Influence of Cracks on the Optical Properties of Silver Nanocrystals Supracrystal Films.

Physical properties of nanocrystals self-assembled into 3D superlattices called supracrystals are highly specific with unexpected behavior. The best example to support such a claim was given through STM/STS experiments at low temperature of very thick supracrystals (around 1000 layers) where it was possible to image the surpracrystal surface and study their electronic properties. From previous studies, we know the optical properties of Ag nanocrystals self-assembled in a hexagonal network two-dimensional (2D) or by forming small 3D superlattices (from around 2 to 7 layers) are governed by dipolar interactions. Here, we challenge to study the optical properties of Ag supracrystals film characterized by large thicknesses (from around 27 to 180 Ag nanocrystals layers). In such experimental conditions, according to the classical Beer-Lambert law, the absorption of Ag films is expected to be very large, and the film transmission is close to zero. Very surprisingly, we observe reduced transmission intensity with an increase of the notch line width, in the 300-800 nm wavelength range, as the supracrystal film thickness increased. By calculating the transmission through the supracrystal films, we deduced that the films were dominated by the presence of cracks with wetting layers existing at their bottoms. This result was also confirmed by optical micrographs. The cracks widths increased with increasing the film thickness leading to more complex wetting layers. We also demonstrated the formation of small Ag clusters at the nanocrystal surface. These results provide some implications toward the design of plasmonic materials.