Introducing plasmonic antenna nanojunctions into nanocavity structures to confine and enhance their local electromagnetic fields

Abstract

Plasmonic nanocrystals can interact with external photons to generate local electromagnetic fields around them, with potential applications in photocatalysis and sensing. Since these applications largely rely on their local electromagnetic fields, some strategies can be used to enhance the local electromagnetic fields around plasmonic nanocrystals to improve their application performance. By self-assembling plasmonic nanocrystals into nanojunctions, their local electromagnetic fields can be coupled with each other, thereby achieving an enhanced effect. Another promising strategy is to use plasmonic nanocrystals to form nanocavity structures for confining and amplifying electromagnetic fields. Here, plasmonic nanocrystals are stacked into nanojunctions on a substrate consisting of a dielectric layer and a metal film to produce a plasmonic nanocavity with a metal–dielectric–metal (MDM) structure. For steeple-like nanojunctions produced on this substrate, two effects, plasmonic nanoantenna and plasmonic nanocavity, can be used to confine and enhance local electromagnetic fields. Compared with the wire-like nanojunction produced on this substrate (only the plasmonic nanocavity effect) and the steeple-like nanojunction produced on the Si substrate (only the plasmonic nanoantenna effect), this special nanostructure exhibits a narrower dark field scattering peak (confinement effect) and a stronger dark-field scattering signal (enhancement effect). These results are consistent with those obtained via finite-difference time-domain (FDTD) simulations of the local electric fields in nanostructures. And the Raman enhancement factor of the sample with the plasmonic nanoantenna effect and plasmonic nanocavity (steeple-like nanojunction on a SiO₂–Ag–Si substrate) is 5 to 10 times higher than that of the sample without these two effects (single plasmonic nanocrystal on a Si substrate). This study demonstrates that introducing plasmonic nanojunctions with nanoantenna effects into nanocavity structures can effectively confine and enhance their local electromagnetic fields, making them suitable for catalysis and sensing applications.