Optical response and SERS properties of individual large scale supracrystals made of small silver nanocrystals

There is a considerable interest in producing and understanding the optical and spectroscopic properties of ordered nanoparticle assemblies. Herein, we describe and interpret the optical absorbance and Raman properties of 5.9 nm +/- 0.3 nm diameter silver nanocrystals coated with dodecanethiol and organized in highly ordered 3D superlattices of different heights. Each superlattice was studied individually, which allowed to elaborate a model based on Maxwell-Garnett theory to reproduce qualitatively the height and wavelength dependence of the absorbance. Importantly, because of their small size compared to that of traditional nanoparticles used in Surface Enhanced Raman Spectroscopy (SERS), the large 3D distribution of hot spots generated by the silver superlattices allowed to easily obtain SERS spectra of the surrounding ligands despite their intrinsic low Raman cross section. Accordingly, traces of thiophenol could be detected very easily.