Theoretical prediction of superconductivity in intrinsic and hydrogenated transition metal mononitride monolayers

Two-dimensional superconductors have garnered significant attention due to their exceptional quantum properties, particularly tunable superconductivity, which holds great potential for advanced quantum technologies. In this study, first-principles calculations reveal two stable transition metal mononitride monolayers---1T- and 1H-Mo₂N₂---featuring chemically distinct Mo- and N-terminated surfaces. Electron-phonon coupling calculations predict the superconducting transition temperatures (Tc) of ~13.6 K for the 1T phase and ~13.5 K for the 1H phase. The coupling between Mo d_z² orbital electrons at the Fermi level and out-of-plane acoustic phonon modes associated with Mo atoms is identified as a key contributor to superconductivity. Hydrogenation on the Mo-terminated surface boosts Tc to 25.4 K in 1T phase, driven by the increase of Mo d_z² electronic states at the Fermi level and the softening of the out-of-plane acoustic mode induced by the activation of Mo-H bond stretching vibrations. In contrast, hydrogenation on the N-terminated surface suppresses superconductivity due to the inhibition of out-of-plane Mo vibrations. Furthermore, double-sided hydrogenation on both surfaces reduces Tc to 16.9 K in the 1T phase and 8.2 K in the 1H phase. These findings provide valuable insights into tunable superconductivity in two-dimensional materials through targeted site-dependent hydrogenation.