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The size and size-distribution of CdS nanocrystallites is controlled by the microwave-assisted reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF). The absorption onset of the CdS nanocrystallites shifts to longer wavelength with increasing irradiation time, indicating particle size growth under prolonged irradiation. However, when the microwave irradiation of the solution is periodically interrupted and then repeated, keeping the solution at ambient temperature before each irradiation, the absorption onset remains at the same wavelength and only the optical density of the absorption band is increased, suggesting that the particle growth occurs only during the continuous irradiation and stops once the system is cooled down. Photoluminescence observed for the CdS nanocrystallites changes with the irradiation time and the repetition of the irradiation, suggesting a structural change of the nanocrystallite surface due to the consumption of excess Cd2+.
Wada et al. (Mon,) studied this question.