Phosphorene analogues: Isoelectronic two-dimensional group-IV monochalcogenides with orthorhombic structure

The group-IV monochalcogenides SnS, SnSe, GeS, and GeSe form a family within the wider group of semiconductor ``phosphorene analogues.'' Here, we used first-principles calculations to investigate systematically their structural, electronic, and optical properties, analyzing the changes associated with the reduction of dimensionality, from bulk to monolayer or bilayer form. We show that all those binary phosphorene analogues are semiconducting, with band-gap energies covering part of the infrared and visible range, and in most cases higher than phosphorene. Further, we found that they have multiple valleys in the valence and conduction band, the latter with spin-orbit splitting of the order of 19--86 meV.