NANOSYM · Simulation of directed self-assembly of nanocrystals

Simulation of directed self-assembly of nanocrystals

Modern technologies require a diversity of new metamaterials with highly specific functions that cannot be created using traditional synthetic methods. Fortunately, under certain conditions, natural forces such as entropy, electrostatics and van der Waals interactions can be recruited to drive the assembly of nanoparticles into ordered novel structures. The growth mechanism depends on a complicated interplay of interparticle forces, packing constraints and dynamics that are yet poorly understood. At present, experiments are well ahead of theory and simulation, yet a lack of control over design and functionality of metamaterials reduces their potential applicability. Theoretical and numerical studies can provide helpful guidelines to assist in the design of nanoparticle-based crystals. In particular, the formation probability of a critical nucleus can be computed via Monte Carlo umbrella sampling techniques, whereas an interfacial free energy between crystalline nanocluster and surrounding medium, that defines the height of the nucleation free energy barrier, cannot be estimated with direct methods. Therefore, during the first funding period the participant was actively collaborating with the group of Prof. Binder in Johannes Gutenberg University, Mainz, and Prof. Tosatti in the International Centre for Theoretical Physics, International School for Advanced Studies (ICTP/SISSA), Trieste, on the projects dedicated to the development of advanced theoretical and computational techniques for the solid-liquid interfacial free energies. In order to define the interfacial free energies we calculated the chemical potential excess relative to the coexistence curve as a function of order parameters and compared the results to the capillary wave fluctuations. The methods were tested on a variety of systems including Lennard-Jones, hard spheres and polymer-colloid mixtures and gave rise to several publications. We subsequently applied a state of the art computational machinery to explore the formation of binary colloid superlattices that were observed by the frontiers in the field. Superlattices with AB-, AB6, AB13, Cu3Au3, CaB6, NaZn13 and cub-AB13 particle stoichiometry with cubic, hexagonal, tetragonal and orthorhombic symmetries were identified. The structural diversity was essentially attributed to the balance of electric charges and dipole moments. At the initial stage we performed an extensive literature review to help us design a realistic interaction model. However, we had to revise our approach after a visit to the nanocolloidal group of Prof. Talapin at the University of Chicago. Our initially formulated model had to be modified and updated according to the recent developments in the experiments. In particular, new data on electrophoretic mobility and strong temperature dependence of binary nanoparticle superlattice (BNSL) structures forced us to deviate from the proposed objectives. As a consequence, some of the project related publications were delayed, but were in progress by the time of NANOSYM completion.

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