A 2D ferromagnetic semiconductor in monolayer Cr-trihalide and its Janus structures

Using first principles calculations, we explored the physical properties of two-dimensional (2D) Cr-trihalide X-3-Cr-2-X-3 or CrX3 (X = Cl, Br, I) and its Janus monolayers X-3-Cr-2-Y-3 (X and Y = Cl, Br, I) where X Y. We found that Janus monolayer X-3-Cr-2-Y-3 materials are dynamically stable and that it is feasible to synthesize X-3-M-2-Y-3 2D-crystals. Both pristine and Janus layers have intrinsic two-dimensional ferromagnetic semiconducting band structures; the largest band gap of 2.3 eV was found in Cl-3-Cr-2-Cl-3, while the band gaps decreased in heavier halide systems. Using Monte Carlo simulations, we found that the Curie temperatures (T-c) showed the same feature of strong dependence on the X and Y halides. In non-polar systems with X = Y, we found no dipole moment, while the polar systems with X Y had induced dipole moments. Thus, the pristine layer has the same function on both sides, while the Janus layer displays dissimilar work functions in two different surface directions; this was related to the dipole moment and the value of electronegativity. We found that both pristine and Janus layer systems displayed rather weak frequency-dependent dielectric functions. Thus, the variation of the refractive index with frequency was small, and almost zero reflectivity was found in all systems.