Abstract
Interdot coherent excitonic dynamics in nanometric colloidal CdSe quantum dots (QD) dimers lead to interdot charge migration and energy transfer. We show by electronic quantum dynamical simulations that the interdot coherent response to ultrashort fs laser pulses can be characterized by pump-probe transient absorption spectroscopy in spite of the inevitable inherent size dispersion of colloidal QDs. The latter, leading to a broadening of the excitonic bands, induce accidental resonances that actually increase the efficiency of the interdot coupling. The optical electronic response is computed by solving the time-dependent Schrodinger equation including the interaction with the oscillating electric field of the pulses for an ensemble of dimers that differ by their size. The excitonic Hamiltonian of each dimer is parameterized by the QD size and interdot distance, using an effective mass approximation. Local and charge transfer excitons are included in the dimer basis set. By tailoring the QD size, the excitonic bands can be tuned to overlap and thus favor interdot coupling. Computed pump-probe transient absorption maps averaged over the ensemble show that the coherence of excitons in QD dimers that lead to interdot charge migration can survive size disorder and could be observed in fs pump-probe, four-wave mixing, or covariance spectroscopy.
Highlights
Colloidal nanometric metallic and semi conducting quantum dots (QDs) have long been used as versatile building blocks for assembling dimers and larger extended arrays [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
Effect of the QD Size Dispersion on Excitonic Level Structure In Figure 4, we show examples of the effect of the size dispersion on the energies of the zero order exciton levels and of the eigen excitons for ensembles of 1000 dimers of three different types: In panel A, a heterodimer built with QDA and QDB of mean diameters equal to DA = 2.35 nm and DB = 3.85 nm with 3% size dispersion and a surface to surface linker distance L = 0.2 nm
There is no exact resonance between excitonic bands on each dot, but the bands h1eA and h2eB overlap, which leads to efficient interdot coupling between these two local excitons
Summary
Colloidal nanometric metallic and semi conducting quantum dots (QDs) have long been used as versatile building blocks for assembling dimers and larger extended arrays [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Sci. 2020, 10, 1328 applications to lasing, sensing, light harvesting, and information processing, they have recently received renewed attention focusing on understanding the role of the fine structure coupling [20,21,22,23,24], electronic and vibrational coherences [25,26,27,28,29,30,31,32] and coupling to biexcitons [30,33,34,35,36,37], electron transport in arrays [38,39,40,41], and the possibility of engineering ultrafast energy transfer between a donor and an acceptor dot [10,11]. We focus in this paper on investigating theoretically coherent energy and charge transfer in ensembles of small (2 to 4 nm diameter) heterodimers and homodimers of CdSe QDs. The inevitable inherent size distribution of colloidal QDs leads to a distribution in electronic properties (exciton energies and wave functions and their couplings). Investigating the effects resulting from the size dispersion in the two dots that constitute a dimer is the main motivation of this paper
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