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A Monte Carlo Analysis of DNA-Mediated Crystallization In Binary Systems of Colloidal Particles

Raynaldo Scarlett1, John C. Crocker2, and Talid Sinno1. (1) Chemical and Biomolecular Engineering, University of Pennsylvania, 220 South 33rd St, Philadelphia, PA 19104, (2) Department of Chemical and Biomolecular Engineering, University of Pennsylvania, 220 South 33rd Street, Philadelphia, PA 19104

Colloidal self-assembly provides a potentially expeditious route for fabrication of structures with unique optical properties [1]. Colloidal systems also provide useful insights into fundamental mechanisms of phase transitions such as crystal nucleation [2], growth [3] and melting that are otherwise difficult to probe in atomic systems.

While there has been extensive study aimed at characterizing thermodynamics of crystallization in colloidal systems, potential kinetic limitations have been largely ignored. In this presentation, we use a recently developed experimental system as a basis for performing detailed Monte Carlo studies of binary crystallization [4]. The system consists of micron-scale polystyrene spheres grafted with single strands of DNA, which interact with each other via a complementary linker DNA molecule present in the surrounding solution. By engineering the DNA sequences on both the grafted and linker strands, it is possible to create binary systems with a variety of crystallization behavior. An important aspect of the simulations is that the effective interaction potential produced by the DNA binding has been measured experimentally and modeled with high accuracy, thereby allowing for a close connection between simulation and experiments [5].

We begin by discussing kinetically limited segregation that can arise during the crystallization of random fcc binary A-B solid solutions in which the A-A attraction is stronger than the A-B or B-B interaction. We then address the richer situation where A-B interactions are stronger than A-A or B-B interactions, which has recently been shown experimentally to lead to the formation of bcc superlattice structures in gold nanoparticle crystallization [6]. A combination of thermodynamic and kinetic factors is shown to lead to a selection between fcc and bcc superlattice structures as the simulation conditions are varied. Moreover, kinetic limitation are shown to strongly affect the defect density in the resulting superlattice structures. The simulations are shown to explain various features observed in the nanoparticle studies. These results should be useful for finding optimal conditions for experimentally growing crystals with desired structures and low defect densities

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