Influence of metal grain size on surface-enhanced Raman scattering

Abstract

The surface roughness of nominally smooth and of randomly roughened thin silver films is characterized using scanning tunneling microscopy and the metal grain size is assessed using transmission electron microscopy. On each type of substrate used, glass or ${\mathrm{CaF}}_{2}$-roughened glass, the silver films are deposited either very slowly (\ensuremath{\sim}0.15 nm ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$) or quite quickly (\ensuremath{\sim}2.0 nm ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$). Only silver films deposited on ${\mathrm{CaF}}_{2}$-roughened glass yield measurable surface-enhanced Raman signals for benzoic acid; the enhancement is brought about by surface field amplification due to the excitation of delocalized surface-plasmon polaritons. However, the surface-enhanced Raman signals obtained from the slow-deposited silver films are significantly better (by about a factor of 3) than those obtained from the fast-deposited silver films on a given ${\mathrm{CaF}}_{2}$-roughened substrate. The explanation of this observation does not lie with different surface roughness; both types of film yield closely similar data on the scanning tunneling microscope. Rather, it is suggested that the relatively small grain size of the fast-deposited silver films leads to increased elastic scattering of surface-plasmon polaritons at the grain boundaries, with a consequent increase of internal damping. This results in a reduction of the scattered Raman signal.