The room-temperature luminescence of single CdSe/ZnS core-shell quantum dots is investigated by spectrally and temporally resolved confocal microscopy. A large (30 nm) blue shift is observed in the emission wavelength during illumination in air. In nitrogen, no blue shift is observed. The blue shift in air is ascribed to a 1 nm shrinkage of the CdSe core by photooxidation. Photobleaching occurs about 4 times faster in air than in nitrogen, indicating the formation of nonradiative recombination centers during photooxidation. The initial light output is higher in air than in nitrogen, which may be due to a reduction of the defect state lifetime by oxygen.

Raman spectroscopy experiments found the V@O stretching frequency for the supported VO 4 species to decrease with increasing catalyst temperature. Calculations on the vibrational frequencies of several models using density functional theory show that a consistent description of the experimental data can be obtained if we assume that the VO 4 species are anchored to the oxidic surface by one V-O bond only, in contrast to the traditional pyramidal model, which assumes three V-O support bonds and one V@O. The proposed VO 3 structure points away from the surface and consists of one V@O unit and an active oxygen ÔmoleculeÕ loosely bound to the vanadium atom, a peroxide species.

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 1 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/CdSeO x 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.

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Strobes are pyrotechnic compositions which show an oscillatory combustion; a dark phase and a flash phase alternate periodically. The strobe effect has applications in various fields, most notably in the fireworks industry and in the military area. All strobe compositions mentioned in the literature were discovered by trial and error methods and the mechanisms involved remain unclear. Many oscillatory systems such as Belousov-Zhabotinsky reactions, cool flames, self-propagating high-temperature synthesis have been observed and theories developed to elucidate their unstable behavior based on chemical interactions or based on physical processes. These systems are compared to experimental observations made on strobe mixtures.