Angular-Resolved Polarized Surface Enhanced Raman Spectroscopy

Surface immobilized gold nanospheres (SIGNs) on a metallic substrate are widely used for surface enhanced Raman scattering (SERS) spectroscopy. Since SIGNs significantly enhance the local electric fields of light, due to localized surface plasmon resonance, it is a highly promising SERS platform. To understand the mechanism of enhancement of Raman scattering in the presence of colloidal metals, it is necessary to know how the molecules are adsorbed on the SIGNs. We performed angular-resolved polarized SERS (AP-SERS) spectroscopy to probe the adsorption structure of the molecules on the SIGN. A theoretical analysis is given for the AP-SERS from the molecules on the SIGNs, based on the quasi-static approximation and the local field approximation. The molecules are adsorbed only in the upper hemisphere region of the SIGN in the high coverage SIGN substrate, whereas at low coverage, the molecules are adsorbed in both upper and lower regions. This is because the solution does not intrude into the space between the nanospheres, as a result of the surface tension of ethanol. This result explains the unstable and irreproducible SERS signal from the aggregated nanospheres.