An Ultrafast Transient Absorption Study of Charge Separation and Recombination Dynamics in CdSe QDs and Methyl Viologen: Dependence on Surface Stoichiometry

The dependence of the dynamics of charge separation and recombination between methyl viologen (MV+2) and photo-excited CdSe quantum dots (QDs) on the surface stoichiometry of the latter is studied for QDs with different Cd:Se mole ratios employing ultrafast time-resolved absorption and emission measurements. The electron transfer rates between photoexcited CdSe QDs and MV+2 are measured directly by monitoring the ultrafast rise and decay of the transient absorption signal due to methyl viologen monocationic radical (MV+center dot). The results show that both forward and back electron transfer rates (5 +/- 1 x 10(12) s(-1) and 1.3 +/- 0.3 x 10(10) s(-1), respectively) are independent of the stoichiometry of the QDs. Interestingly, the efficiency of the electron transfer process, estimated from the yield of MV+center dot, shows significant dependence on the nature of the QDs, with maximum yield (Phi(rs)= 0.52 +/- 0.01) observed in the case of Cd-rich QDs. These findings are explained considering the energetics and surface trap states of these systems.