School of Electrical and Computer Engineering, Cornell University , Ithaca, New York 14850, United States.
Nano Lett. 2015 Jan 14;15(1):339-45. doi: 10.1021/nl503636c. Epub 2014 Dec 31.
In this Letter, we present nondegenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼ 2 ps and a slow time scale that lasts longer than ∼ 100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two-dimensional atomic materials, together with strong electron and hole correlations in two-dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two-dimensional atomic materials.
在这封信件中，我们展示了非简并超快光泵浦探测研究在 MoS2 单层中的载流子复合动力学。通过将探针调谐到远长于激子线的波长，我们使探针传输对光激发电子和空穴的总浓度敏感。我们的测量揭示了两个不同的时间尺度，其中光激发的电子和空穴复合；一个快时间尺度持续约 2 ps，一个慢时间尺度持续超过约 100 ps。观察到的载流子动力学的温度和泵浦强度依赖性与缺陷辅助复合一致，这是电子-空穴复合的主要机制，其中电子和空穴通过俄歇过程被缺陷捕获。二维原子材料中的强库仑相互作用，以及二维金属二卤化物中的强电子和空穴相关性，使得俄歇过程对于缺陷对载流子的捕获特别有效。我们提出了一个载流子复合动力学模型，该模型定量地解释了我们在不同温度和泵浦强度下所有数据的特征。俄歇载流子捕获的速率常数的理论估计与实验确定的值非常吻合。我们的结果强调了俄歇过程在二维原子材料中所扮演的重要角色。
