Theoretical Study of Stability of Halogen‐Defective Trihalide Monolayers: Cases of AlI3, AsI3, and IrBr3

Abstract

A theoretical study is conducted with three MX monolayer 2D materials (AlI, AsI, and IrBr) 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 AlI and AsI have indirect bandgaps of and  eV, respectively. IrBr has a direct bandgap of  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 AlI and AsI 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 IrBr stabilized and enable its displacive phase transition in displacive limit.