Exciton Dynamics in CdS-Ag2S Nanorods with Tunable Composition Probed by Ultrafast Transient Absorption Spectroscopy

Electron relaxation dynamics in CdS-Ag2S nanorods have been measured as a function of the relative fraction of the two semiconductors, which can be tuned via cation exchange between Cd2+ and Ag+. The transient bleach of the first excitonic state of the CdS nanorods is characterized by a biexponential decay corresponding to fast relaxation of the excited electrons into trap states. This signal completely disappears when the nanorods are converted to Ag2S but is fully recovered after a second exchange to convert them back to CdS, demonstrating annealing of the nonradiative trap centers probed and the robustness of the cation exchange reaction. Partial cation exchange produces heterostructures with embedded regions of Ag2S within the CdS nanorods. Transient bleaching of the CdS first excitonic state shows that increasing the fraction of Ag2S produces a greater contribution from the fast component of the biexponential bleach recovery, indicating that new midgap relaxation pathways are created by the Ag2S material. Transient absorption with a mid-infrared probe further confirms the presence of states that preferentially trap electrons on a time scale of 1 ps, 2 orders of magnitude faster than that of the parent CdS nanorods. These results suggest that the Ag2S regions within the heterostucture provide an efficient relaxation pathway for excited electrons in the CdS conduction band.