Abstract
Raman scattering is an inelastic scattering of photons by chemical molecules. Surface-enhanced Raman scattering (SERS) greatly enhances the normally weak Raman signal and thus becomes a sensitive technique for identification of analyte molecules in chemical or biological analysis. The gigantic signal enhancement results from the intense optical field intensity produced around the metal nanostructures on a SERS-active substrate as the incident electromagnetic waves couple to the localized surface plasmons of the metal. In this study, a number of SERS-active substrates with two-dimensional gold nanostructures including plain gold films, nanoporous gold films, and hybrid structures of gold nanoparticles (NPs) on film were prepared. A mechanical scratching method was also employed to create microscale piles of gold sheets. Rhodamine 6G (R6G) was used as the target molecule to evaluate the SERS effect. Plain gold films with thicknesses ranging between 2 and 10 nm were prepared and the strongest SERS signal was obtained from the 5 nm thick film with an average analytical enhancement factor (AEF) of 1.72 × 10^4. It is believed that the semicontinuous framework of the 5 nm film provides abundant ‘hot spots’ and gives the strongest SERS signal. Furthermore, a hybrid film of NPs (10 to 20 nm in size and area density of 0.5/μm2) on the 5 nm thick gold film gives 10 times improvement and the AEF reaches around 1.96 × 10^5. Nanoporous gold films with thicknesses ranging between 10 and 40 nm were prepared, and the sizes of these densely distributed pores range between 10 and 250 nm. An average AEF of 1.15 × 10^4 has been obtained for the 30 nm thick nanoporous film. A hybrid film of NPs on the 30 nm nanoporous film was also prepared, but the SERS effect was improved only marginally. A commercial AFM and a silicon probe were employed to perform mechanical scratching on gold films with thicknesses of 20 to 60 nm. A microscale pile of rolled gold sheets was created at the end of a scratching and became an excellent SERS-active site. By using various scratching distances of 100 to 700 μm on 20 nm plain film, AEFs of 10^4 - 10^6 were obtained. In addition, the SERS signal is stronger when the incident light polarization is parallel to the scratching direction. A highest AEF of 3.80 × 10^5 has been obtained on a 40 nm thick gold film with a scratching of 500 μm.