Cation exchange is quickly becoming a powerful tool for accessing a wide variety of novel nanostructures. Nanocrystal compositions that were previously impossible to make or would have required entirely new synthetic procedures may be made in seconds starting from some of most well-understood materials in nanochemistry. Describing how such reactions take place on an atomistic level will expand the scope of possible products, e.g., doped or alloyed nanocrystals. I will describe our work on imaging single nanocrystals chemically transform from one chemical phase to another and the pathway followed in such a transformation. From fluorescence signatures of these nanocrystals, we find that each nanocrystal undergoes an abrupt transformation from the parent phase to the product phase. Further analyses of the kinetics show that these abrupt transitions are not simply governed by diffusion-limited exchange events but rather are driven by a cooperative effect within each nanocrystal in which early foreign atom incorporations lower the energetic barrier for successive exchange events to occur. We have also observed that early incorporations driving this process can readily be monitored through the blinking of the nanocrystals. There are changes to the single particle blinking behavior that appear to be sensitive to the formation of otherwise unseen reaction intermediates. This understanding of a single nanoparticle's fluorescence during the course of the reaction provides a picture of the reaction mechanism that would otherwise be unseen through ensemble studies.

Cation exchange at the single nanocrystal level: Mechanistic and kinetic insights

251st National Meeting (2016)

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