Synthesis and Assembly of Two-Dimensional Materials in Constrained Geometries: MXenes for All-Liquid Electronics and Nanotube-Encapsulated HfTe2 Nanoribbons

Many chemists and materials scientists strive to understand the structure-propertyrelationship of materials–how the local atomic structure produces the observed optical, thermal and electronic properties. While undoubtedly important, the morphology of the material–how it is shaped or connected beyond the local crystal structure–is also of key importance for applying the material in a given setting. In this dissertation, two examples of morphological control at different size scales are demonstrated. The first focuses on how two-dimensional (2D) materials can be assembled at the liquid-liquid interface and placed in arbitrary shapes using the chemistry of structured liquids. The second focuses on how nanoscale confinement can be used to direct 2D materials towards a 1D morphology through a bottom-up synthesis. Chapter 1 provides a general introduction to these two projects, contextualizing the work for a general audience. After Chapter 1, the dissertation is divided into two parts. Part 1 discusses the field of Structured Liquids, beginning with some background on the field, then discussing recent work involved in using these chemistries to create conductive interfacial films. Chapter 2 provides a background on solubility, the fundamentals of the chemistry of structured liquids, as well as a brief review of how the field has developed since its inception in 2013. From there, information is provided on MXenes and initial attempts towards creating conductive all-liquid pathways. The chapter ends with a detailed discussion of electrical characterization, providing the theory needed to understand the electrical measurements conducted in the next chapter. Chapter 3 discusses the application of MXenes and the structured liquids chemistry to demonstrate all-liquid electronic devices. Building on previous work assembling these materials at liquid-liquid interfaces, the electrical properties of self-assembled MXene networks are investigated and then these interfacial films are used to demonstrate simple electronics applications as a proof of concept. This work shows how specific chemical environments can be used to direct the self-assembly and macroscale morphology for a desired application. Part 2 describes the synthesis of nanotube-encapsulated structures, switching focus from macroscale assemblies of many different nanoparticles to the controlled synthesis of individual nanostructures within carbon nanotubes. Chapter 4 provides a background to this field, introducing the transition metal dichalcogenides and their synthesis, reviewing the group’s work iv on nanotube-encapsulation, and providing some basics on the electron microscopes used to characterize the encapsulated materials. Chapter 5 discusses the synthesis and characterization of HfTe2 nanoribbons encapsulated within carbon nanotubes. Upon imaging the materials, it was noticed the nanoribbons undergo a phase change within the electron microscope so a variety of theoretical calculations are used to elucidate the cause and energetics of this phase change. Chapter 6 summarizes all of this work, emphasizing the connection between the projects and discussing the power of morphological control at various size scales. Future directions are presented for both projects to chart a path for anyone continuing this line of research.

Advisor: Zettl, Alex
Long, Jeffrey