Convective and Evaporative Assembly of Binary Colloidal Films: Structure and Optical Properties.

The self-assembly of colloids is an efficient method for the fabrication of precisely structured nanoarrays, which are of great interest for the development of future devices and materials. Here, the great potential of horizontal deposition (HD) will be demonstrated for the lateral and vertical assembly of gold nanoparticles (AuNPs) and acrylic latex-polymer particles within binary colloidal films. In the first part of this work, it will be demonstrated that a simple and novel method, called Infrared Radiation-Assisted Evaporative Lithography (IRAEL), can be used for the lateral assembly of NPs over a large area. A convective flow, which carries the particles into pre-selected positions, is created through a non-uniform evaporation of the dispersion medium across the surface of the wet film. The resulting binary films show a strong, periodic modulation of the extinction coefficient in the lateral direction of the sample. The optical properties can easily be controlled through the particle size ratio in the blend. Furthermore, it will be shown that the vertical distribution of the particles in binary films prepared through horizontal deposition can actively be controlled through the choice of the experimental parameters. In an overarching study, the role of key parameters including the particle size ratio (RL/S), evaporation rate (E), and particle concentration are investigated. The geometry in the consolidated films, expressed through RL/S, decides if the transport of the smaller AuNPs through the voids of the larger close-packed polymer particles is enabled. The rate of evaporation dictates whether the particle transport is steered by a convective flow, diffusion, or sedimentation. The concentration of the particles sets the timeframe for the assembly process and the strength of particle jamming effects. A clear understanding of all these effects and their combination allow the creation of binary films with very different structures. The created structures include a hybrid structure, consisting of a two-dimensional Au nanoarray, and a three-dimensional binary colloidal crystal, a dot-like Au nanoarray, and samples with a AuNP-enriched layer on the surface and the substrate of the same film. The experimental results allow the precise formulation of design rules for nanostructures prepared from a binary dispersion via the horizontal deposition method. Finally, the optical properties of the hybrid structures were investigated using angle-and polarisation-dependent spectroscopic measurement techniques. The interplay between the optical response of the plasmonic Au nanoarray and the photonic binary colloidal crystal results in very interesting spectra