Finite-Size Effects in Surface-Enhanced Raman Scattering from Molecules Adsorbed on Noble-Metal Nanoparticles

TL;DR

This paper investigates how finite-size effects in small noble-metal nanoparticles influence surface-enhanced Raman scattering, revealing that underscreening in a spillover layer enhances local fields and Raman signals.

Contribution

It introduces a two-region model showing that underscreening in small particles increases Raman enhancement, highlighting size-dependent effects.

Findings

Smaller nanoparticles exhibit greater Raman enhancement due to larger underscreened regions.

Spillover of sp-electron wave-functions reduces d-electron screening near the surface.

Finite-size effects significantly influence surface plasmon local fields and Raman signals.

Abstract

We study the role of strong electron confinement in surface-enhanced Raman scattering from molecules adsorbed on small noble-metal particles. We describe a new source of Raman signal enhancement which originates from different behavior of sp-band and d-band electron densities near the particle boundary. In small particles, a spillover of sp-electron wave-functions beyond the classical radius gives rise to a thin layer with diminished population of d-electrons. In this surface layer, the screening of sp-electrons by d-band electron background is reduced. We demonstrate that the interplay between finite-size and underscreening effects results in an increase of the surface plasmon local field acting on a molecule located in a close proximity to the particle boundary. Our calculations, based on two-region model, show that the additional enhancement of Raman signal gets stronger for smaller nanoparticles due to a larger volume fraction of underscreened region.