Structural distortions in monolayers of binary semiconductors

We examine the structural properties of free standing II-VI and III-V semiconductors at the monolayer limit within first principle density functional theory calculations. A non-polar buckled structure was found to be favoured over a polar buckled structure. While an obvious reason for this may be traced to the contribution from dipole dipole interactions present in the polar structure which would destabilize it with respect to the nonpolar structure, Coulomb interactions between electrons on the cations and anions are found to be the reason for the nonpolar structure to be favoured. A route to tune the Coulomb interaction between the electrons on the cations and anions is through biaxial tensile strain. This allows for a planar graphitic phase in CdS to be stabilized at just 2\% tensile strain. Strain also shifts the valence band maximum from the $\Gamma$ point to the K point opening up opportunities for exploring spin-valley physics in these materials.