Assembling pseudo-trimeric patchy colloids confined to a surface

2D materials have attracted great interest in the past decades. For instance, graphene has exotic mechanical and electronic properties that are not typically found in bulk materials. Unfortunately, the assembly of 2D materials remains hard to study, as direct observation is challenging. However, by using patchy particles we can mimic atomic 2D structures using colloids, which can be studied in real time using microscopy.
Here, we use 4-patch particles with a tetrahedral geometry to build a 2D material. We adsorb one of the patches to the cover glass so that particles can diffuse laterally, but cannot release from the surface. The three remaining, triangularly coordinated patches bond with those of other particles. We thus have particles that are effectively trivalent, and are confined to a plane, much like the carbon atoms in graphene.
Using this colloidal system, we directly observe the formation of a honeycomb crystal structure, and investigate its assembly and structural phases. Interestingly, at finite patch width and bond angles, the phase diagram is much richer than the honeycomb crystal structure alone. Our observations, which are in good agreement with simulation predictions, provide detailed insight into the assembly of graphene, and 2D materials in general.