Probing Surface Saturation Conditions in Alternating Layer Growth of CdSe/CdS Core/Shell Quantum Dots

We monitor effective band gap energy shifts and free reagent concentration during the formation of CdS shells on CdSe nanocrystals to test the hypothesis that alternating addition of stoichiometric doses of precursors can effectively saturate surface sites and thereby enforce conformal shell growth. The selective ionic layer addition and reaction (SILAR) mechanism has been proposed to describe growth under such conditions, and the method is attractive because of the opportunity to (1) avoid cross-reaction of precursors in growing binary films in solution and (2) enforce conformal growth, rather than regioselective growth, by saturating all available surface sites in a self-limiting manner in each half-cycle. The strong red shift that takes place when CdS shells are grown on CdSe cores provides a convenient process monitoring tool that complements scanning transmission electron microscopy imaging and analytical measurements of free reagent concentration. We find that, under commonly used conditions, a cadm...