Self-assembled monolayers (SAMs) are an advantageous platform for probing the fundamental interactions that dictate the spontaneous formation of nanostructures and supramolecular assemblies and directly affect macroscale properties. As such, SAMs provide an avenue for creating surfaces with defined chemical and physical properties. The assembly of these nanoscale constructs is driven by three primary factors: the interface between the substrate and the monolayer, the interactions between the adsorbate molecules, and the interface between the monolayer and the environment. I studied an icosahedral cage boron cluster, the carborane, as a building block for SAMs with properties that we can tune to advantage. Carboranes have several favorable traits, including providing a scaffold for a variety of functional groups. A chalcogenide group, typically a thiol, is used for surface attachment; moreover, bifunctional carboranes also enable control of the valency during assembly and greater reactivity at the environmental interface of the SAM. Additionally, isomers of carboranethiol have distinct dipole moments in terms of orientation and magnitude. The dipoles can lead to the formation of long-range dipole dipole networks within the SAM, which can stabilize the SAM and also modify the surface properties of the material. The rigid, symmetric backbone of the carborane cage results in SAMs that are relatively pristine and defect free. Due to these advantageous traits, carboranes enable the creation of monolayers with tunable interactions at the SAM interfaces. This system not only enables myself and others to study the molecular forces of assembly but also facilitates the simultaneous modification of both chemical and physical properties of surfaces and interfaces.
This thesis describes several carborane based surface assemblies and the variable interactions they have within the SAM interfaces. The introduction of a second thiol group to the carborane cage can be used to modulate the interaction of the SAM with the substrate. Carboranedithiol SAMs exhibit two binding modes, a monovalent state and a divalent state. The presence of these two modes enables tuning of valency using acid base chemistry and thus the ratio of singly bound to dual bound surface molecules can be modified during deposition.
Another avenue to alter the interactions at the substrate-monolayer interface is to use an alternative functional group for surface attachment. A chalcogenide group similar to thiol is selenol, however carboraneselenolate SAMs have a distinct surface morphology compared to carboranethiolate SAMs. Carboraneselenolate SAMs exhibit a dynamic double lattice where surface molecules appear to switch between high- and low-conductance binding modes. This morphology is consistent with other cage molecule selenolate SAMs and is typically associated with substrate-mediated interactions. In contrast, the carboraneselenolate SAMs are resistant to thermal rearrangement and desorption due to the dipole dipole interactions within the monolayer.
Carboranethiols can be modified by adding a carboxylic acid functional group that both alter the interactions within the monolayer and provide a platform for further reactions at the environmental interface. The introduction of a laterally positioned carboxyl functional group increases the steric demands of the molecule, thereby decreasing the packing density, but also enables hydrogen bonding interactions within the monolayer. The pKa of the surface bound carboxylic acid is shifted such that it is approximately two pH units less acidic than in solution. This shift is driven by the dielectric of the environment that the carboxyl group experiences on the surface, which is determined by the intermolecular interactions within the monolayer, partial desolvation, and the proximity to the substrate surface.
The carboxyl group also remains available for further chemistry on the surface and can coordinate with a variety of metal ions or be used as an attachment point for performing chemical lift off lithography (CLL). This lithographic technique was performed successfully on several types of carboxyl carboranethiolate SAMs. The use of these SAMs also enabled the characterization of the post CLL substrate surface via scanning tunneling microscopy. This analysis revealed the molecules left behind during the CLL process are either in small molecular islands or sparsely packed, highly mobile molecules.
There remain many opportunities for further chemistry to be performed with these carboxyl terminated SAMs or with carboranethiol SAMs with other additional functional groups. Carborane-based SAMs are a versatile system that provides a high degree of tunability at all three interfaces of a SAM. The work presented lays the foundation for further application in lithography, like CLL, as well as the use of these SAMs in organic electronics and devices and as interfacial materials.
