Ultrafast Carrier Transfer and Hot Carrier Dynamics in PbS–Au Hybrid Nanostructures

Abstract

Semiconductor−metal hybrid nanostructures (HNs) are promising materials for photovoltaics and photocatalysis because of their efficient charge separation in the excited state. However, analysis of their carrier dynamics has been limited to only a handful of HNs, such as CdSe−Au and CdS−Au. Herein, we synthesized and characterized PbS−Au HNs and examined their carrier dynamics by femtosecond near-IR pump−probe spectroscopy. A decrease in band-edge (1S) bleach yields was clearly observed in PbS−Au HNs as compared to PbS quantum dots (QDs) under 1S excitation, suggesting the existence of ultrafast carrier transfer from the 1S states of PbS QDs to Au nanoparticles (NPs) that was faster than the instrumental response function. A rise time analysis of 1S bleach dynamics determined that no hot carrier transfer was present. In addition to ultrafast carrier transfer, picosecond-scale carrier transfer was observed, with the rate constants increasing with Au NPs diameter. This difference between ultrafast and picosecond-scale carrier transfer is likely due to a difference in rate constants between electron and hole transfer. ■ INTRODUCTIONSemiconductor nanocrystals strongly confine electrons and holes, resulting in significant enhancement of electron−hole Coulomb interaction. This enhancement can lead to highly efficient multiple exciton generation (MEG), 1 a process whereby multiple excitons result from the absorption of only one photon. This process has been observed in several kinds of quantum dots (QDs) 2−5 and could be applied to the development of highly efficient solar cells. However, this strong confinement effect also result in highly efficient Auger recombination, reducing the lifetime of these semiconductor QD excitons to the order of tens of picoseconds. 6 Therefore, it is essential to extract multiple excitons from QDs before Auger recombination for the highly efficient solar cells.In addition, semiconductor QDs have discrete electronic structures while bulk semiconductors have continuous band structures. Since energy separation between 1S and 1P states of CdSe QDs is 10-fold larger than the LO phonon energy, 7 excited carriers in semiconductor nanocrystals relax to bandedge state through different processes from bulk semiconductors such as Auger cooling. 8 The mechanism of intraband relaxation in CdSe QDs has been reported with the state-selective excitation experiments by According to these reports, excited electrons relax primarily via Auger process from the 1P to 1S state, and holes relax primarily by a surface ligand mediated nonadiabatic process. In addition, they reported the hot carrier trapping in CdSe QDs competing with the hot carrier relaxation. 13−15 For bulk semiconductors, excited carriers relax to the band-edge state by phonon emission, and this fast relaxation is an origin of the theoretical efficiency limit of solar cell (Shockley−Queisser limit). 16 In contrast, intraband relaxation time in semiconductor nanocrystals can be controlled by capping reagents, 17 spatial separation between electr...

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