Wave Function Engineering for Ultrafast Charge Separation and Slow Charge Recombination in Type II Core/Shell Quantum Dots

Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
Journal of the American Chemical Society (Impact Factor: 12.11). 06/2011; 133(22):8762-71. DOI: 10.1021/ja202752s
Source: PubMed


The size dependence of optical and electronic properties of semiconductor quantum dots (QDs) have been extensively studied in various applications ranging from solar energy conversion to biological imaging. Core/shell QDs allow further tuning of these properties by controlling the spatial distributions of the conduction-band electron and valence-band hole wave functions through the choice of the core/shell materials and their size/thickness. It is possible to engineer type II core/shell QDs, such as CdTe/CdSe, in which the lowest energy conduction-band electron is largely localized in the shell while the lowest energy valence-band hole is localized in the core. This spatial distribution enables ultrafast electron transfer to the surface-adsorbed electron acceptors due to enhanced electron density on the shell materials, while simultaneously retarding the charge recombination process because the shell acts as a tunneling barrier for the core localized hole. Using ultrafast transient absorption spectroscopy, we show that in CdTe/CdSe-anthraquinone (AQ) complexes, after the initial ultrafast (~770 fs) intra-QD electron transfer from the CdTe core to the CdSe shell, the shell-localized electron is transferred to the adsorbed AQ with a half-life of 2.7 ps. The subsequent charge recombination from the reduced acceptor, AQ(-), to the hole in the CdTe core has a half-life of 92 ns. Compared to CdSe-AQ complexes, the type II band alignment in CdTe/CdSe QDs maintains similar ultrafast charge separation while retarding the charge recombination by 100-fold. This unique ultrafast charge separation and slow recombination property, coupled with longer single and multiple exciton lifetimes in type II QDs, suggests that they are ideal light-harvesting materials for solar energy conversion.

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    • "Electron-hole excitations in semiconductor quantum dots are influenced by their size, shape and chemical composition. Controlling the generation and the dissociation of electronhole (eh) pairs have important technological applications in the field of light-harvesting materials[1] [2] [3] [4], photovoltaics[5– 8], solid-state lighting[9] [10] [11] [12] and lasing[13] [14] [15] [16]. "
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    ABSTRACT: The electron-hole correlation length serves as an intrinsic length scale for analyzing excitonic interactions in semiconductor nanoparticles. In this work, the derivation of electron-hole correlation length using the two-particle reduced density is presented. The correlation length was obtained by first calculating the electron-hole cumulant from the pair density,and then transforming the cumulant into intracular coordinates, and finally then imposing exact sum-rule conditions on the radial integral of the cumulant. The excitonic wave function for the calculation was obtained variationally using the electron-hole explicitly correlated Hartree-Fock method. As a consequence, both the pair density and the cumulant were explicit functions of the electron-hole separation distance. The use of explicitly correlated wave function and the integral sum-rule condition are the two key features of this derivation. The method was applied to a series of CdSe quantum dots with diameters 1-20 nm and the effect of dot size on the correlation length was analyzed.
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    • "This can increase the ratio of charge-transfer rate to the electron–hole recombination rate.86,87 This effect was demonstrated by a comparison of charge transfer and recombination dynamics between CdSe, CdTe, type-I CdSe/ZnS, and type-II CdTe/CdSe core/shell nanocrystals and the electron acceptor AQ (Figure 4).86 In Section 3, we describe how type-II nano-heterostructures are utilized to improve charge separation and increase the yields of photocatalytic reactions. "
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    ABSTRACT: In addition to the size dependent optical and electronic properties of semiconductor quantum dots (QDs), quantum confinement also affects the charge separation and recombination dynamics in QD - charge acceptor complexes. It leads to enhanced amplitudes of electron and hole wave functions at the surface, enabling ultrafast interfacial charge transfer, an important property for the application of QDs in photovoltaic and photocatalytic devices. In this proceeding, we show that both charge separation and recombination are ultrafast in strongly quantum confined PbS QDs adsorbed with electron acceptors. Using CdSe/ZnS type I and CdTe/CdSe type II core/shell QDs as model systems, we show that the spatial distributions of electron and wave functions can be optimized to simultaneously achieve ultrafast charge separation and retard charge recombination.
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