Multi-exciton Dynamics inType II ZnTe/ZnSe Colloidal Quantum Dots

Colloidal quantum dots (CQD) with a core/shell structure are classed as Type II if the electron and hole are confined within different volumes; in contrast, both carriers are confined to the same region in a Type I CQD. The wave-function overlap in this type of structure depends on both the core diameter and shell thickness and hence can be controlled during synthesis. This enables the multi-exciton lifetime1 and the exciton-exciton (X-X) interaction energy2 to be increased by, for instance, growing a thicker shell, and thus enhances the suitability of CQD for applications such as optical amplification3. To date, however, studies have been solely on CQD containing Cd, which is severely restricted because of its toxicity. Here we report on multi-exciton dynamics in a novel Cd-free type II CQD.Ultrafast transient absorption spectroscopy was used to investigate four different ZnTe/ZnSe core/shell samples. In static samples and at high pump power, the transient included a bi-exponential decay which was ascribed to the recombination of both biexcitons and trions within the sample. At moderate pump powers, however, a single exponential decay was observed for vigorously stirred samples and this observation was attributed to suppression of the photo-charging responsible for the formation of trions by the stirring process – see Figure 1. Well-stirred samples yielded biexciton recombination transients with lifetimes that increased with core diameter and shell thickness, as expected. We also show that the magnitude of these decay transients is determined not just by state-filling, as is the case for Type I CQD, but also depends on the spectral shifts produced by the X-X interaction. Finally, significant excited state absorption was observed at energies less than the band gap.