Abstract
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.
Highlights
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, threedimensional array of transparent microspheres arranged like the atoms in a diamond crystal
A favourable three-dimensional metamaterial consists of transparent spheres arranged on a cubic diamond lattice[2], which has led to a multi-decade effort to form diamond structures using lithography[3], micromanipulation[4] or holography[5] as well as self-assembly approaches based upon liquid crystals[6], nanoparticles[7,8,9] or colloidal crystallization[10,11,12,13,14,15]
One example is isomorphic to the MgCu2 Laves phase[12,21] (Fig. 1c), in which the ‘scaffold’ consists of smaller ‘Cu’ spheres[11] arranged into a second diamond lattice of tetrahedral clusters of spheres[15]
Summary
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, threedimensional array of transparent microspheres arranged like the atoms in a diamond crystal. One approach is to use isotropic interactions that combine a long-range repulsion with a short-ranged attraction[10,13,14] (Fig. 1a) While this approach has led to the experimental formation of diamond-like crystals of oppositely charged nanoparticles[7], it does not appear to be adaptable to the larger scales required for photonic materials. Many of the resulting crystals have a well-ordered ‘double diamond’ (DD) or B32 structure— where the ‘scaffold’ is a second diamond lattice of smaller and different-composition spheres (Fig. 1d) interpenetrating the first This structure is isomorphic to the NaTl Zintl phase[32] in atomic solids. Matched simulations fail to nucleate or grow such DD crystals directly from a fluid phase, suggesting non-classical mechanisms for both processes This explanation is supported by the crystallites extreme structural deformability and the experimental observation of reconstructed surfaces. Crosslinking[34] such crystals and dissolving the smaller scaffold species could provide a facile and scalable route for self-assembling diamond crystals that would have interesting and useful metamaterial properties
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