Ding Tao, Liang Guijie, Wang Junhui, Wu Kaifeng
State Key Laboratory of Molecular Reaction Dynamics , Collaborative Innovation Center of Chemistry for Energy Materials (iChEM) , Dalian Institute of Chemical Physics , Chinese Academy of Sciences , Dalian , China 116023 . Email:
Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices , Hubei University of Arts and Science , Xiangyang , Hubei 441053 , China.
Chem Sci. 2018 Aug 1;9(36):7253-7260. doi: 10.1039/c8sc01926f. eCollection 2018 Sep 28.
Heterostructured colloidal nanocrystals, such as core/shells and dot-in-rods, enable new spectral and dynamic properties otherwise unachievable with single-component nanocrystals or quantum dots (QDs). For example, the electron and hole wavefunctions can be engineered such that they are either both confined in the same domain or (partially) separated over different domains in the heterostructures, which are the so-called type I or (quasi-) type II localization regimes, respectively. A critical factor dictating the carrier localization regime is the band alignment or electronic structure of the heterostructure, which, however, is difficult to measure and hence is often ambiguous. In this work, using CdSe@CdS dot-in-rods (DIRs) as a model system, we show that band edge carrier-doping is a simple-yet-powerful tool to probe the electronic structure of heterostructures. By doping an electron into the CdSe core and then observing whether the doped electron bleaches band edge absorption of only the core or those of both the core and shell, we can easily differentiate the type I and quasi-type II structures. A systematic study of DIRs with various dimensions shows that the extent of electron wavefunction delocalization can be tuned by the core sizes and rod diameters. Comparison with the electronic structure determined from transient absorption measurements also reveals the important role of electron-hole binding in affecting the delocalization of electron wavefunction. In addition to probing the electronic structure, the doped electron allows for studying multi-carrier recombination dynamics in these heterostructures which plays a vital role in their many optical and optoelectronic applications. Specifically, by comparing the band edge exciton recombination kinetics of the doped and neutral DIRs, we can extract the negative trion lifetime, which can be further used to derive the positive trion lifetime when combined with biexciton lifetime measurements. These lifetimes also depend sensitively on the core sizes and rod diameters of the DIRs.
异质结构胶体纳米晶体，如核壳结构和棒中量子点结构，能够实现单组分纳米晶体或量子点（QD）无法实现的新光谱和动力学特性。例如，可以设计电子和空穴波函数，使它们要么都限制在同一区域，要么（部分）在异质结构的不同区域分离，这分别是所谓的I型或（准）II型定位机制。决定载流子定位机制的一个关键因素是异质结构的能带排列或电子结构，然而，这很难测量，因此常常不明确。在这项工作中，我们以CdSe@CdS棒中量子点（DIR）作为模型系统，表明带边载流子掺杂是探测异质结构电子结构的一种简单而强大的工具。通过向CdSe核中掺杂一个电子，然后观察掺杂电子是否仅使核的带边吸收漂白，还是使核和壳的带边吸收都漂白，我们可以轻松区分I型和准II型结构。对不同尺寸的DIR进行系统研究表明，电子波函数的离域程度可以通过核尺寸和棒直径进行调节。与通过瞬态吸收测量确定的电子结构进行比较，也揭示了电子-空穴结合在影响电子波函数离域方面的重要作用。除了探测电子结构外，掺杂电子还可以用于研究这些异质结构中的多载流子复合动力学，这在它们的许多光学和光电子应用中起着至关重要的作用。具体而言，通过比较掺杂和中性DIR的带边激子复合动力学，我们可以提取负三激子寿命，当与双激子寿命测量相结合时，该寿命可进一步用于推导正三激子寿命。这些寿命也对DIR的核尺寸和棒直径敏感地依赖。
