Controlled surface trap state photoluminescence from CdS QDs impregnated in poly(methyl methacrylate)

Abstract

A method of microwave (MW) assisted synthesis was employed to prepare cadmium sulfide (CdS) quantum dots (QDs) in dimethylformamide in the presence of poly(methyl methacrylate) (PMMA). The MW irradiation was carried out for a fixed time of 20-30 s and the size of QDs varied from 2.9-5.5 nm. Before each irradiation the solution was cooled down to ambient temperature and the irradiation process was repeated six times. An increase in the intensity and red shift of the characteristic UV-vis absorption peak originating from CdS QDs were observed with repeated MW irradiation, suggesting that the amount of generated CdS QDs increased within the PMMA network and aggregated with repeated MW irradiation. MW irradiation could influence selectively the nucleation and growing rates of PMMA-CdS QDs systems. The broadness and large Stokes shift of the emission from Cd(2+)-rich PMMA-CdS QDs was due to the surface trap state photoluminescence. The recombination of shallow trapped electrons and shallow trapped holes has been considered as the primary source of the surface trap state photoluminescence in Cd(2+)-rich PMMA-CdS QDs. The photoluminescence lifetime was observed to be decreased sharply when the amount of QDs was less, showing the emission decay was dependent on the surface property of PMMA-CdS QDs. The origin of the longer lifetime was due to the involvement of surface trap states and dependent on the amount of CdS QDs present within PMMA and its environment. The effect of the concentration of Cd(2+), S(2-) and PMMA on the generation of CdS QDs within PMMA and the effect of repeated MW irradiation on the optical properties was studied and the results are discussed in this article.