Sol-gel synthesis and spectroscopic properties of thick nanocrystalline CdSe films

Two novel metal alkoxide-derived routes were developed for the synthesis of nanocrystalline CdSe layers with quantum dot sizes between 1 and 4 nm. The first route, where cadmium ethoxy-acetate is reacted with bis(trimethylsilyl)selenium in the presence of aminopropyltriethoxysilane (AMEO), yields highly concentrated alcoholic 0.5 M sols for direct coatings, The second route allows to grow CdSe clusters by infiltrating the selenium precursor into Cd-enriched organosilicate gel layers. The resulting optically transparent films with thicknesses near 10 mu m (obtained in a single-step coating) were characterized by steady-state optical absorption and photoluminescence spectroscopy, high-resolution electron microscopy (HRTEM), X-ray diffraction (XRD), resonance Raman, and time-resolved photoluminescence spectroscopy. The experimental data reveal the presence of nanocrystals exhibiting a tetrahedral shape. The quantum dot films are strongly fluorescing, with a quantum yield near 10%. The decay characteristics of the photoluminescence signal after pulsed excitation is discussed taking into account the splitting of the quantum dot ground state as well as the influence of surface states. Furthermore, a size-dependent shift of the Raman band, attributed to the longitudinal optical phonon of the consolidated CdSe clusters, could be observed. This shift is accompanied by a broadening of the corresponding Raman line width. Both effects, the size-dependent shift as well as the broadening of the Raman line width, indicate that phonon confinement is present for the clusters under consideration.