Speaker: Dr. Junfeng Dai, Southern University of Science and Technology
Time: 10:00 AM - 11:00 AM
With the development of semiconductor industry, scaling of electronic devices in integrated circuits has approached technological and physical limit, such as quantization of electrical conductance in thin transistor channels. Researchers have to search new materials and manipulate new freedom of electrons to overcome this physical limit. So the spin- and valley-based electric devices on two-dimensional materials become an important breakthrough point. In monolayers of transition metal dichalcogenides (TMDCs), the inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics, making possible manipulation of spin and valley in these materials. In this talk, I will discuss the observation of selective photo-excitation of the degenerate valleys by circularly polarized optical pumping in MoS2 and WS2 monolayers, emerging multi-valley two-dimensional semiconductors with remarkable optical and transport properties1. Surprisingly, at room temperature, a valley polarization up to 70% could be sustained in WS2 with a thickness larger than 3 unit layers, which is a significant step towards the development of valley-based optoelectronic devices operating at room temperature2. Moreover, the dynamic processes of intervalley scattering have also been discussed in monolayer and bilayer WS2 with the aid of time-resolved Kerr rotation (TRKR), and polarization-resolved differential reflectance (PRDR) spectroscopies3.