Kinetics of Isovalent (Cd²⁺) and Aliovalent (In³⁺) Cation Exchange in Cd_1-xMn_xSe 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 (E_D) and limiting diffusivities (D₀) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of E_D ≈ 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.