(149b) Templated Assembly of Two-Dimensional Hard-Rod Fluids

Hard rods are interesting building blocks for assembly as they organize themselves into different phases depending on their aspect ratio, concentration, and subtle details of their geometry. Much work has been done to develop an understanding of the various phases exhibited by bulk, hard-rod fluids in two and three dimensions. Work in this area is exciting as the non-symmetric geometries of the particles results in phases that symmetric particles such as disks or spheres do not exhibit. Three-dimensional work on spherocylinders has shown phases such as the lamellar and smectic phases. In addition, short two-dimensional rectangles have shown a phase called tetratic where small domains of rectangles align perpendicular to each other. For certain applications, three-dimensional assembly may not be practical but may provide a framework for understanding particle assembly in two dimensions. For applications in nanoelectronics, it is desirable to control the assembly of nanowires in two dimensions to achieve various structures conducive to nanocircuitry. In our work, we use orientational-bias Monte Carlo simulations to examine the phase behavior of two different systems involving two-dimensional, colloidal nanowires. Our nanowires are modeled as two-dimensional non-interacting rectangles. In the first study, we probe the influence of periodic, hard barriers on the ordering of two-dimensional hard rods. By varying the inter-barrier spacing relative to the rod length, as well as the rod aspect ratio and concentration, we demonstrate that barriers can induce a change from the bulk properties and effectively tune the alignment of the rods. For example, fluids that exhibit an isotropic phase in the bulk can form a uniaxial nematic phase oriented parallel to the barrier walls under confinement. In a second study, we probe the hard-disk templated assembly of colloidal nanowires. We demonstrate that a number of interesting and potentially beneficial phases occur as the rod length relative to the disk diameter, rod aspect ratio and species concentrations are varied. As in the single component rod system, rods and disks are confined to lay between infinitely long, periodically spaced hard barriers.