Superlattice formation in colloidal nanocrystal suspensions: Hard-sphere freezing and depletion effects

Superlattice formation in dried suspensions of colloidal silver nanocrystals (AgNCs) is investigated through a combination of experiment, theory, and simulation. Using microscopic and spectroscopic techniques, we explore the phase behavior of dried AgNC suspensions, and we model the system using Monte Carlo simulations of a coarse-grained model with an effective pair potential that accounts for the composition of the nanocrystal core, the stabilizing ligand shell, and entropic effects associated with unbound ligand in solution. In the absence of free ligands, the effective potential at ligand contact is purely repulsive, and we find superlattice formation at an effective AgNC volume fraction close to that anticipated for hard-sphere freezing. In the presence of free ligands, the effective potential becomes attractive, and crystallization is accompanied by phase separation and multiphase coexistence, as anticipated for colloid-polymer mixtures.