Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots
Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots
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DOI：
10.1063/5.0081192
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发表时间：
2022-02-07
影响因子：
4.4
通讯作者：
Lian, Tianquan
中科院分区：
文献类型：
--
作者：
He, Sheng;Li, Qiuyang;Lian, Tianquan
量子受限半导体纳米晶体的光电性质与其能带边、电子能级、空穴能级及其激子精细结构密切相关。暂态吸收光谱已被广泛用于探测这些材料中光生电子、空穴和激子的动力学，通过它们的态填充引起带边激子跃迁的漂白。原则上，这种效应反映了能带边缘精细结构，并对导带电子有很好的理解。然而，由于价带能级结构的复杂性和许多材料中存在快速空穴捕获，对价带空穴态填充信号的理解仍然很少。本文通过比较不同空穴捕获程度的CdSe量子点(QD)的TA谱和选择性去除吸附的甲基紫精分子的导带电子，研究了价带空穴态填充效应。我们观察到在光致发光量子产率为81%的CdSe/CdS核/壳量子点中，价带空穴贡献了22%的+/-1%的激子漂白，而在光致发光量子产率为17%的CdSe核/壳量子点中观察到可以忽略不计的空穴态填充信号。这种空穴态填充效应可以用一个简化的价带边空穴模型来解释，该模型包含两组双重简并空穴能级，分别负责高能亮激子态和低能暗激子态。我们的结果阐明了价带空穴的TA光谱特征，并为深入了解基于CdSe的量子点的单空穴态的性质提供了理论依据。
The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% +/- 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs.