The Role of Non-Covalent Interactions in the Self-Assembly of Hybrid Organic/Inorganic Nanoparticles into Superlattices

Nanoparticles superlattices can be prepared by self assembly, under strict synthetic control, of hybrid nanoparticles composed of a metallic bulk (from a few to hundreds of namometers in diameter) surrounded by an organic passivating layer. The nanoparticles are arranged with long-range crystalline order. These superlattices are promising materials for applications in various fields such as recording media, light–emitting devices, biological tags, catalysts, solar cells, and sensors. The characterization and elucidation of the size–evolutionary patterns of the properties of these finite size assemblies are among the major challenges in modern materials science. The non-covalent interactions between ligands control the formation of the lattices and their properties. We wish, in particular, to investigate the influence of these interactions on the mechanical properties of the superlattices. The structural, electronic, magnetic, optical, spectroscopic and chemical properties of isolated nanoparticles and their assemblies are intensively investigated but relatively few studies have been devoted to their mechanical properties. As single–crystal nanoparticle superlattices can be prepared with lateral dimensions of ten or more microns and thicknesses of at least several hundred nanometers, nanoindentation can be used to characterize these systems. The observation by atomic force and scanning electron microscopies of the superlattices before and after indentation provides quantitative data on the elastic modulus, plastic deformation (hardness) and fracture properties of the material. Proof-of-principal experiments indenting on superlattices have recently been carried out. We hope, by systematically varying the nature of the nanoparticle ligands, the conditions under which the nanoparticles are assembled (pH, solvent, etc), and the indentation environment (temperature, humidity) to be able to rationalise the effect of the ligand-ligand interactions on the mechanical properties of the superlattices.