Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type II
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