A group of researchers from the University of Pennsylvania, US, have unexpectedly grown colloidal crystals with diamond symmetry, known as double diamond crystals (B32), using a DNA-mediated self-assembly process.

The advancement of current optical materials has promised to do for photonics what semi-conductors did for electronics, but efforts so far have struggled to find the required structure to facilitate this. Such materials require a regular, 3-dimensional network like that of a diamond crystal.

Efforts to produce diamond symmetry in the past have been attempted, and succeeded, but could only be grown from small nanoparticles. Larger particles are needed for actual applications in photonics and no method before now has been able to produce application ready crystals.

It has been notably difficult to form diamond-type lattices through colloidal crystallisation due to issues with the filling fraction and mechanical stability. Many approaches from isotropic interactions, to ‘patchy colloids’ have been attempted, but with no success. The researchers who created the double diamond crystal employed a completely novel approach using a self-assembly approach to growing ‘scaffolded’ diamond crystallites using 400 nm polymer microspheres.

The microspheres were also grafted with complimentary DNA strands onto their surface. This allows for a rapid and reversible DNA-bridge formation to occur that can increase the short-range attraction and drive the spontaneous nucleation growth of large colloidal crystals.

The crystals formed have what is known as a double-diamond structure (also known as a B32 structure)- this is where the supporting scaffold for the crystal acts as a second lattice composed of small and compositionally different microspheres, which interpenetrates the first lattice. The lattice structure is a cuboctahedron composed of square and triangular faces and 104 microspheres. The two interpenetrating lattices also show a lattice structure identical to that of diamond.

The formation of these crystals came as a shock, even to the researchers who created them. Whilst there has been one known case of a double diamond crystal being used in a nanotechnology system, it was unexpected when a mechanism involving DNA colloids was used. It was thought that only next-nearest neighbour interactions were required to form a double diamond crystal through thermodynamic stability. Something which is lacking in DNA colloids.

However, it appears that a new mechanism and molecular interactions can also produce such crystals, which is what has led to the manifestation of DNA colloid double diamond crystals. The crystals are engineered from a fluid phase by a non-classical homogeneous nucleation approach, which produces smaller binding energies and extreme structural deformability. The interactions in play within these double diamond crystals range from reversible, isothermal interaction potentials to DNA bridges and ‘sticky’ particles- particles where length scales are shorter than the particle radius.

An interesting part of this research is that the material never appeared in any of their theoretical calculations and computational models. The reason being that the lattice of the crystal itself is energetically unfavourable, but the experimental was found to be kinetically favourable, not thermodynamically. So, although the simulations would not depict such a structure, the experimental showed otherwise and has prompted the thought that other useful unknown structures could be created using this approach.

Within the scientific community, there has been a recent surge in computational methods to solve experimental problems and predict the molecules before even setting foot into the laboratory. The sobering thought, even with all the focus on computational methods, not all scientific research can be predicted; and the production of materials from spontaneous discoveries opens unseen avenues that bode well for the future to produce inexpensive, mass-produced metamaterials which could be used across a range of photonic applications. By dissolving some of the scaffold and cross-linking parts of the crystal, we could also see a scalable route for the production of self-assembled diamond crystals with interesting metamaterial properties.

Source:

Wang Y., Jenkins I. C., McGinley J. T., Sinno T., Crocker J. C., Colloidal crystals with diamond symmetry at optical lengthscales, Nature Communications, 2017, 8, 14173

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