Self-Assembly And Directed Assembly Of Polymer Grafted Nanocrystals Via Solvent Annealing

Polymer grafted nanocrystals (PGNCs) incarnate a duality of powerful attributes: their inorganic crystal cores hold promise to design nanocrystal solids from superlattice assemblies while their polymeric ligands exhibit the soft colloidal properties of star polymers. From their star polymer analogues, PGNC derive two important, yet sometimes less-exploited attributes: (1) swollen PGNCs are mobile and can be solvent annealed toward thermodynamic equilibrium, and (2) in the semidiluted regime in good solvents, PGNCs display a richer phase diagram compared to that of hard-sphere colloids which can be accessed by controlling the solvent intake during solvent annealing. Here we study the self-assembly of similar to 4 nm Fe3O4 nanocrystals capped with polystyrene ligands under solvent annealing. We confirm swelling behavior and even a phase transition to a hexatic phase, in agreement with models for star polymers. We exploit the polymeric properties of the PGNCs to perform directed self-assembly using thinly textured patterns to achieve long-range orientational order of the superstructure. We also demonstrate the use of the superlattice assembly as a lithographic mask for pattern transfer with a full pitch of 9.5 nm.