Transition metal dichalcogenide heterobilayers (hBLs) have attracted significant attention in the scientific community due to their unique electronic energy band structures and potential applications in optoelectronics. One key aspect of hBLs is the moiré pattern that naturally forms between different monolayers, creating a nanoscale periodic potential that can be manipulated through twist engineering. Twist engineering allows researchers to control the valley degrees of freedom of interlayer excitons, enhancing the controllability of valley properties in these heterostructures.
In a recent study published in Science Advances, Prof. Wang Can and Prof. Xu Xiulai investigated the dependence of valley polarization switching and polarization degree on the moiré period in electrically controlled transition metal dichalcogenide heterobilayers. The researchers demonstrated that the valley polarization of interlayer excitons (IXs) can be effectively controlled by adjusting the twist angle of the hBLs. By manipulating the moiré period, the researchers were able to electrically control both the degree of circular polarization (DCP) and the polarization switching in WSe2/WS2 heterostructure devices.
The study revealed that the twist angle plays a crucial role in determining the valley polarization of IXs. A larger moiré period, resulting from a larger twist angle, leads to a lower interlayer excitonic potential at local minima. This lower potential causes more excitons to be confined, enhancing the DCP of the heterostructures. Additionally, increasing intralayer electron-hole exchange interactions at larger twist angles results in a decrease in intralayer valley lifetime, reducing the initial intralayer valley polarization and ultimately leading to a decrease in interlayer valley polarization.
The researchers supported their experimental findings with theoretical calculations based on first-principle theory. The calculations showed that the difference in excitonic potential between two minima increases with the twist angle, requiring a higher external bias for devices with larger twist angles to switch the polarization. Building on these insights, the researchers demonstrated a valley-addressable encoding device that could pave the way for future non-volatile memories in valleytronic applications.
Overall, this study highlights the significant impact of twist engineering on valley polarization switching in transition metal dichalcogenide heterobilayers. By understanding the mechanisms behind the control of valley properties through twist manipulation, researchers can further explore the potential of hBLs in developing next-generation optoelectronic devices.