Ultrafast Charge Transfer and Trapping Dynamics in a Colloidal Mixture of Similarly Charged CdTe Quantum Dots and Silver Nanoparticles

The interaction between colloidal CdTe quantum dots (QDs) and silver nanoparticles (Ag NPs), both in their negatively charged state, is studied to determine the importance of surface charge and epitaxial coupling of the two components to achieve the potential activity of the metalsemiconductor nanocomposites. An unexpected strong interaction between the two similarly charged species is evident from dramatic quenching of the excitonic emission of the QDs by the Ag NPs. Direct evidence of electron transfer from the photoexcited QDs to Ag NPs is obtained from an accelerated bleach recovery of the first exciton band of the QDs and a faster carrier cooling in the presence of Ag NPs in transient absorption measurements. The evidence of charge carrier trapping is demonstrated by the observation of a new broad positive transient absorption in the visible-near-infrared region, which was absent in their isolated counterparts. The electron transfer and charge carrier trapping processes are further substantiated by the results of similar measurements on core/shell (CdTe/ZnS) QDs. As ultrafast charge carrier (especially the hole) trapping slows the charge recombination process, the present findings open up new possibilities of harnessing solar energy and improving the photocatalytic activity by employing these colloidal mixtures.