Kinetics of Isovalent (Cd 2+ ) and Aliovalent (In 3+ ) Cation Exchange in Cd 1-x Mn x Se Nanocrystals.

Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chemical transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+ . For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad experimental temperature range. At any given temperature, cation exchange by In3+ is approximately 2 orders of magnitude faster than that by Cd2+ , illustrating a critical dependence on the incoming cation. Quantitative analysis of the kinetics data within a Fick's-law diffusion model yields diffusion barriers (ED ) and limiting diffusivities (D0 ) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of ED ≈ 1.1 eV are found for both reactions (In3+ and Cd2+ ). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+ . Overall, these findings provide unique experimental insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.