  As a way to further improve the electronic properties of group V layered
semiconductors, we propose to form in-layer 2D heterostructures of black
phosphorus and grey arsenic. We use \textit{ab initio} density functional
theory to optimize the geometry, determine the electronic structure, and
identify the most stable allotropes as a function of composition. Since pure
black phosphorus and pure grey arsenic monolayers differ in their equilibrium
structure, we predict a structural transition and a change in frontier states,
including a change from a direct-gap to an indirect-gap semiconductor, with
changing composition.
