Institute of Chemistry, The Hebrew University , Jerusalem 91904, Israel.
ACS Nano. 2015 Feb 24;9(2):2138-47. doi: 10.1021/nn5074868. Epub 2015 Feb 10.
Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1Se1Sh transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pump-probe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pump-probe experiments on a sample saturated with single relaxed excitons prove that the resulting 1Se1Sh bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1Se1Sh bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states.
带边光激发 PbSe 纳米晶会诱导强烈的带隙下吸收以及在 1Se1Sh 跃迁中多相的漂白积累。这些特征的幅度和动力学与双激子位移和态填充的预期值不符，这是通常用来解释它们的机制。为了澄清这些差异，本文通过双泵浦探测光谱学研究了相同的跃迁。结果表明，在带隙下再激发可以以高概率产生额外的激子。此外，对单弛豫激子饱和的样品进行泵浦-探测实验证明，所得的 1Se1Sh 漂白与每个纳米晶的激子数不成线性关系。这一发现适用于两个在尺寸上有显著差异的样品，证明了其普遍性。结果分析表明，热激子态下的带边下吸收是由于激发态吸收而不是冷载流子的位移吸收，并且当样品激子的分布包括多激子态时，1Se1Sh 漂白信号不是样品激子的准确计数器。
