No abstract

As luminescence applications of colloidal semiconductor nanocrystals push toward higher excitation flux conditions, there is an increased need to both understand and potentially control emission from multiexciton states. We develop a spectrally resolved correlation method to study the triply excited state that enables direct measurements of the recombination pathway for the triexciton, rather than relying on indirect extraction of rates. We demonstrate that, for core–shell CdSe–CdS nanocrystals, triexciton emission arises exclusively from the band-edge S-like state. Time-dependent density functional theory and extended particle-in-a-sphere calculations demonstrate that reduced carrier overlap induced by the core–shell heterostructure can account for the lack of emission observed from the P-like state. These results provide a potential avenue for the control of nanocrystal luminescence, where core–shell heterostructures can be leveraged to control carrier separation and therefore maintain emission color purity over a broader range of excitation fluxes.

Hybrid perovskites have emerged as a promising material candidate for exciton-polariton (polariton) optoelectronics. Thermodynamically, low-threshold Bose-Einstein condensation requires efficient scattering to the polariton energy dispersion minimum, and many applications demand precise control of polariton interactions. Thus far, the primary mechanisms by which polaritons relax in perovskites remains unclear. In this work, we perform temperature-dependent measurements of polaritons in low-dimensional perovskite wedged microcavities achieving a Rabi splitting of $${{{\hslash }}\Omega }_{{Rabi}}$$ ℏ Ω R a b i  = 260 ± 5 meV. We change the Hopfield coefficients by moving the optical excitation along the cavity wedge and thus tune the strength of the primary polariton relaxation mechanisms in this material. We observe the polariton bottleneck regime and show that it can be overcome by harnessing the interplay between the different excitonic species whose corresponding dynamics are modified by strong coupling. This work provides an understanding of polariton relaxation in perovskites benefiting from efficient, material-specific relaxation pathways and intracavity pumping schemes from thermally brightened excitonic species.

Coupling of excitations between organic fluorophores in J-aggregates leads to coherent delocalization of excitons across multiple molecules, resulting in materials with high extinction coefficients, long-range exciton transport, and, in particular, short radiative lifetimes. Despite these favorable optical properties, uses of J-aggregates as high-speed light sources have been hindered by their low photoluminescence quantum yields. Here, we take a bottom-up approach to design a novel J-aggregate system with a large extinction coefficient, a high quantum yield and a short lifetime. To achieve this goal, we first select a J-aggregating cyanine chromophore and reduce its nonradiative pathways by rigidifying the backbone of the cyanine dye. The resulting conformationally-restrained cyanine dye exhibits strong absorbance at 530 nm and fluorescence at 550 nm with 90% quantum yield and 2.3 ns lifetime. We develop optimal conditions for the self-assembly of highly emissive J-aggregates. Cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) reveal micron-scale extended structures with 2D sheet-like morphology, indicating long-range structural order. These novel J-aggregates have a strong red-shifted absorption at 600 nm, resonant fluorescence with no Stokes shift, 50% quantum yield, and 220 ps lifetime at room temperature. We further stabilize these aggregates in a glassy sugar matrix and study their excitonic behavior using 2 temperature-dependent absorption and fluorescence spectroscopy. These temperaturedependent studies confirm J-type excitonic coupling and superradiance. Our results have implications for the development of a new generation of organic fluorophores that combine high speed, high quantum yield and solution processing.

Interactions between excitons and molecular vibrational modes limit the extent of exciton delocalization and rate of energy transport in organic molecular aggregates, diminishing their performance in many optical device applications. This coupling leads to exciton self-trapping and subsequently changes their emission behavior. Certain amphiphilic cyanine dyes form nanotubular aggregates that demonstrate high exciton transport rates and show no such coupling between excitons and molecular vibrational modes. However, under sustained illumination these aggregates undergo photobrightening (PB) and can show a doubling in quantum yield. We investigate this reversible PB process through spectral- and time-resolved photoluminescence (PL) measurements under low illumination intensities. We observe lengthening exciton lifetimes with no corresponding spectral change. Furthermore, wide-angle X-ray scattering measurements show a change in the aggregate structure following PB. We propose a model of PB through large polaron formation, leading to trapping or shielding of these long coherence length excitons through interactions with supramolecular vibrations rather than the intramolecular vibrations typically observed in other aggregates. These excitons are then less able to access nonradiative recombination sites, which leads to the observed increase in quantum yield. The lattice deformations persist after emission and accumulate over time, resulting in brightening of the aggregate under sustained illumination. We support this model through temperature-, color-, and matrix-dependent photoluminescence measurements and show that the model correctly predicts the changes observed upon addition of a FRET quencher. Finally, we demonstrate control over PB behavior through rigidification of the aggregate with a silica shell, potentially enabling the development of long-term-photobrightened devices utilizing molecular aggregates with significantly higher photoluminescence quantum yields.