Au-Coated Polystyrene Nanoparticles with High-Aspect-Ratio Nanocorrugations via Surface-Carboxylation-Shielded Anisotropic Etching for Significant SERS Signal Enhancement

Abstract

Increasing the hot-spot area with high enhancement ability on SERS-active particles is generally acknowledged as one of the efficient ways to significantly improve the average SERS signal of nanoparticles. A method to create roughness on the surface of nanoparticles was proposed by oxygen plasma etching noncarboxylated polystyrene beads. However, the mechanism of nanocorrugation formation was not clear. Thus, in this paper, we employ argon-based reactive ion etching (RIE) incorporated with carboxylated polystyrene nanoparticles to investigate the roles of nanocorrugations’ morphologies for SERS signal enhancement. The formation mechanism of the nanocorrugations has been investigated thoroughly through a comparison with those formed by oxygen-based RIE processes from their high resolution X-ray photoelectron spectra and surface morphologies with or without hydrazine reduction treatment. Moreover, polystyrene beads with more intrinsic carboxyl groups and etched by argon plasma produce higher nanocorrugations. It is suggested that carbonyl groups with high bond energy become nanomasks on polystyrene bead surfaces and provides high selectivity between carboxyl and polystyrene surfaces under RIE. Raman intensity enhancement on a 20-nm gold coated nanocorrugated polystyrene bead array is summarized by three factors: (1) the effect of plasmonic coupling among neighboring particles, (2) the nanocorrugation-contributed roughness, and (3) the pitch size of nanocorrugations, through the analysis of SEM images, AFM height images, and LSPR signals. Among these factors, the pitch size of nanocorrugations (ranging from ∼6 nm to ∼12 nm on the surface of polystyrene beads) dominates the SERS enhancement. The 870 nm/120s oxygen plasma etched polystyrene beads (OPSBs) with a minimum pitch size of 6 nm provides the highest Raman intensity enhancement (measured by 632.8-nm He–Ne laser), which is 12 times greater than the intensity of nontreated (870 nm/0s) polystyrene beads (while the Au/Ti coating is 20 nm/5 nm).