Electron Extraction Dynamics in CdSe and CdSe/CdS/ZnS Quantum Dots Adsorbed with Methyl Viologen

Semiconductor quantum dots (QDs) composed of multiple components are playing an important role in solar energy conversion as light harvesting materials. Electron extraction dynamics in CdSe and core/shell CdSe/CdS/ZnS colloidal QDs are studied by femtosecond transient absorption spectroscopy in this Article. Our study demonstrates that, in the presence of the commonly used electron acceptor, methyl viologen (MV²⁺), electrons in the 1S state of CdSe QDs can be effectively extracted with a time constant less than 150 fs. With regard to type I core/shell CdSe/CdS/ZnS QDs, 400 nm excitation will mainly populate the CdS first, due to its large absorption cross section at around that wavelength. Electrons from the conduction band of CdS then can be directly extracted by MV²⁺ before transferring to core CdSe. Therefore, MV²⁺ can serve as an efficient bridge to extract electrons from the shell of type I QDs. As compared to the bare QDs, core/shell QDs have slower charge separation and much slower recombination rates. Thus, the core/shell QDs are beneficial for designing solar cells