The properties of inorganic nanoscale particles are largely determined by their surfaces, as the fraction of surface atoms can approach unity as the size approaches 1 nm. As a result, the coordination of ligands to the particle surface can quickly become the dominant energetic contribution to the system and therefore provides an opportunity to use molecular design principles to control the formation of well-defined inorganic materials. However, challenges in characterizing the ligand-particle interface and a lack of mechanistic understanding of the role of ligands in surface reactions has limited the implementation of these structures in a variety of applications. In this talk, I will discuss recent efforts by my group to address fundamental questions in nanoscale surface chemistry and leverage these insights to construct nanoparticle-based materials with novel properties. First, I will show that advanced cryogenic and liquid-phase transmission electron microscopy techniques can be used to map the spatial distribution of ligands on a nanoparticle surface and directly observe the dynamics of symmetry breaking during particle growth. Second, I will report our finding that the “seed” nanoparticle that has been widely used as a precursor in anisotropic gold particle syntheses over the last two decades is, in fact, an atomically-precise inorganic cluster consisting of a 32 atom Au core with 8 halide ligands and 12 neutral ligands constituting a bound ion pair between a halide and the cationic surfactant: Au32X8[AQA+•X-]12 (X = Cl, Br; AQA = alkyl quaternary ammonium). This result establishes a molecular precursor with well-defined surface ligands as the progenitor to larger nanostructures and is a critical first step in understanding particle growth mechanisms. Finally, I will show how control over the surface chemistry of tetrahedron-shaped particles facilitates their assembly into novel superlattices with chiral and quasicrystalline order. These materials, whose construction is enabled by the atomic-scale understanding developed in my lab, will form the basis for future optical and/or mechanical metamaterials, highlighting the power of molecular control over inorganic matter.
