Future optical materials promise to do for photonics what semiconductors did for electronics, but the challenge has long been in creating the structure they require—a regular, three-dimensional array of transparent microspheres arranged like the atoms in a diamond crystal. Here we demonstrate a simple approach for spontaneously growing double-diamond (or B32) crystals that contain a suitable diamond structure, using DNA to direct the self-assembly process. While diamond symmetry crystals have been grown from much smaller nanoparticles, none of those previous methods suffice for the larger particles needed for photonic applications, whose size must be comparable to the wavelength of visible light. Intriguingly, the crystals we observe do not readily form in previously validated simulations; nor have they been predicted theoretically. This finding suggests that other unexpected microstructures may be accessible using this approach and bodes well for future efforts to inexpensively mass-produce metamaterials for an array of photonic applications.

The degree to which DNA-linked particle crystals, particularly those composed of micrometer-scale colloids, are able to dynamically evolve or whether they are kinetically arrested after formation remains poorly understood. Here, we study a recently observed displacive transformation in colloidal binary superlattice crystals, whereby a body-centered cubic to face-centered cubic transformation is found to proceed spontaneously under some annealing conditions. Using a comprehensive suite of computer simulation tools, we develop a framework for analyzing the many displacive transformation pathways corresponding to distinct, but energetically degenerate, random hexagonal close-packed end states. Due to the short-ranged, spherically symmetric nature of the particle interactions the pathways are all barrierless, suggesting that all end states should be equally likely. Instead, we find that hydrodynamic correlations between particles result in anisotropic mobility along the various possible displacive pathways, strongly selecting for pathways that lead to the fcc-CuAu-I configuration, explaining recent experimental observations. This finding may provide clues for discovering new approaches for controlling structure in this emerging class of materials. (10) demonstrated that DNA-mediated interactions can be used to produce large quantities of small, micrometer-scale colloidal clusters that may be useful as building blocks in hierarchical assembly of more complex structures.Current efforts are now directed toward the development of a systematic framework for a priori design of DNA oligomer and particle characteristics given a desired target DNA-linked particle assembly (DLPA). As with any other class of materials, the first step in this process is the calculation of thermodynamic phase diagrams that establish the lowest free energy structure as a function of particle and DNA characteristics (e.g., particle size and shape, DNA hybridization free energy, and DNA graft density and distribution) (examples in refs. 11-13). However, it is increasingly apparent that kinetic factors may play important roles in establishing the products observed in experiments (11,(13)(14)(15)(16). For example, we have previously demonstrated that kinetic limitations in the rates of particle attachment and detachment at the growth interface of binary superlattice colloidal crystallites lead to unexpected competition between different phases and the emergence of nonequilibrium structures (6,11,17). In ref. 8, the intentional introduction of kinetic limitations via a reduction of grafted brush density on Au nanoparticles was similarly shown to lead to metastable phases that transform to lower-energy phases upon annealing, consistent with the presence of polymorphism during nucleation (11, 18-21). Moreover, recent studies have used purposefully induced kinetic arrest to generate binary colloidal gels with controllable mesoscopic structure (15,22).These examples raise questions regarding the stability of nonequilibrium DLPA structures, i.e., whether ...

Transient bridges of DNA have been used to direct the self assembly of microscopic spherical particles into a variety of crystal structures. Here, by selectively reprogramming the strength of the DNA interactions within such crystals we form colloidal clusters with well-defined valence and symmetry at high yield. We first form 'host' crystals containing a small proportion of 'impurity' particles bearing a unique DNA sequence, and then add soluble DNA strands that cause the host crystal to melt while preserving the nearest neighbor bonds around each impurity particle. This yields clusters with cubical and cuboctahedral symmetry from host crystals having BCC and FCC structures, respectively. Annealing of these clusters leads them to transform into lower free energy, but still highly symmetric forms, sometimes accompanied by the ejection of particles. The interactions between such clusters in principle could be further reprogrammed to allow hierarchical assembly processes.

Spherical colloids covered with grafted DNA have been used in the directed self-assembly of a number of distinct crystal and gel structures. Simulation suggests that the use of anisotropic building blocks greatly augments the variety of potential colloidal assemblies that can be formed. Here, we form five distinct symmetries of colloidal clusters from DNA-functionalized spheres using a single type of colloidal crystal as a template. The crystals are formed by simple sedimentation of a binary mixture containing a majority "host" species that forms close-packed crystals with the minority "impurity" species occupying substitutional or interstitial defect sites. After the DNA strands between the two species are hybridized and enzymatically ligated, the results are colloidal clusters, one for each impurity particle, with a symmetry determined by the nearest neighbors in the original crystal template. By adjusting the size ratio of the two spheres and the timing of the ligation, we are able to generate clusters having the symmetry of tetrahedra, octahedra, cuboctahedra, triangular orthobicupola, and icosahedra, which can be readily separated from defective clusters and leftover spheres by centrifugation. We further demonstrate that these clusters, which are uniformly covered in DNA strands, display directional binding with spheres bearing complementary DNA strands, acting in a manner similar to patchy particles or proteins having multiple binding sites. The scalable nature of the fabrication process, along with the reprogrammability and directional nature of their resulting DNA interactions, makes these clusters suitable building blocks for use in further rounds of directed self-assembly.

Many approaches to the self-assembly of interesting microstructures rely on particles with engineered shapes. We create dimpled solid particles by molding oil droplets in the interstices of a close-packed colloidal crystal and polymerizing them in situ, resulting in particles containing multiple spherical dimples arranged with various polyhedral symmetries. Monodisperse micrometer-sized droplets of 3-methacryloxypropyltrimethoxysilane (TPM) are mixed with an excess of polystyrene (PS) microspheres (2.58 μm) and allowed to sediment, forming colloidal crystals with TPM droplets inside their interstitial sites. When these crystals are compressed by partial drying, the trapped droplets wet the multiple microspheres surrounding them, forming a three-dimensional capillary bridge with the symmetry of the interstitial spaces, and then mild heating triggers polymerization in situ. Depending on the initial particle volume fraction and extent of drying, a high yield of dimpled particles having different symmetries including tetrahedra and cubes is obtained. The fractional yield of different shapes varies with the size ratio of the TPM droplets and the PS microspheres forming the template lattice. Sedimentation velocity fractionation methods are effective for enrichment of specific symmetries but not complete purification. Our approach for forming polyhedral particles should be readily scalable to larger samples and smaller sized particles if desired.