Directed nanoparticle assembly in 3D liquid films

Nanoparticles are stabilized by entropic and enthalpic forces when assembling to interfaces in thin polymer films. The forces are so large that it enables the film to follow the surface protrusions instead of dewetting on the substrate. The reason is that disassembling the nanoparticle layer requires more energy than that gained by dewetting from the rough surface. In this work, blends of polystyrene and CdSe nanoparticles were spincoated onto silicon substrates containing sparsely distributed SiO2 particles (circa 120 nm) and thermally annealed. Film profiles of different thicknesses (40-180 nm) were characterized using atomic force microscopy (AFM). Calculations based on a continuum theory were performed and found to be in agreement with the AFM profile data. Cross-sectional transmission electron microscopy (TEM) was performed to provide validation of the three dimensional film profile contour. The structure could be utilized to enlarge the interfacial area for exciton dissociation in polymer-based solar cells.