Raman scattering enhancement contributed from individual gold nanoparticles and interparticle coupling

Gold nanoparticles immobilized on a silicon wafer were used to semi-quantitatively study the Raman enhancement from the individual particles and the interparticle coupling, with p-aminothiophenol (p-ATP) molecules as Raman probes. At low coverage of gold nanoparticles on the surface, the gross intensity of the p-ATP Raman peaks was found to be proportional to the number of particles per unit area, suggesting that the spectra could be a sum of contributions from the isolated particles while the interparticle coupling could be neglected. However, a sharp nonlinear increase in the intensity of the p-ATP Raman peaks versus the number of particles per unit area was observed at higher coverages, indicating that the contribution from the interparticle coupling became remarkable at reduced interparticle distances. By comparing the apparent enhancement factors of the isolated particles with that of particles at saturated surface coverage, a 10-20 times extra-enhancement can be derived which could be attributed to the interparticle electromagnetic coupling. The critical centre-to-centre interparticle distance for significant electromagnetic coupling is found to be about twice the particle diameter. Further efforts will be made in quantitatively correlating the present results with the possible alterations in plasmon resonance of the assembled gold nanoparticles by the substrate and the interparticle coupling.