Nanoparticles as Meta-Atoms: Programmable Matter at the Nanoscale

Matthew Jones from Rice University

Abstract: The a priori design of inorganic solid state materials is often limited by the compatibility of different atomic precursors and a lack of generalized rules for crystallization. An intriguing way to circumvent these challenges is to imagine nanoparticles as ‘meta-atoms’, where the composition of the core dictates material properties and the surface ligands control bonding and structure formation. However, because most syntheses produce nanoparticles that are spherical, assembled materials are often limited to densely-packed, high-symmetry arrangements. Atomic systems, on the other hand, make use of directional bonds and the principle of valency to create low-symmetry molecules and crystals of impressive sophistication. In this talk, I will introduce the concept of nanoparticle shape anisotropy as a means to mimic the highly-directional interactions found in atoms and molecules. In particular, I will show that when functionalized with duplexed DNA strands, the flat facets of anisotropic particles act to bundle and orient molecules in well-defined surface-normal orientations. Thus, the symmetry and valency of the building block can be controlled by the shape and surface faceting of the underlying nanoparticle. This has profound consequences for creating low-dimensionality nanoparticle superlattices when particles are programmed to assemble via the hybridization of complementary DNA strands. By tailoring the length and sequence of the DNA ligands, crystalline nanoparticle-based materials can be assembled that have no atomic analogues and would be difficult, if not impossible, to synthesize using traditional lithographic techniques. The plasmonic properties of these superlattices are especially sensitive to presence of broken-symmetry nanoparticle arrangements which allow for systematic investigations into the origin of coupled optical modes. Finally, I will explore the synthetic origins of shape anisotropy by introducing liquid-phase transmission electron microscopy as a tool for monitoring single-particle reaction dynamics in real time. Specifically, I will show the development of high-energy surface facets under non-equilibrium reaction conditions and explain their appearance through a coordination-number driven mechanism. Taken together, these results contribute new design rules at the atomic, molecular, and nanoparticle levels for the synthesis of inorganic solid state materials and promise a more complete control over the dimensionality, symmetry, and properties of matter.