Room Temperature Bound Excitons and Strain-Tunable Carrier Mobilities in Janus Monolayer Transition-Metal Dichalcogenides.

The successful synthesis of Janus transition metal dichalcogenides offers new opportunities in two-dimensional materials due to its novel properties induced by structural mirror asymmetry. Herein, by using the first-principle calculations, the thermodynamical stability for monolayers MoSSe and WSSe is demonstrated by phonon dispersion with no imaginary frequencies. No longitudinal optical-transverse optical (LO-TO) splitting exists at the Γ point and phonon frequencies are red-shifted, since the 2D Coulomb screening effect is introduced to eliminate the spurious interaction between adjacent layers. An indirect-direct-indirect transition in band gaps for both MoSSe and WSSe is observed, and tunable mobilities can be realized by uniaxial strain, reaching up to 106 cm2 V-1 s-1 when applying 2% tensile strain along the zigzag direction to monolayer MoSSe, which provides a good platform for flexible electronic devices. Large band gaps of 2.569 and 2.666 eV are predicted for monolayers MoSSe and WSSe when considering many-body quasiparticle corrections. The enhanced electron-hole interaction caused by a weak screening effect leads to considerable binding energies for both MoSSe and WSSe, and such tightly binding excitons with large oscillator strengths generate strong absorption peaks in visible region. The remarkable properties of Janus monolayers MoSSe and WSSe make them promising in next-generation microelectronic, optoelectronic, and valleytronic devices.