Insights into the interparticle mixing of CsPbBr3 and CsPbI3 nanocubes: halide ion migration and kinetics

Abstract

Anion exchange of CsPbX₃ nanocrystals (NCs) is an easy pathway to tune the bandgap over the entire visible region. Even the mixing of pre-synthesized CsPbBr₃ and CsPbI₃ NCs at room temperature leads to the formation of mixed halide CsPbBr_3−xI_x NCs. Understanding the reaction mechanism and the kinetics of interparticle mixing is essential for fundamental aspects and device applications. Here, we probed the kinetics of ion migration through time-dependent steady-state photoluminescence (PL) spectroscopy. We found three primary PL peaks after the mixing of NCs—bromide side peak, iodide side peak, and a new peak that emerges during the reaction. The reaction follows first-order kinetics and the activation energy is 0.75 ± 0.05 eV. We propose that the free oleylammonium halides which are in dynamic equilibrium with the NCs, eventually promote interparticle mixing that follows the anion migration from the surface to the core of the nanocrystal, which is the rate-limiting step. Overall, the inherent reaction rate between the halide anions and the nanocrystals governs the reaction kinetics.