The UV-visible absorption spectra of two-dimensional (2D) assemblies of Ag or Au nanocrystals (NCs) are measured in transmission at variable incidence angle, for different average NC diameters ranging from 7.0 nm to 3.9 nm. The absorption spectra dominated by the collective surface plasmon resonance (SPR) absorption band are compared to those calculated using the discrete dipole approximation (DDA) method. The anisotropy of the optical response is inherent to the planar geometry of the NC assembly because of the near-field coupling between nanocrystals. For Ag NCs, the absorption spectra reveal the splitting of the SPR band into two components, namely, the transverse and longitudinal modes. At variance, no SPR band splitting is observed for 2D assemblies of Au NCs with identical size and coating agent. These features were satisfactorily reproduced by DDA simulation even though the SPR mode energies deduced from our calculations slightly overestimate the measured ones. The influence of the coating dodecanethiol molecules on the electron refractive index inside the nanocrystals as well as that of the substrate could explain the mismatch between the measured and calculated spectra.
Source : Collective Surface Plasmon Resonances in Two-Dimensional Assemblies of Au and Ag Nanocrystals: Experiments and Discrete Dipole Approximation Simulation J.Wei, P.Yang, H.Portales, P.A.Albouy and MPPileni J. Phys. Chem. C, 2016, 120, 13732−13738.