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 (NbC) MXene crystals, based on first-principles calculations combined with Eliashberg theory. For bulk layered NbCCl, the calculated superconducting transition temperature () is in very good agreement with the recently measured value of 6 K. We show that is enhanced to 10 K for monolayer NbCCl, 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 for both bulk-layered and monolayer NbCCl crystals, resulting in values of around 38 K. In the S-functionalized NbCCl crystals, our calculations reveal the importance of phonon softening in understanding their superconducting properties. Finally, we predict that NbCS in bulk-layered and monolayer forms is also superconducting, with a of around 28 K. Considering that NbC is not superconducting in pristine form, our findings promote functionalization as a pathway towards robust superconductivity in MXenes.