Nanoporous Silver Films for Surface-Enhanced Raman Scattering-Based Sensing

Nanoporous metallic films have attracted great attention as surface-enhanced Raman scattering (SERS) substrates. The presence of nanosized protrusions in such nanoporous metallic films gives rise to remarkably improved SERS enhancement due to an additional lightning rod effect of protrusions. However, the positive contribution of protrusions was only preliminarily demonstrated by SERS measurements so far, and several important questions remain unaddressed─fundamental understanding into the near-field enhancement of protrusions, qualitative link between the measured SERS enhancement and properties of protrusions, and facile strategy for nanoporous metallic films with protrusions. In this paper, plasma oxidation–reduction of Ag nanofilms was used to fabricate various nanoporous Ag films with nanosized protrusions. The fundamental protrusion size dependence of near-field enhancement was elucidated from both the electrodynamic model and FDTD simulation, with the protrusion approximately treated as a hemiprolate spheroid positioned on a semi-infinite metallic base. With the as-fabricated nanoporous Ag films as the SERS substrates, the qualitative link between the measured SERS enhancement and density/size distribution of protrusions was established. The findings reported here contribute significantly to gaining deeper insight into the SERS enhancement of nanoporous metallic films and guiding future SERS substrate design.