Third party funded individual grant
Start date : 01.01.2016
End date : 31.08.2020
Nanostructuration of a material has profound effects on its properties, leading to both enhancement of existing properties and even entirely new properties not found in the bulk material itself. Fabrication challenges arise due to the small size of the structures of interest. Conventional, top-down nanofabrication requires highly sophisticated and expensive equipment and clean room environments, which limits a wide-spread application of the methodology. An attractive alternative is the design of nanoscale structures and patterns from the bottom up. For example, colloidal particles can be easily synthesized with high precision in the size range between 100 nm and several micrometers. These particles can self-organize into periodic structures with high degrees of order, thus providing easy access to surface structures with high resolution. The main current limitation to the technique is the limited choice of structural motifs that can be created: Predominantly, structures with a hexagonal symmetry based on the packing of isotropic, spherical colloidal particles are formed. Within this project, we aim to develop a methodology to assemble spherical, isotropic colloidal particles into anisotropic structures. It has been shown theoretically that by tailoring the interaction potential, colloidal particles can self-organize into anisotropic phases as a result of a soft repulsion shoulder in the interaction potential. Upon increasing surface coverage, the particles form chain-like assemblies with overlapping repulsion shells within the chain to minimize the repulsive interactions to particles in the neighboring chain. In preliminary experiments, we have discovered that similar phases are formed by the assembly of colloidal particles at the air/water interface in the presence of surface-active agents. The physical origin of the assembly process and the requirements towards colloidal particles, surface active agents and process conditions are not yet clear. The striking similarities between computer simulations of particles with a soft repulsion shell, however, may indicate a similar origin for the chain formation in the experiment. We hypothesize that the surface active agents present at the air/water interface interact with the colloidal particle surface and form a loosely bound shell that induces a soft repulsion barrier to the interaction potential between the particles. Within this proposal, we seek to fundamentally understand and control this assembly phenomenon. We will screen the assembly conditions leading to the formation of anisotropic structures, create of a phase diagram and generate an understanding of the interaction between surface-active moieties and the colloidal particles. Monte Carlo simulations will be performed to support the model, establish phase diagrams for different shapes and ranges of the repulsion shoulder and thus to guide the experimental design towards regions in the parameter space with anisotropic structures.