Tunable Plasmonic Coupling in Core–Shell Planar Tetramers: Composition and Gap Dependent Resonances and Mode Evolution

This study addresses the need for systematic design rules governing the collective plasmonic response of complex, multi-material nanostructures. While core-shell nanoparticles offer enhanced tunability over homogeneous particles, and clustered arrangements can generate intense near-field hot spots, the combined effects of material composition and geometric coupling in planar tetramer core-shell assemblies remain inadequately explored. We numerically investigate planar core-shell tetramers in a square configuration to elucidate how core-shell architecture and interparticle proximity dictate their optical properties. Through full-wave electromagnetic simulations, we analyze the impact of various shell materials (Au, Cu, Al, Ni) paired with Ag or Au cores and interparticle gaps (0–30 nm) on the spectral response and near-field enhancement. Results reveal that the shell material is the primary determinant of resonance quality and spectral position, with noble metal shells (Au, Cu) supporting sharp, high-quality factor modes and strong radiative coupling, while reactive or lossy shells (Al, Ni) lead to broadened, damped responses. Reducing interparticle distance enhances dipolar coupling, forming intense “hot spots” between particles, while larger gaps lead to weaker coupling and localized fields near the substrate. This work establishes clear material- and geometry-dependent guidelines for tailoring the plasmonic response of core-shell tetramers, providing a foundation for optimizing their performance in applications such as enhanced sensing, photocatalysis, and nonlinear optics. • A comprehensive plasmonic study of core–shell nanoparticle quadrumers arranged in a planar square geometry is presented. • Systematic variation of core–shell composition (Ag@Au, Ag@Cu, Ag@Ni, Ag@Al, Au@Cu, Au@Ni) demonstrates how the shell material governs resonance wavelength, linewidth, and radiative efficiency. • The interparticle distance (gap) is identified as a key structural parameter that tunes bonding and antibonding plasmonic modes, modulating near-field enhancement and extinction efficiency. • High-Q resonances and intense electromagnetic hot spots in Ag@Au and Au@Cu quadrumers highlight their potential for plasmonic sensing and photothermal applications.