SYNTHESIS AND OPTICAL PROPERTIES OF LIQUID CRYSTALLINE COMPOSITES OF CADMIUM CAPRYLATE WITH CdSе NANOPARTICLES

Abstract

The conditions for the synthesis of CdSе nanoparticles in ionic liquid crystal melts based on cadmium caprylate have been studied depending on the method of introducing selenide ions (selenourea) into the reaction medium, the reaction time and temperature. Glassy li­quid crystal composites of cadmium caprylate with monodisperse CdSе nanoparticles of various sizes have been obtained. It has been shown that the electronic absorption and fluo­rescence spectra of the obtained composites are related to the size of CdSе nanoparticles, i.e. they demonstrate a quantum-size effect.

It was found that the size of CdSe NPs depends on the synthesis temperature, but practically does not depend on the reaction time. The optimal duration of the synthesis is 2 hours: at 100 °С, you can get CdSe nanoparticles with a size of 1 ± 0.2 nm, at 180 °С – 2 ± 0.2 nm, and at 220 °С – 3 ± 0.2 nm.

A characteristic feature of the absorption spectra of obtained nanocomposites is the dua­lity of the exitonic absorption band maxima, which indicates a special quasi-two-dimensional electronic structure of the synthesized nanoparticles. In this case, the first short-wave absorption peak can be due to the electronic transition from the energy level of the valence band of light holes to the energy level of the conduction band, and the second peak is associated with the same transition of heavier holes.

It was found that fluorescence spectra of the nanocomposites studied have narrow short-wave and wide long-wave regions. The first maxima refer to exciton fluorescence, while it should be noted that the values of the Stokes shift and the half-width of the band are insignificant, which indicate the monodispersity of the sizes of the obtained nanoparticles. The broad long-wavelength band of luminescence refers to impurity luminescence and is caused by the presence of surface traps that bind electron-hole pairs and, thus, cause intense non-radiative trap luminescence. At the same time, with an increase in the size of nanoparticles, the contribution of non-radiative luminescence decreases, which can be explained, in our opinion, both by a decrease in the specific surface area of CdSe NPs and, in part, by an increase in the degree of crystallinity of NPs due to an increase in the temperature of their synthesis.