Prediction of novel semi-conducting two-dimensional MX2 phosphides and chalcogenides (M = Zn, Cd; X = P, S, Se) with 5-membered rings

Abstract

The discovery of novel two-dimensional (2D) materials is a significant obstacle for contemporary materials science. Research in the field of 2D materials has mainly focused on materials possessing 6-membered rings, high symmetry, and isotropic features. The examination of 2D materials presenting 5-membered rings, low symmetry and anisotropic characteristics properties has received scarce attention. In this study, we employed evolutionary algorithms and heuristic approaches combined with first-principles calculations to predict penta-MX₂ structures (M = Zn, Cd; X = P, S, Se). All selected 2D penta-MX₂ phases are dynamically, thermodynamically, mechanically, and thermally stable. Further discussion focuses on their structural, bonding, electronic and optoelectronic features. Our HSE06 calculations reveal that the penta-MP₂, ZnPS, and MSSe structures are semiconductors with a band gap of 0.80–3.08 eV. Conversely, the 2D penta-MPSe (M = Zn, Cd) and CdPS phases are metallic. We additionally note that penta β-ZnP₂ and CdP₂ display direct band gaps (1.39 eV and 1.18 eV, respectively), while the penta α-ZnP₂, ZnPS, ZnSSe, α-CdSSe and β-CdSSe possess indirect band gaps. Remarkably, 2D pentagonal MP₂ (M = Zn, Cd), MSSe (M = Zn, Cd) and ZnPS 2D monolayers exhibit substantial optical absorption (>10⁵ cm⁻¹) throughout a broad range of the visible light spectra. Our results for crystal structure prediction expand the 2D penta-family of phosphides and chalcogenides, and demonstrate the potential of 2D penta-MX₂ materials for optoelectronic applications.