New ordered materials through assembly of heterogeneously charged nanoparticles

The materials of the future will not be fabricated via traditional methods that attempt to impose the structure from the top down. Instead, future materials will be allowed to self-assemble without detailed human intervention; the instructions for the assembly emerge from the nature of the forces acting between the building blocks. Future self-assembled materials may find application in photonics such as optical computers, medical devices, and sensors and will need the precision and reliability of biological self-assembly. In recent years many new types of microscopic building blocks, also called colloids, have been developed. Some of these new building blocks have patches on them such that they do not interact the same way in all directions, very much like Lego blocks. This type of building block the scientists call patchy particles. The challenge is to understand how these interact such that we can put them together in the right way to make the build what we want. In this project we will study a common type of directional interactions that has received limited attention so far: the interaction between heterogeneously charged particles. These interactions give rise to very rich behavior because not only are the attractions direction dependent: also the repulsions are anisotropic. In fact, charged areas of both signs on the particle surface have attracting and repelling counterparts: a unique feature that makes that these interactions might prove crucial for the full control over the self-assembly of novel materials. To highlight the contrast to standard attractive patches, heterogeneously charged particles are also called Inverse Patchy Colloids (IPCs). In computer simulations IPCs have been found to show very rich self-assembly behavior leading to novel structures. These can be either flat or in bulky. This project aims at (i) the production of patchy particles with a well-controlled number of patches on their surface, and (ii) the theoretical and experimental study of their collective assembly. To reach these aims, three partners with strongly complementary skills form the bilateral HotCHpot consortium from France and Austria will collaborate. Together they will produce and characterization of patchy particles over a broad range of sizes and use those to study experimentally how these interact with each other. The broad range of sizes allows for applying very different and complementary experimental techniques for the characterization of the self- assembly behaviour. The experimental results will be compared with computer simulations.

I 3577 HOTCHPOT New ordered materials through assembly of heterogeneously charged nanoparticles How to program particles to organize themselves as desired? We synthesize inverse patchy particles; repulsive (charged) colloidal particles with repulsive (oppositely charged) patches at their poles. These patches are in turn attracted to the rest of the particle surface. This gives rise to intriguing orientation dependent attractions and repulsions that are very different to those between conventional patchy particles. We set out to study the self-assembly of the collective systems. The overall objective is to reproduce at the colloidal scale the ability of atoms to develop directional bonds with a limited number of neighbouring atoms and according to a predefined valence scheme. In practice, it is a question of finding scientific and technical solutions to control in time and space the way in which inverse patchy particles can assemble to form materials with one (or several) precise function(s). Among the possible long-term applications, the fabrication of materials for optics or catalysis with very well-defined architectures could be achieved by simply mixing solutions of "pre-programmed" particles: each elementary particle would 'know' in advance what types of other particles it should be positioned next to in order to give the final structure the expected shape and functions. The scientific and technical challenges include: 1) the synthesis of inverse patchy particles in pure and calibrated batches, both in size and in patch number, at the gram scale; 2) their assembly with the development of robust protocols allowing the particles to assemble in a univocal way according to a predefined stoichiometry and arrangement in space; 3) the use of imaging techniques to analyse the dynamics of the assembly; 4) numerical experiments with the development of versatile simulation tools to robustly and efficiently mimic a variety of experimental conditions, from dynamics under gravity to colloidal epitaxy, thus spanning from quasi two-dimensional to bulk conditions. Major results: Our work has led to significant progress with the production of charged particles with oppositely charged patches. We developed a novel approach based on gaseous ligands to chemically modify part of the particle surfaces. During the development of this method we made the serendipitous discovery of yet another approach which we called contact charge printing. We were the first to successfully apply flow cytometry to characterize and purify patchy particles. However, the purity or the yield was still insufficient to study their self-assembly. Finally, we developed a robust, versatile and efficient simulation method.