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

Two novel metal alkoxide-derived routes ere developed for the synthesis of nanocrystalline CdSe layers with quantum dot sizes between 1 and 4nm. The first route, where cadmium ethoxy-acetate is reacted with bis(trimethylsilyl)selenium in the presence of aminopropyltriethoxysilane (AMEO), yields highly concentrated alcoholic 0.5M sols for direct coatings. The second route allow to grow CdSe clusters by infiltrating the selenium precursor into Cd-enriched organosilicate gel layers. The resulting optically transparent films with thicknesses near 10um (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 per cent

Density functional study on the morphology and photoabsorption of CdSe nanoclusters

The geometrical and electronic structures of a series of small CdSe quantum dots protected by various ligands have been studied by density functional theory. UV-vis spectra have been calculated by time-dependent density functional theory (TDDFT). The goal of this investigation is the rationalization of the basic properties of these systems, in particular, the nature of the exciton peaks. This study has been focused on the (CdSe)(x), x = 13, 19, 33, and 66, "magic-size" clusters that are characterized by high stability and large optical gaps. The geometries of the cluster are relaxed both in vacuum and in the presence of the surfactant ligands. To describe the interaction between the bare clusters and the surfactants, model types of ligands are introduced: fatty acids are modeled using formic and acetic acid and amines are modeled using ammonia and methyl amine. Present calculations demonstrate that the ligands play a crucial role in stabilizing the structure in a bulklike geometry and strongly affect the optical gap of the clusters, due to an optimal coordination of the surface atoms. For these "magic-size" clusters, the UV-vis spectrum is calculated at the TDDFT level. The calculated spectra are in good agreement with the experimental ones for clusters with the same dimension capped with the same type of Uganda. This suggests that our structures are realistic models of the actual quantum dots