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 (Nb 2 C) MXene crystals, based on first-principles calculations combined with Eliashberg theory. For bulk layered Nb 2 CCl 2 , 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 Nb 2 CCl 2 , 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 Nb 2 CCl 2 crystals, resulting in T c values of around 38 K. In the S-functionalized Nb 2 CCl 2 crystals, our calculations reveal the importance of phonon softening in understanding their superconducting properties. Finally, we predict that Nb 3 C 2 S 2 in bulk-layered and monolayer forms is also superconducting, with a T c of around 28 K. Considering that Nb 2 C is not superconducting in pristine form, our findings promote functionalization as a pathway towards robust superconductivity in MXenes.