Highly concentrated 0.5 M II-VI semiconductor quantum dot solutions for coating applications can be synthesized employing chalcogenolysis and condensation of functionalized cluster-like cadmium and zinc ethoxyacetates. Furthermore, in nucleation studies on CdSe solutions, new magic clusters between 0.42 and 1.7 nm in size were found exhibiting sharp HOMO-LUMO resonances (lowest absorption features) in the optical absorption spectra. High resolution small angle X-ray scattering (SAXS) measurements performed on 1.7 and 3.4 nm CdSe clusters corroborate the size. Information on the intra-cluster structure was hard to derive with respect to the small cluster size. These species could be Koch pyramids with a fractal dimension Df=2 as well as non-fractal zincblende pyramids (additionally checked by XRD and HFtTEM). In any case rather chain-like (Dt= 1) aggregates are formed. It further will be shown that in alcoholic CdSe sols the initially nucleated "seeds" are highly reactive. Their sharp HOMO-LUMO transitions are found to be strongly modified by externally induced chemical reactions. For example, aminosilane capped 1.7 nm clusters decompose rapidly upon exposure to phosphines. After a period of few hours, they begin to re-grow to their original size or they reorganize to give smaller 0.85 nm subunits depending on the P/N ratio. In contrast, 0.85 nm phosphinecapped clusters double their size if exposed to amines. The last process liberates cadmium ions into the solution as found in complementary polarographic measurements. ~ allel to this work and the sol-gel chemistry of metal oxides, addressing metal ethoxy-acetate derived synthesis of II-VI chalcogenide quantum dots, to provide insights into the cluster-cluster aggregate evolution mechanism within the strong exciton confinement regime. We shall demonstrate that chemical surface reactions can change both the cluster optical absorption and fluorescence spectra related to structural changes within cluster-cluster aggregates. This spectroscopic work is supported by SAXS-, XRDand HRTEM investigations addressing a fractal character of highly concentrated semiconductor cluster materials. Experimental GeneralAU manipulations involving silylchalcogenides and phosphines were carried out under argon using the Schlenk technique. Cadmium and zinc acetate dihydrate were purchased from Fluka. Bis(trimethylsily1)selenium (TMS)2Se prepared according to the procedure described elsewhere [15] was stored at 240 K under argon. Bis(trimethylsily1)sulfide (TMS)$3 and bis(trimethylsily1)tellurium (TMS)2Te were purchased from Aldrich and Acros respectively. Anhydrous heptane, pyridjne, tetrahydrofuran (THF) and 2-butoxyethanol in addition to aUcyl amines and tributylphosphine (TBP) were purchased from Aldrich. 3-Aminopropyltriethoxy-silane (AMEO) was purchased from ABCR. All chemicals were used without additional purification.

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A new organometallic “cold−slow” route to strongly fluorescing CdTe/CdS (core−shell) colloids and transparent films is presented. Based on the optical absorption, fluorescence, FTIR, micro-Raman, XPS, and XRD data collected on these nanostructures before and after thermal annealing, a mechanistic path of the core−shell formation and thermal break up is proposed and discussed. The processing of the CdTe/CdS nanostructures starts with 0.5 M tributylphosphine (TBP) stabilized CdS colloid in dichloromethane as a solvent. This yellow colloidal oil composed of 3−4 nm CdS clusters is reacted with liquid Bis(trimethylsilyl)-telluride (TMS2Te) in the presence of excess insoluble CdCl2 salt. During this reaction, a rapid chalcogen atom exchange occurs within a few seconds which produces a new CdTe “core”. The expelled sulfide reacts slowly with the CdCl2 salt to form new CdS clusters after several hours. Furthermore, this “CdS-formation-driven CdCl2 salt dissolution” activates a strong green-yellow fluorescence indicating a possible evolution of a “core−shell”-like CdTe/CdS structure. Thermal sintering of the subsequently prepared CdTe/CdS films between 100 and 200 °C completely suppresses the fluorescence and initiates CdTe cluster growth, reflecting a high thermal sensitivity of the “core−shell” interfaces. By further raising the sintering temperature to 300−400 °C, the TBP ligands are released and, consequently, bare CdS- and CdTe nanocrystals, as well as ternary nanocrystalline CdTe x S1 - x phases, start forming. Above 400 °C, the CdTe part of the nanostructures sublimates, yielding (111)-oriented CdTe 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 μm (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.

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