Asymmetric valley polarization and photoluminescence in MoS₂/MoO₃ heterostructure

We investigate circularly polarized photoluminescence (PL) in the MoS₂/MoO₃ heterostructure, which was fabricated by transferring MoS₂ monolayer to cover the MoO₃ few layers on the SiO₂/Si substrate. It is shown that the PL with the same helicity as the excitation light is dominant due to the inherent chiral optical selectivity, which allows exciting one of the valleys in MoS₂ monolayer. The degree of polarization (DP), which characterizes the intensity difference of two chiral components of PL, is unequal for the right-handed and left-handed circularly polarized excitations in the MoS₂/MoO₃ heterostructure. This effect is different from the one in pristine MoS₂. Our Raman spectra results together with ab initio calculations indicate the p-doped features of the MoS₂ when it covers the MoO₃ layers. Thus the possible explanation of the unequal DP is that the p-doping process generates a built-in voltage and therefore brings the difference of electron-hole overlaps between K and K' valleys. Namely the asymmetric valley polarization may be obtained in the MoS₂/MoO₃ heterostructure. Consequently, the circularly polarized PL caused by the electron-hole recombination at K and K' valleys manifests unequal DP for the right-handed and left-handed helix excitations. This asymmetric effect is further enhanced by decreasing the temperature in the MoS₂/MoO₃ heterostructure. Our investigation provides a unique platform for developing novel two-dimensional valleytronic devices.