Composition-tunable valley polarization in monolayer Mo_1-xW_xS₂ alloys

We report a systematic study of excitonic resonance and valley polarization in monolayer Mo_1-xW_xS₂ alloys with compositions x = 0, 0.4, 0.6, and 1.0. Monolayers prepared by mechanical exfoliation were characterized by microscopy, Raman, and X-ray photoelectron spectroscopy. Unpolarized photoluminescence (PL) spectra show a progressive blueshift of the A-exciton peak with increasing W content, consistent with bandgap evolution. Circularly polarized PL measurements reveal valley-selective emission, with the degree of valley polarization (DVP) defined as DVP = (I_co − I_cross)/(I_co + I_cross). The DVP exhibits a maximum near the A-exciton resonance and increases from 3.0 % at x = 0–20.4 % at x = 1.0, accompanied by a blueshift in peak position. The enhancement is attributed to stronger spin–orbit coupling, reduced exciton–phonon scattering, and alloy-modulated relaxation. These findings demonstrate alloy composition as a practical tuning parameter for valley polarization in 2D semiconductors, offering guidance for valleytronic and chiroptical devices.