Light Harvesting Semiconductor Core−Shell Nanocrystals: Ultrafast Charge Transport Dynamics of CdSe−ZnS Quantum Dots

Capping or functionalization of semiconductor quantum dots (QDs) is unavoidable for their photostability in practical use including sensitizer and biological tagging agents. However, the efficiency of the electron/hole transport from the photoexcited QDs to the external environments across the capping shell is not well-understood. In this study we report on the femtosecond carrier dynamics of core−shell type CdSe−ZnS semiconductor QDs of various sizes. Steady-state spectroscopic studies followed by picosecond-resolved time correlated single photon counting (TCSPC) experiments on the complexation of the QDs with a well-known electron acceptor, benzoquinone (BQ), reveal that the complex is essentially static in nature. Femtosecond-resolved fluorescence upconversion experiments on the complex explore the dynamics of electron transport from core CdSe to BQ via ZnS shell. The dependence of the electron transport dynamics on the core size of the QDs has also been explored. We have also studied the dynamics of electron transport from the core CdSe of various sizes to another TiO 2 nanoparticle as an electron acceptor across the shell. Our studies support the relevance of core shell type semiconducting quantum dots in light harvesting devices.