Improvement of Room Temperature Valley Polarization in Transition Metal Dichalcogenides Homojunction via Ionic-Liquid-Gated Modulation.

Due to broken inversion symmetry and strong spin-orbit coupling in monolayers of transition metal dichalcogenides (TMDs), the valleys in momentum space can be selectively controlled by circularly polarized light. This property enables valleytronics applications, where information is encoded in valley states, paving the way for next-generation optoelectronic and quantum devices. In this work, bilayer TMDs homojunctions are fabricated that preserve the intrinsic valley degrees of freedom while maintaining broken spatial inversion symmetry. A significant increase in the degree of valley polarization (DVP) from 0 to 23% and 28% for A (XA) and B (XB) excitons of MoS2/MoS2 homojunction, and 16% for WS2/WS2 homojunction at 300 K is achieved by employing the ionic-liquid gating (ILG) method. Furthermore, in the MoS2/MoS2 homojunction, the room temperature DVP can be further enhanced to 44% and 51% through back-gate-controlled Fermi level modulation, respectively, exceeding the previously reported values for MoS2 monolayers. This improvement is attributed to the high electron concentration, which suppresses intervalley scattering through Coulomb interaction screening, thereby enhancing valley polarization. These findings provide a robust strategy for achieving higher valley polarization in TMDs, advancing the development of practical valley-based electronic devices operable at room temperature.