Self-assembly of binary colloidal nanocrystals

Doctoral thesis (2017)

The synthesis of functional nanoparticles is an important step in the hierarchical construction of hybrid materials for nanotechnological applications. A useful path to build these components is to use ... [more ▼]

The synthesis of functional nanoparticles is an important step in the hierarchical construction of hybrid materials for nanotechnological applications. A useful path to build these components is to use colloidal nanocrystals that can spontaneously agglomerate into ordered structures under confinement. The focus of this thesis is to explore the diversity of superstructures that can be self-assembled using binary dispersions where the dispersed colloids have spherical or quasi-spherical shapes and interact through simple potentials with repulsive cores and short-range attractions. Using computer simulations we demonstrate that agglomeration experiments with heterogeneous binary mixtures of nanoparticles can be exploited for the synthesis of structured clusters which are proposed as potential intermediate building blocks in hierarchical self-assembly of colloidal molecules and crystals. To describe the structural properties of aggregates resulting from confined mixtures of particles with heterogeneous attractions we analyse the structure diagrams of binary Lennard-Jones clusters by means of a basin-hopping global optimisation approach for a broad range of cluster sizes, compositions and interaction energies and present a large database of minimal energy structures. We identify a variety of structures such as core-shell clusters, Janus clusters and clusters in which the minority species is located at the vertices of icosahedra. For a binary mixture with heterogeneous particle diameters we use molecular dynamics simulations to demonstrate that pressure-dependent inter-particle potentials affect the self-assembly route of the confined particles. This is in agreement with experiments where crystalline superlattices, Janus particles, and core-shell particle arrangements form in the same dispersions for moderate changes in the working pressure or the surfactant that sets the Laplace pressure inside the droplets. Comparison of experimental analysis and simulations confirms that the onset of self-assembly depends on particle size and pressure. Finally, we explore regular superlattices into which clusters can arrange by investigating the equilibrium phase behaviour for a monodisperse system of Mackay icosahedra. Monte Carlo simulations show that either a fluid phase, a crystal phase or rotator phases with different degrees of rotational correlations form. We analyse the correlations using the positional and orientational pair correlation functions and find that the densest lattice packing of hard icosahedra is stable at finite temperatures. [less ▲]

Pressure-controlled formation of crystalline, Janus, and core–shell supraparticles

in Nanoscale (2016), (27),

Binary mixtures of nanoparticles self-assemble in the confinement of evaporating oil droplets and form regular supraparticles. We demonstrate that moderate pressure differences on the order of 100 kPa ... [more ▼]

Binary mixtures of nanoparticles self-assemble in the confinement of evaporating oil droplets and form regular supraparticles. We demonstrate that moderate pressure differences on the order of 100 kPa change the particles’ self-assembly behavior. Crystalline superlattices, Janus particles, and core–shell particle arrangements form in the same dispersions when changing the working pressure or the surfactant that sets the Laplace pressure inside the droplets. Molecular dynamics simulations confirm that pressure-dependent interparticle potentials affect the self-assembly route of the confined particles. Optical spectrometry, small-angle X-ray scattering and electron microscopy are used to compare experiments and simulations and confirm that the onset of self-assembly depends on particle size and pressure. The overall formation mechanism reminds of the demixing of binary alloys with different phase diagrams. [less ▲]