Theoretical study of stability of halogen-defective trihalide monolayers:cases of AlI₃, AsI₃, and IrBr₃

A theoretical study is conducted with three MX₃ monolayer 2D materials (AlI₃, AsI₃, and IrBr₃) on their electronic properties and how a halogen monovacancy affects their thermostability. Density functional theory (DFT) calculations are run to obtain the band structures and phonon dispersions for both pristine and defective structures. It is shown that AlI₃ and AsI₃ have indirect bandgaps of 2.40 and 2.23 eV, respectively. IrBr₃ has a direct bandgap of 1.65 eV. Phonon dispersions indicate that they are all thermodynamically stable in pristine state, but their defective counterparts are not. Ab-initio molecular dynamics (AIMD) simulations are conducted for defective ones to further investigate their stability. It is found that AlI₃ and AsI₃ layers are decomposed while IrBr3 layer is bent. Further investigations are conducted by analyzing the bond energies and bond lengths of the three materials. It shows that AlI₃ and AsI₃ have lower bond energy and longer bond length, which makes them dissociated at the ambient temperature while higher bond energy and shorter bond length keep IrBr₃ stabilized and enable its displacive phase transition in displacive limit.