The Effects of Ligand Concentration on the Mechanical Properties of Nanoparticle Films

When dodecanethiol-ligated gold nanopartlices are deposited on an air-water interface, they self-assemble into Langmuir films. Experiments in which the films are compressed uniaxially have demonstrated that as the ligand concentration decreases, the ligand-ligand and ligand-core interactions change and the 2-dimensional compressive and shear moduli of the films increases. However, these experiments were largely done upon films while on an air-water interface – this study focuses on films which are dried and removed from the air-water interface. This is important, since many potential applications of these films are in electronic devices, which require the films to operate in a dry environment. This study examines how variations in the concentrations of ligands in a gold nanoparticle solution and by extension the fractional surface coverage, impact a dried film’s mechanical properties. Specifically, the Young’s Moduli of the films are measured through contact-mode Atomic Force Microscopy (AFM), which demonstrates that low thiol concentrations are conducive to higher young’s moduli.