Cation exchange, in which the parent cations of a solid material are exchanged for a different type of cation while the anionic framework is preserved, is a kinetically driven transformation that occurs at room temperature in chalcogenide nanocrystals.   We are investigating the mechanism of the cation exchange reaction: is this a diffusion-limited cation-by-cation place exchange or a reaction driven by a nucleation stage or an intermediate phase?  Such knowledge can allow us to control the outcome of the reaction and achieve desirable morphologies and compositions.  In particular, we studied the exchange of CdSe nanocrystals with Ag+ and Cu+ ions by means of excitonic absorption spectroscopy and dopant emission spectroscopy.  Preliminary excitonic absorption results exhibit a sigmoidal titration curve, suggesting the presence of co-operative effects in the exchange reaction.  Emission spectroscopy reveals the formation of dopant-related bands induced by the incorporation of the non-native cationic impurities.  The dopant emission band rises with increasing cation concentration, until it reaches a maximum, following which it decreases, eventually reaching zero upon full exchange.  The results strongly suggest that heavy doping of the nanocrystal by the non-native cationic impurities is a precursor to the exchange reaction.  We propose a model for the cation exchange of a nanocrystal as a rapid chemical transition, nucleated by a stage of heavy doping.  The results could be applicable to a wide range of solid-state chemical transitions on the nanoscale.

Probing a dopant-induced transformation in semiconductor nanocrystals

246th National Meeting (2013)

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