Electron Transfer Mediated by Plasmons: Studying Nanosphere Size Contributions via Vibrational Stark Effect

The impact of nanoparticle size on plasmon associated electron transfer processes.

Abstract Text:

The vibrational Stark effect (VSE) has emerged as a powerful spectroscopic tool for probing the influence of local electric fields in complex chemical environments. In particular, plasmon-induced electron transfer processes that occur within the nanoscale junctions (< 1 nm) lead to the generation of rectified optical fields, which can be studied through the VSE. This phenomenon arises when local electric fields perturb the electronic distribution of chemical bonds, thereby inducing measurable shifts in their vibrational frequencies. In this study, we investigate the impact of nanosphere size on the magnitude of VSE using surface-enhanced Raman spectroscopy (SERS) in a nanoparticle on mirror (NPoM) geometry. Gold nanospheres, ranging from 30 nm to 100 nm were synthesized and characterized using dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and scanning electron microscopy (SEM). The NPoM was prepared by depositing the synthesized nanospheres onto a template-stripped ultra flat gold film functionalized with a monolayer of mercaptobenzonitrile (MBN). The nitrile (C≡N), C=C, and aromatic (C–H) vibrational stretches of MBN observed in the SERS spectrum were assessed for frequency shifts induced by the local electric fields generated with single nanoparticles. SEM imaging was employed to correlate with the Raman mapping to identify single nanoparticles, thereby eliminating effect due to interparticle coupling in aggregated nanoparticles. Small nanospheres (32 ± 4 nm) exhibit greater VSE shifts compared to the larger nanospheres (84 ± 8 nm) in nitrile and C=C vibrational frequencies, indicating that the local electric field is influenced by the size of the nanosphere, and the local electric field strength increases with decreasing nanospheres size. A significant vibrational peak shift of ~100 cm^-1 was detected and attributed to electron charging of the molecule in the gap. Quantitative determining the effect of nanosphere size on the VSE can provide a broader understanding of electron transfer processes and the potential use of VSE probes in plasmonic devices.