}_2$ heterobilayer can become an important candidate for various optoelectronic device applications in nanoscale.

• Received 10 March 2016
• Revised 12 May 2016

DOI:https://doi.org/10.1103/PhysRevB.94.195403