Interfacial Electron Transfer Dynamics from Single Molecules and Quantum Dots Studied by Single-Molecule Fluorescence Spectroscopy

Interfacial electron transfer (ET) dynamics from single organic molecules and quantum dots (QDs) to semiconductor nanocrystalline thin films or molecular electron acceptors have been studied by using single-molecule fluorescence spectroscopy.;The photoinduced interfacial ET dynamics in sulforhodamine B (SRhB)-aminosilane-TiO 2/SnO2 nanoparticle (donor-bridge-acceptor) complexes have been studied on the single molecule level. The presence of the silane bridge enabled the complete sampling of these molecules under single molecule conditions. Shorter fluorescence lifetimes for molecules on TiO2 and SnO 2 compared to on ZrO 2 were observed and attributed to ET from SRhB to TiO2 and SnO2. The single molecule fluorescence lifetimes fluctuated with time and varied among single donor-bridge-acceptor complexes, suggesting that both static and dynamic ET rate distributions contribute to the heterogeneity of ET in this system. Computational modeling of the complexes showed a distribution of molecular conformations, leading to a distribution of electronic coupling strengths and ET rates.;The ET dynamics from single CdSe core/multi-shell QDs to adsorbed Fluorescein (F27) molecules and to TiO2 nanoparticles have also been studied by single particle spectroscopy. The QDs in both systems showed intermittent ET dynamics modulated by their blinking activities. The excited state lifetime of QDs in the "on" state reflected the ET rate, whereas in the "off" state, QD excitons decayed by fast non-radiative Auger relaxation. Furthermore, interfacial ET provided an additional pathway for generating "off" states, leading to the intermittent ET dynamics. The ET dynamics from single QDs to TiO2 rutile (110) and (001) single crystals were also studied. The preliminary results have shown that QDs on rutile (110) had much narrower ET rate distributions compared to QDs on TiO 2 nanoparticles. The ET rates for QDs on rutile (110) and (001) were found to differ, probably due to the different surface structures of these two single crystals.;The exciton quenching dynamics of QDs on ITO were also studied and compared with results for QDs on In2O3 and glass. Single QDs on ITO showed suppressed blinking activities and reduced fluorescence lifetimes, which was attributed to negative charging of the QDs. In these negatively charged QDs, the off states were suppressed due to the effective removal of the valence band holes, and their fluorescence lifetimes were shortened because of Auger relaxation processes involving the additional electrons.