SOL-GEL SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF THICK NANOCRYSTALLINE CDSE FILMS

Two novel metal alkoxide-derived routes were developed for the synthes is of nanocrystalline CdSe layers with quantum dot sizes between 1 and 4 nm. The first route, where cadmium ethoxy-acetate is reacted with b is(trimethylsilyl)selenium in the presence of aminopropyltriethoxysila ne (AMEO), yields highly concentrated alcoholic 0.5 M sols for direct coatings, The second route allows to grow CdSe clusters by infiltratin g the selenium precursor into Cd-enriched organosilicate gel layers. T he resulting optically transparent films with thicknesses near 10 mu m (obtained in a single-step coating) were characterized by steady-stat e optical absorption and photoluminescence spectroscopy, high-resoluti on electron microscopy (HRTEM), X-ray diffraction (XRD), resonance Ram an, and time-resolved photoluminescence spectroscopy. The experimental data reveal the presence of nanocrystals exhibiting a tetrahedral sha pe. The quantum dot films are strongly fluorescing, with a quantum yie ld near 10%. The decay characteristics of the photoluminescence signal after pulsed excitation is discussed taking into account the splittin g of the quantum dot ground state as well as the influence of surface states. Furthermore, a size-dependent shift of the Raman band, attribu ted to the longitudinal optical phonon of the consolidated CdSe cluste rs, could be observed. This shift is accompanied by a broadening of th e corresponding Raman line width. Both effects, the size-dependent shi ft as well as the broadening of the Raman line width, indicate that ph onon confinement is present for the clusters under consideration.