Superconductivity in functionalized niobium-carbide MXenes

We detail the effects of Cl and S functionalization on the superconducting properties of layered (bulk) and monolayer niobium carbide (Nb₂C) MXene crystals, based on first-principles calculations combined with Eliashberg theory. For bulk layered Nb₂CCl₂, the calculated superconducting transition temperature (T_c) is in very good agreement with the recently measured value of 6 K. We show that T_c is enhanced to 10 K for monolayer Nb₂CCl₂, due to an increase in the density of states at the Fermi level, and the corresponding electron-phonon coupling. We further demonstrate feasible gate- and strain-induced enhancements of T_c for both bulk-layered and monolayer Nb₂CCl₂ crystals, resulting in T_c values of around 38 K. In the S-functionalized Nb₂CCl₂ crystals, our calculations reveal the importance of phonon softening in understanding their superconducting properties. Finally, we predict that Nb₃C₂S₂ in bulk-layered and monolayer forms is also superconducting, with a T_c of around 28 K. Considering that Nb₂C is not superconducting in pristine form, our findings promote functionalization as a pathway towards robust superconductivity in MXenes.