A mechanistic view of binary colloidal superlattice formation using DNA-directed interactions

The use of grafted, single-stranded DNA oligomer brushes with engineered sequences to effect tunable interactions between colloidal particles has now been demonstrated experimentally in both nano-and microscale systems. The versatility of this technology is highly appealing for realizing self-assembly of complex structures. However, ambiguities remain regarding how operating conditions, such as the rate at which the system temperature is reduced, interact with the other system parameters to produce crystalline assemblies with particular structures and defect densities. In this paper, a computational analysis is presented for the crystallization of binary superlattice crystals comprised of sub-micron colloidal spheres using a realistic model for the DNA-mediated interactions. The binary system consists of two populations of identical spheres that differ only in the sequence of the DNA oligomers grafted onto their surfaces. Metropolis Monte Carlo simulations and perturbation theory for free energy estimation are used to construct a detailed mechanistic picture for binary superlattice formation. The analysis reveals several interesting features of this system, particularly the role of kinetics in dictating not only the quality of the superlattice crystals, but also their crystalline structure. Using the results presented here, we make connections to recent experimental findings in similar binary crystallization systems.