Exciton delocalizing ligands (EDLs) are of interest to researchers due to their ability to allow charge carriers to spread into the ligand shell of semiconductor nanocrystals (NCs). By increasing charge carrier surface accessibility, EDLs may facilitate the extraction of highly photoexcited carriers from NCs prior to their relaxation to the band edge, a process that can boost the performance of NC-based photocatalysts and light harvesting systems. However, hot carrier extraction must compete with carrier cooling, which could be accelerated by the stronger interaction of charge carriers and EDLs. This study describes the influence of the EDL phenyldithiocarbamate (PTC) on the intraband electron and hole cooling rates of CdSe NCs. Using state-resolved transient absorption spectroscopy we find that PTC exchange accelerates hole cooling by a factor of 1.7. However, upon further treatment of these NCs with cadmium(II) acetate, the hole cooling rate reverts to the value measured prior to PTC treatment, yet these NCs maintain a red-shifted absorption spectrum indicative of PTC bound to the NC surface. This result provides strong evidence for the existence of two distinct surface-bound PTC species, one that traps holes before they cool and can be removed by cadmium(II) acetate, and a second species that facilitates exciton delocalization. This conclusion is supported by DFT calculations, as well as photoluminescence and FT-IR measurements which suggest a change in the average binding geometry of PTC upon treatment with cadmium(II) acetate. The outlook from our work is that EDLs do not necessarily lead to an acceleration of carrier cooling, suggesting that they may provide a path for hot carrier extraction. We are currently working to extend this study to understand how EDLs and their binding geometry impact charge carrier delocalization in NC thin films.

Effects of exciton-delocalizing ligands on intraband relaxation in semiconductor nanocrystals

253rd National Meeting (2017)

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