Growth of thiol-coated Au-nanoparticle Langmuir monolayers through a 2D-network of disk-like islands

Abstract

Formation of 2D-networked structures of disk-like islands for ultrathin Langmuir–Schaefer (LS) films of thiol-coated Au-nanoparticles (DT-AuNPs) on H-passivated Si substrates is evidenced for the first time, directly from a broad peak in grazing incidence small angle X-ray scattering data and also from atomic force microscopy images. Theoretical modeling of the system, carried out based on density–density and height–height correlation functions, supports well the formation of such structures. The structural information of the LS films, obtained at different surface pressure, helps to infer the growth of Langmuir monolayers of DT-AuNPs, which is very important in understanding the self-assembly process of nanoparticles at the air–water interface and in controlling the growth of 2D-networked nanostructures in large areas. On the surface of water, DT-AuNPs first self-assembled around different points to form disk-like islands of nanometer size and monolayer height, due to the complex balance of long range van der Waals attractions and short-range steric repulsion of the DT-AuNPs, initiated by solvent evaporation and also to optimize the hydrophobic repulsive force of water. On barrier compression, the size and 2D-network of the islands grow due to a combined effect of collision induced coalescence and solid-like behavior resisting deformation of the islands. On the other hand, the separation between the DT-AuNPs either decreases or increases depending upon the competitive effects of packing or buckling.