Helical Structure-Dependent Surface-Enhanced Raman Spectroscopy Enhancement in Gold Nanohelices

Electrical field (e-field) enhancement in gold nanohelices (AuNHs) with approximately 130 nm helical diameter (80 nm wire diameter) and various helical pitches (80-170 nm) is investigated with finite-difference time-domain (FDTD) simulation. The dimensions of the AuNHs are empirically determined from electron microscopic images of AuNHs that were synthesized via surfactant-assisted seed-mediated growth. In contrast to a Au cylinder with 80 nm diameter, the e-field is effectively enhanced in the AuNH by a longitudinally incident electromagnetic wave (? of approximately 600 nm) to the AuNH axis. In particular, the dipole distribution of the AuNHs is distinguished by transverse and longitudinal incidence: higher volume and more uniform hot spots exist in AuNHs with longitudinal incidence. A maximum surface-enhanced Raman scattering (SERS) enhancement of 10 ⁶ is obtained in AuNHs with a 100 nm pitch. SERS enhancement values that are obtained from the simulations are compared with experimentally measured SERS enhancement of 4-mercaptobenzoic acid that is loaded on the AuNHs. In addition, the power law dependence of the helical gap on SERS enhancement is relatively stronger compared to the nanogap in gold dimer nanostructures. Hence, AuNHs are satisfactory SERS templates with resonance tuning ability because of their unique structural characteristic (helical gap).