The discrete dipole approximation method is used to simulate the optical response of ultrafine Au nanoparticles « NPs » with a diameter of 5 nm assembled in a planar hexagonal array. Similar calculations performed for Ag NPs arrays are also presented for comparison. For both cases, the absorption spectra are calculated for various incidence angles and interparticle distances to reveal the optical anisotropy related to the geometry of the system through the splitting of the surface-plasmon resonance « SPR » into the transverse and longitudinal modes. This effect usually results in the emergence of two SPR bands in the absorption spectrum as observed for Ag NPs arrays when the border-to-border interparticle distance becomes smaller than around one particle radius. Conversely, such a splitting is shown to be undistinguishable for Au NPs arrays whatever the interparticle distance and the incidence angle are. The different behaviors pointed out between Au and Ag NPs arrays are ascribed to the intrinsic dielectric properties of these two metals.
Source : Ability to discern the splitting between longitudinal and transverse plasmon resonances in Au compared to Ag nanoparticles in close-packed planar arrays. Yang, H. Portalès and M.P. Pileni Phys. Rev.B, 2010, 81, 205405.