Plasmon-resonant Raman spectroscopy in metallic nanoparticles: Surface-enhanced scattering by electronic excitations

Abstract

Since the discovery of surface-enhanced Raman scattering (SERS) 40 years ago, the origin of the "background" that is systematically observed in SERS spectra has remained questionable. To deeply analyze this phenomenon, plasmon-resonant Raman scattering was recorded under specific experimental conditions on a panel of composite multilayer samples containing noble metal (Ag and Au) nanoparticles. Stokes, anti-Stokes, and wide, including very low, frequency ranges have been explored. The effects of temperature, size (in the nm range), embedding medium (SiO 2 , Si 3 N 4 , or TiO 2 ) or ligands have been successively analyzed. Both lattice (Lamb modes and bulk phonons) and electron (plasmon mode and electron-hole excitations) dynamics have been investigated. This work confirms that in Ag-based nanoplasmonics composite layers, only Raman scattering by single-particle electronic excitations accounts for the background. This latter appears as an intrinsic phenomenon independently of the presence of molecules on the metallic surface. Its spectral shape is well described by revisiting a model developed in the 1990s for analyzing electron scattering in dirty metals, and used later in superconductors. The g s factor, that determines the effective mean-free path of free carriers, is evaluated, g expt s = 0.33 ± 0.04, in good agreement with a recent evaluation based on time-dependent local density approximation g theor s = 0.32. Confinement and interface roughness effects at the nanometer range thus appear crucial to understand and control SERS enhancement and more generally plasmon-enhanced processes on metallic surfaces.

Vibrational density of states and thermodynamics at the nanoscale: the 3D-2D transition in gold nanostructures

“…3(b) that the corrected spectra are perfectly symmetrical with regard to the Rayleigh line testifying that the recorded signal obeys the thermodynamic balance equation and thus originates in thermalized Raman processes. As already discussed in details recently282934, the inelastic scattering contributions are due to either atomic or electronic excitations. Among the contributions of atomic motions, Lamb modes imply each NP as a whole and they are well described within the elastic approximation of a continuous medium17.…”

“…3(a) for both Stokes and anti-Stokes ranges. Using specific dark-field geometry and filtering3437, the spectra have been recorded with frequency shifts down to few cm −1 . Some very weak structures are visible around 80 and 140 cm −1 superimposed on a strong quasi-elastic scattering tail which intensity increases rapidly with the Au equivalent thickness (from A to D).…”

“…Some very weak structures are visible around 80 and 140 cm −1 superimposed on a strong quasi-elastic scattering tail which intensity increases rapidly with the Au equivalent thickness (from A to D). In order to verify that the collected signal comes from Raman processes, and not luminescence ones, the rough signal I exp has been corrected by the Bose statistical population factor and by the spectral response f of the whole experimental set-up34. One thus obtains a “corrected” intensity:…”

“…The “coupling coefficient” C ( ν ) is expected to be weakly dependent on the vibrational frequency out of very selective electronic transitions, and I ( ν ) has been shown to give a very accurate image of the VDOS282934.…”

“…However, plasmon-resonant Raman scattering has been recently revealed as an original way to access the VDOS of embedded Ag NPs or deposited Au colloids2829. To circumvent, here also, the limits of a very low scattering volume and efficiency, spectrally and spatially localized surface plasmon resonance, optical amplification and dark-field geometry have to be simultaneously used34. Using atomic simulations, the effect of specific atomic arrangements has been checked by fitting the corresponding calculated VDOS with these experimental results12.…”