Title: Assembly of Multifunctional Materials Using Molecular Cluster Building Blocks
Description: This thesis explores the synthesis, properties, and potential applications of molecular clusters and the hierarchical solids that form when complementary clusters are combined. Chapter 1 introduces the diverse set of molecular clusters that I employ as nanoscale building blocks in the assembly of multifunctional materials. The core structure of the molecular clusters is closely related to the superconducting Chevrel phases. In discrete clusters, however, the core is passivated by organic ligands, which add stability and important functionalities. The molecular clusters have rich physical and chemical properties of their own, and I present some of the techniques used to investigate their intrinsic electronic properties. Finally, I review some of the modes by which the molecular clusters interact with another to assemble into hierarchical solids. The structural tunability and complexity embedded in the molecular clusters will enable the design of modular, well-defined, multifunctional materials with desirable electronic and magnetic properties.

Chapter 2 details the synthesis and characterization of a family of manganese telluride molecular clusters. By varying the ligands that decorate the surface of the inorganic core, I show that the core structures can be tuned. The study of molecular clusters provides insight into how extended solids form. As such, I make structural comparisons of the clusters to known solid-state compounds. Being structurally varied and chemically flexible, the clusters reported in this chapter present an exciting new class of building blocks for the assembly of solid-state compounds. In Chapters 3-4, I present a nanoscale approach to investigate the electronic behaviors of individual molecular clusters. By using a scanning tunneling microscope-based break-junction technique and density-functional theory calculations, I study the effects of the junction environment and the redox properties of the molecular clusters on the conductance of single-cluster junction. Importantly, current blockade effect is observed at room temperature in the single-cluster junctions, allowing for the conductance to be turned on or off by varying the bias potential.

Chapters 5-7 explore the synthesis and properties of the hierarchical solids comprised of molecular cluster building blocks. Chapter 5 unveils an approach to create a three-dimensional (3D) coordination network of molecular clusters by using a bifunctional cyanide ligand. The cyanide ligand is appended to the metal sites of the cluster through the carbon terminus, leaving the nitrogen end available for coordination by a divalent metal cation. Whereas the molecular cluster itself is paramagnetic across a temperature range of 3-300 K, the 3D coordination compound shows a ferromagnetic transition at ~25 K. In Chapter 6, I describe the importance of a molecular recognition feature on the molecular cluster that contributes to the assembly of a layered, van der Waals solid. The bulk material contains monolayers of fullerene and can be mechanically exfoliated to thinner layers, providing a key templated strategy to isolate free monolayers of fullerene. Lastly, Chapter 7 details layered, van der Waals solids of rhenium and molybdenum synthesized using traditional solid-state reactions.

Because the neighboring cluster units are covalently bound together, the inter-cluster coupling is much stronger in the plane of these materials than that of the self-assembled solid described in Chapter 6. The strong two-dimensional (2D) character in these layered materials allows for the exfoliation of bulk crystals into robust, low-defect monolayers. The surfaces of these monolayers are covered with substitutionally labile ligands, which is an atypical yet valuable feature among 2D materials. I demonstrate that the electronic properties of the monolayers can be tuned by exchanging the surface ligands.