Computational discovery and characterization of polymorphic two-dimensional IV-V materials

First-principles calculations predict the stability and properties of two-dimensional (2D) group IV-V MX compounds (M = Si, Ge, Sn, Pb; X = P, As, Sb, Bi) in two distinct crystallographic configurations: a low symmetry Cm layer inspired by the layered bulk structures of SiP, SiAs, GeP, and GeAs, and a high symmetry P6¯m2 structure previously predicted for 2D SiP. The calculations predict that the Cm structure is more stable for X = As, Sb, and Bi, and the P6¯m2 structure for X = P. The electronic properties of the two structures are quite different. Specifically, the Cm band gaps are systematically larger by about 15% for most compounds, and the band offsets indicate that all of these compounds in the Cm crystal structure will form type II heterojunctions in contact with their P6¯m2 polymorphs. Pourbaix diagrams predict that a few of the 2D IV-V compounds are stable when exposed to water.