Strain tuning of the electronic structure and optical properties of novel Janus MgBrI monolayer: Insights from first-principles calculations

Abstract

Using first-principles calculations in the framework of density functional theory (DFT), we systematically carry out the dynamic stability, electronic and optical properties of the Janus MgBrI monolayer and its parent MgI2 and MgBr2 monolayers. Our findings show that these systems have no imaginary frequencies in the entire Brillouin zone (BZ), which means they are very stable. At the equilibrium state, the results indicate that the Janus MgBrI monolayer is a direct band gap insulator with an energy gap of 3.475 eV, which is smaller than the bandgap of MgI2 and MgBr2 monolayers. The studied optical properties including the complex dielectric function, refractive index, reflectivity, extinction coefficient and absorption coefficient as a function of the photon energy have been investigated. The absorption coefficient shows a strong absorption of the light in the ultraviolet (UV) region. We studied how biaxial strain effects the electronic and optical properties of the Janus MgBrI monolayer. When the strain ranged from −10% to 10%, the Janus MgBrI monolayer showed an insulator characteristic with a direct band gap, which changed to an indirect band gap when the biaxial compressive strain was −10%. In addition, the Janus MgBrI displays an absorption of around 105 cm−1 in the ultra-violet (UV) region. Our results highlight an intriguing new two-dimensional (2D) material that could be used in optoelectronic devices and UV absorbers.