Gap Plasmons Multiple Mirroring from Spheres in Pyramids for Surface-Enhanced Raman Scattering

A reliable fabrication technique for obtaining a high density of regular nanogaps is critical for ultrasensitive surface-enhanced Raman scattering (SEAS). However, nano gaps produced between nanostructures have suffered from the lack of controllability. Taking the sphere-film structure as a starting point for its straightforward fabrication technique and well-controlled gaps, we propose a novel nanostructure by combining Au nanospheres and inverse pyramidal holes. The proposed nanostructure is obtained by trapping Au nano spheres in inverse pyramidal hole arrays to create multiple, uniform, and reproducible geometrical gaps near the contact points between the nanospheres and the inverse pyramids. The combined nanostructure, referred to as SIP (spheres in pyramids), supports augmented plasmon hybridization due to mirroring of gap plasmons and induces much stronger electromagnetic enhancement at normal incidence when compared to the contacting sphere-film structure. In contrast to the sphere-film structure exhibiting a maximum enhancement for slant incidence, the SIP achieves a maximum enhancement at normal incidence. The effective plasmon hybridization from SIP arrays sustains SERS intensities 9 times stronger than those of conventional contacting sphere-film structures and achieves 64% of the total electromagnetic field enhancement obtained from the sphere-spacer-film structure with a 0.6-0.8 nm thick spacer.