Photoinduced Plasmon-Driven Chemistry in trans-1,2-Bis(4-pyridyl)ethylene Gold Nanosphere Oligomers.

Continuous wave (CW) pump-probe surface-enhanced Raman spectroscopy (SERS) is used to examine a range of plasmon-driven chemical behavior in the molecular SERS signal of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on individual Au nanosphere oligomers (viz., dimers, trimers, tetramers, etc.). Well-defined new transient modes are caused by high fluence CW pumping at 532 nm and are monitored on the seconds time scale using a low intensity CW probe field at 785 nm. Comparison of time-dependent density functional theory (TD-DFT) calculations with the experimental data leads to the conclusion that three independent chemical processes are operative: (1) plasmon-driven electron transfer to form the BPE anion radical; (2) BPE hopping between two adsorption sites; and (3) trans-to- cis-BPE isomerization. Resonance Raman and electron paramagnetic resonance (EPR) spectroscopy measurements provide further substantiation for the observation of an anion radical species formed via a plasmon-driven electron transfer reaction. Applications of these findings will greatly impact the design of novel plasmonic devices with the future ability to harness new and efficient energetic pathways for both chemical transformation and photocatalysis at the nanoscale level.

Surface-enhanced Raman trajectories on a nano-dumbbell: transition from field to charge transfer plasmons as the spheres fuse.

By taking advantage of the tensor nature of surface-enhanced Raman scattering (SERS), we track trajectories of the linker molecule and a CO molecule chemisorbed at the hot spot of a nano-dumbbell consisting of dibenzyldithio-linked silver nanospheres. The linear Stark shift of CO serves as an absolute gauge of the local field, while the polyatomic spectra characterize the vector components of the local field. We identify surface-enhanced Raman optical activity due to a transient asperity in the nanojunction in an otherwise uneventful SERS trajectory. During fusion of the spheres, we observe sequential evolution of the enhanced spectra from dipole-coupled Raman to quadrupole- and magnetic dipole-coupled Raman, followed by a transition from line spectra to band spectra, and the full reversal of the sequence. From the spectrum of CO, the sequence can be understood to track the evolution of the junction plasmon resonance from dipolar to quadrupolar to charge transfer as a function of intersphere separation, which evolves at a speed of ∼1 Å/min. The crossover to the conduction limit is marked by the transition of line spectra to Stark-broadened and shifted band spectra. As the junction closes on CO, the local field reaches 1 V/Å, limited to a current of 1 electron per vibrational cycle passing through the molecule, with associated Raman enhancement factor via the charge transfer plasmon resonance of 10(12). The local field identifies that a sharp protrusion is responsible for room-temperature chemisorption of CO on silver. The asymmetric phototunneling junction, Ag-CO-Ag, driven by the frequency-tunable charge transfer plasmon of the dumbbell antenna, combines the design elements of an ideal rectifying photocollector.