Quantized aggregation phenomena in II‐VI‐semiconductor colloids

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Abstract
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 ethoxy‐acetates. 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 HRTEM). In any case rather chain‐like (Df= 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.