Plasmonic Percolation-Induced Dielectric-to-Metal-like Optical Transition in Sub-Monolayers of Silver Nanoparticles on Gold Substrates

The optical properties of nanostructured films govern the performance of plasmonic sensors and optoelectronic devices. Here, we report a systematic investigation of the surface plasmon resonance (SPR) response of submonolayers of hexanethiol-functionalized silver nanoparticles (AgNPs, diameter ≈20 nm) self-assembled on gold substrates, with surface coverage ( S o ) varied from ≈4% to ≈97%. Combining angular-interrogation SPR experiments with finite-difference time-domain (FDTD) simulations, we identify a plasmonic percolation transition at a critical coverage of S o ≈ 40%. Below this threshold, the AgNPs act as isolated high-index scatterers and the resonance angle shifts to higher values (red shift), in quantitative agreement with the Maxwell–Garnett effective medium approximation (EMA). Above the threshold, near-field plasmon hybridization between neighboring particles drives a reversal to lower resonance angles (blue shift), and the EMA breaks down, confirming the onset of collective metal-like optical behavior. A derivative analysis of the resonance angle shift provides a model-independent marker of the transition at S o ≈ 35–40%. The transition is further characterized by a monotonically decreasing SPR dip depth and a pronounced maximum in refractive index sensitivity near the coupling threshold. The observed critical coverage of S o ≈ 40% is substantially lower than the classical 2D hard-disc percolation threshold of S c = 67.6%, explained by an effective plasmonic coupling radius of r eff = 13.0 nm consistent with the known LSPR near-field decay length for 20 nm Ag nanoparticles. These results provide a quantitative framework linking nanoparticle self-assembly, plasmonic percolation, and SPR sensitivity, with direct implications for rational design of nanoparticle-modified plasmonic sensing platforms.