A First-Principles Study on Electronic, Magnetic and Optical Properties of Two-Dimensional Janus FeSX (X = Se, Te)

Abstract

In this work, we study with first-principles methods the electronic material properties of two-dimensional (2D) Janus iron dichalcogenides FeSSe and FeSTe. Using density functional theory (DFT) calculations with the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, we study 2D Janus structures. Structural optimization shows a P3m1 structure for the said materials characterized by a central iron layer covalently bonded between two distinct chalcogen layers. Both FeSSe and FeSTe exhibit metallic character with complex, multi-band Fermi surfaces dominated by Fe 3d orbitals and van Hove singularities. A significant finding is the presence of spontaneous magnetization and exchange splitting, confirming magnetically ordered ground states. FeSSe displays a ferromagnetic coupling with a total magnetization of 1.96 μB, whereas FeSTe exhibits a quenched magnetic state of 0.47 μB due to ferrimagnetic-like tendencies of the Te atoms. Optical calculations show impressive light-harvesting capabilities, with absorption coefficients 1.5 − 1.8×106 cm−1 in the ultraviolet region (αz peaks around 113−174 nm for FeSSe and 114−158 nm for FeSTe) and a broad response extending into the near-infrared to visible region. Furthermore, the inherent structural asymmetry induces a pronounced out-of-plane charge disparity, resulting in an intrinsic electric dipole moment. These findings suggest that 2D Janus Fe-based materials are promising candidates for valleytronics, spintronic injectors, polarization-sensitive photodetectors, catalysis, and the exploration of emergent quantum phases, such as unconventional superconductivity.