Department of Physics, North Carolina State University , Raleigh, North Carolina 27695, United States.
Nano Lett. 2014 Jan 8;14(1):202-6. doi: 10.1021/nl403742j. Epub 2013 Dec 13.
Single layer MoS2 is an ideal material for the emerging field of "valleytronics" in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron-hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.
单层 MoS2 是新兴的“谷电子学”领域的理想材料，在该领域中，通过光激发可以精细控制载流子动量。众所周知，该体系还表现出很强的多体相互作用，如紧密束缚的激子和三激子。在这里，我们通过超快瞬态吸收光谱报告了单层 MoS2 中谷弛豫动力学的直接测量结果。我们的结果表明，强库仑相互作用显著影响谷群动力学。圆偏振光的初始激发在 K 谷内产生电子-空穴对。这些激子与暗的谷间激子态相干耦合，从而促进了快速电子谷去极化。由于价带自旋态的非简并性，空穴谷弛豫延迟高达约 10 ps。谷间双激子的形成揭示了空穴谷弛豫动力学。我们观察到，与传统半导体相比，双激子的形成具有大一个数量级以上的结合能。这些测量为二维半导体中特定谷的过程提供了重要的见解。因此，它们可以用来建议设计半导体材料的途径，以实现对谷极化的控制。
