| Photoluminescence spectra and fluorescence quantum yields of CdSe nanocrystals capped with octyldecylamine ligands were measured as a function of nanocrystal concentration. It was found that higher concentrations of nanocrystals caused both a drop in the fluorescence quantum yield and a red-shift of the peak emission intensity. Utilizing a 2mm micro-cuvette for photoluminescence measurements along with a micro-translational stage, this effect was determined to be a consequence of reabsorption of the emission. Employing a novel experimental setup, we were able to almost completely remove the reabsorption effect from the experimental measurements. The resulting system yielded fluorescence that still showed quenching at higher concentrations, but also showed a significant blue-shift in the emission spectrum. These results were explained as being due to collisional self-quenching and shown to fit well with the Stern-Volmer equation. Additional measurements showed that the Stern-Volmer constant increases with increasing nanocrystal size. The blue-shift with increasing concentration rules out possible Forster energy transfer in solution, and is consistent with observed size-dependent collisional quenching. This is the first reported observation of size-dependent collisional self-quenching in nanocrystal solutions. Some additional solvent effects on the fluorescence are presented.; Nanocrystal-polymer multilayer light emitting diodes were fabricated using CdSe/CdS core-shell nanocrystals that demonstrated uniform pixel emission and narrow electroluminescent spectral profiles (∼25 nm FWHM). Such devices exhibited an initial extended turn-on time of up to 20 hours to reach maximum brightness accompanied by monotonic increases in resistance and temperature over the operational lifetime. Operating the devices at reverse bias results in a "dark" period of little or no luminescence during subsequent forward bias operations, after which, the luminescence recovers to the permanently decayed value. This recovery turn-on time varies linearly with the reverse bias duration and exponentially with the reverse bias current. A simple charge trapping model with an exponential energy distribution of traps is proposed to explain the experimental results. Some additional results concerning observations of enhanced emission of a secondary higher energy peak for CdSe/CdS nanocrystals with thick (∼4 monolayers) US shells is presented and discussed. |
