Quantized aggregation phenomena in II-VI-semiconductor colloids

Highly concentrated 0.5 M IT-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 D-f=2 as well as non-fractal zincblende pyramids (additionally checked by XRD and HRTEM). In any case rather chain-like (D-f=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 phosphine-capped clusters double their size if exposed to amines. The last process liberates cadmium ions into the solution as found in complementary polarographic measurements.