Research on the valley-related properties based on multilayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDCs), owing to the unique valley-related phenomena, have aroused great attention. However, in multilayer TMDCs, for the presence of the spatial-inversion symmetry, the valley-related phenomena are absent, which limits the developments of valleytronics. In this report, by utilizing the ionic liquid (IL) covering on top of the multilayer TMDC samples, the spatial-inversion symmetry is broken by the out-of-plane electric field induced by IL. The valley-related properties are therefore induced. Firstly, we investigated the circular photogalvanic effect (CPGE) in multilayer MoS₂. Upon the electric field induced by IL, the valley-coupled CPGE signals emerge. Via the wavelength-dependent measurements, it is confirmed only with resonant excitation of K (K’) valley can the CPGE signals be detected. Furtherly, we studied the circular photon assisted valley Hall effect in multilayer WSe₂. With the application of IL, a Hall-oriented photocurrent signal in proportional to the drain-source voltage is detected, illustrating the valley Hall effect is successfully observed. These results imply the valley degree of freedom could be reborn in multilayer TMDCs.