Exciton-exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton-exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, with projected pump threshold densities below 1 kW cm(-2) for continuous wave lasing.

Intensity instabilities are a common trademark of the photoluminescence of nanoemitters. This general behavior is commonly attributed to random fluctuations of free charges and activation of charge traps reducing the emission yield intermittently. However, the actual physical origin of this phenomenon is actively debated. Here we devise an experiment, variable pulse rate photoluminescence, to control the accumulation of charges and the activation of charge traps. The dynamics of these states is studied, with pulse repetition frequencies from the single-pulse to the megahertz regime, by monitoring photoluminescence spectrograms with picosecond temporal resolution. We find that both photocharging and charge trapping contribute to photoluminescence quenching, and both processes can be reversibly induced by light. Our spectroscopic technique demonstrates that charge accumulation and trap formation are strongly sensitive to the environment, showing different dynamics when nanocrystals are dispersed in solution or deposited as a film.

We investigate a promising organic/inorganic hybrid composite for solution-processable optoelectronics made by lead sulphide nanoparticles and fullerene derivatives, which combine the sensitivity of PbS to the infrared spectrum with the good electron transport properties of fullerenes. Charge separation is the crucial process that determines whether the heterojunction can be the building block for devices converting photogenerated excitons into free charges flowing in a circuit. Subpicosecond spectroscopy techniques on bulk heterojunctions between PbS nanocrystals of various sizes and [6,6]-phenyl-61-butyric acid methyl ester (PCBM) were employed to reveal the ultrafast dynamics of photoexcited carriers, particularly transfer of photoexcited electrons from nanocrystals to PCBM. Electron transfer is found to critically depend on nanoparticle size, occurring for nanocrystals with diameter 4.4 nm and smaller, not for larger ones. Our findings are relevant to the engineering of hybrid solar cells and light detectors based on PbS nanocrystal/fullerene bulk heterojunctions

CdSe/CdS colloidal nanocrystals are members of a novel class of light-emitting nanoparticles with remarkable optical properties such as suppressed fluorescence blinking and enhanced emission from multiexciton states. These properties have been linked to the suppression of non-radiative Auger recombination. In this work we employ ultrafast spectroscopy techniques to identify optical signatures of neutral and charged excitonic and multiexcitonic states. We show that Auger recombination of biexcitons is not suppressed, while we observe optical gain and amplified spontaneous emission from multiexciton states and from long-lived charged-exciton states.

The spectroscopic features of dye aggregates in organic-inorganic Rhodamine 6G-silica hybrids are investigated in sol-gel prepared type II bulk and thick film samples in the 10(-4)-10(-3) mol l(-1) concentration range. No aggregates are observed in film samples irrespective of the dye concentration and the spectroscopic features are ascribed to fluorescent monomers. On the contrary, by means of excitation and emission fluorescence measurements and the analysis of fluorescence decay kinetics the formation of fluorescent aggregates is reported in bulk samples and the geometry of the formed aggregates at the silica surface is discussed on the single exciton theory framework. Beside the distribution of a single emitting specie observed in bulk samples and individuated as oblique J dimers, the analysis of the excitation spectra indicate also the presence of "dark'' monomer units. An energy transfer mechanism is hypothesized to explain the interaction of the monomer-dimer pair