Effect of Electron-Hole Overlap and Exchange Interaction on Excitation Radiative Lifetimes of CdTe/CdSe Heteronanocrystals

Abstract

Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core–shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e–h pairs (excitons) that are responsible for the light emission, but also modifies the e–h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e–h exchange interaction, independently of the e–h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron–hole separation is systematically varied. We are able to unravel the separate effects of e–h overlap and e–h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields.