Blueing, bleaching, and blinking of single CdSe/ZnS quantum dots

Room temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) are presented. Luminescence spectra were recorded over time periods of up to 30 min with a time resolution down to 6 ms. A clear 30 - 40 nm blue shift in the emission wavelength is observed for QDs in ambient air, before the luminescence stops after about 2 - 3 min due to photobleaching. The blue shift is absent in a nitrogen atmosphere and photo-bleaching occurs after much longer times, 10 - 15 min. These observations are explained by photoinduced oxidation. During illumination in the presence of oxygen the surface of CdSe is oxidized, which effectively results in shrinkage of the CdSe core diameter by almost 1 nm and, consequently, in a blue shift. This also influences the blinking behavior. The faster fading of the luminescence in air suggests that photoinduced oxidation results in the formation of nonradiative recombination centers at the CdSe/ CdSeO_x interface. In a nitrogen atmosphere photoinduced oxidation is prevented by the absence of oxygen. A surprising observation is a higher initial light output for CdSe/ZnS QDs in air. We explain this higher light output by a fast reduction of the lifetime of the long-lived defect states of CdSe/ZnS QDs by oxygen.