Three-dimensional liquid surfaces through nanoparticleself-assembly

Abstract

Nanoparticles blended into thin polymer films are driven to assemble at interfaces by both entropic and enthalpic forces. When nanoparticles assemble at a substrate–polymer film interface, these forces are so large that they enable the film to follow surface protrusions and form a three-dimensional surface, instead of dewetting from the substrate. In other words, disassembling the nanoparticle layer requires more energy than that gained by the film dewetting from the rough surface. Here we studied blends of linear polystyrene and CdSe nanoparticles spin coated onto silicon substrates containing sparsely distributed SiO₂ particles (ca. 120 nm diameter). The films were then thermally annealed for periods of up to 24 h, well above the glass transition temperature of the polymer. The profiles of different film thicknesses (40–180 nm) were characterized using atomic force microscopy (AFM) before and after being annealed and were found to become much smoother, yet remain three-dimensional after annealing with a profile that gradually decayed away from the SiO₂ particles. Calculations based on a continuum theory using a balance of assembly, dispersion and surface tension forces were performed and found to be in agreement with the AFM profile data demonstrating the strength of the assembly forces.