The electrical and valley properties of monolayer MoSe₂

We studied the electrical, optical and valley properties of monolayer MoSe₂. The measured PL circular polarization of monolayer MoSe₂ was negligibly small, compared with the more-than 60% circular polarization of monolayer MoS₂. Doping-dependent PL measurement and Kelvin probe microscopy show that the small circular polarization because of its longer exciton lifetime in monolayer MoSe₂. Furthermore, it is found that the longer exciton lifetime is due to the nearly intrinsic electrical property of monolayer MoSe₂, where non-radiative trion decay, which is the dominant exciton decay mechanism in n-type monolayer MoS₂, is minimized due to the low density of excess electrons (holes). We believe that S and Se vacancies play an important role in determining the Fermi level position for these materials. Our experimental results show that monolayer MoSe₂ can be a better valleytronics material than monolayer MoS₂ in spite of its small circular polarization.