Theoretical prediction of two-dimensional II-V compounds

Graphene has attracted significant attention as a pioneer of two-dimensional zero gap semiconductors, but the development of new two-dimensional materials with a finite band gap has been actively pursued. In this study, the structural stability of double bilayers (DBs) of group II-V compounds (II: Be, Zn, and Cd; V: P, As, and Sb) has been systematically investigated using first-principles calculations based on density functional theory. The thermodynamic calculations have confirmed that BeP, BeAs, ZnP, and ZnAs can be produced through exothermic reactions from their constituent bulk systems. It has also been confirmed that all the compounds have phonon dynamical stabilities. Only CdP and CdAs have been found to have an AB-stacked DB structure with threefold symmetry, while the other compounds have ${\text{AB}}^{\ensuremath{'}}$-stacked DB structure with broken symmetry. The difference in atomic radii between group II and group V results in the so-called size effect, which determines the stacking pattern. The structural stability of II-V DB thin films is explained by analogy with the surface structural stability of compound semiconductors: The change in the atomic arrangement of the DB structure alters the electronegativity of the surface orbitals of the II-V thin film, which does not result in any unsaturated bonds, i.e., no metallic bands across the Fermi level appear. The various DB II-V compounds proposed in this study will join the ranks of atomic-level 2D semiconductor materials.