View article

Breaking the symmetry in the out-of-plane direction in two-dimensional materials to trigger distinctive electronic properties has long been predicted. Inspired by the recent progress in the experimental synthesis of a sandwiched S–Mo–Se structure (Janus SMoSe) at the monolayer limit [Zhang et al., ACS Nano, 2017, 11, 8192–8198], we investigate the transport and electronic structure of two-faced XMoY monolayers (X, Y = O, S, Se and Te) through first-principles calculations. It is found that all the monolayers are semiconductors except OMoTe, which is metallic. Interestingly, the “parents” of OMoTe (MoO2 and MoTe2) are both semiconductors. Further analysis shows that it is the out-of-plane asymmetry-induced strain that results in the metal–semiconductor transition between Janus OMoTe and its parents. By increasing the ratio of O atoms in one face of MoTe2, a progressive decreasing trend of the bandgap, as …