Diffusionless transformations in DNA-mediated alloys of nano-colloids

Nanoscale self-assembly is investigated using the specific interaction of DNA hybridization. Ordered arrays produced using the sequence-specific binding properties of DNA are expected to possess useful optical and electronic properties, which are not easily achieved through traditional fabrication methods. This thesis demonstrates that if specific highly ordered structures are to be targeted, an ability to understand and experimentally control the interaction matrix among the particles involved in the self-assembly process is essential. We first propose a novel methodology for homogeneously nucleating DNA-mediated binary alloys of one-sized colloids of several hundreds of nanometers in diameter. We explain how several critical experimental variables are carefully adjusted for optimizing the nucleation kinetics. Since nucleation rates involve exponentials of rapidly varying free energies, they are extremely sensitive to experimental conditions. The beauty of working with colloidal systems is that their phenomenology can be readily observed using standard macroscopic tools. This motivates us to develop a fluorescence based methodology, which uses non-polar fluorophores, to unmistakably distinguish the two populations of spheres from one another. Combining this with differential interference contrast microscopy allows us to determine the type of crystal lattice along with its compositional ordering. The interplay between the different attractive interactions is observed to have a profound kinetic effect on the assembly process, dictating the ordering of the thermodynamically favored structures. In fact, varying pairs of binding energies can produce both body-centered cubic and close-packed superlattice structures with various levels of ordering. A qualitative closure is obtained with a simulation framework that quantitatively predicts the required experimental conditions for the formation of well-equilibrated superlattices. This thesis reports the successful self-assembly of perfect binary face-centered-cubic lattices reminiscent of CuAu-like crystal structures. These structures are kinetically extremely difficult to grow but experimental evidences demonstrate that they actually originate from diffusionless transformations of CsCl-like crystals. These structural transformations are widely observed in metals. In fact, steel technology uses this property to produce a broad variety of microstructures with different physical characteristics. Finally, our experimental observations are compared to well-established orientation relationships between the two phases, which in turn motivates the proposal of a possible lattice correspondence between CsCl and CuAu-like crystals, which has yet to be seen in the literature