Effect of Polymer Confinement:  Tuning Self-Assembled Growth of Monodisperse Au Nanoparticles on Polystyrene Films

X-ray reflectivity and tapping mode atomic force microscopy reveal that, Au, sputter-deposited on polystyrene films for film thickness ≤4 R g ( R g being unperturbed polymer gyration radius), diffuse and coalesce slowly, under ambient conditions and predominantly along the polymer surface, to form nanoparticles. The nanoparticles are highly monodisperse, and their in-plane dimensions increase with decrease in polystyrene film thickness. The observed directed coalescence is caused by sharply defined, higher surface energy zones or “traps”, corresponding to lower cohesion, between gyration spheres on polystyrene film surface. Lowering of in-plane cohesion and coalescence are found only when gyration spheres are layered along film depth due to thinning of the polystyrene film. The coalescing potential of these traps is given by the spatially localized increase in surface energy and can be increased by confinement. This fact is the principle behind this tunable self-assembly a new and generalized mode of self-assembly of Au nanoparticles with monodispersity and tunability in size.