Tuning high-order multiexciton properties of CdSe quantum dots via size and surface modification

Excitonic properties of colloidal quantum dots are strongly dependent on size and surface properties. The same is predicted to be valid for the properties of multiexcitons. To realize applications exploiting the generation of multiexcitons in colloidal nanocrystals, it requires a comprehensive understanding of size and surface influence e.g., on the lifetimes of multiexciton species. In this study, we employ intensity-dependent transient absorption spectroscopy to probe multiexcitons in long-chained organic (phosphonic acids and trioctylphosphine oxide), and short inorganic ligand (sulfide, S2−) capped colloidal CdSe quantum dots of four different sizes. To analyse the intensity-dependent transient absorption data a global fit method based on Markov Chain Monte Carlo sampling was employed. Applying a simple Auger recombination model lifetimes and spectra of multiexciton species were analysed. The spectra obtained for different multiexciton species exhibit both size and surface functionalization dependent features, which allow us to distinguish between different species. Independent of the surface modification, we find that the multiexciton lifetimes follow the volume scaling laws established earlier. Owing to the strong surface hole trapping induced by the S2− ligands, S2−-capped QDs show prolonged multiexciton lifetimes compared to the QDs capped with the native organic ligands with long alkyl chains. Deconvoluting contributions of bleach, stimulated emission and photoinduced absorption in the species spectra enables us to determine multiexciton binding energies. We observe a decrease of binding energies with increasing size and a clear reduction in multiexciton binding energies for the S2− -capped QDs. The observed trends can be explained by changes in the overlap of electron and hole wave functions depending on the QDs diameter and the charge carrier localization which can be induced by trapping in surface defect sites.