The Influence of Fractional Surface Coverage on Core-Core Separation in Monolayers of Thiol-Ligated Gold Nanoparticles*

The way in which thiol-ligated gold nanoparticles self-assemble into ordered monolayers when deposited on a surface is contingent on the thiol's length and concentration. The influence of ligand composition on the properties of nanoparticle films is well-reported. However, the understanding of the molecular and structural origins of these properties is incomplete, particularly when examining the number of ligands on a nanoparticle surface. It has been assumed that the concentration of ligands in solution, within typical ranges, always generates the maximum possible nanoparticle surface packing. We demonstrate through Grazing Incidence X-Ray Diffraction and Transmission Electron Microscopy that the nanoparticle separation and correlation length of these films increases linearly with thiol concentration in solution, indicating that the bulk thiol is in equilibrium with thiol on the core surface. Our findings thus challenge the assumption that the free energy of binding of an alkanethiol to a gold nanoparticle is so large that its surface is consistently saturated with ligands.