Guiding Self-Assembly of Functionalized Nanoparticles by Computational Modeling of Effective Interactions*

Nanoparticles have attracted much attention due to their unusual optical and electronic properties, which vary in scale from molecular to bulk. Practical applications include optoelectronic materials and photovoltaic cells, which exploit self-assembly into crystalline arrays (superlattices). Bulk dispersions can be sterically stabilized against aggregation by functionalizing the particles with ligand brushes. Experiments have shown that silver nanoparticles coated with adsorbed oleylamine ligands can self-assemble into equilibrium superlattices in the presence of free ligands. However, the interplay between repulsive steric and attractive depletion interactions is not well understood. To better understand the role of adsorbed and free ligands in self-assembly, we perform molecular dynamics simulations to microscopically model silver nanocrystals coated with ligand chains immersed in a solution of free ligands. We extract the effective pair potential and input it into simulations of a coarse-grained model of nanoparticle dispersions to explore structure and phase stability. Our results offer potential insight into the design of experiments and fabrication of nanocrystal superlattices from other materials of practical interest, such as silicon.