Coherent Exciton Dynamics in Ensembles of Size-Dispersed CdSe Quantum Dot Dimers Probed via Ultrafast Spectroscopy: A Quantum Computational Study

Abstract

Featured Application: Our results on the quantum electronic dynamics of quantum dots pave the way to several applications in nanoscience. The large density of purely excitonic coherences occurring on a wide range of time scales can be controlled by selecting the mean size of the two quantum dots (QDs) used to build the dimer and adjust to applications, for example, to light harvesting and information-processing devices.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.

Room Temperature Inter-Dot Coherent Dynamics in Multilayers Quantum Dot Materials

“…51 More details about the modeling and the values of the parameters are given in the SI and ref. 51 .…”

Massively parallel classical logic via coherent dynamics of an ensemble of quantum systems with dispersion in size

“…We chose to use a model Hamiltonian based on the effective mass model (EMA) with the k.p method (38,40,41,45,46) to describe an individual dot and describe the coupling with an excitonic Hamiltonian including both local and CT excitons. As discussed before (47) . this enables us to realistically model the effect of size variation of the individual dots and also account for variations in the interdot distance.…”

“…The eigenexcitons can be designated by their zero order label because the intra and interdot Coulomb interactions are weak. In an ensemble of size dispersed dimers, each level in panel C is the mean value of an energy distribution that for moderate size dispersion (up to 5% in diameter) remain Gaussian, see SI sections 1 and 2 and ref (47) for additional details, validation of the energy levels model and figures.…”

“…See section S1, Model excitonic Hamiltonian, in the SI and ref. (47) for additional details. The level structure of the eigenstates shown in Figure 1C is computed for a dot separation of 0.55 nm and dot diameters = 2.4 nm and = 3.6 nm.…”

Featuring the State of the Art of Nanosciences in Belgium

“…Effective mass approximation combined with the k • p method was used by Gattuso et al [5] to predict exciton dynamics in a large assembly of CdSe quantum dot dimers. In close connection with the experiment, they calculated the optical response of the dimer assembly to sequences of femtosecond laser pulses similar to what is used in pump-probe transient absorption spectroscopy.…”