Semiconductor quantum dot (QD) assemblies are promising systems for light harvesting and energy conversion and transfer, as they have a superior photostability compared to classical dyes and their absorption and emission properties can be tuned during synthesis. Here, we investigate excitonic energy transfer in self-assembled dentrite-type fractal structures consisting of QDs by microscopically mapping their fluorescence spectra and lifetimes. The behaviors of CdSe/ZnS and CdTe QD assemblies are compared; in particular, the energy transfer probability is found to be stronger in CdTe-based structures, scaling with their radiation quantum yield. Our results indicate Förster-type energy transfer in both systems, although with a higher efficiency in CdTe. The energy transfer is caused by near-field (nonradiative) dipole–dipole coupling between the individual QDs within a dendrite, with the excitation migrating from the edges to the center of the structure. The experimental findings are supported by theoretical modeling results obtained by using master equations for exciton migration/decay kinetics in diffusion-limited fractal aggregates composed of identical particles.

Hybrid materials formed by semiconductor quantum dots and J-aggregates of cyanine dyes provide a unique combination of enhanced absorption in inorganic constituents with large oscillator strength and extremely narrow exciton bands of the organic component. The optical properties of dendrite structures with fractal dimension 1.7-1.8, formed from J-aggregates integrated with CdTe quantum dots (QDs), have been investigated by photoluminescence spectroscopy and fluorescence lifetime imaging microscopy. Our results demonstrate that (i) J-aggregates are coupled to QDs by Förster-type resonant energy transfer and (ii) there are energy fluxes from the periphery to the centre of the structure, where the QD density is higher than in the periphery of the dendrite. Such an anisotropic energy transport can be only observed when dendrites are formed from QDs integrated with J-aggregates. These QD/J-aggregate hybrid systems can have applications in light harvesting systems and optical sensors with extended absorption spectra.

No abstract

We implement an efficient numerical method to calculate response functions of complex impurities based on the Density Matrix Renormalization Group (DMRG) and use it as the impurity-solver of the Dynamical Mean Field Theory (DMFT). This method uses the correction vector to obtain precise Green's functions on the real frequency axis at zero temperature. By using a self-consistent bath configuration with very low entanglement, we take full advantage of the DMRG to calculate dynamical response functions paving the way to treat large effective impurities such as those corresponding to multi-orbital interacting models and multi-site or multi-momenta clusters. This method leads to reliable calculations of non-local self energies at arbitrary dopings and interactions and at any energy scale.