Recent fabrication of a Janus MoSSe monolayer has raised exciting prospects of developing polar two-dimensional (2D) materials that exhibit excellent properties for nanodevice applications. Here, we proposed MoSSe as a superior gas sensing material by investigating the adsorption of CO, CO₂, NH₃, NO and NO₂ on the Janus layer by first-principles calculations. Due to the presence of out-of-plane polarization originating from the asymmetrical structure, it is found that NH₃ adopts distinct opposite orientations when it is adsorbed on the Se or S side of the Janus layer. The binding strengths of all the molecules adsorbed on the Se surface are generally much stronger than those on the S surface. More interestingly, upon strain deformation, the adsorption strengths of NH₃ and NO₂ molecules on the Se side of MoSSe can be remarkably enhanced, but gradually lowered on the S side. We revealed that the strain-dependent adsorption behavior is driven by a significant change of electrostatic potential difference between the Se and S surfaces under tensile strain. Corresponding to the distinct adsorption behaviors on the two sides, different electronic variations are also revealed. With higher gas sensitivity, and surface and strain selectivity, Janus MoSSe is proposed as an ideal material for constructing ultrahigh-sensitivity nanoscale sensors.

The successful fabrication of Janus transition metal dichalcogenide (TMD) monolayer has sparked extensive research interests in various fields, such as nanoelectronics, optoelectronics, valleytronics, and catalysis. Janus TMDs can not only inherit the advantages of conventional TMDs but also produce novel properties which are different from their counterparts. The breaking of vertical mirror symmetry can induce a variety of novel properties, such as Rashba spin splitting, vertical piezoelectricity, and long exciton lifetime. Moreover, the intrinsic electric field that originates from the vertical asymmetry can serve as a superior platform for tuning the interlayer coupling when forming van der Waals structures. In this mini review, the recent key research progresses of 2D Janus TMDs, including the fundamental properties and potential applications, are briefly summarized, and the existing challenges are also presented.