Network of nano-objects
The mastering of metallic/ligand surface interactions and the factors that govern the growth of particles has proven to be fundamental in synthesising particles with defined sizes and shapes, organising the particles in super-networks, and controlling their physical properties. The formation of super-networks is particularly important since these will grant access to identical particles in the form of dense material, but where the particles are electrically isolated. Therefore, the use of thiol or oxygenated ligands that are strongly bound to the metallic surface, enable metallic nanoparticles to be assembled to form a two or three-dimension networks (refer to below).
It was shown that the stabilisation of nanoparticles could be assured by ligands that are strongly bound to their surface, or by weakly coordinated ligands. In the first case, spherical nanoparticles that are monodispersed in size are obtained, while in the second case, changes in size and shape are possible, depending upon the organisation of the solution around the particle.
With the goal of obtaining nano-objects with controlled size and shape and therefore perfectly controlled physical properties, different types of ligands can be associated to the solution, behaving in the same manner as cationic systems in water/oil mixtures. In the case of tin nanoparticles, it appears that the monodispersity is related to the formation of a super-network, or the cristallisation phenomenon, induced by the presence of the ligand/surfactant mixture. This phenomenon appears to be general and was extended to other metallic systems. A long-distance organisation is obtained in the form of super-networks containing varied forms of nanoparticles: spherical, ovate, cubic and parallelepiped, but always identical within the super-network.
In the case of cobalt particles, the substitution of amine oleyl with hexadecylamine produces nano-rods with dimensions of 120 x 5,5 nm. By modifying the nature of the acid, it was possible to obtain a self-organisation of the nano-rods within the super-network (refer to the above figure).
An alternative approach for the organisation of functionalised nanostructures in 2D networks is to use substrates that have been functionalised. Therefore, the very low-energy deposit of aggregates preformed in gaseous phase, enables ordered networks to be created, for which the geometry and elementary cell parameters may be controlled through the trapping of defects already inscribed on the substrate surface.