Nucleation phenomenon in nanoparticle self-assemblies
Dispersed nanoparticles self-assemble into complex structures when segregated from the solvent either by evaporation or precipitation. Thus, different micro- and macroscopic structures (opals, fractals, liquid crystals) formed by nanoparticles are observed as a result of the balance between electrostatic forces, surface tension, volume exclusion, substrate topography and affinity, and size, shape and concentration of the particles. The formation process is close to the hard-sphere case with two special features: nanoparticles are coated with surfactant molecules of roughly 1 nm in length, and nanoparticles are in the nanometric scale resulting in an increased mobility. Moreover, self-assembly does not take place in solution; hence, a minimum concentration is needed in order to start self-assembly. This process occurs in contact with the substrate, when the solvent cannot re-disperse the particles anymore.