Superconductivity in Janus M₂X MXenes (M = W, Mo, Nb, and X = C, N) functionalized with hydrogen and bromine

Surface functionalization is a promising approach to modify the electronic characteristics of two-dimensional materials. Inspired by the successful Janus syntheses observed in graphene and transition metal dichalcogenides, we have developed M₂X MXenes (where M = W, Mo, Nb, and X = C, N) that are asymmetrically functionalized with hydrogen (H) and bromine (Br) atoms on opposite surfaces. We have examined their structural stability and superconductivity properties through a series of first-principles calculations alongside McMillan's equation. Notably, H/Br functionalization triggers superconductivity in non-superconducting pristine MXenes (such as Nb₂C) and boosts the transition temperatures (T_c) in others. For Mo₂CHBr, we predict a T_c of approximately 15 K, which is considerably higher than that of its pristine counterpart. Our findings suggest that the enhancement in electron-phonon coupling, driven by the asymmetric surface chemistry and lattice dynamics, is the primary reason for this increase in T_c. This study establishes Janus MXenes as a promising platform for the advancement of novel high-temperature 2D superconductors.