Synthesis and Transmission Electron Microscopy of Hexagonal Boron Nitride Nanostructures and Transition Metal Trichalcogenides in the Single Chain Limit

Hexagonal boron nitride (h-BN) nanomaterials exhibit peculiar morphologies and properties making them uniquely suitable for a host of applications. In this dissertation, I will introduce different approaches to synthesize various h-BN nanostructures, ranging from aerogels (three-dimensional porous network) and sheets (two-dimensional layers) to tubes (one-dimensional tubular material). The materials synthesis includes high-temperature carbothermic conversion (for h-BN aerogels and sheets) and extended pressure inductively-coupled (EPIC) nitrogen plasma (for h-BN nanotubes). The as-synthesized nanomaterials are characterized by different techniques with the emphasis on transmission electron microscopy (TEM), including both in-situ TEM and aberration-corrected TEM.In the second part of this dissertation, I focus on the discovery and structural characterization of the single chain limit of transition metal trichalcogenides (such as niobium triselenide, NbSe3), quasi one-dimensional van der Waals materials. By using carbon and boron nitride nanotube as nano-reactors, few-to-single chain NbSe3 can be isolated and stabilized. An aberration-corrected scanning/transmission electron microscopy (S/TEM) study shows that a single chain of NbSe3 exhibits structural torsional waves not found in bulk NbSe3 crystals. Density functional theory calculations reveal that charge transferred from nanotubes to the encapsulated single chain is responsible for the torsional instability. We term the phenomenon Charge-induced Torsional Wave (CTW). The study is extended to explore other transition metal trichalcogenides in the single chain limit, for example hafnium tritelluride, HfTe3. A newly segmented chain structure of HfTe3 is observed with potential non-trivial topological edge states