Structural, electronic and mechanical properties of two-dimensional Janus transition metal carbides and nitrides

Surface functionalization can be used to tune the electronic and mechanical properties of MXenes. In this work, the structural stability, electronic and mechanical properties of monolayer M2X (M = Sc, Ti, V, Mn, Nb, Mo, Hf; X = C, N) with asymmetrical functionalization to form Janus MXenes were investigated by first-principles calculations, and the results were compared with the symmetrical functionalization. Results show that asymmetric functionalization has a consequential effect on the structure stability, electronic, elastic properties of the MXenes. For pristine monolayer M2X (M = Sc, Ti, V, Mn, Nb, Hf; X = C, N) systems, T-phase is energetically stable than H-phase, whereas the stable structures of monolayers Mo2C and Mo2N are H-phase. All the asymmetrically functionalized monolayers M2X (M = Sc, Ti, V, Mn, Mo, X = C, N) except for Sc2C are metallic materials. A metallic to semiconductor transition occurs in monolayer Sc2C upon surface functionalization. The mechanical stability of monolayer M2X can be improved by Janus surface functionalization.