Single-Nanocrystal Reaction Trajectories Reveal Sharp Cooperative Transitions

Whereas pathways of chemical reactions
involving small molecules
are well-understood, the dynamics of reactions in extended solids
remain difficult to elucidate. Frequently, kinetic studies on bulk
materials provide a picture averaged over multiple domains or grains,
smearing out interesting dynamics such as critical nucleation phenomena
or sharp phase transitions occurring within individual, often nanoscale,
grains, or domains. By optically monitoring a solid-state reaction
with single nanocrystal resolution, we directly identified a unique,
previously unknown, reaction pathway. Reaction trajectories of single
cadmium selenide nanocrystals undergoing ion exchange with silver
reveal that each individual nanocrystal waits a unique amount of time
before making an abrupt switch to the silver selenide phase on a few
hundred millisecond time scale. The gradual reaction progress of ensemble-scale
cation exchange is actually comprised of these sharp single-nanocrystal
switching events. Statistical distributions of waiting times suggest
that the reaction is a cooperative transition rather than a diffusion-limited
cation-by-cation exchange, which is confirmed by a stochastic reaction
model. Such insight, achievable from single nanocrystal reaction studies,
furthers mechanistic understanding of heterogeneous reactions, solid-state
catalysis, bottom-up nanostructure growth, and materials’ transformations
and degradation in reactive environments.