Surface-enhanced Raman scattering properties of highly ordered self-assemblies of gold nanorods with different aspect ratios

Gold nanorods with aspect ratios of from 1 (particles) to 31.6 were synthesized by the seed-mediated method and packed in a highly ordered structure on a large scale on silicon substrates through capillary force induced self-assembly behaviour during solvent evaporation. The gold nanorod surface exhibits a strong enhancing effect on Raman scattering spectroscopy. The enhancement of Raman scattering for two model molecules (2-naphthalenethiol and rhodamine 6G) is about 5-6 orders of magnitude. By changing the aspect ratio of the Au nanorods, we found that the enhancement factors decreased with the increase of aspect ratios. The observed Raman scattering enhancement is strong and should be ascribed to the surface plasmon coupling between closely packed nanorods, which may result in huge local electromagnetic field enhancements in those confined junctions.