Ultrafast hole transfer in CdSe/ZnTe type II core−shell nanostructure

Abstract

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-II 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.