First principles prediction of the stable MgFCl Janus monolayer with tunable properties

Herein, we introduce a new Janus monolayer, namely MgFCl, where its structural, electronic, and optical properties are systematically investigated by means of first-principles calculations. It is demonstrated that MgFCl single layer is structurally and dynamically stable. The electronic band structure indicates a wide Γ−Γ direct gap of 4.32(5.61) eV as determined by PBE(HSE06) functional. Consequently it exhibits a good absorption in the far and extreme ultraviolet region. In addition, the physical properties tuning via external strain and alkaline earths substitution is also studied. Results show a strong strain-dependence of the energy gap, however the direct gap nature is retained. The alkaline earths incorporation induces a strong structural local distortion due to the atomic size difference. A band gap reduction of 0.19 eV is caused by Ca atom, while the Sr and Ba atoms induce insignificant change of this important parameter. Calculated spectra suggest that the MgFCl monolayer absorption may be extended to a wider energy range and enhanced by applying proper strain and alloying. Results presented herein may recommend new two-dimensional (2D) materials for applications in optoelectronic nano devices, specifically, those working under high energy conditions. •MgFCl Janus monolayer exhibits good stability.•MgFCl Janus monolayer is a direct gap insulator with band gap of 5.61 eV.•The external strain can tune effectively the electronic structure.•Ca incorporation reduces the energy gap, while Sr and Ba presence induces insignificant change.•Optical properties of MgFCl monolayer can be enhanced by either external strains or alkaline earth substitution.