(727i) Realizing Non-Close-Packed Crystal Structures through Directional Binding of DNA-Functionalized Colloidal Clusters

A diverse range of ordered configurations have been demonstrated using self-assembling particles driven by one or more of a number of strategies, including external fields, depletion, and functionalized particles. In spite of this success, it is apparent from consideration of atomic and molecular systems that many interesting and useful configurations require anisotropy for their realization. Anisotropy has been engineered into colloidal systems through a variety of means. For example, anisotropically-shaped nanoparticles with simple interparticle interactions have been assembled into numerous crystalline, quasicrystalline, and disordered configurations [1]. Here, it has been shown that directional entropic interactions between particles are primarily responsible for the resulting configurations, making it somewhat difficult to predict a priori the assembly outcome [2]. A more direct approach is to functionalize spherical particles in such a way as to produce directional interactions, resulting in so-called â??patchy particlesâ?? [3]. While conceptually appealing because of the atomic orbital analogy, it is generally difficult to accomplish such patterned functionalization in practice.

An alternative approach for engineering assembling entities that exhibit anisotropy is to create small clusters out of spherical colloidal particles. Here, we study computationally the self-assembly of large numbers of colloidal clusters formed by a recently-introduced crystal templating method [4]. In this technique, bulk crystals are first assembled with DNA-functionalized sub-micron sized polystyrene beads. A small number of beads are designed to bind particularly strongly with their neighbors so that upon subsequent melting of the bulk crystal, these particles form the center of individual small clusters with a specific geometry. Rather than attempt to assemble these clusters directly into superstructures, we study the assembly of clusters mediated by the addition of spherical â??bondâ?? particles. In particular, the â??bondâ?? spheres are DNA-functionalized so that they interact attractively with the clusters. In this construct, the differing number of particle-particle contacts available between a cluster and bond particle as a function of orientation creates directional bonding.

A variety of simulation techniques are employed to study the thermodynamics and kinetics of the assembly process. Specifically, we compute nucleation barriers for several crystalline configurations using tetrahedral, octahedral, and cubic clusters. We show that some cluster types lead to facile growth of crystalline superstructures, while others lead to structures that are highly susceptible to defect formation. Crystal growth kinetics are probed using molecular dynamics and Brownian dynamics simulations and again demonstrate a wide range of kinetic limitations depending on the cluster geometry.

[1] X. Ye, J. E. Collins, J. C. Y. Kang, D. T. N. Chen, A. G. Yodh, and C. B. Murray, PNAS 107, 22430 (2010).

[2] P. F. Damasceno, M. Engel, and S. C. Glotzer, Science 337, 453 (2012).

[3] F. Romano and F. Sciortino, Nat. Commun. 3, 1968 (2012).

[4] J. T. McGinley, Y. Wang, I. C. Jenkins, T. Sinno, and J. C. Crocker, ACS Nano 9, 10817 (2015).