Ultrafast Hole Transfer in CdSe/ZnTe Type II Core-Shell Nanostructure

We have synthesized thiol-capped CdSe/ZnTe quantum dot core-shell nanostructures by colloidal methods, have characterized them by steady-state absorption and photoluminescence (PL) spectroscopy and further confirmed by high resolution transmission electron microscopy and X-ray diffraction measurements. Clear red shift on shell formation was observed in optical absorption and photoluminescence studies. Time-resolved emission studies indicate longer emission lifetime of CdSe/ZnTe core-shell as compared to CdSe QD material where in both cases only CdSe gets excited, which indicates spatial charge separation in type-II core-shell. Ultrafast photoinduced charge transfer dynamics in type-H CdSe/ZnTe donor-acceptor core-shell were studied in real-time using femtosecond broadband pump-probe spectroscopy. Our transient absorption studies suggests that on photoexcitation core-shell hole transfer from CdSe core to ZnTe shell takes place in pulse-width limited time scale as evidenced by an increase in cooling dynamics of the charge carriers from 150 fs for CdSe to 300 fs for thickest CdSe/ZnTe core shell. Increase in cooling dynamics in core shell has been explained due to decoupling of electron and hole in photoexcited core-shell. Trapping dynamics play a major role in the excited dynamics of the photoexcited charge carriers of quantum dot materials. Bleach recovery kinetics of the photoexcited QD materials fitted multi-exponentially where 2.5 ps (first component) has been attributed to the electron trapping dynamics and the longer components (30-50 ps and >400 ps) attributed to the charge recombination dynamics.