SERS Study of the Mechanism of Plasmon-Driven Hot Electron Transfer between Gold Nanoparticles and PCBM

Plasmonic nanostructures have been widely used in photochemical reactions to enhance reaction rates, decrease energy barriers, or change reaction pathways, but the detailed mechanisms of these plasmon-driven processes are not well understood. In this work, continuous-wave (CW) pump-probe surface-enhanced Raman spectroscopy (SEAS) was used to systematically investigate the plasmon-driven hot electron transfer mechanism between gold nanoparticles and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM). The Raman peak shift of the PCBM pentagonal pinch mode was used as an indicator of the electron transfer process, which is verified using electrochemical SERS (EC-SERS) and density functional theory (DFT) calculations. Using wavelength-scanned pump-probe SERS and DFT calculations, we found that hot electrons generated from both intraband and interband transitions can transfer from Au nanoparticles to PCBM through an indirect electron transfer mechanism. Photothermal effects do not play a significant role in the process, as demonstrated by an effective vibrational temperature calculated using the anti-Stokes/Stokes Raman intensity ratio. These experiments in combination with DFT calculations not only clarify the detailed reaction mechanism, but also provide more insight into rules for designing rational reaction pathways for plasmon-driven chemistry.