Tuning high-order multiexciton properties of colloidal 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, a comprehensive understanding of size and surface influence, e.g., on the lifetimes of multiexciton species, is required. In this study, we employ intensity-dependent transient absorption spectroscopy to probe multiexcitons in colloidal CdSe quantum dots of four different sizes capped with long-chained organic (majorly trioctylphosphine oxide) or short inorganic (sulfide, S^2-) ligands. To analyze 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, the lifetimes and spectra of multiexciton species were analyzed. The spectra obtained for different multiexciton species exhibit both size and surface functionalization dependent features, allowing 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 S^2- ligands, S^2--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 S^2--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.