Ultrafast Exciton Dynamics in Colloidal CdSe/CdS Octapod Shaped Nanocrystals

We studied the carrier dynamics in colloidal octapod-shaped cadmium selenide/cadmium sulfide (CdSe/CdS) nanocrystals in the solution phase via pump-probe optical techniques with subpicosecond resolution. We could resolve bleaching from two different types of electronic states having distinct dynamics and assigned them to states delocalized in the pods and mildly localized in the core based on the good agreement of energies found with effective mass modeling. Contrary to other CdSe/CdS core/shell nanocrystals, such a mild localization has geometrical origins as the best agreement was found for negligible conduction band offset. Moreover, even though the large surface of the CdSe/CdS heterointerface results into a weak signature of electron trapping in the bleaching spectrum, we found that a relevant fraction of electrons do remain delocalized in pod states for long times and are thus available for diffusion in photovoltaic applications where the highly branched geometry is expected to advantageously yield to effective percolation in dense assemblies.