Assembly of trivalent particles under confinement: from an exotic solid phase to a liquid phase at low temperature

Phases of surface-confined trivalent colloidal particles

Patchy colloids promise the design and modelling of complex materials, but the realization of equilibrium patchy particle structures remains challenging. Here, we assemble pseudo-trivalent particles and elucidate their phase behaviour when confined to a plane. We observe the honeycomb phase, as well as more complex amorphous network and triangular phases. Structural analysis performed on the three condensed phases reveals their shared structural motifs. Using a combined experimental and simulation approach, we elucidate the energetics of these phases and construct the phase diagram of this system, using order parameters to determine the phase coexistence lines. Our results reveal the rich phase behaviour that a relatively simple patchy particle system can display, and open the door to a larger joined simulation and experimental exploration of the full patchy-particle phase space.

Visualizing defect dynamics by assembling the colloidal graphene lattice

Graphene has been under intense scientific interest because of its remarkable optical, mechanical and electronic properties. Its honeycomb structure makes it an archetypical two-dimensional material exhibiting a photonic and phononic band gap with topologically protected states. Here, we assemble colloidal graphene, the analogue of atomic graphene using pseudo-trivalent patchy particles, allowing particle-scale insight into crystal growth and defect dynamics. We directly observe the formation and healing of common defects, like grain boundaries and vacancies using confocal microscopy. We identify a pentagonal defect motif that is kinetically favoured in the early stages of growth, and acts as seed for more extended defects in the later stages. We determine the conformational energy of the crystal from the bond saturation and bond angle distortions, and follow its evolution through the energy landscape upon defect rearrangement and healing. These direct observations reveal that the origins of the most common defects lie in the early stages of graphene assembly, where pentagons are kinetically favoured over the equilibrium hexagons of the honeycomb lattice, subsequently stabilized during further growth. Our results open the door to the assembly of complex 2D colloidal materials and investigation of their dynamical, mechanical and optical properties.

Kagome lattice made by impenetrable ellipses with attractive walls

Low-dimensional structures, such as the kagome lattice, are experiencing renewed interest within the physics, chemistry and materials science communities in terms of both basic and applied research. Herein, we show that stable kagome lattices can be made by hard-core ellipses with attractive walls. We study a model in which hard-core ellipse is covered uniformly by an attractive square-well layer. Analytical calculations predict that for certain combinations of the asphericity aspect ratio and the attraction range, the kagome lattice is the ground-state conformation of this model. For one specific set of parameters computer simulations prove that the kagome lattice is the lowest free energy structure at low temperatures. At high temperatures, the conformational ensemble is dominated by liquid states. The temperature at which transition from the liquid to the kagome structure occurs has a maximum as a function of density, indicating that the underlying phase transformation is re-entrant. The maximum is attributed to the energy difference between the liquid and crystalline states. Our study reveals that the kagome lattice can be produced by means of very simple models. No specifically designed molecular shapes or interactions are required. Instead, very basic physical characteristics, such as asphericity and uniform attraction, are sufficient to induce spontaneous transition into this structure. In the context of the general understanding of the self-assembly processes, this finding is encouraging, giving one hope that the requirements for the assembly of other low-dimensional structures could be equally simple.

Magnetic Coupling in Colloidal Clusters for Hierarchical Self-Assembly

Manipulating the way in which colloidal particles self-organize is a central challenge in the design of functional soft materials. Meeting this challenge requires the use of building blocks that interact with one another in a highly specific manner. Their fabrication, however, is limited by the complexity of the available synthesis procedures. Here, we demonstrate that, starting from experimentally available magnetic colloids, we can create a variety of complex building blocks suitable for hierarchical self-organization through a simple scalable process. Using computer simulations, we compress spherical and cubic magnetic colloids in spherical confinement, and investigate their suitability to form small clusters with reproducible structural and magnetic properties. We find that, while the structure of these clusters is highly reproducible, their magnetic character depends on the particle shape. Only spherical particles have the rotational degrees of freedom to produce consistent magnetic configurations, whereas cubic particles frustrate the minimization of the cluster energy, resulting in various magnetic configurations. To highlight their potential for self-assembly, we demonstrate that already clusters of three magnetic particles form highly nontrivial Archimedean lattices, namely, staggered kagome, bounce, and honeycomb, when focusing on different aspects of the same monolayer structure. The work presented here offers a conceptually different way to design materials by utilizing preassembled magnetic building blocks that can readily self-organize into complex structures.

Colloidal molecules and patchy particles: complementary concepts, synthesis and self-assembly

About the latest developments regarding self-assembly of textured colloids and its prospects.