Polymorphism and transformations of DNA-grafted colloids yield a panoply of crystal structures

While the spontaneous self-assembly of colloidal building blocks into opal-like crystals has long been considered a promising route for the fabrication of meta-materials, rather few structures have been demonstrated to form. Here we find that, contrary to simulations, binary DNA-grafted microspheres often simultaneously nucleate and grow different crystal forms (polymorphs) in the same sample, and that these unstable ‘floppy’ crystals, during slow cooling, transform into still more crystal structures. Remarkably, up to seven different crystal structures could be observed in a single sample. Varying the interactions and particle sizes, we found a total of ten different crystalline structures, including 6 not previously predicted or observed. Simulations with validated potentials predict that such transformations should result only in poorly ordered final states. We find that the crystals transform in a coherent manner into well-ordered final structures across all the parameters we explored, presumably controlled by kinetic factors, such as hydrodynamic interactions and the kinetics of the DNA bridges between particles. Our results indicate that a previously unsuspected range of crystal structures are accessible if the physics controlling their formation can be understood.