Simultaneous interfacial and precipitated supracrystals of Au nanocrystals: experiments and simulations.

Under solvent saturation, a precipitation of full-grown supracrystals on the one hand and the formation of well-defined supracrystalline films at the air-liquid interface on the other hand were previously observed for the first time (J. Am. Chem. Soc.2012, 134, 3714-3719). Here, these two simultaneous growth processes are studied by additional experiments and by Brownian dynamics simulations. The thickness of the supracrystalline films and the concentration of free nanocrystals within the solution are measured as a function of the nanocrystal size. The simulations show that the first process of supracrystal growth is due to a homogeneous nucleation favored by solvent-mediated ligand interactions, while the second one is explained in terms of a diffusion process caused by a decrease in the surface energy when the particles penetrate the air-liquid interface. It is also verified that the presence of thiol molecules at the air-solution interface does not hinder the formation of supracrystalline films.