Photon Antibunching in Small Clusters of CdSe/ZnS Core/Shell Quantum Dots

Photon correlations in the collective emission of hybrid gold-(CdSe/CdS/CdZnS) nanocrystal supraparticles

We investigate the photon statistics of the light emitted by single self-assembled hybrid gold-CdSe/CdS/CdZnS colloidal nanocrystal supraparticles through the detailed analysis of the intensity autocorrelation functiong(2)(τ). We first reveal that, despite the large number of nanocrystals involved in the supraparticle emission, antibunching can be observed. We then present a model based on non-coherent Förster energy transfer and Auger recombination that well captures photon antibunching. Finally, we demonstrate that some supraparticles exhibit a bunching effect at short time scales corresponding to coherent collective emission.

Symmetry-Breaking Charge Separation in a Chiral Bis(perylenediimide) Probed at Ensemble and Single-Molecule Levels

Chiral molecular assemblies exhibiting symmetry-breaking charge separation (SB-CS) are potential candidates for the development of chiral organic semiconductors. Herein, we explore the excited-state dynamics of a helically chiral perylenediimide bichromophore (Cy-PDI2) exhibiting SB-CS at the ensemble and single-molecule levels. Solvent polarity-tunable interchromophoric excitonic coupling in chiral Cy-PDI2 facilitates the interplay of SB-CS and excimer formation in the ensemble domain. Analogous to the excited-state dynamics of Cy-PDI2 at the ensemble level, single-molecule fluorescence lifetime traces of Cy-PDI2 depicted long-lived off-states characteristic of the radical ion pair-mediated dark states. The discrete electron transfer and charge separation dynamics in Cy-PDI2 at the single-molecule level are governed by the distinct influence of the local environment. The present study aims at understanding the fundamental excited-state dynamics in chiral organic bichromophores for designing efficient chiral organic semiconductors and applications toward charge transport materials.

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

Photon antibunching in a cluster of giant CdSe/CdS nanocrystals

When closely packed into a high-density film, semiconductor nanocrystals (NCs) can interact with each other to yield collective optical behaviours, which are normally difficult to characterize due to the ensemble average effect. Here we synthesized semiconductor NC clusters and performed single-particle spectroscopic measurements to probe the electronic couplings of several giant CdSe/CdS NCs contained in one cluster with nanometer-scale separations. Such a single cluster exhibits multiple emission peaks at the cryogenic temperature with nearly identical photoluminescence decay dynamics, suggesting that the Förster-type energy transfer does not occur among the composing NCs. Surprisingly, strong photon antibunching is still observed from a single cluster, which can be attributed to the Auger annihilation of photo-excited excitons from different NCs. The isolation of several nearby NCs interacting with the above novel mechanism has marked a solid progress towards a full understanding and an efficient control of the operation parameters in NC-based optoelectronic devices.