Structural phases of colloids interacting via a flat-well potential

Using Langevin dynamics simulations we investigate the self-assembly of colloidal particles in two dimensions interacting via an isotropic potential, which comprises both a hard-core repulsion and an additional softened square-well potential of controllable width alpha. In dilute concentrations, the particles assemble in small clusters with a well-defined crystalline order. For small values of alpha the particles form triangular lattices. As alpha is increased, more particles can be captured by the potential well giving rise to different crystalline symmetries and the structural phase transitions between them. The main structures observed are triangular, square, and a mixture of square and triangular cells forming an Archimedean tiling. In the concentrated regime the particles form a single percolated cluster with essentially the same orderings at the same ranges of alpha values as observed in the dilute regime, thus showing that cluster boundary effects have a minor influence on the cluster crystal symmetry. By using energy analysis and geometry arguments we discuss how the different observed structures minimize the system energy at different values of alpha.