Valley Exciton Modulation in Janus MoSeS via Phase-Engineered Intrinsic Polarization Fields

Janus transition metal dichalcogenides host a unique structural asymmetry that enhances their potential for spintronics and valleytronics. Here, using a phase engineering strategy, a hybrid H/T-MoSeS monolayer is synthesized via in situ plasma substitution. Theoretical analysis predicts a valley splitting of 7.35 meV, which is experimentally realized, with a 2.7-fold enhancement in the g-factor compared to the H-MoSeS. The substantial valley splitting originates from the additional exchange interactions and spin imbalance introduced by the T-phase. The coupling with the T phase further results in electrically tunable valley polarization, which reveals a sharp increase within ±20 V, gradually saturates at higher voltages, and reaches a high ratio of up to 43.8% for trions at room temperature. Such an enhancement is accompanied by elongated exciton lifetimes due to the increased electron concentration and phase-modulated Janus polarization field. All these findings reveal the synergistic interplay between phase engineering and the polarization field in Janus structures, providing a development strategy for 2D spin-valley devices with designed phase components and structural symmetry.