Spectroscopy and Femtosecond Dynamics of Water Soluble Type I CdSe/ZnS Core–Shell Quantum Dot

Abstract

Thiol-capped type I CdSe/ZnS core–shell quantum dot nanostructures have been synthesized at low temperature in water; and then characterized by steady-state absorption and photoluminescence (PL) studies and high resolution TEM (HRTEM) measurements. On excitation of CdSe quantum dot predominantly surface state emission was detected, however on ZnS shell formation prominent exciton emission of CdSe with much higher overall quantum yield was observed. Femtosecond up-conversion measurements reveal that exciton emission found to decay much faster as compared to that of surface state emission. Lifetime of CdSe exciton emission found to increases with ZnS shell thickness. Femtosecond transient absorption studies have been carried out on these QD and core–shell material at 400 nm laser light and monitored the transients in the visible region to study charge carrier dynamics in ultrafast time scale. On laser excitation electron–hole pairs are generated which are detected by induced absorption signal for the charge carriers in visible region and immediate bleach at excitonic position for both QD and QD core–shell. Carrier quenching studies has been carried out for both CdSe and CdSe/ZnS by using benzoquinone (BQ, electron quencher) and pyridine (Py, hole quencher) suggest that although CdSe/ZnS form type I core–shell, still both electron and hole found to be leaked through ZnS shell from CdSe core.