Optical properties of dye molecules adsorbed on single gold and silver nanoparticles

Despite the well-known relationship between the resonance Raman excitation profile and the absorption line shape, there is scant experimental evidence for effects in absorption or fluorescence spectroscopy related to the observations of surface-enhanced Raman scattering (SERS). On the other hand, numerous Raman studies have been done on the SERS phenomenon, where large, enhancement factors have been determined. In this work, the absorption properties,of molecules adsorbed on single gold and silver nanoparticles (monomers) have been investigated, with particular emphasis on an examination of the effect on the spectrum of the adsorbate. A number of the adsorbates studied are similar to those reported in SERS studies. The adsorbates can be divided into two classes according to the nature of the interaction with the adsorbent. Class I shows little change in the absorption spectrum. Class 11 shows a large reduction in absorption. The only examples of an increase in absorption arise from solvatochromic effects. The implication of these observations for the mechanism of SERS is discussed.