Spectroscopic and Computational Studies on Ligand-Capped Metal Nanoparticles and Clusters

Metal nanoparticles represent a bridge between single atoms and bulk materials, presenting peculiar chemical and optical properties. Under irradiation with an appropriate electromagnetic wave, the conduction electrons do not oscillate freely, because they are trapped in the nanometric size of the metal particles, which exhibit collective excitations called “localized plasmons.” These latter are needed to promote enhancements for both the Raman signal and the fluorescence emission of molecules adhering to the metal surface, when the exciting radiation wavelengths match those of the plasmon bands. Hence, Raman enhancements up to 107 factors are generally observed for molecules adsorbed on silver or gold nanoparticles in the SERS (surface-enhanced Raman scattering) measurements. When, instead, metal particles have sizes below about 2 nm, they do not have metallic properties owing to the existence of discrete electronic energy levels and the loss of overlapping electronic bands. These metal clusters exhibit a typical quantum size behavior, with optical and electronic properties different from those relative to plasmons. In this work, the spectroscopic properties of silver and gold nanoparticles and clusters, capped with organic ligands, are investigated by Raman scattering, absorption, and fluorescence measurements and interpreted by different computational approaches.