Photochemistry at the Surface of Gold Nanoprisms from Surface-Enhanced Raman Scattering Blinking

In this work, we investigate the optical properties of triangular gold nanoprisms synthesized following a one-pot chemical process and using triethylene glycol (TREG) and polyvinylpyrrolidone (PVP) as solvent and capping agents. The nanoprisms sustain a strong localized surface plasmon resonance (LSPR) which is exploited in surface-enhanced Raman scattering (SERS) by the PVP and TREG molecules present at the nanoparticle surface. The work is focused on the temporal fluctuations of the SERS signal. The assignment of the observed Raman features is based on density functional theory calculations, performed for various interaction states between the PVP or the TREG molecules and the gold surface, in a simple model system. The SERS blinking is analyzed quantitatively using the autocorrelation of spectrally selected temporal SERS traces. We also use SERS covariance maps in order to investigate the cross-correlation between the relevant Raman features. We show that the fluctuations of the PVP and TREG SERS signals are not random. We found that the Raman features associated with the CO stretching vibrations are fingerprints of the interaction between the organic molecules and the nanoparticle surface. In particular, we observe quasi-periodic adsorption/desorption events of the PVP molecules that reveal their dynamical interaction with the gold surface. From the SERS covariance maps, we also point out correlations between the TREG and PVP signals which we interpret as occasional exchange of the two molecular species. The dependence of the SERS blinking on the detuning between the Raman excitation and the surface plasmon resonance is also investigated. We show that the optical excitation plays an important role in the photoinduced chemical processes that occur close to the nanoparticle surface.