In situ investigation of two-step nucleation and growth of CdS nanoparticles from solution.

We report on a combined ultra-fast in situ SAXS and WAXS study along a free-jet providing insight into the evolution of the morphology and crystalline structure of CdS quantum dots in the very early stage of nucleation between 100 µs and 2.5 ms with a time resolution down to 10 µs. Accessing this yet unexplored time regime provides direct evidence of a two-step mechanism via formation of prenucleation clusters followed by nanoparticle nucleation from coalescing precursors. Using ab initio calculations, the latter species is identified as Cd13S4(SH)18 clusters, the stability of which results from a compact surface and inner structure.