Glenna Drisko – Controlling nanomaterial deposition on 2D surfaces using solution properties and nanochemistry

ICMCB – Bordeaux

Monolayers of particles or perforations deposited on a 2D substrate are relevant to applications in optics, as scattering from both individual and collections of nano-objects impact light reflection, absorption and transmission. The degree of control over nano-object placement and density determines the possible applications of the 2D material. The process of convective particle self-assembly into thin films is an efficient and low-cost route to material fabrication, however, it suffers from low reproducibility due to its high dependency on particle concentration, as well as a variety of interactions and physical parameters. inhomogeneities in flow rates, and instabilities at the air–liquid interface, are mostly responsible for
reproducibility issues. Thus, currently convective particle assembly is not used to produce materials requiring a high degree of control over pattern formation, meaning that many nanostructured surfaces must be achieved by expensive and low-throughput top-down fabrication techniques.
We present a study of the role of viscosity in convective self-assembly of particles using a series of alcohols to show its impact on stick-slip behavior and interband distances. Silica particles were used as a model system, both spherical and in the form of nanoribbons, assembled using dip-coating, in a range of single and binary dispersant phases. Viscosity was found to affect the patterning of particles during assembly, but was not the dominate solvent property dictating surface coverage. We then studied the impact of dip-coating withdrawal rate on the orientation of nanoribbons, finding that we could change between parallel and
perpendicular depositions, with regards to the meniscus. We also studied particles suspended in binary mixtures of solvents. We show that mixtures of glycerol and alcohol or poly(acrylic acid) and alcohol lead to more complex patterning, that in single dispersent systems. Evaporative flux in a binary solvent systems lead to strong differences in self-assembly patterns. We also found that surface tension gradients formed during evaporation in the binary systems heavily impacted assembly patterns. Evidence of Marangoni currents were only observed in some instances of the co-solvent systems, being dependent on the rate of solvent evaporation. To produce homogeneous particle assemblies and control stick-slip behavior, gradients must be avoided, the surface tension should be minimized and the viscosity maximized.
Dip-coating has been applied to the deposition of thin films using sol-gel chemistry, where the reaction mixture can be modified to either deposit a homogenous silica layer, or a perforated layer. The density and the size of the perforations depends on the reaction conditions, impacting the optical properties of such films. Finally, continuous films and nanoparticle deposition have been combined in order to tune the visual appearance of the 2D surfaces.
By mastering the chemistry of the solution, the physical parameters during deposition and the environmental conditions, homogeneous films with a sufficient level of control can be achieved using inexpensive, large-surface deposition via dip-coating.