Exciton formation in monolayer transition metal dichalcogenides
Exciton formation in monolayer transition metal dichalcogenides
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DOI：
10.1039/c6nr02516a
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发表时间：
2016-01-01
期刊：
影响因子：
6.7
通讯作者：
Zhao, Hui
中科院分区：
文献类型：
--
作者：
Ceballos, Frank;Cui, Qiannan;Zhao, Hui
二维过渡金属二硫属化物为研究受限结构中的激子提供了独特的平台。最近，人们对这些材料中激子的几个重要方面进行了详细研究。然而，自由载流子形成激子的过程仍有待了解。在这里，我们报告了 MoS2、MoSe2、WS2 和 WSe2 单层样品中激子形成过程的时间分辨测量。由超短激光脉冲注入的自由电子-空穴对立即感应出调谐到激子共振的探针脉冲的瞬态吸收信号。信号在 1 ps 内迅速下降约两倍，随后是较慢的衰减过程。相反，当激子共振注入时，不存在快速衰减分量。基于其激发过剩能量和强度依赖性，这种快速衰减过程归因于电子-空穴对形成激子。这种解释也与一个模型一致，该模型显示自由电子-空穴对在改变激子吸收强度方面比激子有效大约两倍。根据我们的测量和结果分析，我们确定这些单层中的激子形成时间短于 1 ps。
Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps.