Graphene nanoribbons (GNRs) are pseudo-1D carbon nanostructures that display similar, unique electronic properties to 2D graphene. However, lateral confinement of electrons within GNRs generates a sizeable bandgap thus elevating these materials to a privileged position for high-performance electronic device applications. While traditional synthetic approaches toward GNRs (i.e. top-down techniques) lack complete control over edge structure, width, and chemical functionality, recent examples of bottom-up synthetic routes, both in solution and on catalytic metal surfaces, have provided an avenue toward atomically defined, smooth edge GNRs. Nonetheless, there are still many major challenges that need to be addressed in order to apply these materials into the next generation of electronic device applications. Several of those challenges are related to: (1) the deposition, proper alignment, and orientation of solution synthesized GNRs (Chapter 2), (2) the design and synthesis of low band gap GNR structures for uses in traditional field effect transistor applications (Chapter 3), and (3) the rational synthesis of GNR heterostructures (block polymers) in order to access exotic physical phenomena (Chapter 4). Herein, I have developed several methods to address all three of these challenges, providing a major steppingstone toward incorporation of GNRs into next generation electronic devices.

Condensed matter physics today stands at the forefront of exploring unusual emergent material properties and exotic phenomena. At UC Berkeley, my research has been driven by a fundamental question: Can we engineer fundamental properties of novel organic quantum materials to address real-world problems? I believe the answer to this question lies at the intersection of various fundamental disciplines. In chemistry, my focus has been developing on-surface reactions that enable the assembly of unprecedented chemical structures, giving access to exotic materials with uncommon electronic structures. These synthetic innovations have been translated into developing organic quantum materials with unique properties in condensed matter physics.

In Part 1 of this dissertation, we start with the tight-binding approximation and density functional theory to illustrate the theoretical principles for constructing organic quantum materials (Chapter 1). We then focus on the carbon material system under investigation, examining the quantum confinement effect in Chapter 2, followed by an introduction to the scanning tunneling microscope that we used in this research.

In Part 2, we present several successful examples of constructing novel organic quantum materials based on the theoretical and instrumental background from Part 1. To this end, we have developed the low-dimensional N-heterocyclic carbene lattices, utilizing the frontier orbital to engineer one-dimensional to two-dimensional low-workfunction materials (Chapter 4). Additionally, we have employed orbital engineering concepts to construct a Dirac nodal-line semimetal in a dual-square covalent-organic framework (Chapter 5). The final example in Chapter 6 discusses the spontaneous symmetry-breaking Jahn-Teller distortion effect in an unconventional π-conjugated polymer.

The exponentially increasing demand for smaller, faster, and more energy efficient electronic devices represents a monumental challenge in designing the next generation of post-silicon functional electronic materials. Traditionally, device architectures based on organic/inorganic semiconductors are fabricated using a top-down approach; their performance is inherently limited by the spatial resolution of photolithographic techniques. Fischer group’s goal is to understand, control, and to be able to harness the exceptional electronic/magnetic properties emerging from nanoscale graphitic materials by developing novel synthetic strategies toward graphene nanoribbons (GNR) via bottom-up synthesis from atomically defined molecular precursors. The semiconducting properties of GNRs can be modulated by varying their width, edge structure and the identity of atoms in the periodic lattice.

My thesis work will detail the power of organic synthesis in design of molecular precursors en route to GNRs, the nanoribbon formation analysis techniques, and the outlook toward application of GNRs in a low-power, tunneling field-effect transistor device architecture (Chapter 1). A large part of my work has aimed at developing new synthetic strategies to access heteroatom-substituted GNRs. After making advances toward BN-substituted GNR precursors, I have discovered a molecular scaffold which furnishes the first example of a surface-mediated C–N bond formation in the context of a 7-AGNR segment (Chapter 2). I have also prepared a series of molecular precursors that have led to the formation of a new family of nitrogen-substituted zigzag GNRs, which possess intriguing spin-separated states along the edges (Chapter 3). In addition to exploring heteroatom substitution, I investigated all-carbon GNR scaffolds that utilize topological band engineering concepts in order to explore new variables that will allow rational tuning of electronic properties in GNRs. I have prepared multiple precursors to continue the realization of topologically protected interface states in GNRs, which are generated by construction of a superlattice of well-defined ribbon segment heterojunctions (Chapter 4).

While the discovery of 2D graphene offered incredible promise for next generation electronics, device fabrication was severely hindered by the semi metallic nature of graphene. Graphene nanoribbons (GNRs) are quasi-1D nanostructures with similar electronic properties to the parent graphene, and the additional benefit of a widely tunable band gap. The ability to modulate the band gap in GNRs makes them particularly promising candidates for high-performance electronic devices. The band gap in these materials is intimately linked to the atomic structure of the GNR and it is essential to understand the connection between electronic structure and various parameters such a width, dopant position/concentration, and edge topology. To determine this relationship, the synthesis of atomically precise GNRs is crucial. Traditional synthetic techniques for GNR synthesis (e.g. lithographic patterning or the unzipping of carbon nanotubes) fail to meet this requirement. Instead, this thesis will focus on bottom-up synthetic techniques where a molecular precursor is sublimed onto a metallic substrate to yield the desired GNR. Moreover, the metallic substrate provides the ideal platform for electronic and atomic structure characterization using Scanned Probe Microscopy (SPM) techniques.

In this thesis, I will begin by explaining exotic magnetic phenomena that emerge from graphene nanoribbons with zigzag edge topology (Chapter 2). This chapter will discuss all-carbon GNRs and GNRs with nitrogen dopants to ascertain the effect of the dopant atom. Next, I provide information about low band gap GNRs that could be ideal candidates for next generation electronic devices (Chapter 3). Afterwards, I will consider the potential to form Nitrogen bonds with the underlying metallic substrate (Chapter 4). The final chapters in this thesis will take advantage of topological engineering to access low band gap GNRs (Chapter 5) and metallic GNRs (Chapter 6) that can be used to construct high-performance electronic devices entirely from GNRs.

Enediyne chemistry is a field with growing potential especially in the industries of material synthesis, catalysis, and therapeutic drug design. Enediynes offer a flexible synthetic handle with which to design these materials. However reactivity of enediynes is complex and in need of extensive and unambiguous characterization (Chapter 1). Imaging techniques like non-contact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM) can be utilized for unambiguous determination of reaction products. Using, nc-AFM and STM, supported with ab initio density functional theory (DFT), we provide a glimpse into the reactive capabilities of enediynes and precisely characterize the complex product mixture from a reaction of 1,2-bis((2-ethynylphenyl)ethynyl)benzene (Chapter 2). Tailoring reaction conditions, including enediyne structure, can also result in tailoring the product of enediyne cyclization reactions. We characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1’-bi(indenylidene), a poly-acetylene derivative that we have obtained through cooperative C1–C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1’-bi(indenylidene) diradical intermediates (Chapter 3). These chemical transformations offer contrast to those from our initial investigation prompting a comprehensive study of the molecular energetics and conformational dynamics underlying these transformations. We report the detailed investigation of a surface-catalyzed cross-coupling and sequential cyclization cascade of 1,2-bis(2-ethynyl phenyl)ethyne on Ag(100) supported by theoretical simulations. These simulations indicate that the kinetic stabilization of experimentally observable intermediates is observably influenced by selective energy dissipation to the substrate and entropic changes associated with key transformations along the reaction pathway (Chapter 4). Finally, we begin to build on a motif by which to synthesize one-dimensional materials using the reactivity lessons learned from surface studies of enediynes. Synthesis of new small molecule enediynes is discussed, the thermodynamics of Bergman cyclization is considered and evaluated, and polymers are synthesized using Bergman Cyclization Polymerization (BCP) (Chapter 5).

Abstract

Synthesis of Doped Graphene Nanoribbons from Molecular and Polymeric Precursors

by

Ryan Randal Cloke

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Felix Fischer, Chair

As electronic devices continue to shrink and energy problems continue to grow, nanoscale materials are becoming increasingly important. Graphene is a material with exceptional promise to complement silicon in next-generation electronics because of its extraordinary charge carrier mobility, while also finding a role in cutting-edge energy solutions due to its high surface area and conductivity. Improving on this material even further by reducing the width of graphene to nanoscale dimensions with atomically-precise dopant patterns is the subject of this thesis. Nanometer-wide strips of graphene, known as graphene nanoribbons (GNRs), offer the advantages of semiconducting behavior, combined with more accessible surface area compared to bulk graphene (Chapter 1). Additionally, it is demonstrated that GNRs can be doped with atomic precision, allowing for intricate modulation of the electronic properties of this material, which was studied by STM, STS, and nc-AFM (Chapter 2). Controlled growth of GNRs on surfaces is still an outstanding challenge within the field, and to this end, a variety of porphyrin-GNR template materials were synthesized (Chapter 3). The GNRs obtained in this work were also synthesized in solution, and it was shown that these materials possess excellent properties for applications in hydrogen storage, carbon dioxide reduction, and Li-ion batteries (Chapter 4). A prerequisite for solution-synthesized GNRs, conjugated aromatic polymers are an important class of materials in their own right. Therefore, Ring-Opening Alkyne Metathesis Polymerization was developed using conjugated, strained diynes (Chapter 5). The resulting conjugated polymers were explored both for their own materials properties due to a remarkable self-assembly process that was discovered, and also as precursors to GNRs (Chapter 6). This work advances the fundamental understanding of carbon-based nanostructures, as well as the large-scale production of GNRs for next-generation energy and electronics applications.

Graphene nanoribbons (GNRs) have been predicted to have exceptional traits for electronic, semiconductor, qubit, and logic applications, as well as representing a versatile platform for fundamental investigations in exotic physical phenomena. To access the remarkable electronic and magnetic properties predicted in these scaffolds, materials need to be synthesized with extreme precision. Bottom-up synthesized GNRs offer exceptional control of the resultant structure, down to the atomic level. This uniquely allows access to probe structures designed with exact molecular specifications. As the field develops and new increasingly complex scaffolds with designer attributes are envisioned, new methodologies and monomers must be designed to meet these increasing demands.Chapter 2 of this dissertation investigates the synthesis of structures that can utilize topological state engineering to influence the electronic properties of nanographenes and GNRs. This involved detailed investigations into pyrylium and xanthylium chemistry and the reactions of sterically hindered systems. Chapter 3 investigates potentially metallic GNR scaffolds for integration with easily accessible semiconducting GNRs. This work utilizes cyclization chemistry in the synthesis of cyclopentane rings, and led to the synthesis of a GNR that shows two distinct morphologies upon STM tip activation. Chapter 4 expands upon the reported synthesis of 6N-ZGNR to create new monomers that were predicted to form GNRs with novel properties. This work led to the development of a complimentary synthetic route that enables the synthesis of acridines previously inaccessible with the reported monomer synthesis. Finally, chapter 5 examines the synthesis of asymmetric N-ZGNR polymer precursors, labile solubilizing groups to facilitate MAD transfer, and 9-AGNR terminators.