Time-resolved fluorescence spectroscopy study on the photophysical behavior of quantum dots

Room-temperature time-resolved luminescence measurements on single CdSe/ZnS quantum dots (QDs) are presented. Fluorescence emission spectra were recorded over periods of up to 30 minutes with a time resolution as small as 6 ms. For QDs in ambient air, a clear 30-40 nm blue shift in the emission wavelength is observed, before the luminescence stops after about 2-3 minutes because of photobleaching. In a nitrogen atmosphere, the blue shift is absent while photobleaching occurs after much longer times (i.e., 10-15 minutes). These observations are explained by photoinduced oxidation. The CdSe surface is oxidized during illumination in the presence of oxygen. This effectively results in shrinkage of the CdSe core diameter by almost I nm and consequently in a blue shift, The faster fading of the luminescence in air suggests that photoinduced oxidation results in the formation of non-radiative recombination centers at the CdSe/CdSeOx interface. In a nitrogen atmosphere, photoinduced oxidation is prevented by the absence of oxygen. Additionally, a higher initial light output for CdSe/ZnS QDs in air is observed. This can be explained by a fast reduction of the lifetime of the long-lived defect states of CdSe QDs by oxygen.