Distance-Dependent Electron Transfer in Tethered Assemblies of CdS Quantum Dots and TiO2 Nanoparticles

Abstract

We have characterized electron injection from photoexcited CdS quantum dots (QDs) to TiO 2 nanoparticles as a function of the interparticle separation within molecularly linked assemblies. CdS QDs were tethered to TiO 2 nanoparticles through bifunctional mercaptoalkanoic acids (MAAs). Electron injection and interfacial charge recombination were characterized by steady-state emission quenching, nanosecond time-resolved emission, and nanosecond transient absorption. The electron injection yield decreased with increasing MAA chain length and interparticle separation. Electron injection occurred on multiple timescales. A fast component (<10 −8 s) accounted for the majority of injection, while the remainder occurred on the microsecond time scale. We attribute the multiexponential injection kinetics to electron transfer from a range of conduction-band and trap states. Interfacial charge recombination occurred on the microsecond time scale, and the kinetics were independent of the MAA chain length. Our findings reveal that the excited-state deactivation pathways and interfacial electron-transfer reactivity of tethered assemblies of nanoparticles can be tuned systematically by varying the interparticle separation.