Quasi-Type II Carrier Distribution in CdSe/CdS Core/Shell Quantum Dots with Type I Band Alignment
Quasi-Type II Carrier Distribution in CdSe/CdS Core/Shell Quantum Dots with Type I Band Alignment
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DOI：
10.1021/acs.jpcc.7b11684
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发表时间：
2018-06-07
影响因子：
3.7
通讯作者：
Tamai, Naoto
中科院分区：
文献类型：
--
作者：
Wang, Li;Nonaka, Kouhei;Tamai, Naoto
带对准对于理解核/壳量子点的光学性质和载流子转移是必不可少的。随着壳层厚度的增加，CdSe/CdS核壳量子点的吸收光谱和发光光谱发生红移，最低电子跃迁振荡强度减弱，发光寿命延长，属于准II型带取向。然而，态选择激发的飞秒瞬态吸收光谱显示具有薄CdS壳层的CdSe/CdS量子点具有I型波段排列，其中激发态电子在核心激发的CdSe核心中定位，而在壳层激发的整个量子点中脱域，尽管在稳态光谱中观察到准II型载流子分布。在I型核壳量子点中，CdS壳层在表面电子和空穴捕获过程中起着能量屏障的作用。CdSe核的空穴捕获过程的时间常数（类似于10ps）由于通过CdS壳层高能势垒的隧道效应而延长了10倍，这是由双激子诱导的光谱位移相关的衰减估计的。在1S(e)-2S(3/2)(h)跃迁中也观察到类似于100 ps空穴捕获过程引起的双激子谱移。本文的研究结果有助于阐明异质纳米结构的能带对准和载流子分布，有助于客观地提取光伏应用中的载流子。
Band alignments are essential for understanding the optical properties and carrier transfer of core/shell QDs. As CdSe/CdS core/shell QDs with increasing shell thickness represent red shifted absorption and luminescence spectra, weakened oscillator strength of the lowest electronic transition, and elongated luminescence lifetime, they are assigned to quasi-type II band alignment. However, femtosecond transient absorption spectroscopy with state-selective excitation revealed a type I band alignment of the CdSe/CdS QDs with a thin CdS shell, in which the excited electron is localized in the CdSe core with core excitation while delocalized in the whole QDs with shell excitation, even though a quasi-type II carrier distribution was observed with steady-state spectroscopy. In the type I core/shell QDs, the CdS shell acts as an energy barrier in surface electron and hole-trapping processes. The time constant of the hole-trapping process of the CdSe core (similar to 10 ps) was elongated 10 times owing to a tunnel effect through the high energy barrier of the CdS shell, which was estimated from the decay related to the biexcitonic induced spectral shift. The biexcitonic spectral shift induced by a,similar to 100 ps hole-trapping process was also observed at the 1S(e)-2S(3/2)(h) transition. Our results from transient absorption spectroscopy with state-selective excitation are useful to clarify band alignment and carrier distribution of hetero-nanostructures, which could help to objectively extract charge carriers in photovoltaic applications.