Curvature driven phase separation in mixed ligand coated gold nanoparticles

Abstract

Monolayer-coated gold nanoparticles have been the subject of extensive studies in fields ranging from physics to medicine. The properties of these nanomaterials such as solubility and surface energy are often attributed solely to the chemical functionalities of the ligand head-groups. However, the morphology of these monomolecular layers on gold nanoparticles plays as important of a role as the surface chemistry. Intriguing phase-separation phenomena have been observed for mixed self-assembled monolayers (SAM) of octanethiol (OT) and mercaptopropionic acid (MPA) on the surface of gold nanoparticles. These ordered structures are studied here through scanning tunneling microscope (STM) images, as a function of the gold core diameter, which is a measure of the particle's curvature. The packing of OT homoligand nanoparticles is found to have a head-group spacing of 0.54 nm, which differs from that on flat gold (111) surfaces, 0.5 nm. The OT:MPA heteroligand nanoparticles are observed to phase-separate into ordered ribbon-like domains, with spacings that depend on the nanoparticle diameter. A geometric framework that includes a continuous and crystallographic description is established to best describe the observed behaviors.