UCLA

Two-dimensional (2D) surfaces, whether self-assembled monolayers (SAMs) or layered materials, are essential components of electrical and thermal systems, and as such, detailed understanding of these systems is of utmost importance. Scanning tunneling microscopy (STM) is the primary mode of investigation in each chapter. Self-assembled monolayers are ideal systems for exploring several classes of interactions: substrate-substrate, substrate-head group, and interface-environment. In the first half of this dissertation, I focus on the first two using carborane chalcogenides as model systems. Carboranes are cages composed of two boron and ten carbon atoms ideal for studying substrate-substrate interactions due to their rigid cages and ability to differ in intermolecular forces with a consistent footprint and pristine monolayer. Identification of individual isomers in a mixed monolayer had not yet been achieved, but using carboranethiols we are able to achieve this goal by exploiting an increased dipole from methylation without changing the footprint of the adsorbate. This solution was selected as STM records a convolution of physical and electronic height, and methylation affects both. In the same vein, carboraneselenols enable control over monolayer dynamics and long-range ordering of high-conductance molecules in a stochastic mix of high- and low-conductance molecules. By altering the direction and magnitude of the dipole in the carboraneselenol as well as the electronics of the cage-selenium attachment, the kinetic versus thermodynamic monolayers display different degrees of ordering linking low- and high-conductance molecules. While experimentation is still required, preliminary results suggest significantly different behavior for the two different carboraneselenols studied. The second half of this dissertation concerns 2D materials, namely Moiré patterns generated by twisted materials and MXenes, layered 2D transition metal carbides or nitrides. Moiré patterns are interference patterns arising from mismatched lattice and may be formed by twisting one sheet of graphene or molybdenum disulfide relative to another in a bilayer. Thermal transport, governed by phonons, is tuned by more than 600% as a function of the Moiré pattern and twist angle in bilayer graphene. Current efforts are focused on similar experiments in molybdenum disulfide. The final chapter explores the surfaces of Ti3C2Tx MXene with low temperature, ultrahigh vacuum STM for the first time. Surface chemistry is revealed at the atomic level and the effects of oxidation on the surface arrangement and electronics are discussed in conjunction with theoretical support. This project is a great first step toward the identification and control of surface groups and defects on MXene surfaces for applications in green energy storage, catalysis, and more.