Femtosecond (fs) broad-band transient absorption (TA) is used to study the intra-band relaxation and depopulated dynamics of electron and hole quantized states in CdSe nanocrystals (NC's) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a "phonon bottleneck", we observe a fast sub-picosecond 1P-to-1S relaxation, with the rate enhanced in NC's of smaller radius. We use fs IR TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole pathways leading to the depopulation of NC quantized states. In contrast to electron relaxation, which is controlled by NC surface passivation, depopulated of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at surface defects, but rather arise from relaxation into intrinsic NC states.
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机译：飞秒（fs）宽带瞬态吸收（TA）用于研究具有各种表面特性的CdSe纳米晶体（NC）中的电子和空穴量子态的带内弛豫和失种群动力学。我们观察到的不是由于“声子瓶颈”而导致的能量弛豫率的急剧降低，而是观察到了皮秒级的1P到1S的快速弛豫，在较小半径的NC中，弛豫率得到了提高。我们使用fs IR TA来探测电子和空穴的带内跃迁，这使我们能够区分导致NC量化状态减少的电子和空穴通路。与通过NC表面钝化控制的电子弛豫相反，在所有类型的样品中，空穴量子化状态的减少都非常快（sub-ps-ps时标），而与NC表面处理（包括用ZnS覆盖的NC's）无关层）。我们的结果表明，超快孔动力学不是由于表面缺陷处的陷阱所致，而是由松弛到固有的NC状态引起的。
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