Core–Shell Plasmonic Nanostructures on Au Films as SERS Substrates: Thickness of Film and Quality Factor of Nanoparticle Matter

Assembling plasmonic nanoparticles on metal films is an elegant method for the design of SERS platforms with dense hot spots and amplified electric fields. Interaction between the localized surface plasmons of metal nanoparticles within assemblies induces collective plasmonic modes which can further couple with the propagating surface plasmons prevailing on the metal films. Herein, we report on the electric field effects as well as Raman signal enhancements arising due to the sandwiching of Au and Ag core–shell nanoparticle assemblies on Au films with varying thicknesses of the underlying metal film. The sandwich plasmonic platforms are prepared by linking Ag@SiO₂ as well as Au@SiO₂ nanoparticles on Au films using (3-mercaptopropyl)­trimethoxysilane (3-MPTS). The interaction between the SiO₂ shell on the Ag/Au nanoparticles and free silanol groups on 3-MPTS provides a monolayer of core–shell systems on the Au films, as corroborated by SEM images. Finite-difference time-domain simulations with heptamer models of Ag@SiO₂ and Au@SiO₂ particles on Au films confirm an enhancement in the electric field upon sandwiching the nanoparticle aggregates on the Au films. The Raman signal enhancement factors for the dye Rhodamine 6G are estimated, and the enhancement in the Raman signal intensities on Ag@SiO₂ over Au@SiO₂ assembled on a 20 nm Au film is attributed to the higher Q-factor of Ag. The largest measured Raman signal intensity on Ag@SiO₂ on a 60 nm thick Au film, ∼10⁷, is reasoned based on the variation of the electric field intensity of the Au film as a function of its thickness.