Thin photonic crystal templates for enhancing the SERS signal: a case study using very low concentrations of dye molecules

A single monolayer photonic crystal (PhC) of polystyrene(PS) colloidal particles with gold nanoparticles drop-casted on the surface have been employed as templates for the detection of dye molecules (methylene blue is taken as analyte for the case study) at very low concentrations using the surface-enhanced Raman scattering (SERS) technique. Results obtained from the SERS investigations clearly reveal that PhC templates significantly enhance the Raman peak, lowering the detection limits of dye molecules (viz., Methylene Blue, Rhodamine 6G, and Malachite Green) to similar to 50 nM. The results are understood by performing finite element method (FEM) based electromagnetic numerical simulations. Simulations have shown that the electric field locally gets redistributed, and significant enhancement occurs at wedge regions formed by touching PS spheres, which gets further enhanced in the presence of a gold nanoparticle at the wedge region. Gold nanoparticles sitting at the wedge regions serve as hot spots and are responsible for enhanced SERS signals.