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

Electron transfer (ET) dynamics from the 1Se electron state in quasi-type II CdSe/CdS core/shell quantum dots (QDs) to adsorbed methyl viologen (MV(2+)) were measured using femtosecond transient absorption spectroscopy. The intrinsic ET rate kET was determined from the measured average number of ET-active MV(2+) per QD, which permits reliable comparisons of variant shell thickness and different hole states. The 1Se electron was extracted efficiently from the CdSe core, even for CdS shells up to 20 Å thick. The ET rate decayed exponentially from 10(10) to 10(9) 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 1Se1S3/2 the electron coupled to the 2S3/2 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.