On the Thermodynamic Stability of Binary Superlattices of Polystyrene-Functionalized Nanocrystals

Assembly of two types of functionalized nanocrystals by tuning the solvent quality always leads to phase-separated single-component superlattices, while if directed by solvent evaporation, it may result in cocrystallization into binary superlattices, thus raising the following obvious question of: What phase is the thermodynamic equilibrium? We address this fundamental problem by evaluating the free energies of single-component and binary superlattices of nanocrystals functionalized with polystyrene. We show that most binary superlattices reported in the literature are metastable against phase separation. We also characterize the structure of the different superlattices and show that they are accurately described by the orbifold topological model (OTM). We discuss the implications of the results for ongoing experiments and for the general assembly of nanostructures.