Interfacial electron transfer from CdSe/ZnS quantum dots to TiO2 nanoparticles: size dependence at the single-molecule level.

Electron transfer (ET) kinetics of CdSe/ZnS core/shell quantum dots (QDs) on bare coverslips and a TiO(2) nanoparticle-coated thin film has been investigated at the single-molecule level. The QDs prepared have three different diameters of 3.6, 4.6, and 6.4 nm. The trajectories of fluorescence intensity are acquired with respect to the arrival time. The on-time events and subsequent fluorescence lifetimes are shorter with decreasing size. Given the lifetime measurements for QDs on glass and TiO(2), the rate constant of ET from QDs to TiO(2) may be determined to be 1.3×10(7), 6.0×10(6), and 4.7×10(6) s(-1) for the increasing sizes of the QDs. The plot of on-time probability density versus arrival time is characterized by power-law statistics in the short time region and a bending tail in the long time region. Marcus's ET model is employed to satisfactorily fit the bending tail behavior and to further calculate the ET rate constants. The theoretical counterparts for the different sizes are 1.4×10(7), 6.4×10(6), and 1.9×10(6) s(-1), showing good agreement with the experimental results.