Comparative study of electronic and optical properties of monolayer MoSi2N4 with adsorbed functional groups

Abstract

The electronic and optical properties of monolayer MoSi₂N₄, modified by the adsorption of eight distinct functional groups—hydrogen, halogens (fluorine, chlorine, bromine, iodine), oxygen, hydroxyl, and amino—were systematically explored using first-principles simulations. The adsorption coverage was varied (n = 1–16), revealing profound modifications in the electronic structure, particularly when all silicon or nitrogen atoms on a single surface were fully functionalized (16 adsorbed atoms). Notably, the bandgap diminished to 0 eV upon adsorption of hydrogen (H), fluorine (F), or iodine (I), while chlorine (Cl) and bromine (Br) adsorption reduced the bandgap from 1.79 eV (pristine MoSi₂N₄) to 0.92 eV (MoSi₂N₄-Cl₁₆) and 1.12 eV (MoSi₂N₄-Br₁₆), respectively. In contrast, oxygen (O), hydroxyl (OH), and amino (NH₂) groups resulted in bandgaps of 1.25 eV (MoSi₂N₄-O₁₆), 1.19 eV (MoSi₂N₄-OH₁₆), and 1.05 eV (MoSi₂N₄-NH2₁₆). A comparative analysis of the optical absorption spectra demonstrated that surface adsorption significantly broadened the absorption range, extending it into the infrared region while encompassing the ultraviolet and visible domains. Among the functional groups studied, iodine adsorption induced the most pronounced enhancement in the absorption intensity. Following the adsorption of 16 iodine atoms, the maximum ultraviolet absorption coefficient reached 1.07 × 10⁵ cm⁻¹, more than doubling the intrinsic value. These results highlight the efficacy of functional group adsorption in tuning the optoelectronic properties of MoSi₂N₄, with H, F, and I atoms exerting the most substantial influence on the bandgap and iodine atoms exhibiting the most significant and tunable optical absorption characteristics.