Chen Xin, Zheng Shu-Wen, Wang Xue-Peng, Wang Hai-Yu
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China.
Phys Chem Chem Phys. 2022 Jul 13;24(27):16538-16544. doi: 10.1039/d2cp02105f.
Excitonic devices based on two-dimensional (2D) transition metal dichalcogenides (TMDCs) can combine spintronics with valleytronics due to its special energy band structure. In this work, we studied the generation and relaxation processes of spin/valley polarized excitons dynamics in monolayer WSe and its van der Waals (vdW) heterojunction with graphene using a circularly polarized femtosecond pump-probe system. The spin/valley depolarization dynamics of the A exciton in monolayer WSe is found to exhibit a biexponential decay. The fast relaxation process is due to the ultrafast intervalley electron-hole spin-flip exchange coupling and electron-phonon scattering. And the slow relaxation process originates from the recombination and relaxation of the trion states. Graphene has an electron extraction effect on WSe, which prevents the formation of trions. Therefore, the spin/valley depolarization process of the A exciton in the heterojunction exhibits only a fast relaxation process. In both monolayer WSe and its heterojunction with graphene, B/A' excitons exhibit a negative spin/valley polarization which is mainly due to two-photon absorption and excited Bose scattering. Our work systematically studied the spin/valley depolarization dynamics of excitons and revealed possible mechanisms of their differences in isolated 2D WSe and vdW heterojunctions.
基于二维（2D）过渡金属二硫属化物（TMDCs）的激子器件由于其特殊的能带结构，可以将自旋电子学与谷电子学结合起来。在这项工作中，我们使用圆偏振飞秒泵浦 - 探测系统研究了单层WSe及其与石墨烯的范德华（vdW）异质结中自旋/谷极化激子动力学的产生和弛豫过程。发现单层WSe中A激子的自旋/谷去极化动力学表现出双指数衰减。快速弛豫过程是由于超快的谷间电子 - 空穴自旋翻转交换耦合和电子 - 声子散射。而缓慢弛豫过程源于三重激子态的复合和弛豫。石墨烯对WSe有电子提取作用，这阻止了三重激子的形成。因此，异质结中A激子的自旋/谷去极化过程仅表现出快速弛豫过程。在单层WSe及其与石墨烯的异质结中，B/A'激子表现出负的自旋/谷极化，这主要是由于双光子吸收和激发玻色子散射。我们的工作系统地研究了激子的自旋/谷去极化动力学，并揭示了它们在孤立的二维WSe和vdW异质结中差异的可能机制。
