Quasiparticle Band Structure, Exciton, and Optical Property in Janus Structures of Transition-Metal Dichalcogenide Monolayers.

Since the first fabrication of the Janus MoSSe monolayer, this kind of polar structure has garnered incredible interest as an emerging class of two-dimensional (2D) materials. The vertical dipole in Janus MoSSe enables a range of novel applications in optics and optoelectronics. Besides MoSSe, the broad family of Janus transition-metal dichalcogenides (TMDCs) offers numerous options for diverse electronic and optical properties. Meanwhile, the coupling between strong spin-orbit effect and exciton effect could induce spin-valley excitons, which deserve further investigation in these Janus systems. In this paper, based on the first-principles density functional theory (DFT) integrated with many-body perturbation method, i.e., -BSE method, we study the quasiparticle electronic structures, excitons, and optical properties of six different Janus 2H-phase Mo/WXY monolayers (X and Y stand for S, Se, or Te). When Se is replaced by Te and subsequently S is replaced by Se, the global band gap and direct band gap at the point decrease in both Mo and W series. The optical gap monotonically decreases from 1.70 to 1.35 eV in Janus TMDCs containing Mo, covering the wavelength from red to infrared light. This range from 1.87 to 1.44 eV is a bit wider in the systems with W. Regarding the spin-orbit effect, the splitting between A peak and B peak, which are dominated by strong bound excitons, shows a variation in compounds of Mo (W) about 40 (20) meV. Meanwhile, this splitting in the W series is much larger than that in the Mo series, consistent with the strong splitting of the valence band maximum at the point observed in the electronic structure. The broad optical spectrum, combined with the unique Janus structure and strong spin-orbit effects at the valley, provides precise optical control, paving the way for advanced applications in photovoltaics and valleytronics.