Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type ll and CdS/CdSe Inverted Type I Core-Shell: A Structural Analysis through Carrier-Quenching Study

We have employed femtosecond transient absorption spectrocopy to monitor charge carrier delocalization in CdSe/CdS quasi-type II and CdS/CdSe inverted type I core-shell nanocrystals (NCs). Interestingly, CdSe and CdS QD pairs can make both type I and quasi-type II core-shell structures, depending on their band alignment and charge carrier localization. Steady-state optical absorption and luminescence studies show a gradual red-shift in both optical absorption and emission spectra in CdSe/CdS core-shell with increasing CdS shell thickness. The luminescence quantum yield in CdSe/CdS core-shell drastically increases with shell thickness. Notably, CdS/CdSe inverted core-shell shows a huge red-shift both in absorption and luminescence which closely matches with the band edge photoluminescence (PL) of pure CdSe QDs (shell). However, the luminescence quantum yield does not change much with shell thickness. Depending on their band energy level alignment, the charge carrier (electron and hole) delocalization in both the core-shells have been demonstrated using electron (benzoquinone, BQ) and hole (pyridine, Py) quencher. The bleach recovery kinetics of CdS/CdSe core-shell recovers faster in the presence of both BQ and Py. However, for CdSe/CdS core-shell, the bleach recovers faster only in the presence of BQ while the bleach dynamics remain unaffected in the presence of Py. Our experimental observations suggest that in CdSe/CdS quasi-type II core-shell, photoexcited electrons are localized in CdS shell and holes are localized in CdSe core; however, in CdS/CdSe inverted core-shell both electrons and holes are localized in the CdSe shell.