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

Abstract

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.