Effect of CdS shell thickness on the optical properties of water-soluble, amphiphilic polymer-encapsulated PbS/CdS core/shell quantum dots

Cation exchange, recently explored for synthesizing core/shell quantum dots (QDs), causes continuous core size change during shell formation. By carefully varying parent PbS QD size and cation exchange conditions, we have synthesized PbS/CdS core/shell QDs with a similar PbS core size of ∼4.5 nm yet a different CdS shell thickness. This enables us to study the effect of shell thickness on the properties of PbS QDs after their transfer from chloroform into watervia poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). It was found that the quantum yield (QY) of PbS cores in water firstly increases with shell thickness up to ∼0.7 nm, reaching 33%, owing to better surface passivation and then decreases to 1.7% when the shell thickness reaches 2.3 nm. Such decline is due to the formation of new defects with shell deposition. In contrast, the variation amplitude of QY during water transfer monotonically decreases and QD photostability monotonically improves with shell thickness. It is clear that although newly introduced defects play a fundamental role in the absolute QY, they do not show any overwhelmingly negative effects on the variation of QY with environments and photostability. The colloidal stability of QDs in buffers containing different salt concentrations seems to be not affected by the shell thickness, possibly due to the same steric stabilization effect of the amphiphilic polymer in all samples. Further investigation on a series of core/shell samples confirms that ∼0.7 nm is an optimal shell thickness for various core sizes investigated herein, consistently yielding the maximum QY and reasonably good photostability.