Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale
Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale
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DOI：
10.1021/jp2058673
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发表时间：
2011-11-17
影响因子：
3.7
通讯作者：
Kambhampati, Patanjali
中科院分区：
文献类型：
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作者：
Kambhampati, Patanjali
在半导体量子点（QDs）中以激子的形式限制电子和空穴的能力创造了一种既新颖又有潜在用途的电子结构，可用于各种应用。在点的光激发下，初始激子态可能是电子热的。热激子的弛豫动力学是控制光学增益、热载流子提取和多激子产生等关键过程的主要事件。在这里，我们描述了在胶体CdSe量子点上的飞秒状态分辨泵浦/探针实验，该实验提供了激子态到态跃迁速率的第一个定量测量。这里的测量和建模揭示了热电子和热空穴的弛豫有多条路径，直接的结果是电子和激子在量子点中没有声子瓶颈。这种不存在的声子弛豫是由点的各种激子态与光学声子和声学声子之间的弱激子声子耦合的独立测量证实的。我们表明，采用这种热激子弛豫动力学的多通道图可以调和先前结果的分歧。这幅图为设计针对特定应用的具有松弛特性的材料建立了一个框架。我们最后与热激子表面俘获有关。表面捕获过程是产生光产物的关键步骤，它可以模糊光增益、多激子复合、多激子产生和单点闪烁的测量。我们发现热激子表面捕获可以有效地与热激子弛豫竞争，从而混淆了这些过程。
The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton phonon coupling between the various excitonic states of the dot and the optical and acoustic phonons. We show that the divergence of prior results can be reconciled by adopting this multichannel picture of hot exciton relaxation dynamics. This picture establishes a framework for designing materials with relaxation properties targeted for specific applications. We conclude with connection to hot exciton surface trapping. The process of surface trapping is the key step in creation of the photoproduct which can obscure measurements of optical gain, multiexciton recombination, multiple exciton generation, and single dot blinking. We show that hot exciton surface trapping can effectively compete with hot exciton relaxation, thereby obfuscating these processes.