Recent studies on plasmon-assisted chemical reactions postulate that the hot carriers of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, a quantitative validation at the level of molecular quantum states is currently missing. I will present our recent spectroscopic studies on metal-bound reactants, which directly proves the non-thermal vibrational molecular excitations caused by plasmons.
We report the first direct visualization of gap-SPPs propagating on a AgNW dimer. A self-assembled AgNW dimer loaded with a monolayer of molecules is locally excited to launch the SPPs, and the wide-field microscopy maps of surface-enhanced Raman scattering (SERS) of the molecules are acquired. The SERS images, representing the gap-field intensity distributions, reveal that the gap-plasmons of AgNW dimers with a few nm of gap can propagate up to ~8 um. The images also show oscillating components with periods of 400 ~ 800 nm, arising from the mode-beating of the two gap-SPP modes. Through a close comparison with electrodynamics simulations of NWs, we identify that the two modes are the monopole-monopole gap-modes with a propagation length of 0.5 ~ 2 um), and the dipole-dipole gap-modes with a propagation length of 5 ~ 8 um.
The existence of sub-nm plasmonic hot-spots and its relevance in spectroscopy, microscopy, and photo-chemical applications remain elusive despite a few recent theoretical and experimental evidence. I will present new spectroscopic evidence that angstrom-sized hot-spots do exist on the surfaces of plasmon-excited nanostructures, and that these hot-spots enhance metal - molecule charge-transfer rates. Surface-enhanced Raman scattering (SERS) spectra of 4, 4’-biphenyl dithiols placed in metallic junctions reveal simultaneously blinking Stokes and anti-Stokes spectra, some of which exhibit only one prominent vibrational peak. The activated vibrational modes are found to vary widely between junction-sites. Such site-specific, single-peak spectra could be successfully modeled using single-molecule SERS induced by a hot-spot with a diameter no larger than 3.5 Å, located at specific molecular sites. The model, which assumes the stochastic creation of hot-spots on locally flat metallic surfaces, consistently reproduces the intensity distributions and occurrence statistics of the blinking SERS peaks. We also observe time-resolved Stokes and anti-Stokes SERS spectra of nitrobenzenethiols placed at plasmonic junctions strongly suggesting that such atomic-scale hotspots accelerate the hot-electron transfer between the metallic surface and the molecules, and thus promotes photo-reduction processes on metallic surfaces. This unusual photo-chemical activity of the hot-spots may provide new insight into “chemical” enhancement mechanism in SERS.
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