The mechanical, electronic and optical properties of two-dimensional transition metal chalcogenides MX2 and M2X3 (M = Ni, Pd; X = S, Se, Te) with hexagonal and orthorhombic structures

Abstract

In this paper, we perform first-principle studies on the mechanical, electronic and optical properties of two-dimensional transition metal chalcogenides MX₂ and M₂X₃ where M = Ni or Pd and X = S, Se or Te. For MX₂, besides the common hexagonal forms, it presents a stable orthorhombic structure with highly in-plane anisotropic properties and giant negative Poisson's ratios. For M₂X₃, on the other hand, it presents no hexagonal but another orthorhombic structure with highly anisotropic mechanical and electronic features. These orthorhombic MX₂ and M₂X₃ are semiconducting with band gaps ranging from 1.05 to 2.40 eV, and they are hyperbolic materials operating in a broad range of the electromagnetic spectrum from infrared to ultraviolet. Moreover, by joint analysis of band gaps, band edges and optical absorption, Ni₂Se₃, Pd₂S₃ and orthorhombic MX₂ are found to be highly efficient for solar-driven photocatalytic water splitting.