Electronic and spintronic properties of Janus MSi2PxAsy ( M = Mo, W) monolayers

Very recently, Janus two-dimensional (2D) materials have gathered enormous attention due to possessing unique properties caused by the lack of out-of-plane symmetry. Herein, based on the successful fabrication of 2D ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$ monolayer, we have reported 12 novel Janus ${M\mathrm{Si}}_{2}{\mathrm{P}}_{x}{\mathrm{As}}_{y}$ ($M$ = Mo, W; and $x+y=4$) structures with excellent dynamic stability and great potentials in spintronics and valleytronics applications. First of all, our first-principles calculations show that all structures demonstrate semiconductor characteristics with a direct band gap. The phonon dispersions confirm their dynamic stability. These Janus monolayers illustrate in-plane piezoelectric coefficients up to 4.53--6.89 pm/V and the broken mirror symmetry of these Janus structures induces out-of-plane piezoelectric coefficients up to 0.024 pm/V. Strong spin-orbit coupling (SOC) in these structures results in valley polarization at the edge of the valence band, which is stemmed from the breaking of the inversion symmetry. Constructing Janus structures induces an intrinsic electric field that gives rise to the Rashba-type spin splitting around the $\mathrm{\ensuremath{\Gamma}}$ point of the valence band and the $M$ valley of the conduction band. Comprehensive studies show that spin splitting is higher in compounds with heavier atoms like ${\mathrm{WSi}}_{2}{\mathrm{PAs}}_{3}$. According to the results, ${\mathrm{WSi}}_{2}{\mathrm{P}}_{3}\mathrm{As}$ and ${\mathrm{WSi}}_{2}\mathrm{PAsPAs}$ exhibit the largest values of the Rashba coefficient at the $\mathrm{\ensuremath{\Gamma}}$ point of the valence band. In summary, our predicted Janus ${M\mathrm{Si}}_{2}{\mathrm{P}}_{x}{\mathrm{As}}_{y}$ structures enhance the Janus 2D materials family and are promising candidates for future spintronics and valleytronics applications and also motivate related experimental works in the future.