Title: Structure-Conductivity Relationships in Group 14-Based Molecular Wires
Description: Single-molecule electronics is an emerging subfield of nanoelectronics where the ultimate goal is to use individual molecules as the active components in electronic circuitry. Over the past century, chemists have developed a rich understanding of how a molecule’s structure determines its electronic properties; transposing the paradigms of chemistry into the design and understanding of single-molecule electronic devices can thus provide a tremendous impetus for growth in the field. This dissertation describes how we can harness the principles of organosilicon and organogermanium chemistry to control charge transport and function in single-molecule devices. We use a scanning tunneling microscope-based break-junction (STM-BJ) technique to probe structure-conductivity relationships in silicon- and germanium-based wires. Our studies ultimately demonstrate that charge transport in these systems is dictated by the conformation, conjugation, and bond polarity of the σ-backbone. Furthermore, we exploit principles from reaction chemistry such as strain-induced Lewis acidity and σ-bond stereoelectronics to create new types of digital conductance switches.

These studies highlight the vast opportunities that exist at the intersection between chemical principles and single-molecule electronics. Chapter 1 introduces the fields of single-molecule electronics, silicon microelectronics, and physical organosilane chemistry and our motivation for bridging these three worlds. Chapters 2-6 elaborate on the specific approach taken in this dissertation work, which is to deconstruct the molecular wire into three structural modules – the linker, backbone, and substituent – then synthetically manipulate each component to elucidate fundamental conductance properties and create new types of molecular conductance switches. Chapter 2 describes the first single-molecule switch that operates through a stereoelectronic effect. We demonstrate this behavior in permethyloligosilanes with methylthiomethyl electrode linkers; the strong σ-conjugation in the oligosilane backbone couples the stereoelectronic properties of the sulfur-methylene σ-bonds that terminate the molecule.

Chapter 3 describes the electric field breakdown properties of C-C, Si-Si, Ge-Ge, Si-O, and Si-C bonds. The robust covalent linkage that the methylthiol endgroup forms with the electrodes enables us to study molecular junctions under high voltage biases. Chapter 4 unveils a new approach for synthesizing atomically discrete wires of germanium and presents the first conductance measurements of molecular germanium. Our findings show that germanium and silicon wires are nearly identical in conductivity at the molecular scale, and that both are much more conductive than aliphatic carbon. Chapter 5 describes a series of molecular wires with π–σ–π backbone structures, where the π–moiety is an electrode–binding thioanisole ring and the σ–moiety is a triatomic α–β–α chain composed of C, Si, or Ge atoms. We find that placing heavy atoms at the α–position decreases conductance, whereas placing them at the β–position increases conductance. Chapter 6 demonstrates that silanes with strained substituent groups can couple directly to gold electrodes.

We can switch off the high conducting Au-silacycle interaction by altering the environment of the electrode surface. These chapters outline new molecular design concepts for tuning conductance and incorporating switching functions in single–molecule electrical devices.