Shell effects on hole-coupled electron transfer dynamics from CdSe/CdS quantum dots to methyl viologen

Abstract

Electron transfer (ET) dynamics from the 1S_e electron state in quasi-type II CdSe/CdS core/shell quantum dots (QDs) to adsorbed methyl viologen (MV²⁺) were measured using femtosecond transient absorption spectroscopy. The intrinsic ET rate k_ET was determined from the measured average number of ET-active MV²⁺ per QD, which permits reliable comparisons of variant shell thickness and different hole states. The 1S_e electron was extracted efficiently from the CdSe core, even for CdS shells up to 20 Å thick. The ET rate decayed exponentially from 10¹⁰ to 10⁹ s⁻¹ for increasing CdS shell thicknesses with an attenuation factor β ≈ 0.13 Å⁻¹. We observed that compared to the ground state exciton 1S_e1S_3/2 the electron coupled to the 2S_3/2 hot hole state exhibited slower ET rates for thin CdS shells. We attribute this behaviour to an Auger-assisted ET process (AAET), which depends on electron–hole coupling controlled by the CdS shell thickness.