This article reports on a project that observed size-dependent surface-enhanced Raman spectra (SERS) for crystal violet (CV) adsorbed on a monolayer of hexagonally close-packed silica spheres.
Enhancement factors on the order of 104 were estimated based on the ratio of the number of CV molecules excited in solution to those on the surface of the monolayer array. The proposed enhancement mechanism was that of delocalized surface plasmon resonance over the monolayer array. Plasmon resonances of silica spheres were calculated using Mie theory for light scattering and experimentally validated with UV–vis absorption spectra. There was an excellent agreement between the observed spectrum from the monolayer arrays and a single particle calculated spectrum. This work points the way to used silica particles as a viable alternative to current SERS substrates, as well as illustrates the importance of surface plasmon resonances from photonic crystals in the SERS of semiconductors. (publisher abstract modified)