First-principles study of binary group IV-V polymorphs in 2D tetrahex structure

Abstract

Tetrahex materials are a novel family of 2D materials, made of an ordered se-quence of tetragons and hexagons, with exotic electronic, optical and mechanical properties. Motivated by the promising photocatalytic and auxetic properties found in the Tetrahex-carbon, we investigate the behavior of binary group IV-V polymorphs in this structure (denoted by th-XY2 where X = C and Si, and Y = N, P, As, and Sb), through first-principles methods. We demonstrate that these compounds exhibit robust energetic, dynamical, thermal, and mechanical stabilities. Our calculations show that the intrinsic structural anisotropy within this family induces strongly anisotropic mechanical, electronic, and optical behav-ior. These materials offer high ultimate strain, comparable to that of graphene. The majority are semiconductors in nature, where th-CAs2 and th-CP2 possess direct and quasi-direct band gaps, respectively, and the rest have indirect band gaps. Besides, an indirect-to-direct band gap transition can be induced in th-CSb2 through strain engineering. Studied compounds have good optical absorption in the visible and ultraviolet regions of the light spectrum, suitable for optoelectron-ics applications. Their band gaps are wide enough to provide the photogenerated energy required for the splitting of water. In th-CAs2 and th-CP2, the positions of band edges are perfectly compatible with the water oxidation and reduction potentials. Besides, they offer anisotropic high charge carrier mobilities, which prolongs the average lifetime of charge carrier drift. Having all these features in one package, these compounds, especially th-CAs2 and th-CP2, can be consid-ered promising candidates for high-performance photocatalytic water splitting. Moreover, we found auxetic behavior in th-CN2 and th-SiN2 around their equilib-rium structure. Having this rare feature endows these compounds with potential applicability in numerous areas, from biotechnology to defense.