Enhanced Near-Field Focusing via Interior Cavity and Nanogap Coupling in Disk-Cavity-Trench Nanostructures.

We report the synthesis of plasmonic complex Disk-in-Trenched nano-Dome (DiTD) nanostructures that combine a nanodisk core, an internal hollow cavity, and a surrounding trench shell in a single architecture. Their construction involves a multi-step synthetic method beginning with Au@Pt nanodisks, followed by facet-controlled Ag overgrowth as a sacrificial layer and subsequent formation of the inner cavity and Au trenches. This architecture confines light within the trench nanogaps and between the nanodisk core and the hollow cavity. Structural parameters such as nanogap and shell thickness can be systematically tuned to clarify e-field enhancement mechanisms within a single particle. Single-particle surface-enhanced Raman spectroscopy (SERS) and finite element method simulations reveal that DiTDs with an optimal trench gap (12.8 nm) and thin shell (15.9 nm), which enable effective coupling between the inner cavity and nanogap at 785 nm excitation, exhibit strong near-field focusing. These findings indicate that SERS enhancement is governed by both gap miniaturization and three-dimensional plasmonic interactions throughout the nanoparticle.