Light interactions with supracrystals either deposited on a substrate or dispersed in water
Nanocrystals with low size distribution are able to self-assemble into a 3D crystalline structure called colloidal crystals or super/supracrystals. A rather large number of supracrystal specific properties have been achieved showing promising potential applications. Here, we compared intrinsic properties induced by light interacting with fcc supracrystals of hydrophobic metal nanocrystals either deposited on a substrate or dispersed in aqueous solution. We first describe the formation of a dried supracrystal film grown via a heterogeneous process with cracks formed due to the shrinking of the film caused by restriction of its adhesion on the surface. We also describe the method to fabricate hydrophobic supracrystals dispersed in aqueous solution. The optical properties of the thick dried supracrystal film are determined from the wetting layers formed at the bottom of the cracks whereas, for water dispersed suprastructures, both the collective optical photonic mode and absorption of dispersed nanocrystals used as build blocks are observed. Ag nanocrystals used as building blocks in a dried supracrystal film vibrate coherently as atoms in a nanocrystal. However, it is impossible to determine the oscillation period of the whole assembly. Conversely from a dynamic study, the breathing period of the assemblies dispersed in aqueous solution is found to be around 300 ps. Whatever experimental conditions, nanocrystals exposed to light breath coherently in a supracrystal. In aqueous solution, supracrystals behave as nanoheaters.