Coupling of Localized Surface Plasmon Resonance in Self-Organized Polystyrene-Capped Gold Nanoparticle Films

Glass-deposited monolayers of polystyrene-coated gold nanoparticles with controlled interparticle distance have been prepared. Normal incidence extinction spectra show a progressive red shift of the plasmon resonance wavelength as the interparticle gap within the film is reduced. Polarized extinction spectra were measured at in-plane incidence using an optical waveguide lightmode spectroscopy (OWLS) setup. The in-plane (TE)-polarized spectra show a red shift with decreasing interparticle gap, whereas the out-of-plane (TM)-polarized component shows no visible change. These observations are typical of dipolar near-field interparticle plasmon coupling. Simulations using the discrete dipole approximation have been conducted to compare the decay rate of the red shift with increasing interparticle gap for different particle arrangements (pair, row of 5 and 2D hexagonal array of 19). The calculations show that the variation of the relative red shift with the relative interparticle spacing follows a first-order exponential decay law in all three structures, with a decay constant similar to results previously reported. A secondary, slower decay rate, occurring at relative interparticle gap values above 1.25, is found for the 2D array. The decay constant of this large-distance regime is close to that of the measured extinction spectra of the polystyrene-coated gold nanoparticles monolayers, which present relative interparticle distances within this range. This second decay constant may be the result of an increased contribution of higher-order modes.