Effects of gold film morphology on surface plasmon resonance using periodic P3HT:PMMA/Au nanostructures on silicon substrate for surface-enhanced Raman scattering

We study the effects of the morphology of gold film on the unusual modulation of surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS) using the periodic nanostructures of P3HT:PMMA/Au on the silicon substrate. The periodic structures of cylindrical holes with triangular lattice were fabricated first by e-beam lithography using a photoluminescent resist of P3HT:PMMA at a thickness of ∼100-200 nm, and the structures were then coated with gold of ∼20 nm thickness. The geometries and structures of the samples were studied by atomic force microscopy and scanning electron microscopy. The relationships between the geometry and the resonance were investigated by the extinction spectra and confocal Raman mapping. The results show that the resonance wavelength of the extinction peak is blue shifted, and its width becomes wider when the hole diameter of the structure increases or the lattice constant decreases. However, the extinction peak is red shifted when the thickness of the periodic nanostructures increases due to the surface plasmon resonance, localized surface plasmon resonances, and coupling between neighboring holes. The finite-difference time-domain (FDTD) method was adopted to simulate different nanostructures, and we found that the morphology and location of gold film on the periodic structure of P3HT:PMMA film played a vital role in the extinction spectra of the composite film of P3HT:PMMA/Au. A large enhancement of Raman scattering was observed when the SERS and SPR were correlated in the nanostructure at the exciting wavelength of 632.8 nm. This study provides a useful strategy to modulate the extinction spectra and enhance the intensity of Raman spectra by changing the nanostructures. The observed SERS will be useful for the design and fabrication of functional devices and sensors.