Understanding the correlation between nanoscale architecture and its macroscopic function provides incredible insight to the development of next-generation electronic and photonic devices. Integration of functional molecular materials into structured devices requires in-depth understanding of the structure, reactivity, and acid-base properties of the adsorbed molecules on the surface. Self-assembled monolayers (SAMs) have been used extensively to probe and to tune the properties of these interfaces. Here, we examine different SAMs of diverse carborane derivatives. Due to the high axial symmetry and robust nature of these molecules, many of the common defects observed in n-alkanethiolate monolayers are eliminated, enabling controlled platforms to study the properties at substrate/environment interfaces. In addition to the presentation of relatively pristine and defect free monolayers, the carborane scaffold offers many synthetic opportunities to alter other physical and chemical characteristics. These characteristics include but are not limited to dipole magnitude and direction, a multiplicity of additional functionalizations, and diversity among anchoring groups. Therefore, we tested these three properties and the effects conferred on the substrate/monolayer interface, the interaction between adsorbate molecules, and lastly the interactivity between the monolayer and environmental interface.
This thesis describes several examples of carborane surface assembly and how they can be used to explore different facets of self-assembly. Working from the top-down and first exploring the adsorbate-environment interface we use a carboranethiol with a laterally positioned carboxylic acid. The addition of the carboxylic acid introduces changes within the monolayer such as an increased nearest neighbor spacing to maximize hydrogen-bonding interactions. Despite the lateral positioning of the carboxylic acid, the functional group still partakes in environment interactions that shift the pKa approximately two pH units less acidic in comparison to the molecule in free solution. This shift was attributed to several factors including the change in dielectric environment that the functional group experiences, the proximity of the carboxyl to the substrate, and partial desolvation.
To examine effects of adsorbate-adsorbate interactions within the monolayer we looked at monolayers composed of two o-carboranedithiol isomers. By orienting the two thiol groups on the carbon vertices or directly across from them, we demonstrate a controlled system of heterogeneously mixed oppositely oriented neighboring dipoles and analyze how they interact and assemble under ambient conditions.
To alter the interactions at the substrate-monolayer interface, switching the attachment functionality can be used as a method to directly probe differences in surface attachment behavior. In comparison to thiol anchoring groups, other chalcogens such as selenols, are great candidates to make this correlation. Carboraneselenol SAMs exhibit similar packing density when compared side by side to their carboranethiol counterparts, however, they also exhibit a dynamic double lattice. This double lattice demonstrates stochastic switching between molecules of high and low conductance states which has been observed in other cage molecule selenolate SAMs.
By utilizing an array of techniques such as scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and contact angle goniometry, we elucidate the roles of monolayer, substrate and environmental conditions, and their effects on the underlying mechanisms of self-assembly yet, many questions remain unanswered. These unanswered questions serve as useful outlets to pursue further routes of investigation on self-assembly and the characteristics and properties that have the potential to greatly further the field of interfacial electronic design and application.
